AIR DISTRIBUTIONSYSTEM DESIGNGood Duct Design Increases EfficiencyBUILDING TECHNOLOGIES PROGRAMOFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGYBuildings for the21st CenturyBuildings that are moreenergy efficient, comfortable,and affordable… that’s thegoal of DOE’s BuildingTechnologies Program.To accelerate the developmentand wide application of energyefficiency measures, theBuilding Technologies Program:• Conducts R&D on technologiesand concepts for energyefficiency, working closely withthe building industry and withmanufacturers of materials,equipment, and appliances• Promotes energy/moneysaving opportunities to bothbuilders and buyers of homesand commercial buildings• Works with state and localregulatory groups to improvebuilding codes, appliancestandards, and guidelines forefficient energy useT e c h n o l o g y F a c t S h e e tINTRODUCTIONCentral heating and cooling systems use an airdistribution or duct system to circulate heatedand/or cooled air to all the conditioned roomsin a house. Properly designed duct systemscan maintain uniform temperaturesthroughout the house, efficiently and quietly.WHY DUCT DESIGN IS IMPORTANTThe efficiency of air distribution systems hasbeen found to be 60-75% or less in manyhouses because of insufficient and/or poorlyinstalled duct insulation and leaks in the ductsystem. Properly designed and installed ductsystems can have efficiencies of 80% or morefor little or no additional cost, potentiallysaving a homeowner $50-200 or more per yearin heating and cooling costs. Moreover,efficient duct system designs can reduceequipment size, further saving money for newor replacement equipment.Duct systems that leak and/or do notdistribute air properly throughout the housemay make some rooms too hot and others too

cold. Leaky and unbalanced duct systems mayforce conditioned air outside andunconditioned air into the house. Thisincreases heating and cooling costs and mayalso draw humidity, dust, mold spores, andother contaminants into a home from the attic,crawlspace, or garage and radon gas from thesoil. In extreme cases, poorly designed andinstalled duct systems can inducebackdrafting—spillage of flue gases fromcombustion appliances (e.g., furnace, waterheater, fireplace) into the living space—primarily when atmospheric or natural-draftflues are used rather than poweredcombustion systems.Duct systems that are undersized, are pinched,or have numerous bends and turns may leadto low air flow rates and high air velocities.Low air flow rates cause the heating andcooling equipment to operate inefficiently.High air velocities increase noise.DUCT DESIGN OBJECTIVESThe objectives of good duct design areoccupant comfort, proper air distribution,economical heating and cooling systemoperation, and economical duct installation.The outcome of the duct design process willbe a duct system (supply and return plenums,ducts, fittings, boots, grilles, and registers)that• Provides conditioned air to meet all roomheating and cooling loads.• Is properly sized so that the pressure dropacross the air handler is within manufacturerand design specifications.• Is sealed to provide proper air flow and toprevent air from entering the house or ductsystem from polluted zones.• Has balanced supply and return air flows tomaintain a neutral pressure in the house.• Minimizes duct air temperature gains orlosses between the air handler and supplyoutlets, and between the return register andair handler.

Page 2SUPPLY DUCT CONFIGURATIONSSUPPLY DUCT SYSTEMSSupply ducts deliver air to the spaces that are to beconditioned. The two most common supply duct systems forresidences are the trunk and branch system and the radialsystem because of their versatility, performance, and economy.The spider and perimeter loop systems are other options.TRUNK AND

BRANCH

SYSTEM

In the trunk and branch system, a large main supply trunk isconnected directly to the air handler or its supply plenum and

serves as a supply plenum or an extension to the supply plenum.Smaller branch ducts and runouts are connected to the trunk.The trunk and branch system is adaptable to most houses, butit has more places where leaks can occur. It provides air flowsthat are easily balanced and can be easily designed to belocated inside the conditioned space of the house.There are several variations of the trunk and branch system. Anextended plenum system uses a main supply trunk that is onesize and is the simplest and most popular design. The length ofthe trunk is usually limited to about 24 feet because otherwisethe velocity of the air in the trunk gets too low and air flow intobranches and runouts close to the air handler becomes poor.Therefore, with a centrally located air handler, this duct systemcan be installed in homes up to approximately 50 feet long. Areducing plenum system uses a trunk reduction periodically tomaintain a more uniform pressure and air velocity in the trunk,which improves air flow in branches and runouts closer to theair handler. Similarly, a reducing trunk system reduces thecross-sectional area of the trunk after every branch duct orrunout, but it is the most complex system to design.SPIDER

SYSTEM

A spider system is a more distinct variation of the trunk andbranch system. Large supply trunks (usually large-diameterflexible ducts) connect remote mixing boxes to a small, centralsupply plenum. Smaller branch ducts or runouts take air from theremote mixing boxes to the individual supply outlets. This systemis difficult to locate within the conditioned space of the house.RADIAL

SYSTEM

In a radial system, there is no main supply trunk; branch ductsor runouts that deliver conditioned air to individual supplyoutlets are essentially connected directly to the air handler,usually using a small supply plenum. The short, direct ductruns maximize air flow. The radial system is most adaptable tosingle-story homes. Traditionally, this system is associatedwith an air handler that is centrally located so that ducts arearranged in a radial pattern. However, symmetry is notmandatory, and designs using parallel runouts can bedesigned so that duct runs remain in the conditioned space(e.g., installed above a dropped ceiling).PERIMETER

LOOP

SYSTEM

A perimeter loop system uses a perimeter duct fed from acentral supply plenum using several feeder ducts. This systemis typically limited to houses built on slab in cold climates andis more difficult to design and install.SpiderRadialTrunk and BranchPerimeter Loop

Page 3○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○

DUCT MATERIALS

Air distribution ducts are commonly constructed from sheetmetal, rigid fiberglass duct board, or flexible nonmetallic duct.Selection of duct material is based on price, performance, andinstallation requirements.Designs that use the house structure or building framing (e.g.,building cavities, closets, raised-floor air handler plenums,platform returns, wall stud spaces, panned floor joists) assupply or return ducts can be relatively inexpensive to install.However, they should be avoided because they are difficult toseal and cannot always be insulated. In addition, because suchsystems tend to be rough and have many twists and turns, it isdifficult to design them so as to ensure good air distribution.Even return plenums built under a stairway or in a closet, forexample, should be avoided if a completely ducted system ispossible.SHEET

METAL

Sheet metal is the most common duct material and can be usedon most all supply and return duct applications (for plenums,trunks, branches, and runouts). Sheet metal ducts have asmooth interior surface that offers the least resistance to airflow. When located in an unconditioned space, they must beinsulated with either an interior duct liner or exterior insulation.They must also be carefully and completely sealed duringconstruction/installation, using approved tapes or preferablymastic, because each connection, joint, and seam has potentialleakage. Screws should be used to mechanically fasten alljoints.RETURN AIR TECHNIQUESClosed interior doors create a return-airblockage in systems with only one or tworeturns. Grilles through doors or walls orjumper ducts can reduce house pressuresand improve circulation.RETURN DUCT SYSTEMSReturn ducts remove room air and deliver it back to the heatingand cooling equipment for filtering and reconditioning. Returnduct systems are generally classified as either central ormultiple-room return.MULTIPLE

-ROOM

RETURN

SYSTEM

A multiple-room return system is designed to return air fromeach room supplied with conditioned air, especially those thatcan be isolated from the rest of the house (except bathroomsand perhaps kitchens and mechanical rooms). When properlydesigned and installed, this is the ultimate return duct systembecause it ensures that air flow is returned from all rooms(even with doors closed), minimizes pressure imbalances,improves privacy, and is quiet. However, design andinstallation costs of a multi-room return system are generallyhigher than costs for a central return system, and higherfriction losses can increase blower requirements.CENTRAL

RETURN

SYSTEM

A central return system consists of one or more large grilleslocated in central areas of the house (e.g., hallway, understairway) and often close to the air handler. In multi-storyhouses, a central return is often located on each floor. Toensure proper air flow from all rooms, especially when doorsare closed, transfer grilles or jumper ducts must be installed ineach room (undercutting interior doors to provide 1 inch ofclearance to the floor is usually not sufficient by itself).Transfer grilles are through-the-wall vents that are oftenlocated above the interior door frames, although they can beinstalled in a full wall cavity to reduce noise transmission. Thewall cavity must be well sealed to prevent air leakage. Jumperducts are short ducts routed through the ceiling to minimizenoise transfer.Transfer grilleJumper ductDoor undercutWall grilleDoor grilleSupply air○○○○○○○○○○○○

Page 4AIR DISTRIBUTION SYSTEM DESIGNFIBERGLASS

DUCT

BOARD

Fiberglass duct board is insulated and sealed as part of itsconstruction. It is usually used to form rectangular supply andreturn trunks, branches, and plenums, although it can be usedfor runouts as well. Connections should be mechanicallyfastened using shiplap or V-groove joints and stapling andsealed with pressure-activated tapes and mastic. Fiberglass ductboard provides excellent sound attenuation, but its longevity ishighly dependent on its closure and fastening systems.FLEXIBLE

NONMETALLIC

DUCT

Flexible nonmetallic duct (or flex duct) consists of a duct innerliner supported on the inside by a helix wire coil and coveredby blanket insulation with a flexible vapor-barrier jacket on theoutside. Flex duct is often used for runouts, with metal collarsused to connect the flexible duct to supply plenums, trunks,and branches constructed from sheet metal or duct board. Flexduct is also commonly used as a return duct. Flex duct isfactory-insulated and has fewer duct connections and joints.However, these connections and joints must be mechanicallyfastened using straps and sealed using mastic. Flex duct iseasily torn, crushed, pinched, or damaged during installation. Ithas the highest resistance to air flow. Consequently, if used, itmust be properly specified and installed.DUCT AND REGISTER LOCATIONS

Locating the air handler unit and air distribution system insidethe conditioned space of the house is the best way to improveduct system efficiency and is highly recommended. With thisdesign, any duct leakage will be to the inside of the house. Itwill not significantly affect the energy efficiency of the heatingand cooling system because the conditioned air remains insidethe house, although air distribution may suffer. Also, ductslocated inside the conditioned space need minimal insulation(in hot and humid climates), if any at all. The cost of movingducts into the conditioned space can be offset by smallerheating and cooling equipment, smaller and less duct work,reduced duct insulation, and lower operating costs.There are several methods for locating ducts inside theconditioned space.• Place the ducts in a furred-down chase below the ceiling (e.g.,dropped ceiling in a hallway), a chase furred-up in the attic, orother such chases. These chases must be speciallyconstructed, air-sealed, and insulated to ensure they are notconnected to unconditioned spaces.• Locate ducts between the floors of a multi-story home (runthrough the floor trusses or joists). The exterior walls of thesefloor cavities must be insulated and sealed to ensure they arewithin the conditioned space. Holes in the cavity for wiring,plumbing, etc., must be sealed to prevent air exchange withunconditioned spaces.• Locate ducts in a specially-constructed sealed and insulatedcrawlspace (where the walls of the crawlspace are insulatedrather than the ceiling).Ducts should not be run in exterior walls as a means of movingthem into the conditioned space because this reduces theamount of insulation that can be applied to the duct and thewall itself.A supply outlet is positioned to mix conditioned air with room airand is responsible for most of the air movement within a room.Occupant comfort requires that supply register locations becarefully selected for each room. In cold climates, perimeter flooroutlets that blanket portions of the exterior wall (usuallywindows) with supply air are generally preferred. However, intoday’s better insulated homes, the need to locate outlets nearDUCTS INSIDE CONDITIONED SPACE

Page 5AIR DISTRIBUTION SYSTEM DESIGNthe perimeter where heat loss occurs is becoming lessimportant. In hot climates, ceiling diffusers or high wall outletsthat discharge air parallel to the ceiling are typically installed. Inmoderate climates, outlet location is less critical. Outlet locationsnear interior walls can significantly reduce duct lengths(decreasing costs), thermal losses (if ducts are located outsidethe conditioned space), and blower requirements. To preventsupply air from being swept directly up by kitchen, bathroom, orother exhaust fans, the distance between supply registers andexhaust vents should be kept as large as possible.The location of the return register has only a secondary effecton room air motion. However, returns can help defeatstratification and improve mixing of room air if they are placedhigh when cooling is the dominant space-conditioning needand low when heating is dominant. In multi-story homes withboth heating and cooling, upper-level returns should be placed

high and lower-level returns should be placed low. Otherwise,the location of the return register can be determined by whatwill minimize duct runs, improve air circulation and mixing ofsupply air, and impact other considerations such as aesthetics.DUCT DESIGN METHODThe air distribution system should be designed at the same timethe house plans are being developed, following the proceduresin the Air Conditioning Contractors of America’s (ACCA’s)Manual D: Residential Duct Systems. Planning locations forductwork, structural framing, plumbing, and electrical wiringsimultaneously avoids conflicts between these systems.The following eight steps should be followed in the design ofan air distribution system to ensure efficiency and comfort:1. Select the general type of heating and cooling equipment(e.g., furnace, heat pump, air conditioner). The heating andcooling equipment should be selected based on occupantpreferences, availability of different fuels (e.g., natural gas,electricity), installation costs, and operating costs.2. Select the general type of air distribution system (supply andreturn duct systems). The general designs and duct materialsfor the supply and return duct systems should be selectedafter considering the type of equipment selected and itslocation, the local climate, the architectural and structuralfeatures of the house, zoning requirements, and installationand operating costs. ACCA’s Manual G: Selection ofDistribution Systems and Manual RS: Comfort, Air Quality, andEfficiency by Design can assist in this selection.3. Calculate the design heating and cooling loads of each roomof the house and the loads that are associated with the entirehouse using ACCA’s Manual J: Residential Load Calculation(eighth edition). Room loads are used to determine the air flowneeded for each room, and the house loads are used to sizeand select specific heating and cooling equipment models.4. Size and select the specific models of the heating and coolingequipment using ACCA’s Manual S: Residential EquipmentSelection. This precedes the duct sizing calculations because,in residential applications, the blower (fan) data of the selectedequipment establish the duct design criteria. In addition,identify any component or device (e.g., filter, humidifier,electric resistance heater, cooling coil) that was not includedwhen the blower data and their associated pressure dropswere developed.5. Develop a scale drawing or rough sketch of the air distributionsystem showing the location of the air handling equipment,supply outlets, return openings, loads and air flow ratesassociated with each supply and return register, location ofduct runs, lengths of straight duct runs, fitting types, andequivalent lengths of the fittings. Be sure to account for thedirection of joists, roof hips, and other potential obstructionssuch as two-story foyers or rooms.6. Determine the size of all the ducts based on the room loads,blower data, pressure drops of additional components ordevices, and equivalent duct lengths following the proceduresin ACCA’s Manual D: Residential Duct Systems. Several ductlayouts may need to be examined before reaching a final design.7. Select and size the air distribution system devices (returngrilles and supply air diffusers, grilles, and registers) usingACCA’s Manual T: Air Distribution Basics for Residential andSmall Commercial Buildings. These must be selected tomaintain air velocities below values that will cause noise but,

in the case of supply outlets, sufficiently high so that air isdisbursed to exterior walls or ceilings as desired.8. Select the insulation levels for the duct system in accordancewith the 2000 International Energy Conservation Code.

Page 6For more information, contact:Energy Efficiency andRenewable EnergyClearinghouse (EREC)1-800-DOE-3732www.eere.energy.govOr visit the Building TechnologiesProgram Web site atwww.buildings.govOr refer to A Builder’s Guide toResidential HVAC SystemsNAHB Research Center800-638-8556www.nahbrc.orgOr refer to the Residential DuctDesign: A Practical Handbook(Report CU-7391)Electric Power Research Institute800-313-3774 press 2www.epri.comWritten and prepared for theU.S. Department of Energy by:Southface Energy Institute404-872-3549www.southface.orgU.S. Department of Energy’sOak Ridge National LaboratoryBuildings Technology Center865-574-5206, www.ornl.gov/btcManuals D, G, J, RS, S, and Tcan be obtained from theAir Conditioning Contractorsof America1712 New Hampshire Ave., NW,Washington, DC 20009202-483-9370, www.acca.orgThe International EnergyConservation Code can beobtained from the InternationalCode Council, 703-931-4533www.intlcode.orgNOTICE: Neither the United Statesgovernment nor any agencythereof, nor any of their employees,makes any warranty, express orimplied, or assumes any legal liabil-ity or responsibility for the accu-racy, completeness,or usefulness of any information,apparatus, product, or process dis-closed. The views and opinions ofauthors expressed herein do notnecessarily state or reflect those ofthe United States government orany agency thereof.

AIR DISTRIBUTION SYSTEM DESIGNPrinted with a renewable-source ink on paper containing at least50% wastepaper, including 20% postconsumer waste.March 2003 DOE/GO102002-0782

DESIGN RECOMMENDATIONS ANDKEY DESIGN ELEMENTSIn designing the air distribution using ACCA’sManual D: Residential Duct Systems, considerthe following recommendations beforefinalizing the design:

• Design the air distribution system to belocated inside the conditioned space of thehouse to the greatest extent possible. Do notlocate ducts in exterior walls.• The entire air distribution system should be“hard” ducted, including returns(i.e., building cavities, closets, raised-floor airhandler plenums, platform returns, wall studspaces, panned floor joists, etc., should notbe used).• In two-story and very large houses, considerusing two or more separate heating andcooling systems, each with its own ductsystem. In two-story homes, for example,upper stories tend to gain more heat insummer and lose more heat in winter, so thebest comfort and performance is oftenachieved by using separate systems for theupper and lower stories.• Consider supply outlet locations near interiorwalls to reduce duct lengths.• Locate supply outlets as far away fromexhaust vents as possible in bathrooms andkitchens to prevent supply air from beingswept directly up by the exhaust fans.• Consider installing volume dampers locatedat the takeoff end of the duct rather than atthe supply register to facilitate manualbalancing of the system after installation.Volume dampers should have a means offixing the position of the damper after the airdistribution system is balanced.• When using a central return system, include(a) a return on each level of a multi-storyhouse, (b) a specification to install transfergrilles or jumper ducts in each room with adoor (undercutting interior doors to allow1 inch of clearance to the floor is usually notsufficient), and (c) if at all possible, a returnin all rooms with doors that require two ormore supply ducts.• Specify higher duct insulation levels in ductslocated outside the conditioned space thanthose specified by the 2000 InternationalEnergy Conservation Code, especially whenvariable-speed air handling equipment isbeing used. Lower air flows provided byvariable-speed heating and cooling systemsto improve operating efficiency increase theresident time of air within the air distributionsystem, which in turn increases thermallosses in the winter and thermal gains in thesummer. Attic insulation placed over ductshelps where it is possible.• Specify that all duct joints must bemechanically fastened and sealed prior toinsulation to prevent air leakage, preferablywith mastic and fiberglass mesh. Considertesting of ducts using a duct blower toensure that the air distribution system is

tight, especially if ducts are unavoidablylocated in an unconditioned space. A typicalrequirement is that duct leakage (measuredusing a duct blower in units of cubic feet perminute when the ducts are pressurized to25 Pascals) should not exceed 5% of thesystem air flow rate.

Air DistributionBy: Robert J. Tsal, Ph.D. and Geoffrey C. Bell, PE

Energy Efficiency and Air Distribution

Air distribution through a laboratory is critical to the facility's safety and energy efficiency; nonetheless, air distribution systems are typically treated as an afterthought in the design process. Small ductwork is often routed circuitously, resulting in significant energy waste. In addition, the system air velocity is usually selected by rule-of-thumb and its noise impact is addressed afterward. However, the design of an energy-efficient air distribution system should be an iterative process, facilitated with the "T-Method," which incorporates life-cycle cost. A key to saving energy is to reduce the friction loss of the air distribution system by using large-diameter, round ductwork, efficient fittings, lower coil and filter face velocities, and energy-efficient noise attenuators. [Houghton, et al, 1992]

Air distribution components typically used in the research laboratory include:

Air handler with fan,

Cooling/heating coils,

Air filters,

Sound attenuators,

Ductwork or plenums,

Variable air volume (VAV) terminals or air balancing devices,

Duct fittings,

Fire and smoke protection devices (supply side only), and

Fume hoods, biological safety cabinets, or other exhaust devices. [Naughton, "HVAC Systems… Part 1," 1990]

Laboratory cleanrooms require special consideration because of the need to move large, laminar volumes of air for contaminant removal. For cleanrooms, energy efficiency is increased with efficient ductwork design and lower face velocities for coils, dampers, and filters. Naughton, in "HVAC Systems for Semiconductor Cleanrooms - Part 1" (1990) notes that,

When hundreds of thousands of cfm are involved, the reduction in fan static pressure of just 0.1 in. WC (24.9 Pa) can result in $7,200 per year of savings for a 10,000 ft2 (929m2) clean room. In addition to fan horsepower savings, each 0.1 in WC (24.9 Pa) will also produce 3.9 tons (13.7 kW) of air-conditioning savings due to the reduced fan heat load.

More:Ductwork Design FundamentalsLow-velocity Duct DesignDuctwork System EffectDuctwork Pressure BalancingDuctwork Air LeakageDuct Construction and LeakageDuct Leakage and Laboratory IsolationDuctwork Material and ConstructionDuctwork Layout RecommendationsDuct Shape ConsiderationsDuct FittingsDuct Fittings and EconomicsDuct Insulation GuidanceDuctwork SizingComputerized Ductwork SimulationDuctwork OptimizationEconomic Duct Optimization for California LaboratoriesDisplacement Air FlowCleanroom Air DistributionCleanrooms—Pressurized Plenum vs. Ducted Designs

Computerized Ductwork Simulation

Various methods and computer programs can be used to simulate airflow through a duct system. However, simulation methods can only model ductwork systems. No simulation method by itself produces an optimized air distribution system and no "standard" optimization program currently exists. [Scott, 1986]

The ASHRAE Handbook recommends the T-Method, which allows a user to select duct sizes and fan pressure, for duct simulation and "generalized" optimization that minimizes life-cycle cost. This design technique was incorporated into the 2001 ASHRAE Fundamentals Handbook. The T-method integrates the life-cycle cost of the air distribution system—first-cost, energy cost, and hours of operation—into the analysis of ductwork and fan selection. The system total pressure is optimally derived while costs are minimized. According to Shepard et al. (1995), "…the size and the energy use of the fan can be about 45 to 75 percent smaller with [the] T-method…" than with the conventional "equal friction" sizing method. However, acceptance of the T-method has not been widespread because hand calculation of the results is time-consuming. [Shepard et al., 1995; Tsal, 1995]

Quality duct design can be achieved only by using a comprehensive computer simulation program. A comprehensive simulation program allows an engineer to model changing cross sections; closing and opening dampers; removing, adding, and modifying fittings and duct-mounted equipment; selecting different diffusers; and changing fans or motors. A good computer program adjusts the fan-system operating point and shows new airflow quantities, velocities, pressure profiles, and how they differ from a preceding design. In other words, a comprehensive computer simulation program allows design engineers to model "what-if" in order to design laboratory environments that are as energy-efficient and flexible as possible. Ideally, a simulation program is user-friendly, window-driven, and capable of calculating supply, exhaust, and return systems using ducts of any shape. A comprehensive simulation program requires an extensive library of fittings and duct roughness. Finally, a good simulation program performs air balancing based on mass flow rate rather than on volumetric flow. The program should provide an opportunity to model control dampers, and

fire/smoke dampers, and it should help evaluate and improve ductwork layout. [Tsal, 1999] [T-DUCT, 1994]

More:Existing Duct Simulation MethodsT-Method Computerized Duct SimulationT-Method Ductwork Simulation ProgramT-Method Capabilities

Duct Fittings

The resistance of duct fittings must be determined carefully so air flow at outlets will be the same as called for in the building design. The DFDB [ASHRAE, 1994] lists the dynamic loss coefficient, C, for 228 duct fittings. When more than one type of fitting can be used equally, the fitting with the smaller loss coefficient should be chosen. However, a higher-pressure-loss fitting may be desirable if the fitting is located in a branch near a fan or helps provide pressure balancing. Specific examples of fitting performance based on energy consumption are provided elsewhere in this chapter.

More:Branch take offsElbowsDampersDiffusersStack discharge fittings

Branch take offsConnecting a straight duct to the side of a trunk duct is similar to connecting a branch take off; a dynamic loss is experienced when the air must turn 90°. Devices to reduce this dynamic loss include splitter dampers, extractors, scoops, cones, elbows, and 45° and radiused tap-ins. Listed in order of improving performance; a basic straight 90° take off falls in the middle of the list with C=1.2. Splitter dampers, extractors, and scoops are counterproductive; a 45° tap in is both economical and efficient (C=0.7). Coefficients will vary with area ratios and velocity ratios.

ElbowsElbow types range from mitered to long radius. Long radiused elbows are most efficient. An elbow with a centerline radius (r/D or r/W) of 1.5 is very efficient (C=0.19) and should be used in cases where duct air velocity is 13 m/s (2,500 fpm) or higher. A standard radiused elbow (r/D of 1.0) is more economical and only slightly less efficient (C=0.23); it is generally preferred. Sometimes only a mitered elbow (C=1.15) will fit into the space provided. In this case, a turning vane will reduce pressure losses if properly installed. However, a turning vane should not be used in a transitional (drop check) elbow or an elbow with an angle other than 90°. Accurate turning vane installation is critical to performance, so factory-made or carefully fabricated shop units are necessary. Improperly installed turning vanes can be counterproductive. A double thickness turning vane is more costly and its performance is much poorer than that of a single vane; however, a double thickness vane is required when spanning 0.9 m (3 ft) or more because of structural needs. Turning vanes have been known to become dislodged and have substantially blocked airflow because of seismic disturbance. There are almost always alternatives to the mitered elbow and vane combination. A very short radiused elbow with a single fully radiused vane (C=0.43) is often a good compromise. [Rozell, 1974]

Dampers A typical HVAC system contains numerous dampers. Many dampers are installed to maintain airflow balance but could be eliminated if different design principles were used. Most dampers are designed to restrict or stop airflow. These include flow-control, balancing, economizer, back-draft, face and bypass, and splitter dampers.

The rule of thumb for splitter dampers is: never use them. Another device will always work better. Their biggest problem is a ripple effect on system balance if the splitter moves. Balancing dampers should always be loose fitting in order to restrict but not stop air flow. A 25-mm (1") clearance around the closed balancing damper and duct is usually acceptable and very easy to install. Other dampers are built to stop airflow and should be factory manufactured so that performance data are available. Opposed blade dampers, the best choice for airflow are effective only through the center one-third of the 90° travel. At low angles, their dampening effect is negligible, and at high angles, flow rates change too rapidly compared to the angular displacement or travel of the damper. Dampers can be a source of noise, particularly if a system is poorly balanced. Computer modeling is recommended for studying these damper effects.

Dampers must be installed in places where airflow needs to be controlled and/or blocked. Dampers located directly behind an outlet tend to be noisy. A better location is in the final branch near the connection to the trunk duct. Wherever a balancing or volume damper is located, it should be accessible. Lay-in ceiling tiles provide good access; in a fixed ceiling, an access door is needed. Dampers should not be installed in hood exhaust systems even if the exhaust duct passes through a firewall. Use the UL approved alternative -- a properly supported, heavy-gauge steel, unobstructed duct.

Dampers have to withstand the maximum static pressure in a system. The maximum static pressure is the maximum that can be experienced in a system, not simply the pressure introduced by the fan during normal operation. Maximum static pressure usually occurs when all dampers in a system are closed except those on one flow path. Simulation computer modeling (T-Duct, 1994) can calculate maximum conditions.

DiffusersA duct diffuser is different from a room outlet. A duct diffuser is an expansion of duct size where the duct takes in or discharges air to a large space (usually the atmosphere). A stack discharge diffuser with a 14° included angle that doubles the duct area at the discharge reduces the C from 1.00 to 0.33. The performance of a vertical air inlet with a conical cap (C=1.2) can be improved by installing a 1.0D - 1.25D cone diffuser on the inlet end of the duct (C=0.3). Fan discharge diffusers and other connections are discussed elsewhere in this Section.

Stack discharge fittingsThe purpose of a stack discharge fitting is to keep rain out of a discharge stack. Although commonly used, cone caps have negative effects on the exhaust airflow path. Placing an inverted cone inside a cone cap helps somewhat but not usually enough. A vertical stack head (Figure 3) 25 mm (1") larger in diameter than the stack and four diameters long is recommended. It keeps almost 100% of rain out (rain usually falls at an angle) at virtually no dynamic loss other than that caused by the velocity head, C=1.0. Stack discharge diffusers are discussed below.

Figure 7.3

Duct Fittings and Economics

Clearly, if two fittings can be used interchangeably and their performance is equal but one costs less to install than the other, then the less costly fitting has better economic performance. However, the choice is rarely this simple. Some specific examples of economic considerations installation and performance of in duct fittings are:

More:TransitionsFilter-Bank ConnectionsFlexible duct

TransitionsTransitions are usually placed in a trunk or branch duct following a tap-in or branch because the airflow rate is reduced. An expanding transition at the fan discharge is usually used to reduce the main trunk velocity and its associated pressure drop. The least expensive transition maintains three sides straight while changing the fourth side. The changing of two or more sides presents both a layout and an installation challenge. Automated fitting fabricating machines have reduced the magnitude of this problem but it still exists. Ideally, a change in one dimension on one side in a one-section transition should create no more than a 5° angle (C=0.02).

Filter-Bank ConnectionsTypically, large changes in duct size occur in connections from a filter bank to a duct trunk. Building a tapered connection in a length of less than 1.2 m (4 ft) (Figure 4) causes sharply angled (60°) sides in a four-sided connection, C=0.85 (C varies with angle and area ratio). To improve construction economy, while reducing pressure drop, install a plenum box on the filter bank, and add a cone, C=0.085, or bell mouth, C=0.03, for the duct entry (Figure 5).

Figure 7.4

Filter-Bank ConnectionsTypically, large changes in duct size occur in connections from a filter bank to a duct trunk. Building a tapered connection in a length of less than 1.2 m (4 ft) (Figure 4) causes sharply angled (60°) sides in a four-sided connection, C=0.85 (C varies with angle and area ratio). To improve construction economy, while reducing pressure drop, install a plenum box on the filter bank, and add a cone, C=0.085, or bell mouth, C=0.03, for the duct entry (Figure 5).

Figure 7.4

Figure 7.5

Flexible ductCeiling outlets need to be well coordinated with duct locations to prevent excessive offsets in duct drops. Using flexible duct for the drop relieves contractors of the fear of missing an outlet with a duct-run centerline. Additionally, most ceiling outlets can be equipped with a round collar for connection to a flexible duct. If a flexible duct is to be run horizontally through a confined space to an outlet, a small box with a round collar facing the flexible duct can be provided; however, outlet performance will be compromised. Flexible duct lengths should be limited to 2 m (6 ft) and sharp bends should be avoided.

Duct Insulation Guidance

Insulation is applied to ductwork to enhance thermal performance and prevent condensation and dripping. Duct thermal performance needs enhancement since air transported through a supply duct is at a temperature different than that of the surroundings. Insulation reduces the rate of thermal loss to those surroundings. Without insulation, the air would need extra heating or cooling in order to arrive at the design supply air temperature. Return air ducts only need to be insulated if they pass through environments that adversely affect the return air temperature. Exhaust air ducts normally do not need insulation. Supply air ducts may be left un-insulated if they run exposed through the space being conditioned; this arrangement also reduces system first cost.

Insulation prevents condensation and dripping from ducts. Un-insulated cold air ducts very often have surface temperatures below the local dew point. At this temperature, condensate will form and

eventually drip off, causing an uncontrolled accumulation of moisture on the outside surface of the duct. Duct insulation eliminates the formation of condensate and consequently prevents rusting and staining.

Extra heating (or cooling) energy required to compensate for reduced thermal performance of un-insulated duct has a negative effect on the HVAC system's life-cycle cost. Therefore, duct insulation always presents an optimization problem. Since insulated duct costs much more than un-insulated, the recommended air velocity becomes a key factor in optimization. For instance, a higher air velocity reduces duct surface area and thus insulation cost.

Because of the relatively small temperature differences between supply air ducts and the spaces through which they ductwork are routed, a one-inch-thick fiberglass blanket is almost always sufficient. Insulation should be wrapped around the duct's exterior. A protective cover with a vapor barrier such as an aluminum foil, referred to as FKS, should be included in insulation specifications. Care must be exercised to protect exterior insulation integrity where insulation comes in contact with hangers, supports, and other structural members. Interior duct insulation (lining) should not be used in laboratory or cleanroom applications because the insulation tends to entrain microscopic particles into the airflow.

Special consideration must be given to ducts exposed to weather. Lagging materials or heavy metal covers over the insulation are commonly used to protect ductwork. A life-cycle cost analysis may be necessary to determine optimum insulation thickness when ducts encounter temperature extremes.

Ductwork Sizing

A large number of different duct sizing methods use arbitrary initial parameters based on engineering experience. These parameters are either initial velocity or pressure loss per unit of length. Two of the most widely used duct sizing methods, presented in the ASHRAE 2001 Fundamentals Handbook [ASHRAE, 2001], are the Equal Friction and the Static Regain methods.

The static regain ductwork design method has been the choice of engineers for many years even though it is more difficult and time-consuming than the equal friction technique. Static regain designs have been attributed to yielding more balanced systems that have better flow characteristics than equal friction systems. However, the equal friction method can provide equally efficient designs when experienced engineers use careful initial design assumptions.

A correctly sized duct system appropriately distributes design airflows throughout the facility. Sizing the duct system requires selecting all duct cross sections to result in a pressure-balanced system for the facility. Because there are practically an unlimited number of duct sizes and arrangements that will satisfy a facility's design air-flow requirements, sizing duct is still an art. The mathematical technique of "numerical analysis" incorporates iteration procedures that can provide a solution to duct-sizing problems. However, the solution is "constrained" by the "initial guess" which is provided by design engineers based on their individual experience. Also, it is important to remember that manual duct design methods, such as Equal Friction or Static Regain, do not minimize system life-cycle cost.

More:Pressure BalancingDuct oversizingIteration ProcessInitial guessExisting Duct Sizing Methods

Pressure BalancingAccording to the ASHRAE 2001 Fundamentals Handbook [ASHRAE, 2001], an engineer should after the initial sizing, "calculate the total pressure loss for all duct sections, then resize sections to balance pressure losses at each junction." The handbook does not explain a pressure balancing procedure. However, an experienced engineer can calculate and design a duct system that satisfies duct design requirements. Regardless of the method used, it is necessary to check duct systems for pressure balancing because an unbalanced system will not perform as intended. Typically, an imbalance results from higher than expected air flow in branch duct runs. For energy, economic, and noise considerations, it is generally advisable to reduce high air flows in ductwork by making ducts smaller rather than relying on dampers or orifices.

Duct oversizingIn a typical design process, ductwork size is minimized. In an integrated system design process that uses the concept of "right sizing", ductwork "oversizing" can be justified. Relatively small duct sizes require larger pressure drops and more fan energy than larger duct sizes. Therefore, for instance, extending a duct plenum makes a larger but effective lower-pressure distribution system. In addition, the end of a critical duct path run should be slightly oversized to keep velocity and pressure losses lower. It is usually more cost effective to maintain a duct size to the next branch or take-off rather than installing a fitting that would only reduce the trunk by 50 mm (2 inches). Another place where oversizing comes into play is noise elimination. In general, lower velocities prevent rumbling or whistles that would otherwise require energy-consuming sound attenuators to eliminate.

Iteration ProcessEfficient duct sizing is only performed through iterations because of the non-linear hydraulic characteristics of ducts caused by the interactions of pressures, flows, and cross-sectional areas. Unfortunately, existing duct-sizing methods do not explain how to perform iterations. The most difficult and important part of the calculation is that iteration leads to convergence that results in solutions for all variables in the equations.

Initial guessThe first step of any mathematical iteration procedure is known as the "initial guess." This is where initial values of unknown variables are selected. For HVAC applications, existing duct-sizing methods have established guidelines for initial guesses. Major unknown variables are duct cross-sectional areas and fan pressure. Duct-sizing methods search for duct cross sections by using arbitrary data, based on "engineering experience." These include assigned pressure losses per 100 ft of length for the system's longest branch according to the Equal Friction Method. Sometimes an initial guess is based on a table in which diameters correspond to flow rates, or on the following formula [Coad, 1985]:

Diameter = 0.85 (Flow)0.4

The initial guess is the starting point for a duct size calculation. Iteration is then conducted for cross-sectional areas that will satisfy all system requirements including design airflow, pressure balancing, velocity ranges, and geometrical constraints. Existing sizing methods are arranged to calculate based on an "initial guess."

Existing Duct Sizing MethodsThe traditional duct sizing methods are Equal Friction and Static Regain [ASHRAE, 2001]. Both methods were developed as expedient practical procedures and neither addresses optimization. Available computer programs are simply automated versions of these manual procedures.

The Equal Friction method creates an "initial guess" for duct sizing by establishing a constant pressure loss per unit of duct length. A number of sources recommend using 25 Pa (0.1 in. WG) pressure loss per 30 m (100 ft) total length. This length is selected for the "critical path," which is the longest branch in an air distribution system. It is assumed that the longest run will have the highest sum of total pressure loss. However, the longest run is not necessarily the run with the greatest friction loss, however, because shorter runs may have more elbows, fittings, and other flow restrictions. The procedure for using the Equal Friction method for duct design, including system balancing, follows:

Step 1. Select the "critical path" as the longest branch between fan and terminal outlets.

Step 2.Assign total pressure losses to each section of the "critical path" as the recommended pressure loss per unit of length multiplied by the actual section length.

Step 3. Calculate cross sections for the "critical path" using previously assigned total pressure losses, and correct these if necessary in order to satisfy velocity and geometrical constraints. Pressure loss in junctions cannot be calculated until branched cross sections are assigned. At this time, the pressure loss in junctions can be ignored; a constant pressure loss can be assumed for any junction, or the same cross sections can be used in branches as in trunk ducts.

Step 4.Sum the pressure losses in the "critical path" and select a fan so that fan total pressure is close to the sum of total pressure losses in the critical path. This pressure is called the "root pressure." At this step the root pressure is the same as the fan pressure. If the selected fan does not satisfy the pressure requirement, change the assigned pressure loss per unit length and repeat the process from Step 2.

Step 5.Assign a total pressure at each node of the critical path. To achieve pressure balancing, node pressures must be dissipated in corresponding branch sections.

Step 6. Exclude sections that belong to the critical path and select the longest branch from the remaining sections. This will be the new critical path and the node pressure is its root pressure.

Step 7.Calculate the total pressure loss per unit length of the new branch as its root pressure divided by its length. This pressure loss should be larger than the initial pressure loss per unit length assumed for the main critical path in Step 2.

Step 8.Repeat the calculation process for the new critical path, starting from Step 2.

Step 9.Continue this process until cross sections are calculated for all sections.

The engineer should achieve pressure balancing by selecting proper duct cross-sections rather than by using dampers.

Note that during such a calculation process, the pressure loss in the "critical path," which is already calculated, will change because of the change of cross sections in the branches of junctions. A major problem in this process is to satisfy the noise and geometry criteria. For example, a short section located close to the fan must be balanced with the long "critical path." Often, this can only be done by dampening flow. However, this creates noise caused by high velocities in the damper. Occasionally, lowering fan pressure can prevent noise, but more often it indicates that the layout of the system must be modified.

The Static Regain method of duct sizing is based on Bernoulli's equation, which states that when a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs. This is used as the major principle for sizing the ducts so that the increase in static pressure at each branch offsets the friction loss in the succeeding section of the duct. The static pressure should then be the same before each terminal and at each branch. This method provides a convenient means of designing a long duct run with several take offs so that the same static pressure exists at the entrance to each branch, outlet, or terminal take off. The Static Regain method applies to supply systems only. This method is also based on an arbitrary parameter, which is the velocity for the root section. The ASHRAE 2001 Fundamentals Handbook, Chapter 34, Table 10 [ASHRAE, 2001] gives the suggested range of velocities based on "engineering experience." When energy cost is high and installed ductwork cost is low, a lower initial air velocity is most economical. For lower energy costs and high duct costs, higher air velocity is most economical.

Like the Equal Friction method, the Static Regain method requires iterations. The major difference between the Static Regain and Equal Friction methods is that one uses the ratio of pressure loss to the length, and, in the other, the succeeding cross section is selected as a function of previously established air velocities at junctions:

(Pressure loss)1-2 = [(Velocity)12 - (Velocity)22] x (Density) / 2

Both methods are based on an initial guess.

The Static Regain method has been shown to have a number of deficiencies [Tsal and Behls, 1988]. The method has been partially modified [Brooks,1995] to compensate for some of these problems.

Popular traditional duct design methods, including Equal Friction and Static Regain [ASHRAE, 1997], provide engineers with design tools. However, these methods involve some engineering judgment and extensive manual recalculations, so air distribution systems designed by different engineers for identical situations will turn out to have different fans, duct sizes, costs, and overall system energy demands.

Tsal and Behls (1986) conducted a comprehensive analysis of existing duct-sizing methods . This analysis shows that these methods, after a number of iterations, can select cross sections that deliver the designed amount of flow to terminals; these methods cannot, however, select the most economically efficient design.

A large number of duct-sizing computer programs are available commercially. Most are based on manual sizing techniques. One example, the DUCTSIZE computer program developed by Elite Software, can size a duct system up to 500 sections using the Equal Friction, Static Regain, or Constant Velocity techniques. Ducts can be round, rectangular, or flat oval. DUCTSIZE calculates noise levels and required attenuation and presents a list of required materials. In addition, input data can be taken directly from a duct drawing file created by AutoCAD.

Computerized Ductwork Simulation

Various methods and computer programs can be used to simulate airflow through a duct system. However, simulation methods can only model ductwork systems. No simulation method by itself produces an optimized air distribution system and no "standard" optimization program currently exists. [Scott, 1986]

The ASHRAE Handbook recommends the T-Method, which allows a user to select duct sizes and fan pressure, for duct simulation and "generalized" optimization that minimizes life-cycle cost. This design technique was incorporated into the 2001 ASHRAE Fundamentals Handbook. The T-method integrates the life-cycle cost of the air distribution system—first-cost, energy cost, and hours of operation—into the analysis of ductwork and fan selection. The system total pressure is optimally derived while costs are minimized. According to Shepard et al. (1995), "…the size and the energy use of the fan can be about 45 to 75 percent smaller with [the] T-method…" than with the conventional

"equal friction" sizing method. However, acceptance of the T-method has not been widespread because hand calculation of the results is time-consuming. [Shepard et al., 1995; Tsal, 1995]

Quality duct design can be achieved only by using a comprehensive computer simulation program. A comprehensive simulation program allows an engineer to model changing cross sections; closing and opening dampers; removing, adding, and modifying fittings and duct-mounted equipment; selecting different diffusers; and changing fans or motors. A good computer program adjusts the fan-system operating point and shows new airflow quantities, velocities, pressure profiles, and how they differ from a preceding design. In other words, a comprehensive computer simulation program allows design engineers to model "what-if" in order to design laboratory environments that are as energy-efficient and flexible as possible. Ideally, a simulation program is user-friendly, window-driven, and capable of calculating supply, exhaust, and return systems using ducts of any shape. A comprehensive simulation program requires an extensive library of fittings and duct roughness. Finally, a good simulation program performs air balancing based on mass flow rate rather than on volumetric flow. The program should provide an opportunity to model control dampers, and fire/smoke dampers, and it should help evaluate and improve ductwork layout. [Tsal, 1999] [T-DUCT, 1994]

More:Existing Duct Simulation MethodsT-Method Computerized Duct SimulationT-Method Ductwork Simulation ProgramT-Method Capabilities

Existing Duct Simulation Methods There are a few numerical methods for calculating flow distribution in a duct system. The oldest method, called the equivalent nozzles method, was developed in Germany at the end of the 19th century by Bless [Lobaev, 1959]. The intent of this method is to replace the resistance of the ductwork with the equivalent resistance of a nozzle. The method is based on the quadratic law of resistance.

Kamenev (1938) developed the unit flow method. This method assumes flow through the terminal section is equal to one unit of flow. This method, as well as the equivalent nozzle method, is used in cases of quadratic law friction, which applies only when duct velocity is greater than 70 m/s (13,700 fpm). This velocity is impractical for HVAC ducts.

Butakov (1949) ref165 developed the duct characteristics method. Butakov [Butakov, 1949]used the old friction coefficient formula developed by Bless and substituted it into the Darcy-Weisbach equation. An important shortcoming of this method is that the use of Bless's formula results in pressure losses that differ by 20 percent from those found with the more accurate Colebrook (1938) ref168 or Altshul-Tsal [ASHRAE, 2001] equations.

Lobaev (1959) developed the equivalent resistance method that can be used for duct sizing and system simulation. This is the one of the best analytical methods for duct optimization.

Tsal and Shor (1967) used the steepest descent method for duct simulation and implemented it in a computer program. The descent step is normalized at each iteration as a function of maximum gradient-vector. The computer program calculates the flow distribution in branches; corrects to the fan operating point in the case of a change of flow; and calculates the required brake horsepower. Major applications include industrial exhaust systems that convey dust and where dampers are prohibited.

Tsal and Chechik (1968) developed the algorithm for the dynamic programming method for flow distribution . This method is more difficult to implement than the steepest descent method, but, unlike some other methods, it has no convergence problems.

The Newton-Raphson method was first used for network simulation by Stoecker, et al. (1974) ref205 for simulating central chilled-water systems and Gregory et al. (1975) ref181 for duct systems; the method was later translated into a computer code called TVENT1P. The main purpose of this program is the dynamic modeling of a duct system for tornado conditions using an electrical system analog for the airflow system in order to simulate the system's dynamics. TVENT1P uses only fixed resistance coefficients. After each iteration, the program must be interrupted; then, C-coefficients based on output flows must be recalculated and used as input data for the next iteration. Revised C-coefficients have to be calculated manually for all junctions, transitions, and elbows when the C-coefficients are a function of flow, velocity, or Reynolds number.

The well-known Equal Friction and Static Regain methods cannot simulate airflow.

T-Method Computerized Duct SimulationThe objective of duct simulation is to model airflows by obtaining pressure balancing [Tsal, et al.1990]. The system simulation solution is obtained when the total pressure loss for each system path is equal to the fan total pressure. The following requirements must be satisfied:

Kirchoff's first law. For each node, the mass flow in and out must be equal. Pressure balancing. The total pressure loss in each pathmust be equal to the fan total

pressure. In other words, for any node the total pressure losses for all paths must be the same.

Fan and system characteristics. Available fan pressure and flow depend on the fan characteristic curve. Fan flow and pressure must match the system flow rate and resistance.

The T-Method computerized duct simulation determines the flow within each section of a duct system for known duct sizes and fan characteristics. The T-Method duct simulation is based on the same tee-staging idea as Dynamic Programming [Bellman 1957 ref159; Tsal and Chechik, 1968]. The T-Method incorporates the following major procedures:

System condensing. The branched tree system is condensed into a single imaginary duct section with identical hydraulic characteristics and the same life-cycle cost as the entire system.

Selection of an operating point. The actual system flow and pressure are determined by locating the intersection of the fan and system curves.

System expansion. The condensed imaginary duct section is expanded into the original flow distribution system with appropriate airflow rates. The expansion procedure distributes the fan airflow throughout the system's sections. Unlike the condensing procedure, the expansion procedure starts at the root section where the fan is located and continues in the direction of the outlets.

To simulate a combined supply-return system, the distribution of the pressure losses between subsystems must be found. The T-Method can find the distribution by first condensing each subsystem separately and then expanding both condensed sub-roots, which are interpreted as two sections (supply and return) connected in series. [Farajian, et al, 1992]

T-Method Ductwork Simulation ProgramThe commercially available T-Method simulation program, T-Duct, consists of four major parts: an interactive window-driven preprocessor, a calculation processor, a numerical/graphical post-processor, and a database of fittings. The program calculates the actual airflow distribution throughout a system, adjusts the fan operating point, and checks the brake horsepower of the existing fan. The program output also shows the percent airflow difference between design and actual performance for each duct section, so the user can determine the need for corrective action. [T-Duct Program, 1994]

The program's interactive preprocessor accepts a wide variety of data, particularly for fittings and duct-mounted equipment. It presents only fittings suitable for a particular duct section and can display fittings on the screen. A full range of browse, edit, copy, move, erase, and error-detection techniques saves time for inputting identical sections or branches. The program can automatically convert data among three unit systems (SI, Inch-Pound, and Metric), and it has many help screens.

The processor is an iterative procedure. Iterations allow for matching the fitting and equipment local resistance coefficients with the actual airflows for each section of the duct. The convergence is very efficient, usually requiring few iterations.

The post-processor is partially interactive. It shows the actual operating point on fan and system curves, total and static pressures, and sectional and fitting data, including actual airflows and air velocities. The post-processor allows for correcting cross-sections, closing or opening dampers, and changing fan characteristics. Calculations can be repeated without leaving the program.

The Data Base has a number of libraries containing fittings and duct-mounted equipment from the ASHRAE Fundamentals Handbook [ASHRAE, 1997], the "HVAC Systems - Duct Design" [SMACNA, 1990], and various other sources.

The T-Method has a number of advantages compared to other simulation methods:

It uses similar techniques for duct optimization and simulation, It is appropriate for any duct shape, material, and air density, It is appropriate for supply, return, and exhaust systems, It recognizes variable C-coefficients, It can accommodate any fan characteristic curve, and It has an efficient convergence process.

The best way to perform duct calculations for laboratories is by using an appropriate computer program such as T-Duct.

T-Method CapabilitiesAnother example that illustrates the T-Method's capability is a simulation of the example in the Fundamentals Handbook Chapter, "Duct Design" [ASHRAE, 2001]. The example was analyzed to determine the airflow rates when system dampers are fully opened. Drastic differences were discovered where airflows changed from 23% to 37% compared to the original system design [Tsal, et al., 1990]. System analysis can also be done for situations such as partially occupied buildings, so engineers and contractors can predict flow distribution, locate dampers, and save energy by balancing the system, repositioning dampers and adjusting fan speed. The results of the simulation calculations are flow rate and pressure resistance of each section of duct, pressure at each node, and an operating point on the fan performance curve. The T-Method for duct simulation has also been used to analyze systems with: (1) fans working in parallel when one fan is shut down, and (2) a VAV system working in the minimum and maximum flow regimes.

Duct system simulation is needed in several situations for HVAC designs, including:

Analyzing air-flow redistribution in a multiple-fan system when one or more fans shut down, Analyzing pressure differences between adjacent confined spaces in a nuclear facility when a

DBA occurs, Analyzing air-flow distribution in a VAV system when there is terminal box flow diversity, Analyzing air flow redistribution resulting from modifications to the HVAC system, Analyzing system air flow for partially occupied laboratories, Determining the need for fan or motor replacement during retrofitting of an air distribution

system, Analyzing smoke control system performance during a fire when some fire/smoke dampers

close and others remain open,

Analyzing pneumatic conveying systems and manifolds, Analyzing the influence of a change in damper blade angle on air flow at existing terminal

outlets, Finding the operating point on the fan performance curve when duct size or damper blades

angles are changed, Connecting additional terminal outlets to an existing system, Analyzing the possibility for damper-generated noise, Determining the maximum static pressure that can be experienced by dampers when all

system dampers are closed except one.

Ductwork Optimization

Inefficient ductwork system design results in either wasted energy or installation of excessive ductwork. One could theoretically design a particular ductwork system as large as subway tunnels or as small as cocktail straws. A subway-tunnel system's cost is extremely high but energy cost is minimal. A cocktail-straw system's energy cost is extremely high but construction cost is minimal. Somewhere in between the tunnel and the straw is the optimum system and duct size.

A duct optimization method determines duct sizes and selects a fan that minimizes system life-cycle cost. Optimization can compare system costs for different fan pressures. Data variables needed for optimization include: initial cost, energy cost, operating time period, escalation rate, and interest rate. According to Tsal and Behls (1986) the three major optimization objectives for a ductwork system are:

1)Optimum total fan pressure. This is the operating point on a fan's curve that assures that a system supplies the necessary airflow to each terminal at a minimum life-cycle cost.

2)Optimum duct velocity ratio. This is the ratio between air velocities in all sections of a ductwork system that satisfies the requirements for minimum life- cycle cost. A further explanation of this ratio is presented in the referenced work by Tsal and Behls (1986).

3)System pressure balancing. The pressure provided by a system fan which delivers airflow to each outlet, is dissipated by ductwork and fittings. The best way to attain design airflow to each outlet is by pressure balancing through changing duct sizes rather than using dampers or other devices.

Dean and Ratzenberger [Dean, et al., 1985 ref171] compare a "not optimum" duct design, created using the Equal Friction method, with a computerized optimum design. The computer program selects optimum duct sizes, helps improve ductwork layout, and fits superior-performing round ducts into tight spaces in addition to maximizing the use of round ducts throughout the system. It selects all trunk duct diameters, fittings, and terminal boxes with run-out fittings in order to obtain approximately equal pressures at all terminal boxes for design load conditions. The computer program analyzes both the air handling system and exhaust/return ductwork in combination. When compared, the computerized design had substantially lower: first cost, operating cost, and noise level than the Equal Friction method design.

More:Duct Optimization PrinciplesExisting Tree-Network Optimization MethodsT-Method Duct OptimizationOptimization Calculation FormsEconomical Analysis From Beta Software

Duct Optimization PrinciplesMathematical programming states that any optimization problem can be defined as a process of minimization or maximization of an objective function in a space restricted by constraints. [Fox, 1971]

The objective function for duct optimization is the life-cycle cost, which is given by:

Life-cycle cost = (First year energy cost) (PWEF) + (Initial cost)

where,

PWEF is the present worth escalation factor.

The PWEF is:

PWEF = x 100%

where,

AER= annual escalation rate

AIR = annual interest rate

a= amortization period, years.

First year energy cost is determined by:

Energy cost = x (Fan Pressure)

Laboratory electrical unit energy cost depends on local industrial retail prices of electricity, including demand charges and consumption costs. The unit energy cost or electrical energy retail prices for all U.S. electric utilities can be obtained from Electric Sales and Revenue [EIA, 1995]. The costs are adjusted for 500-kW demand for industrial consumers, which includes laboratories. Data for the annual escalation rate (AER) are predicted by Utility Costs Forecasting [EIA, 1985 ref176] and Data Research Utility Costs Forecasting [Data Resources, Inc., 1985 ref170]. The accuracy of any calculation cannot be greater than the accuracy of the input data. Economic data are good only for current periods and cannot predict situations such as oil embargoes or Persian Gulf crises. Therefore, precise economic data are not needed for duct design. If the annual interest rate (AIR) is unknown, an interest rate of 6% can be used. If the amortization period (a) is unknown, 10 years can be used.

The initial cost includes the cost of ducts and HVAC equipment. The duct cost is presented as a function of the cost per unit area of duct surface, adjusted for straight ducts and fittings.

Installed duct prices are available from "Mechanical Cost Data" [Means, 1997]. These values are based upon a typical system layout, 25% of which is fittings. Duct costs include material, shop labor, field labor, shop drawings, shipping, and a 35% markup on costs for overhead and profit. Labor is figured at $26.50 per hour. More accurate optimization can be obtained by separating the cost of straight ducts from the cost of fittings. Cost data for fittings are also available from "Mechanical Cost Data" [Means, 1997].

The main equipment included in the objective function is a system's central air-handling unit. The pressure loss of duct-mounted equipment (coils, silencers, terminal control boxes) is included in the duct sections where this equipment is located.

An important factor in duct optimization is the cost of space required by ducts and equipment. This cost can be ignored if the space cannot be otherwise utilized. However, if saved space could be utilized, its cost must be included in the objective function. Including this additional cost could lead to reducing the size of ducts and thus increasing energy consumption.

Electrical energy retail prices vary widely. The maximum difference in electric energy costs between industrial customers in Saint Paul City, Alaska (50.66 c/kWh) and Douglas County, Washington state (1.62 c/kWh) is a factor of 31 to 1. Costs for ductwork range from $12.02 per square foot for 10-gauge galvanized iron to $3.10 per square foot for 26-gauge spiral ducts, a ratio of 3.9 to 1 (Wendes, 1986). Combining the two ratios yields a potential factor of 122 to 1 depending on locale and type of ductwork. Because of the electrical energy and ductwork price variations, there is a great potential for reducing the life-cycle cost of different duct systems.

It is important in duct optimization to satisfy all necessary constraints. A detailed explanation of each constraint can be found in Tsal and Adler (1987). The constraints are:

Kirchoff's first law. For each node, the mass flow rate "in" is equal to the mass flow rate "out."

Pressure balancing. The total pressure loss in each path must be equal to the fan total pressure. In other words, for any node the total pressure loss for each path is the same.

Nominal duct sizes. Each diameter of a round duct or height and width of a rectangular duct is rounded to the near nominal lower or upper size. Nominal duct sizes normally depend on manufacturers' standard increments. Ducts are available in 1-inch diameter increments to 20-inches and 2-inch increments for sizes larger than 20-inches Standard sizes can differ by country.

Air velocity restriction. This is an acoustic (ductwork regenerated noise) or particle conveyance limitation.

Pre-selected sizes. Duct diameters, heights, and/or widths can be pre-selected. Construction restrictions. Architectural space limitations may restrict duct sizes. Equipment. Central air-handling units and duct-mounted equipment must be selected from

those produced by industry.

Existing Tree-Network Optimization MethodsMany analytical and numerical methods for pipe and duct optimization have been developed during the last century. A comprehensive survey of existing numerical duct optimization methods was conducted by Tsal and Adler (1987). The first optimization method was developed by Grashoff in 1875 for a single pipeline. Several of the calculation procedures for duct optimization attempt to minimize total cost by establishing optimum velocities or friction rates. These procedures are based on the classical calculus minimization technique of setting the first derivative to zero in order to find the diameter of the pipe or duct or to determine air velocity.

The classical method of optimization for multi-path district heating systems was first applied by Shifrinson (1937) ref198 and for multi-path duct systems by Lobaev (1959) ref190. These techniques are impractical for manual calculation. According to Tsal and Adler (1987), analytical approaches can be effectively used only to identify trends in system behavior. A comprehensive analysis of a multi-path duct system was published by Bouwman (1982) ref162.

Computer-aided numerical optimization methods are divided into two categories, discrete methods, (coordinate descent, dynamic programming, and T-Method) and continuous methods (penalty function, Lagrange multipliers, reduced gradient, and quadratic search).

The coordinate descent method is the most common technique for duct optimization [Tsal and Chechik, 1968]. A number of different techniques are based on this method for selecting initial conditions, searching for the next duct section to be changed, and "freezing" selected diameters. Dynamic programming is one of the most powerful methods for multi-path tree network optimization [Tsal and Chechik, 1968]. The penalty function method transforms constrained problems into non-constrained problems, by adding penalty coefficients to the objective function.

This method has been applied to different networks by Tsal and Chechik (1968). Another way to directly optimize a network is by the Lagrange multipliers method [Zanfirov, 1933 ref223; Bertschi ref161, 1969; Kovaric, 1971 ref187; Stoecker et al., 1971]. The modified Lagrange multipliers method has been applied for network optimization by Murtagh (1972) ref194. The reduced gradient method [Arklin and Shitzer, 1979 ref151] is one of the best computerized-techniques for application to rectangular duct optimization. It performs nonlinear optimization with equality constraints and then applies the Newton-Raphson technique to find an optimum solution. A technique called quadratic search was introduced for a concave problem optimization by Leah et al. (1987) ref189. It was applied for chilled water system optimization.

Many of these methods can find the minimum of an unconstrained concave problem, but most fail to yield a solution that can be successfully used in practice. In general, the objective function is not uniformly concave. An example in Tsal and Adler (1987) explains this phenomenon. There is no analytical or numerical method that can easily find the global minimum and satisfy all duct system constraints.

T-Method Duct OptimizationAn advanced duct design optimization technique based on the T-Method is being developed by Tsal, Behls, and Mangel through cooperative research with ASHRAE [Tsal et al., 1986]. The economic analysis of the example from the Duct Design chapter of the ASHRAE Fundamentals Handbook(2001) re152 showed that significant initial or operating cost reductions are obtainable. In addition, the three requirements for optimized designs -- optimum fan selection, pressure balancing, and optimum sectional velocity ratios [Tsal and Adler, 1987] -- are satisfied by the T-method.

The T-Method's advantages over other optimization methods are:

It is appropriate for any duct shape and material; It is appropriate for supply, return, exhaust, and combined supply-return systems; It eliminates critical path selection; It acknowledges constraints (velocity and space limitations, pre-selected or maximum sizes); It includes pressure balancing; It rounds to nominal duct sizes; It selects optimum fan and motor or central air-handling unit; It optimizes the design for a pre-selected fan; and It has an efficient convergence process.

The T-Method has been expanded and found to be capable of optimizing duct systems with air leakage [Tsal et. al., 1998]. The T-Method incorporates the following major procedures:

1)System condensing: Condensing a branched tree system into a single imaginary duct section with identical hydraulic characteristics and the same life-cycle cost as the entire system.

2)Air-handling unit selection: Selecting an optimal fan and establishing optimal system pressure loss.

3) System expansion: Expanding the condensed imaginary duct section into the original system with optimal distribution of pressure selected in step 2.

The T-Method demonstrates that two duct sections connected in series can be condensed into a single imaginary duct section. The imaginary section will have the same: flow, pressure loss sum, and initial cost, as the individual sections. The T-Method shows that K-coefficients, the hydraulics characteristics of each section (which depend on flows, sizes, lengths and fittings) must satisfy the following:

K1-2 = ( K10.833 + K20.833 )1.2

Similarly, two duct sections connected in parallel can be condensed into an imaginary duct section that has the same pressure loss as individual sections, the sum of their flows, and the sum of their initial costs. The relationship between K-coefficients must satisfy the following:

K1-2 = K1 + K2

The T-Method identifies a duct tree system as a sequential number of sections connected in series and in parallel. In a tree system, all individual sections can be condensed into a single imaginary duct section. This process is performed numerically by a recursive procedure that starts from terminals and moves from tee to tee until it hits the root. This movement from tee to tee is the source of the T-method's name.

Most decisions about selecting an air-handling unit will fit into one of the following three categories.

Case 1. The optimum fan pressure is calculated using the classical method where the first derivative is made to equal zero. Once the desired fan pressure and flow are known, the fan can be selected from a catalog.

Case 2. A number of central air-handling units or fans with motors are being considered. The cost of each fan and motor, the total fan pressure, and coefficients of efficiency are known. A comparison is made of life-cycle costs of the system equipped with each fan. The optimum fan is then selected at minimum cost.

Case 3. Fan and motor are pre-selected based on necessary fan pressure.

In the T-method an expansion procedure distributes available fan pressure throughout the system sections. Unlike the condensing procedure, the expansion procedure starts at the root section and continues in the direction of the terminals.

An important advantage of the T-Method is that it can handle constrained optimization processes including non-linearity and integer duct-size rounding. Rounding means selecting a lower or higher nominal duct size. If the lower nominal size is selected, the initial cost decreases, but the pressure loss increases and may cause fan pressure to increase. If the upper nominal size is selected, the initial cost increases but the section pressure loss decreases. This saved pressure means a smaller nominal size can be used in the next sections in the duct network. Therefore, size rounding is also relevant to optimization. The T-Method contains a procedure that predicts the influence of the initial cost of different duct sizes for both a specific duct section and the remaining system. The rounding procedure is efficient but complicates the calculations. For manual calculation, a simplified procedure called the 1/3 boundary procedure is recommended. For this procedure, if a choice is to be made between two commercially available duct sizes where duct "A" is smaller than duct "B," the size difference between A and B is first divided into thirds. Then, if the calculated size is less than A + 1/3, duct A is chosen. If the calculated size is equal to or greater than A + 1/3, duct B is chosen. However, the 1/3 boundary procedure is just a rough approach. If the lower size is selected for a long duct with many local resistances, the pressure loss in the corresponding path may exceed the fan pressure capability. A final advantage of the T-Method shows that it can optimize a duct system with air leakage [Tsal et. al., 1998REFERENCES: Distribution Systems].

All existing analytical and numerical duct design methods except dynamic programming are iterative [Tsal and Adler, 1987]. The T-Method is iterative but relatively simple; it is also able to select the optimum pressure for each system section while incorporating pressure balancing. Many parameters such as C-coefficients for junctions and transitions depend on duct size and are not known at the beginning of the calculation process; they have to be defined during the iterations. The T-Method converges efficiently. Usually, five iterations are sufficient to obtain the optimum solution with a high degree of accuracy.

To optimize a combined supply-return system, the distribution of the pressure losses between the supply and return subsystems must be optimized. The T-Method does this by first condensing each of the subsystems. Next, both condensed sub-roots are interpreted as two sections connected in

series and a condensed root section is substituted for them. Then, a fan and motor or central air-handling unit is selected, and the pressure is distributed for the supply and return subsystem as in an expansion procedure. The T-Method can optimize both the supply and return subsystems as one system.

Abstract: Energy Efficiency and Architectural Programming

Modern research facilities provide usable space for laboratories, laboratory support areas, offices, and interactive spaces for formal and informal gatherings. The special equipment and environments required for research make these facilities complex and expensive to build and operate. Complying with building codes and considering building standards are part of the architectural programming process. The research organization priorities will set the tone for the incorporation of the energy-efficiency measures (EEMs) for the facility. It is important that the facility be able to accommodate changes in use by including flexibility in the original design. However, the facility's near-term energy use must not be overlooked even though the facility may plan for larger system capacity in the future. Architectural arrangements that provide laboratory isolation can result in energy efficiency benefits by using a design concept that includes modular degrees of isolation for the required controlled environments. The modular research laboratory provides an opportunity to arrange the environmental conditioning systems efficiently. Utility service coordination, by providing orderly pathways and routing, will reduce energy use by streamlining their layout and configuration. Minienvironments can reduce energy consumption greatly with their ability to confine energy-intense environments to small volumes.

CodesEnergy Efficiency and Codes

While it is a fact that codes and energy efficiency are inexorably linked, the codes have few requirements that affect the facility's energy efficiency directly. Recommended standards (see below) have a greater influence on the energy consumption of the laboratory facility. The building standards that apply to all occupancies throughout the state of California appear in the California Code of Regulations and the California Health and Safety Code. Laboratory facilities, per the Uniform Building Code (UBC), typically fall into one of three classifications; B, H-8, or H-7.

The occupancy classification is the key to any impacts of a building standard on the facility's energy efficiency. The energy engineer rarely will have a say in the determination of the facility's UBC classification. However, the indirect benefits of a less stringent classification can reduce energy consumption, e.g., by allowing the recirculation of air within a laboratory rather than requiring 100% outside air at all times. Therefore, the energy engineer should study the requirements of each classification to be familiar with their potential energy impacts and relate these findings to the project design team. For a general evaluation of codes and building energy-efficiency programs, see Lee, 1997.

More:California Health and Safety CodeUniform Building CodeUniform Fire CodeCalifornia Code of Regulations

Research laboratories exist to provide the precise environmental conditions required for research. These conditions require sophisticated, expensive, energy-intensive HVAC systems. Laboratories typically consume 300,000 to 400,000 BTUs per square foot per year or more, six to 10 times the number of BTUs consumed in a typical office building. However, energy consumption and operating costs can be reduced through "right sizing," choosing the most efficient and cost effective combinations of equipment and equipment sizes as well as managing the laboratory load, all to achieve energy efficiency. A comprehensive example of incorporating right-sizing techniques is provided in a report by Wrons (1998)ref324 on Sandia National Laboratories' Process and Environmental Technology Laboratory (PETL) located in Albuquerque, New Mexico. Right sizing is an iterative process; although new techniques are developed continuously, the basic elements are:

Life-Cycle Cost Analysis,

Conditioning System Capacity Analysis,

Diversity Analysis, and

Load Management Analysis. [Cooper, 1994]

Life-cycle cost analysis

Energy intensive environmental conditioning systems have high operational and first costs. Therefore, it is very important for the energy engineer to consider the optimum mix of operational and first costs to determine the system's life-cycle cost. Life-cycle cost (LCC) analysis accounts for all costs incurred for the HVAC system from installation through a chosen period of time, usually 20 years. Life-cycle cost analysis is a "yard stick" to measure the relative benefits of the choices available to the design team. When an energy-efficiency measure (EEM) happens to have the lowest first cost, an LCC analysis is not necessary.

Conditioning system capacity analysis

Estimating the conditioning capacity necessary for a laboratory includes a myriad of choices to determine the laboratory's HVAC system type and size. To make these choices intelligently, the engineer must understand the variability of the laboratory facility's load profile. Airflow rate through the facility is a subject of considerable debate that is primarily driven by the air change rate per hour (ACH) and the design fume hood face velocity.

Diversity analysis

Diversity analysis in a laboratory ventilation system accounts for the fact that not all laboratory spaces or fume hoods are operated at 100 percent, 24 hours per day. The larger the facility, the smaller the probability of simultaneous use of all available capacity. Studies and practical experience have shown that, for large laboratories with many fume hoods, at least 20 to 30 percent are closed or only partially used at any one time. Therefore, HVAC systems can be sized for 70 to 80 percent of peak ventilation capacity. Sizing the HVAC system at 70 percent of peak load decreases operational and first costs, gives better system control, increases system stability, and reduces mechanical space requirements. Taking advantage of diversity is particularly valuable when retrofitting existing facilities where available space is limited. Therefore, it is very important to consider diversity when sizing a large laboratory HVAC system. Small, single-room laboratories should always be sized for full 100 percent capacity without downsizing. [Lentz and Smith, 1989; Cooper, 1994]

Load management analysis

A comprehensive analysis of the laboratory loads should include an interview between the researchers and the energy engineer. Such interviews often produce unexpected results and increased energy efficiency; identification of equipment and occupancy schedules helps clarify system capacity needs, and, in some cases, reveals that demand-controlled ventilation is a viable option.

Finally, control is the single most important design variable in an HVAC system that meets a laboratory's exacting environmental requirements. The control scheme must address temperatures as well as safe ventilation and stable control of building pressures, duct static pressures, and air migration patterns. An in-depth examination of control systems is presented in Chapter 4. [Lentz, and Smith, 1989]

Abstract: Energy Efficiency and Direct Digital Control

The overall impact of a DDC system upon the energy efficiency of a research laboratory is considerable and includes a broad range of positive benefits. Some impacts have a direct energy influence, e.g., precise temperature control, while others have indirect energy consequences, e.g., consumption reporting. While the structure/architecture of the DDC system is very important, the energy engineer should give greater consideration to the operator interface, called the Person/Machine Interface (PMI). There are particular advantages that the distributed DDC system has over a pneumatic system and individual controllers. The building of a comprehensive sequence of operations is the first step in the implementation of the laboratory Energy Monitoring and Control System (EMCS). Depending upon budget and scope constraints, a DDC system can provide the core of a growing, flexible system that can provide Total Laboratory Energy Management (TLEM) which is further described in Section 4.4, below. A highlighting of DDC advantages and benefits follows:

Centralized User Interface

Dynamic, Precise Facility Control

Coordination of Facility Systems Operation

Speed and Reliability from Computational Power

Optimizing Facility Diversity

Durability and Flexibility

Troubleshooting/Easing of maintenance

Trending and History Data Logging

Customized Energy Reporting

Direct Digital Control (DDC) AdvantagesEnergy Efficiency and DDC Advantages

DDC EMCSs replace conventional pneumatic or electromechanical HVAC control systems with equipment capable of performing not only control but energy management and system diagnostic functions in the environment of a centralized computer network. An EMCS accepts analog, discrete, and digital input from remote sensors and devices, processes the data, and then controls remote mechanical equipment. An EMCS inherently has more accurate control because it reduces the drift, maintenance and recalibration problems common with pneumatic control systems. Additionally, an EMCS can make all facilities function more efficiently when the gathered data are compiled into useful, pertinent reports. Quality control, production, research, and maintenance will all benefit from the increased information flow when it is properly managed. [Ruys, 1990]

Compared to conventional control systems, DDC offers the following advantages:

Control Precision,

Systems Coordination,

Optimum Start,

Diversity Analyses,

Retrofit Identification,

System Load Tracking,

Monitoring and Maintenance Information,

Trend Information and History data, and

Energy Reporting.

More:DDC vs. conventional pneumatic controlsDDC control integrationMonitoring and maintenanceReporting

Abstract: Energy Efficiency and Laboratory Supply Systems

Energy engineers designing laboratory supply systems now have numerous opportunities for better contamination control and energy efficiency than in the past. More than 60 percent of the energy consumed by a conventional lab or cleanroom is used to circulate air and to supply heating, cooling, humidity, and clean air, so energy-efficient designs can result in substantial savings. Generally, annual supply system energy costs are ranked from highest to lowest as follows:

Cooling/Chillers,

Fan energy,

Humidification, and

Heating.

In the case of cleanrooms, the energy consumed by humidification and heating varies depending on climatic conditions and can shift from season to season. In large, specialized cleanrooms (Class 1000 and cleaner), the largest amount of energy, aside from that used for manufacturing equipment, is to supply vertical unidirectional air flow.

This chapter looks at four areas of supply systems that should be considered for energy efficiency:

Plant Devices,

Air Systems,

Air-Handling Units, and

Energy Recovery.

We analyze these categories separately; however, design decisions for one category affect the energy use of the others, so the cumulative effects of interactions among them must be considered. [Takenami et al., 1989; Brown, 1990; Naughton, 1990a; Naughton, 1990b]

Air SystemsEnergy Efficiency and Air Systems

The HVAC Air System can be considered the "lungs" of the facility. Many major energy-using components are necessary to provide the desired environment. As in the cases of chillers and boilers, energy savings can be realized in air-handling by modularizing these systems. We examine energy efficiency by reviewing major system requirements for laboratory and cleanroom environments. [Charneux, 2001]

VAV systems

Laboratory-type facilities benefit especially from VAV systems. VAV systems reduce both operating energy costs and capital costs. By continuously adjusting to match the environmental conditioning required by the facility, VAV systems save operating energy. When diversity or varying loads are taken into account, the additional first cost of VAV systems can have life-cycle paybacks, including operational energy savings, in less than six months. [Atwell and McGeddy, 1989; Neuman and Guven, 1994; Parker et al., 1993] [Basso, 1997]

When air-handling equipment is operated at low air-flow rates, the reduction of the pressure loss and the higher degree of efficiency of the heat exchangers can more than compensate for the higher purchase costs of the VAV system. Outside working hours, the air-flow rate can be reduced to 50 percent of the design value. The resulting energy consumption of the VAV system for conveying the air decreases to less than 25 percent of peak load. [Schicht, 1991]

Make-up air systems

One of the largest subsystem energy users in a laboratory's space conditioning system is the make-up air-handling system. Make-up air units can use tremendous amounts of energy unnecessarily in part because of basic design decisions regarding the temperature and humidity tolerance allowed in the laboratory or cleanroom. The energy requirements to heat, cool, dehumidify, or humidify the make-up air are considerable and can represent 30 percent to 65 percent of the total energy required to maintain the laboratory or cleanroom environment. Charneux (2001) describes an interesting laboratory design in which classroom and office area airflow is combined with supply make-up air for the facility's lab spaces, resulting in an overall outside air demand reduction of 30 percent. As noted by Lacey (1997), an innovative "focused" make-up ventilation system is used in an animal anatomy lab to provide "spot" ventilation. This system, which also uses air-to-air energy recovery, consumes 10 percent of a conventional bulk ventilation system. [Charneux, 2001] [Lacey, 1997] [Kruse, 1991; Naughton, 1990a; Brown, 1990]

Cleanroom recirculation air systems

Cleanrooms of class 1000 and cleaner have air change rates of 600 to 900 per hour. Large amounts of energy are necessary to transport these huge quantities of cleanroom air and remove fan heat. Recirculation air systems for cleanroom designs can maximize energy savings by reducing both the unidirectional air-flow rate and the pressure drop in the air recirculation loop. Significant energy savings are also possible when high-efficiency components are used for circulating these large quantities of air. [Naughton, "HVAC Systems … Part 1, 1990]

In cleanrooms, air flow is a generally fixed parameter based on the air velocity desired. Required fan horsepower can be reduced by one-third if the clean room is provided with a mixed HEPA filter air velocity and only the product and the production equipment are covered with 90 fpm (0.457 m/s) air flow while the remainder of the cleanroom operates at a lower velocity of 60 fpm (0.305 m/s). [Naughton, "HVAC Systems … Part 1, 1990]

Major energy savings can be achieved by lowering system static pressure and improving fan efficiency. The energy required to overcome the system static pressure rises at a cubed rate, thus increasing energy requirements exponentially. [Ciborowski and Pluemer, 1991]

More:VAV systemsMake-up air systemsAir recirculation systems

Abstract: Energy Efficiency and Exhaust Systems

In laboratory-type facilities, a fundamental goal of energy engineers is to reduce the amount of exhaust air to the lowest safe level for any particular design because conditioned exhaust air is very energy intensive. Code and certification requirements that determine the amount of exhaust need to be verified with the authority that has jurisdiction over the facility design and operation; however, there are surprisingly few codes that stipulate the actual amount of exhaust for laboratory-type facilities. Certification standards must be carefully understood to insure that they are appropriate for the actual activities for which laboratory equipment and space are being used. Devices that exhaust air from a laboratory have evolved in response to concerns about safety and energy consumption. For fume hoods, the most important energy-efficiency measure is to incorporate variable volume exhaust airflow that changes with the position of the protective sash. Manifolding fume hoods, when appropriate, reduces exhaust system energy consumption. Manifolding can also reduce first costs and increase system flexibility. Optimizing stack heights and air stream exit velocities can minimize required energy to disperse exhaust stack effluent. Finally, even the most sophisticated, energy-efficient exhaust system can be rendered ineffective if operators are not trained and motivated to use the system to its maximum potential.

Overview of Exhaust Systems

As presented in the Laboratory Control and Safety Solutions Application Guide – Rev. 2, 1994 all exhaust systems for laboratory-type facilities must meet the following four fundamental requirements:

The system capacity and air velocity must transport all hazardous airborne substances away from their origin and discharge them sufficiently high above the facility. These substances include one or a combination of chemical fumes, vapors, airborne biological substances and various particulate and radioactive elements.

The system must not leak or allow the exhaust air stream to re-enter the facility. The system's components such as ducts, fans, and dampers must be able to

withstand the corrosive or other adverse effects of the transported substances. The system operation must not generate an unacceptable sound level or excessive

vibration.

In selecting an exhaust system, one must also consider: first costs, life-cycle costs, maintainability, space requirements, expansion possibilities, and component reliability (see Chapters 2 and 3). [Lunneberg, 1998]

More:Exhaust System ConfigurationExhaust air cleaning systemsSpecialized exhaust systems

Room Pressure ControlEnergy Efficiency and Laboratory Pressure Control

By insuring that the laboratory is safely and correctly isolated from adjacent spaces, the air pressure control system contributes to the overall energy efficiency of the laboratory facility. An energy-efficient VAV supply and exhaust can be used to control this pressure. The techniques to provide laboratory pressure control include differential pressure sensing, air-flow tracking, and combination pressure sensing/tracking.

The advantages and drawbacks of these techniques make clear that the best choice for safe and energy-efficient laboratory operation is a combination of pressure sensing and air-flow tracking. The pressure control system's efficient maintenance of laboratory conditions has a large effect on heating, cooling, and air moving expenses. According to Grossman (1995), "Depending on the technology used, each laboratory air-flow control system may require a different maximum volume of air measured in cfm, to do the job properly. At an average cost of $3/cfm [to $7/cfm] each year, the differences between the energy costs associated with systems can often be quite dramatic."

More:Laboratory pressure control objectivesStatic pressure forceVAV and laboratory pressure controlApplication of pressure sensingApplication of air-flow trackingCombined pressure sensing and air-flow trackingEnthalpy stabilization

Noise AttenuationEnergy Efficiency and Noise Attenuation

Noise attenuation devices typically increase the pressure drop of the air distribution system, increasing its energy consumption. Numerous strategies exist for eliminating noise; however, it is preferable not to design a noisy system in the first place. Noise is caused by air movement and

transmitted by vibrations to the ductwork from fans, dampers, and other components, especially fume hoods. An engineer can make a system quieter by selecting low-noise fans, incorporating round ductwork, and reducing air-flow velocity by oversizing the ductwork. Active noise cancellation technology can reproduce low-frequency fan noise electronically and reintroduce the noise 180° out of phase, canceling it without restricting air flow within the ductwork. [Wise and Dineen, 1995; Micro-Electronics Facility Efficiency Workshop, 1995; Handbook of Facilities Planning, 1990]

More:Fans and noiseFume hoods and exhaust ductwork noiseActive noise attenuation

Abstract: Energy Efficiency and Air Filtration

The first step in energy-efficient air filtration design is to determine accurately rather than estimate the filtration required for the laboratory's process needs and for safety. Close attention to filtration efficiency will result in significant energy use reductions over the life of the facility, especially when the optimization of the filter's final pressure drop is calculated.

The next step in designing energy-efficient filtration is reducing pressure loss in filter systems by selecting filters with the lowest pressure drop available, usually those with deep, extended surfaces; underrating filter bank(s) by sizing for reduced volume compared to the rated filter volume; and designing the filter bank for a low face velocity of no more than 300 feet per minute (100 feet per minute is best for energy-efficient design). The Micro-Electronics Facility Efficiency Workshop (1995) points out that, "...since filter life is inversely proportional to the square of velocity, cutting velocity in half can extend filter life by a factor of four." Some High-Efficiency Particulate Air (HEPA) filters cause less pressure drop than the filters typically included in conventional supply systems; HEPA filtration does not necessitate large pressure drops.

The features described above will cost more than conventional designs partly because of their requirements for increased duct size and filter area. However, these recommendations are usually shown to be cost effective when life-cycle cost analysis is done.

Degree of FiltrationEnergy Efficiency and Degree of Filtration

The degree of air filtration needed is determined primarily by the process that the air stream serves and is typically stipulated by codes or researcher requirements. In a typical laboratory, high-efficiency filtration is not normally required. Filters with 30 percent ASHRAE efficiency (atmospheric dust spot test method) provide adequate filtration for a reasonable first cost if maintenance is provided at appropriate intervals. The type of laboratory isolation required, e.g., hazardous or

protective (see Chapter 2), will also determine the degree of filtration necessary. In a laboratory isolated for hazardous research, the exhaust air steam may need to be filtered with High Efficiency Particulate Air (HEPA) and activated-carbon filters. A research laboratory that is protectively isolated may also require HEPA filtration of the supply air, as in the case of a cleanroom. For energy efficiency, the filter system should be "underrated." [McIlvaine, 1992; NAFA Guide..., 1993; Bas, 1995]

Underrating filters

Underrating a filter system means passing less air through it than its rated capacity allows, that is, less volume of air per unit time than the clean filter can manage at a specified pressure drop. Because underrating means a lower pressure drop and increased dust holding capacity compared to operation at rated capacity, the filter will have a longer life and a lower energy consumption during its life. The NAFA Guide to Air Filtration (1993) points out that underrating means, "The time required for a pressure drop increase due to captured dust will be extended."

More:Filtration overviewCleanroom filtration

Abstract: Energy Efficiency and LightingBy Doug Avery, Michael Siminovitch, Ph.D., and Geoffrey C. Bell. P.E.

Typically 10 to 20 percent less energy is consumed by lighting in laboratory-type facilities than by the HVAC system. Nonetheless, efficient lighting systems provide significant energy savings. Efficient lighting design begins with understanding the tasks to be performed in the laboratory. A design that incorporates both dedicated task illumination and general ambient lighting is most energy efficient. High-efficiency lighting components, such as T8 fluorescent lamps and electronic ballasts, are the starting point in energy-efficient lighting designs. Lighting energy is also dramatically reduced by control systems that turn off lights based on occupancy or adjusts lighting in response to available natural light. In some laboratories, a remote lighting system provides the benefit of isolating a large portion of the lighting system from the laboratory space.

Lighting DesignEnergy Efficiency and Lighting Design

Efficient laboratory lighting design first considers the task that occupants will perform in the space. We review two lighting design approaches: general lighting and task-ambient lighting. The lighting design approach determines the type of lighting calculation used. Lighting design affects other energy-consuming systems in the facility. For instance, in cleanrooms, large ceiling luminaires reduce ceiling area for HEPA filters. As filter area is reduced, filter exit velocity increases, increasing the

static pressure within the system, which causes fans to consume more energy. Finally, all heat generated by the luminaires and the harder working fans affects cooling equipment sizing. [McIlvaine, 1992; Eley et al., 1993]

More:Task identificationGeneral lighting designTask-ambient lightingDaylighting

High-Efficiency Lighting ComponentsEnergy Efficiency and Lighting Components

A number of lighting system components are available for installation in research laboratories. This section will review the various lamps, ballast, and fixtures that are typically utilized to provide general space illumination, with particular attention to the efficacy of specific lamp and ballast combinations. Components designed to provide task lighting will also be examined, including discussions of the relative efficiency of various products. [Catone, 2001]

According to Eley et al. (1993) ref145,

There are three primary means of improving the efficiency of a fluorescent lamp-ballast system:

Reduce the ballast losses. Operate the lamp(s) at a high frequency. Reduce losses attributable to the lamp electrodes.

More:LampsElectronic ballastsFixtures

Diagonal air-distribution system for operating rooms: experiment and modeling

Monika Woloszyna, , , Joseph Virgonea and Stéphane Mélenb aCentre de Thermique de Lyon:UCBL, CNRS UMR 5008, INSA de Lyon, bat. 307 20, av. A. Einstein, 69621 Villeurbanne Cedex, FrancebAir Liquide, Centre de Recherche Claude Delorme, Jouy-en-Josas, France Received 19 March 2003;  revised 17 March 2004;  accepted 24 March 2004.  Available online 5 June 2004.

Abstract

The airflow patterns and the diffusion of contaminants in an operating room with a diagonal air-distribution system were subjected to both experimental measurements and numerical modeling. The experiments were carried out in MINIBAT test cell equipped with an operating table, a medical lamp and a manikin representing the surgeon. Air velocity and tracer-gas concentration were measured automatically at more than 700 points. The numerical simulations were performed using EXP′AIR software developed by Air Liquide for analyzing air quality in operating rooms. Only isothermal

conditions were investigated in this comparison with the numerical software. The results showed that the contaminant distribution depended strongly on the presence of obstacles such as medical equipment 3

1Technology & Servicesa report by William LawranceProduct Manager, Fläkt WoodsFläkt Woods Group has been supplying air-handlingunits for hospitals for several decades. Indeed, eventoday, a EU 2000 air-handling unit is being installedsomewhere in the world every 10 minutes. The EU 2000 generation of air-handling units havenow been on the market for over 10 years and areinstalled in many hospitals all over Northern Europeand Scandinavia. Fläkt Woods have been manufacturing air-handlingunits for over four decades and some of the first unitsinstalled are still running well today. The air handling unit and the air distributionterminal devices are important parts of the ventilationsystem and now Fläkt Woods is introducing a furtherdevelopment in air distribution for demandingindoor environments.Generally, hospitals tend to have a large number ofrelatively small air-handling units, each serving aspecific function within the building. The specialtechnical demands include hygiene, reliability, safetyand energy-related issues.Fläkt Woods has some interesting solutions for filterframes, high-efficiency particulate air (HEPA) filterinstallation, fans, cooling coils and heat recovery, aswell as silencers and the casing itself.In a hospital environment, there tend to be highconcentrations of harmful micro-organisms. Theirroutes to humans are either by physical contact or byairbourne routes. In this environment they areparticularly dangerous because of reduced immunitylevels in patients. The risk of being infected through the airbourneroute is a function of particle concentration. Thechance of a particle that is carrying an organismfalling into an open wound increases with particleconcentration.By reducing the concentration we reduce thechance of infection and, hence, the number ofpatients infected.Four main factors affect the local concentrationaround a person in a room:• firstly, the concentration of particles would tendto increase with rate of production of particles inthe room;• secondly, the proportion of supply and exhaust airquantity in relation to the size of the room; • thirdly, the level of filtration of the supplied airwill affect the ability of the ventilation system todilute the room air particle concentration; and • fourthly, air turbulence and air movement in theroom can transport particles so the method of airdistribution will affect local concentrations.The last three of these are attributes of the ventilationsystem that can be engineered to limit the effect ofthe first and the Fläkt Woods Group have systemsand products to meet these engineering problems.The group has supplied equipment to a very largenumber of hospitals, which has given us very longand wide experience; we are able to offer relevant

advice based on that experience.Air-handling unit filters have the task of, as aminimum, limiting the concentration of particlesentering the room from outdoors but also of keepingthe air-handling unit components as clean as possiblein order to reduce the risk of biological growthwithin the unit itself. Ideally, the air handling unitshould not produce any dust itself, but that is moreor less impossible where moving parts are concernedi.e. the fan set. Fläkt Woods have some well-tested solutions thatreduce the production of dust generated by the fanset. For example, flat belts produce considerably less

Air Handling and Ventilation Systems for HospitalsClassCollecting Efficiency (%) at Most Penetrating Particle SizeH10>85H12>99,5Table 1

dust than normal v-belts and are more efficient attransmitting the power from the motor to the fan.Direct driven fans avoid the use of belts and bothplug fans and axial fans can be offered.The motor can also be mounted on the outside ofthe air-handling unit casing so that we avoidmounting both the motor and the belt drive in theair stream. This involves the use of a special fan withan extended shaft. Care must be taken whenengineering such a solution since increasing the sizeof the shaft while avoiding the critical speedcompensates for the additional forces on the shaft. Inaddition, the fan and motor assembly must also befully isolated from the building structure. Final filtration can be used to substantially reduce theconcentration of dust particles in the supply air andhere the design of the filter frame and its installationcan be decisive. The Fläkt Woods patented HEPAfilter frame eliminates the risk of bypass leakage whileavoiding the use of liquid seals and offering clean andrapid filter replacement. HEPA filters are available indifferent grades.The standards used for determining the type ofcollecting efficiency of filters often differ betweenEurope and the US. Even within Europe there aredifferent standards in different countries. EuropeanCommittee for Standardization (CEN) EN 1822 givesa common standard within Europe and is based on thefilters’ ability to collect the most penetrating particlesize (MPPS) or, in other words, how good the filter isat stopping the particles that are most difficult to catch.Military Standard 282, dioctyl phthalate (DOP) 0.3µmis used in the US. Particles with a size of 0.3µm are notnecessarily the most difficult to catch so for any givenindividual filter the test result will appear to be betterwhen tested in accordance with the US standard thanif tested to the European standard. Figure 1 indicatesthe collecting efficiency for DOP 0.3µm as a functionof the airflow for one whole cassette.As a result, care should be taken when comparing theperformance data for HEPA filters from differentmanufacturers.Note that the collecting efficiency is inverselyproportional to the air velocity, while air pressuredrop is proportional to air velocity. That means that

increasing the velocity (or reducing the overall filterarea) may be a false economy since although theinitial installation costs may seem lower, theoperating cost will be much higher and the filtrationperformance reduced.Heat RecoveryFläkt Woods offers a full range of heat recoverysystems and for hospitals would recommend eitherliquid coupled or plate heat exchangers. Theadvantage of the liquid-coupled system is that thereis no risk of transfer of air and contamination fromthe extract to the supply side. The system also offersconsiderable flexibility since the supply and extractneed not be near each other and multiple heatexchangers can be used on either or both of thesupply and extract.Econet®

is a liquid-coupled heat recovery systemwith some additional benefits. The system isdelivered complete with a speed-controlled pump,valves, sensors and control system. We alsomaintain the system after the contractor hasinstalled it. The control system is programmed witha Fläkt Woods developed optimiser. The pump iscontrolled to circulate the right quantity of waterfor the prevailing conditions of temperature and airflow rate.Econet®

is selected to give you a high temperatureefficiency and the optimiser ensures that the systemgives peak performance continually, even if you areoperating a variable air volume system.Another important advantage of Econet®

is that thesystem can be connected to a source of low-gradeheat. It is possible to feed water with a temperatureas low as 35°C into the heat recovery circuit toprovide additional heating when the heat recoverysystem does not meet the demand. The return waterwill be as low as 15–20°C, which can give substantialbenefits in district heating systems. Econet®

can beintegrated with the chiller system by using warmwater from the condensor.Econet®

can also be used for cooling. Cool water can be injected into the circuit during thewarm season. Life-cycle CostLife-cycle cost (LCC) is the total cost of purchasing,installing and running an item of equipment for adesignated number of years. The environmental costcan also be included in this sum. Technology & Services2B U S I N E S S B R I E F I N G : H O S P I T A L E N G I N E E R I N G & F A C I L I T I E S M A N A G E M E N T 2 0 0 3

Figure 1

Page 3Air Handling and Ventilation Systems for HospitalsLCC = Investment cost + Energy cost + Servicecost + Environmental cost + Taxes (if applicable)Over a 10-year period, the overall LCC is dominatedby the cost of energy, which can typically add up to

over 80% of the total cost. Installing effective heat recovery equipment canreduce heating and cooling energy costs. As wemanufacture the three most important types of heatrecovery, we can advise on the best system for yourapplication and discuss the advantages of thedifferent systems.However, the electrical energy used to drive the fanmotors is often the largest part of the total energycost. Efficient fans and motors will obviously reduceenergy cost but selecting a bigger unit will often havea greater impact on the specific fan power. Our air-handling unit selection program canautomatically calculate LCC costs for units selected –ask your local Fläkt Woods representative todemonstrate the LCC program for you and ask for anLCC calculation with your tender bids.It goes without saying that reliability and longevityare important issues in hospital systems. The EU2000 unit is a standardised product with flexibilityand variety built into the design platform, which isspecifically engineered for reliability in operation.We manufacture all our major components withinthe Fläkt Woods Group and have full control overquality and performance. All components are fullytested in our modern research and development(R&D) facility to the most rigorous and up-to-dateinternational standards.If an air-handling unit is easy to maintain then thereis a good chance that it will be maintained. That iswhy, when this generation of air-handling units wasbeing designed, we consulted facilities managementcompanies and maintenance engineers for their inputand advice as to the best possible features.A well-maintained unit performs reliably and gives along service life as well as good quality supply air.Regular and well carried out maintenance will alsokeep energy costs down.All air-handling units are supplied with order-specificinstallation instructions. Maintenance instructionsand spare parts lists are also supplied, normally afterdelivery. The documents can be supplied in paperform or in the form of an Acrobat file.Fläkt Woods catalogues provide detailed informationabout all standard components and the company’sselection programs provide all performance data forthe specific unit.Certificates of various types are also available on request. The EU 2000 unit meets all relevant CEN standardsand is certified under Eurovent (www.eurovent-certification.com).Naturally, design and production is quality-certifiedunder International Organization for Standardization(ISO) 9000 and Fläkt Woods have also attainedcertification under environmental standard ISO 14000.The EU 2000 unit can also be selected to meet therequirements of the German hygiene standard VDI(Association of German Engineers) 6022.Customised Environment®– Elea CAREOver the years Fläkt Woods has been a pioneer in airdistribution. The well-known Floormaster displace-ment ventilation system was an innovative develop-ment that has revolutionised comfort ventilation inmany different building applications. Now it is timeto take another huge step forward in thedevelopment in this system.

Elea CARE is a new air distribution system fromFläkt Woods. While taking advantage of thedisplacement ventilation method, Elea is uniquelyequipped with newly developed low resistanceHEPA filters and a room air re-circulation system.Both the supply and the re-circulated air passthrough the filters so that the supply air is clean andparticles produced in the room either from thepeople in it or from a process are filtered out.Elea units can easily replace the existing displacementterminal units without any need for further alterationto the existing ducting and air handling system.With conventional methods, HEPA filters anddiffusers have only partly satisfied the need to controlparticle concentration in rooms. Such systems seek toensure that the air entering the room is clean bydiluting the concentration of particles in the room. 3B U S I N E S S B R I E F I N G : H O S P I T A L E N G I N E E R I N G & F A C I L I T I E S M A N A G E M E N T 2 0 0 3

Figure 2

Page 4The problem is that people, tools and machines etc.,generate the majority of particles in the occupancyzone itself and mixing systems tend to spread thatpollution all over the room and even transmit it toother spaces in the building. This is an importantissue and there is a lot of interest in findingimprovements in hospital ventilation because ofgrowing concern over hospital-acquired infections.Cleanrooms with HEPA filter ceilings can solve theproblem since they provide a laminar flow of airvertically down through the room and allcontamination created within the room is carriedaway in the exhaust air. Cleanrooms are, however,rather expensive to install and run.Displacement systems use the heat sources in the roomto create a temperature gradient and a slow movementof air from the floor-mounted displacement terminaltowards the ceiling. Particles are carried up to theceiling by the upward currents of air from the variousheat sources in the room and the dirty air is extractedat ceiling level. The depth of dirty air near the ceilingdepends on the airflow rate and the rate of particleproduction in the room. If the air supplied to thedisplacement terminal has been through a HEPA filterthen a relatively clean zone is created near to theterminal itself. Increasing the grade of filtration in theair-handling system does not improve the air qualitysignificantly if the room has a high particle load. Customised Environment®Elea CARE addresses this problem by adding air re-circulation with HEPA filtration within the room.Fresh air is passed through a HEPA filter before beingsupplied to the room using normal displacementtechnique while a small fan re-circulates room airthrough a second HEPA filter. Both the supply andthe re-circulation filters are grade H12, which meansthat 99,95 % of all particles will be taken out of theair. The device also features an integrated exhaust airsection with a third HEPA filter to clean the airbefore removing it from the room.The pressure drop is low and the device is designedso that it can replace existing displacement terminalswithout any need to increase the supply airflow.Elea CARE is available in five sizes for airflows up to540m3/h, covering most requirements.

With this system, a conventional ventilation systemcan be used to achieve clean air zones without theneed to build expensive cleanrooms.To help you design with Elea CARE, a computerfluid dynamics (CFD) system-simulation tool hasbeen developed by Fläkt Woods. The software toolsimulates the spread of temperature, air velocity andparticle concentration in a room with a highaccuracy and is backed up by well-documentedlaboratory testing that has been used to calibrate theCFD model. sTechnology & Services4B U S I N E S S B R I E F I N G : H O S P I T A L E N G I N E E R I N G & F A C I L I T I E S M A N A G E M E N T 2 0 0 3

Figure 3: Mixing VentilationFigure 4: Displacement VentilationFigure 5

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Construction Methods, Materials, and Techniques

 By William Perkins Spence

Page 1045    Exit full

screen ContentsTable of Contents

The Enclosing Walls

Review Questions 26

Properties of Materials 49

PART II 63

vi 38

Soils 91

Foundations 101

Concrete and Masonry 119

Finishing Concrete 153

Concrete Reinforcing Ma... 161

Review Questions 195

Precast Concrete Slabs 203

Erecting Precast Concrete 213

Additional Information 219

Clay Brick and Tile 232

Concrete Masonry 250

Stone 263

Masonry Construction 269

Nonferrous Metals 342

Steel Frame Construction 376

Building a Structural Cla... 457

Suggested Activities 632

Additional Information 693

viii 407

CSI Division 422

Products Manufactured f... 459

Wood and Metal Light F... 488

Evolution of Wood Ligh... 499

Heavy Timber Constructi... 539

Finishing the Exterior an... 564

Finishing the Interior 579

Interior Finish Carpentry 586

Review Questions 596

Plastics 604

Thermal and Moisture Pr... 619

Vapor Barriers 631

Sealers for Exterior Mate... 637

Review Questions 648

Roofing Systems 664

Doors and Windows 694

Doors Windows Entrances 709

Windows 718

Entrances and Storefronts 728

Cladding Systems 734

Asphalt 785

Other Products Made wit... 1153

CSI Division 770

Protective and Decorativ... 777

Acoustical Materials 801

Interior Walls Partitions 816

Flooring 845

Carpeting 862

CSI Division 874

CSI Division 897

Mercantile Equipment 903

CSI Division 909

PART VII 921

xii 14

Conveying Systems 935

PART VIII 971

Plumbing Systems 981

Electrical Equipment and... 1089

Electrical Power Conduct... 1095

Meters 1102

Overcurrent Protection D... 1109

Electrical Supply 1117

Appendixes 1137

Metric Information 1151

Coefficients of Thermal ... 1157

References 1136

Index 1177

Copyright

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Air delivery system for hospital rooms and the like Document Type and Number:United States Patent 4489881 Link to this page:http://www.freepatentsonline.com/4489881.html Abstract:A method and apparatus for delivering conditioned air to hospital patient rooms or clean rooms which are maintained at a higher or lower pressure level than the adjacent halls. A constant volume delivery duct is provided to counter the constant volume toilet exhaust, and the flow rates in the constant volume ducts are selected to maintain the desired pressure level in the room. The room temperature is maintained by a variable volume system including a variable volume supply duct and a variable volume return duct. Dampers in the variable volume ducts are controlled in unison to control the room temperature as desired while always maintaining equal inflow and outflow in the variable volume system to maintain the pressure level in the room. A specially constructed and partitioned terminal unit provides a terminal for the variable volume supply and return ducts and the constant volume supply duct.

Hospital Operating Rooms   [Component Selection] Hospital Operating Rooms    Air distribution for hospital operating rooms is much more critical and specialized than for a typical air conditioned office building space. In an office building the air distribution system is designed to entrain room air into the supply air stream so as to rapidly mix the two air masses and create a more uniform temperature in a draftless occupied zone.

This type of air distribution system is not suitable for a hospital operating room as it would cause the uncontrolled spread of airborne contaminants.

In the hospital operating room, the control of airborne contaminants is a consideration in addition to the room comfort conditions. Opinions vary regarding the importance of airborne contamination with respect to post-operative infection. It is generally agreed that the majority of infections are caused

 

 The largest sources of contamination in a sterilized operating room, with a clean air supply and isolation from adjacent areas, are the surgical team and patient. The function of the operating room’s air distribution system, therefore must be to carry away any contaminants expelled into the air by either the surgical team or the patient on the operating table. The system must also isolate and

by contact contamination from the patient themselves or the surgical team. Studies have also indicated a relationship between the incidence of infection and the level of air contamination. While the validity of these results can be questioned due to changes in surgical gowns, surgical techniques, antibiotics, etc., the consensus is simply that the air should be kept as clean as possible.

The two primary sources of airborne contamination are generally considered to be micro-organisms present within the operating room and particles introduced into the operating room by ventilation or infiltration. Particles entering the operating room by ventilation can be controlled with the use of high efficiency particulate filters, while infiltration is controlled by maintaining a positive pressure in the operating room as compared to the surrounding spaces.

remove this contaminated air so it cannot mix with the clean supply air. The simplest way to reduce the airborne contaminants present in the operating room is to increase the fresh air ventilation rate. This practice of dilution has led to air supply exchange rates much in excess of those typically required for thermal control. In fact, these increased air exchange rates can lead to thermal discomfort due to drafts. As a result, an air distribution system for the operating room must be capable of introducing a large volume of supply air into the space in a controlled manner while maintaining an acceptable comfort level in the occupied zone. An effective method of controlling the transport of airborne contamination is the introduction of supply air into the operating room at a low uniform velocity to promote a stable downward flow of air.

Laminar Flow Systems    The laminar flow ventilation system was developed to provide a method of controlling the transport of air contamination by introducing the supply air into the operating room at low uniform velocities promoting a stable downward flow of air. The most effective laminar flow ventilation system would have the entire ceiling consisting of laminar flow diffusers to prevent entrainment. A complete ceiling of laminar flow panels would require much more air to develop a proper air pattern than is required to achieve the specified number of air changes per hour. The high air change rates required to produce laminar flow over the entire room normally rule out this system due to high energy costs.

By reducing the area of laminar airflow to the critical zone around the operating table, the total air requirements of the system can be reduced. Although laminar flow diffusers discharge air at low face velocity, some entrainment of room air still occurs. This entrainment in combination with the temperature differential of the supply air causes the air pattern to angle towards the center of the discharge air envelope. As a result the clean zone is reduced as the distance from the face of the diffuser is increased. This should be considered when laying out the location for the laminar flow

  Laminar Flow - Full Ceiling Supply

 Laminar Flow - Partial Ceiling Supply

 The supply air for a laminar flow ventilation system is filtered by a HEPA filter bank located upstream of the operating room air distribution system, or by HEPA filters which are an integral part of each of the laminar flow diffusers.

With the HEPA filters located in a bank upstream of the operating room, filter service and maintenance

diffusers. can be performed without entering the sterile environment of the operating room.

Supply diffuser with room side replaceable integral HEPA filters offer ease of accessibility for filter service and change-out, but must be accessed from inside the sterile operating room.

Laminar Flow with Air Curtain    In many cases, the ongoing energy costs associated with a full ceiling laminar flow ventilation system can be reduced by reducing the size of the area requiring laminar airflow. Essentially, creating a clean zone around the operating table within the operating room. This is achieved by surrounding the operating table with an air curtain.

This air curtain is created using linear slot diffusers on each of the four sides around the operating table. The linear slot diffusers are installed in the ceiling a minimum of 3 feet out from the sides of the operating table, allowing room for the surgical staff and equipment to move and still be contained in the clean zone. The linear air diffusers discharge the supply air at an angle of approximately 15° from vertical, maintaining a barrier between the clean zone around the operating table and the surrounding operating room. The air curtain presents a physical barrier, in the form of a clean air curtain, between the laminar flow diffusers and the contaminated room air at the ceiling level, where the laminar flow diffuser is most likely to entrain room air.

The air curtain entrains contaminated room air to its outer boundary layer and carries it way from the operating table work area, toward exhaust grilles, thus speeding dilution of the contaminated room air.

Laminar flow diffusers installed in the ceiling inside the air curtain provide low velocity, laminar flow of clean air over the surgical staff, patient and operating table.

The supply air for this type of system is typically filtered using HEPA filters located upstream of the operating room air distribution system. Of the total supply air, 65 – 75% of the supply air is delivered through the air curtain and the remaining 25 – 35% is distributed through

 

The Price HORD is an integrated system of laminar flow and linear slot diffusers that minimizes mixing of room and supply air to create a controlled operating room work area.  Contamination entering the operating room by infiltration is controlled by keeping the operating room at a positive pressure in relation to the surrounding areas. For this reason, the return air volume must be slightly less than the supply air volume. Care must be taken that the differential between return and supply air volumes is not too great as this could impede the dilution of the contaminated air. Typically the return system is sized for approximately 85% of the total supply airflow.

The return grilles are mounted at low level, approximately three to six inches above the floor. In this location they exhaust both the contaminated air and any heavier-than-air gases.

The operating room return air system ideally consists of four return grilles, one located in the center of each wall. In the case where it is not possible to have a return grille in each wall, the next best option is to have two return grilles, located on opposite sides of the air curtain. Alternatively, the grilles could be located in opposite corners of the room. When using only two return grilles if they were located on adjacent walls, this could result in the migration of contaminated air back into the operating area.

the laminar flow diffusers. Ceiling Construction    The role of the ceiling in an operating room is to seal the room from the ceiling plenum. This is to prevent infiltration of contaminants from the ceiling space and to allow for pressurization of the operating room.

There are typically three ceiling systems used in hospital operating rooms. These are drywall ceilings, gasketed t-bar ceilings and a combination of drywall and gasketed t-bar ceilings.

The drywall ceiling works well for sealing the operating room from the ceiling plenum, but can pose a problem when ceiling space access is required. Sealed access doors in the drywall ceiling are installed near equipment requiring periodic maintenance and service. If equipment must ever be removed from the ceiling space, the access doors may not be large enough to facilitate this, requiring removal of large portions of the drywall ceiling.

A gasketed T-bar ceiling also works well for sealing the operating room from the ceiling plenum and has the added advantage of allowing access to the ceiling space when required. Ceiling panels are clipped in place, compressing the gasket between the panel and tee, forming the seal. When it is necessary to get into the ceiling space, the clips are removed and the panels are lifted allowing access to the equipment installed above the operating room. In the event equipment must be removed, panels and tees can be removed to allow access. Panels are normally constructed of painted metal to facilitate cleaning.

The third type of ceiling system is the combination of drywall and gasketed t-bar ceilings. This typically consists of the perimeter of the room being drywall and the center, above the operating table being gasketed t-bar as shown in the drawing. This system provides easy access to equipment located above the t-bar system.

 

Combination Drywall / T-Bar Ceiling

 

Component Selection   [Hospital Operating Rooms] 

     Air Curtain    

 HORDHospital Operating Room Diffuser

Linear slot discharges vertical curtain of clean air.

Creates a “room within a room” around perimeter of operating table work area.

Single or multiple side feeds from supply air plenum.

LFD / LFDSS / LFD2Laminar Flow Diffuser

Perforated face discharges non-aspirating (non-mixing) vertical flow of clean air.

Air pattern “flows” over the operating table on its way to the floor.

Creates a “washing” and“rinsing” effect.

Terminal Units for Cleanrooms

A variety of liner options are available.

Reduces risk of micro-organism growth.

Prevents fibrous particles from entering supply air stream.

  730 / 735 SeriesStainless Steel Return Grilles

Low level exhaust grilles remove contaminated air and heavier-than-air gases from O.R.

Stainless steel construction ensures strength and ease of cleaning.

Exhaust volume should be 15% lower than supply to ensure positive room pressure.

  Unitee CRCleanroom Ceiling Systems

Prevents air leakage betweenplenum and operating room.

Utilizes unique hold-down clip and gasketed tee design.

Ceiling panels available specific to applications.

System Overview

Price offers a wide variety of products which meet the air distribution requirements of modern hospitals and medical facilities.

Illustrated is a typical installation of a hospital operating room to meet stringent ventilation needs.

Price specialized environment products and engineering expertise have been created to handle any critical hospital applications (Intensive Care units, Burn Wards, Recovery Rooms, etc.)

 Laminar Flow

 

LFDC / LFDCDLaminar Flow Diffuser with High Efficiency Filters

Provides a laminar or uni-directional flow of clean air over the operating table.

Houses a high efficiency filter with extraction efficiencies from 95% to 99.999%.

Terminal Units for Cleanrooms

A variety of liner options are available. Reduces risk of micro-organism growth.

Prevents fibrous particles from entering supply air stream.

730 / 735 SeriesStainless Steel Return Grilles

Low level return grilles remove contaminated air from the room.

Stainless steel construction ensures strength and ease of cleaning.

 

UNITEE CR / UNITEE HDCRClean Room Ceiling System

Prevents air leakage between plenum and cleanroom.

Utilizes unique hold-down clip and gasketed tee design.

Ceiling panels available specific to cleanroom applications.

System OverviewPrice has developed a series of components which can provide a supply of clean filtered air to today’s modern cleanrooms. Cleanroom components ensure that air is cleaned utilizing high efficiency filters at terminal diffusers.

This illustration is an example of a cleanroom where dust and micro-organisms are removed from the supply air and the required level of cleanliness is maintained.

Component Selection   [Hospital Operating Rooms] 

     Air Curtain    

 HORDHospital Operating Room Diffuser

Linear slot discharges vertical curtain of clean air.

Creates a “room within a room” around perimeter of operating table work area.

Single or multiple side feeds from supply air plenum.

LFD / LFDSS / LFD2Laminar Flow Diffuser

Perforated face discharges non-aspirating (non-mixing) vertical flow of clean air.

Air pattern “flows” over the operating table on its way to the floor.

Creates a “washing” and“rinsing” effect.

Terminal Units for Cleanrooms

  730 / 735 SeriesStainless Steel Return Grilles

  Unitee CRCleanroom Ceiling Systems

A variety of liner options are available.

Reduces risk of micro-organism growth.

Prevents fibrous particles from entering supply air stream.

Low level exhaust grilles remove contaminated air and heavier-than-air gases from O.R.

Stainless steel construction ensures strength and ease of cleaning.

Exhaust volume should be 15% lower than supply to ensure positive room pressure.

Prevents air leakage betweenplenum and operating room.

Utilizes unique hold-down clip and gasketed tee design.

Ceiling panels available specific to applications.

System Overview

Price offers a wide variety of products which meet the air distribution requirements of modern hospitals and medical facilities.

Illustrated is a typical installation of a hospital operating room to meet stringent ventilation needs.

Price specialized environment products and engineering expertise have been created to handle any critical hospital applications (Intensive Care units, Burn Wards, Recovery Rooms, etc.)

 Laminar Flow

 

LFDC / LFDCDLaminar Flow Diffuser with High Efficiency Filters

Provides a laminar or uni-directional flow of clean air over the operating table.

Houses a high efficiency filter with extraction efficiencies from 95% to 99.999%.

Terminal Units for Cleanrooms

A variety of liner options are available. Reduces risk of micro-organism growth.

Prevents fibrous particles from entering supply air stream.

730 / 735 SeriesStainless Steel Return Grilles

Low level return grilles remove contaminated air from the room.

Stainless steel construction ensures strength and ease of cleaning.

 

UNITEE CR / UNITEE HDCRClean Room Ceiling System

Prevents air leakage between plenum and cleanroom.

Utilizes unique hold-down clip and gasketed tee design.

Ceiling panels available specific to cleanroom applications.

System OverviewPrice has developed a series of components which can provide a supply of clean filtered air to today’s modern cleanrooms. Cleanroom components ensure that air is cleaned utilizing high efficiency filters at terminal diffusers.

This illustration is an example of a cleanroom where dust and micro-organisms are removed from the supply air and the required level of cleanliness is maintained.

Apollo Cancer Institute

Medical Oncology Pediatric Oncology Radiation Oncology Surgical Oncology

Apollo Cancer Institute (Ext: 1970, 1971) The Apollo Cancer Institute, which is a comprehensive, multidisciplinary, advanced by latest technology with the most competent and highly skilled health care professionals. The institute provides the treatment in the following major specialties: » Medical Oncology » Surgical Oncology » Pediatric Oncology » Radiation Oncology

The word "Cancer" is widely associated with impending death amongst a large number of people. This is understandable as Cancer is a killer disease. According to the W.H.O., globally one out of three women and one out of four men are likely to get cancer in their lifetime. With such a high probability rate, cancer can affect anyone, anytime and anywhere. However very few

people know that cancer is curable in a large number of cases if detected early and a patient can lead a normal life. Although the prognosis of the disease is bad in an advanced stage, today the use of modern technology has brought the cure rate of cancer to almost 70-80%.

The Indraprastha Apollo Hospital, New Delhi has recently started the Apollo Cancer Institute, which is a comprehensive, multidisciplinary, state-of-the-art facility. It brings in the latest technology with the most competent and highly skilled health care professionals. This institute has the unique advantage of not being a stand-alone cancer unit but also having the most modern backup from all super specialties and diagnostics.

Advanced Radiation TherapyTreatments at the Rajiv GandhiCancer Institute, New DelhiThe Rajiv Gandhi Cancer Institute (RGCI),a dedicated oncology center in New Delhi, India,is one of the few medical centers in the countryto have Intensity-Modulated Radiation Therapy(IMRT) technology. An interview with Dr. Y. P. Bhatia and Dr. Anil Kumar Anand.Interview conducted by Lalitha Maheshwaran, Siemens Ltd., Indiaadditional topics of importance regarding advanced radiationtherapy treatment in India today. Following are some excerptsfrom the interview:MEDICAL SOLUTIONS: What is the overall mission of yourinstitution? How does IMRT help to fulfill your objectives?DR. BHATIA: We observed long ago that oncology serviceswere lacking in Northern India, and the demand for theseDr. Anil Kumar Anand (left) and Dr. Y. P. Bhatia (right) at the interview with Medical Solutions.

Page 2SCIENCERADIATION THERAPYMEDICAL SOLUTIONS RSNA 2004

93services was increasing steadily. We therefore wanted tobuild a facility that would be compatible with global standardsand, at the same time, offer medical care at an affordablecost to patients. From the beginning, this was our overallobjective and IMRT was an important development becauseof its high-precision technique, in which the associated morbidity is much lower than with other modalities oftreatment.MEDICAL SOLUTIONS: How has Siemens partnered withyour institute in meeting these goals?THE RAJIV GANDHI CANCER Institute in New Delhi is one of the few oncology centers in India to provide IMRT. It is one of the most modern oncology centers in India.DR. BHATIA: The history of our association with Siemensdates back to the inception of the institute. At that stage, we

were assisted by a soft loan from the German government tohelp purchase the technologies we required. Today, Siemensadvanced radiation therapy equipment is an integral part ofour system.We handle approximately 25 to 30 new patients each day.The linear accelerator, for example, treats over 140 patientsper day and, clearly, we cannot afford a breakdown of theequipment – even for one day – as there would be sheer

Page 3SCIENCERADIATION THERAPY94MEDICAL SOLUTIONS RSNA 2004

chaos if that were to happen. Because of Siemens’ tremen-dous uptime commitment, we are able to keep the downtimein our facility to a bare minimum.MEDICAL SOLUTIONS: How has the demand for radiationoncology services increased compared to existing treatmentfacilities currently available in India?DR. BHATIA: The demand is steadily increasing. When we setup our institute, we expected to handle seven or eight newpatients per day, but we actually receive approximately threeto four times that number. The situation in other institutionsis similar, because cancer patients requiring radiation therapyare underserved in our area.MEDICAL SOLUTIONS: Is there need for major expansion ofthese facilities? If yes, in what way?DR. BHATIA: Certainly there is a need for expansion, and wehave already moved ahead in that direction. For example, anew extension is opening up in the same campus, which willhave 120 additional beds and two new linear accelerators. Itis expected that this will be completed in June or July of2005. Additionally, we are putting together an outreachprogram, and there are ongoing discussions with the government of Meghalaya (a province in Eastern India) tobuild their own cancer control program. This would be a natural expansion because we have already trained some oftheir doctors in clinical oncology.MEDICAL SOLUTIONS: What are your major challenges?DR. BHATIA: There are two issues that we need to handlesimultaneously. One is the growing number of people whoneed oncology care services in the wide area that we have tocover in Northern India. The second issue is the rapid obso-lescence of technology. In order to remain compatible withworld standards, we have to offer the latest equipment andtechnology to our patients. We need to continuouslyadvance ourselves at every opportunity. Apart from this,IN INDIA, CANCERS OF THE HEAD, neck and brain are the most common tumors. Dr. Anand, Dr. Kataria (radiation oncologist) and physicist Mr. Munjal (from left to right) discuss the treatment planning of a patient.

Page 4SCIENCERADIATION THERAPYMEDICAL SOLUTIONS RSNA 2004

95there is the challenge of keeping spare parts available for ourexisting equipment. I spoke earlier about the need for maxi-mum uptime, and how the absence of this can become ahindrance to our goal of maximum treatment time.MEDICAL SOLUTIONS: Could you explain your views on therole of health insurance providers in India, and their impacton patient care and treatment economics?DR. BHATIA: Health insurance in this country needs tomature a lot – basically, it has a long way to go to becomeacceptable. If you look at all types of insurance, including theschemes run by the Indian government, only 12 percent ofthe population is covered. For the rest, any medical expenseis an out-of-pocket expense to the patient. If the insurancesystem were to become more widespread, it would signifi-cantly help out a lot of people.

MEDICAL SOLUTIONS: What are the most important tech-nological advantages that IMRT offers compared to earliertechniques?DR. ANAND: In radiation therapy, IMRT is the most importantdevelopment since the introduction of the linear accelerator,because it offers a number of advantages. For example, thedose distribution is better and we can handle irregulartumors more effectively, with minimum damage to the sur-rounding normal tissue. We can design the treatment in such away that the maximum dose follows the contour of the tumor.MEDICAL SOLUTIONS: What were the main considerationsthat prompted you to choose Siemens equipment?DR. ANAND: We have had two previous experiences in deal-ing with Siemens over the past seven years, and we werequite happy – a certain comfort level had developed over the years. More importantly, their technology is superior to others. The SIMTEC IM-MAXX technology allows us toprovide faster delivery of IMRT, so the patient is on the treat-ment table for a shorter period of time. Hence the patientremains comfortable and we can handle a greater number of patients. And, Siemens has a significant presence inNorthern India with very reliable service, which results invery high uptime for our equipment.MEDICAL SOLUTIONS: Head and neck cancer is one of themost common cancers in India. Could you explain the impactof IMRT in these cases?DR. ANAND: IMRT is most useful particularly in head, neckand brain tumors. We can increase the dose even if there are critical structures in the close vicinity. We now have experience with IMRT treatment in more than 250 cases,putting us fairly high up the learning curve.One of the most common problems in head and neck canceris dryness of the mouth. With earlier types of treatment, thiscondition would distort the internal structure of the oralcavity, but with IMRT, the parotid glands are spared and mouthdryness is avoided. In a study of 19 cases, we were able tocompletely avoid this problem for 78 percent of the patients.MEDICAL SOLUTIONS: What was the most dramatic patientexperience in terms of therapeutic effectiveness at yourcenter?DR. ANAND: We have had several cases, but the one whichparticularly comes to mind is that of a 56-year-old womanwho had a recurrent tumor in the paranasal sinus. The tumorwas very close to the optic nerve, requiring us to beextremely cautious. With IMRT, she was able to recover completely; with previous treatments, this would not havebeen possible.140 PATIENTS per day are treated with the PRIMUS linear accelerator.BEFORE THE RADIATION treatment begins, radiologictechnicians verify the individual settings for each patient.