Duct

Ducts are used inheating, ventilation, and air conditioning(HVAC) to deliver and remove air. The needed airflows include, for example,supply air,return air, andexhaust air. Ducts commonly also deliverventilation airas part of the supply air. As such, air ducts are one method of ensuring acceptableindoor air qualityas well asthermal comfort.

A duct system is also calledductwork. Planning (laying out), sizing, optimizing, detailing, and finding the pressure losses through a duct system is calledduct design.

MaterialsDucts can be made out of the following materials:

Galvanized steelGalvanized mild steel is the standard and most common material used in fabricating ductwork. For insulation purposes, metal ducts are typically lined with faced fiber glass blankets (duct liner) or wrapped externally with fiber glass blankets (duct wrap). When called for, a double walled duct is used. This will usually have an inner perferated liner, then a 1-2" layer of fiberglass insulation contained inside an outer soild pipe.Rectangular ductwork commonly is fabricated to suit by specialized metal shops. For ease of handeling, it most often comes in 4' sections (or joints). Round duct is made using a continuous spiral forming machiene which can make round duct in nearly any diameter when using the right forming die and to any length to suite, but the most common stock sizes range evenly from 4" to 24" with 6"-12" being most commonly used. Stock pipe is usually sold in 10' joints. There are also 5' joints of non-spiral type pipe available, which is commonly used in residential applications. These are snapped together with a pipe-lock seam and uses a crimped and beaded small end (the outlet of the direction of airflow) for connections.

AluminiumAluminium ductwork is lightweight and quick to install. Also, custom or special shapes of ducts can be easily fabricated in the shop or on site.The ductwork construction starts with the tracing of the duct outline onto the aluminium preinsulated panel. The parts are then typically cut at 45, bent if required to obtain the different fittings (i.e. elbows, tapers) and finally assembled with glue. Aluminium tape is applied to all seams where the external surface of the aluminium foil has been cut. A variety of flanges are available to suit various installation requirements. All internal joints are sealed with sealant.Aluminum is also used to make round spiral duct, but it is much less common than galvinzed steel.Polyurethane and phenolic insulation panels (pre-insulated air ducts)Traditionally, air ductwork is made of sheet metal which was installed first and then lagged with insulation. Today, a sheet metal fabrication shop would commonly fabricate the galvanized steel duct and insulate with duct wrap prior to installation. However, ductwork manufactured from rigid insulation panels does not need any further insulation and can be installed in a single step. Both polyurethane and phenolic foam panels are manufactured with factory applied aluminium facings on both sides. The thickness of the aluminium foil can vary from 25 micrometres for indoor use to 200 micrometres for external use or for higher mechanical characteristics.There are various types of rigid polyurethane foam panels available, including a water formulated panel for which the foaming process is obtained through the use of water and CO2instead ofCFC,HCFC,HFCandHCgasses. Most manufacturers of rigid polyurethane or phenolic foam panels usepentaneas foaming agent instead of the aforementioned gasses.A rigid phenolic insulation ductwork system is listed as a class 1[clarification needed]air duct to UL 181 Standard for Safety.

Fiberglass duct board (preinsulated non-metallic ductwork)Fiberglass duct board panels provide built-in thermal insulation and the interior surface absorbssound, helping to provide quiet operation of the HVAC system.The duct board is formed by sliding a specially-designedknifealong the board using a straightedge as a guide. The knife automatically trims out a groove with 45 sides which does not quite penetrate the entire depth of the duct board, thus providing a thin section acting as ahinge. The duct board can then be folded along the groove to produce 90 folds, making the rectangular duct shape in the fabricator's desired size. The duct is then closed with outward-clinching staples and special aluminum or similar metal-backed tape.

Flexible ducting

Flexible ducts (also known asflex) are typically made of flexible plastic over a metal wire coil to shape a tube. They have a variety of configurations. In the United States, the insulation is usually glass wool, but other markets such as Australia, use bothpolyesterfibre and glass wool for thermal insulation. A protective layer surrounds the insulation, and is usually composed ofpolyethyleneor metalisedPET. It is commonly sold boxes containg 25' of duct compressed into a 5' length. It is available in diamters ranging from as small as 4" to as big as 18", but the most commonly used are even sizes ranging from 6" to 12".Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. It is commonly attached with long zip ties or metal band claps. However, the pressure loss is higher than for most other types of ducts. As such, designers and installers attempt to keep their installed lengths (runs) short, e.g. less than 15 feet or so, and try to minimize turns. Kinks in flexible ducting must be avoided. Some flexible duct markets prefer to avoid using flexible duct on the return air portions of HVAC systems, however flexible duct can tolerate moderatenegative pressures. The UL181 test requires a negative pressure of 200 Pa.[3]

Fabric ductingThis is actually an air distribution device and is not intended as a conduit for conditioned air. The term fabricductis therefore somehow misleading;fabric air dispersion systemwould be the more definitive name. However, as it often replaces hard ductwork, it is easy to perceive it simply as a duct. Usually made of polyester material, fabric ducts can provide a more even distribution and blending of the conditioned air in a given space than a conventional duct system. They may also be manufactured with vents or orifices.Fabric ducts are available in various colours, with options for silk screening or other forms of decoration, or in porous (air-permeable) and non-porous fabric. The determination which fabric is appropriate (i.e. air-permeable or not) can be made by considering if the application would require an insulated metal duct. If so, an air-permeable fabric is recommended because it will not commonly create condensation on its surface and can therefore be used where air is supplied below thedew point. Material that eliminates moisture may be healthier for the occupants. It can also be treated with an anti-microbial agent to inhibit bacterial growth. Porous material also tends to require less maintenance as it repels dust and other airborne contaminants.Fabric made of more than 50% recycled material is also available, allowing it to be certified as green product. The material can also befire retardant, which means that the fabric can still burn, but will extinguish when the heat source is removed.Fabric ducts are not rated for use in ceilings or concealed attic spaces. However, products for use inraised floorapplications are available. Fabric ducting usually weighs less than other conventional ducting and will therefore put less stress on the building's structure. The lower weight allows for easier installation.

WaterproofingThe finish for external ductwork exposed to the weather can be sheet steel coated with aluminium or an aluminium/zinc alloy, a multilayer laminate, a fibre reinforced polymer or other waterproof coating.

Duct system componentsBesides the ducts themselves, complete ducting systems contain many other components.

Vibration isolatorsA duct system often begins at anair handler. Theblowersin the air handler can create substantialvibration, and the large area of the duct system would transmit thisnoiseand vibration to the inhabitants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler. The rubberized canvas-like material of these sections allows the air handler to vibrate without transmitting much vibration to the attached ducts. The same flexible section can reduce the noise that can occur when the blower engages and positive air pressure is introduced to the ductwork.Take-offsDownstream of the air handler, the supply airtrunk ductwill commonly fork, providing air to many individual air outlets such asdiffusers, grilles, and registers. When the system is designed with a main duct branching into many subsidiary branch ducts, fittings calledtake-offsallow a small portion of the flow in the main duct to be diverted into each branch duct. Take-offs may be fitted into round or rectangular openings cut into the wall of the main duct. The take-off commonly has many small metal tabs that are then bent to attach the take-off to the main duct. Round versions are calledspin-in fittings. Other take-off designs use a snap-in attachment method, sometimes coupled with an adhesive foam gasket for improved sealing. The outlet of the take-off then connects to the rectangular, oval, or round branch duct.

Stack boots and headsDucts, especially in homes, must often allow air to travel vertically within relatively thin walls. These vertical ducts are calledstacksand are formed with either very wide and relatively thin rectangular sections or oval sections. At the bottom of the stack, astack bootprovides a transition from an ordinary large round or rectangular duct to the thin wall-mounted duct. At the top, astack headcan provide a transition back to ordinary ducting while aregister headallows the transition to a wall-mounted air register.

Volume control dampersDucting systems must often provide a method of adjusting the volume of air flow to various parts of the system. Volume control dampers (VCDs; not to be confused with smoke/fire dampers) provide this function. Besides the regulation provided at the registers or diffusers that spread air into individual rooms,damperscan be fitted within the ducts themselves. These dampers may be manual or automatic.Zone dampersprovide automatic control in simple systems whilevariable air volume(VAV) allows control in sophisticated systems.

Smoke and fire dampersSmoke and fire dampers are found in ductwork where the duct passes through a firewall or firecurtain.Smoke dampers are driven by a motor, referred to as anactuator. A probe connected to the motor is installed in the run of the duct and detects smoke, either in the air which has been extracted from or is being supplied to a room, or elsewhere within the run of the duct. Once smoke is detected, the actuator will automatically close the smoke damper until it is manually re-opened.Fire dampers can be found in the same places as smoke dampers, depending on the application of the area after the firewall. Unlike smoke dampers, they are not triggered by any electrical system (which is an advantage in case of an electrical failure where the smoke dampers would fail to close). Vertically mounted fire dampers are gravity operated, while horizontal fire dampers are spring powered. A fire damper's most important feature is a mechanicalfusible linkwhich is a piece of metal that will melt or break at a specified temperature. This allows the damper to close (either from gravity or spring power), effectively sealing the duct, containing the fire, and blocking the necessary air to burn.

Turning vanes

Turning vanesare installed inside of ductwork at changes of direction (e.g. at 90 turns) in order to minimise turbulence and resistance to the air flow. The vanes guide the air so it can follow the change of direction more easily.

PlenumsPlenumsare the central distribution and collection units for an HVAC system. The return plenum carries the air from several large returngrilles(vents) orbell mouthsto a central air handler. The supply plenum directs air from the central unit to the rooms which the system is designed to heat or cool. They must be carefully planned in ventilation design

Terminal unitsWhile single-zoneconstant air volumesystems typically do not have these, multi-zone systems often have terminal units in the branch ducts. Usually there is one terminal unit perthermal zone. Some types of terminal units areVAVboxes (single or dual duct), fan-powered mixing boxes (in parallel or series arrangement), and induction terminal units. Terminal units may also include a heating or cooling coil.

Air terminalsAir terminals are the supply air outlets and return or exhaust air inlets. For supply,diffusersare most common, butgrilles, and for very small HVAC systems (such as in residences) registers are also used widely. Return or exhaust grilles are used primarily for appearance reasons, but some also incorporate an air filter and are known as filter returns.[4]Duct cleaningThe position of theU.S. Environmental Protection Agency(EPA) is that "If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary."[5]A thorough duct cleaning done by a professional duct cleaner will removedust,cobwebs,debris, pet hair,rodenthair and droppings, paper clips,calciumdeposits, children'stoys, and whatever else might collect inside. Ideally, the interior surface will be shiny and bright after cleaning. Insulated fiber glass duct liner and duct board can be cleaned with special non-metallic bristles. Fabric ducting can be washed or vacuumed using typical household appliances.Duct cleaning may be personally justifiable for that very reason: occupants may not want to have their house air circulated through a duct passage that is not as clean as the rest of the house. However, duct cleaning will not usually change the quality of the breathing air, nor will it significantly affect airflows or heating costs.

References1. The Fundamentals volume of theASHRAE Handbook,ASHRAE, Inc., Atlanta, GA, USA, 2005

2. HVAC Systems -- Duct Design, 3rd Ed.,SMACNA, 1990

3. "Factory-Made Air Ducts and Air Connectors UL 181", UL Standards, retrieved September 2, 2009

4. Designer's Guide to Ceiling-Based Room Air Diffusion, Rock and Zhu,ASHRAE, Inc., Atlanta, GA, USA, 2002

5. "Should You Have the Air Ducts in Your Home Cleaned?", U.S. Environmental Protection Agency, retrieved April 17, 2008