variable air volume system

Variable Air VolumeVAV Systems
Jeff Wotnosky
Trane Sales Engineer
Raleigh, North Carolina
This presentation can be used to re-introduce rooftop VAV systems to technical customers, and to introduce Trane’s new “Rooftop VAV Systems” application manual (lit number SYS-APM007-EN):
http://tranenetlax1/GrpApplicationsEngineering/Home/Appl_man/SYS-APM007-EN.pdf
Notes to the presenter:
Some slides in this presentation include animation. The script under the animated slides includes a cue word (CLICK) to instruct the presenter when to press the space bar, or click the mouse, in order to activate the next step of the animation.
Special notes to the presenter are in red. Further reference information is listed at the bottom of some slides.
The most up-to-date version of this preso can be found at: http://tranenetlax1/GrpApplicationsEngineering/Home/PowerPointPresos.htm
File Name - Page * Version: 9/00
VAV systems
Why(/Not) design VAV systems
What buildings utilize VAV
Changeover Bypass ( Varitrac)
True VAV ( Varitrane)
Single Zone VAV
System control considerations
Leed and VAV
Questions?
So how does a Trane EarthWise rooftop VAV system help if the building is going for LEED certification?
What is Variable Air Volume(VAV)
?
Yesterday we spoke about VariTrac. Today we will be changing gears and discussing VariTrane
VAV System
VAV
terminal
units
supply
ductwork
return
ductwork
supply-air
diffusers
A rooftop VAV system consists of a packaged rooftop air conditioner that can serve multiple, individually-controlled zones. Conventional practice is to use a standard-efficiency rooftop unit with air-cooled condensing and gas heat. The system is typically designed to deliver 55°F, and often has an airside economizer.
Each zone has a VAV terminal unit that is controlled to maintain the desired temperature in that zone. Interior zones typically use cooing-only VAV terminals, while VAV reheat boxes are typically used for the perimeter zones and for some conference rooms.
A building automation system is often included to startup and shutdown the system based on a time-of-day schedule, but the level of optimization is usually minimal.
Characterstics of VAV systems
Vary the air volume as load requires
Why use VAV?
Provides multiple zones of comfort
Life cycle cost will be less than other HVAC Systems trying to accomplish similar comfort levels
Load diversity
Smaller equipment ( lower AC unit first cost compared to Constant Volume)
Less supply air ( less energy consumption)
Able to adapt to changes in building use
Go to black board and draw diagram showing pressure dependent, then pressure independent.
Why Not ??
Controls, equipment, commissioning
Increased maintenance cost
Terminal units might be located in occupied space
Common places to use VAV systems
Commercial/Medical Office buildings
Schools ( all levels)
Houses of worship
Conference Centers
VAV System Types
Changeover Bypass (Varitrac)
“True VAV” (Varitrane)
Single Zone VAV
VariTrac Changeover / Bypass System
Cost Effective way to zone Standard packaged Rooftop units or Split Systems
Often Driven by Light Commercial contractors:
Used to “Package Rooftop Equipment”
Desire somewhat simple installations and want to get on and off the job quickly.
Want to get a little more flexibility than their typical applications
What is VariTrac?
Pressure dependent - does not read flow - control is dependent on pressure in the duct.
Changeover/Bypass VAV System Layout
SA
Like a typical VAV system, a changeover/bypass system contains an airflow modulation device for each individually controlled space. This VariTrac damper modulates supply airflow in response to the space load. However, instead of modulating the central supply fan, this system supplies constant primary airflow. Any unneeded air is diverted to the return airstream, allowing individual comfort control of the spaces.
At part load conditions, when more of the primary air bypasses the space, the mixture of previously conditioned primary air and recirculated return air cuts energy use at the cooling and heating equipment. This explains the use of the term “bypass” in the name of this system. However, due to the fan providing a constant airflow, no fan energy savings is realized at part load conditions.
The term “changeover” refers to how this system handles the building’s cooling and heating requirements. The central air handler can provide either cooled or heated primary air to the space terminal units and it makes this decision by periodically “polling” the spaces. Since it can only provide heating or cooling at a given time, this system is best applied to smaller buildings with a minimal number of incidences where heating is required in some spaces and cooling is simultaneously required in others.
Where would you use VariTrac?
Ideal candidates for VariTrac Systems are:
one-story office buildings, clinics, movie theaters, strip malls, light manufacturing centers.
Examples are offices within a church, school, or manufacturing facility seeking individual (zoned) temperature control.
Popular in restaurants. Gives VAV or zone control at an affordable price.
Light Commerical RTU
Light Commercial Split System
VariTrac Changeover Bypass System
Odyssey Split System (ReliaTel)
VaiTrac or VariTrane
Units with reheat
Cooling only standard
Remote reheat control
Round or Rectangular
Pressure Dependent System (damper position control- no direct CFM measurement)
Up to 1.75” system pressure
VariTrac Dampers
The current system uses three direct wired (non-communicating) components, the bypass damper, the supply air temperature sensor, and the duct air pressure sensor.
The new system uses a communicating bypass assembly that combines these three functions into one device that resides on theCOM 4 bus with the zone dampers and Voyager RTU. The damper information, duct pressure, and supply air temperature are communicated back to the new CCP via the bus.
The new assembly eliminates the option for velocity and is available only configured as a static pressure sensor.
In retrofit jobs (where an existing CCP is replaced by a new CCP), the existing bypass damper, duct pressure sensor, and supply air temperature sensor must also be replaced by a communicating bypass damper assembly.
Central Control Panel
Simple PC interface with Windows™ based software
Interfaces with Main AHU via 2H/2C Control
Scheduling
How is changeover achieved?
CCP polls UCM’s
On a minimum calls for changeover the CCP will disable heat or cool and enable heat or cool
CCP then communicates to UCM’s new control mode and UCM’s control to different setpoints
True VAV System – (Varitrane)
VAV
terminal
units
supply
ductwork
return
ductwork
supply-air
diffusers
A rooftop VAV system consists of a packaged rooftop air conditioner that can serve multiple, individually-controlled zones. Conventional practice is to use a standard-efficiency rooftop unit with air-cooled condensing and gas heat. The system is typically designed to deliver 55°F, and often has an airside economizer.
Each zone has a VAV terminal unit that is controlled to maintain the desired temperature in that zone. Interior zones typically use cooing-only VAV terminals, while VAV reheat boxes are typically used for the perimeter zones and for some conference rooms.
A building automation system is often included to startup and shutdown the system based on a time-of-day schedule, but the level of optimization is usually minimal.
True Variable-Air-Volume System
supply
fan
cooling
coil
thermostat
VAV
box
PA
SA
A variable-air-volume (VAV) system delivers the primary air at a constant temperature and varies the airflow to maintain the required space temperature at all load conditions.
All VAV units have a controller attached to them and are designed to control temperature in the space… applications beyond “zone temperature control”, should be discussed with product support.
VAV Terminal Units
2. Go into each type individually and explain their features.
VCCF
No auxiliary heat
Air valve modulates open or closed to deliver primary air. Always trying to satisfy zone temperature.
This VAV terminal unit is typically used for those zones that require year-round cooling, like the interior zones of a building. It is the most common and basic type of single-duct VAV terminal unit.
Go over Model #
VCCF
primary
air
supply
air
VAV Terminal Units
A VAV terminal unit is a sheet-metal assembly installed upstream of its respective space diffusers. The unit consists of an air-modulation device, control hardware and, depending on the system application, possibly a heating coil, a filter, and a small terminal mixing fan.
Modulating the airflow to each individual space is accomplished using a temperature-controlled mechanical device that varies the airflow resistance in the supply duct to that space. The rotating blade damper changes airflow resistance by rotating the damper into the air stream, restricting the size of the air passage to the space. It is very cost-effective and flexible. Typically, either a pneumatic or electric controller can be used to adjust the damper.
An understanding of the common VAV terminal unit types is important to understanding VAV systems.
VCWF
Hot Water Coils
TOPSS always selects the smallest coil that meets capacity.
Model #
VCWF
primary
air
supply
air
VCEF
Interlocking door disconnect
Heater line fuse
Element removal through control enclosure
Electric heat units now (beginning in 2Q07) have the ability to remove the elements without disturbing downstream ductwork. This is handy if the heater elements fail for some reason…
Terminal Reheat System
EA
PA
OA
RA
supply
fan
cooling
coil
reheat
coil
thermostat
SA
The terminal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces. Each space has its own heating coil to temper the air to satisfy its load.
parallel,
Fan-Powered VAV
Similarly, fan-powered terminal units can be used in perimeter spaces that require seasonal cooling and heating.
In the heating mode, the parallel, fan-powered unit turns on its small fan as the first stage of heating. Doing so allows it to temper the supply air with the heat of the building and lights that is carried by the return air rather than using “new” energy. When activated, the small fan also increases airflow to the space, improving the mixing of supply and space air to prevent stagnation.
Series, fan-powered terminal units may also be used in this manner. They offer the added advantage of supplying constant airflow to the space in both cooling and heating modes.
Parallel, Fan-Powered
primary air
plenum
air
A parallel, fan-powered terminal unit consists of a primary airflow modulation device and a small, integral, constant-volume fan packaged to provide parallel airflow paths.
Parallel Fan-Powered VAV
cool primary air
from rooftop unit
warm air recirculated
from ceiling plenum
(second stage of heat)
Heat generated by light fixtures warms the air in the ceiling plenum. Fan-powered VAV boxes use this warm air from the plenum as a form of energy recovery.
In a parallel, fan-powered VAV box, primary airflow is reduced as the cooling load decreases. When the box reaches its minimum airflow setting, CLICK the small fan activates to draw warm air from the ceiling plenum and mix it with cool primary air from the rooftop unit. CLICK If more heat is required, the heating coil can be activated.
Parallel, fan-powered VAV boxes can save energy because they use warm air from the ceiling as the first stage of heating. This delays the need to use new energy by activating the heating coil.
And, since the terminal fan only operates when the zone needs heat, these boxes use much less fan energy than series fan-powered boxes, in which the terminal fan runs whenever the zone is occupied.
[Further information: SYS-APM007-EN, pp. 28-31]
supply
air
primary air
A series, fan-powered terminal unit consists of an airflow modulation device and a small, constant-volume fan, packaged so that their airflow paths are in series. The terminal unit fan operates continuously whenever the space is occupied. The fan draws air from either the primary air stream or the plenum, based on the thermostat in the space. This results in a constant volume of supply air delivered to the space at all times.
Parallel vs. Series
Intermittent fan
Fan only runs in the heat mode (Variable Volume to the space)
Fan is the first stage of heat
Fan CFM < Max Cooling
Most energy efficient design
Continuous fan
Fan runs continuously in the occupied mode (constant volume to the space)
Fan CFM = Max Cooling
Smaller main supply fan?
fan-powered VAV
series FPVAV with ECM
This chart shows the impact of using parallel versus series fan-powered units in the same example office building. The “base case” uses single-duct, VAV reheat terminal units. Parallel fan-powered VAV reduced the overall HVAC energy use in each of the three locations, with the biggest impact in the heating-dominated climate of Minneapolis.
CLICK Series fan-powered VAV terminals—even when they are equipped with ECMs (electrically-commutated motors)—actually increased HVAC energy use in Atlanta and Los Angeles. For the building in Minneapolis, series fan-powered VAV reduced energy use compared to VAV reheat, but still used more energy than parallel fan-powered VAV.
Fan Powered Units: Cooling Only
Plenum Air provides “free” reheat from lights, etc.
Parallel units (only when fan energized)
Series (increases as air valve closes)
Cooling only (no reheat) - fan is only heat source.
Fan Powered Units: Cooling with Hot Water Reheat
Model VPWF/LPWF (Parallel)
Model VSWF, LSWF (Series)
Coil Offerings
1 Row
2 Row
2. 1 & 2 row coils
Different mounting options
Water coils can have a pressure drop which impacts system efficiency and fan airflow. Parallel units are MOST efficient when the water coil is mounted on the plenum inlet. Why would that be?
Fan Powered Units: Cooling with Electric Reheat
Model VPEF / LPEF(Parallel)
Discharge mounted
UL listed
Pressure drop through electric heaters is negligible. That is why the heater is located ONLY on the unit discharge.
Fan Powered Units: Electric Heater Options
8 different voltages
2 stages of heat (plus the fan as stage 1)
Interlocking door disconnect
Heater line fuse
Air flow switch
Differences between VariTrac and VariTrane
VariTrac
Simpler System Control
Up to 24 zones
Very Flexible System Control (more complex sequences)
Larger Variable Volume AHU/RTU equipment (up to 130 Tons)
Up to 120+ zones
Pressure Independent
VAV System Types
Single Zone VAV
Single Zone VAV
AHU is your VAV Box !!
Application for Single Zone VAV
Large areas ( conference rooms, assembly halls…)
Control air flow off room sensor
Controls
Analog - commonly ordered in error - needs DDC
In the last 20 years pneumatics have gone from 90 to 80% to less than 20% with the introduction of DDC.
Unit Controls
No- controls- field installation of others controllers- about 5% (ENON)
Factory Installation of others Controls- about 20% (FM0*)
DDC Controls - around 70% (DD**)
Trane DDC (Communicates to a Trane-Trane-Trane System)
Trane LonTalk DDC (designed to talk a language which is not proprietary to Trane)
Analog - commonly ordered in error - needs DDC
In the last 20 years pneumatics have gone from 90 to 80% to less than 20% with the introduction of DDC.
VAV systems
Optimal start and optimal stop
Fan-pressure optimization
Supply-air-temperature reset
occupied hours
Optimal Start
6 AM
mid
mid
system
on
system
off
occupied
heating
setpoint
unoccupied
heating
setpoint
optimal
start
In many applications, a simple time clock or time-of-day schedule is used to start and stop the HVAC system.
CLICK During the unoccupied hours, the system is shut off and the temperature is allowed to drift away from the occupied setpoint. The time at which the system starts in the morning is typically set to ensure that the indoor temperature reaches occupied setpoint on either the coldest or warmest morning of the year. The result is that, for most days, the system starts much earlier than it needs to. This increases the number of system operating hours and increases energy use.
An alternative approach is a strategy called “optimal start.” A building automation system is used to determine the length of time required to bring each zone from its current temperature to the occupied setpoint temperature.
CLICK Then, the system waits as long as possible before starting, so that the temperature in each zone reaches occupied setpoint just in time for occupancy.
This reduces the number of hours that the system needs to operate, and saves energy by avoiding the need to maintain the indoor temperature at occupied setpoint even though the building is still unoccupied.
[Further information: SYS-APM007-EN (pp. 129-130) and BAS-APG003-EN (pp. 128-131)]
occupied hours
Optimal Stop
6 AM
occupied
setpoint
A related strategy is “optimal stop”. As I mentioned, at the end of the occupied period the system is shut off and the temperature is allowed to drift away from the occupied setpoint. The building occupants, however, may not mind if the zone temperature drifts just a few degrees before they leave for the day.
Optimal stop uses the same building automation system to determine how early the heating and cooling can be shut off for each zone, CLICK so that the indoor temperature drifts only a few degrees from occupied setpoint. The supply fan continues to operate, and outdoor air is still provided to the building for ventilation. Only cooling and heating are turned off.
The optimal stop strategy also reduces the number of hours that the system needs to operate, saving energy by allowing indoor temperatures to drift early.
communicating BAS
VAV boxes
P
In a VAV system, as the cooling loads in the zones change, the VAV boxes modulate to vary airflow supplied to those zones. This causes the pressure inside the supply ductwork to change. The rooftop unit modulates the supply fan to maintain a certain static pressure in the ductwork. In most systems, this pressure sensor is located about two-thirds of the distance down the main supply duct. But when no zone is at design airflow, the system is generating more static pressure than required because all the dampers are partially closed.
When communicating controllers are used on the VAV boxes, its possible to optimize this control function to provide just enough pressure in the duct, and minimize fan energy consumption.
Each VAV box controller knows the current position of its damper. The BAS continually polls these individual box controllers, looking for the one with the most-open damper. If all dampers are somewhat closed, the fan’s static-pressure setpoint can be reset lower.
The result is that the supply fan generates only enough static pressure to push the required quantity of air through this “critical” VAV box. Same airflow, just less pressure.
surge
This concept, which Trane calls fan-pressure optimization, has several benefits.
First is that it reduces supply fan energy use. CLICK At part-load conditions, the supply fan operates at a lower static pressure and, therefore, uses less energy.
Second, because the supply fan doesn’t need to generate as much pressure, it typically generates less noise. And, with lower pressures in the ductwork, the dampers in the VAV boxes will be more open, resulting in less noise generated at the boxes.
A third benefit is the reduced risk of fan surge. By allowing the fan to operate at lower pressures when delivering reduced airflows, the fan operating point is kept further away from the surge region.
The fourth benefit is the ability to have the pressure sensor factory-installed and tested at the outlet of the fan inside the rooftop unit.
ASHRAE Standard 62.1-2004
in response to:
(ventilation reset)
The 2004 version of ASHRAE Standard 62 now explicitly permits dynamic reset of outdoor airflow as operating conditions change.
The standard lists examples, including these two:
You can reset intake airflow in response to variations in zone population, which is often referred to as “demand-controlled ventilation.” DCV is commonly implemented using carbon dioxide (CO2) sensors, occupancy sensors, or time-of-day schedules.
In addition, you can reset intake airflow based on variations in ventilation efficiency, which Trane refers to as “ventilation reset.”
The goal of each of these strategies is to bring in only the minimum amount of outdoor air needed for the current operating conditions. In most applications, reducing the outdoor airflow will save energy.
ventilation optimization
elevators
vestibule
corridor
mech
room
BAS
CO2
CO2
OCC
OCC
AHU
TOD
TOD
For a VAV system, the best approach often combines several demand-controlled ventilation strategies at the zone level, using each where it best fits; and then couple these with ventilation reset at the system level.
CLICK With this strategy, CO2 sensors are installed only in those zones that are densely occupied and experience widely varying population. In this example, sensors are installed only in the conference room and the lounge. These are good candidates for CO2 sensors, and provide the biggest bang for the buck.
CLICK However, zones that are less densely-occupied or have a population that varies only a little are probably better suited for occupancy sensors. In this example, each of the private offices has an occupancy sensor to indicate when it is unoccupied, which then lowers the ventilation requirement for that zone. Occupancy sensors are relatively inexpensive, don’t need to be calibrated, and are already used in many zones to control the lights.
CLICK Finally, zones that are sparsely populated or have predictable occupancy patterns may be best controlled using a time-of-day schedule in the building automation system. This schedule can either indicate when the zone would be normally occupied, or may even be used to vary the zone’s ventilation requirement based on anticipated population.
ventilation optimization
DDC/VAV terminals
rooftop unit
with controls
communicating BAS
CO2
TOD
TOD
These various zone-level DCV strategies are used to reset the ventilation requirement for their respective zones for any given hour. In addition, the flow sensor on each VAV terminal continuously monitors primary airflow being delivered to the zone.
CLICK This zone-level control is then tied together using ventilation reset at the system level. The BAS periodically gathers this data from all the VAV terminals and solves the ventilation reset equations (from ASHRAE 62) to determine how much outdoor air must be brought in at the rooftop unit to satisfy all of the zones served.
CLICK Finally, the BAS sends this calculated outdoor airflow setpoint to the rooftop unit, which modulates a flow-measuring outdoor-air damper to maintain this new setpoint.
rooftop VAV system
HVAC Energy Savings
% of base
optimized controls
conventional system
The results show that there is potential to save energy in rooftop VAV systems through the use of optimized controls.
The blue column to the left for each city is the conventional system, and the green column to the right is the optimized system. The Y axis is overall HVAC energy consumption, shown as a percentage of the base case.
The optimized controls reduced HVAC energy use by 24% for the building in Atlanta, by 19% in Los Angeles, and by 27% in Minneapolis.
Trane Control Systems Architecture
VAV systems
Sustaining Performance
Centralized alarming, diagnostics, trending
Features like centralized alarms or diagnostic messages alert the building operator or outside service contractor, and even help them pinpoint the cause of a problem. This allows them to respond to problems faster.
With remote communications and remote access, these messages can be transmitted, even allowing problems to be fixed before people return to work on Monday morning.
VAV systems
Building operations, energy management, commissioning, validation
Various facility management services can help you operate your facility more efficiently. In addition to the standard energy management features available in Tracer Summit, add-on packages and software products are also available.
Computerized maintenance management systems schedule preventive maintenance, issue and track work orders, and manage maintenance-related purchasing and inventory. By integrating these tools with the building automation system, work orders can be automatically “triggered.”
Tools like Tracer Summit Energy Services can allow you, or a service provider, to use data to spot anomalies or unusual conditions, and take corrective action to reduce energy costs before they become too big of a drain on the business.
[Further information: BAS-SLM015-EN]
VAV systems
Periodic commissioning and calibration exercises the different components of the system, testing them to make sure that they continue to work as intended.
For example, the system can direct the VAV boxes to step through various tests to make sure the flow sensor is calibrated, and that the damper, heating valve, and terminal fan all operating properly.
This avoids wasting energy and assures that the system is able to provide occupant comfort.
[Further information: BAS-APG003-EN (pp. 145-152)]
VAV systems
for flexibility
A wireless zone sensor can be easily placed in the best location to provide comfort. This might be on a cubicle wall, a concrete or brick wall, or some other difficult-to-wire location.
In addition, a wireless sensor is easy and inexpensive to move when the layout or use of the zone changes.
Or if the planned location of the sensor turns out to be a bad spot, it can be easily moved until the right location is found.
[Further information: BAS-SLM023-EN]
EA credit 1: Optimize Energy Performance
TRACE 700
Ventilation optimization
CO2 sensors only in densely-occupied zones
Traq dampers in IntelliPak rooftop unit
Certainly, one of the key goals of an EarthWise system is to save energy, so it helps achieve points under EA credit 1. Because this credit typically requires whole-building energy simulation, tools like TRACE 700 can help.
In addition, Trane’s new IntelliPak II rooftop unit is the most efficient in the marketplace, with a full-load EER of 9.7.
CLICK Tracer Summit’s ventilation optimization strategy can help achieve EQ credit 1, by installing CO2 sensors only in the densely-occupied zones, and then using Traq outdoor airflow measurement dampers in the Trane IntelliPak rooftop unit.
[EA credit 4: Enhanced Refrigerant Management – Some sizes of IntelliPak I units can achieve this point with R-407c, but some sizes cannot. Some sizes of IntelliPak II units (with R-410a), can achieve this point, but many cannot.]
Summary
Why(/Not) design VAV systems
What buildings utilize VAV
Changeover Bypass ( Varitrac)
True VAV ( Varitrane)
Single Zone VAV
System control considerations
Leed and VAV
VAV systems
QUESTIONS ??
VAV systems
THANK YOU
919-781-0458

variable air volume system

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