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VAV System
Heating, Ventilation, and Air Conditioning Systems

AE390

Team #1: Ed Asbury, Jordan Wood, Marc Kuchler

 

Diagram of VAV system
Figure 4.30. Schematic drawing of a variable-air-volume HVAC system.

http://www.cyberus.ca/~twp/exch4-1.htm

System Explanation:

The Variable Air Volume (VAV) system makes it possible for the users to control the volume of conditioned air that arrives in any defined space or zone.  The conditioned air mixes with the room air and cools or heats the zone independent of the zones in the building.  This is achieved by incorporating a system dampers, air valves, and fans into the delivery subsystems.  When a loads for a given space is reduced then the system will respond by turning off a fan or closing a damper such that air is diverted away from that space and is sent only the those rooms that need cooling or heating. This results in a more efficient system that requires less power to operate and saves money. 

Air Damper

http://auction.smarthome.com/XcAPViewItem.asp?ID=46877

Explanation of Subsystems:

  • Ductwork Delivery system - Sections of ductwork are hoisted into the ceiling and vertical ductwork chases placed in the walls to create an air delivery network.  Ducts work in the same way that water pipes do.
  • Thermostat - to allow the users of a room to adjust the temperature a system of sensors and electronics are placed in a room to send signals to the air handler telling it to turn on or off.
  • multi-port pressure sensing ring - provides both accurate airflow sensing and control in response to duct static pressure.

Applications:

There are six popular VAV systems. They are:

  • Shutoff

The shutoff simply is comprised of a VAV system which has an air valve that opens and closes.  The users can simply open or shut off the air flow traveling through the ductwork allowing for the desired temperature to be achieved in a given space.
 

  • VAV Reheat

 

http://tristate.apogee.net/cool/csvrd.asp

 This type of VAV system is used primarily to cool a space which does not normally require heating.  An example of this might be a space with lots of windows providing a surplus of solar heat.  Instead of locating the heating equipment inside of the space hot water, steam, or an electric coil can be run through the VAV outlet terminals and reheat the air before it enters a space.  This option is chosen primarily when cost is the primary criteria being taken under consideration.

  • Parallel Fan Powered


http://tristate.apogee.net/cool/csvpd.asp

The parallel fan powered VAV terminal system is commonly used to condition the air for a space because it takes into consideration minimum ventilation and air changes requirements.  The conditioned air first passes through a fan which accelerates the air velocity to a rate needed to provide minimum CFM for a space.  The second local fan then provides an additional boost to the air velocity if more cold air is called for to cool a space.  If the second fan is turned off then the system will automatically operate at the minimum required air delivery rate.  In this situation the second fan does not block the incoming air from the primary fan as it travels through the ductwork and into the room.  The fans work together to service a given space and velocity is the key factor for the success of this system.  It is important to make sure that the design takes into account a fluctuating air velocity into the space. 

  • Series Fan Powered

http://tristate.apogee.net/cool/csvsd.asp

The Series fan powered terminal unit system is also a popular VAV system.  It is similar the the Parallel Fan VAV system except that all of the air passes through both of the fans.  The secondary fan is sized to handle 100 percent of the airflow and is turned on only when additional air flow is called for.  This ensures that the air enters the space at a constant flow with only the temperature varying.  As a room cools the dampers will close and allowing for more room air to be recirculated and less and less incoming cool air to be introduced into a space.  These systems can be commonly found in bathrooms, entrance ways, hallways, and conference rooms. 

  • Dual Duct

http://tristate.apogee.net/cool/cxsvv.asp

A Dual Duct terminal unit has two air valves which connect into a VAV box.  One of the ducts transports warm air while the other transports cool air into a space.  The air mixes in the VAV box and is delivered to a space.  This system allows for a variable temperature and air volume.  In many cases this system works well however it is more expensive up front because more ductwork needs to be installed into the system.
 

  • Changeover/Bypass

http://tristate.apogee.net/cool/csvcd.asp

When the initial cost is the deciding factor the changeover/bypass systems may be the cheapest option.  It is called the changeover/bypass system because it changes over between heating and cooling operation and uses a bypass loop to allow constant volume.  A monitoring system controls the

Considerations:

The VAV system could be considered inappropriate for a building with only one zone required or needed.  In that case a CAV system would be a better option because you would not need to condition air in multiple zone and it is cheaper.

Limitations:

  • Air velocity is an important factor to take into consideration when designing a VAV system.  Some diffusers work best when there is a constant air velocity and some do not work well when the velocities fluctuate.  The proper discharge geometry in a linear slot diffuser will make for good air distribution in a space.

 

  • Some buildings require a certain number of air changes for a given space and this could become problematic if a zone has already reached the desired temperature.  Ventilation would require more air to be brought into a space and further condition the space.  This must be taken into account in the design of the system. 

 

  • Building pressurization is also a significant factor that must be taken into consideration with a VAV system.  Some rooms need to be pressurized such that when a door opens untreated air does not rush into a space.  If the pressurization of the room continually changes it must be such that it still maintains a higher pressurization from the rest surrounding.

 

  • VAV systems do not provide the precision of humidity control that some specialized laboratory or refrigeration spaces may require.

 

Types of materials:

The ducts are normally constructed out of aluminum or stainless steel sheets.

Numeric Parameters:

Types of VAV Systems

Model Type

Type

UNHT

Primary CFM

Fan CFM

Cooling-Only, Parrellel

PAR

NONE

600-3200

400-2000

Electric Reheat Parrellel

PAR

ELEC

600-3200

400-2000

Hot Water Reheat Parrellel

PAR

1RHW/2RHW

600-3200

400-2000

Cooling-Only, Series

SER

NONE

300-3200

400-3000

Electric Reheat Series

SER

ELEC

300-3200

400-3000

Hot Water Reheat Series

SER

1RHW/2RHW

300-3200

400-3000

                                Source: Basic Numerical Parameters for VAV System

http://www.pages.drexel.edu/~gkl23/AE_Design_I/HVAC_Analysis/

 

VAV Sizes and Capacity’s

    

VAV Size

AIR FLOW RANGE L/S

DIM A

DIM B

OPTIONAL INLET

6

50-225

150

203

150RO

8

100-450

250

304

200RO

10

150-650

300

335

205FO

12

350-1100

400

457

300FO

14

550-1250

500

558

350FO

16

700-1750

600

660

400FO

12D

900-2200

850

918

N/A

14D

1000-2500

1050

1120

N/A

16D

1400-3500

1250

1324

N/A

http://www.pages.drexel.edu/~rg22/aed1/

Links to Relevant Resources:

http://tristate.apogee.net/cool/home.asp

http://www.epa.gov/iaq/largebldgs/eiaq_page.htm

 

VAV System Plenum System Radiant Floor Comparisons

Send mail to ea38@drexel.edu with questions or comments about this web site.
Last modified: 05/03/05

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