V V T System Outside Air

[Fayaz]

I am having trouble satisfying outside air requiremetns with VVT (VAV-bypass)system. Here is an example

Class room 500 sq. ft
Students: 25
OA required = 25x15 = 375
HVAC Unit = 20-ton with 40% OA (8000 cfm total air, 3200 cfm OA)

Summer Operation:
Supply Air = 1000 (vav box fully open)
OA supplied to the class = 1000*40% = 400
OA requriements are met? YES

Winter Operation:
Box closes 50%
Supply air = 500 cfm
Outside air supplied = 500*40% = 200 cfm
OA requirements met? NO

If it was true VAV system, my outside air would be 80% on the unit, and will satisfy room OA requirements. But
this is V V T system, which has constant air flow with by-pass damper. So, we have constant flow of 8000 cfm and 3200 cfm OA at the unit.


Sincerely,

Fayaz Din

 

[walkes]

In most VVT systems that I have seen the unit cycles to meet the demands of the spaces. Each space votes on the mode of the unit. If come spaces are calling for heat then the unit will heat, the other boxes will close off until the initial call is met. The units usually cycle between heat, cool and ventilation mode. If your zones are similar the unit should be in the same mode for each of the zones and the zone valves will modulate to meet the space demand.

With a VVT you will have unvitiated air in the return path when space air valves close and air is bypassed to the return plenum. Your O/A % will increase in your supply air.

With the VVT system your air to the classroom will probably not be at 50% continually but the valve will begin to close when the unit is cooling while the space requires heating.

That's probably not very clear but I've been awake too long today.

See the following

http://www.zoneall.ca/

there should be an explanation there.

 

[tombmech]

That's not very clear to me, either - but I feel your pain.

If I understand the description, the OA% and airflow remain constant at the AHU. Technically speaking, the whole reason for introducing outside air is to displace inside air with air that has a lower CO2 concentration. CO2 levels rise as occupants increase, and no air exchange takes place.

Even if the OA percentage is bypassed to the return when the box closes down, the air displacement (CO2 dilution) is still taking place.

Another way to look at this is to draw a control volume around the whole building - does the air exchange take place in that instance? If so, then only a comparative difference between rooms should be a concern, and that may be minimal.

This scenario sounds a lot better than systems with a true VAV, where the AHU airflow is reduced with OA at a constant percentage. Ventilation definitely suffers in those cases.

 

[GMcD]

And that is why an "all-air" system for this application is not going to work (even though it is the design standard in almost all of North America (and many other places). Instead - design the room perimeter envelope for minimal heat gains and heat losses, use hydronic radiant heating/cooling for as much of the transient loads (sensible only with radiant cooling) as possible, and then provide a 100% outdoor air displacement ventilation system with heat recovery (or energy recovery depending on local climate)