Direct supply (without VAV box) in a space served by VAV system

[orator]

Is it correct to have direct supply (without VAV box) in a space that is served by VAV system.
For example : A building is having 10 different spaces and all are provided with VAV boxes. There are some internal spaces like corridors, Toilets etc. which actually doesn’t need VAV control. Constant flow will be okay for these spaces as there will be no frequent change in the heat gain/loss. Can these spaces have direct supply (without VAV box) from the same AHU which is also serving other spaces with VAV boxes.

 

[SAK9]

As the duct pressure varies,it is not advisable to supply air directly without some sort of control.I have in the past used a CAV box for such areas.You can use use a common CAV box for corridors,lobby etc

 

[orator]

Thanks SAK9. Yes the duct pressure varies but we have VFD for fan which will maintain the duct pressure by modulating the fan speed in case any of the VAV box modulates. And why CAV box? normal Volume damper can do the same work in these spaces as that of CAV box. Please correct me if I am wrong.

 

[lukaiENG]

SAK9's point (I believe) is that there will be duct pressure variations as VAV boxes open and close on the system. just because the fan is maintaining a constant* pressure at one point in the system, this does not mean all other points in the medium pressure duct system will also be constant.
As an extreme example if all of the boxes downstream of your toilets were 100% open and all the boxes upstream of your toilets were at minimum then you will have a lower static pressure if the situation was reversed.
As loads shift, pressures will change.

A CAV box will modulate to maintain a constant flow with a variable static pressure differential.

*it won't be constant

 

[SAK9]

I should have said I was using static pressure reset method for VFD control wherein the static pressure in the ductwork varies depending on the location of a 100% open VAV box. There is no way you can manually balance one branch of a system when the pressure behind the damper can vary a fair bit.

 

[pocketengineer]

For simplicity, let take a very simple VAV system with 4 outlets. 2 outlets (say outlets 1 and 2) are fitted with VAV boxes, and the other 2 outlets (say outlets 3 & 4) have nothing (just have fixed volume control damper). The moment outlet 1 and/or 2 are closed partially, the excess air will rush out from outlet 3 and 4 instantly (no delay). VFD control will have no chance to work as intended. If you agree with this principle, then you would agree with SAK9 to have some sort control.

So will your hybrid VAV system work? I would say yes, but it is not a good engineering practice. Also, I will not know how much excess air will come out from those outlets without VAV boxes. The whole principle behind the VAV system is to have a better control, yet save energy.

 

[317069]

Pocketengineer, let me revise a little in your example as follow, if all four outlets have VAV boxes, but two of them (outlet 3&4) have a constant load. (they will stay 100% opened all the time), now, what would happen if outlets 1&2 have modulated to the minimum position.

 

[317069]

SAk9, did you mean by CAV box "series fan-powered mixing box"

 

[pocketengineer]

For ease of reference, let say outlets 1 & 2 serve Room A. Outlets 3 & 4 serve Room B.

1) Pressure Dependent VAV box:
VAV systems are dynamic. The resulting flow at any given moment through the VAV boxes is very dependent upon the system conditions at that moment. When cooling load in Room A decreases, VAV boxes start to close. Air flow reduces. The result will be an increase in the system static pressure which will increase the air flow to Room B. Room B will start to over-cool until the thermostat sensed it. Then, Room B will start to modulate the VAV boxes to reduce the air flow.

2) Pressure Independent VAV box:
Room B's VAV boxes will quickly sense the increased air flow and start to modulate the VAV boxes to maintain the required air flow.

 

[EnergyProfessional]

For some "random" cooling of hallways etc. i typically see a small duct just bleeding off from a VAV zone. Yes, not really correct, but also simple. for a hallway it doesn't matter if the temp is 100% correct.
you also can just not condition those areas if they don't have much load. for example corridor between conditioned offices will approximate the temperature of the offices unless the corridor has perimeter wall, or large roof. Determine the loads.