Pressure build-up Calculation in Rooms

[slynk]

Hi everyone,

Is there any way that I can calculate the pressure build-up in a room by just knowing the inlet + exhaust flow rates and room volume?

I understand leakages through door gaps should be taken into consideration and they can be calculated using Q = 0.827*A*P^(1/N) [This is in SI units]. However, in my case, I don't know the value of P, the differential pressure across the adjacent rooms currently since there isn't any PDT around.

I do know the design pressure differential of the rooms, but after 10 years of running the plant with no one to work on HVAC optimisation, and no PDTs for certain rooms, we figured it's most likely that the DPs have deviated from design.

Any miraculous way of calculating this without using instruments?

 

[Zesti]

Short answer: No, you can not calculate a roompressure(difference) from just knowing the inlet and exhaust flow rates.

The difference between inlet and exhaust flows through cracks and there is no way of knowing what the characteristics of the cracks and openings are.

 

[Zesti]

(This whole subject of room pressures keeps popping up a lot... do we need a FAQ for this subject?)

 

[TwinScrew]

^^ Couldn't hurt. I'd find it of use as well.

 

[slynk]

Yeah, that would be great. There aren't a lot of resources online that teach how to calculate it. Google on the topic and this forum will be the first on the list of search results.

Thanks anyway!

 

[mauricestoker]

If the room is existing, there is no sense in bothering with any form of calculation-just cut straight to the TAB job. Place a magnahelic across the perimeter, set supply constant, and vary the exhuast damper until you get the dP you want, then mark each damper.

 

[slynk]

Calculations are for cost benefits tabulation when we convert both supply and extract fan motors from fixed speed to VSD.

In this case, we suspect that the fans may be oversized and are trying to find out what are the actual airflows supplied and extracted.