Calculation of leakage from a sealed room

[Ioannis]

Hello Everyone,

I am trying to calculate the openings of a sealed room according to a pressure drop in a specific time.
I have the following equation:

Q = 840*A*√(dP)

Q: differential airflow in liters/sec
A: room leakage in m²
dP: differential pressure in Pa

So, if for example we have a room of 500m³ at atmospheric pressure 101325 Pa and we increase its pressure** by 80 Pa and then measure how much time it will need to decrease for 30Pa we can calculate the differential airflow by the following:

(P_si + P_start) * V₁ = (P_si + P_finish) * V₂ <=>
(101325+80) * V₁ = (101325+50) * V₂ <=>
V2 = 500.148m³

So because of a dP of 30Pa we get 148 liters difference.
If that dP occurred in 180sec then we get Q= 148/180 = 0.82 liters/sec
If we then put this number to the initial equation we get the room leakage.

My problem is that the results that I get are different from another method that is used to find the room leakage. So obviously there is a mistake in the above calculations but I can't find what.

**The pressure increase of the room is achieved by using a blower and the temperature is stable.

Does anyone have an idea?
Thank you in advance!

 

[jrmichler]

Do an error analysis of your method. Do an error analysis of the other method. Compare. If the error bands from two different methods do not overlap, there is a mistake somewhere. Keep digging until you find it.

 

[russ_watters]

I'm not following what you did; it looks like you posted an equation and then did somethjing else instead...which doesn't make a lot of sense to me. Can you explain what you did?

In either case, room leakage is difficult because it isn't just a function of leak area, but leak geometry; there is less flow through long skinny cracks than short fat ones.

 

[Ioannis]

This equation Q = 840*A*√(dP) comes from the 1999 Ashrae Application Handbook, Fire and Smoke Management. Since I am interested on finding the A which is the room leakage I need to find a way to calculate the Q which is the differential airflow.

That is where the second part of the calculation comes where I presume that if I know the initial volume of the room and the dP in a specific amount of time then I can calculate the differential air volume.

 

[jrmichler]

An error analysis will show that your method of measuring leakage is subject to large errors from multiple sources.

Your equation assumes turbulent flow through the leakage path, which is correct for large leaks. Small leaks will have laminar flow. I don't have a reference at hand, but if you search blower door testing of buildings, you will find a discussion of this. The effect of laminar vs turbulent flow is significant in a structure with leak rate over 100 times higher than your sealed room.

If you measure leak rate vs pressure difference, I expect you will find that leak rate is linearly proportional to pressure difference. Since your leak rate is so low, you might be better off to measure flow rate using compressed air with an orifice. A little 1/2 hp air compressor will flow the liter per second that you need.