Assessing Room Differential Pressure

[Tinker79]

This may seem like a repeat thread, but I have some questions.
I have a large room in a smoke exhaust condition. There is mechanical exhaust and mechanical makeup, and, by design, a flow differential between the two.
I am being asked to provide the differential pressure between the room and atmosphere, to assist in calculations for door weight during egress.

I have the equation for door opening forces from ASHRAE Applicaitons, 52.6: F=Fdc+((5.20*W*A*deltaP)/2(W-d))

I have all the other variables but need the differential.

I note that user "quark" quoted this:

"The pressure of the room is [cfm(in)-cfm(exh.)]/room volume*101327. The pressure terms are in Pascals."

I have a cfm balance of -36,000cfm and a room volume of 84,285 cu. ft.

This gives me a differential pressure in pascals of 4.21E-6, or 1.69x10^-8 inches water gage...I have issues believing this!

This is an approximation so I am happy to disregard leakage in this case.

Can anyone point me at a pressure assessment calc for the room? Thanks in advance.

 

[Tinker79]

Okay, so I've thought about this a little more.

There is a flow imbalance but this cannot be permanent or total; a flow equilibrium must be obtained otherwise the pressure in the room would drop by a value related to 36,000cfm with each minute that passed.

So leakage is essential in flow balance, but a negative pressure in the space must still exist as the motive force FOR the leakage....which I would still like to find!

Thanks

 

[MintJulep]

Measure or guess.

Pretty much your only choices.

 

[Tinker79]

I think I have my number now, and I think perhaps forum members would like to see how I arrived here. I have some automatic opening doors for the space.

Using BS: EN 12101-6, which details an equation for calculation of air flow under a given pressure, I have
Q(flow)=0.83*leakage area*dp^0.5

(metric calc, using m^3/s and m^2, and Pascals)

I calculated the leakage area based on NFPA 92A construction standards per unit area of room and flipped the equation to

dP=(Q/(0.83*leakage area))^2

I threw in my automatic opening doors and a safety factor to the leakage area (halving it for safety means the pressure differential is greater, more conservative for egress).

I'm pretty satisfied with this - but let me know if you see any gaping flaws.

 

[MintJulep]

It's a guess.

Q(flow)=0.83*leakage area*dp^0.5

Is essentially the formula for gas flow through an orifice. In effect it is treating the leakage area as an equivalent orifice where 0.83 is the nozzle coefficient. Is that really equivalent to the flow characteristics of your leaks?

Is the leakage area of your room really in accordance with the NFPA's guess as to what "standard" leakage should be?

Your safety factor is a guess on your part.

So you wind up with a number based on two guesses from people you don't know and one of your own. Or three of your own if you count your guess that the two standards you cite are appropriate for your application.

It's a nice defensible guess, since you can nicely point to standards, but don't deceive yourself. It's a guess.

 

[Tinker79]

I wasn't attempting to disprove you! I'm fully aware of the inaccuracies built in at the assumption level and I've declared those, but at this point a conservative "guess" based on approximate variable relationships to help out with the assessment of door weight for code compliance is better than nothing.

The formula for gas flow through an orifice with a similar safety factor (in the other direction) is used to calculated pressurization requirements for egress stairs so I felt that this was a suitable parallel application.

To answer your questions in detail:

"Is that really equivalent to the flow characteristics of your leaks?"
Not precisely, but how much will they deviate from this value? Hence safety factor...

"Is the leakage area of your room really in accordance with the NFPA's guess as to what "standard" leakage should be?"
I chose NFPA 92A 'tight' construction for leakage. Worst case. Also probably as close as possible given the case, new, high end construction.

The safety factor is there to combat the variability implicit in the assumptions of the other two stages. I guessed this based on my assessment on the magnitude of that variability.

To me, pointing to standards is less important than have a logical calc.


Thanks for the feedback and pointing out the risks of generating confidence from standards.

 

[MintJulep]

Ok, we're on the same page.

If it were me I'd be inclined to caveat the hell out of this result though.

Document the methodology and sources. Something about best practices and standards. Something about the variability of construction. Something about commissioning of the HVAC system. A firm recommendation to confirm the value by test after everything is done.

 

[Tinker79]

Thanks for the tips! I definitely echoed a few of your caveat suggestions...

 

[ChasBean1]

It is an estimation, not a guess. With a guess you might start with a differential of -200 cfm, -2,000 cfm or -100,000 cfm and need to replace your fans after failing testing. You went about figuring it the right way.

But what comes first in the sequence - your automatic doors opening or your fans? If the doors don't open first, they probably won't - I've seen this before with atrium smoke control systems. I'd do the calc with them closed and shoot for a -0.05 in. w.c. differential (NFPA 92A standard differential for a smoke partition).