Stair pressurization with open doors

[Ahmedbadr87]

I'm trying to design pressurization system for stairwell.actually, We follow IBC AND ASHRAE IN WHICH I can not find and specific number of doors to be open while designing the system to calculate the pressurization fan capacity.My question is regarding the open door airflow.I have seen many engineers design the system so that the airflow velocity through the open door is around .75 m/s to hold smoke back from entering the stairwell which would increase the fan capacity to meet this demand.But i can not find such a criteria in Ashrae at all.A solved example of pressurization system using Contam software shows that, it is only enough to have a positive pressure difference across open door to guarantee that smoke is held back from entering the stairwell.Am i getting it right? because i'm confused.

Thanks in advance

 

[joecampbell]

https://www.cedengineering.com/userfiles/Stairwell Pressurization Systems.pdf

 

[ChasBean1]

Go for zero doors open.

If you do, you'll have a well designed stair pressurization system that maintains the .10 to .35 range that you need. For a typical 7-15 floor building, it might be a 5000-7000 cfm unit.

When you design for 1 door open (about 21 ft2 of leakage plus normal construction leakage) or 2 doors open (about 42 ft2 of leakage added to construction leakage) your 7,000 cfm fan turns into a 30,000 or 40,000 cfm fan. With quite a bit of HP difference.

In essence, no. Don't try to size fans based on open stair doors. If any AHJ wants you to do otherwise, they need to go to ChasBean1 school..

 

[lilliput1]

I used 2 doors open. Foolish to think about saving money when lives could be at stake. Use constant speed fan but with bypass damper to exhaust or recirculate air, controlled by stairwell pressure sensor.

 

[chicopee]

But if pressurization is too high, can any stairwell door be opened for evacuation?

 

[ChasBean1]

Chicopee, that's the reason for the upper limit of .35". When you start to approach ~.3", doors that lead into stairwells start to exceed 30 lbf (life safety code) door opening force.