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shilpasree (Mechanical) (OP)
27 Mar 02 16:26
We are using stair case pressurization system in one of our sub station project. The capacity of fan is calculated by assumimg that one door(ground floor-opening outwards) is opend and the other door (first floor-opening inwards) closed and to maintain 15 pa inside the stairwell. But when both doors are closed pressure is building up inside the space and we are facing difficulty in opening the first floor door. How can we design a relief damper to maintain a pressure of 50 pa inside thee stairwell when both door are closed.
KenRad (Mechanical)
28 Mar 02 8:23
50 PA, or 0.2" WG seems kind of high;  you might still have problems with the doors at that pressure.  If 15 PA (0.06" WG) is sufficient pressurization when one of the doors is open, you may want to keep pressure closer to this value when both of the doors are closed.  This could be accomplished inexpensively by using one or more counterbalanced dampers in the doors or walls.  Ruskin is one manufacturer of these dampers.  You could adjust the counterbalance such that the damper is shut when one of the doors is open, and starts to open as the door shuts to relieve pressure.  I have used these in the past to provide room makeup air in emergency exhaust systems.
ChasBean1 (Mechanical)
28 Mar 02 19:38
shilp - I think the best answer is to abort your original design idea and design the system to meet code with the doors closed. In the US, the 50 Pa is a good target. Code mandates .15 to .35 inches water column (37-87 Pa). I've found that if you exceed 60-70 Pa, your door force becomes excessive and exceeds the NFPA life safety code requirement (US) of 30 lbf (133 N).

The stairwell undergoes changes in pressure in fractions of seconds as doors open and shut during normal operation or during egress. In systems that are maintaining pressure in the right range, say 50 Pa, one door opening will drop that pressure to 15 Pa (give or take), a second door will drop it more. Then both doors shut, and stair pressure is instantly 50 Pa again. If you try to incorporate an HVAC control to respond to this type of rapid fluctuation, you would likely need to use a mechanical response, such as by a spring-loaded or gravity relief. Using electronically controlled dampers and fan variable speed drives will likely not have the response time required. Tuned to operate at that speed, wild setpoint hunting could result.

Re-sheave the fans and adjust stairwell door sweeps to achieve the 50 Pa pressure with the doors shut. Accept the fact that the system will not be at that pressure with doors open, but air flow will still be in the proper direction.

The following is from the Engineered Systems magazine archives and gets into some problems I've found in commissioning these systems and proposed resolutions. See the "description/problem/resolution" section. Some of this advice might help from a design perspective.

http://www.esmagazine.com/es/cda/articleinformation/coverstory/bnpcoverstoryitem/0,2500,65962,00.html
stevenw (Mechanical)
2 Apr 02 14:02
We use a 24"X24" barometric relief damper at the top of the stairs with and adjustable counterweight.
Add air to the stairs every other floor.
Be sure the contractor has sealed the stairwell. If the doors do not have seals you can blow a lot of air and not get the pressure needed.
Goorah (Mechanical)
7 Apr 02 17:45
ChasBean and Ken:

Fantastic answers! Ones for my technical Scrap book.

Paul/Goorah

Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."

paulkeating (Mechanical)
5 Jun 04 10:26
We (in Australia) use variable speed drives (ok, frequency inverters to some of you) for ALL new stair pressurisation systems.  Pressure sensor in stair well is your point of control, fan speeds up or slows down to try to maintan pressure, regardless of doors opening or closing ...
imok2 (Mechanical)
5 Jun 04 17:12
Another system to consider is a supply fan bypass.  In this system, the supply fan is sized to provide at least the minimum air velocity when the design number
Of doors are open. The flow rate of air into the stairwell is varied by modulating bypass dampers, which are controlled by one or more static pressure sensors that sense the pressure difference between the stairwell and the building.
When all the stairwell doors are closed, the pressure difference increases and the bypass damper opens to increase the bypass air and decrease the flow of supply air to the stairwell. In this manner, excessive stairwell pressures and excessive pressure differences
between the stairwell and the building are prevented.

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