Stairwell Pressurization

[mechfp]

Hi All, M new to HVAC designing. Wanna know more about Stairwell Pressurization. Gone through many threads but need some basic explanation. Please provide some link or document.

 

[cdxx139]

Whats your local code? It will tell you what to do.

knowledge is power

 

[stookeyfpe]

Go purchase the SFPE/ICC Handbook of Smoke Control Engineering. It has everything you need.

 

[KiwiMace]

The basic concept is you need to provide enough air to maintain a minimum airflow from the stairwell through a given amount of doors open, but not too much that it might be difficult to open doors when they are all shut. Your code will tell you how many doors, max opening force, airflow required across open doors, etc.

 

[ChasBean1]

You can’t design around open doors. This is because a stairwell pressurized to these values will immediately (in under two seconds) be less than a tenth of these values when a door is open.

For example, if you supply a sealed and properly constructed stairwell that may have 4 total square feet of leakage with 5,000 cfm, this airflow causes stair pressure to be within the code criteria. Now if you open a stairwell door, the 4 square feet becomes 25 square feet. To achieve the same pressure, your initial 5,000 cfm now becomes about 36,000 cfm.

If two doors are open, you’d need a 76,000 cfm fan.

It needs to be accepted that the code mandated pressure cannot be met with egress doors open. It is not practical to buy a fan that is ten times more expensive to meet conditions with impractical criteria. Design around a sealed stairwell with all doors closed.

 

[ElSid1]

ChasBean1, that is NOT 100% true. The code and ASHRAE typically say closed doors. I just did a system where the AHJ wanted positive pressure if the doors were open. I'd need to go look at my calculations, put the fan size GREW astronomically! So the client and the AHJ settled on a "piece of paper blowing through the doors when they were open". Again, I'd have to go back through some calculations to confirm the "pressure" we used with the open doors.

 

[KiwiMace]

I have experience of doing this under NZ/AUST and Singapore codes which both had/have an open door flow requirement of 1 m/s average velocity across two doors open - ie. the escape doors floor-stairwell and stairwell-outdoors.

Pressurisation is only relevant when the door are closed, which we controlled to 50 Pa (code req'd pressurisation) with a CV fan and a VFD fan feeding from the bottom and top respectively. Without the pressure control, it can be tricky to balance the door opening force constraint in the stairwell and still maintain the flow demand.

The concept of the flow criteria is to prevent a complete loss of effectiveness when the doors are open, and stop smoke from flooding into the stairwell as people escape. This without designers trying to achieve some given pressure and exploding the fan sizing per chasbean1's comment.

 

[urgross]

I would recommend looking at NFPA 92. That also discusses the open door and closed door, or compensated and non-compensated conditions.

 

[ElSid1]

urgross
You beat me to it. IT was a year ago ang I couldn't remember the code we used ... it was NFPA 92

 

[KiwiMace]

I just read the section of NFPA 92 and it is quite different. It seems that practically, you only have to maintain the minimum pressure, which for a sprinklered building is 0.05 inWG (only 12.5 Pa). It does not specify the number of open doors, which might come from the building code (?).

 

[urgross]

Maintaining 0.05 IWG is not so easy in a large building. Getting the AHU control interlocks for 90A and 92 requirements for multipple smoke zones for multiple AHU's is definitely not easy.

 

[ElSid1]

urgross is correct, the controls are a pain

KiwiMace, you need to look at the building code, the fire code and above all the AHJ requirements. As you have experience with open door scenarios, you clearly understand how quickly the CFM goes up to maintain ANY semblance of possitive pressure.

The system I worked on ... the "floating" paper pressure test was "adequate" to show possitive pressure to the FD. I believe we used 0.005 in the calculations as a starting point. Our 8000 CFM closed door system quickly grew to 70,000 cfm due to the need of 4 doors being open for egress was in the requirements. See chasebean1's comment above

 

[ChasBean1]

Ok, you need to maintain stairwells +0.15 to +0.35" w.c. Stairwells and smokeproof enclosures are defined to the tee based on non-bendable code criteria and you have no wiggle room. You CAN'T design to these pressures with any door open. ElSid, sorry, it's 100% true. I don't usually post 50% true things.

 

[ElSid1]

ChasBean1, ASHRAE, NFPA, IBC, etc ... they ALL reference a pressure and testing with the doors CLOSED. As an engineer, we follow these standards. In my case, the AHJ MANDATED an open door test. They didn't care what the standard was. Remember, all of the standards are guides and are not mandatory until picked up by the AHJ. The AHJ "modified" the code (i.e. it was LAW) to test with open doors. I ran into your example above ... 5000 cfm fan growing into 76,000 cfm fan. This was for an underground station where keeping smoke out of the staircase was, as stated by

to prevent loss of effectiveness when the doors are open, and stop smoke from flooding into the stairwell as people escape

. I argued it was not code and LOST! They effectively pointed me to the code implementations with addendums, that were LAW! Just be mindfull of the LAW as that is what the AHJ will enforce. Now, as to your statement

You CAN'T design to these pressures with any door open

I COMPLETELY agree. That is why the AHJ agreed to a "floating paper test". On paper, I think we went to 0.0005" to compute an answer. I need to go back to the calculations.

 

[KiwiMace]

I think your AHJ is smoking something if he is asking for an enforced dp across an open door and then testing it with toilet paper. Try suggesting an average flow velocity that gives you a reasonable fan capacity.

 

[DRWeig]

Listen to Kiwi, ElSid1, and ChasBean1.

I have dealt for many years with the issue of measuring delta-p in stairwells, isolation rooms, hazardous storage areas, and all that stuff. My company supplies a number of different brands of room pressure switch for alarming.

The issue of pressure difference with an open door causing false trips on the sensitive pressure switches cannot be solved without some logic. If supply or exhaust airflow doesn't change much but the dp suddenly drops to an unmeasureably low value, assume a door is open. After a time period, put out a different alarm ("Hey! Shut the door!). For highest reliability, include a door status reed switch like burglar alarms have. They're about $5.00 tops. Then you can safely ignore the dp when the door is open. Again, if it stays open too long then it needs investigating. Somebody has propped it, or is standing there holding a conversation with the thing open.

Attempting to measure dp across an open door is nuts, unless you're in a wind tunnel. In certain hospital isolation rooms, we've been made to do a smoke test with the door open. That can prove that the supply to a positive room is greater than the exhaust or return.

But a dangling toilet paper test? Heavens to Betsy.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[urgross]

Even for biological agent labs, I never had to demonstrate open door dP, only inward direction of flow. When smoke, viruses or bacteria can go against the flow, we're all toast anyway.

 

[ozmosis]

mechfp
It depends a lot on where you are in the world as to how subjects such as this are treated and acted upon.
I was in Malaysia recently and was asked to present at an institute (mechE) monthly meeting a paper referring to the use of VFDs in Fire & Smoke environments (VFDs are my business), and I used a presentation I have made in Australia many times. It refers to AS/NZ 1668.1 and AS/NZ 1851 as a guide (these Aussie standards are linked to ASHRAE standards). Fairly standard stuff.
It was quite scary the questions coming out during and after about how, in Malaysia, they design their buildings, and Malaysia is one of the better countries for building safety, or so I thought. It made me check out the exits in my hotel that night!

Regarding the discussion about open doors pressure test. Closed doors are tested but the requirement to open the doors without engaging superman is the criteria. You need to keep smoke out of the stairwell but you need to be able to get people into the stairwell, safely. The design of controls and equipment to maintain pressure with doors open would be practically impossible, the span of the dp would be too great. As kiwimace points out, 50pa is typically the accepted pressure to maintain both requirements. I recall it was used as the 'granny test': if poor old granny can get out into the stairwell without too much heaving on the door, then it is ok.

 

[DRWeig]

From 2012 California Building Code:

1133B.2.5 Door opening force.

The maximum force required to push or pull open a door shall comply with this section. Push or pull force for a hinged door shall be measured perpendicular to the door face at the door opening hardware or 30 inches (762 mm) from the hinged side, whichever is farther from the hinge. Push or pull force for a sliding or folding door shall be measured parallel to the door at the door pull or latch. Compensating devices or automatic door operators complying with Section 1133B.2.3.2 may be used to meet the maximum force limits.

1. Required fire doors shall have the minimum opening force allowable by the appropriate administrative authority, not to exceed 15 lbf (66.7N).

2. Other than required fire doors, interior doors shall have a maximum opening force of 5 lbf (22.2 N).

3. Other than required fire doors, exterior doors shall have a maximum opening force of 5 lb (22.2 N).

Regarding item 1, the AHJs in my experience, southeastern US, have had a minimum of 7.5 lbf.

A simplified calculation of the differential pressure needed to exert 7.5 lbf on a door is:

lbf to open = Door area (sq ft) X 2.66 X dp ("WC)

Solving for dp,

dp ("WC) = lbf to open / door area (sq ft) / 2.6

So for a 7' x 3' door, 7.5 lbf yields 0.13 "WC differential pressure.

15 lbf will be twice that pressure.

The 2.66 factor is based on the simplifying assumption that the class 2 lever advantage is 2, and the distance from the latch edge to the handle is neglected as well. I think it's pretty close to correct. Easy to test with a dp gauge and a carry-along fish scale (kidding -- better use an accurate pulling force gauge). The force drops precipitously at the first crack.

Of course, you need to add in hinge friction, spring closers, and anything else that gets in the way, then adjust your dp to keep the opening force in the range.

Sorry for not making the metric conversions in my calculation above. I hope nobody is offended.

I got a good chuckle out of some engineer friends yesterday with the line about the flapping toilet paper test.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[ElSid1]

I got a good chuckle out of some engineer friends yesterday with the line about the flapping toilet paper test.

Laugh all you want. It is a true story and currently being constructed for an underground light rail station where the AHJ criteria was

to prevent loss of effectiveness when the doors are open, and stop smoke from flooding into the stairwell as people escape

Believe me, doing the calc was a pain and an even bigger pain was to set up the controls!