Positive Pressure - How much is enough

[jwdundee]

I am evaluating the existing Air Conditioning system for an MCC (motor control center) room at a steel mill. The system has a supply air flow of 14,600 scfm and an exhaust air flow of 13,600 scfm. The room is 125'x25'x14'. This appears to equate to 20 air changes per hour. However, they are experiencing too much infiltration of dust and dirt. How much excess supply air is required to provide adequate positive pressure in the room? Is there an ASHRAE standard that spells this out? I know that for room purging per NFPA 496 a face velocity of 60 fpm is required and that this equates to approx. 1300 cfm for a standard door. Any help is appreciated.

 

I am also working on a similar problem for a paper mill control room. I was looking at some Westvaco (vapor adsorber equipment company) and they suggest the following:

Air exchanges: 2 to 5 per hour, depending on the freguency of passage into doorways (a room rarely entered being on th lower end of the range)

Room Pressurization: 0.08 to 0.25 inches of water

I hope this helps.
Mike Thompson
pager: 256-920-2931

 

[butelja]

I believe that the NFPA stadards call out 0.1" WC pressurization. Unfortunately, I'm not in my office where I can check the standard to verify this value.

 

[malz]

I maintain between 1' & 1.25 ' static in my facility & have no problems. All of my older static operated devices need at least 1" to function normally. Anything below an inch is trouble around here.

 

[ChasBean1]

jw - how do you know the the dust/dirt is being infiltered and that the supply air isn't the source? What kind of filters are on the supply and how are they maintained? I think your flow differentials should be fine. Don't pressurize your room to an inch unless you have very small doors... they might be tough to operate.

 

[MPE]

I agree with ChasBean1

A steel mill environment isn't going to be the cleanest and if you are exhausting 13600 cfm and supplying 14600 cfm i would question the quality of the air being introduced not the pressurization of the room.

what level of filtration is being used? maybe an approach such as a "clean" room with high recirculation and high efficiency filtration using minimum outside air. you may have to provide additional mechanical cooling due to the heat to be removed but it could solve you dust problem.

You also do not mention the room construction - is it "tight"?

If you use high pressurization, consider a small "door" within the door to relieve the pressure differential enough to open the larger door.

 

I recently did a clean space with about 15 Pa positive pressure maintained. I approached the design by calculating the leakage flow by the formula

leakage flow thru door (Cu.M/S)= 0.84*Door gap in Sq.M* Square root(Delta Pr diff in Pa). You must calculate for every door of the room and add the exhaust flow . You can include some safety factor if you are sure about the leakage area of the door.To relieve the excess flow we installed few gravity dampers. The system is working fine.
regards
VVVM