Positive Pressure - How much is enough
Positive Pressure - How much is enough
(OP)
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.





RE: Positive Pressure - How much is enough
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
RE: Positive Pressure - How much is enough
RE: Positive Pressure - How much is enough
RE: Positive Pressure - How much is enough
RE: Positive Pressure - How much is enough
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.
RE: Positive Pressure - How much is enough
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