Positive Pressure in A/C Room

If you're talking about clean rooms, here's a recommendation:

"In order to maintain air quality in controlled areas, it is important to achieve a sufficient airflow and a positive pressure differential relative to adjacent uncontrolled areas. In this regard, airflow sufficient to achieve at least 20 air changes per hour and, in general, a pressure differential of at least 0.05 inch of water (with all doors closed), are acceptable. When doors are open, outward airflow should be sufficient to minimize ingress of contamination'."

(Click anywhere on the paragraph above for more clean room information).

I added to the GA's for the answer above. However there may be additional considerations, such as if you have a known hole in the room walls somewhere (sometimes there is a conveyor exiting a "clean" room and entering a "dirty" room (process to packaging or something similar). Then you may want to offer more pressure and/or (more important) volume to the system. You just don't want to go so high that the wind is whistling through the cracks in the door and you get smacked in the rump with the door as you walk through it!

I once designed a very nice and efficient filtration room on the top of an air intake shaft with a HUGE fan pushing the air into the building after it pulled it in through the filters. Of course, as these things seem to go, the engineer can give all the recommendations in the world, but the owner has to follow them - and that is where most of these things break down...

Long story short, the operators did no cleaning of the filters for a very long time - in a very dusty environment - and no one paid any attention to the magnehelics associated with the system. Finally one day someone decided it might be time to do something so they went up on the roof and tried to open the door into the filtration shed. Called me and asked for the key - door was locked and couldn't turn the handle. I said that the door knob was provided by the owner, and I couldn't possibly help with the problem.

Next day I get a call that they got the door open. It was not locked, but pulling so hard against the strike plate that they couldn't turn the handle... Finally when they got "Big Jim" to turn the knob, the door almost jerked his arm out of the shoulder, knocked him on his face and the door continued around the circle and smashed into the wall behind it putting a nice transverse crease in it! A little too much suction caused by the large fan and the filters that were completely plugged to the point of nearly caving in. After that they checked the filters personally every week, and they actually paid attention to the magnehelic.

Some times you learn the hard way...