How to calculate server room air flow requirements

physical-environment

I have a small server closet that sits on a climate-controlled manufacturing floor. Since the main room is air conditioned, I want to use the air from that space to inexpensively cool the server closet by exhausting the server closet to the main room. How can I calculate the CFM requirements for an exhaust fan to cool to a certain temp if I know the size of the room, the BTUs/hr of the equipment and the temp of the air coming in?

FYI, the room is 800 ft³, my heat output is about 5000 BTUs/hr and the air intake is at 72 °F.

Best Answer

The basic formula for translating is:

      BTU/hr
CFM = -------
      1.1 * delta-T

Delta-T is the temperature differential between the two sides of the barrier in Celsius. The 1.1 value is a fudge factor for the air's heat-carrying capacity, which we can't figure without percent-humidity. Since we don't know the temperature of the room being cooled, here are some values based on 5K BTU/hr and a 72 degree intake.

 Room T | CFM
----------------
| 74    | 4132 |
| 75    | 3030 |
| 76    | 2066 |
| 77    | 1623 |
| 78    | 1420 |
| 79    | 1165 |
| 80    | 1033 |
----------------

For comparison a certain portable AC unit I just looked at is rated for 10,000 BTU/Hr and runs at about 220 CFM. It can get such low flow rates because for that unit delta-T is a lot higher than what you're attempting here.

I understand why that is an undesirable option though. Those need drains, or a scheduled bucket-brigade, as well as routine maintenance to keep running.

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