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.

  • Make sure you take precautions to prevent any dust from the manufacturing processes from getting pumped into your server room. – user62491 Apr 29 '13 at 20:28
  • Yes, the intake is filtered. That's why everything is in a room and not on the open floor. – fwrawx Apr 29 '13 at 21:59

The basic formula for translating is:

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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  • How does the size of the room factor into this equation? – fwrawx Apr 29 '13 at 21:58
  • @fwrawx That's the thing, it doesn't. This formula assumes perfect mixing of air/heat, and you really do know how much heat is generated by everything in the room. Reality will differ due to air-flow, and predicting where hot-spots will form based on where inlet/outlet locations are and CFMs is well beyond my skill. – sysadmin1138 Apr 29 '13 at 22:02
  • Or put a bit differently, room-size affects the rate of temperature increase, but does not affect the total heat in the system. Get a big enough room and you can survive a 12 hour AC shutdown w/o going too hot. In a wee closet, all that heat will get you into trouble a lot faster. – sysadmin1138 Apr 30 '13 at 11:32
  • Are you sure Delta-T is in Celsius when you are using BTUs in the numerator? Perhaps the 1.1 constant (i.e., your "fudge factor") adjusts for this? – HVACer Nov 3 '16 at 15:48

If you decide to go with a portable AC unit, there are models that re use the water that comes from cooling, to cool the coils and as such does not need to have a bucket emptied or drained, the water simply evaporates through the exhaust hose.

One of those companies is Danby http://www.danby.com/en/US/, have a look you might be able to find what you need at this site, good luck

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