Take the 2-minute tour ×
Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

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.

share|improve this question
    
Make sure you take precautions to prevent any dust from the manufacturing processes from getting pumped into your server room. –  kce 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

2 Answers 2

up vote 11 down vote accepted

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.

share|improve this answer
    
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
    
Works for me, thanks! –  fwrawx Apr 29 '13 at 22:16
    
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

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

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.