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  1. #1
    Join Date
    Mar 2007
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    Please Help Confirm Cooling Requirements


    This is a follow-up to a post from about two years ago. Thank you in advance for reading this. It is quite long and full of details! I am looking for advice on how much air exchange I need in a server closet I have in my San Diego, California home.

    First let me describe my cooling needs. I have a 24U rack in the room. It has two servers in it, a UPS, switch, and various networking gear. I used APC's runtime calculator to calculate the thermal heat for all of the rack components. I ended up with 2,300 BTUs/hour. I just upgraded my two servers in the past three months so it is unlikely that the above figure will change too much over the next few years.

    Now, in terms of layout. The closet is part of a 380 square foot, one-story room. The closet itself is 12 feet long by 3 feet wide by 9 feet tall. In addition to the server rack I have three metal file cabinets (two in one corner, one in the other). The closet has two sets of double-doors, (4 total doors), all 8 feet high. We installed a 150 CFM bathroom fan in the ceiling above the back of the rack. This fan is more of an emergency backup. It is connected to a thermostat and turns on when the closet temperature exceeds a set temperature (currently set at 90 degrees). The main part of the 380 square foot room is cooled by a dedicated 2-ton Fujitsu Halcyon heat pump.

    Shophound gave me great advice two years ago. He wrote: You need air exchange to happen in that closet so cooler air entering the closet will absorb heat, be expelled, then find its way to the a/c to have its heat removed and dumped outside.

    This is what I want to do. To help keep the rack itself cool with good air flow, I just ordered two rack fan panels. Each panel blows 64 CFM of air, has two fans, and sits 2U high. My plan is to place one fan on top of each of my two rackmount servers. Depending on how they work out, I may buy a third panel to place above my UPS. Once these are installed, I will have between 128 and 192 CFM of air moving from the front to the back of the rack (not including the servers' own fans). I will block out with filler panels as much open space in front of my rack as possible to force the airflow in one direction.

    I have a LaCross weather station that monitors the temperature in the closet. When I close the doors the temperature has gone as high as 93 degrees. I want to be able to keep the doors closed for noise control and to unclutter the room.

    Now I am looking to implement Shophound's suggestion of active air exchange. I looked online and unless I am missing something, all of the door fans are too small and/or noisy for my needs. The place where I bought the rack fans will make a custom panel for me that I can mount inside my door. I would have one unit near the floor at the front of the rack. This will force cool room air into the closet in front of the rack. I would have another unit on the top of the door at the back-end of the rack. This would force the hot air in closet at the back of the rack out into my room. I should point out that my rack is enclosed on top and on both sides.

    I have a bunch of options for fans and am looking for advice. I am currently looking at two different 120mm fans from SilenX. One blows 72 CFM at 14db while the other blows 90 CFM at 18db. My desk sits just 4 feet from where the intake fans would be placed. So I need to make sure it is not too noisy.

    I tried calculating the temperature difference at various CFM rates. I used the formula posted here that says DT = BTU / (1.08 x CFM).

    Applying the 2,300 BTUs and using three of the 72 CFM fans (or 216 CFM total), I calculate the temp difference as 9.9 degrees. Am I correct that if I keep my office at 76 degrees, the closet will be roughly 86 degrees?

    How do my rack fans affect this equation?

    I want to have enough fans to keep the closet temp below 88 degrees. I want to balance that with the need to have a quiet solution that will not interfere with my work. So fewer fans are better for noise.

    Thanks again for reading all of this. I look forward to any recommendations. Shophound are you still around?


  2. #2
    Join Date
    Jul 2008
    Washington, DC
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    take alook at this

    Liebert stuff is not cheap but it's about the best HVAC equipment made. I really don't know what a 24U rack is so this is somewhat a shot in the dark, but it may be a real simple solution for you.

    Good luck

  3. #3
    Join Date
    Dec 2005
    San Jose, CA
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    consider installing a raised floor system. This would mean making you'refloor the supply plenum. then you could direct the conditioned air where neede including directly below server. and do not forgfet heat rises. You,re return would be uo high and you could use traditional equipment with low ambient modifications.

  4. #4
    Join Date
    Feb 2008
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    Your calculations are accurate.
    The rack fans will add a small load to the equation.
    Do you keep your office at 76 F continuously?
    Do you run the system at night?


  5. #5
    Join Date
    Sep 2008
    Cedar Rapids, IA
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    In general the noise level of a fan depends on its speed. I designed an exhaust system for a Bingo hall (too many cigarette smokers) They had an existing exhaust but it was too noisy (they couldn't hear the numbers called out.)

    We replaced 2 small fans with 6 huge ones and ran them at slow speeds. You could not hear them even when the room was empty.

    I would undercut the door to let cool air enter at the bottom, then put a grill at the top w a 24" box fan running slowly. Build a baffle behind the fan to contain the noise from the servers and enclose it at the sides and bottom to make it draw air from the top.

  6. #6
    Join Date
    Oct 2008
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    I specialize in data center cooling, usually on a much larger scale, but have done a few closets like yours. If noise is a huge consideration look at some of the ducted fans from Fantech. For this low of a cfm requirement they will be perfect, are extremely quiet, give good pressure, and with the ducting, very flexible in their usage. More $ than a standard fan but if you want low noise it's worth considering.

    As was mentioned earlier pull the cold air in from down low and duct the warm air out of the room to a place near a return. Do as much to keep the cold air in front of the servers and the warm behind them and move ~150-200 CFM of the warm air out of the room and you will be more than fine.

    you can contact me at for some free advice if you are still having trouble keeping the closet cool but I think you are on the right track

  7. #7
    Join Date
    Jun 2008
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    As mentioned warm air recirculation can be a big problem and difficult to control especially in a small space like that. Also using a higher supply temp. from conditioned room space will make it more critical to prevent recirculating hot air mixing with supply air. Sounds like you are taking steps to avoid this by isolating your entering and leaving air (if im picturing it correctly).

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