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Posted by Bennett Price on April 19, 2007, 2:49 am
Jonathan Kamens wrote:
> Greetings,
>
> I'm trying to figure out how much cooling capacity we need
> for our small machine room. The numbers I'm getting seem
> absurdly high, so I thought I'd run it past the denizens of
> this newsgroup and see if anyone has any suggestions.
>
> The room is 9 ft. x 17 ft. x 10 ft. high. There's no drop
> ceiling or raised floor. Two vents from the house A/C
> discharge into the room, and there's a return vent venting
> into the drop ceiling in the hallway. It's not a designed
> machine room per se; it's more of a big closet that we're
> using to hold our racks.
>
> There are 4 long flourescent bulbs in the room, which I
> suspect don't add much to the heat load.
>
> We've got two racks of hardware, a couple of small desktops
> and a CRT monitor. One of the racks is about 2/3 full of
> various pieces of network hardware -- PoE switches, switches,
> minihubs, CSU/DSU, and a couple of breadbox PCs that are our
> firewalls.
>
> The other rack is pretty much full of servers, most of which
> are 1U servers whose power load is 560W and whose specs claim
> a BTU rating of 2750 (which seems a bit high, but that's what
> the specs say).
>
> When we were relying on just the house A/C vents to cool the
> room, it was always very warm in front of the racks, and
> downright hot in back of them. When the temperature behind
> the racks hit 100 deg. F and the HDD temperature monitor I
> was running on one of the desktops hit 54 deg. C, I decided
> it was time to do something about the problem. I called Spot
> Coolers and described the situation, and they suggested a
> 1.5-ton MovinCool unit, which we agreed to. We ran 12-inch
> flexible plastic ducting from the cooler to the return vent.
>
> The unit managed to get the temperature in the room
> (according to its thermometer) down to around 74 degrees, and
> the temperature behind the racks got down to around 80. Then
> we turned on a few more servers, and now the room temperature
> is hovering around 79 and the behind the racks it's up to
> around 90. I find it hard to believe that the few servers we
> turned on made that big of a difference, so I suspect that
> there are other environmental differences going on, although
> I'm not sure what they could be.
>
> Since we would ideally like to keep the room around 68
> degrees, clearly the 1.5-ton unit isn't big enough. The
> question is, how much capacity do we need?
>
> I took an inventory of all the hardware in the room and
> recorded from each item's specifications the BTU if given or
> the wattage * 3.5 (a metric I found in several places on-line)
> otherwise. When I totaled everything, I got 75,103 BTU, or
> 6.3 tons. I find it extremely hard to believe that less than
> two full racks of hardware can generate that much heat, but
> perhaps I'm hearkening back in the days when most servers only
> had one CPU :-).
>
> I guess the question I need sanity-checked by the experts here
> is, can two racks of hardware reasonably generate that much
> heat? If so, and we want to leave open the possibility of
> adding a third rack of hardware to the room, it seems to me
> that we're going to have to get a 10-ton unit. Are there even
> 10-ton coolers that'll fit in a small machine room?
>
> Thanks for any help and advice you can provide.
>
The nameplates can be deceiving as they generally quote worst case. For
example, each 1 U server may vary in how much memory is installed, how
many discs. To determine your actual load, you need an ammeter; an
electrician can help with that. Or get a clamp-on ammeter from Radio
Shack, connect it to one 1U server, and then multiply.
As for your steep sudden rise, it may have little to do with adding a
few servers; instead it may be due to temp or humidity outside building
coincidentally going up, perhaps overtaxing building's AC.
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