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BTU and AC Ton Sizing

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Greg Stark_1
Frequent Advisor

BTU and AC Ton Sizing

Does anyone have any experience building data rooms? We are about to move our office and the new data room has a 4-ton Liebart AC. Is there a way to know how many BTU's this type/size of AC can handle?

I have roughly estimated the total BTU's for the equipment will be about 30,000 BTU/hr.

Thanks again,
Greg
11 REPLIES
A. Clay Stephenson
Acclaimed Contributor
Solution

Re: BTU and AC Ton Sizing

Easy,

1 Ton = 12000 BTU/hr. so your unit will handle 48000BTU/hr.
If it ain't broke, I can fix that.
A. Clay Stephenson
Acclaimed Contributor

Re: BTU and AC Ton Sizing

One other point. It would scare me to death to move into a data center with only 1 HVAC unit. If it dies, your boxes die. Nothing will damage equipment like overheating. Even running for a few minutes at hi-temperature can drastically reduce the life of equipment. I would much rather see you have 2 3-ton units than your single 4-ton unit.

At the very least, you need to monitor temperature and shutdown all power if a set point is exceeded.

I actually always have fully redundant units and monitor temperature as well --- that way I don't need the extra stuff. It's one of God's little jokes.

If it ain't broke, I can fix that.
Greg Stark_1
Frequent Advisor

Re: BTU and AC Ton Sizing

Thanks Clay!

I agree, this not the most redundant of cooling strategies. Most of the equipment in this room is old/test equipment. For those servers that are productive/expensive we use our Symmetra to shut them down if the temp in the room gets to high.

As for getting another Liebert, you know what they say about lead balloons!
Chris Vail
Honored Contributor

Re: BTU and AC Ton Sizing

Actually, a single 4 ton unit should handle a bunch of servers and such. As Clay mentioned, a ton of A/C equals about 12000 BTUS (so how does that weigh a ton?). But instead of getting a second A/C unit, just have plans in place in case of failure.

You can do this by getting a laser thermometer. These are now cheap--less than $100. Start by measuring & recording the input and output temperatures of all of your gear. The difference between these two numbers is the temperature rise for that equipment. This should be in the 5-15 degrees range, but can go into the 20's and even 30's. Look for the hardware with the highest temperature gain. If this is, say, 18 degrees, subtract that number from 100, and this will be the temperature at which you must start shedding load should the A/C fail. The reason for this is that silicon gets very, very wierd at temperatures above 100 degrees (some as low as 90!) so be conservative. Without that data, figure that you start shutting down hardware when the temperature of the air going into a computer gets to 85 degrees F.
Have some strong, STRONG fans nearby, and know where you can find ice--LOTS of ice. When/if the A/C fails, set up your fans at the entrance(s) of the data center, and blow as much air across the ice as possible. The idea is to melt the ice--so be ready to catch the drips.
I did this a few years ago when the A/C in the data center went out for 6 hours--at no point did any temperature rise above 82 degrees, and we never shut anything down. But it did get REAL interesting that day....



Chris
Pete Randall
Outstanding Contributor

Re: BTU and AC Ton Sizing

Greg,

If adopting the fallback fan strategy, make *very* sure you know which way the air moves through the equipment when you place the fans. I've heard horror stories of people frying boxes by putting the fan in the seemingly logical postition of behind the box. If the air flow goes in the front and out the back, your fan is now preventing the unit from cooling itself because it can no longer exhaust the heat!!


Pete

Pete
A. Clay Stephenson
Acclaimed Contributor

Re: BTU and AC Ton Sizing

I must say that relying upon fans and bags of ice sounds a lot like "M I C K E Y M O U S E" especially if you consider the origin the thermal unit "Ton". It's not "about" 12000 BTU / hr; it's exactly 12,000 BTU / hr and it comes from the amount of heat absorbed by freezing 1 Ton of water (or melting -- it's the same amount of energy) at constant temperature. The "heat of fusion" for water is 144 BTU / lb. The energy required to change cause the change of state for 2,000 lbs is thus 144 BTU / lb X 2000 lb or 288,000 BTU's. This is assumed to occur over a day so 1 Ton = 288000 BTU / 24 hr = 12,000 BTU / hr.

To put this in perspective, if we assume that your your estimated thermal load of 30,000 BTU / hr. is correct, we would need to melt 30000 BTU / hr / 144 BTU / lb or ~ 208 lbs. of ice / hr to maintain constant temperature! That would result in about 25 gallons of meltwater per hr. Even if you let the ambient temperature rise to 100F or so, it really hardly affects the numbers. The real cooling comes from bringing in outside air and exhausting the heated air. I don't even want to think about the condensation problems that might arise when you finally do begin to cool this 100% humidity air.

The problem with all these stopgap measures is detecting them and having someone constantly on-site to deal with them. There is really no substitute for either redundant cooling or automatic power shutoff. If your suits balk at this, ask how valuable is the data? How valuable is uptime?

Note that the 2nd unit need not be a data-center class unit. That will lower the cost of the unit by about a factor of 3-4 and you can set it's thermostate to perhaps 5F above "normal" so that this unit never runs -- except when you need it most. It also then allows you to do maintenance on your primary unit.

If it ain't broke, I can fix that.
Chris Vail
Honored Contributor

Re: BTU and AC Ton Sizing

Yo Clay....
I never, claimed my "fans and bags of ice" solution WASN'T a mickey mouse solution. In fact, I thought it was pretty shabby.
However, the alternative was to shut down all functions of a Fortune 500 company and send 2000+ employees home for the rest of the day. It was a last-ditch effort to keep everything functioning. And we did melt a lot of ice--I bought a bunch of powerful fans to move the air in and out. I spent a good part of the day with a wet/dry vacuum cleaning up the drips. The humidity wasn't really an issue: when it finally came back online, the Liebert did a fine job of dehumidifying the air. Fortunately, the room had two doors, so I had 4 fans blowing in, 4 fans blowing out, and 6 blowing air around in the room. All this because some large bird (maybe a buzzard) had built its nest in the outside heat exchanger!
Further, when the suits (in the construction business: its jeans & cowboy boots) found out how much a 2nd Liebert cost, they decided that my spur-of-the-moment plan would remain the fall back! The Data Center really wasn't big enough for 2 air handlers, and building a new Data Center was out of the question.
The last I checked, those fans, trays and wet/dry vacuum were still securely stored against just such an emergency. As long as there IS a plan, then WTH? There is an easy way, a cheap way, but no easy/cheap way to do stuff. As long as senior management is aware of the risks, then its THEIR problem, not mine or any sysadmins.

Sysadmins administer SYSTEMS. And sometimes those systems include buzzard nests.

Chris
mprensky
Occasional Contributor

Re: BTU and AC Ton Sizing

Where did you get the information on the BTU's/hr. I have some HP UNIX servers and can not seem to find any specs on them.
Angus Crome
Honored Contributor

Re: BTU and AC Ton Sizing

Chris, I too have taken this approach in the past. After it was all over, I went to upper management and asked for a second air handler (the room was big enough), but they wouldn't go for the expense. At least, not until I asked them all to sign a piece of paper stating that they were aware of the risks. You cannot imagine how fast that system went in, once they were aware of "their" liability. The moral, "get it in writing", otherwise, it is still on you, and you "will" find yourself on the street, if another failure occurs, and your backup strategy doesn't work as well.
There are 10 types of people in the world, those who understand binary and those who don't - Author Unknown
Greg Stark_1
Frequent Advisor

Re: BTU and AC Ton Sizing

For BTU, I found a formula of 1 Watt = 3.413 BTU. I got the number of Watts for each device from the manufacturer or from the UPS sizing tool at www.apcc.com.

Greg
Daryl_21
Occasional Visitor

Re: BTU and AC Ton Sizing

.. as I've mentioned before just sizing based on equipment load will get you in trouble. Especially if you equate the equipment BTU load directly to unit sizing.

Equipment does both latent and sensible work. If you size your load based on your sensible, you will be short as a 36,000 btu machine may not even have 30,000 btu of sensible. (sensible is a change in temperature)

Additionally, you aren't factoring in room sizing, insulation, exposure, or any people loading.