cooling load for server room 2

[scha0786]

Quick question,

I was asked to quickly size a cooling system (for budget purposes) for a server room. Can I take the amp draw at the UPS supply the server room. Convert to watts. Convert to BTU's and divide by 12,000 to get my tonnage.

thanks,

P.S. - this equipment is in and running right now and I will put some safety factor on my unit selection.

 

[KiwiMace]

Yes, but:

Use about 90% x full capacity of the UPS, with some engineering judgement as to whether it is appropriately sized for the room. You don't want to be putting another one in later on.

Size the cooling system using the sensible heat capacity of the equipment.

Price based on high reliability equipment, like Liebert, Data-aire etc. A DX fan coil unit from Carrier will not offer as good a life with a 24h/day operation.

Make sure you understand what the plans are for the room. Typically, IT people want old systems to be available forever just in case, so the new systems just add on and on.

 

[DRWeig]

Along with Kiwi's fine answer, I assume that you are also including lights, people, any ventilation air (that one's important), and any building envelope gains if the data center has a roof, exterior walls, and fenestration.

But in my experience (energy modeling, not design), heat gains from the stuff connected to the UPS, the UPS heat loss, and ventilation air have dominated. The others take on more significance if the room is large.

Best to you,

Goober Dave

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[scha0786]

Ok sounds good. Yes I will use Trane Trace 700 to model the room and input the misc heat load from the equipment. No vent air, no people etc just server equip.

This server room is located on the north slope of Alaska. So I was thinking of going with the Liebert DS series with a drycooler outside. I would opt to get the GLYCOOL option they offer so in the winter time they can utilize free cooling. Of course the glycol loop will have 60% E.G. mixture so its good down to -70F (design is -50F but I have seen it up here -55F).

what do you think of my idea?

 

[KiwiMace]

Sounds fine to me. Liebert DS is a good machine. When most people say 'server room' I think of an IT closet with 4-5 RT, but it must be a decent sized room/load if you are putting in one of these.

Consider the airflow, well.

 

[scha0786]

yeah it is a pretty big room.

Currently they have a liebert DH192 in it (10 ton). they have a false floor with floor supply set up. Unfortunately they only have one unit, this past summer the unit sprung a leak on the glycol side and they had no way to shut it down for repairs or the whole camp lost internet! So we told them to order one of those portable cooling units on wheels to get them by for the repairs. Now that the unit was fixed and back up and running they want a 100% redundant system in case that unit ever goes down again.

Fun stuff, I am really interested in the Glycool technology that is out there. It is a perfect application for them to utilize free cooling. thanks for the help everyone.

 

[ScottyUK]

Don't forget to provide cooling for the UPS itself. I know it is obvious, but the number of installations where the server room is cooled and the UPS is left to roast itself in an annexe suggests it isn't obvious to everyone.

Batteries like roughly 20°C ambient - too cold and they can't deliver their stored energy, too hot and they have a short life. The battery installation is a big fraction of the overall UPS cost, so maximising its useful life makes economic sense.

Consider what happens during a power outage, when the server room continues running on battery reserve from the UPS. The cooling plant needs to be supplied from the utility - generator transfer switch so it keeps running during a utility power outage, and not from the utility service to the building. Obvious? Yes, I know. Frequently overlooked? Oh yes...

 

[tys90]

Great suggestions here. I would also suggest getting a humidifier on the Liebert unit (it may come standard with it?). It's not as much of an issue with the water/glycol system since you can control your supply air temperature better than a refrigerant system but for a small additional price it's worth having. Especially if your ventilation air isn't humidified.

 

[scha0786]

ok so now I am kinda confused. I asked the IT people what there total amp draw is at the UPS. They told me 90 amps. So do take 120 * 90 = 10800 watts divide by 1000 = 10.8 kW * 3,412 = 36849.6 btu/h divide by 12,000 = 3.07 tons?

I am confused if I use 480, 208 or 120 to get the watts?

The UPS is a METEK and on the front nameplate it has the following (Main AC input = 480, 59.1 amp max @ 60 Hz and 3 phase) (DC Input 120 VDC @ 229.9 amps) (AC output 120/208 60 Hz 83.4 amps 3 phase 30 KVA)

Did I convert to get the correct watts?

thanks for the help everyone

 

[MintJulep]

If I had to guess - and I do - I would guess that UPS feeds a 90 amp input breaker on the power distribution panel for the servers.

 

[RossABQ]

I believe your UPS peak heat load would be 480 * 59.1 * 1.77 (3-ph conversion) times 5% to 7% losses. You have already accounted for the power that goes to the servers, right? So you only need to account for the losses in the UPS.

 

[scha0786]

mintjulep, Thats what I would say but I will have to get my electrical engineer involved i guess, lol. He'll be able to tell me what voltage is going on.

 

[KiwiMace]

Most rack servers are set up to run line-line 208VAC as it is more efficient. Two breakers per feed on the power distribution unit.

Use the UPS output data: 30 kVA x 0.9 pf = 27 kW peak power output of the UPS.

In reality you do not want to exceed 90% of this because the UPS needs some headroom to charge the batteries on a long draw. So you should reasonably target 24 kW IT cooling and whatever for the room load - closer to 7 RT sensible. The Liebert DH192 you already have is good for around 48-50kW - a little oversized but perhaps they expected more growth, or you have (real) windows in your server room. The actual capacity should follow a nice long user requirements talk with the whole chain of command on the IT and management side.

 

[KiwiMace]

Whoops, I assumed that the UPS was not installed in the server room. The very best UPS units add about 8% of power output as heat to the room. I would go at least 15% for a small 30kVA. If you look at the input power and output power there is a 60% overhead - I wouldn't be too bothered about that. Most manufacturers publish the heat loss.

 

[DRWeig]

Scha0786, I suggest doing some measuring of current.

As Mint implied, asking IT people for equipment loads is pretty futile. They'll either give you breaker size, UPS size, or some other number that is not directly related to actual cooling load.

Best to you,

Goober Dave

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[sspeare]

When you have calculated all your loads, you'll have to select the equipment with adequate SENSIBLE capacity. Typically in a server room, almost all the cooling load will be sensible (no people = no latent load). So, when you select your cooling equipment, make sure you select something that can handle the Sensible Load. For example, let's say you calculate 10 Tons of SENSIBLE LOAD. Assume, for all practical purposes, you have 0 Tons latent load (no people = no latent). Your load is 100% sensible. Don't just select a machine with a total capacity of 10 tons/120,000 BTUH, because the 120,000 BTUH capacity probably has a SENSIBLE/latent split of approx 75% SENSIBLE/25% latent.

 

[scha0786]

I agree with KiwiMac,

I just looked at the picture I have from the UPS nameplate and AC output = (AC output 120/208 30 KVA 60 Hz 83.4 amps 3 phase 24 KW at 0.8 PF)

See attached pic

So what I am going to do is use the 24 KW as my server load, I looked up the data sheet for the UPS and the heat loss for that UPS is 5.663KW. I will take all of that and input it into my Trane Trace 700 program as misc load and model the rest of the room.

Then I will make sure my AC unit can handle the sensible load.

Make sense?

thanks for the help

 

[DRWeig]

I think that will be fine, a bit on the high side but OK. The IT folks might actually load it to the max before long.

To cover yourself, though, pay attention to KiwiMace's first reply above about engineering judgement. If the server room has a whole bunch of empty space, it won't stay empty for long. Part of it will be filled by a second UPS if needed, and more racks of equipment. It's worth asking the IT folks about expansion plans within the room, and whether they want to cover future loads with this system. I would think they would wait and budget for more A/C at the same time they procure the equipment, but I would rather have them say it than just assume.

Best to you,

Goober Dave

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[scha0786]

Ok,

so I did my Trane Trace 700 load calc and it spits out 8.9 tons of cooling required. Going with a 10ton Liebert DS

THANK YOU FOR ALL THE HELP!!! now I just have to sell this to the customer, lol