Server Room AC Sizing Questions

[autospy1]

Looking for advice on server room cooling sizing. I'm hoping to get some unbiased opinions from other pros who aren't trying to sell me something.

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Part A:

We are moving 129 servers (4 racks), each with a nameplate rating of 350W, into a small room. Based on our measurements, each server is pulling 200W at peak load -- or a total of 26kW for all systems. There are no windows, and no one will be permanently stationed in the room (only in there to service the equipment). I would like to have some room for expansion. According to my calculations, I would require around 8 tons of cooling. However, I'm getting wildly differing opinions from the pros about the size:-

1. Previous owner of equipment is specifying that he was using 4 tons.
2. HVAC Engineer is specifying - a 5 or 6 ton "Liebert" Unit
3. Data Aire Rep is specifying - a 8 ton air-cooled ceiling mount unit
4. Liebert Rep is specifying - 10 tons (possibly 2 - 5 ton units)

Any contractors/engineers out there agree with any of these numbers?

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Part B:

The room is only 124 sq ft. It is a bit small, so I'm worried that the system might short cycle. There will be a LOT of air moving around in this room. I had a few contractors in yesterday, and one of them jokingly said I might have problems opening the door.

Any advice about this? I might be able to convince management to make the room bigger (knock down a few walls around it), but I need some compelling opinions. However, I'd rather not make the room any bigger.

Thanks!

 

[Zesti]

If the server-equipment is using 26kW then that is the minimum cooling-power that is required from the cooling-unit. (I think the power that goes outside the room via the wires is quite neglible)

Why you Imperial folk like to use "tons" (whatever that is) when you are talking about cooling-power is beyond me...
Just convert "tons" to "kW" and you will know what unit you need.

However:
30kW , dT=10K --> 9000 m3/h
room 11 m2 , height 2,5m = 28 m3

That is over 300 airchanges per hour...

 

[walkes]

Part A: 8-tons seems to be about right based on you measuring 26 kW.

Part B: The precision cooling units should be able to control the space temperature without short cycling. If you need future capacity you may be tight for space to install a second unit. (The Liebert rep may have suggested 2 five ton units for some redundancy and some future capacity)

 

[imok2]

If you use 226 watts per sq.ft. x 124 sq.ft you get:
Total Watts 28,024
Total BTUs 95,562
Total Sensible Tons of Air 7.96
Total BTUs Per Square Foot 770.66
Total Square Feet Per Ton of Air 15.57

Does the calculations include lights, shell load, UPS loss and HVAC Equipment itself. However, it is quite satisfactory for electrical and mechanical program requirements. If it was me I would go for 2-5ton units because of mayainance and repair issues from time to time

 

[autospy1]

Thanks for the information. Pardon my basic questions...I'm an EE by training.

The Liebert rep is telling me I need a total of 10 Tons...because I am only considering sensible heat...not latent heat. Does this make sense to anyone?

I didn't consider the room/shell load..it seems to be about .25 ton. If we expand the room, the total size will be 240 sq ft. Will it change the size of my unit significantly? I know it will help with airflow. Is 300 ACH an issue?

 

[imok2]

That's correct, there will be very little if any latent heat to contend with but I always like some redundency for data centers

 

[imok2]

I believe that 5 tons will handle the load most of the time so with 2 -5 ton units you will have a redundant unit and also have more cooling for expansion and service with out a shut down. Look for units with variable speed compressors and supply fan drives so if you want to you can run with both units or one unit and still keep the air changes at a reasonable rate for the smaller room if thats what you decide

 

[imok2]

In an attempt to clear up the debate on how much energy the Internet uses, the Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (LBL) has released a report on data centers, also called server farms. The report concludes that data centers signif-icantly exaggerate their power requirements, in part due to optimistic growth projections and a need to make their operations look more substantial than competitors’. Power estimates also assume that computer equipment will use its rated power at all times, which is not true. Estimates also tend to be based on total floor space rather than computer-room floor space.
Data centers use, at most, 50 W/sq ft, says the report, including energy needed for cooling.

 

[RossABQ]

Data centers use, at most, 50 W/sq ft, says the report, including energy needed for cooling.

I just love generalizations like that; what exactly is a "data center"? Blade servers? Is this the load on the server floor itself, or averaged over all DC spaces? I don't disagree that IT folks want as much cooling as they can get, whether it's justified or not.

This case is more of a closet, not a server farm. Within data centers, there are at least 4 different types, based on the data being processed which affects the load characteristics.

For this load and this size room, air management could well be an issue. Note that 4 standard cabinets and a floor-mounted CRAC unit will take up nearly half of the floor space. Is this on a raised floor with underfloor air distribution, or hard floor with overhead air distribution?