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  #1  
Old 04-01-2007, 10:49 PM
Zitibake Zitibake is offline
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Default semi-enclosed cold aisle?

I assist a facility that doesn't use a warm-air return plenum, doesn't have a particularly high ceiling, and the CRACs intake warm return air at head-height. There are some cold aisles that end relatively near the downflow CRACs. It appears that the CRACs could be sucking some cool air out of the ends of the cold aisles. I was wondering if anyone installs doors or other air barriers at the ends of cold aisles, to help keep CRACs from sucking cold air?

Similarly, I have seen instances where bladeservers placed near the top of a cabinet suck warm air down into the cold aisle. Totally contained hot aisles can keep the warm air from recirculation, but require in-row cooling, or ducting the warm air to the CRAC intakes. In a datacenter with legacy downflow CRACs and no warm plenum, I am wondering if it would make sense to repurpose some "warm aisle containment" hardware to cover-over part of a cold aisle, where bladeververs are mounted high, to help keep the bladeservers from sucking overhead air.
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Old 04-02-2007, 02:48 AM
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KenB KenB is offline
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How big is this place? A CFD analysis is your most accurate tool, but if you want to experiment, here are some suggestions:
  • Re-mount blade chassis low in racks
  • Chatsworth (and probably others) make rack chimneys to direct exhaust air higher overhead
  • If there is a suspended ceiling, install grilles and use the area above it for return air (also re-duct CRAC intake)
  • Measure air delivered via cold aisles. If <150 cfm/kW, add supplemental cold air where needed
  • Install vinyl strip curtains at ends of hot aisles to prevent air recirculation
Ken

Last edited by KenB; 04-02-2007 at 01:34 PM.
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Old 04-03-2007, 01:22 AM
Zitibake Zitibake is offline
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Quote:
Originally Posted by KenB View Post
How big is this place?
Thousands of square feet is in the low teens.

Quote:
Install vinyl strip curtains at ends of hot aisles to prevent air recirculation
I hadn't seen those. Thanks for the tip.

I think moving to a warm plenum is the right answer in this facility, but it will take some work to get there. During the transition from above-cabinet air return to warm plenum, cooling may be sub-optimal.
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Old 04-03-2007, 04:02 AM
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Ken,
Very interesting recommendation on the Vinyl curtains. I assume you are talking about the ones that are seen on larger walk in refrigerators/freezers to contain the cold air inside while the main door is open and goods are being transported?

Zitibake,
You are likely seeing a major increase of rebreathing in your racks due to having the blade servers towards the top. I am assuming you have them up top to deliver the hot air to the top of the cabinet in order to be sucked in to the crac unit faster. It is important to remember that since you are blowing air out faster at that level, you are potentially introducing warm air in an area where the crac unit will likely not be able to take all of the warm air as quickly as you are producing it. This problem gets compounded when the blade servers suck in the warmest air from the rebreathed air and exhausts even warmer air. It is kind of a chain reaction.

It is also important to remember the servers under the blade chassis. If you are creating a heavy curtain of air at the top of the cabinet (the curtain created by hot exhaust of the blades at a high velocity) they will not be able to exhaust straight up as efficiently. This will most likely result in the hot air going horizontal and likely rebreathing through the side of the cabinets or gaps in between the servers.

I strongly recommend that you reconsider the placement of the blade chassis. Give them the coldest air (at the bottom) and don't suffocate the servers underneath the blade chassis. Also be sure to use blank filler panels to cover and gaps within the cabinet. Also consider using side panels on the cabinets to help eliminate rebreathing. If you have rear doors and excessive cabling masses on the back of the cabinets, try removing them to see if you are promoting proper air flow.

Cooling can be a real pain in the you know where. To be quite honest, I love hunting down hot spots and fixing the issues. It can be so easy to mitigate problems and when they aren't you can go on a mini scavenger hunt! I always carry my trusty infrared thermometer and digital psychrometer.

Hope all this rambling has helped.
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Old 04-03-2007, 03:08 PM
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Quote:
Very interesting recommendation on the Vinyl curtains. I assume you are talking about the ones that are seen on larger walk in refrigerators/freezers to contain the cold air inside while the main door is open and goods are being transported?
Yep. I was talking with some folks recently that use these to cap the ends of their hot aisles and provide a barrier above each row (using above ceiling plenum for return air, I assume), to prevent hot air rebreathing. They said they could cool 9kW racks with only subfloor air .

Ken
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Old 06-14-2007, 02:17 AM
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Those fans seem pretty cool! I do have 1 question though. The picture you attached shows what appears to be the cold row. If this is truly the cold row, why are you using the fans in the plenum space of the cold row? If this is the hot row, whats up with the floor tiles? Seems a little counter productive in either scenario...

Also, where did you get those fans?
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Old 11-28-2007, 02:40 AM
Zitibake Zitibake is offline
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Quote:
Originally Posted by hmsee1 View Post
The photo is indeed a cold aisle. The overhead units are not fans; but, ducted cold air from a 40 Ton top exhaust Liebert CRAC. Each 40 Ton CRAC supplies four cold rows.
Are you using both a hot plenum, and also ducted cold air through the plenum?

40 tons per 4 rows... does that handle about 30kw of heat per row? (each row has 5 cabinets = 6kw per cabinet cooled from the top?)

With ducting, how would you deal with having a CRAC unit down?
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  #8  
Old 11-29-2007, 07:57 PM
cernst cernst is offline
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I've also been talking to folks about curtains above the racks to mitigate hot air from going over the top of the racks into the cold aisle. Some say it is a good idea...some think it is a bandaid...but then again, everyone has their own ideas and think all other ideas are bandaids!
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  #9  
Old 12-02-2007, 09:04 PM
Zitibake Zitibake is offline
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Curtains on the ends of aisles, and above cabinets should help... but then the fire marshall may start asking how your fire supression is going to flow. If you had sprinkler heads in every cold and hot aisle, then I imagine it shouldn't be a problem.

Last edited by Zitibake; 12-02-2007 at 09:15 PM.
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