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Data Centre Cooling

For decades most Data Centre's utilised raised floors to inject cool air from underneath up to the racks in a back to back arrangement, this is called the Hot aisle Cold aisle method, but for some years now this method is considered outdated and rather inefficient for Data Centre cooling.

inefficient hot aisle - cold aisle approach


This method, as seen at left, involves hot air from hardware released from the rear door of a rack into the general Data Centre airspace, to assist with some form of hot air containment, most rows of racks will be so two rows are back to back, but, this still allows for hot air mixing with cold air as they are not truly contained for exhaust.

Many modern Data Centre builders have got it right in what makes far more sense using the all Cold aisle method, which involves an overhead plenum for the hot air to be expelled into, this means only cold air in your DC, and no mixing of cold and hot air, since the idea is to keep everything cool so your valuable hardware stays at a safe operating temperate.

The cold only method works by, as mentioned above, using an overhead return plenum to capture the hot air, and in most buildings this is already in place by means of false ceilings which can easily be sealed and used for this purpose. The racks will often have a low current fan that pulls the hot air out into the plenum by way of a type of boot, think of it as like a house chimney expelling smoke into the outside air, rather than filling up your lounge room. The cold air can be pumped in from overhead or the side so your racks can sit on that nice firm slab of concrete, and in this day and age with the world experiencing a horrific number of earthquakes, it also just might save your bacon if you are in a quake zone.

efficient all cold aisles approach


The method works best by using a grated front cage door on the racks to allow the cold air in, with the two sides and rear door fully sealed so the rear hot air rises to be expelled into the plenum. It's also more efficient if all blank RU's have blank panels attached to trap that hot air in the rear, back in 2007, I even saw pictures of some rack RU's covered and taped over with plastic, some racks even with cardboard... well... so long as it does the job I suppose :-)

This approach has the benefit of keeping your entire Data Centre cooler, and given the CRAC units have an intake inside the plenum, they have less work to do, anything that needs to cool 27/28c into 22c must be better than something that needs to cool mid-high 30's and in Brisbane, that's more summer days than not, even days around 40, which, although not having the same humidity level, even Sydney can see a good number of days up that high. This makes your DC cooler, Greener since you'll have less of a carbon footprint, and more cost efficient.

It is also important I think to have a fresh air intake to your CRAC's for OH&S, some DC's in the U.S I've heard inject outside air for five minutes every 30 minutes, sure, that kind of contradicts the above advantage injecting some hot air, but overall it's cooler temps to be cooled and the idea of this is after all to keep the DC cool, the other is just beneficial side affects.

Sidenote
The entire idea of this is hot air containment, so if you only have a small room, with a few racks in it and use in-row CRAC units, you can take a similar approach, using sealed rack sides, sealed spare RU's and front grated doors, but with rear grated doors to expel the hot air into the small area behind, sealing above and the sides of your row of racks (even cheap perspex will do the job) forcing all hot air into that small isolated area at the rear of the racks where your in-row CRAC unit has its inlet, or, by still using duct cut outs into the plenum to allow the hot air to escape for a building CRAC, so long as that rear area is isolating the hot air from the server rooms cool air, the net effect is pretty much the same, and its much cheaper idea if you only have a few racks using in-row air cooling.


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