Hot Aisles, Cold Aisles

Hot Aisles, Cold Aisles

The creation of "server farms" comprising hundreds of individual file servers has

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become quite commonplace in the new e-commerce economy, while other businesses spawn farms by moving equipment previously in closets or under desktops into a centralized data center environment. However, many of these farms are hastily planned and implemented as the needed equipment must be quickly installed on a rush schedule. The typical result is a somewhat haphazard layout on the raised floor that can have disastrous consequences due to environmental temperature disparities. Unfortunately, this lack of floor-layout planning is not apparent until after serious reliability problems have already occurred.

Dealing with environmental temperature problems in a data center having no downtime windows can be quite challenging. In large areas, the only remedy may be a new physical layout of the equipment racks. This changeover may take several years to complete because the work can only be done when computer technology rolls over. During this period of delay, the potential for an increased rate of hardware failures is very high, and unplanned downtime due to heat problems should be expected.

One solution for avoiding downtime is to have a strategic plan for how the server farm will be implemented. A Best Practice is to use rows of equipment racks in an alternating arrangement of "cold aisles" and "hot aisles." This is best accomplished when the layout of the file-server farm area is first being planned, and it is exceedingly more difficult to accomplish when the computer room is already populated with operating hardware.

A cold aisle is defined as having perforated floor tiles that allow cooling air to come up from the plenum under the raised floor, and a hot aisle has no perforated tiles. In the cold aisle, the equipment racks are arranged face to face so the cooling air discharged up through the perforated floor tiles is sucked into the face of the computer hardware and exhausted out the back of the equipment rack onto the adjacent hot aisles.

Hot aisles are literally hot because the objective of the alternating cold and hot aisle design is to separate the source of cooling air from hot air discharge which returns to the computer-room cooling unit. Therefore, no perforated tiles should be placed in the hot aisles, as this would mix hot and cold air and thereby lower the temperature of the air returning to the cooling units, which reduces their useable capacity. Hot aisles should be hot! Cold aisles should be cold!

This system is not perfect because some equipment manufacturers bring cooling air out through the top and/or sides. And some equipment is designed to bring air in from the sides and exhaust out the top, or bring air in from the top and exhaust out the bottom.

For this system to work well, especially for loads exceeding 50 watts/ft 2 gross:

	The cold aisle must be two tiles wide (aisle must allow both tiles to be removable so two perforated tiles can installed and their airflow not be obstructed)
	The load dictates the number of perforated tiles to be installed in the cold aisle only
	Cable cutouts must be sealed to sustain static pressure and prevent cold air bypass
	Blanking plates must be installed within racks to prevent re-circulation of hot exhaust air within racks
	Depending upon static pressure, grates instead of perforated tiles will be required for loads above 3 to 4 kW per rack or frame
	Unobstructed space around free standing equipment must not allow bypass of air flow between the cold and hot aisle, rows of equipment must be laid out perpendicular to the face of cooling units to minimize the re-circulation of exhaust air flowing over the top of the racks
	Finally there must be at least three feet of non-obstructed clearance between the top of the rack and the top of the return air path back to the cooling units.

See the whole paper here: Hot Aisles, Cold Aisles

How do you know if you've got the right Air Flow? There is a means by which to get quick and accurate prediction of the air flow distribution in your raised floor data center. Go to: Raised Floor Data Center Air Flow Distribution for more information.

Topics: Computer room air conditioning, crac units, thermal reliability modeling, heat build-up. Design of new data centers and reconfiguring of existing data centers to achieve environments that are efficient, and reliable.

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