Effective computer room cooling - Infrastructure

Communications News, August, 2003 by Ian Seaton

Computer equipment cooling air, delivered from a raised-floor air source, can be optimized by applying some basic thermal management concepts. This checklist reviews key considerations in applying these principles:

[check] Manage the air movement under the floor tiles.

Sealing off all the escape points for bypass air--air that is escaping from under the floor--increases the static pressure under the raised floor, increasing the air flow (CFM) at the points of need. Most computer rooms allow 50%-80% bypass air, resulting in static pressure under the raised floor of approximately 0.01" of water, which will deliver around 200 cubic feet of air per minute through a 25% open-perforated floor tile, and inadequate cooling for a normally loaded cabinet. Blocking off enough of the bypass air to raise that static pressure to 0.09" will more than triple CFM delivered through a floor tile and quadruple the cooling delivered to a cabinet.

[check] Manage how the cooled air is actually delivered to equipment.

Do not locate computer room air conditioning units (CRAC) at right angles to each other. The results will be misinformed CRACs that will often deliver colder air to the cold spots and warmer air to the hot spots. A computer room with all the air and cable running in the same parallel direction is the best way to make air direction both below and above the floor predictable.

Avoid the normal inclination to locate the hottest equipment closest to the CRAC. This may result in raising the temperature of the chilled air delivered into the entire room.

[check] Apply the same management principles to cabinets/enclosures.

If the bypass air elimination produces adequate CFM through all the perforated floor tiles, the cabinet should either have no doors or, if required for security, doors with a minimum 60% open mesh for maximum airflow, with no top-mounted fan kits. Do not use top-mounted fan kits, which can he counter-productive by allowing return air and source air in the cold aisle to mix.

If cabling obstacles in the rear of the cabinet make pushing the hot air out difficult for small server fans (10-30 CFM), use rear door-mounted fans to push air directly into the hot aisle and keep the cabinet between the source air and the return air.

If the optimal CFM cannot be maintained, air must be pumped directly into the cabinet and distributed with a booster, such as centrifugal blowers that direct the chilled under-floor air through a nozzle up the front of the equipment or fans that direct the air into a "plenum" door, with vents or baffles that will direct the air into the cabinet to points of need.

A solid-panel front door, filler panels in unused spaces between equipment and some means of sealing between the equipment and the cabinet side panels also will increase static pressure. When cabinets are bayed together, two cabinets should share a side panel. Top-mounted fans may he deployed in this configuration because their output will not mix with the equipment input air.

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