Making Sure Cooler Racks Prevail

In the past, security system installations of any appreciable size included the installation of a fire-resistant sheet of plywood installed in a utility closet, and on that wall would be mounted power supplies, alarm cabinets and distribution devices. This common installation configuration could be found in both residential and commercial installations.

Today, the installation norm is quickly becoming an equipment rack configuration. Equipment rack enclosures offer a very compact way for the security dealer to meet the rapidly changing demands of today’s integrated systems. While these systems offer compactness and convenience, they also offer a new challenge of keeping high-performance equipment sufficiently cooled for long-term reliability and performance. 

Recently, I have heard technicians commenting on a variety of home-brewed theories on how to keep rack-mounted equipment cool. I thought it was a perfect opportunity to check with the experts and learn some new advice on keeping your rack-mounted equipment the appropriate temperature.

Controlling the Flow
First off, you need to know how to control the flow; airflow that is. Equipment such as rack-mounted RAIDs (redundant array of independent disks) can have air-blowing fan banks of up to six or eight fans pushing a large volume of hot air. There is a special reason for such air force power: to move the hot air away from the equipment. So, the main design question is: What is the best method to move hot air quickly away from the equipment? 

Care has to be taken as to how the hot equipment exhaust is directed out of an enclosed rack; the emphasis here being on the word enclosed. Would you have a chimney in your house with holes in it? I have noticed several low-end designs in which the racks are open all over and everyone seems to pray that the heat will find its way out and up somewhere.

I have even witnessed older board-mounted equipment in a closed utility room, with standard air conditioning vents, get so hot that the equipment started shutting down due to heat overload. Make sure to do an overall BTU (British thermal units) heat calculation of your overall equipment room and that equipment is either cooled and/or air removed.

According to Bob Schluter, president and chief engineer for Middle Atlantic Products Inc. of Riverdale, N.J., “Most studies have shown that for every 10° F rise over 85° F, digital equipment life is reduced by approximately 40 percent! The Uptime Institute states, ‘For every 18° F increase above 70° F, long-term reliability is reduced by 50 percent.’ Clearly, there is a need to control the temperature inside electronic enclosures.”

Equipment Room Design Tips
As most of us know, equipment racks are vertically divided into increments of rack units (RU) or numbered Us such as 1U, 2U, etc. (“Tech Talk” tidbit: 1 rack unit is 44.45mm, or 1 3/4 inches, high.) It is a common belief among installers that at least 1U spacing between rack-mounted equipment is good for cooling and airflow.

The big error is that the open spaces on the exterior of a rack can allow hot exhaust air to be sucked back in and cause a recirculation of air. Since the overall air is not vented properly, hot spots can build up in the interior of the rack enclosure. Exterior spaces, such as in the front of a rack, should be covered with airflow management blanking panels. (TIP: Save assembly time by using snap-on, rather than screw-on, covers. An ample supply of 1U snap-on covers will accommodate any size openings between equipment.)

Again, it would seem good common sense to place a vent opening at the top of the back panel on an equipment rack. Since heat rises, it would escape from the high open vent or perhaps even from vent openings in the top of the rack enclosure. However, this practice can be mixed with the use of parallel exhaust fan arrays on the top of equipment racks. In this case, cold air will be sucked out by the top fans from the top vents, causing most of the warm air to recirculate in the cabinet.

It would be better for top exhaust fan combinations to forgo the top vent opening and have intake vents at the bottom of the cabinet. A rule of thumb is to keep inlet and exhaust air far apart. Avoid vented rack panels in the upper six spaces of the rack. Another rule of thumb for sealed equipment racks with top exhaust fan arrays is that the combined cubic feet per minute (CFM) rating of the top fan must equal the total CFM of the internal equipment fans in the rack. Make sure not to use rack rails that are inset from the side of the enclosure. The open side area allows for hot exhaust air to return to equipment air intakes and causes hot spots. 

Depending on the utility room exhaust duct locations, it may be better to have exhaust venting that goes directly from the rack enclosure top to a dedicated ceiling or wall exhaust portal. Incoming AC vents should also be placed away from the rack so that cool air can settle and be drawn in from the bottom of the rack. Since AC is expensive, the manager of an equipment room located in the cooler northern states may want to look at taking advantage of outside air for cooling assistance. Another suggestion is to not have racks directly under supply ductwork. Cold air falls, and this can impede the flow of the hot air that rises from the top of the rack and affect if getting back to the return air duct. 

Data center cooling is typically very inefficient, with some estimating as much as 78 percent of all cooled air vents exiting the equipment room without doing anything to reduce the heat circulating around the equipment. A suggestion is if you have an open floor that is forcing cool air into the racks, then seal up cabling holes with fire-stop pillows to better control room cool airflow so it is not lost out of the bottom hole.

Where to Find Out More
For those who are interested in more research material on equipment rack and room cooling design, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has a new publication called “Datacom Equipment Power Trends and Cooling Applications.” Additionally, equipment manufacturers such as Middle Atlantic Products and American Power Conversion (APC) have some very good educational documentation on equipment cooling planning and design.

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