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Keeping the Closet Cool

The Closet as a Mini-Data Center
If you count up the square footage of all your network closets, it may exceed the size of your data center. Looking at your network closets from this perspective, you can see how it would be important to pay attention to the cooling requirements of your closets. In other words, the same consideration that's taken for the data center must be applied, on a smaller scale, to the network closets.

Cooling IT technology can account for as much as 45% of the IT power bill: The lower the closet temperature, the higher the power bill. Rising electrical rates makes it even more important to deliver cooling efficiently.

How Hot Can It Get?
You have to determine which closets are crucial and which are non-critical to your network operation. The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Technical Committee 9.9 publishes recommendations that temperatures should be kept within 68 to 77 degrees F. The ASHRAE provides for an allowable temperature range of 59 to 90 degreees F for IT equipment. The maximum temperature is 104 degrees F.

When less critical equipment is used, 90 degrees F is an acceptable temperature. Ensure that you check the temperature within the equipment racks, as this temperature may be much higher than ambient room temperature.

Hot technology also means less reliable technology. According to the Uptime Institute, once the IT equipment operates at or above 88 degrees F, reliability drops by 50%--another reason to manage cooling properly.

Four Ways to Remove the Heat
The removal of heat generated by IT equipment can range from passive to active solutions:

* Conduction--This passive solution has the heat passing through walls and doors without any action taken to cool the closet. It is the cheapest solution but has limited heat removal capability.

* Passive Ventilation--This solution depends on passive cooling through vents in the room and door. The room or space on the other side of the vent is assumed to be cooler than the closet. This is also a low-cost solution that can remove more heat than a conduction solution.

* Fan Assist Ventilation--This active solution requires a fan to assist in the heat removal to a cooler space outside the closet. This is a little more expensive but may be all that is needed for a small amount of equipment or for non-critical equipment.

* Air Conditioning--This active solution can be achieved by using the building's existing air conditioning vents in the closet. This increases the cooling cost. The disadvantage is that the closet may remain warmer than the rest of the building space if there is considerable IT equipment in the closet. Also, it may be normal practice to allow the building space to operate at a higher temperature overnight to reduce the cooling costs, but this will make the closet warmer, possibly too warm for reliable operation.

The most expensive solution is to vent dedicated air conditioning into the closet. This ensures that the cooling system will not be affected by the change in temperature of the rest of the building.

"Cooling Method Guide Based on Power Load and Target Room Temperature"
Chart from, "Cooling Strategies for IT Wiring Closets and Small Rooms" by Schneider Electric

See reference at the end of this blog.

Closet Size vs. Cooling
The size of the closet has an influence on the cooling capability. The larger the closet, the greater the floor, ceiling, and wall space that can dissipate the heat. Also, the equipment density plays a role in cooling. The less equipment per square foot, the better the dissipation of heat.

Three more conclusions are important when it comes to the closet design:

* Temperature increases as the construction materials' thermal resistance increases for the wall, ceiling, and floor materials. Better insulation properties reduces the heat dissipation.

* Every degree increase in building air conditioning during evening and weekend also increases closet temperature by same amount.

* If one or more walls are subject to sun exposure and outdoor temperature on a hot sunny day, this will reduce heat dissipation and may even add heat to the closet.

How Much UPS for Cooling?
The important question is, "How long does the closet have to operate during power failure?" UPS systems can be sized for minutes to hours of operation. The longer the UPS power has to operate, the greater the cost and space required in the closet. The heat from the UPS is quite small, so it will not really affect the cooling required in the closet.

If the closet has to operate for about 10 minutes, then no UPS power will be required for cooling. If the operational time needed exceeds 10 minutes, then the UPS has to be sized to cover the cooling system as well. How long the UPS should support the closet is a business decision.

If the closet design depends on conduction or passive ventilation, then no extra UPS power is required. A small amount of UPS power is required for a fan-assisted cooling design. The amount of UPS power required increases considerably when air conditioning is required.

See my previous blog on this subject, "Cool Technology."

An excellent detailed white paper is available from Schneider Electric, "Cooling Strategies for IT Wiring Closets and Small Rooms."