Calculating Total Cooling Requirements for Data Centers

White Paper 25     Summary Revision 3     By Neil Rasmussen

This paper describes how to estimate heat output from Information Technology equipment and other devices in a data center such as UPS, for purposes of sizing air conditioning systems. A number of common conversion factors and design guideline values are also included.

Heat is energy and is commonly expressed in Joules, BTU, Tons, or Calories.  Common measures of heat output rate for equipment are BTU per hour, Tons per day, and Joules per second. There is no compelling reason why all of these different measures are used to express the same thing.  The mixed use of these measures causes a great deal of confusion. Fortunately, there is a worldwide trend to move all power and cooling capacity measurements to a common standard, the Watt.  For this reason, we will discuss cooling and power capacities in Watts.

The power consumed from the AC power mains is essentially all converted to heat. This fact allows the thermal output of IT equipment in Watts to simply equal its power consumption in Watts. BTU per hour, as is sometimes provided in datasheets, is not necessary in determining the thermal output of equipment. The thermal output is simply the same as the power input except in the case of Voice over IP (VoIP) Routers.

The total heat output of a system is the sum of the heat outputs of the components. The complete system includes the IT equipment, plus other items such as UPS, power distribution, air conditioning units, lighting, and people. Fortunately, the heat output rates of these items can be determined via simple and standardized rules.

A detailed thermal analysis using thermal output data for every item in the data center is possible, but a quick estimate using simple rules gives results that are within the typical margin of error of the more complicated analysis. The quick estimate also has the advantage that it can be performed by anyone without specialized knowledge. A worksheet that allows the rapid calculation of the heat load is provided in the full white paper.

The prior analysis ignores sources of environmental heat such as sunlight through windows and heat conducted in from outside walls. Many small data centers and network rooms do not have walls or windows to the outside, so there is no error resulting from this assumption. However, for large data centers with walls or a roof exposed to the outdoors, additional heat enters the data center which must be removed by the air conditioning system.

In addition to removing heat, an air conditioner system for a data center is designed to control humidity. Ideally, when the desired humidity is attained, the system would operate with a constant amount of water in the air and there would be no need for ongoing humidification. Unfortunately, in most air conditioning systems the air-cooling function of the air conditioning system causes significant condensation of water vapor and consequent humidity loss. Therefore, supple-mental humidification is required to maintain the desired humidity level.  For more information on humidification see White Paper 58, Humidification Strategies for Data Centers and Network Rooms.

Once the cooling requirements are determined, it is possible to size an air conditioning system. The Watt loads of each of the factors described earlier in this summary can be summed to determine the total thermal load. Expressing all measures of power and cooling in Watts simplifies the process. A general rule is that a CRAC system rating must be 1.3 times the anticipated IT load rating plus any capacity added for redundancy. This approach works well with smaller network rooms of under 4,000 ft2 (372 m2).

For larger data centers, the cooling requirements alone are typically not sufficient to select an air conditioner. Typically, the effects of other heat sources such as walls and roof, along with recirculation, are significant and must be examined for a particular installation. The design of the air handling ductwork or raised floor has a significant effect on the overall system performance, and also greatly affects the uniformity of temperature within the data center. The adoption of a simple, standardized, and modular air distribution system architecture, combined with the simple heat load estimation method described, could significantly reduce the engineering requirements for data center design.

For more information on this topic including common conversion factors, design guideline values, and calculation worksheet, please download White Paper 25, Calculating Total Cooling Requirements for Data Centers

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