In data centers the entire electrical-kilowatt-power input to computer equipment is converted to heat and discharged into a conditioned space. Most small and medium-size data-processing centers utilize direct-expansion computer-room air conditioners - typically, split-air, water, or glycol-cooled. Large data centers predominantly utilize chilled-water-room units for several reasons, including:

Demands relating to safety, the autarky of the entire system and power management are also rising steadily. The remote maintenance of a computer center or of individual server locations is the current requirement of the industry. Practically all the cabinet parameters from temperature, moisture, smoke and vibration to door contacts and access check and mains-phase monitoring should be recorded and documented in a single unit.

Per American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards, typical data-center HVAC design conditions (68 to 77°F, 40- to 45-percent relative humidity) are required day and night, 365 days a year. Chilled-water temperature usually is required to be 45 to 50°F, depending on the computer-room-air-conditioning (CRAC) unit coil design for cooling and dehumidification. This continuous temperature- and humidity-control requirement also allows the possibility of substantial energy savings.

Two significant energy-saving possibilities for chilled-water systems are the use of free-cooling chillers and simultaneous heat-recovery chillers for dehumidification.

The continuous cooling demand of data centers is largely unchanged during winter because winter building losses do not have a significant effect on total heat load. With the application of a free-cooling circuit to a central chiller, refrigeration compressors can be switched off for long periods of time during winter, spring, and fall. A water-cooled central chiller can be designed to switch automatically to free cooling as soon as a cooling tower's cooling water or glycol is at or below the return chilled-water temperature, while an air-cooled chiller with an integrated or separate free-cooling coil can switch to free cooling when the outside ambient temperature is lower than the return chilled-water temperature. Both of these energy-efficient designs for large data center HVAC systems have similar advantages, including a direct reduction in energy costs and fewer compressor running hours. Further, the refrigeration compressors in an air-cooled chiller are never required to operate in very cold ambient temperatures, which eliminates any special low-temperature refrigeration controls and extends the equipment's life expectancy by eliminating the most difficult operating conditions for an air-cooled system.

Example:

Consider the savings possible in a New York City data center with 20 25-ton chilled-water CRAC units. This represents an installed cooling capacity of 500 tons. ASHRAE's historical weather data in New York City suggests a 99-percent winter design dry-bulb temperature of 11°F and an average of 3,750 hr of dry-bulb temperatures below 40°F per year. The chilled-water CRAC units typically require approximately 50°F chilled-water-supply and 60°F chilled-water-return temperatures. This means an ideal situation exists for winter free-cooling chiller savings.

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