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Packaged chillers from 70 to 300 tons are available with dual screw compressors in independent refrigeration circuits to provide added reliability and redundancy. Step or stepless unloading compressor controls allow for changing data-center load conditions and can closely match the power input to an applied load when compressor operation is required.

Chillers above 300 to 350 tons typically have a centrifugal design with a single compressor. They generally are only water-cooled and can be utilized with a cooling-tower free-cooling system as described previously.

HEAT-RECOVERY CHILLERS

While free cooling is possible in warmer climates, savings are limited by the need for ambient-air or cooling-water temperature to be lower than return chilled-water temperature. This reduces the number of available free-cooling hours, but a careful study of temperature-bin data can reveal possible savings at any location. For data-center HVAC systems in the South and West, however, the demand for a dehumidification cycle (simultaneous cooling and reheating) provides yet another opportunity to save on electrical demand charges and continuous electrical energy.

While the use of reheat is not recommended because of its waste of energy, it is utilized in many southern locations because of the influence of high-humidity makeup air and/or vapor-pressure equalization when the vapor barrier around a data center is not secure. Computer-room units in these installations can be specified with hot-water-reheat coils (in lieu of electric-resistance heaters), and the chillers can be specified with up to 90-percent continuous (series) heat recovery to a hot-water loop at 90 to 110°F for reheating purposes.

This is preferred to parallel heat recovery, which typically is available as “all or nothing” heat. Series heat recovery means hot discharge refrigerant gas always passes through a heat-recovery exchanger before entering a condenser. No additional refrigeration controls are required, and the maximum available heat always is supplied to the hot-water loop. Therefore, sufficient (free) reheat always is available to compensate for any cooling required during the dehumidification cycle because the total available condenser rejected heat is approximately 130 percent of the evaporator's capacity. Greater than 90-percent recovery of 130-percent evaporator capacity equals reheat available at greater than 117 percent of total cooling capacity. Heat recovery is available from air- or water-cooled chillers, and most existing chillers can be retrofitted.

Consider the savings on any large CRAC installation: The same New York City example data center located in Atlanta, with 20 25-ton chilled-water CRAC units, requires 500 tons of cooling capacity. If reheat were required, each of these room units would require approximately 22.5 kw of electrical-resistance heat for dehumidification reheat. This would represent 562 kw of installed electrical heat, which would be added to the electrical demand charge of the building.

If dehumidification were required for about 20 percent of the year at a cost of 8 cents per kilowatt-hour, the annual power cost would be 450 kw times 24 hr times 365 days times 0.2 (20-percent assumed dehumidification) times 8 cents per kilowatt-hour, which equals $63,072. In addition, the demand charge would be approximately $10,000 per year, for a total annual savings of $73,072.

The cost of installing low-pressure hot-water heating coils with control valves in CRAC units is a little higher than the cost of installing electrical-resistance-heater coils and associated controls, but the continuous demand and energy savings far outweigh the initial cost. It should be noted that specifying excessively high water temperatures for reheat should be avoided because chiller efficiency is reduced by high condensing temperatures.

CONCLUSION

Large data centers represent an opportunity for major HVAC energy savings by utilizing chillers with integrated winter free cooling or heat-recovery chillers for free reheat during dehumidification. In either case, the payback period typically will be less than one year. All of these systems can help a project earn Leadership in Energy and Environmental Design credits.

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The president of Motivair Corp., Graham Whitmore has specialized in refrigeration and HVAC in England and the United States for 40 years. Previously, he was the U.S. marketing director for Hiross Inc., specializing in computer-room air-conditioning units.

Green Refrigerants

In the evolving world of non-ozone-depleting refrigerants, it is a wise and socially responsible position to specify any of the previously mentioned energy-saving chillers for data centers with a suitable “green” refrigerant. Data centers epitomize the latest technology in industry today. Therefore, it is appropriate that major energy savings are realized whenever possible and that the latest refrigerants are employed in these installations.

The estimations and suggested applications in this article are based on the use of screw compressors with R-407C refrigerant, which is a non-ozone-depleting replacement for R-22. R-407C qualifies for Leadership in Energy and Environmental Design (LEED) credits based on an ozone-depletion potential of 10-5 and a global-warming potential of 1,700. Other common “green” refrigerants include R-134a and R-410A.

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