Chillers are a type of cooling equipment that produces chilled water to cool air. The chilled water is distributed throughout the building by pipes. The two major categories of chillers are water-cooled and air-cooled. Water-cooled chillers use water to transport away the heat rejected in their condensers. The water, called condenser water, is cooled in a cooling tower. Air-cooled chillers have condensers that are cooled with ambient air.

In large facilities, the equipment used to produce chilled water for HVAC systems can account for up to 35% of a facility's electrical energy use. If replacement is determined to be the most cost-effective option, there are some excellent new chillers on the market.

The most efficient chillers currently available operate at efficiencies of 0.50 kilowatts per ton (kW/ton) a savings of 0.15 to 0.30 kW/ton over most existing equipment. When considering chiller types and specific products, part-load efficiencies must also be compared. If existing chiller equipment is to be kept, there are a number of measures that can be carried out to improve performance.

Consider chiller replacement when existing equipment is more than ten years old and the life-cycle cost analysis confirms that replacement is worthwhile. New chillers can be 30% to 40% more efficient than existing equipment. First-cost and energy performance are the major components of life-cycle costing, but refrigerant fluids may also be a factor. Older chillers using CFCs may be very expensive to recharge if a refrigerant leak occurs (and loss of refrigerant is environmentally damaging).

An excellent time to consider chiller replacement is when lighting retrofits, glazing replacement, or other modifications are being done to the building that will reduce cooling loads. Conversely, when a chiller is being replaced, consider whether such energy improvements should be carried out; in some situations those energy improvements can be essentially done for free because they will be paid for from savings achieved in downsizing the chiller (see Integrated Building Design). Be aware that there can be lead times of six months or more for delivery of new chillers.

Electric chillers use a vapor compression refrigerant cycle to transfer heat. The basic components of an electric chiller include an electric motor, refrigerant compressor, condenser, evaporator, expansion device, and controls. Electric chiller classification is based on the type of compressor used: common types include centrifugal, screw, and reciprocating. The scroll compressor is another type frequently used for smaller applications of 20 to 60 tons. Hydraulic compressors are a fifth type (still under development).

Both the heat rejection system and building distribution loop can use water or air as the working fluid. Wet condensers usually incorporate one or several cooling towers. Evaporative condensers can be used in certain (generally dry) climates. Air-cooled condensers incorporate one or more fans to cool refrigerant coils and are common on smaller, packaged rooftop units. Air-cooled condensers may also be located remotely from the chillers.

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