Power input performance for screw chillers has been improving over the years as a result of better designs and compressor configurations. Overall mechanical and compression efficiencies vary with the compression ratio (absolute discharge pressure divided by suction pressure in psia). These efficiencies range from 75 to 82%, including the losses associated with the hermetic-type refrigerant cooled motor.

At ARI Standard rating conditions (44°F leaving chilled water, 85°F entering condenser water), typical chiller operation will be about 40°F evaporating and 100°F condensing gauge pressures. A modern screw compressor has an energy efficiency ratio (EER) in the 14 to 17 range, equal to 0.85 to 0.70 kW per ton at full load. But remember, chillers don't operate at full load that often and screw compressors are more efficient at part load. Integrated part load performance (a weighted average operating condition) for screw units can be as low as .42 kW per ton.

The chiller's power requirement is likely to be rated in brake horsepower (bhp) in air-cooled applications. To convert from bhp to electric input in kW, estimate the motor efficiency (typically about 90 percent efficient) and use the following example of a 100 bhp rating. It would translate to:

(100 bhp x 0.746 kW/bhp) = 82 kW.
.9 eff.

Assembled into air-cooled chiller packages or split units in the 20 to 200 ton capacity range screw chillers can be expected to perform at about 9 to 10 EER, equivalent to 1.15 to 1.06 kW per ton with an average of about 1.1 kW per ton.

While they tend to cost more than reciprocating units, air-cooled screw chillers offer better efficiency, infinitely variable capacity control, and a higher tolerance to liquid refrigerant entering the compressor.