Chiller Systems

Reciprocating Compressors - Power Requirements

General

Due to better valve designs and configurations that reduce pressure losses, power requirements for reciprocating chillers have been improving over the years. Overall mechanical and compression efficiencies vary with the compression ratio, but are generally in the 72 to 78% range including the hermetic-type refrigerant-cooled 1,750 rpm motor. Compression ratio is computed by dividing the absolute discharge pressure by the suction pressure both measured in psia.

At ARI Standard rating conditions (44°F leaving chilled water, 85°F entering condenser water), typical chillers operate around 40°F evaporating and 100°F condensing temperatures equivalent to pressures. A modern reciprocating compressor has an energy efficiency ratio (EER) of about 15, equal to 0.79 kW per ton. However, in air-cooled conditions the condensing pressure is likely to run up to a 130°F temperature corresponding to pressure, with EERs ranging from about 10.4 up to 11.3, which equate to 1.15 to 1.06 kW per ton.

Assembled into chiller packages in the 20 to 200-plus ton capacities, air-cooled units will typically have EERs ranging from 9.0 to 10.9, equal to 1.33 to 1.10 kW per ton with an average of about 1.22 kW per ton. Similar water-cooled chiller packages will have EERs ranging from 13.1 to as high as 15.8, which equates to 0.92 to 0.76 kW per ton with an average of about 0.82 kW per ton.

Manufacturers continue to develop more efficient models. In some cases, scroll compressors are being used, in place of reciprocating.