Reciprocating Compressors - Power Requirements & Costs
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.
Reciprocating Compressors - First Cost
While they are the least efficient of the chiller package options, reciprocating or scroll compressor chillers have a definite first cost advantage in the smaller chiller sizes. The first cost of reciprocating chiller packages is the lowest of the various electric chiller options, certainly when expressed in $ per ton. The compressors are competitively priced since they are used in many different chiller models. Plus, many more reciprocating chillers are produced than larger centrifugal and screw type chillers. These economies of scale result in a lower unit cost, especially for models up to about 200 tons.
Reciprocating Compressors - Operating Costs
For example, an air-cooled chiller serving a hospital operating room suite that operates year-round could well have a lower annual electric cost than a comparable water-cooled unit, due to the large number of operating hours the unit will be operating at part-load and low-ambient temperature conditions. Only a careful energy use analysis of each application performed by a qualified professional can identify the most economical equipment choice.
Maintenance costs must also be factored in. Here are some typical mid-1995 $ per ton annual values.
|Chiller Type||20 Tons||50 Tons||75 Tons||100 Tons||150 Tons||200 Tons|
If the chiller is driven by a natural gas engine, the added maintenance costs of the engine must also be included. Typically this amounts to about $0.012 per ton per operating hour.