Because of their small size, these units are less energy efficient than other designs. Screw compressors generally use two intermeshed screws for compression. They are not as cost effective as either the small reciprocating units or the large centrifugal chillers. Most of the applications are in the range of 100 to 300 tons of cooling capacity, but even here they make up only a small portion of the market. Their energy efficiency is between that of reciprocating and centrifugal units.
Centrifugal chillers are the most widely used.
A centrifugal compressor rotates at high speed, compressing the refrigerant by centrifugal force. Chillers include a compression, evaporator and condenser along with ancillary equipment to move air. Often a large building will use several chillers. For example, the World Trade Center in New York City uses seven centrifugal chillers. Five new units were installed in 1993 to replace those damaged in the World Trade Center bombing.