Many different types and sizes of compressors are used in refrigeration and air conditioning systems. They vary from the small hermetic units used in drinking fountains and refrigerators, to the large centrifugal units used for air conditioning. One of the most commonly encountered compressors on todays modern ships is a high speed, continuous running unit with a variable capacity. This compressor is of the multicylinder, reciprocating design, with a positive unloaded system built into the compressor to control the compressors capacity.