Compressors are preferable on reliability grounds to units primarily designed for the smaller range of temperatures required in air conditioning or cooling applications. In small equipment where cost is a major factor and on-site installation is preferably kept to a minimum, such as hermetically sealed motor/compressor combinations (Figure 3.3), there are no rotating seals separating motor and compressor, and the internal components are not accessible for maintenance, the casing being factory welded.
In these compressors, which are available for small capacities, motor and drive are sealed in compact welded housing. The refrigerant and lubricating oil are contained in this housing. Almost all small motor-compressor pairs used in domestic refrigerators, freezers, and air conditioners are of the hermetic type. An internal view of a hermetic type refrigeration compressor is shown in Figure 3.3. The capacities of these compressors are identified with their motor capacities. For example, the compressor capacity ranges from 1/12 HP to 30 BG in household refrigerators. Their revolutions per minute are either 1450 or 2800 rpm. Hermetic compressors can work for a long time in small-capacity refrigeration systems without any maintenance requirement and without any gas leakage, but they are sensitive to electric voltage fluctuations, which may make the copper coils of the motor burn. The cost of these compressors is very low. Also, Figure 3.4 shows two air- cooled condensing units using a hermetic-type refrigeration compressor.
