The most expensive part in the condensing unit is the compressor. It is the heart, the pump that circulates the refrigerant as the heart does with the blood. A hermetic compressor is the most common one found in residential and light commercial air conditioning
and refrigeration systems. In Fig. 3-8 a typical hermetic compressor is illustrated. They are sealed units and cannot be serviced internally in the field. There are re-building shops that have the equipment to cut them open, replace defective parts an weld the shell together again. These compressors are suction cooled. This means that enough cool gas must return to the compressor from the evaporator coil to maintain a desired compressor operating temperature. An electric motor sealed in the shell drives a crankshaft and one or more pistons to operate a reciprocal compressor. In the case of a hermetic rotary type of compressor, vanes similar to those on a water pump impeller or vanes on an oil pump do the pumping instead of the pistons.
Located on the shell are pieces of pipe that have been welded to the body to give access to the high side and low side of the compressor. There might also be small tubes that can be used to charge the system or install pressure operated controls. The larger of the pipes is the low side, or suction side of the system. The smaller pipe is the discharge side, or hot gas line. These short stubs of pipe may be either steel or copper. The refrigeration piping is soldered to them.
Hermetic compressors can be used in small reach in refrigerators and are rated at fractional hp (horsepower). They range from 1/14 to 60 ton cooling capacity. The larger tonnages are used in commercial units. When deciding whether you should have a compressor re-built, first check the price of a new one. Make sure the condition of the rest of the condensing unit warrants an investment as large as a new compressor. If the condenser coil is rotten and the whole cabinet is held together by rust, the customer might want to replace the whole thing. Good customer relations builds confidence in a technician.