The main types of compressors used in industrial refrigeration are screw, reciprocating, centrifugal, and rotary vane. Another type that is manufactured in large numbers is the scroll compressor, but so far these compressors are not available in the size normally encountered in industrial refrigeration. Rotaryvane compressors are still in use as low-stage compressors, but very few new ones are installed.
Centrifugal compressors have been standard in large-capacity chemical and process industry plants where they are driven by electric motors or by steam or gas turbines. Centrifugal compressors are also widely used for chilling water in air-conditioning applications. Manufacturers use such refrigerants as R-123 and R-134a in these packages. After condensing the longer list of compressor types, the two that remain, and the ones that will be addressed in this book, are reciprocating and screw compressors. This chapter focuses on reciprocating compressors, while Chapter 5 concentrates on screw compressors.
Two styles of construction of compressors are open and hermetically sealed. In the open-type compressor, as in Figure 4.1, the shaft extends out of the compressor and is connected externally to the electric motor which drives the compressor. In the hermetically-sealed motor-compressor unit, the entire assembly is encapsulated and only the refrigerant lines and electrical connections, and no rotating shaft, penetrate the housing. These hermetically sealed units are always used for domestic refrigerators, window-type air conditioners, and other small appliances. They are expected to run for decades with no leaks of refrigerant.
Some larger-size compressors are semi-hermetic, where the heads of the compressors can be removed to gain access to the pistons and valves for servicing. Semi-hermetic compressors available on the market are generally limited to a maximum refrigeration capacity of about 150 kW (40 tons of refrigeration).
In either a hermetic or semi-hermetic unit, the refrigerant is in contact with the windings of the motor, so halocarbon refrigerants which do not attack copper are used in this type of compressor. Since ammonia will react with copper, ammonia compressors are of the open type. At one time the shaft seal was a troublesome cause of ammonia leaks, but the quality of shaft seals has progressively improved. Nevertheless, the development of a hermetic ammonia compressor remains an attractive goal, and such approaches as using aluminum windings for the motor, encapsulating the rotor so that ammonia does not contact the motor windings1, or connecting the motor and compressor through a magnetic drive2 are all approaches being considered. On the other hand, an open-type compressor is usually more efficient than the hermetic type, because the suction vapor in a hermetic compressor passes over the motor to cool it, and in so doing the vapor is superheated, thereby requiring more power for compression.
A central emphasis of this chapter is the effect of suction and discharge pressures on the refrigeration capacity and power requirement of the compressor. The suction and discharge pressures are the primary influences of the rest of the system on the compressor. The nature of the refrigeration load at the evaporator strongly influences the suction pressure, and the ambient conditions translate through the condenser to control the discharge pressure. The performance characteristics that are most important to the user of the compressor are the refrigerating capacity and the power requirement. The compressor has a dominating influence on system performance, so the ability to predict influences of changes in conditions imposed on the system, such as fluid temperatures at the evaporator or air temperatures at the condenser, requires an understanding of compressor performance. Certain idealizations will initially be the basis of explaining the performance characteristics, but then the chapter shows that these idealized trends prevail in real compressors. The latter part of the chapter explains some capabilities and limitations of the reciprocating compressor and also describes auxiliaries necessary for reliable operation.