The refrigeration professional who selects and operates compressors should have a clear understanding of how the evaporating and condensing temperatures affect the refrigerating capacity and power requirements of the compressor. The compressor is often called the heart of the refrigeration system, but it must interact with the other components of the system. Each component affects the performance of the total system, and the two operating variables to which the compressor is most sensitive are the evaporating and condensing temperatures. The two performance characteristics of most importance to the designer and operator are the refrigerating capacity and the power requirement.
Throughout this book and throughout the industry the compressor is characterized as having refrigerating capacity. Of course, refrigeration takes place at the evaporator, not the compressor. Indicating that the compressor possesses a certain refrigerating capacity in kW (tons of refrigeration) means that the compressor is capable of pumping the flow rate of refrigerant that will provide the stated refrigeration capacity at the evaporator.
A portion of this chapter is devoted to showing and explaining the influences of evaporating and condensing temperatures on the refrigerating capacity and power requirement of the compressor. The designer and operator of refrigeration systems must constantly reckon with how the conditions imposed on the compressor—the saturated suction temperature and the condensing temperature—influence the two key performance factors, the refrigeration capacity and the power requirement. Beyond knowing the trends, the implications of these trends are crucial. In addition to a citing these performance trends, the next several sections will explain from thermodynamics and the pumping characteristics of reciprocating compressors why the trends occur.