Refrigeration Compressors

In a refrigeration-cycle system, the compressor raises the pressure of the refrigerant gas from evaporator pressure to condenser pressure. The compressor delivers the refrigerant to the condenser at a pressure and temperature at which the condensing process can be readily accomplished. Since thermal losses through the compressor casing are usually small and cannot be measured accurately, they are usually neglected. Hence, both polytropic (input) work and irreversible adiabatic (actual gas) work may be considered the same.

There are several types of compressors and each type can be of hermetic or open-drive design:

Hermetic (or closed-drive) compressors (Figure 372) consist of an electric motor and compressor built into an integral housing using a common shaft and bearings. Since the motor is built into the housing, the windings run in a lower temperature environment that is cooled by the refrigerant. The motor, therefore, can be smaller than in other configurations. Typically, the motor is cooled with suction gas as it passes through the windings. Motors may also be cooled by liquid refrigerant and economizer gas. A penalty in thermal efficiency occurs as refrigeration capacity is used to absorb the motor heat. This does, however, eliminate the need to remove the motor heat from the mechanical room.

An advantage of hermetic compressors is that they eliminate the need for mounting the motor and aligning the couplings, and minimize other site work during installation. They also eliminate the potential for refrigerant leaks at the shaft seal.

Open-drive compressors (Figure 37-3) operate with an external or exposed electric motor or prime mover that drives the compressor. Large motors are common and driver heat gain is discharged into the surrounding atmosphere. In some cases, mechanical energy must be expended (i.e., a fan) to rid the mechanical room of excessive heat.

An advantage of the open-drive arrangement is that it offers slightly greater thermal efficiency than hermetic compressors because driver cooling with suction gas or water is eliminated, allowing more refrigerating effect to be applied to the actual load. Refrigeration changeover is also less cumbersome because the driver is open and accessible. Disadvantages include size and the potential leak path occurring at the shaft seal.

Fig. 37-2 Cutaway View of Hermetic Compressor. Source: The Trane Company

Suction Discharge

Valve Valve

Suction Discharge

Valve Valve

Fig. 37-3 Cutaway View of Open-Type Compressor. Source: The Trane Company
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