Vapor Compression System Drivers

Vapor compression system drivers, which produce shaft rotational power, are essentially the same as those used for air compressors and other mechanical drive services applications. As with air compressors, the vapor compressor is indifferent to the type of driver, except with respect to the operating speed. Its brake power requirement is matched by the torque and rotational speed at the output coupling of the drive shaft. The energy requirement of the driver will be a function of the compressor's brake power requirement and the performance of the driver in providing that power to the compressor at the particular operating speed, plus any gearing efficiency losses.

While there is some component variation, vapor compression systems are quite similar, regardless of the driver. This is illustrated by Figure 37-28, which shows two factory-assembled large-capacity compressor/driver packages. On the bottom is an electric motor-driven unit and on the top is a steam turbine-driven unit. Between the electric motor and the compressor is a speed increaser gear. With this particular unit, no gearing is required for the steam turbine driver.

Following is a comparative discussion of commonly used drivers for vapor compression systems — electric motors, reciprocating engines, gas turbines, and steam turbines. Given the commonality with other mechanical drive applications, the focus here is on driver characteristics as they are specifically applied to vapor compression systems. A review of Chapter 29, which provides greater detail on mechanical drive service for each driver type, is, therefore, recommended prior to proceeding with the following section. For additional detail on driver performance expressions, refer to Chapters 2 and 33. For additional detail on each of the prime mover types, refer to the applicable chapters in Section III.

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