Page 456 - Mechanical Engineers' Handbook (Volume 4)

7 System Components 445

Figure 9 Volume–pressure relationships for a reciprocating compressor.

volumetric flow and pressure ratios are balanced. Capacity can be controlled with cylinder
unloading.
Open systems and many processes require nearly fixed compressor suction and discharge
pressure levels. This load characteristic is represented by the horizontal typical open-system
line in Fig. 9. In contrast, condenser operation in many closed systems is related to ambient
conditions, for example, through cooling towers, so that on cooler days the condenser pres-
sure can be reduced. When the refrigeration load is lower, less refrigerant circulation is
required. The resulting load characteristic is represented by the typical closed-system line in
Fig. 9.
The compressor must be capable of matching the pressure and flow requirements im-
posed upon it by the system in which it operates. The reciprocating compressor matches the
imposed discharge pressure at any level up to its limiting pressure ratio. Varying capacity
requirements can be met by providing devices that unload individual or multiple cylinders.
This unloading is accomplished by blocking the suction or discharge valves that open either
manually or automatically. Capacity can also be controlled through the use of variable speed
or multispeed motors. When capacity control is implemented on a compressor, other factors
at part-load conditions need to considered, such as (1) effect on compressor vibration and
sound when unloaders are used, (2) the need for good oil return because of lower refrigerant
velocities, and (3) proper functioning of expansion devices at the lower capacities.
Reciprocating compressors employ a lubricant. Oil is pumped into the refrigeration
system during operation. Systems must be designed carefully to return oil to the compressor
crankcase to provide for continuous lubrication and also to avoid contaminating heat-
exchanger surfaces. At very low temperatures ( 50 C or lower, depending on refrigerant
used) oil becomes too viscous to return, and provision must be made for periodic plant
shutdown and warm-up to allow manual transfer of the oil.
Reciprocating compressors usually are arranged to start with the cylinders unloaded so
that normal torque motors are adequate for starting. When gas engines are used for recip-
rocating compressor drives, careful torsional analysis is essential.

Rotary Compressors
Rotary compressors include both rolling piston and rotary vane compressors. Rotary vane
compressors are primarily used in transportation air-conditioning applications, while rolling