Almost all refrigeration plants are subject to varying refrigeration loads and, in general, plants operate at part load. The capacity must be reduced, otherwise the evaporating temperature would drop until the plant capacity matched the load. An evaporating temperature that is too low may damage the product. Small plants simply cycle the compressor on and off to provide a time-average match of capacity to load. In larger plants the capability to modulate the capacity down to at least 25% of full capacity is desirable. As the cost of variable-frequency drives decreases and their reliability improves, this method of capacity modulation may become more prominent. For the time being, however, cylinder unloading is the standard technique for adjusting the pumping capacity of reciprocating compressors.
Multicylinder compressors can be unloaded by holding open the suction valve on a cylinder. During its intake stroke, the piston draws suction gas into the compressor, but then on its return stroke, instead of compressing the vapor, the piston pushes the refrigerant back into the suction manifold. The suction valve can be held open by valve-lifting pins which in turn are actuated by oil from the lubricating oil pump or discharge gas that is controlled by solenoid valves. In some designs, the normal position of the unloaders is such that oil pressure is required to activate compression. This arrangement automatically starts the compressor with the cylinders unloaded, because oil pressure is not available until the compressor is at least partially up to speed. The two variables most frequently sensed to regulate the operation of the cylinder unloaders are the suction pressure or the outlet temperature of the liquid being chilled in the evaporator.
To indicate part-load efficiency, compressor manufacturers typically suggest a power-capacity relationship during cylinder unloading, like that shown in Figure 4-20. This graph could apply to an 8-cylinder compressor that can unload 2, 4, and 6 cylinders. In order to operate at 60% capacity, for example, the compressor would switch back and forth between the half-unloaded and the one-fourth-unloaded conditions. The percent of full capacity closely matches the percent of total cylinders pumping, but the power requirement runs several percent higher than the linear relationship. One report5 on field behavior of compressors during unloading indicates, as in Figure 4-21, a slightly lower efficiency than that indicated in Figure 4-20.
Power to overcome friction in pumping an idle piston is to be expected, but there is a compensating positive influence. At part-load operation the condenser and evaporator heat transfer rates drop so the condensing temperature decreases and the evaporating temperature increases. Both effects reduce power and compensate for the losses of the idle cylinders.