Chapter 6—REFRIGERATION AND AIR CONDITIONINGmanufacturer’s technical manual or the directionsenclosed with every new seal.CAPACITY CONTROLControlling the capacity of the compressor isaccomplished by unloading and loading thecylinders. This is a very desirable design featureof the unit, because if the compressor is to bestarted under a load (all cylinders are working),there is a much greater amount of torque requiredand it is necessary to have a much larger drivemotor. Also, if the compressor is running at a con-stant capacity or output, it will reach the lowtemperature or pressure limits and will be con-stantly starting and stopping, thereby putting ex-cessive work on the unit.Unloading of the cylinders in the compressoris accomplished by lifting the suction valves offtheir seats and holding them open. This methodof capacity control unloads the cylinders com-pletely and allows the compressor to work at aslittle as 25% of its rated capacity.Unloader MechanismWhen the compressor is not in operation, theunloader power element mechanism, which isoperated by oil pressure from the capacity con-trol valve, is in the unloaded position (figure 6-6).The unloader spring pushes against the unloaderpiston. This action moves the unloader rod to theleft, thereby rotating the cam rings. As the camrings are rotated, the lifting pins are forced up-ward, raising the suction valve off its seat. Thesuction valve is held in this position until the com-pressor is started and oil pressure of approxi-mately 30 psi is reached. At this time, the oilpressure from the capacity control valve pushesthe unloader piston back to the right against theunloader spring. The motion transmitted throughthe push rod rotates the cam ring, thus loweringthe lifting pins and allowing the suction valve toclose or operate normally and the cylinder tobecome loaded (figure 6-7). On most compressorsunloaders are connected to the cylinders in pairs.Figure 6-7.—Unloader mechanism in loaded position.6-5
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