Controlling the capacity of the compressor is accomplished by unloading and loading the cylinders. This is a very desirable design feature of the unit, because if the compressor is to be started under a load (all cylinders are working), there is a much greater amount of torque required and it is necessary to have a much larger drive motor. Also, if the compressor is running at a constant capacity or output, it will reach the low temperature or pressure limits and will be constantly starting and stopping, thereby putting excessive work on the unit.

Unloading of the cylinders in the compressor is accomplished by lifting the suction valves off their seats and holding them open. This method of capacity control unloads the cylinders completely and allows the compressor to work at as little as 25% of its rated capacity.

Unloader Mechanism

When the compressor is not in operation, the unloader power element mechanism, which is operated by oil pressure from the capacity control valve, is in the unloaded position (figure 6-6). The unloader spring pushes against the unloader piston. This action moves the unloader rod to the left, thereby rotating the cam rings. As the cam rings are rotated, the lifting pins are forced up-ward, raising the suction valve off its seat. The suction valve is held in this position until the compressor is started and oil pressure of approximately 30 psi is reached. At this time, the oil pressure from the capacity control valve pushes the unloader piston back to the right against the unloader spring. The motion transmitted through the push rod rotates the cam ring, thus lowering the lifting pins and allowing the suction valve to close or operate normally and the cylinder to become loaded (figure 6-7). On most compressors unloaders are connected to the cylinders in pairs.

Figure 6-7.Unloader mechanism in loaded position.