Where a compressor does not have any capacity reduction device and on-off switching will not give the degree of control required by the process, the cool­ing capacity can be regulated by injecting discharge gas back into the suction (see Figure 9.2). It has the effect of keeping the evaporator pressure constant, regardless of the load, and can have a wide range of capacity reduction, down to 10% of full load. It is a constant pressure valve, balancing the suction pres­sure against a pre-set spring.

They may be either direct or pilot operated. They are often equipped with an external equalizer which works just like an externally equalized expansion valve. Injecting the hot gas upstream of the evaporator enables the expansion valve to open and admit more cool refrigerant which maintains gas velocity in the evaporator and aids oil return.

Note the solenoid valve in Figure 9.2 . If the system is set up with a pump down, then this solenoid must be wired in parallel with the pump down sole­noid in order to achieve pump down.

This control method is wasteful of energy.

Posted in Refrigeration and Air Conditioning