In a refrigeration system designed to maintain a predetermined temperature at full load, any decrease in load would tend to lower below fullload temperature the temperature of the medium being cooled.
To maintain constant temperatures in applications having varying loads, means must be provided to change refrigerant temperature to meet varying-load requirements.
Refrigerant temperature is a function of evaporator pressure. Thus,the most direct means of changing refrigerant temperature to meet varying-load requirements is to vary the system pressure. This variation of system pressure is accomplished by adjusting the setting of a back-pressure regulator. A number of back-pressure valve controls are
available. Some of them are mentioned here.