The most important elements of a system are the components that control the flow of refrigerant in accordance with the requirements of the refrigeration load. Next in importance are the high-pressure (safety) control and the low-pressure (operating) control. The refrigeration compressor, condenser, and receiver are all pressure vessels. As such, they have maximum pressure limitations based on their construction. Several adverse system conditions can raise the normal pressure level in a system that exceeds the maximum pressure limits of the components. The best way to handle an excessive pressure problem is to use a high-pressure cut-out. It should be set to open the control circuit to the compressor motor sufficiently above the operating range of the equipment, yet far enough below the possible bursting point of the pressure vessels in the system.
The low-pressure control is often referred to as the “operating” control. It serves that function in many applications. It controls the temperature of an area or products by responding to pressures that correspond to evaporator temperatures. The control acts when the suction pressure reaches the point at which it corresponds to the design application temperature. At this point, the control opens the circuit to the compressor motor controller, or to the solenoid valve in the liquid line if the system operates on a pump-down cycle. It closes the circuit again when the suction pressure shows a predetermined temperature rise in the refrigerated area or products.