The control system of a reciprocating air compressor may include one or more control devices. These control mechanisms may include start-stop control, constant-speed control, cool-ing water failure switches, and automatic high-temperature shutdown devices.

Control or regulating systems for air com-pressors in use by the Navy are largely of the start-stop type. In the start-stop design, the compressor starts and stops automatically as the receiver pressure falls or rises within predetermined set points. On electrically driven compressors, the system is very simple. As air pressure in the receiver increases, it actuates a pressure switch that opens the electrical control circuit when the pressure acting upon the switch reaches a given value, commonly called a SET POINT. The switch closes the control circuit when the pressure drops a predetermined amount. Because of their high horsepower rating, centrifugal compressors do not have automatic start-stop controls. In-stead, an automatic load and unload control system is used.

Some electrically driven units, such as the medium-pressure system, are required to start at either of two pressures. In these units, there are two pressure switches. One of these switches has a three-way valve that admits pressure from the air accumulator to the selected pressure switch. In other electrically driven units, the air is directed from the receiver through a three-way valve to either of two control valves adjusted for the required range of pressure settings. A line connects each control valve to a single pressure switch. This switch may be set to any convenient pressure. The setting of the control valve selected will determine the operation of the switch.

The CONSTANT-SPEED CONTROL regulates the pressure in the air receiver by controlling the output of the compressor. This control works without stopping or changing the speed of the unit. The constant-speed control prevents frequent starting and stopping of compressors when there is a fairly constant but low demand for air. Control is provided when air is directed to unloading devices through a control valve that is set to operate at a predetermined pressure.

AUTOMATIC HIGH-TEMPERATURE SHUTDOWN DEVICES are fitted on almost all high-pressure air compressors. If the cooling water temperature rises above a safe limit, the compressor will stop and will not restart automatically. Some compressors are equipped with a device that will shut down the compressor if the temperature of the air leaving any stage exceeds a preset value.