Page 209 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 195

Manual Control

With manual control, the operator is responsible for adjusting com-
pressor capacity either up or down in accordance with pressure varia-
tions. He may control the compressor by starting or stopping the driver,
throttling the driver, or manually adjusting the capacity. Other control
modes are occasionally employed in special applications.

In order to manually adjust the capacity, the compressor cylinders can
be equipped with suction valve unloaders, suction valve lifters, and fixed
or variable clearance pockets.

Automatic Control

In automatic control schemes, the basic functions of sensing, relaying,
and activating are performed by a combination of standard devices.
These devices usually employ mechanical and electrical operations and
principles. Most of these devices are extremely simple and have been in
common use since the dawn of the machine age.

The elements found in most automatic control schemes make use of
five basic mechanisms:

1. Pistons and cylinders to perform work. Inside unloaders use pistons
and cylinders for operation.
2. Diaphragms. The outside-type unloaders and some pressure switch-
es are diaphragm-operated.
3. Solenoids. These devices use the basic principles of electromagnet-
ism to perform work and are used widely for relaying. Solenoid-
operated relays control the flow of water and air and are used exten-
sively in the circuits of modern control schemes.
4. Valves of all types. Both manual and automatic valves are used
throughout automatic control schemes to control and regulate the
flow of air, water, and oil.
5. Electronic and electric switches and relays. These perform the sim-
ple task of opening and closing contacts. They are applied in most
electronic and electrical circuits found on modern capacity controls.