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 occurs more often than every 3 minutes because this will cause excessive wear
on the bearings and valves.
1.1.2.5 Butterfly Valves. Centrifugal compressor control may also be
accomplished by throttling the suction or discharge of the compressor with a
butterfly valve.
1.1.3 Rotary Sliding Vane Compressor. There are several common methods for
maintaining constant speed control of rotary sliding vane compressors.
1.1.3.1 Automatic Start/Stop at Predetermined Pressures. Speed control is
maintained automatically according to predetermined pressures.
1.1.3.2 Blocking the Compressor Inlet. Blocking or unblocking of the
compressor inlet maintains speed control.
1.1.3.3 Intake Unloaders.Intake unloading consists of an automatic valve
located at the compressor suction. This valve closes off the compressor
intake, thereby preventing the compressor from taking in air. The intake
unloader (figure 5-7) incorporates two valves in one body, an air
piston-operated inlet valve and a pressure release valve. When the compressor
is delivering air, the inlet valve is in its normal open position and the
release valve is closed. When receiver pressure builds up to the unloading
pressure, a pressure switch deenergizes a three-way solenoid pilot valve which
admits receiver pressure to the piston of the inlet valve. The piston rises,
closing the inlet valve and opening the pressure release valve. The pressure
release valve relieves air pressure from the portion of the compressor
discharge line between the compressor and the discharge check valve. This
permits unloaded operation of the compressor at atmospheric pressure. When
receiver pressure drops to the lower setting of the pressure switch, the
contacts close, energizing the solenoid pilot valve. This cuts off receiver
pressure and vents the pressure in the unloader to atmosphere. The inlet
valve is opened, permitting the compressor to load.
1.1.4 Rotary Twin-Lobe Compressor. Control methods for the rotary twin-lobe
are the same as for the rotary sliding vane compressor.
1.1.5 Rotary Liquid Piston Compressor. Control methods for the rotary liquid
piston compressor are the same as for the rotary sliding vane compressor.
1.1.6 Axial Flow Compressor. Control methods for the axial flow compressor
are the same as for the centrifugal compressor.
1.2 Variable Speed Control. There are many methods available to vary the
speed of the compressor to produce a more efficient system. Those listed
below may not apply to all types of compressors, but generally, more than one
method deserves consideration during a system design or retrofit. The
following methods vary in applicability depending on energy source,
horsepower, weight, revolutions per minute, and cost requirements.
1.2.1 Gasoline, Steam, and Diesel Engines. These engines can be manually
controlled or load controlled by the use of a governor.
5-12
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