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CHAPTER 5. CONTROLS
Section 1. PRIME MOVER CONTROLS
1 DESCRIPTION.  Compressed air delivery is regulated by control of the
compressor drive. This is accomplished by varying the drive speed to regulate
air output in response to load variations.
1.1 Steam Engine Controls.  The steam engine is still used to a limited
extent as a prime mover at some Navy installations. When steam engines
require replacement,they are usually replaced by electric motors, steam
turbines, and internal combustion engines. Where the steam engine is used
with reciprocating compressors, it is generally integral to the compressor.
Control is normally furnished with steam-driven air compressors by a
combination of speed and pressure governors. These governors are of two
types, throttling and automatic cutoff.
1.1.1 Throttling Governors. A change in steam pressure changes in compressor
speed. This change varies the delivery of the oil pump which is chain-driven
from the compressor shaft. Oil discharging from the pump passes through a
variable  restriction. The back pressure restriction is applied to the
diaphragm of the throttle valve. Increased oil delivery and compressor speed
raises the pressure on the diaphragm causing the throttle valve to close,
restoring the speed of the compressor.  A small orifice and return line is
provided from the throttle valve diaphragm to permit a small amount of oil to
be recirculated to prevent any air from accumulating in the diaphragm. When
receiver pressure changes, he pressure control valve varies the restriction
t
in the oil line from the pump. This changes the back pressure applied to the
diaphragm of the throttle valve,which changes the compressor speed to restore
receiver pressure. This control is used with steam engines of 150 hp or less.
1.1.2 Automatic Cutoff Governors.  In the automatic cutoff governor,
hydraulic pressure is supplied by the oil pump which is driven from the
compressor shaft. Any variation in steam pressure, which changes the speed of
the compressor, causes a change in oil pressure from the pump. This pressure
change either raises or lowers the piston. The rack on the piston rotates the
sprocket, which in turn rotates the cutoff valve rod to reset the cutoff and
adjust the speed. Any change in receiver pressure also affects the hydraulic
pressure. The pressure control valve, which senses receiver pressure,
bypasses some of the oil flow to increase or decrease hydraulic pressure.
1.1.3 Automatic Start-Stop Governors. Automatic start-stop governors (figure
5-l) are also used on steam-driven compressors. This governor has a
spring-loaded diaphragm, the underside of which is always open to receiver
pressure.  When receiver pressure becomes greater than the spring pressure
above the diaphragm, the diaphragm is raised, unseating the needle valve. Air
then flows to the chamber above the piston attached to the steam valve. Air
pressure on this piston forces the steam valve to its seat, cutting off the
flow of steam to the compressor. With a decrease in steam pressure, the valve
operates in reverse to readmit steam to the cylinder.
1.2 Motor Controls.  Where air demand is intermittent, a start-stop control
may be provided for the air compressor.  This system consists basically of a
pressure operated switch connected to the motor starter circuit. When air
5-1





 


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