A. The one having two pipe connections and a light regulating spring is
known as the excess pressure head; the other, with a single pipe
connection and heavy regulating spring, as the maximum pressure head.
5. Q. When does the excess pressure head control the flow of steam to
the compressor?
A. When the automatic brake valve is in any one of its first three
positions; namely, release, running and holding positions.
6. Q. With the automatic brake valve in release, running or holding
position, what pressure is in chamber "f" above the diaphragm? In
chamber "d" below the diaphragm?
[Illustration: Fig. 1. The SF-4 Compressor Governor.
Connections: FVP, Feed Valve Pipe. ABV, Automatic Brake Valve.
MR, Main Reservoir. B, From Boiler. P, To Air Pump.]
A. Air, at feed valve pipe pressure, enters at the connection marked
"FVP" and flows to chamber "f" above the diaphragm; this pressure acts
in conjunction with the regulating spring 27 in creating the total
pressure on the diaphragm. Air at main reservoir pressure flows through
the automatic brake valve to the connection marked "ABV" to chamber "d"
under the diaphragm.
7. Q. At what pressure is the regulating spring in the excess pressure
head adjusted?
A. Usually twenty pounds.
8. Q. With the spring adjusted at twenty pounds, what will be the total
pressure on the upper side of the diaphragm?
A. Twenty pounds, plus the pressure in the feed valve pipe.
9. Q. With the feed valve adjusted at seventy pounds, and the regulating
spring at twenty pounds, what pressure will be had in the main reservoir
when the governor stops the compressor?
A. Ninety pounds.
10. Q. Explain the operation of the governor in controlling the
compressor when a main reservoir pressure of ninety pounds is reached.
A. When the main reservoir pressure in chamber "d" slightly exceeds the
pressure on top of the diaphragm it will move upward, carrying the pin
valve with it. The air in chamber "d" passes by the unseated pin valve
through port "b" into chamber "b" above the governor piston, forcing it
downward, seating the steam valve 5, thus shutting off the steam to the
compressor.