against a light load. This increases the life of the drive motor and belts and also reduces the
possibility of tripping the motor overload relay. The unloader incorporates a pilot valve
which is actuated centrifugally by unloader weights attached to the end of the crankshaft.
1-20. INTERCOOLER AND AFTERCOOLER. The compressor is equipped with an inter-
cooler (Figure 1-2) between the first and second stages which removes most of the heat of
compression from the air before it enters the second stage, thus improving efficiency and
d e c r e a s i n g the final discharge air temperature. The intercooler consists of finned tubing
connecting the discharge of the first stage to the inlet of the second. The compressed air
flows through these tubes and its heat is transferred to the cooling fins. The ambient air
from the fan-type flywheel passes over the fins, d i s s i p a t i n g the heat to atmosphere. The
aftercooler (Figure 1-2) consists of finned tubing connecting the second stage discharge to the
receiver, and functions in the same manner as the intercooler.
1-21. SAFETY VALVES. Safety valves are provided for both the first and second stages
to
p r o t e c t against damage from over pressure. The first stage safety valve (Figure 1-2)
is
mounted on the intercooler outlet manifold, and the second stage safety valve (Figure l-l)
is
located in the service line of the receiver. The first stage safety valve is set to blow at
60
PSIG, and the second stage is set at 200 PSIG.
1-22. STARTING SWITCH. The compressor starting switch is incorporated as a part of the
magnetic controller (Figure 1-1) furnished with the driving motor. The controller incorpo-
r a t e s an ON-OFF snapswitch for starting and stopping the compressor. Complete details
will be found in Section VI..
1-23. CHECK VALVE. A check valve (Figure 5-13) is located in the discharge line and acts
to check the flow of air from the air receiver to the cylinder when the compressor is stopped.
Stop Control, which makes or breaks electrical contact to the motor at predetermined pres-
s u r e s . This type of regulation is used when the demand for air is small or intermittent, but
where pressure must be continuously maintained.
1-25. AUTOMATIC START AND STOP CONTROL .
1-25.1. Automatic Start and Stop Control is obtained by means of a pressure switch (Figure
thereby maintaining the air receiver pressure within prescribed limits. The pressure switch
is piped to the receiver and is actuated by changes in receiver pressure.
1 - 2 5 . 2 . The pressure switch has a cut-out adjustment and a differential adjustment. The
cut-out is the pressure at which the switch contacts open, and the differential is the span be-
tween the cut-in and cut-out settings. A fairly wide differential setting of the pressure switch
should be maintained to prevent frequent starting and stopping of the compressor.
