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 (b) Three-Way Solenoid Valve. The unloader valve (figure 5-5) is a
three-way solenoid-operated valve actuated by operation of the pressure
switch. Connection A of the valve is piped to the inlet valve unloader,
connection B is a vent to atmosphere, and connection C is connected to the air
receiver. When receiver pressure has reached its preset maximum, the pressure
switch contacts open, deenergizing the solenoid. The core of the solenoid
moves the operating lever downward to close connection B of the valve and open
connection C, allowing receiver pressure to act on the inlet valve unloader.
Connection C is held open until receiver pressure drops to the minimum setting
of the pressure switch. The switch then closes, energizing the solenoid.
Connection C closes, cutting off pressure to the inlet unloader, and
connection B opens,releasing the pressure on the unloader to atmosphere.
1.1.1.2 Clearance Pockets. Another method of unloading a compressor is by
the use of clearance pockets built into the cylinder. Normal clearance is the
volume at the end of the piston and under the valves when the piston is at the
top of the compression stroke. Each end of the cylinder is fitted with two
clearance pockets which are connected with, but cut off from, the cylinder by
air-operated clearance valves. Each clearance pocket can hold one-quarter of
the air compressed by the cylinder in one stroke. When both pockets at one
end of the cylinder are open, no air is taken into that end of the cylinder.
The clearance valves are separate from, and work independently of, the main
compressor valves. They are similar to the regular inlet valves but have an
air-operated lifting yoke which operates in the same manner as the inlet valve
unloader described in paragraph 1.1.1.1. Figure 5-6 illustrates the operation
of the clearance pockets and the corresponding indicator diagrams of an air
compressor under five-step clearance control.
FIGURE 5-5. Three-Way Solenoid Valve
5-10
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