Figure 37-9. Pilot-Operated Absolute (POA) Valve.
compressor speed, evaporator temperature, and
opening a thermal fuse. A compressor discharge
changes in altitude. The POA valve is operated by a
pressure switch sometimes is used to protect against a
bellows containing an almost perfect vacuum. The
low refrigerant condition (fig. 37-10). This switch
expanding and contracting action of the bellows operates
disengages the compressor drive to protect the system
a needle valve, regulating its surrounding pressure. As
when discharge pressure drops below approximately 35
inlet and outlet pressure are equalized, spring pressure
psi (241 kPa). A muffler often is used on the outlet side
closes the valve. The pressure differential across the
of the compressor (fig. 37-10). The muffler helps
valve then forces the piston toward the lower pressure,
reduce compressor pumping noise and line vibrations.
therefore opening the valve to allow refrigerant to flow.
a. Two-Cylinder Axial. The two-cylinder axial
compressor (fig. 37-12) has two reciprocating pistons
37-7. Compressor. The compressor is designed to
fitted into cylinders. A special valve plate, operated by
increase the pressure of vaporized refrigerant exiting the
differential pressures, is used to control gas flow.
evaporator. When the system is activated, a coil
produces a magnetic field that engages the drive pulley
b. Four-Cylinder Radial. The four-cylinder
to operate the compressor (fig. 37-10). Some
radial compressor (fig. 37-13) positions four pistons at
compressors are protected from overheating by a
right angles to each other. The pistons are driven by a
superheat switch located inside the compressor (fig. 37-
central shaft connected to the engine by the electric
Should the compressor develop an excess
clutch assembly and V-belt. The radial compressor's
amount of heat due to a loss of refrigerant or oil, the
compact design is very popular on today's vehicles.
superheat switch disengages the compressor by
completing a circuit and