Suction Throttling Valve
Figure 13-10.Pilot-operated absolute (POA) suction throttling valve.
The pilot-operated absolute (POA) suction
throttling valve (fig. 13-10) maintains the proper
minimum evaprator pressure regardless of compressor
speed, evaporator temperature, and changes in altitude.
The POA suction throttling valve is operated by a
bellows containing an almost perfect vacuum. The
expanding and contracting action of the bellows
operates a needle valve, regulating its surrounding
pressure. As inlet and outlet pressure are equalized,
spring pressure closes the valve. The pressure
differential across the valve then forces the piston
toward the lower pressure, therefore, opening the valve
to allow refrigerant to flow.
The compressor increases the pressure of vaporized
refrigerant exiting the evaporator. When the system is
activated, a coil produces a magnetic field that engages
the drive pulley to operate the compressor (fig. 13-11).
Some compressors tie protected from overheating by a
superheat switch located inside the compressor (fig.
13-12). Should the compressor develop an excess
amount of heat due to a loss of refrigerant or oil, the
superheat switch disengages the compressor by
completing a circuit and opening a thermal fuse.
Sometimes a compressor discharge pressure switch is
used to protect against a low refrigerant condition. (See
fig. 13-10.) This switch disengages the compressor drive
to protect the system when discharge pressure drops
below approximately 35 psi (241 kPa). Often a muffler
is used on the outlet side of the compressor (fig. 13-11).
The muffler helps reduce compressor pumping noise
and line vibrations.
Two-Cylinder Reciprocating Compressor
The two-cylinder reciprocating compressor
13-13) has two reciprocating pistons fitted
cylinders. A special valve plate, operated by differential
pressures, is used to control gas flow.