Section 22: HEATING AND AIR CONDITIONING
PA1593
27
FIGURE 32: REFRIGERANT SOLENOID VALVE 22044
5. Replace the coil as stated previously.
6. Add a small quantity of refrigerant R-134a to
the low side of the system. Check for leaks.
Return the system to normal operation.
7.8 EXPANSION VALVE
7.8.1 Passenger’s Section HVAC Unit
The expansion valve for the passenger’s section
HVAC unit is a thermo-sensitive valve with a
remote control bulb head attached to the
evaporator outlet line and is accessible by the
evaporator coil access door (Fig. 16 & 33). The
valve regulates the flow of refrigerant liquid into
the evaporator coils and is controlled by the
suction gas temperature leaving the evaporator.
The bulb head senses the refrigerant gas
temperature as it leaves the evaporator. High
temperature will cause expansion and pressure
on the power head and spring. Such action
causes the assembly valve to open, allowing a
flow of refrigerant liquid into the evaporator.
The remote bulb and power assembly is a
closed system. The pressure within the remote
bulb and power assembly corresponds to the
saturation pressure of the refrigerant
temperature leaving the evaporator and moves
the valve pin in the opening direction. Opposed
to this force, on the under side of the diaphragm
and acting in the closing direction, is the force
exerted by the superheat spring.
As the temperature of the refrigerant gas at the
evaporator outlet increases above the saturation
temperature corresponding to the evaporator
pressure, it becomes superheated. The
pressure thus generated in the remote bulb and
power assembly surpasses the combined
pressures of the evaporator pressure and the
superheat spring, causing the valve pin to move
in the opening direction. Conversely, as the
temperature of the refrigerant gas leaving the
evaporator decreases, the pressure in the
remote bulb and power assembly also
decreases and the combined evaporator and
spring pressures cause the valve pin to move in
the closing position.
FIGURE 33: EXPANSION VALVE 22045
As the operating superheat is raised, the
evaporator capacity decreases, since more of
the evaporator surface is required to produce
the superheat necessary to open the valve. It is
obvious, then, that it is most important to adjust
the operating superheat correctly and that a
minimum change in superheat to move the valve
pin to full open position, is of vital importance
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