Section 22: HEATING AND AIR CONDITIONING
PA1553
30
sleeve in the bonnet enters the mating hole
in the body.
4. Insert the four socket head screws and
tighten evenly.
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.
FIGURE 33: REFRIGERANT SOLENOID VALVE 22044
9.8 EXPANSION VALVE
9.8.1 Cabin HVAC Unit
The expansion valve for the cabin HVAC unit is
a thermo-sensitive valve with a remote control
bulb head attached to the evaporator outlet line
and is accessible by the cabin air filters access
door (Fig. 15 & 16). 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.
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
because it provides savings in both initial
evaporator cost of operation. Accurate and
sensitive control of the refrigerant liquid flowing
to the evaporator is necessary to provide
maximum evaporator capacity under load
conditions. The spring is adjusted to give 12 to
16
o
F (-11.1 to -8.8
o
C) of superheat at the
evaporator outlet.
This ensures that the refrigerant leaving the
evaporator is in a completely gaseous state
when drawn into the suction side of the
compressor. Liquid would damage the
compressor valve, piston and heads if allowed
to return in the suction line.
A vapor is said to be superheated when its
temperature is higher than the saturation
temperature corresponding to its pressure. The
amount of the superheat is, of course, the
temperature increase above the saturation
temperature at the existing pressure.
As the refrigerant moves along in the
evaporator, the liquid boils off into a vapor and
the amount of liquid decreases until all the liquid
has evaporated due to the absorption of a
quantity of heat from the surrounding
atmosphere equal to the latent heat of
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