SECTION 22: HEATING AND AIR CONDITIONING
DOB 2490-2789 | X3-45 Commuter PA1605 Maintenance Manual (Final Version – Rev.2 April 2016)
35
The filter-dryer must be replaced after a
severe system failure or if a line in the
system has been opened over a prolonged
period of time. The line will then have to be
properly evacuated.
Best practice would be to replace the filter-
dryer each time a line is opened.
CAUTION
Be careful not to damage the machined faces
while the valve is apart.
7.10 THERMOSTATIC EXPANSION VALVE
7.10.1 Passengers’ Section HVAC Unit
The expansion valve for the passengers’ 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 (Figure 45). 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 underside of the diaphragm
and acting in the closing direction, is the force
exerted by the 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 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 45: 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
because it provides savings in 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 should be
adjusted to give 12°F to 16°F 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
