Section 22: HEATING AND AIR CONDITIONING
PA1562
27
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 32: 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 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
vaporization of the refrigerant. The gas
continues along in the evaporator and remains
at the same pressure. However, its temperature
increases due to the continued absorption of
heat from the surrounding atmosphere. The
degree to which the gas refrigerant is
superheated is related to the amount of
refrigerant being fed to the evaporator and the
load to which the evaporator is exposed.
Superheat Adjustment
The starting method of adjusting the superheat
is to unscrew completely the main evaporator
expansion valve adjusting screw, then screw in
13 turns clockwise for 134A (Fig. 33).
Afterwards, the following procedure should be
followed:
1. Operate coach for at least one-half hour at
fast idle with temperature control set at 82
o
F
(27,7
o
C), Then set temperature to minimum
to keep the compressor on 6 cylinders.
2. Install pressure gauge at the evaporator
suction header. You may install the
pressure gauge at compressor suction, but
then add 3 psi to reading.
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