MCi Crusader II 1989 Maintenance Manual page 486

Date
1-1-89
Page __
1_6-_1_6 _ _
MC-9 MAINTENANCE MANUAL
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 underneath side of the diaphragm and
acting in the closing direction, is the force exerted by the
evaporator pressure, and the pressure 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
bulb and power element increases above the combined
pressures of the evaporator pressure and the superheat
spring, causing the valve pin to move in the opening direc-
tion. Conversely, as the temperature of the refrigerant gas
leaving the evaporator decreases, the pressure in the bulb
and power assembly also decreases and the combined eva-
porator 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 most important to adjust the operating superheat.
correctly. A minimum change in superheat required to
move the valve pin to full open position, is of vital impor-
tance because it provides savings in both initial evaporator
costs and cost of operation. Accurate and sensitive control
of the liquid refrigerant flow to the evaporator is necessary
to provide maximum evaporator capacity under all load
conditions.
The spring is adjusted to give 8°-12°F. (5°-7°C.) of
superheat at the evaporator outlet. This ensures the refrig-
erant leaving the evaporator is in a completely gaseous
state when drawn into the suction side of the compressor.
Liquid would damage the compressor valves, pistons and
heads if allowed to return in the suction line.
Vapor is 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 refrigerant gas continues along in the eva-
porator and remains at the same pressure; however, its
temperature increases due to the continued absorption of
heat from the surrounding atmosphere. Tt.e degree to
which the refrigerant gas is superheated is a function of the
amount of refrigerant being fed to the evaporator and the
load to which the evaporator is exposed.
CAUTION: If the expansion valve is sus-
pected of being out
of
adjustment. check
the
A/C systein for a restricted suction line,
plugged filter-dryer or
a
partially open
valve before conducting this procedure.
PRESSURE
GAUGE
BEFORE CHECKING OPEN SYSTEM
Figure 16-19: Gauges Installed
for
Superheat Adjustment
SUPERHEAT ADJUSTMENT - Fittings are provided on
both evaporators to adjust superheat temperature. Before
checking the superheat in either the dr.iver's or the main
evaporator, turn the respective evaporator expansion valve
adjustment control full counterclockwise and then 11 full
turns clockwise. Perform the following procedure to adjust
the superheat setting of the expansion valves:
1. Operate coach engine for at least one-half hour at fast
idle with temperature controls set at 82°F. (28°C.).
2. Refer to figures 16-19 and 16-20. Install a pressure
gauge at the fitting of the evaporator expansion valve. The
hose end with the checkvalve depressor is connected to the
fitting with the valve stem.
3. Install a remote reading thermometer to the evapora-
tor outlet near the existing remote bulb. Refer to the illustra-
tions. Thermostatic tape must be wrapped around the bulb,
evaporator outlet line and gauge probe to get a true reading
of the line temperature.
4. Over an 8-minute operating period, check and record
the following gauge readings:
a. At 2-minute intervals record the pressure reading
and the temperature reading (figure 16-21 ).
b. Record the highest temperature reading (during the
8 minutes) and the lowest temperature reading.
5. Refer to the Pressure-to-Temperature Chart. For each
pressure reading recorded (a
in
step 4), record the equival-
ent temperature. Example: