PREVOST X3-45 2016 Maintenance Manual Download

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Section 22: HEATING AND AIR CONDITIONING

PA1593

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

F (-11.1 to -8.8

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. 34).

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

(27,7

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.

FIGURE 34: SUPERHEAT ADJUSTMENT INSTALLATION

22046

3. Install a remote reading thermometer to the

evaporator outlet line near the existing
remote bulb (Fig. 34).

4. Apply thermostatic tape around the bulb

and evaporator outlet line to get a true
reading of the line temperature.

5. Block condenser if necessary to keep

pressure over 150 psi.

FIGURE 35: HIGH & LOW SWING TEMPERATURE AT
REMOTE BULB

22047

6. Check approximately 5 readings of

pressure at 2-minute intervals and convert
to temperature using the temperatures &
pressures table (page 35). Likewise check
the temperature reading at the remote bulb
at the same 2-minute intervals and record
the low and high swing readings of the
needle (refer to Fig. 35).

PREVOST X3-45 2016, Maintenance Manual

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