Bleeding Heating System; Cooling; Refrigeration Cycle; Refrigerant - PREVOST H3-41 Maintenance Manual

6.4.5

Bleeding Heating System

Whenever the heating system has been drained
and refilled, or the system has run low on
coolant and coolant has been added, it is
necessary to bleed air from heating system.
Locate the purge valve illustrated in Figure 74 &
Figure 79, and open them momentarily until no
air escapes from the lines. If present, open the
manual vent located inside the HVAC unit, on
the driver's side (Figure 73).
NOTE
In test mode (see paragraph 5.4 TEST MODE
FOR SWITCHES AND SENSORS), with the
parking brake applied and the passenger set
point set to a value higher than 64°F (18°C),
the hot water circulating pump is not set to
OFF as it would normally do when the outside
temperature gets above 50°F (10°C). This
feature is useful when working on the heating
system to remove air pockets trapped in the
system.
6.5

COOLING

The central system is equipped with 4-cylinder
4NFCY Bitzer compressor
conditioning capacity of 7½ tons. The receiver
tank and filter-dryer are mounted inside the
condenser compartment.
The schematics of FIGURE 4 and FIGURE 7
show the central HVAC system's cooling function
arrangement.
6.5.1

Refrigeration Cycle

Refrigeration may be defined as the transfer of
heat from a place where it is not wanted to a
place where it is unobjectionable. The air
conditioning system used is the "closed" type
using R134a refrigerant.
1. The refrigerant flowing to the compressor is
compressed to high pressure and reaches a
temperature higher than the surrounding air.
It is passed through the air-cooled fins and
tubes of the condenser causing the hot,
high pressure gas to be condensed into a
liquid form.
2. The liquid refrigerant leaves the condenser
then, flows to the receiver tank.
SECTION 22: HEATING AND AIR CONDITIONING
with an air
PA1621 Maintenance Manual All Series | Revised February 2019
H3 Series only: The liquid refrigerant
3.
leaves the receiver tank and then flows
back to the condenser for sub-cooling and
return to the receiver tank.
4. The liquid refrigerant leaves the receiver
tank and passes through a filter-dryer where
moisture, acids and dirt are removed and
then through a moisture indicator which
indicates if any moisture is present in the
system.
5. By its own pressure, the liquid refrigerant
flows through a thermal expansion valve
where the pressure drop causes the
refrigerant to vaporize in a vapor-liquid state
at a low temperature pressure.
6. The cold low pressure refrigerant passes
through the passenger's and driver's unit
evaporator coils which absorb heat from the
air passing over the fins and tubes, and
changes into gas.
refrigerant is drawn into the compressor to
repeat the air conditioning cycle.
7. The success of the air-conditioning system
depends on retaining the conditioned air
within the vehicle. All windows and intake
vents should be closed. An opening of
approximately 8 in
2
neutralize the total capacity of the system.
8. Other causes of inadequate cooling are di
Series only:rty coils or filter. Dirt acts as
insulation and is also serves as a restriction
to the air flow.
9. The refrigeration load is not constant and
varies. It is also affected by outside
temperature, relative humidity, passenger
load, compressor speed, the number of
stops, etc.
The compressor will
operating conditions.
6.5.2

Refrigerant

The A/C system of this vehicle has been
designed to use R134a refrigerant as a medium.
Regardless of the brand, only R134a must be
used in this system. The chemical name for this
refrigerant is 1,1,1,2-Tetrafluoroethane.
WARNING
Refrigerant in itself is nonflammable, but if it
comes in contact with an open flame, it will
In this form, the
(5162 mm ) could easily
2
load depending on
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