1-10
62-10630
1.7 REFRIGERANT CIRCUIT DURING COOLING
When cooling (See Figure 1-7), the unit operates as a
vapor compression refrigeration system. The main
components of the system are the reciprocating
compressor,
air-cooled
condenser,
thermostatic
expansion valve and direct expansion evaporator.
The compressor raises the pressure and temperature of
the refrigerant and forces it through the discharge check
valve and condenser pressure control valve into the
condenser tubes. The discharge check valves prevent
reverse flow through the non operating compressor.
When operating on the road compressor, the flow also
passes through the oil separator where oil is removed
and returned to the compressors.
The condenser fan circulates surrounding air over the
outside of the condenser tubes. Heat transfer is thus
established from the refrigerant gas (inside the tubes) to
the condenser air (flowing over the tubes). The
condenser tubes have fins designed to improve the
transfer of heat. This removal of heat causes the
refrigerant to liquefy; liquid refrigerant flows from the
condenser to the receiver.
The receiver stores any excess charge due to surges in
load on the system and stores refrigerant during low
side service.
The refrigerant then flows through the subcooler,
filter-drier, sight glass, liquid line check valve (option for
40S) and liquid / gas heat exchanger to the thermostatic
expansion valve.
The subcooler and heat exchanger remove additional
heat form the liquid to provide improvement in system
efficiency. The filter--drier contains an absorbent and
filter screen that keeps the refrigerant dry and clean.
The sight glass is fitted with an indicator that changes
color to indicate moisture content of the refrigerant. The
check valve serves to prevent reverse flow of refrigerant
during the heating/defrost cycle.
The liquid than enters the thermostatic expansion valve
(with external pressure equalizer) which regulates the
flow rate of refrigerant towards the evaporator in order to
obtain maximum use of the evaporator heat transfer
surface.
The evaporator tubes have aluminium fins to increase
heat transfer; therefore heat is removed from the air
circulated through the evaporator. This cold air is
circulated throughout the truck to maintain the cargo at
the desired temperature.
The transfer of heat from the air to the low temperature
liquid refrigerant causes the liquid to vaporize. The
vapor at low temperature and pressure passes back
through the heat exchanger then enters the compressor
pressure regulating valve (CPR) which regulates
refrigerant pressure entering the compressor to prevent
overloading of the vehicle engine. From this point, the
cycle starts over.
The quench valve (BPV) opens as required to maintain
a maximum discharge temperature of 127
_
C (260
°
F).
1.8 REFRIGERANT CIRCUIT DURING HEAT AND
DEFROST
When refrigerant vapor is compressed to a high
pressure and temperature in a compressor, the
mechanical energy necessary to operate the
compressor is transferred to the gas as it is being
compressed. This energy is referred to as the ”heat of
compression ” and is used as the source of heat during
the heating or defrost cycle (See Figure 1-8).
The main difference between heating and defrosting is
that, when in heating mode the evaporator fans continue
to run circulating the air throughout the truck to heat the
product. When defrosting, the evaporator fans stop,
allowing the heated vapor to defrost any ice build-up on
the coil.
When heating or defrost is required, the hot gas
solenoid valve is energized (opened) to allow heated
refrigerant vapor to flow directly to the evaporator coil.
The system is fitted with a condenser pressure control
valve (see Figure 1-8). The valve closes when pressure
is above the setting of the condenser pressure control
switch to prevent additional pressure rise in the system.
When pressure is below the setting of the condenser
pressure control switch, the valve is opened to
pressurize the receiver and force additional refrigerant
into the system and increase heating capacity.