Air Conditioning Refrigerant Cycle - Carrier 68G5-105 Series Operation & Service Manual

1.8 AIR CONDITIONING REFRIGERANT CYCLE

When air conditioning (cooling) is selected by the main
controller, the unit operates as a vapor compression
system using R-134a as a refrigerant.
components of the system are the reciprocating
compressor, air-cooled condenser coil, subcooler,
filter-drier, thermostatic expansion valves, liquid line
solenoid valves and evaporator coils. (See Figure 1-12)
The compressor raises the pressure and the
temperature of the refrigerant and forces it through the
discharge line, and the check valve into the condenser
tubes. The condenser fan circulates surrounding air
(which is at a temperature lower than the refrigerant)
over the outside of the condenser tubes. Heat transfer is
established from the refrigerant (inside the tubes) to the
condenser air (flowing over the tubes). The condenser
tubes have fins designed to improve the transfer of heat
from the refrigerant gas to the air; this removal of heat
causes the refrigerant to liquefy.
The refrigerant leaves the condenser and then flows
through the subcooler, which subcools the refrigerant
before it enters the thermal expansion valves; this
reduces flash gas in the evaporator.
subcooler, the liquid refrigerant passes through the
liquid line (King) service valve, and then through a
filter-drier where an absorbent keeps the refrigerant
clean and free of water.
From the filter-drier, the liquid refrigerant then flows
through the main liquid line solenoid valve to the main
evaporator and thermal expansion valve and to the
T-312
parcel rack evaporator solenoid valve (if this option is
active) the parcel rack thermal expansion valve and
through the driver solenoid valve (if this option is active)
and to the driver thermal expansion valve. The solenoid
The main
valves open during cooling to allow refrigerant to flow to
the thermal expansion valves. The main liquid solenoid
valve and the parcel rack liquid line solenoid valve
(when so equipped) closes during shutdown to prevent
refrigerant migration. The thermal expansion valves
reduce the pressure and temperature of the liquid and
meters the flow of liquid refrigerant to the evaporator to
obtain maximum use of the evaporator heat transfer
surface.
The low pressure, low temperature liquid that flows into
the evaporator tubes is colder than the air that is
circulated over the evaporator tubes by the evaporator
blower (fan). Heat transfer is established from the
evaporator air (flowing over the tubes) to the refrigerant
(flowing inside the tubes). The evaporator tubes have
aluminum fins to increase heat transfer from the air to
the refrigerant; therefore the cooler air is circulated to
the interior of the coach.
The transfer of heat from the air to the low temperature
From the
liquid refrigerant in the evaporator causes the liquid to
vaporize. This low temperature, low pressure vapor
passes through the suction line and returns to the
compressor where the cycle repeats.
When ventilation only is selected by the main controller,
only the evaporator fans function to circulate air
throughout the coach. The refrigerant cycle will remain
off.
1-12