Safety Devices; Air Conditioning Refrigeration Cycle - Carrier EM-17 Operation And Service

1.9 SAFETY DEVICES

System components are protected from damage
caused by unsafe operating conditions with safety
devices. Safety devices with Carrier Transicold supplied
equipment include high pressure switch (HPS), low
pressure switch (LPS), circuit breakers and fuses.
a. Pressure Switches
High Pressure Switch (HPS)
During the A/C mode, compressor operation will
automatically stop if the HPS switch contacts open due
to an unsafe operating condition. Opening HPS
contacts de--energizes the compressor clutch, shutting
down the compressor. The high pressure switch (HPS)
is installed in the center head of the compressor.
Low Pressure Switch (LPS)
The low pressure switch is installed in the compressor
and opens on a pressure drop to shut down the
compressor clutch when a low pressure condition
occurs.
b. Ambient Lockout
The ambient temperature sensor located in the
condenser section measures the condenser inlet air
temperature. When the temperature has fallen below
the cut out set point the compressor is locked out until
the temperature rises above the cut in setting. The set
point will be to cut out at 50°F ± 3° and cut in at 60°F
±3°. This setting protects the compressor from
damage caused by operation in low ambient conditions.
1.10 AIR CONDITIONING REFRIGERATION
CYCLE
When air conditioning (cooling) is selected by the
thermostat, the unit operates as a vapor compression
system using R--134a as the refrigerant (See
Figure 1--4 OR Figure 1--5). The main components of
the system are the reciprocating compressor, air--
cooled condenser coils,
thermostatic expansion valve, liquid line solenoid valve
and evaporator coils.
The compressor raises the pressure and the
temperature of the refrigerant and forces it through the
discharge check valve into the condenser tubes. The
T--336
receiver, filter--drier,
1--6
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 condense, thus liquid refrigerant leaves
the condenser and flows to the receiver.
The receiver serves as a liquid refrigerant reservoir so
that a constant supply of liquid is available to the
evaporator as needed, and acts as a storage space
when pumping down the system.
The refrigerant leaves the receiver and flows through
the filter--drier where an absorbent keeps the refrigerant
clean and dry. The liquid line may be equipped with a
sight glass to observe the refrigerant for restricted flow
and the correct charge level.
From the filter--drier, the liquid refrigerant then flows
through the liquid line solenoid valve towards the
thermostatic expansion valve. For the CM--5 condenser
this flow is a direct path. For the KR--4 condenser, the
refrigerant also passes through a subcooling circuit in
the leaving condenser. The thermal expansion valve
reduces pressure and temperature of the liquid and
meters the flow of liquid (saturated mix) refrigerant to
the evaporator to obtain maximum use of the evaporator
heat transfer surface.
The low pressure, low temperature liquid (saturated
mix) that flows into the evaporator tubes is colder than
the air that is circulated over the evaporator tubes by the
evaporator fans. 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 vehicle. Liquid line solenoid valve
closes during shutdown to prevent refrigerant migration
to the compressor.
The transfer of heat from the air to the low temperature
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.
05/09