United Technologies Carrier TRANSICOLD 68RM35-604-20, Операция и обслуживание

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1-9 T-288
Boost Pump Relay (BPR)
The boost pump relay (BPR) is plugged into the
relay board on the electrical control panel located
in the rear compartment of the unit. (See
Figure 1-3.) This relay is energized during the
reheat and heating modes. When the Boost Pump
Relay (BPR) is energized, a set of internal
contacts will close to activate the Water Pump
Relay (WPR), which is O.E.M. supplied.
1.6 MOISTURE/LIQUID INDICATOR
The moisture/liquid indicator is located on the
receiver. The element in the indicator is highly
sensitive to moisture and will gradually change
color in direct relation to an increase or decrease
in the moisture content of the system. The
dry-caution-wet system operating conditions are
then easily determined by matching the element
color with the two colors displayed on the
reference label. Colors change as often as the
system moisture content changes.
1.7 AIR CONDITIONING REFRIGERANT CYCLE
The refrigerant cycle is off during the vent mode
and only the evaporator blowers operate to
circulate air throughout the bus. When thermostat
is calling for cooling, the unit operates as a vapor
compression
system using R-134a as a
refrigerant. The main components of the system
are the reciprocating compressor, air-cooled
condenser coil, thermostatic expansion valve, and
evaporator coil. (See Figure 1-5.)
The refrigeration cycle begins when the
compressor clutch is engaged. The compressor
raises the pressure and the temperature of the
refrigerant and forces it into the condenser coil
tubes. The condenser fan circulates surrounding
air (which is at a temperature lower than the
refrigerant) over the outside of the condenser coil
tubes. Heat transfer is established from the
refrigerant (inside the tubes) to the condenser air
(flowing over the outside of the tubes). The
condenser coil 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; thus liquid refrigerant leaves
the condenser coil 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 coil as needed and as
a storage space when pumping down the system.
The receiver is equipped with a sight glass, to
observe
correct charge level, and a
moisture/liquid indicator.
The liquid refrigerant leaves the receiver and flows
through the subcooler where it is subcooled
before entering the liquid-suction heat exchanger.
The liquid-suction heat exchanger (LSHX), used
on R-134a systems only, increases system
operating
efficiency by subcooling liquid
refrigerant before it enters the thermostatic
expansion valve; this reduces flash gas. It also
serves to ensure there is no liquid refrigerant in the
suction gas returning to the compressor. If there is
low temperature liquid refrigerant present in the
suction line returning to the compressor, it will
evaporate in the heat exchanger from heat
absorbed from the liquid line.
The refrigerant leaves the LSHX and flows
through the filter-drier inlet valve and through a
filter-drier
where an absorbent keeps the
refrigerant clean and dry. It then flows out of the
filter-drier and through the filter-drier outlet valve
and liquid line solenoid valve.
The liquid then flows to an externally equalized
thermostatic expansion valve which reduces
pressure and temperature of the liquid and meters
the flow of liquid refrigerant to the evaporator coil
to obtain maximum use of the evaporator coil heat
transfer surface.
The low pressure, low temperature mixture of
liquid and vapor refrigerant that flows into the
evaporator coil tubes is colder than the air that is
circulated over the evaporator coil tubes by the
evaporator blower. Heat transfer is established
from the evaporator air (flowing over the tubes) to
the refrigerant (inside the tubes). The evaporator
coil tubes have aluminum fins to increase heat
transfer from the air to the refrigerant. The cooler
air is then circulated to the interior of the bus by the
evaporator blowers.
The transfer of heat from the air to the low
temperature liquid refrigerant in the evaporator
coil causes the liquid to vaporize. This low
temperature, low pressure vapor passes through
the liquid-suction heat exchanger and the suction
line back to the compressor.
The low pressure refrigerant vapor is now drawn
into the compressor where the cycle repeats.