Refrigerant Circuit During Cooling; Refrigerant Circuit - Heating And Defrosting - Carrier VECTOR 6500 Operation & Service Manual

2.14 REFRIGERANT CIRCUIT DURING COOLING

When cooling, (See Figure 2−14) the unit operates as a
vapor compression refrigeration system. The main
components of the system are: the (1) reciprocating
compressor, (2) air-cooled condenser, (3) expansion
valves, and (4) direct expansion evaporator.
The compressor raises the pressure and the tempera-
ture of the refrigerant and forces it into the condenser
channels. The condenser fans circulate surrounding air
over the outside of the channels. The channels have fins
designed to improve the transfer of heat from the refrig-
erant gas to the air. This removal of heat causes the re-
frigerant to liquify. Liquid refrigerant leaves the condens-
er and flows to the receiver.
The receiver stores the additional charge necessary for
low ambient operation. The receiver is equipped with a
fusible plug which melts on occurrence of very high tem-
perature to relieve the refrigerant pressure.
The refrigerant leaves the receiver and flows through
the liquid line service valve to the subcooler. The sub-
cooler occupies a portion of the main condensing coil
surface and gives off further heat to the passing air.
The refrigerant then flows through a filter-drier where an
absorbent keeps the refrigerant clean and dry.
The refrigerant flows to the suction line heat exchanger.
Here the liquid is further reduced in temperature by giv-
ing off some of its heat to the suction gas.
The liquid then flows to an electronic expansion valve
which reduces the pressure of the liquid and meters the
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flow of liquid refrigerant to the evaporator to obtain maxi-
mum use of the evaporator heat transfer surface.
The refrigerant pressure drop caused by the expansion
valve is accompanied by a drop in temperature so the
low pressure, low temperature fluid that flows into the
evaporator tubes is colder than the air that is circulated
over the tubes by the evaporator fan. The evaporator
tubes have aluminum fins to increase heat transfer;
therefore heat is removed from the air circulated over
the evaporator. This cold air is circulated throughout the
refrigerated compartment 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.
This low temperature, low pressure vapor passes
through the evaporator outlet. The vapor then passes
through the heat exchanger where it absorbs more heat
from the high pressure / high temperature liquid and
then returns to the compressor through the compressor
suction modulation valve (CSMV). The CSMV controls
the compressor suction pressure thereby matching the
compressor capacity to the load.
2.15 REFRIGERANT CIRCUIT - HEATING AND DE-
FROSTING
The compressor is turned off for all heating and defrost
cycles. In heat, the coil mounted heaters and evaporator
fan(s) are energized. Air flowing over the warm heater
elements is circulated through the compartment to
maintain the cargo at the desired temperature. Defrost
may be accomplished using either electric heaters or re-
turn air (refer to Section 4.6).
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