Carrier NDK33BN0KB, Руководство по эксплуатации и обслуживанию

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62-61753-21 2-20
The receiver stores the additional charge necessary
for low ambient operation. The receiver is equipped
with a relief valve.
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 liquid then flows to an evaporator expansion valve
(EVXV) which reduces the pressure of the liquid and
meters the flow of liquid refrigerant to the evaporator to
obtain maximum use of the evaporator heat transfer
surface.
The refrigerant pressure drop caused by the expan-
sion valve is accompanied by a drop in temperature so
the low pressure, low temperature liquid that flows into
the evaporator tubes is colder than the air that is circu-
lated over the evaporator 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 circulat-
ed throughout the refrigerated compartment to main-
tain 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 the
evaporator outlet temperature and pressure sensors
(EVOT and EVOP) which aid in calculation of super-
heat. The vapor then returns to the compressor
through the compressor suction modulation valve
(CSMV). The CSMV controls the flow of refrigerant
entering the suction line, thereby matching the load to
the compressor capacity.
In the bypass mode, the unloader solenoid valve is
energized (opened) by the microprocessor. This action
reduces compressor capacity by allowing flow from the
mid-point of the scroll, through the economizer con-
nection into the suction line.
b. Standard Mode
The system will operate in Standard Mode when addi-
tional cooling is required but generator current is lim-
ited to the point that economized mode cannot be
supported.
In the standard mode (see Figure 2-10), the system
operates the same as in Bypass Mode except the un-
loader solenoid valve (USV) is de-energized (closed)
by the microprocessor. This action closes off the flow
from the mid-point of the scroll into the suction line,
bringing the compressor to full capacity.
c. Economized Mode
In the economized mode the "main" refrigeration sys-
tem operates the same as in the Standard Mode ex-
cept the microprocessor energizes (opens) the econ-
omizer solenoid valve (ESV). With the economizer
solenoid valve open (see Figure 2-11), the capacity of
the unit is increased by subcooling the liquid refriger-
ant entering the evaporator expansion valve. Overall
efficiency is increased because the gas leaving the
economizer heat exchanger enters the compressor at
a higher pressure, therefore requiring less energy to
compress it to the required condensing conditions.
Liquid refrigerant for use in the economizer circuit is
taken from the main liquid line as it leaves the filter
drier.
The liquid refrigerant flows through the economizer
thermal expansion valve (TXV) and the economizer
heat exchanger internal passages absorbing heat from
the liquid refrigerant flowing to the evaporator expan-
sion valve. The resultant "medium" tempera-
ture/pressure gas enters the compressor at the econ-
omizer connection.
d. Null Mode
The system will enter null mode during periods where
no cooling or heating is required and the system is in
continuous run. In null mode the compressor and
heaters are shut down while the evaporator fans con-
tinue to operate. The condenser fan will operate if
engine cooling is required.
The system may also enter the null mode while in start
stop if required time, coolant temperature or battery
condition parameters have not been met.
2.16 HEAT AND DEFROST
In the Heat and Defrost Modes, the compressor is de-
energized. In Heat Mode, the coil mounted heaters
and evaporator fans are energized. Air flowing over
the warm coil is circulated throughout the refrigerated
compartment to maintain the cargo at the desired tem-
perature.
If the microprocessor is Configured to allow Natural
Defrost and the compartment temperature is above
freezing defrost may be accomplished using a combi-
nation of coil heaters and return air.
If compartment temperature is below freezing or not
sufficiently warm enough to defrost the coil, the evapo-
rator fans are de-energized and the coil mounted
heaters energized.