62-61753-21
2.4.10 Ambient Air Temperature Sensor (AAT)
The AAT provides microprocessor with condenser
entering air temperature information to be displayed,
recorded in the Data Recorder and used to control the
refrigeration system. The sensor is located behind the
front grill.
2.4.11 Filter Drier
The drier is a cylinder shell containing a drying agent
and screen. It is installed in the liquid line and func-
tions to keep the system clean and remove moisture
from the refrigerant.
2.4.12 Receiver
Liquid refrigerant from the condenser drains into the
receiver. The receiver serves as a liquid reservoir
when there are surges due to load changes in the
system; as a storage space when pumping down the
system and as a liquid seal against the entrance of
refrigerant gas into the liquid line. The receiver is pro-
vided with two bull’s-eye sight glasses, for the obser-
vation of liquid level & moisture content, and a pres-
sure relief valve.
2.4.13 Economizer Circuit
The economizer circuit consist of a compact braze
plate heat exchanger, a solenoid valve (ESV) and a
thermal expansion valve (TXV). It is located above the
compressor.
In the economized mode, the frozen and pull down
capacity of the unit is increased by subcooling the
liquid refrigerant entering the evaporator expansion
valve. Overall efficiency is increased because the gas
leaving the economizer enters the compressor at a
higher pressure, therefore requiring less energy to
compress it to the required condensing conditions.
To place the unit in the economized mode, the micro-
processor energizes (opens) the economizer solenoid
valve (ESV). Refrigerant flows from the main liquid
line, through the ESV to the economizer thermal ex-
pansion valve (TXV). The TXV controls the flow of
liquid to the economizer heat exchanger according to
changes in superheat of the refrigerant leaving the
economizer heat exchanger.
2.5 EVAPORATOR SECTION
The evaporator (see Figure 2-3) fits into a rectangular
opening in the upper portion of the trailer front wall.
When installed, the evaporator section is located in-
side this refrigerated compartment, and the condens-
ing section is outside.
The evaporator assembly consists of an evaporator
coil, evaporator fan motors, evaporator expansion
valve, evaporator coil & drain pan heaters, defrost
termination temperature sensor, supply & return air
sensors and a high temperature switch. Evaporator
equipment also includes the suction modulating valve,
located in the condensing section.
Heating is accomplished by electric heaters mounted
on the evaporator coil.
Automatic evaporator coil defrosting is initiated by
either a differential air switch (which senses the pres-
sure drop across the coil) or by the defrost timer in the
microprocessor.
2.5.1 Evaporator Coil
The unit evaporator is a tube in fin type. The operation
of the compressor maintains a reduced pressure within
the coil. At this reduced pressure, the liquid refrigerant
evaporates at a temperature sufficiently low enough to
absorb heat from the air.
2.5.2 Evaporator Expansion Valve (EVXV)
The EVXV is an electronic device which controls the
flow of liquid to the evaporator according to changes in
superheat of the refrigerant leaving the evaporator.
The expansion valve maintains a relatively constant
degree of superheat in the gas leaving the evaporator
regardless of suction pressure. The valve has a dual
function - automatic expansion control and prevention
of liquid return to the compressor.
2.5.3 Compressor Suction Modulation Valve
(CSMV)
The CSMV opens and closes as required for capacity
control of the refrigeration system cooling cycle. It is
located in the suction line at the exit of the evaporator
assembly.
2.5.4 Evaporator Switches, Transducers and
Sensors
a. Evaporator Outlet Pressure Transducer (EVOP)
The EVOP provides the microprocessor with evapora-
tor outlet pressure information to be displayed, record-
ed in the Data Recorder and used to control the refrig-
eration system. It is located in the evaporator section
near the evaporator expansion valve in the suction
line.
b. Evaporator High Temperature Switch (EVHTS)
EVHTS is a safety switch which opens on a tempera-
ture rise to de-energize the electric heaters if the tem-
perature in the evaporator compartment reaches the
cut-out point. It is located on the center tube sheet of
the evaporator coil
c. Evaporator Outlet Temperature Sensor (EVOT)
The EVOT provides the microprocessor with evapora-
tor outlet temperature information to be displayed,
recorded in the Data Recorder and used to control the
refrigeration system.
It is located on the suction line near the evaporator
expansion valve.
d. Defrost Termination Temperature Sensor (DTT)
The DTT is located on the center tube sheet of the
evaporator coil. It provides the microprocessor with
evaporator temperature information to be displayed,
recorded in the Data Recorder and used to determine
termination of defrost.
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