Thermo King UT-1200, Manual

Page: 36 / 169

Unit Description
36
Heat/Defrost Mode
The refrigerant flow in Heat/Defrost Mode is a
conventional hot gas heat cycle. The CIS is closed
and the HGS is open so high pressure refrigerant
vapor leaves the compressor and flows through
the DPR (which maintains a minimum discharge
pressure) and the HGS to the hot has line. Most of
the refrigerant flows through the hot gas line and
the drain pan heater to the evaporator. There, the
refrigerant heats the evaporator and returns to the
compressor through the heat exchanger,
accumulator, and SPR (or ETV). Some of the
refrigerant flows through the receiver tank
pressure check valve and forces any liquid
refrigerant out of receiver tank through the drier,
heat exchanger, and expansion valve to the
evaporator. From which it returns to the
compressor through the heat exchanger, and
accumulator.
Solenoid and Check Valves
1. Condenser Inlet Solenoid (CIS)-Closed/
Energized
2. Liquid Injection Solenoid (LIS)-Closed/
De-energized but the microprocessor will
open it if necessary to control compressor
temperature.
3. Condenser Check Valve-Closed
4. Receiver Tank Pressure Check Valve-Open
5. Purge Valve (PV)-Open/Energized
6. Purge Check Valve-Closed
7. Hot Gas Solenoid (HGS)-Open/Energized
Figure 9: Heat/Defrost Mode
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