Thermo King MD-MT, Operator'S Manual

Page: 27 / 192

Unit Description
27
Unit Operation
Several operating characteristics of the system
during engine operation are:
• Both evaporators can Cool, Heat, or Defrost at
the same time.
• One evaporator can Cool, Heat, or Defrost
while the other evaporator cools, Heats, or
Defrosts.
• An evaporator will shift to null if its
thermostat calls for Low Speed Heat. When
the evaporator is in null, it does not heat or
cool and the fans do not run.
• If a thermostat is set below Heat lockout, its
evaporator will not heat.
• If a thermostat calls for high speed, it will
force the condensing unit to run in high speed.
• If one evaporator is in Heat or Defrost while
the other evaporator is in Cool, the condenser
inlet solenoid will be closed and the
evaporator that is in Heat or Defrost will act as
the condenser.
• The condenser inlet solenoid and the suction
line solenoids are normally open solenoid
valves that close when energized.
• The condenser inlet solenoid is energized
(closed) whenever either evaporator is in Heat
or Defrost.
• The liquid line solenoids, the hot gas
solenoids, the receiver tank pressure solenoid,
and the purge valve are normally closed
solenoid valves that open when energized.
• The flow of refrigerant through the system is
controlled by the solenoid valves and the
check valves.
• The solenoid circuits are switched by the
thermostat and defrost relays.
• The receiver tank pressure solenoid is used to
control the discharge pressure when the
condenser inlet solenoid is closed. The
receiver tank pressure solenoid is controlled
by the receiver tank pressure cutout switch.
• The purge valve is used to increase the heating
capacity of the unit by making more
refrigerant available when either evaporator is
in Heat or Defrost. The purge valve is
energized along with the receiver tank
pressure solenoid. This allows any liquid
refrigerant that is trapped in the condenser to
flow into the accumulator where it can be used
in Heat or Defrost.
During electric operation, the operating
characteristics of the system are basically the
same with the following exception:
• If the Zone 1 and the Zone 2 thermostats are
both calling for Low Speed Heat, the
condensing unit will shift to null, the electric
motor will stop, and the unit will not operate.
• The condensing unit will stay in null until one
(or both) of the thermostats call for high speed
or until Defrost is initiated in one of the
evaporators.
The mode of operation charts show the modes of
operation that result from various combinations of
thermostat and defrost demands.