T-334
4--9
Defrost may be initiated any time the defrost
temperature sensor reading falls below the controller
defrost termination thermostat set point. Defrost will
terminate when the defrost temperature sensor reading
rises above the defrost termination thermostat set point.
The defrost termination thermostat is not a physical
component. It is a controller setting that acts as a
thermostat, “closing” (allowing defrost) when the defrost
temperature sensor reading is below the set point and
“opening” (terminating or preventing defrost) when the
sensor temperature reading is above set point. When
the unit is operating in bulb mode (refer to paragraph
3.3.10), special settings may be applicable.
If the controller is programmed with the Lower DTT
setting option the defrost termination thermostat set
point may be configured to the default of 25.6C (78F) or
lowered to 18C (64F). When a request for defrost is
made by use of the manual defrost switch,
communications or probe check the unit will enter
defrost if the defrost temperature thermostat reading is
at or below the defrost termination thermostat setting.
Defrost will terminate when the defrost temperature
sensor reading rises above the defrost termination
thermostat setting. When a request for defrost is made
by the defrost intermale timer or by demand defrost, the
defrost temperature setting setting must be below 10C
(50F).
When the defrost mode is initiated the controller opens
contacts TC, TN and TE (or TV) to de-energize the
compressor, condenser fan and evaporator fans. The
COOL light is also de-energized.
The controller then closes contacts TH to supply power
to the heaters. The defrost light is illuminated.
When the defrost temperature sensor reading rises to
the defrost termination thermostat setting, the de-icing
operation is terminated.
If defrost does not terminate correctly and temperature
reaches the set point of the heat termination thermostat
(HTT) the thermostat will open to de-energize the
heaters. If termination does not occur within 2.0 hours,
the controller will terminate defrost. An alarm will be
given of a possible DTS failure.
If probe check (controller function code CnF31) is
configured to special, the unit will proceed to the next
operation (snap freeze or terminate defrost). If the code
is configured to standard, the unit will perform a probe
check. The purpose of the probe check is to detect
malfunctions or drift in the sensed temperature that is
too small to be detected by the normal sensor out of
range tests. The system will run for eight minutes in this
condition. At the end of the eight minutes, probe alarms
will be set or cleared based on the conditions seen.
When the return air falls to 7C (45F), the controller
checks to ensure the defrost temperature sensor (DTS)
reading has dropped to 10C or below. If it has not, a DTS
failure alarm is given and the defrost mode is operated
by the return temperature sensor (RTS).
If controller function code CnF33 is configured to snap
freeze, the controller will sequence to this operation.
The snap freeze consists of running the compressor
without the evaporator fans in operation for a period of
four minutes with the suction modulation valve fully
open. When the snap freeze is completed, defrost is
formally terminated.
4.10.6 Emergency Bypass Operation
To place the unit in the emergency bypass mode, cut the
wire tie installed at the switch mounting and place the
EMERGENCY BYPASS switch in the ON position
which will in turn activate the Emergency Bypass
System (EBS) control module.
To operate the fans only the MODE switch must be in
the FANS ONLY position and the EMERGENCY
BYPASS Switch must be in the ON position.
The EBS module uses the system’s safety devices (high
pressure switch, motor internal protectors, and heat
termination thermostat) to protect the system while in
Emergency Bypass Mode.
CAUTION
The unit will remain in the full cooling mode
as long as the emergency bypass switch is
in the BYPASS position and the MODE
SWITCH is in the FULL COOL position. If
the cargo may be damaged by low tempera-
tures, the operator must monitor container
temperature and manually cycle operation
as required to maintain temperature within
required limits.
In the ON position the EBS will be enabled. With the
MODE SWITCH in the FULL COOL MODE. The
following will occur simultaneously:
1. The EBS switch will enable EBS input.
2. The phase detection circuit will detect the phase
rotation and close to provide power to the compres-
sor contactor.
3. The condenser fan contact will close to energize the
condenser contactor and provide power to the con-
denser fan motor.
4. The evaporator fan contact will close to energize the
high speed evaporator contactor and provide power
to the evaporator fan motor.
5. The EBS electronic module will open the SMV to
100%.
To return the unit to normal operation, place the EBS
switch in the NORMAL OPERATION position. When
emergency operation is no longer required, re-install the
wire tie at the switch mounting.
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