Thermo King SB-210, Manual

Page: 27 / 159

Unit Description
27
Operating Modes
The microprocessor uses a complex program to
determine which operating mode the unit should
be in. Therefore, it is difficult to predict which
operating mode the unit should be in by
comparing the setpoint to the box temperature.
In diesel operation the diesel engine operates at
either low speed or high speed as determined by
the microprocessor. The unit will Cool or Heat in
either high or low speed. The unit will Defrost in
low speed only. Heat and Defrost consists of hot
gas delivered to the evaporator coil distributor.
In electric operation (Model 50 units only) the
drive motor operates at a single speed. It does not
run in Null. Electric evaporator heaters are also
used to increase the Heat and Defrost capacities
during electric operation.
Diesel Operation
In diesel operation the microprocessor will select
the operating mode from the following:
• High Speed Cool
• Low Speed Cool
• Low Speed Modulated Cool (ETV units only)
• Null (CYCLE-SENTRY operation only)
• Low Speed Modulated Heat (ETV units only)
• Low Speed Heat
• High Speed Heat
• Defrost
Electric Operation
In electric operation the microprocessor will
select the operating mode from the following:
• Cool
• Modulated Cool
• Null (CYCLE-SENTRY operation only)
• Modulated Heat (Hot Gas only)
• Hot Gas Heat
• Full Heat (Hot Gas and Electric Heat)
• Defrost (Hot Gas and Electric Heat)
Defrost
Frost gradually builds-up on evaporator coils as a
result of normal operation. The unit uses hot
refrigerant to defrost the evaporator coils. Hot
refrigerant gas passes through the evaporator coil
and melts the frost. The water flows through
collection drain tubes onto the ground. The
methods of Defrost initiation are Automatic, and
Manual.
Automatic Defrost:
The controller is programmed
to automatically initiate timed or demand defrost
cycles. The unit can enter defrost cycles as often
as every 30 minutes if required.
Manual Defrost:
In Manual Defrost Mode, the
operator initiates a defrost cycle.
NOTE: The unit will not perform a Manual
Defrost cycle unless the unit has been turned on
with the
O
N
key, the unit is running in
Continuous or CYCLE-SENTRY mode (or shut
down in CYCLE-SENTRY Null mode), and the
coil temperature is below 45 F (7 C).
Defrost is initiated manually using the
microprocessor
D
EFROST
key. Defrost is initiated
automatically on demand by the microprocessor
or by a defrost timer.
The evaporator coil temperature must be below
45 F (7 C) to allow defrost. A Demand Defrost
cycle will occur if the differences between the
return air temperature, discharge air temperature,
and coil temperature are greater than
predetermined values.
Two defrost timers are used. When the unit is
In-Range (within a few degrees of setpoint),
Defrost intervals are controlled by the Defrost
Interval In-Range timer (DEFI). This timer can be
set for 4, 6, 8 or 12 hours. The standard setting is 6
hours. When the unit is not In-Range, Defrost
intervals are determined by the Defrost Interval
Not In-Range timer (DEFN). This timer can be set
for 2, 4, 6, 8 or 12 hours. The standard setting is 4
hours. This feature allows a shorter Defrost
interval to be used when the unit is out of range
during a pull-down and more frequent Defrost
cycles may be beneficial.