Thermo King SB-190 30, Manual

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Unit Description (Rev. 12/03)
11
Refrigeration Modulation System
The ETV system provides precise control of the refrigera-
tion system and the temperature of the cargo area. As the
temperature of the return air begins to approach the setpoint,
the microprocessor begins to close the ETV valve in the
suction line between the evaporator and the heat exchanger.
The microprocessor closes the ETV valve more as the
return air temperature gets closer to the setpoint. When the
temperature is near setpoint, the ETV valve is closed to its
limit and the hot gas bypass valve is opened. When the tem-
perature begins to move away from the setpoint, the ETV
valve begins to open. The hot gas bypass valve remains
open until the ETV valve is completely open, then the hot
gas bypass valve closes. This provides very smooth and
steady temperature control and the temperature does not
oscillate above and below setpoint as much as it does in a
unit that does not have modulation.
Electronic Throttling Valve (ETV) (Optional)
The optional Electronic Throttling Valve (ETV) is a vari-
able position valve operated by a stepper motor. The ETV is
located in the suction line between the evaporator and the
heat exchanger. Discharge and suction pressure transducers
supply pressure information to the µP-VI microprocessor
control system. The microprocessor controls the electronic
throttling valve directly. The ETV replaces both the throt-
tling valve and the modulation valve used in other units.
The ETV system also uses a hot gas bypass valve like the
one used with the modulation valve in other units.
The ETV system provides enhanced control of the refrigera-
tion system as follows:
• Engine Horsepower Control. The engine horsepower
control algorithm is the primary control used to obtain
maximum capacity. This allows the refrigeration sys-
tem to fully utilize the power capabilities of the engine
under varying conditions.
• Discharge Pressure Protection. This protection algo-
rithm provides an additional measure of protection
against high discharge pressures and possible compres-
sor damage. It will prevent shutdowns in high ambient
temperatures by allowing continued operation of the
unit at a temporarily reduced refrigeration capacity.
• Engine Coolant Temperature Protection. This pro-
tection algorithm protects the engine from high coolant
temperature shutdowns and possible engine damage. It
will reduce the load on the engine by temporarily
reducing refrigeration capacity. This lowers the engine
temperature while still allowing continued unit opera-
tion.
• ETV Control. The ETV system replaces the modula-
tion valve. The ETV control algorithm operates much
the same as modulation on other units.
SEQUENCE OF OPERATION
When the On/Off switch is turned on, the LCD display
(which normally shows the setpoint, the return air
temperature, and the operating icons) is illuminated. If the
CYCLE-SENTRY mode has been selected, the unit will
start and stop automatically. If the Continuous mode has
been selected, the unit will start and run automatically.
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.