Modes Of Operation; Temperature Control; Cooling Mode; Heating Mode - Carrier GR-45 Operation And Service

2.3 MODES OF OPERATION

The system is operated by a Carrier Transicold
Micromax microprocessor controller which consists of
a logic board (Figure 1-10), relay board (Figure 1-8 or
Figure 1-9), and manual operator switches. The logic
board regulates operational cycles of the system by
energizing or de- -energizing Relay Board relays in
response to deviations in interior temperature. Modes of
operation include Cooling, Heat and Vent. Refer
toFigure 2-1 and the following paragraphs for a
description of each mode.
Figure 2-1 shows the Logic Board actions at various
temperature deviations from setpoint. On rising
temperature, changes occur when the temperature rises
above Logic Board setpoints, On falling temperature,
changes occur when temperatures falls below Logic
Board set point. The system will operate in these modes
unless pressures override the Logic Board settings.

2.3.1 Temperature Control

Temperature is controlled by maintaining the return air
temperature measured at the return air grille.

2.3.2 Cooling Mode

Cooling is accomplished by energizing the compressor
and condenser fans, opening the liquid line solenoid
valve and closing the heating valve. Once interior
temperature reaches the desired set point, the system
may operate in the clutch cycle or reheat mode.
Selection of clutch cycle or reheat is factory
programmed in accordance with the bus purchase
specification.
A controller programmed for clutch cycle will
de- -energize the compressor clutch and allow the system
to operate in the vent mode until further cooling is
required.
A controller programmed for reheat will maintain
compressor operation and cycle the heat valve to allow
reheating of the return air. In the reheat mode interior
temperature is maintained at the desired set point while
additional dehumidification takes place.

2.3.3 Heating Mode

In the heat mode the liquid line solenoid is closed and
the compressor and condenser fans are shut down. The
heat valve is opened to allow a flow of engine coolant
through the heat section of the evaporator coil. The
evaporator fans speed is varied as required to circulate
air over the evaporator coil based on the temperature
difference from setpoint.
Heating will not start until the water temperature switch
(WTS) closes. The WTS is located on the block of the
vehicle and is provided by the OEM. It senses the engine
coolant temperature and closes on temperature rise at
105F . The switch prevents the circulation of cooler air
throughout the vehicle as the engine comes up to
temperature.

2.3.4 Boost Pump

When the unit is in heat the boost pump relay is
energized, providing 24 VDC to activate the boost
pump.

2.3.5 Vent Mode

In the vent mode the evaporator fans are operated to
circulate air in the bus interior.

2.3.6 Fresh Air System

The fresh air damper is opened to allow entrance of
ambient air into the air entering the evaporator coil. The
damper is operated by the controller to open when return
air temperature is within +/- -5F (+/- -2.8C) of set point.
2.3.7 Compressor Unloader Control
When operating in cooling, the unloaders are used to
reduce system capacity as return air temperature
approaches set point. Operation of the unloaders
balances system capacity with the load and thereby
prevents overshoot from set point.
Relay Board mounted unloader outputs control the
capacity of the compressor by energizing or
de-energizing unloader solenoid valves. The model
05K (GR45) has two banks of two cylinders each while
the model 05G compressor (GR60) has three banks of
two cylinders each. Energizing a valve de-activates a
bank of cylinders. The 05K right cylinder bank (looking
at the pump end) and the outboard cylinder banks of the
05G are equipped with unloader valves (UV1 and, for
the 05G, UV2), each controlling two cylinders; this
allows the 05K to be operated with two or four cylinders
and the 05G to be operated with two, four or six
cylinders.
Whenever the compressor is started, the unloaders are
energized for thirty seconds to reduce starting torque.
After thirty seconds, unloaders may be de-energized.
Any subsequent changes between energizing and
de-energizing the unloaders for temperature control
must be staged with a thirty second delay. Once an
unloader is energized for pressure control, it remains
energized for two seconds to prevent short cycling.
Only one unloader may change state at a time when
staging is required. Operating parameters for
temperature control, suction pressure control and
discharge pressure control are as follows.
a. Temperature Control
The unloaders are used to control system capacity by
controlling compressor capacity.
1 Compressor Unloader UV1 Relay. When return air
temperature falls to less than 2F (1.1C) above set
point unloader UV1 is energized. If temperature
rises to greater than 3F (1.7C) above set point,
UV- -1 will be de- -energized to place the compressor
at 100% capacity.
Compressor Unloader UV2 Relay. When return air
2
temperature falls to less than 1F (0.6C) above set
point unloader UV2 is energized. If temperature
rises to greater than 2F (1.1C) above set point,
UV- -2 will be de- -energized to place the compressor
(GR60 only) at 66% capacity.
b. Suction Pressure
2-3
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