Carrier 60 Service Manual download pdf (Page 24)

2-3

T--295

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 B oard relays in

responsetodeviationsin interiortemperature. Modesof

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 operatein 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 compressorclutchand allow t he 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 i s 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

vehicleandis providedby theOEM. It sensestheengine

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

damperisoperated bythe controllerto openwhenreturn

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 s et point.

Relay Board mounted unloader outputs control the

capacity of the compressor by energizing or

de-energizing unloader solenoid valves. The m odel

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. The05K rightcylinder bank(looking

at the pump end) and t he 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 twoor 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 t he 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 C ompressor 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.

2 Compressor Unloader UV2 Relay. When return air

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

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Carrier 60 Service Manual Page 24

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