Air conditioning (Description): Description and operation

Description

General

Where fitted, the air conditioning system supplies cooled and dehumidified, fresh or recirculated air to the interior of the vehicle. Air is cooled by drawing it through the matrix of an evaporator. The air is then ducted into the heater assembly, from where it is distributed to the vehicle interior through the heating and ventilation system air ducts.

In the heater assembly, the temperature of the air distributed to the vehicle interior can be adjusted by passing a proportion, or all, of the cooled air through the heater matrix. The volume of air being distributed is controlled by the variable speed blower in the heater assembly. For details of temperature control and distribution. HEATING AND VENTILATION, DESCRIPTION AND OPERATION, Description.

The air conditioning system uses a pressure sensor and evaporator temperature sensor to provide operating condition feedback to the engine management system to enable the ECM to predict engine load and run the cooling fans in response to changing atmospheric conditions and driver demand.

Refrigerant System

The refrigerant system is a sealed closed loop system which is charged with Refrigerant R134a as the heat transfer medium. It works in combination with a blower unit, blend unit and control system to achieve the desired air temperature. ND-8 oil is added to the refrigerant to lubricate the internal components of the compressor. The refrigerant system comprises of the following main components connected together by refrigerant lines: l Compressor (variable load) l Condenser (with modulator) l Thermostatic expansion valve l Evaporator

To accomplish the transfer of heat, the refrigerant is circulated around the system, where it passes through two pressure/temperature regimes. In each of the pressure/temperature regimes, the refrigerant changes state, during which process maximum heat absorption or release occurs. The low pressure/temperature regime is from the thermostatic expansion valve, through the evaporator to the compressor; the refrigerant decreases in pressure and temperature at the thermostatic expansion valve, then changes state from liquid to vapour in the evaporator, to absorb the heat. The high pressure/temperature regime is from the compressor, through the condenser and modulator (receiver/drier), back into the condenser where it is supercooled and then to the thermostatic expansion valve. The refrigerant increases in pressure and temperature as it passes through the compressor, then releases heat and changes state from vapour to liquid in the condenser.

Fan blown air is passed through the evaporator where it is cooled by absorption due to the low temperature refrigerant in the evaporator. Most of the moisture held in the air is condensed into water by the evaporator and drains away beneath the vehicle via a drain tube.

The compressor receives the returned low pressure, warm, vaporised refrigerant from the evaporator to complete the refrigeration cycle.