Air Conditioning

Air conditioning

General




The air conditioning only functions at ambient temperatures above 0 °C. At temperatures lower than this, the pressure in the system is too low and the compressor is never engaged. This setting is made to prevent ice building up on the evaporator.
The air conditioning only functions when the engine is running. The air conditioning functions regardless of engine speed (RPM) and vehicle speed. The compressor is dependent on the engine speed, the higher the engine speed the more cooling that can be provided.
The blower fan must be operating before the climate control system can be engaged.
The air conditioning system has only two positions, off and on. If it is too cold in the passenger compartment, the temperature can be controlled with the normal heat and blower fan controls.
Air conditioning is based on the same principle as that used in refrigerators.
The rear climate control unit in the seven seat XC90 supplies the passengers in the third row of seats with cooled air. For the rear climate control system to function the rear climate control system blower fan motor must be running and the front climate control system must be switched on.

Refrigerant




1. Compressor
2. Condenser
3. Engine cooling fan (FC)
4. Receiver drier
5. Expansion valve
6. Evaporator
7. Blower fan.
Explanation:
A - High pressure, warm fluid
B - Low pressure, cold fluid
C - Low pressure, cold gas
D - High pressure, warm gas
Dark arrow - Warm air
Light arrow - Cold air
By allowing the refrigerant to circulate in the closed system and changing the pressure and volume it will change temperature and boil (evaporate). At the pressure found in the system, approximately 170 - 320 kPa (1.7 - 3.2 bar), the refrigerant boils at approximately 0 °C to +4 °C.
A condition of the refrigerant boiling is that the heat is accessible. This heat is taken from the air around the evaporator (6) where boiling occurs. When the heat is taken up by the refrigerant the surrounding air becomes cooler. It is this cooled air that is blown into the passenger compartment by the climate control system blower fan (7). The heat taken up by the refrigerant in the evaporator (6) is transported out to the engine compartment, where it is transferred to the air by the condenser (2). The condenser is cooled by the airflow and the engine cooling fan (FC) (3).

Compressor




The task of the compressor is to:
- draw gaseous refrigerant from the evaporator
- compress the gas thereby increasing its pressure and temperature
- expel the gas with high pressure and high temperature to the condenser.
The compressor takes in cold refrigerant gas from the evaporator on the intake side through the low pressure connection. In ideal conditions, the compressor compresses the refrigerant from approximately 200 kPa (2 bar) to between 1.2 MPa and 2.1 MPa (12 and 21 bar). During the process, the refrigerant heats up from 0 °C to between 70 °C and 110 °C. These pressure and temperature values apply when the system is operating under optimal conditions.
The relief valve, located on the rear section of the compressor, functions as an additional safety device. The valve opens and releases refrigerant when the pressure in the system is too high. The valve then shuts again when the pressure has returned to normal. The temperature of refrigerant gas can reach as high as 125 °C.
The compressor can only compress gases, as liquid would damage the compressor.

Expansion valve




The expansion valve checks the refrigerant flow in the evaporator in relation to the temperature and pressure. The expansion valve gives an even temperature control because the amount of refrigerant is controlled as necessary.

Evaporator




The task of the evaporator is to cool and dry the air.
The refrigerant (the volume that is measured by the expansion valve) is sprayed into the evaporator. As soon as the refrigerant enters the cooling pipes, it evaporates due to the drop in pressure. During this process, heat is also extracted from the cooling pipes. The cooling pipes and the entire evaporator cool immediately. The refrigerant gas is then drawn out using the compressor.
The airflow is routed past the evaporator and is cooled and dried, and then forwarded to the passenger compartment via the air distribution ducts.
When the air entering the passenger compartment encounters the cold evaporator, the humidity in the air condenses on the evaporator. The water drains out under the vehicle via a drain hose. In the event of high humidity, this may be considerable quantities of water, which is often mistaken for a water leak from the engine for example.

Receiver drier




The task of the receiver drier is to:
- function as a filter, taking up and binding moisture (water) found in the system
- function as a storage container for leak detection agent.
The leak detection dye mixes with the oil. When a leak occurs, it is possible to locate the leak with UV light.

Condenser (air cooled heat exchanger)




The task of the condenser is to cool, so converting the hot, gaseous refrigerant to a liquid.
The compressor pumps in hot refrigerant gas under high pressure. The gas has a temperature of between 70 and 110 °C in the intake on top of the condenser. When the hot gas flows through the pipe loop, it is cooled down and condensed. The heat that is removed from the gas during this process is transferred to the surrounding air. The engine cooling fan (FC), which is the same fan used by the engine cooling system, increases the airflow through the condenser. The air flow increases the transfer of heat from the refrigerant to the outside air.
The engine cooling fan (FC) is controlled by the linear pressure sensor (high-pressure and safety valve), located on the drier, via the engine control module (ECM) and via the control module for the engine cooling fan (FC). The engine cooling fan (FC) is also controlled by the engine coolant temperature (ECT).

Service valves




1. The service valve for draining, vacuum pumping and filling with refrigerant
2. Service valve for draining and vacuum pumping.
The service valves are used to:
- check the system through manometer connections
- drain the system of refrigerant
- vacuum pumping
- filling with refrigerant, high-pressure side only (1).

Caution! Filling with refrigerant must only be carried out on the high-pressure side to prevent the risk of liquid refrigerant in the compressor.