Air Conditioning

Air conditioning

Note! Some variations may occur in the illustrations, however the essential information is always correct.




The illustration shows the placement in a vehicle with engine B6324S. The main difference in vehicles with other engines is the placement of the compressor.
The air conditioning system only works when the outdoor temperature is above 0 °C. At lower temperatures the pressure in the system is too low and the compressor is never activated.
The air conditioning system only works when the engine is running. It works irrespective of the revs and the vehicle's speed. However, at full throttle, Kick down, the compressor is deactivated, to give improved acceleration power when e.g. overtaking.
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, you control the temperature with the normal heat and blower fan controls.
There are five main components in the climate control system:
- evaporator
- receiver drier (integrated with the condenser)
- compressor
- condenser
- expansion valve.
The system is filled with a suitable quantity of refrigerant and oil. The role of the oil is to lubricate and seal the compressor (piston and cylinder).
A fluorescent leak-tracing agent is added to the refrigerant. This makes it possible to trace leaks using UV light.

Principle sketch of the climate control system














The cooling system is divided into:
- a low pressure side (B, C)
- a high pressure side (A, D).
The two sides are separated by the compressor (1) and the expansion valve (5). The evaporator (6) is located on the low pressure side and the condenser (2) is on the high pressure side.
The design of the climate control system in a vehicle is based on the laws of nature, which mean that liquids evaporate when they are subjected to a temperature increase or a reduction in pressure and that heat is absorbed during this process.
If the hot vapor is then cooled again, the heat that has been absorbed is released and the gas reverts to liquid form. This process is repeated as many times as necessary, so that "cooling is produced" continually. This is the same type of process as used in refrigerators, for example.

Refrigerant
In order to remove heat from the passenger compartment a medium that has a lower evaporating temperature than air is used, because heat will always move from a hotter body to a cooler body. The medium that is used is refrigerant R134a.
R134a is a gaseous fluorocarbon. It is chlorine free and does not damage the ozone layer. However, R134a is still environmentally hazardous as it contributes to the greenhouse effect. It is therefore important that all service work is performed by trained personnel.
R134a retains its gaseous form at normal atmospheric pressure, and only condenses if it is cooled down to below -26 °C.
R134a has the following properties:
- can only be mixed with synthetic PAG oils (polyalkylene glycol) and not with mineral oils
- does not affect metals
- affects some types of plastic, so only special seals that are intended for R134a should be used
- is not explosive
- is odorless
- is not toxic in low doses
- effectively absorbs moisture
- is not flammable
- is heavier than air in gaseous form.
By allowing the refrigerant to circulate in a closed system and changing its pressure and volume, you can change its temperature to make it boil (vaporize). At the working pressure prevailing in the system's low-pressure side, approximately 170 - 320 kPa (1.7 - 3.2 bar), the refrigerant boils at about 0 to +4 °C.
A prerequisite in order for the refrigerant to boil is that heat is available. The heat is taken from the air around the evaporator in which the boiling occurs. When the heat is taken up by the refrigerant the surrounding air becomes colder. It is this cold air that is blown out into the passenger compartment by the climate system's fan. The heat taken up by the refrigerant in the evaporator is transported to the engine compartment where it is discharged to air by the condenser. The condenser is cooled by the speed wind and the engine cooling fan.

Compressor




The compressor is mechanical and is driven by the vehicle engine. When the compressor is operating it "steals" 0.5-8 kw (0.7-11 bhp) from the engine. This may be noticed as slight jerks when driving as the compressor is engaged / disengaged.
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.
In ideal conditions the compressor compresses the refrigerant from about 200 kPa (2 bar) to between 1.2 MPa and 2.1 MPa (12 and 21 bar). During the process the refrigerant is heated up from 0 °C to between 70 °C and 110 °C. These pressure and temperature values apply to normal working pressure in the system's high-pressure side, when the system works under optimal conditions.
The pressure relief valve located at the rear of the compressor, acts as an extra safety device. The valve opens and releases refrigerant when the pressure in the system becomes too high (at about 3.5 MPa (35 bar)). The valve closes once again when the pressure returns to its normal value.
The temperature of the refrigerant gas can increase to as much as 125 °C.
The compressor can only compress gases, as liquid would damage the compressor.
The compressor is in the refrigerant circuit, located between the evaporator and the condenser.
The compressor has a variable displacement. The compressor mounting and pipe connections vary depending on the engine in the vehicle.
Compressors with variable displacement usually do not switch off during normal operation. The flow of refrigerant is continually adapted based on need. The compressor works between min. and max. displacement due to the following:
- the pistons are driven by a cam disc with a variable angle
- the angle is determined by springs if the compressor is disengaged
- if the compressor is engaged, the angle is determined by the pressure exerted on the top of the pistons (= intake pressure) and the bottom of the pistons (= pressure in the crankcase) during the intake phase
- the pressure on the underside of the pistons (= in the crankcase) is regulated by a valve, which attempts to keep the intake pressure constant
High intake pressure = large displacement.
- The valve opens and lowers the pressure in the crankcase. The counterpressure on the back of the pistons then reduces and the cam disc angle increases.
- The increased angle generates increased stroke, which causes a greater amount of refrigerant to be "sucked in" and a drop in intake pressure.
Low intake pressure = small displacement.
- The valve closes and pressure in the crankcase increases. The pressure is built up by refrigerant that is led from the outlet side to the crankcase via a calibrated duct. The counterpressure on the back of the pistons then increases and the cam disc angle decreases.
- The decreased angle generates a reduced stroke, which causes a smaller amount of refrigerant to be "drawn in" and an increase in intake pressure.
The compressor is lubricated with specially developed refrigerant oil. This oil (synthetic PAG oil) is mixed with the refrigerant when the air conditioning system is in operation.

A/C switch




The compressor is controlled by a linear sensor. It acts as a switch than switches off the compressor if the pressure in the cooling system becomes too high or too low. The A/C switch also controls the engine cooling fan. The A/C switch is located on the cooling system's high-pressure side directly before the evaporator. In the engine compartment the A/C switch is located in the cold zone under the plenum cover.

Magnetic clutch assembly









The compressor is driven by the engine's crankshaft via the drive belt. As soon as the engine starts, the belt pulley (4) on the compressor's drive shaft runs unimpeded. When the climate system is switched on, the current runs through the magnetic coil (5) which becomes magnetic. This presses the carrier disc (1) on the compressor's drive shaft forward against the pulley.
The clutch engages and the compressor accelerates at engine speed.
When the current to the magnetic coil is interrupted, the carrier disc (1) is released from the pulley (4) with the aid of return springs.
When the compressor is disengaged the pulley runs freely without any influence from the compressor. When the current flows through the magnetic coil, the flexible plate is drawn in and engages with the pulley. Power is then transferred from the pulley via the plate to the compressor shaft.
In order for the magnetic clutch to function correctly, the distance between the carrier disc (1) and the pulley (4) must be exactly correct. Shim discs (2) are therefore placed between them.

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.
The expansion valve is mounted on the intake and outlet pipes of the evaporator. In the refrigerant circuit, it is located on the high pressure side between the receiver drier and the evaporator.
The expansion valve is a pressure and temperature dependent flow regulator of the constriction type. At low cooling demand the amount of refrigerant is reduced by the valve closing. If the cooling demand increases the valve opens slightly to allow more refrigerant to the evaporator.
Systems with an expansion valve have a greater register than systems with a fixed choke because the amount of refrigerant can be better regulated to the cooling demand.
The expansion valve consists of a diaphragm (1) and a valve housing (2).

Cross section of the expansion valve




1. Valve inlet from receiver drier
2. Valve slide
3. Valve outlet to compressor
4. Temperature sensor
5. Diaphragm (filled with refrigerant)
6. Diaphragm
7. Valve inlet from evaporator
8. Valve outlet to evaporator
9. Ball valve
10. Spring
Liquid refrigerant comes from the receiver drier and flows through the valve inlet (1). The ball valve (9) mists the refrigerant before the refrigerant flows on through the valve outlet (8) and into the evaporator.
The ball valve opening is controlled by the temperature and pressure in the refrigerant gas coming out of the evaporator.
If for example the temperature of the refrigerant gas, coming from the evaporator in the expansion valve intake (7), increases the temperature sensor (4) and increases the temperature of the refrigerant in the diaphragm head.
The refrigerant in the diaphragm head expands and compresses the ball valve in the valve seat downwards using the diaphragm (6) and a valve slide (2) so that the valve seat opens. The flow of the refrigerant to the evaporator increases. The evaporator is cooled and the temperature of the refrigerant gas is reduced.
The ball valve closes as soon as the temperature in the evaporator falls and therefore the temperature of the refrigerant in the diaphragm head also falls.

Evaporator




The evaporator is placed in the climate system's distribution housing. It is reached from the right-hand side of the distribution housing.
In the refrigerant circuit, the evaporator is located on the low pressure side between the expansion valve and the compressor.
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.
When the hot 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.
The heat (energy) released during condensing is transferred to the refrigerant which vaporizes, the temperature difference between the air and the refrigerant drops. High humidity therefore results in an increased cooling requirement.
The surface of the evaporator is polyurethane coated to reduce bad odors. The polyurethane coating reduces the surface tension of the water and makes it easier for the water to run off.
The evaporator consist of a tubular coil in which the refrigerant flows. The coils are equipped with flanges to increase the heat absorbing area. The evaporator is tilted to make it easier for the condensation water to drain off.
There is a low pressure in the evaporator on account of the restriction on the expansion valve and the compressor's suction effect.
To obtain sufficient cooling capacity, the refrigerant's evaporation temperature must be considerably lower than the temperature desired in the passenger compartment. But to prevent the moisture that has condensed on the evaporator from freezing to ice, the air is not cooled below approximately +3 °C (at which stage the temperature of the refrigerant in the evaporator is approximately -3 °C).
There is a temperature sensor after the evaporator. The temperature sensor detects the temperature of the air that has passed through the evaporator. When the temperature is too low, the compressor is switched off. This is to prevent ice forming on the evaporator.
It is important that there is the correct amount of refrigerant in the climate control system.
- Too much refrigerant in the evaporator causes the refrigerant to heat up, but it only evaporates partially. This means that a smaller amount of heat is absorbed from the air, which results in reduced cooling capacity
- Too little refrigerant in the evaporator causes the refrigerant to evaporate and the vapor to overheat. This means that a smaller amount of heat is absorbed from the air, which results in reduced cooling capacity.

Condenser (air cooled heat exchanger)




The condenser is placed in front of the engine's radiator. In the cooling system it is placed on the high pressure side opposite the compressor and receiver drier.
The task of the condenser is to cool, so converting the hot, gaseous refrigerant to a liquid.
The compressor pumps refrigerant gas under high pressure , which has a temperature of between 70 and 110 °C, into the intake on the top of the condenser. When the hot gas flows through the tubular coil it is cooled and changes into liquid (condenses). The heat removed from the gas during this process is emitted to the surrounding air.
The engine cooling fan increases the air flow through the condenser. This increases the transfer of heat from the refrigerant to the ambient air.
The amount of heat emitted = the amount of heat taken up by the evaporator + the amount of heat supplied by the compressor. In order to receive sufficient cooling capacity, the refrigerant's condensation temperature must lie above the ambient air's temperature.
The fan is controlled by the A/C switch and the fan thermostat, which is located on the engine's refrigerant circuit.
The condenser consists of pipe loops in which the refrigerant flows. The loops have flanges to increase the heat radiating surface.
The pipe connection is made of aluminium. The receiver drier is integrated with the condenser on the left-hand side of the condenser.

Receiver drier (integrated with the condenser)




The receiver drier is integrated with the condenser. It is located on the high-pressure side, between the condenser and the expansion valve, on the refrigerant circuit.
The receiver drier consist of a replaceable filter cartridge. It has the task of taking up and binding moisture (water) found in the system as well as filtering foreign substances.
The refrigerant in a liquid state passes at high pressure from the condenser into the receiver drier. The refrigerant then flows on to the expansion valve.
If the cooling system has been opened (unplugged) for 10 minutes or leaked for a period, so that moisture has been able to enter the system, the receiver drier must be replaced. When replacing the receiver drier, the new receiver drier contains leakage indication color that spreads through the system as soon as the system is started.
The receiver drier can only absorb a limited amount of moisture. If the system contains more moisture than the receiver drier can handle it may lead to icing which blocks the expansion valve.

Service valves




1. The service valve for draining, vacuum pumping and filling with refrigerant
2. Service valve for draining and vacuum pumping.
There is a service valve on the low-pressure side (2), under the plenum cover, and one on the high-pressure side (1). The valves are in different sizes to avoid mix-ups.

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

Hoses, pipes
All connections, except to the expansion valve, have double O-rings and seals. The connection to the expansion valve has a single O-ring.
The O-rings are manufactured in a specially developed material (HNBR = hydrated nitrile rubber).