The Principle of Car Airconditioning system

Principles of air conditioning (AC)

Function of vehicle air conditioning systems

• Vehicle air conditioning systems are designed to perform two basic functions, to cool the passenger compartment air to a lower temperature than the ambient (outside) air and to de-humidify the passenger compartment air to a comfortable level.
• In order to carry out these functions successfully the system must be able to counter the effects of unwanted heat and relative air humidity in the vehicle interior.

Unwanted heat

Unwanted heat may enter the vehicle by radiation, e.g.

• Heat radiated by the sun is conducted through the roof, windows and body panels.
• Heat which radiates upwards from the road is conducted through the floor panel.
• Occupants' body heat which radiates into the interior.

• Or alternatively by conduction e.g.
• Engine heat conducted through the bulkhead panel.
• Exhaust heat conducted through the floor panel.

Relative air humidity

• Relative air humidity is very important for driver and passenger comfort. A level of around 60% is desirable as the air can easily absorb the moisture given off by the human body. The role of the air conditioning system is therefore to maintain a comfortable level of humidity at all times.

Evaporation

• Evaporation is the process whereby the state of a liquid is changed to a vapour. If for example water is boiled it evaporates and changes its state to steam, as the evaporation takes place the vapour produced absorbs heat. If more heat is applied the temperature of the water will not increase, but the rate at which it evaporates will.

Condensation

• Condensation is the process whereby a vapour when cooled, changes its state to a liquid. If for example the vapour given off by boiling water is allowed to cool it will return to a liquid state.

Refrigerant evaporation and condensation

• Water is not a suitable refrigerant for air conditioning as its boiling point is too high. Even at low pressures no usable cooling effect could be achieved when it evaporates. For that reason special refrigerants are used that evaporate at very low temperatures (R134a boils at approximately -27°C at normal atmospheric pressure). The temperature at which refrigerant evaporation takes place varies according to pressure.

Refrigerant circuit

• The heat from the car interior, that is absorbed during evaporation, must be dispelled into the atmosphere and the refrigerant vapour condensed back to a liquid. Therefore the refrigerant in an automotive air conditioning system needs to be circulated continuously.

---

Expansion valve system

When the AC system is in operation the refrigerant is circulated continuously, passing through the following stages:

1. Relatively cold, low pressure refrigerant vapour is drawn into the compressor and is compressed. During its compression the temperature of the vapour is greatly increased.
2. Hot high pressure refrigerant vapour from the compressor enters the top of the condenser and passes down through the cooling tubes. The heat created during compression and that absorbed by the refrigerant during the previous cooling cycle is extracted. The refrigerant temperature lowers and it condenses before it exits the bottom of the condenser as high pressure liquid refrigerant.
3. Cool air is forced through the condenser fins, either by the speed of the vehicle, or by the condenser blower. This ensures the condenser can function under all operating conditions.
4. Liquid refrigerant from the condenser is collected in the receiver/drier where it is filtered and dried. The refrigerant vapour rises to the top of the receiver/drier and the liquid refrigerant collects at the bottom. The liquid refrigerant leaves through the central tube.
5. High pressure liquid refrigerant from the receiver/drier enters the expansion valve and a metered quantity is injected into the evaporator. The expansion valve senses the temperature of the vapour leaving the evaporator and the quantity is varied to maintain complete evaporation of the refrigerant.
6. As the pressure drops, the liquid refrigerant evaporates and absorbs heat from the evaporator.
7. Warm air from the car's interior is forced through the evaporator fins by the heater blower and is cooled as its heat is absorbed by the evaporating refrigerant. The cooled, de-humidified air is blown into the car's interior. The moisture in the warm air condenses on the surface of the evaporator and is allowed to drain away from the evaporator housing. Relatively cold low pressure refrigerant vapour leaves the evaporator and is then recycled.
8. High pressure refrigerant temperatures are monitored by switches or sensors located in the system.

1. High pressure refrigerant vapour
2. High pressure refrigerant liquid
3. Low pressure refrigerant liquid/vapour
4. Low pressure refrigerant vapour

---

Fixed orifice tube system

When the AC system is in operation the refrigerant is circulated continuously, passing through the following stages:

1. The compressor draws in relatively cold, low pressure refrigerant vapour and compresses it, causing a significant temperature increase.
2. Hot high pressure refrigerant vapour from the compressor enters at the top of the condenser and passes down through the cooling tubes. The heat created during compression and that absorbed by the refrigerant during the previous cooling cycle is extracted. The refrigerant temperature lowers and it condenses before it exits the bottom of the condenser as high pressure liquid refrigerant.
3. Cool air is forced through the condenser fins, either by the speed of the vehicle, or by the condenser blower. This ensures the condenser can function under all operating conditions.
4. High pressure liquid refrigerant from the condenser enters the orifice tube which allows only a metered quantity to be injected into the evaporator.
5. As the pressure drops, the liquid refrigerant evaporates and absorbs heat from the evaporator.
6. Warm air from the car's interior is forced through the evaporator fins by the heater blower and is cooled as its heat is absorbed by the evaporating refrigerant. This cooled, de-humidified air is then blown back into the car. The moisture in the warm air condenses on the surface of the evaporator and is allowed to drain away from the evaporator housing. Relatively cold low pressure refrigerant vapour (and possibly liquid) leaves the evaporator and is then recycled.
7. Because the orifice tube is of a fixed size it cannot ensure that the refrigerant leaving the evaporator is completely evaporated. Therefore it is passed through the accumulator/drier where it is filtered and dried and is allowed to completely evaporate (liquid refrigerant would damage the compressor). It leaves the accumulator/drier and is then recycled.
8. Low pressure refrigerant temperatures are monitored by switches or sensors located in the accumulator/drier.
9. High pressure refrigerant temperatures are monitored by switches or sensors located in the line before the fixed orifice tube.

1. High pressure refrigerant vapour
2. High pressure refrigerant liquid
3. Low pressure refrigerant liquid/vapour
4. Low pressure refrigerant vapour

---

Fixed orifice tube system with refrigerant heating - cooling mode

This system is used in some diesel powered vehicles during the engine warm up period to aid the heating of the passenger compartment in cold weather. When the AC system is in operation refrigerant is circulated continuously, passing through the following stages:

NOTE:  In this mode the AC refrigerant control valve [A] is open.

1. The compressor draws in relatively cold, low pressure refrigerant vapour and compresses it, causing a significant increase in temperature.
2. Hot high pressure refrigerant vapour from the compressor enters at the top of the condenser and passes down through the cooling tubes. The heat created during compression and that absorbed by the refrigerant during the previous cooling cycle is extracted.
3. The refrigerant enters an integral receiver drier (also called a modulator) where it is filtered and dried before it exits the bottom of the condenser as high pressure liquid refrigerant.
4. Cool air is forced through the condenser fins, either by the speed of the vehicle, or by the condenser blower. This ensures the condenser can function under all operating conditions.
5. High pressure liquid refrigerant from the condenser enters the orifice tube which allows only a metered quantity to be injected into the evaporator via the AC refrigerant control valve.
6. As the pressure drops, the liquid refrigerant evaporates and absorbs heat from the evaporator.
7. Warm air from the car's interior is forced through the evaporator fins by the heater blower and is cooled as its heat is absorbed by the evaporating refrigerant. This cooled, de-humidified air is then blown back into the car. The moisture in the warm air condenses on the surface of the evaporator and is allowed to drain away from the evaporator housing. Relatively cold low pressure refrigerant vapour (and possibly liquid) leaves the evaporator and is then recycled.
8. Because the orifice tube is of a fixed size it cannot ensure that the refrigerant leaving the evaporator is completely evaporated. Therefore it is passed through the AC refrigerant hot gas tank where it is allowed to completely evaporate (liquid refrigerant would damage the compressor). The refrigerant vapour leaves the AC refrigerant hot gas tank and is then recycled.
9. Refrigerant pressure is monitored by a refrigerant pressure sensor located in the high pressure line.

1. High pressure refrigerant vapour
2. High pressure refrigerant liquid
3. Low pressure refrigerant liquid/vapour
4. Low pressure refrigerant vapour

---

Fixed orifice tube system with refrigerant heating - heating mode

This system is used in some diesel powered vehicles during the engine warm up period to aid the heating of the passenger compartment in cold weather. When the refrigerant heating system is in operation refrigerant is circulated continuously, passing through the following stages:

NOTE:  In this mode the AC refrigerant control valve [A] is closed.

1. The compressor draws in low pressure refrigerant vapour and compresses it. This results in a significantly increased refrigerant temperature.
2. Hot high pressure refrigerant vapour from the compressor bypasses the condenser via the AC refrigerant control valve and passes directly to the evaporator.
3. Cold air from the car's interior is forced through the evaporator fins by the heater blower and is heated by the hot refrigerant vapour. Relatively cold low pressure refrigerant vapour (and possibly liquid) leaves the evaporator.
4. Relatively cold low pressure refrigerant vapour enters the AC refrigerant hot gas tank where any liquid can completely evaporate. Some models use an electrical AC refrigerant heater around the AC refrigerant hot gas tank to raise the temperature of the refrigerant before it returns to the compressor to be recycled.
5. Refrigerant pressure is monitored by a refrigerant pressure sensor located in the high pressure line.
6. The AC refrigerant heater is controlled by a thermal switch.

1. Low pressure refrigerant liquid/vapour
2. Low pressure refrigerant vapour