Air Conditioning
Refrigerant Circuit:
Refrigerant Circuit with a Fixed Orifice Tube
Operation
The compressor draws the gaseous refrigerant in from the front and compresses it. In so doing it is heated to a temperature of between 70°C and 110°C.
The gas is then pumped to the condenser. The condenser consists of numerous cooling fins, through which the gas passes. The refrigerant gas is cooled both by the external air flow and depending on the vehicle model by the auxiliary or radiator fan until it condenses.
The condensed liquid refrigerant then passes through a fixed orifice tube with a constant diameter. The pressure behind the orifice tube drops, the temperature falls rapidly as a result, and the refrigerant partially vaporizes.
The fixed orifice tube is mounted in front of the evaporator within the refrigerant line, in which complete evaporation takes place. As a result, evaporator and the air flowing past it are both cooled.
The cooled air is blown by the heater/air conditioning blower through the air distribution system into the vehicle interior.
The refrigerant gas then passes through the suction accumulator before it is drawn into the compressor again.
Condenser with fan
Location
The condenser is located forward in front of the radiator.
Function
Liquefies the gaseous refrigerant by cooling.
Construction and operation
The condenser consists of a coiled line connected firmly by a series of fins. Hot refrigerant gas under high pressure is pumped at 80-125°C from the compressor into the upper inlet line of the condenser. When the hot gas passes through the coiled lines it cools until it condenses. The heat extracted from the gas during this process is transferred into the surrounding air.
Fan
The fan(s) provide a quicker means of removing the heat from the condenser and as a result a more rapid cooling of the refrigerant. The fan is controlled by means of the high pressure switch in the refrigerant line and the temperature sensitive radiator fan switch in the coolant circuit.
Compressor:
Compressor
Function
The compressor has the following functions:
- Raising the pressure of the refrigerant gas.
- Pumping refrigerant gas.
Operation
The compressor draws in cool refrigerant gas from the suction accumulator through the suction connection.
The compressor compresses the refrigerant gas ideally from approximate 200 kPa to 2250 kPa (29 psi to 326 psi) and at the same time warms it from 0°C to 110°C (32°F to 230°F). This temperature and pressure data is based on an "ideal system". When the vehicle is running the suction pressure is between 120-300 kPa (17.5-43.5 psi) and under normal conditions the high pressure reaches approximately 1400-2000 kPa (200-225 psi), but can reach 2800 kPa (406 psi) in extreme cases. At the same time, the temperature of the refrigerant gas can rise to 125°C (257°F). The hot gas is pumped under high pressure to the condenser through the pressure connection. The compressor is only able to compress gaseous material, any liquid sucked in would damage the compressor.
High pressure switch
Location
In the line from the compressor to the condenser.
Function
Protection of the air conditioning system against a pressure overload by cutting out the compressor.
Operation
The function of the high pressure switch is to protect the high pressure section of the refrigerant circuit. When pressure exceeds the maximum value of 2965 kPa (430 psi) because, for instance, the air flow through the condenser is impeded or the high-pressure line is blocked, the high pressure switch switches the compressor off.
Only when the pressure has fallen once more to 2241 kPa (325 psi) bar is the compressor switched on again. At a pressure level of 2241 kPa (325 psi) the PCM switches to high fan speed.
Only when the pressure has fallen once more to 1717 kPa (249 psi) does the PCM switch to low fan speed.
Fixed Orifice Tube:
Fixed orifice tube
Position in the refrigerant circuit
Line between the condenser and the evaporator.
Function
The fixed orifice tube has the following functions:
- Regulating the amount of flow to the evaporator
- Separating the high-pressure side from the low-pressure side
The liquid refrigerant flows under high pressure from the condenser to the inlet side of the fixed orifice tube. Two O-rings prevent the refrigerant from flowing past the fixed orifice tube. Two filters at the entrance and outlet of the fixed orifice tube remove any particles from the refrigerant. The outlet filter has the additional function of further atomizing the refrigerant. Only a quantity of refrigerant commensurate with the pressure can ever flow through the calibrated internal diameter of the fixed orifice tube. As a result the amount of flow through is restricted. The internal diameter of the fixed orifice tube varies according to the required cooling performance of the air conditioning system. The internal diameter can be determined by means of the color of the fixed orifice tube housing and an appropriate color code chart.
Evaporator:
Evaporator
Position in the refrigerant circuit
On the low-pressure side between the fixed orifice tube and the suction accumulator.
Function
Cooling, drying and cleaning the air.
Construction and operation
The evaporator is mounted in the evaporator housing behind the heater/air conditioning blower and operates like a heat exchanger. It consists of numerous cooling coils which are connected by fins.
The refrigerant metered by the fixed orifice tube is sprayed as a low pressure liquid into the evaporator. As soon as the refrigerant comes into contact with the inner surface of the cooling coils it vaporizes and extracts heat from the coils. The cooling coils and the entire evaporator undergo considerable cooling. The vaporous refrigerant is then drawn in by the compressor.
The air flowing through the evaporator is cooled and dried and fed into the passenger compartment by the heater/air conditioning blower.
At the same time any moisture released is precipitated on the evaporator fins and condenses. The condensed water is drawn off through a drainage line. Any dust or pollen particles stuck to the evaporator are washed away during this process. As a result, the air is cleaned and dried.
A/C Cycling Switch:
A/C cycling switch
Location
On top of the suction accumulator.
Function
The A/C cycling switch has the following functions:
- Protecting the evaporator against icing
- Protecting the air conditioning system against reduced pressure or an empty system.
Operation
The A/C cycling switch has the function of a de-icing switch and switches the compressor clutch on/off.
The pressure and the temperature inside the suction accumulator and the evaporator are the same. If the pressure and, therefore, the temperature, drops in the suction accumulator, the temperature in the evaporator approaches freezing point. The evaporator threatens to ice up and lose its ability to cool. The A/C cycling switch then opens the compressor clutch when the pressure drops to a certain level and only closes it again when there is an appropriate rise in pressure.
Suction Accumulator:
Suction Accumulator
Position in the refrigerant circuit
On the low-pressure side between the evaporator and the compressor.
Function
The suction accumulator has the following functions:
- Reservoir for gaseous refrigerant.
- Filter and dehydrator for the refrigerant.
- Protection device for the compressor.
Operation
The refrigerant gas is fed from the evaporator into the suction accumulator through the inlet line. It flows around the plastic cap and in so doing develops a swirling motion.
The desiccant bag absorbs any moisture from the refrigerant.
The refrigerant gas collects underneath the plastic cap, where it is drawn into the opening of the outlet line. The refrigerant oil which has collected at the bottom of the suction accumulator is drawn through a filter mounted in a U-pipe and mixed with the refrigerant gas (ratio of oil/gas mixture: 3 %). In this way adequate provision is made for the lubrication of the compressor's moving parts.