A/C Refrigerant System
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 airflow 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 fixed orifice tube drops, the temperature falls rapidly as a result and the refrigerant partially vaporizes.
The fixed orifice tube is installed in the refrigerant line and is located upstream of the evaporator in which full evaporation takes place. Because of this, the evaporator is cooled and, therefore, the air which passes over it is also cooled.
The cooled air is blown by the heater/air conditioning blower through the air distribution system into the vehicle interior.
The refrigerant gas is then transferred through the accumulator/dehydrator before being drawn back in by the compressor.
Electrical System
Control
The high-pressure and low-pressure switches must be closed when the air conditioning is activated. Once a certain pressure is exceeded in the high-pressure section of the air conditioning system, the high-pressure switch opens the air conditioning clutch power circuit.
The second pressure switch in the high-pressure circuit closes the high speed fan control circuit when a maximum pressure is reached, so that the air conditioning fan motor is supplied with current from the high speed fan relay. This increases the cooling power.
Air Conditioning System Low Pressure Switch
Control
When the air conditioning is switched on, the clutch only receives 12 V if the low pressure switch is closed. When the pressure falls below 1.6 bar, the low pressure switch is opened and the supply of current to the compressor clutch is interrupted.
Air Distribution Operating Switch:
Air distribution rotary control
Task and Function
If the air distribution rotary control is turned to "Defrost" or "Footwell/Defrost" position, the air conditioning is switched on regardless of the position of the air conditioning control switch or blower switch. As a result, the air in the vehicle used to defrost the windows is dried beforehand.
Refrigerant Circuit
Installation position - condenser and fan
The condenser is located at the front upstream of the radiator.
Condenser - installation position in the refrigerant circuit
On the high-pressure side between the compressor and fixed orifice tube
Task - Condenser
Liquefaction of the gaseous refrigerant by cooling
Construction and operation
The condenser comprises several parallel pipes which are rigidly fixed to numerous fins. Hot refrigerant gas at a temperature of 80-125°C is pumped under pressure from the compressor into the inlet pipe at the top of the condenser. As it passes through the pipes, the hot gas cools down so much that it condenses. The heat produced by this process is given up to the surrounding air.
Fan
The fans ensure that the heat given up by the condenser is dissipated more quickly, thus allowing the refrigerant to cool down more quickly. The fan is controlled by the high-pressure switch in the refrigerant line and by the temperature-dependent cooler fan switch in the coolant circuit.
Compressor, Part 1:
Compressor, Part 2:
Compressor
Installation position in the refrigerant circuit
Between the lines leading from the accumulator/dehydrator to the condenser.
Function
The air conditioning compressor has the following functions:
- Compressing the refrigerant.
- Circulating the refrigerant.
Operation
The compressor draws in cool refrigerant gas from the accumulator/dehydrator via the suction connection and forces it into the condenser.
Ideally, the compressor condenses the refrigerant gas from approx. 2 bar to 12-18 bar, the gas heating up from 0°C to approx. 70-110°C during this process. In extreme climatic conditions, the pressure on the input side can be up to 3 bar and on the high-pressure side up to 24 bar. The hot gas is pumped to the condenser under high-pressure via the high-pressure connection. The compressor is only able to compress gaseous material; any liquid sucked in would destroy the compressor. This is why the air conditioning system may only be filled with gaseous material at the low-pressure end.
High-pressure switch
Installation position
In the line between the condenser and evaporator.
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 31.4 bar 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.
The compressor will not be switched on again until the pressure has dropped back to 17.2 bar.
The fan speed increases when the pressure is more than 20.7 bar. The EECV module will not switch the fan to a lower speed until the pressure has dropped back to 17.2 bar.
Fixed Orifice Tube:
Fixed orifice tube
Installation position in the refrigerant circuit
In the line between the condenser and the evaporator.
Function
The fixed orifice tube has the following functions:
- Limits the amount of flow into the evaporator.
- Expands the refrigerant to low pressure.
Function
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 filter at the outlet also atomizes the refrigerant. Only the quantity of refrigerant corresponding to the pressure can ever flow through the calibrated internal diameter of the fixed orifice tube. As a result, the amount of flow through the tube is restricted. The internal diameter of the fixed orifice tube corresponds to the required refrigeration power in the air conditioning system.
Evaporator:
Evaporator
Installation position in refrigerant circuit
At the low-pressure end between the fixed orifice tube and the accumulator/dehydrator. The blower box has to be dismantled in order to remove the evaporator.
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, 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 resulting condensed water is drawn off via 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.
Air Conditioning System - Low Pressure Switch:
Air Conditioning System - Low Pressure Switch
Installation position
On the low-pressure line between the evaporator and the wheel housing.
Function
The air conditioning low-pressure switch has the following functions:
- Protecting the evaporator against icing
- Protects the air conditioning system when the pressure is too low or the system is empty in the event of leaks.
Operation
The low-pressure switch functions as a de-icing switch and controls the compressor clutch on/off switch.
The pressures in the low-pressure line and evaporator are practically equal. As the pressure in the low-pressure line drops, the temperature in the evaporator approaches freezing point. There is a risk that the evaporator will ice up and loose its cooling power. The low-pressure switch opens the compressor clutch when the pressure drops to a certain level and will not close it again until the pressure has risen to a suitable level.
Accumulator/Dehydrator With Air Conditioning Low Pressure Switch (Zetec Engine Version Shown):
Suction Accumulator:
Accumulator/dehydrator
Installation position in the refrigerant circuit
On the low-pressure side between the evaporator and the compressor.
Function
The accumulator/dehydrator performs the following functions:
- Reservoir for gaseous refrigerant.
- Filter and dehydrator for the refrigerant.
- Protects the compressor from sucking in liquid refrigerant.
Operation
The refrigerant gas is transferred from the evaporator to the accumulator/dehydrator via the inlet line. It flows around the plastic cap and is swirled in the process.
The plastic cap inside the accumulator/dehydrator prevents the air conditioning compressor from drawing in liquid refrigerant.
The dehydrator element absorbs the moisture removed from the refrigerant.
The refrigerant gas collects beneath the plastic cap where it is sucked through the opening in the outlet line. The refrigerant oil at the bottom of the accumulator/dehydrator is sucked out through a filter fixed to the U-pipe and mixed with the refrigerant gas (oil/gas mix: 3 %). This ensures that the moving parts in the compressor are adequately lubricated.