Glossary

Glossary

A/C clutch
This makes the connection between the ribbed belt pulley and the compressor drive shaft.

A/C compressor
The A/C compressor is driven via a belt or shaft by the vehicle engine. The A/C compressor extracts the refrigerant gas from the evaporator, compresses it and relays it to the condenser.

A/C Compressor Regulator Valve -N280-
With an externally regulated compressor, the regulator valve is actuated by the control module to control the pressure on the low-pressure side and thus the evaporator temperature.

A/C Compressor Speed Sensor
A control module uses the sensor pulses (4 per compressor revolution) and the engine speed to calculate the belt slip. If the belt slip exceeds a specified value, the compressor is switched off by the control module via the A/C clutch.

A/C Evaporator Temperature Switch
The temperature switch determines the temperature between the cooling fins of the evaporator to stop the evaporator icing up.

Boiling point
R134a boils at -26.5 degrees C at normal atmospheric pressure (1 bar).

Charge factor
To avoid overfilling of vessels, regulations governing compressed gases specify the number of kilograms of refrigerant with which a vessel may be filled per liter of internal vessel volume. The product of multiplying this "charge factor" by the internal volume of the vessel is the permissible capacity. For refrigerant the figure is 1.15 kg/ltr.

Combustibility
Refrigerant is non-flammable. In fact, it has a fire-retardant or extinguishing effect.

Condenser
The condenser conducts heat from compressed refrigerant gas to the ambient air.

Critical temperature/critical pressure
The refrigerant R134a remains chemically stable up to a gas pressure of 39.5 bar (corresponding to a temperature of 101 degrees C), above this temperature, the refrigerant decomposes.

Evaporator
The liquid refrigerant evaporates in the evaporator pipe coils. The heat required for this is extracted from the air flowing on the evaporator ribbing.

Expansion valve
The expansion valve atomizes the streaming refrigerant and controls the flow quantity so that the vapor is gaseous only at the evaporator outlet, depending on the heat transmission.

Fin comb
This is used to straighten bent condenser fins.

Frigen/Freon
Trade names also applying to refrigerants which are not to be used in vehicles.

High Pressure Sensor
The high-pressure sensor generates a square-wave signal or data telegram when voltage is applied. This signal changes along with pressure in the system. The downstream control modules (radiator fan control module, Engine Control Module (ECM), A/C control head etc.) use this signal to calculate the pressure in the refrigerant circuit and to actuate the A/C clutch, A/C Compressor Regulator Valve N280, coolant fans and fan motor accordingly.

Highly water-absorbing (example: refrigerant oil)

Leak detector
Minor leaks can only be traced by way of an electronic leak detector or by first rendering them visible in UV light, e.g. using a special additive. Start up leak detector in line with relevant operating instructions.

Nitrogen
Is used to flush refrigerant circuits.

Notes for repairing
When working on the refrigerant circuit, always observe the generally valid safety precautions and the pressure vessel regulations.

O-Rings
These rings seal off the connection points between individual components of the refrigerant circuit.

Pressure gauge
The high-pressure gauge measures the pressure distributed evenly from the compressor outlet via the condenser to the constriction (restrictor, expansion valve) when the air conditioning system is switched on. The low-pressure gauge measures the pressure distributed evenly from the constriction (restrictor, expansion valve) via the evaporator to the inlet of the compressor when the air conditioning system is switched on.

Pressure gauge set for nitrogen
The function of the pressure gauges in this unit is to reduce the nitrogen vapor pressure. The vapor pressure in the nitrogen cylinder is approx. 50 bar at workshop temperature. The set can also be used to check for leaks in the refrigerant circuit.

Pressure relief valve
The valve opens at approx. 38 bar. The valves of certain compressors are equipped with a disc which breaks off when the valve responds.

Pressure switch
This switches the coolant fan to the appropriate speeds and the compressor "off" in the event of loss of refrigerant or excess pressure.

Receiver
The receiver collects the droplets of liquid and conveys them in a continuous stream to the expansion valve, as otherwise the expansion valve would not be able to fill the evaporator properly. Moisture which has entered the refrigerant circuit during repairs will be collected by a filter (desiccant bag) in the vessel. The receiver is installed in conjunction with the expansion valve.

Record sheet
This is used to register the amount of refrigerant used.

Refrigerant oil
In this case, one works with a substance with a low boiling point, designated as refrigerant. The refrigerant R134a used is tetrafluoroethane. It boils at -26.5 degrees C at normal atmospheric pressure (1 bar).

Reservoir
The reservoir collects the mixture from the evaporator to ensure that the compressor only draws in dry refrigerant in gas form. Moisture which has entered the refrigerant circuit during repairs will be collected by a filter (desiccant bag) in the reservoir. The reservoir is always installed in conjunction with a restrictor.

Restrictor
The restrictor creates a constriction. This constriction restricts the flow rate, thus governing the quantity of refrigerant for the evaporator. The restrictor also divides the refrigerant circuit into high and low-pressure side.

Retrofit procedure
This refers to the conversion of an air conditioning system from refrigerant R12 to R134a.

Schrader valve
These are valves installed in the connections, e.g. for service work in the refrigerant circuit.

Service station (e.g. A/C service station VAS 6007A)
The service station makes it possible to extract and clean the refrigerant, separate the refrigerant oil and re-charge the refrigerant circuit.

Sight glass
This is found on air conditioning systems converted from R12 to R134a refrigerant. It is not suitable for R134a, as the refrigerant does not entirely mix with the refrigerant oil.

Solenoid valve
The flow of refrigerant in the second evaporator is switched on or off by the shut-off valve.

Solidification point
-101.6 degrees C for R134a at normal atmospheric pressure (1 bar)

tc
tc stands for critical temperature (also known as critical point). This means, above the critical temperature there is no longer a boundary between liquid and gas. A substance above its critical temperature is always gaseous. At temperatures below the critical temperature, all types of refrigerant in pressure vessels exhibit both a liquid and a gas phase, i.e. there is a layer of gas above the liquid. As long as there is still gas present in the container next to the liquid, pressure is dependent on ambient temperature.

Vapor pressure
Is the pressure acting on the surface of a liquid. Vapor pressure is dependent on temperature.

Water content
Water destroys the air conditioner as it combines with other impurities at high pressures and temperatures to form acids.