System With TEV-Valve (Thermostatic Expansion Valve)
TEV Valve
Active flow regulator (= variable choke valve).
Consists of:
- a valve which is affected by a spring and a membrane.
- A capillary pipe with a resistor positioned on the evaporator outlet pipe.
- The position of the resistor has been carefully tested.
- The capillary pipe is filled with refrigerant.
- a pressure equalization pipe is connected to the evaporator outlet pipe.
Governs the volume of refrigerant which is released from the evaporator.
Too little refrigerant = the temperature at the outlet pipe increases the refrigerant in the capillary pipe expands and exerts pressure on the membrane the valve is opened and releases more refrigerant to the evaporator.
Too much refrigerant = the temperature at the outlet pipe falls the refrigerant in the capillary pipe reduces in volume the spring presses the valve back the valve is shut and releases less refrigerant to the evaporator.
The temperature and pressure in the system are related to each other. Assumes that there is a drop in pressure in the evaporator, in which case the temperature at the outlet pipe also falls. This means that the valve is closed and there will be too little refrigerant - the capacity reduces. The pressure in the outlet pipe is led via a pressure equalization pipe to the underside of the membrane and compensates for any fall in pressure.
The TEV valve is carefully adapted for its system and for the type of refrigerant that is used.
The valve is calibrated to release through exactly the correct amount of refrigerant which can be evaporated completely and to keep the amount of super heating to a minimum (super heating is normally approximately 6-10°C (43-50°F). This is to obtain maximum cooling capacity and to prevent refrigerant in liquid form leaving the evaporator and reaching the compressor.
Freeze protection
Thermostat with capillary pipes or a temperature sensor depending on the car and model year.
Positioned where the evaporator is coldest (where frost forms first) The location is extremely important and is often tested manually.
The thermostat/pressure sensor interrupts a current circuit and disengages the compressor when the temperature becomes too low. This is so that the water that condenses on the evaporator does not freeze to ice and obstruct the airflow through the evaporator. Heavy build up of ice can also cause damage to the evaporator.
When the temperature increases again, the thermostat/temperature sensor closes the current circuit and the compressor is engaged.
The thermostat/temperature normally opens at approximately 2°C (36°F) and closes at approximately 6°C (43°F).
Receiver drier
The function of the TEV valve can be interfered with by gas bubbles in the refrigerant. Therefore the receiver drier must be positioned on the high pressure side and be able to separate gas from liquid.
The receiver drier outlet is located so that only refrigerant in liquid form can leave the receiver drier.
Refrigerant in gas form is collected in the top of the receiver drier. When the heat is transferred, the refrigerant will also condense.
There is a filter in the receiver drier which collects any impurities in the system.
Control of the temperature in the passenger compartment
Certain cars have an adjustable thermostat which can be adjusted for the evaporator temperature at which the compressor will be disconnected.
The maximum position (marked red) should however only be used briefly after start to rapidly lower the temperature in the passenger compartment, and if applicable in dry desert climates. Otherwise there is a risk that the condensation will freeze to is.
On most cars the air conditioning has only two positions, off and on.
If it is too cold in the passenger compartment, the temperature can be controlled with the normal heat and blower fan controls.
Service socket
On systems with a TEV valve, there is two nipples (one on the high pressure side and one on the low pressure side) to which the A/C station or pressure gauge can be connected.