Subjects:
- General
- Capillary (Fixed Restriction Expander) with Accumulator
- TEV (Thermostatic Expansion Valve)
General:
An expander is part of the air conditioning. It is basically a narrowing in the air conditioning line between the dryer / filter element and the evaporator. The change from high to low pressure takes place from this expander.
After the refrigerant has passed the filter / dryer element, it enters the expansion device with a (high) pressure of around 15 bar and a temperature of around 55 degrees. There is a constriction in the expansion member. The refrigerant is forced through this constriction. Due to this constriction, the refrigerant loses its pressure (the pressure drops from 15 bar to 2 bar). This pressure reduction also immediately lowers the boiling point of the refrigerant, so that the liquid almost completely changes into vapor form due to the sudden increase in temperature. To enable the transition from liquid to vapor, heat is extracted from the air flowing through the evaporator. This air is then cooled and then flows into the interior. This is the cooled and dried air for which an air conditioner is used.
The refrigerant's job of cooling the air flowing through the evaporator is now complete. Further in the system, after passing the compressor, the temperature will decrease again, causing the refrigerant to condense again in vapor form. The refrigerant then becomes a liquid again. The process will continue in this way continuously. See the chapter states of aggregation for more information about the different processes and state changes.
There are different types of expansion devices, namely the capillary and the thermostatic expansion valve (TEV). The TEV is also often referred to as a “block valve”. These are described below.
Capillary (Fixed Restriction Expander) with Accumulator:
The capillary is mounted in a high pressure pipe between the condenser and the evaporator (see the picture below). Often it is not even clear to see where it is mounted and it will only be found when the temperature of the pipe is measured or felt. The pipe between the condenser and the capillary will feel cold. It will feel warm between the capillary and the evaporator. The temperature difference is due to a large pressure drop in this “tube with a narrowing”.
As previously described, this drop in pressure causes a lower boiling point of the refrigerant, raising the temperature and turning the liquid into vapour. The vapor then passes through the evaporator. The heat from the air that the interior fan blows through the evaporator is absorbed into the refrigerant. The air is thereby strongly cooled and continues cold and dried its way to the interior.
Short info about the accumulator:
In an air conditioning system that is equipped with a capillary, there is no filter / dryer in the system. There is, however, a separate accumulator (in the low-pressure side). This accumulator ensures that the last liquid droplets change into gaseous form before they enter the compressor. If there is liquid in the refrigerant, the compressor can break due to a liquid hammer. After all, the compressor can only compress and move gaseous refrigerant.
See the page accumulator.
Filters are mounted before and after the capillary tube. Except for these filters and the constriction, the capillary is not really much.
The disadvantage of the capillary is that it cannot be adjusted and regulated, as is the case with the Thermostatic Expansion Valve (TEV).
Replacing a capillary with a different size (a different diameter of the constriction) changes the cooling capacities of the system. For example, when the evaporator freezes, a capillary with a smaller constriction can be chosen. The degree of pressure reduction and the temperature of the refrigerant therefore depend on how the capillary is constructed. The advantage of the capillary is that the construction is cheaper than the TEV.
TEV (Thermostatic Expansion Valve):
A thermostatically controlled expansion valve controls the flow opening to the evaporator on the basis of temperature, which could not be regulated with the capillary.
We now deal with the thermostatic expansion valve with remote sensor.
There are several types, including an expansion valve with a distance sensor and internal and external pressure equalization, but also an expansion valve without a distance sensor (block valve) with internal and external diaphragm. The versions are all slightly different, but the operation comes down to the same; namely making the passage of the refrigerant through the constriction larger or smaller, so that more or less refrigerant can flow through the evaporator. To avoid confusion, we only deal with the type with distance sensor.
Based on the temperature of the gas leaving the evaporator (on the top left side in the picture), the measured values of the remote sensor control the thermostatic expansion valve further open or further closed. If the outgoing gases being measured are too cold, it means that the evaporator is getting too cold. That results in frostbite. The temperature should therefore not fall below 4 degrees Celsius. When the temperature of the refrigerant drops when leaving the evaporator, this will be measured by the remote sensor. The thermostatic expansion valve is closed slightly more, so that less refrigerant can flow into the evaporator. The temperature of the evaporator, so also the temperature of the outgoing refrigerant, will drop. With sufficient temperature increase, the sensor will control the TEV to allow some more refrigerant to enter the evaporator. More refrigerant is then evaporated, so that the outgoing gases become colder again. This is a process that keeps repeating itself to keep the temperature as constant as possible.
The moment the evaporator freezes, it will no longer be possible to blow air through it. A recognizable complaint is that the air conditioning works very well for a while, then blows less and less air and after that there is no more air flow, while the fan motor is running. After the air conditioning has been switched off, or the heating has been switched on for a while, the airflow becomes stronger again.
The evaporator freezes because the refrigerant falls below 4 degrees Celsius. This can be due to a faulty distance sensor, a fault with the TEV or a software error. Sometimes the complaint can be resolved with a software update, with which the computer increases the temperature of the refrigerant in the evaporator.
Click here to go to the air conditioning (main) page, where the operation of the whole system with the parts is described.
