• Operation of the evaporator
  • Expanding the refrigerant
  • Freezing the evaporator

Operation of the evaporator:
The evaporator is located under the dashboard in the stove house built-in. The evaporator itself is constructed of curved tubes with aluminum heat plates, or rows of aluminum tubes. The refrigerant flows through these pipes in vapor form, which has cooled down strongly due to the pressure drop of the expansion valve. The temperature of the refrigerant at the beginning of the evaporator is about 3º C. The outside air passing through the interior fan blown through, it cools down. The heat from this air is transferred to the refrigerant. This heats up the refrigerant and will have reached a temperature of at least 5º C when it leaves the evaporator.

The evaporator must not freeze. The outside air can no longer pass through it. To prevent freezing, the flow of the refrigerant through the Thermal Expansion Valve constantly arranged. In the next chapter more about the freezing phenomena of the evaporator.

Expanding the refrigerant:
The evaporator's job is to cool the passing outside air on its way to the interior. The air is blown through the ice-cold evaporator, causing it to cool down. The refrigerant enters the evaporator in gaseous form. This is due to the expansion valve, which is mounted just in front of the evaporator. An expansion valve creates a narrowing in the air conditioning line between the condenser (or dryer / filter in the case of a TEV) and the evaporator. The change from high to low pressure takes place from this expander. Two types of expansion valves can be fitted, namely the Thermal Expansion Valve (TEV) and the Capillary.
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 narrowing, the refrigerant loses its pressure, which 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 where an air conditioner is used.

On the page expander everything about the Thermal Expansion Valve (TEV) and the Capillary is explained.

Freezing the evaporator:
It happens in some cars that the evaporator freezes. At that moment no more air can pass through the evaporator and no more air is blown through the ventilation grilles. This would then happen after the air conditioning has been on for a while. Due to the very low temperature of the evaporator and excessive moisture, the moisture in the evaporator freezes, causing it to clog. A possible cause is that the water drain is clogged. In air conditioning, moisture is extracted from the outside air, which is discharged as condensation from the evaporator through the water outlet on the street. That is why there is always a puddle of water under the car when the outside temperature is very hot and the air conditioning is switched on.
When the discharge is in order, a possibility can be sought to raise the evaporator temperature. With some cars this can be done electronically with the aid of diagnostic equipment in the garage, but for systems with a capillary the mounting of another expansion device offers the solution. Mounting another capillary with a larger constriction means less pressure drop than with a smaller constriction. Less pressure drop also means a drop in refrigerant temperature. This may be enough to stop the evaporator from freezing.

Click here to go to the air conditioning (main) page, where the operation of the whole system with the parts is described.

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