Evaporator:
The air that is blown into the interior by the interior fan undergoes a cooling process through the activated air conditioning. This cooling process of the air takes place in the evaporator. The evaporator is built under the dashboard, in the heating box. It consists of bent pipes with aluminum heat plates or rows of aluminum tubes.
Gaseous, evaporated refrigerant flows through the evaporator. The evaporator extracts heat and moisture from the air blown through it by the interior fan or attracted by the evaporation of the refrigerant. You can feel this same effect when you get disinfectant on your skin; it feels cold due to the rapid evaporation of the disinfectant on your skin. A small portion of body heat is transferred to the surrounding air.
The air from the interior fan gives up its heat to the evaporator. The cooled and dehumidified air is then directed through the heater flaps to the appropriate discharge openings, reaching the interior.
When the air conditioning is turned off, refrigerant remains in the evaporator, but it is not circulated by the air conditioning compressor. The interior fan blows or sucks external air through it at that moment. The evaporator then takes on the temperature of the outside air.
Expansion of the refrigerant:
During the circulation of refrigerant in the air conditioning system, the refrigerant goes through various states: liquid, vapor, or a combination of both. These are referred to as the phases of matter. In the evaporator, as the name suggests, the refrigerant will evaporate. The evaporation occurs after the expansion. We will delve deeper into this in the following paragraph.
After the refrigerant passes through the filter/drier element, it reaches the expansion valve with a (high) pressure of about 15 bar and a temperature of about 55 degrees. This valve is directly mounted on the intake and outlet of the evaporator. In the expansion valve, the transition from high to low pressure occurs. Inside the expansion valve there is a restriction through which the refrigerant is forced. This restriction causes a pressure drop of the refrigerant from approximately 15 bar to 2 bar. This sudden pressure drop results in a lowering of the boiling point of the refrigerant. Consequently, the refrigerant transitions from liquid to saturated vapor. This indicates the presence of both vapor and liquid particles.
Heat is needed for the evaporation of liquid. To enable the transition from liquid to gas, the refrigerant extracts heat from the air flowing through the evaporator. This air cools down and then flows into the interior. This heat prompts the liquid particles to transform into vapor.
There are two types of expansion valves: the Thermal Expansion Valve (TEV) and the Capillary. On the page about the expansion valve, both types are described.
Freezing of the evaporator:
In some cars, the evaporator can freeze. When this occurs, no air can flow through the evaporator, resulting in no air being blown through the ventilation openings. This phenomenon often arises after prolonged use of the air conditioning. Due to the extremely low temperature of the evaporator and the presence of excessive moisture, the moisture in the evaporator can freeze, leading to blockages. A potential cause for this might be a clogged water drain.
In air conditioning systems, moisture is removed from the outside air and forms as condensate on the evaporator, which is drained onto the street through the water drain. This is why a puddle of water is often seen under the car when the outside temperature is high and the air conditioning is on.
If the water drain is functioning correctly, one can look at ways to increase the evaporator’s temperature. In certain cars, this can be electronically controlled using diagnostic equipment in a garage. For systems with a capillary, replacing the expansion device might be a solution. A capillary with a larger restriction can be installed, which results in less pressure drop than a capillary with a smaller restriction. Less pressure drop also results in a less significant decrease in the refrigerant temperature. This might be sufficient to prevent the evaporator from freezing.
Possible malfunctions and defects:
When an air conditioning system is not functioning properly, the technician often first checks the pressures in the system. Depending on the malfunction, the evaporator could be the culprit. These are the most common malfunctions and defects of the evaporator:
- Leakage: One of the most common issues is leakage. This can be caused by corrosion, expansion and contraction, and ageing of the material. The evaporator is built behind the dashboard in the heating box. To dismantle the evaporator, often the entire dashboard and heating box need to be disassembled. This is a labor-intensive task. In the image below, we see a dismantled dashboard and heating box, along with the leaking evaporator of a BMW 1-series (2012). Leaking refrigerant can decrease the performance of the air conditioning system, ultimately leading to an empty system. When all refrigerant has escaped and the pressures are too low, the compressor will not engage for safety reasons.
When checking for leaks, we use a leak detector. When the system is filled with a UV additive, yellow/green streaks are visible around the leak. However, the evaporator is often difficult to access, making visual inspection impossible. The leak detector should provide a solution. - Narrowing or blockage: Contaminants or material deformations can obstruct the airflow through the evaporator. By measuring pressures and temperatures, we can identify if this issue is present.
Related page: