Evaporator:
The air blown into the cabin by the interior blower fan undergoes a cooling process when the air conditioning is switched on. This air-cooling process takes place in the evaporator. The evaporator is installed under the dashboard, in the HVAC housing. It is made up of bent pipes with aluminum heat transfer plates or rows of aluminum tubes.
Gaseous, evaporated refrigerant flows through the evaporator. The evaporator extracts heat and moisture from the air that the interior blower fan blows or draws through it, due to the evaporation of the refrigerant. You notice the same effect when disinfectant liquid gets on your skin; it feels cold because the disinfectant evaporates quickly on your skin. A small part of your body heat is transferred to the surrounding air.
The air from the interior blower fan transfers its heat to the evaporator. The cooled and dehumidified air is then directed by the HVAC flaps to the correct outlet openings, reaching the cabin.
When the A/C is switched off, refrigerant is still present in the evaporator, but it is not circulated by the A/C compressor. At that moment, the interior blower fan blows or draws outside air through it. The evaporator then takes on the temperature of the outside air.
Expanding the refrigerant:
As the refrigerant circulates through the A/C system, it goes through different states: liquid, vapor, or a combination of both. These are the so-called states of matter. In the evaporator—as the name suggests—the refrigerant will evaporate. Evaporation occurs after expansion. In the next paragraph we will go into this in more detail.
After the refrigerant has passed the filter/drier element, it reaches the expansion valve at a (high) pressure of about 15 bar and a temperature of about 55 degrees. This valve is mounted directly on the inlet and outlet of the evaporator. In the expansion valve, the transition from high to low pressure takes place. Inside the expansion valve there is a restriction that the refrigerant is forced through. This restriction causes the refrigerant pressure to drop from about 15 bar to 2 bar. This sudden pressure drop results in a lower boiling point of the refrigerant. As a result, the refrigerant changes from liquid to saturated vapor. This means that both vapor and liquid particles are present.
Heat is required for the evaporation of liquid. To make the transition from liquid to gas possible, the refrigerant extracts heat from the air flowing through the evaporator. This air cools down and then flows into the cabin. This heat causes the liquid particles to turn into vapor.
There are two types of expansion valves: the Thermal Expansion Valve (TXV) and the capillary tube. On the page about the expansion valve, both types are described.
Evaporator freezing:
In some cars, the evaporator may freeze. At that point, no more air can flow through the evaporator, so no air is blown through the air vents. This phenomenon often occurs 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 and cause blockages. A potential cause of this can be a clogged water drain.
In air conditioning systems, moisture is extracted from the outside air, which forms as condensation on the evaporator and is drained onto the street via the water drain. That is why you often see a puddle of water under the car when the outside temperature is high and the A/C is switched on.
If the water drain is functioning correctly, you can look at ways to increase the evaporator temperature. In certain cars, this can be controlled electronically using diagnostic equipment at a repair shop. For systems with a capillary tube, replacing the expansion device can be a solution. A capillary tube with a larger restriction can be installed, resulting in less pressure drop than with a capillary tube with a smaller restriction. Less pressure drop also results in a less severe drop in refrigerant temperature. This may be enough to prevent the evaporator from freezing.
Possible malfunctions and defects:
When an air conditioning system is not working properly, the technician often first checks the pressures in the system. Depending on the fault, the evaporator may be the culprit. These are the most common malfunctions and defects of the evaporator:
- Leakage: One of the most common problems is leakage. This can be caused by corrosion, expansion and contraction, and therefore aging of the material. The evaporator is installed behind the dashboard in the HVAC housing. Therefore, to remove the evaporator, the entire dashboard and HVAC housing often have to be removed. This is a labor-intensive job. In the image below, we see a removed dashboard and HVAC housing, alongside the leaking evaporator of a BMW 1 Series (2012). Leaking refrigerant can reduce the performance of the A/C system, eventually resulting in an empty system. When all refrigerant has escaped and the pressures are too low, the compressor will no longer switch on for safety.
When checking for leaks, we use a leak detector. When the system has been filled with a UV additive, yellow/green spots can be observed around the leak. However, the evaporator is often so difficult to access that a visual inspection is not possible. The leak detector must then provide the answer. - Restriction or blockage: Contamination or deformation of the material can restrict the airflow through the evaporator. By measuring pressures and temperatures, we can identify whether this is the case.
Related page: