Table of Contents
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Where is the expansion valve located?
The expansion valve can be located either in front of the bulkhead (in the engine compartment) or behind the bulkhead (in the interior) of the vehicle.
This location allows the refrigerant to flow from the condenser to the evaporator through the filter dryer and the expansion valve. Then, it returns to the expansion valve to finally reach the compressor.
Here is the complete process:
1. The refrigerant flows from the condenser, via the filter dryer, to the expansion valve.
2. Then, the refrigerant flows to the evaporator.
3. When the refrigerant leaves the evaporator, it flows through the measuring side of the expansion valve to the compressor
What is the function of an automotive expansion valve?
The expansion valve allows the refrigerant to reach the evaporator in the right conditions. Because the valve is variable, both the state and the amount of refrigerant are adjusted.
When the refrigerant flows through the expansion valve, the pressure, temperature and boiling point decrease. As a result, the refrigerant is about to change state. Once it arrives in the evaporator, the refrigerant evaporates with the help of the heat from the airflow. By removing this heat, the temperature of the airflow drops.
The outside temperature is not always the same. If cold air flows through the evaporator, less refrigerant can change state than when it is warm outside. The expansion valve allows the maximum amount of refrigerant that can evaporate through, which prevents liquid refrigerant from leaving the evaporator.
What is the structure of the expansion valve?
The valve block is a commonly used implementation of the expansion valve.
- The lower half of the valve block takes care of the pressure and temperature drop.
- The upper half is the measuring side of the valve block.
- On top of the valve block, there is a metal housing containing a temperature sensitive element and a membrane.
- The membrane is connected to a control pin.
- This control pin rests on a ball, which is pressed by a return spring against the seat of the passage.
The space between the ball and the seat is the valve.
How does an automotive expansion valve work?
The available space increases when the refrigerant leaves the valve in the lower half of the valve block. The refrigerant suddenly has much more space, so the pressure drops sharply. With the lowering of the pressure, the temperature and boiling point of the refrigerant also decrease.
The boiling point of the refrigerant must not be too high. The heat from the airflow should be sufficient to reach the refrigerant’s boiling point so that the refrigerant evaporates. During the evaporation, the refrigerant withdraws a lot of heat from the airflow.
The measuring element
The refrigerant changes state as it flows through the evaporator. In addition to changing state, the temperature increases slightly.
This temperature increase expands the measuring element, whereby the membrane moves downward. The control pin follows the movement of the membrane and pushes the ball down against the spring force.
When the valve opens further, more liquid refrigerant flows to the evaporator. As a result, the temperature of the gaseous refrigerant leaving the evaporator drops.
The measuring element cools down again, moving the membrane upwards, which makes the valve opening smaller.
After that, the temperature of the gaseous refrigerant rises again, and the cycle repeats itself until a balance is reached. This process is how the expansion valve works, controlling pressure, flow rate, and temperature in a vehicle’s air con system.
Knowing the basics of an automotive expansion valve is one of the first steps students must take to understand, diagnose, and repair a car’s air conditioning system.
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