Introduction:
An expansion valve is an essential component of air conditioning systems. It functions as a restriction in the line between the dryer/filter element and the evaporator, resulting in a transition from high to low pressure. In the image below, the expansion valve (executed as a block valve) is outlined in green.
After the refrigerant passes the filter/dryer element from the compressor, it reaches the expansion valve with a pressure of approximately 15 bar and a temperature of about 45 degrees Celsius. From the expansion valve, the refrigerant enters the evaporator. As the refrigerant flows through the restriction of the expansion valve, it undergoes a significant pressure reduction. When the pressure drops, the boiling point of the refrigerant also decreases. The refrigerant begins to evaporate and changes from liquid to gas form. In this phase change from liquid to vapor, the refrigerant absorbs heat from the surroundings. This released heat is extracted from the passing air that flows through the evaporator, resulting in the cooling of the air. This cooled air is directed into the interior, resulting in the cooled and dehumidified air that air conditioning produces.
There are different types of expansion valves, namely the capillary expansion valve and the thermostatic expansion valve (TEV), which is also often referred to as a “block valve.” These are described below.
Capillary:
In air conditioning systems, you may sometimes encounter a simple type of expansion valve, called a capillary or orifice. In newer vehicles, expansion valves are usually equipped with a thermostatic (controlled) expansion valve instead of a capillary.
In an air conditioning system with a capillary, the cooling capacity cannot be precisely adjusted. If the pressure becomes too high or the evaporator becomes too cold, the air conditioning compressor usually shuts off.
The exterior of the capillary expansion valve is usually made of plastic, and there is a special tube inside. Filters are present before and after this tube. The capillary causes a sudden pressure drop, which quickly lowers the boiling temperature of the refrigerant and changes it from liquid to gas. How the capillary is constructed determines how much the pressure drops, and this affects the temperature when the refrigerant enters the evaporator. The capillary can be found in various sizes, and if you install one with different dimensions, it changes the system’s cooling capacity. If less evaporation occurs in the evaporator, it usually means less cooling.
In air conditioning systems with a capillary, we usually also find an accumulator in the low-pressure section. This prevents liquid from being sucked into the compressor, as the capillary has a fixed opening. The accumulator also has other important tasks, such as filtering, removing moisture (drying), and storing refrigerant. The refrigerant enters the accumulator from the evaporator as a gas, with some liquid droplets remaining. A separation screen in the accumulator ensures that the liquid particles settle down along the side. A desiccant removes moisture from the refrigerant. Furthermore, the vapor is drawn through a small opening of about 1 millimeter by the compressor at the top, carrying a bit of oil with it.
In air conditioning systems with a capillary, the following malfunctions may occur:
- Blockage: If the capillary becomes clogged with contaminants in the refrigerant, it can reduce the cooling capacity;
- Incorrect dimensions: In certain cases, it may be necessary to replace the capillary with one of different dimensions to adjust the system’s cooling capacity. This may be required in system modifications or if the original specifications do not meet the required performance, such as a freezing evaporator or insufficient cooling.
- Systematic problems: If the air conditioning system continually shows performance issues and other components have been checked and are in good condition, the capillary may be a possible cause. The capillary may be damaged, which is not easy to see.
Thermostatic Expansion Valve (TEV):
An air conditioning system that we typically find in modern vehicles is a system with a thermostatically controlled expansion valve, shortened to TEV. The thermostatic expansion valve replaces the capillary system and is essentially a restriction whose opening size is regulated by the temperature of the gas exiting the evaporator.
There are various configurations. Besides replacing the capillary, the filter/dryer element is also different. The filter/dryer is located directly after the condenser and deals with the refrigerant in liquid form. The temperature is measured after the evaporator. If the evaporator temperature becomes too high because too little refrigerant is flowing through, the opening is enlarged, allowing more refrigerant into the evaporator and lowering the temperature again. The thermostatic expansion valve keeps the temperature (and pressure) within certain limits constant. This also means we can be sure that the refrigerant is drawn in vapor form by the compressor, which eliminates the need for an accumulator in the low-pressure section.
The thermostatic expansion valve can be divided into three types:
- Expansion valve with remote bulb-control with internal or external pressure equalization.
- Block valve with internal or external diaphragm.
- Electronically controlled expansion valve.
Thermostatic expansion valve with remote sensor and internal pressure equalization:
The thermostatic expansion valve consists of two parts, namely the sensing section and the sensor or bulb, which is connected to the actual expansion valve. The sensing section is filled with gas and is located at the outlet of the evaporator. When the temperature at the outlet of the evaporator rises because too little refrigerant passes, the gas expands and increases the pressure. The pin then releases the ball, allowing more refrigerant to enter the evaporator, and the temperature at the outlet drops again. The ball is released as soon as the force on the diaphragm from the sensor becomes greater than the sum of the spring force and the refrigerant pressure at the evaporator inlet. When the temperature after the evaporator becomes too low, the opposite happens. The spring force presses the ball back on the seat, narrowing the opening and reducing the flow of refrigerant. The TEV valve thus keeps the refrigerant temperature constant. The thermostatic expansion valve measures the temperature and converts it into pressure. The pressure regulation activates the valve.
Thermostatic expansion valve with remote sensor and external pressure equalization:
Pressure equalization relates to the pressure under the diaphragm. If the space under the diaphragm is connected to the inlet side of the evaporator, we do not account for the pressure drop that occurs in the evaporator. After all, the temperature measurement takes place on the outlet side of the evaporator, while the regulation takes place on the inlet side. When the pressure drop exceeds 0.2 bar, it is recommended to use an expansion valve with external pressure equalization. If the space under the diaphragm is connected to the outlet side of the evaporator, the pressure drop is compensated. External pressure equalization is typically applied to larger systems.
Block valve with external control diaphragm
The block valve is installed at the inlet and outlet line of the evaporator. The inlet line is located next to the outlet line at the evaporator. At the bottom of the block valve, the refrigerant enters in liquid form from the filter/dryer (condenser) and passes the ball valve on its way to the evaporator. Above the diaphragm is a specified amount of gaseous refrigerant. This gas will take the temperature of the gas exiting the evaporator. If the temperature rises, the pressure increase will push the stem downward, resulting in a larger flow opening in the supply line. This allows more refrigerant into the evaporator, lowering the temperature. In reverse situations, the ball valve will close, allowing less refrigerant into the evaporator, and raising the temperature.
Block valve with internal control diaphragm:
In the block valve with an internal control diaphragm, there is a thermocouple with refrigerant at the outlet side of the evaporator. The refrigerant in the thermocouple assumes the temperature of the refrigerant exiting the evaporator. At a high temperature, the refrigerant expands, causing the capsule diaphragm to push the rod down and enlarge the ball valve’s opening. Conversely, a lower temperature will cause the diaphragm to move upward, reducing the opening. These two situations are shown in the images below.
Electronically controlled thermostatic expansion valve:
With the help of the climate control ECU, the Electronically Controlled Expansion Valve (abbreviated as EEV) can be controlled. A stepper motor may be used for this purpose. This stepper motor allows the needle to gradually enlarge or reduce the opening. Depending on the desired interior temperature, the ECU can very quickly regulate the capacity by utilizing the electrically controlled air conditioning compressor and the expansion valve.
Possible Faults:
In the workshop, we encounter problems with the expansion valve. Usually, problems arise due to contamination, causing the expansion valve to become clogged or remain open.
- The valve is clogged:
Clogging is caused by impurities in the refrigerant. As a result of the clogging, too little refrigerant enters the evaporator, leading to rising pressure with the risk of overheating the compressor. - The valve remains open:
Because the valve remains open, too much refrigerant can enter the compressor. If not all refrigerant has turned into gas in the evaporator, there is a risk that a (too large) amount of liquid refrigerant enters the compressor, leading the compressor to a liquid lock.
Contamination is easy to prevent: replace the filter/dryer periodically.
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