BETWEEN TWO WORLDS

So small and yet so vital: expansion and throttle valves separate the high-pressure from the low-pressure section in the air conditioning circuit. They sit upstream of the evaporator and regulate the refrigerant flow as a function of temperature. If they are defective, the air conditioning system simply won’t work.

The expansion/throttle valve is the fourth component on the high-pressure side of the air conditioning circuit after the compressor, the condenser, and the filter-drier. The liquid refrigerant, which has been cleaned by the filter-drier, still reaches the expansion/throttle valve under pressure. Since this valve represents a “bottleneck,” the refrigerant expands abruptly. The pressure drop causes the refrigerant to transform into a gaseous state. Now it has arrived on the low-pressure side.

Purpose of Function

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The expansion valve is seated in a small, usually cube-shaped housing. It has four connections for the low-pressure and high-pressure refrigerant lines. Inside, a needle regulates the flow rate of the high-pressure line. The needle is controlled by a diaphragm. A spring provides the necessary resetting force.

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The throttle valve essentially has the same but somewhat simplified function. As the name suggests, this valve throttles the flow of refrigerant. The rapid loss of pressure causes the refrigerant to cool down. A fine sieve in the throttle valve then sprays the refrigerant into the evaporator. Instead of a filter-drier, air conditioning systems with a throttle valve have an accumulator with a similar filtering and storage function. However, the accumulator is located on the low-pressure side between the evaporator and the air conditioning compressor. This ensures that no liquid refrigerant enters the air conditioning compressor and thus causes damage.

What expansion and throttle valves do is pure physics: The pressurized liquid refrigerant expands downstream of the valve. As a result, it is cooled down and injected into the evaporator as a fine mist. The drop in pressure reduces the boiling point of the refrigerant, causing it to change into a gaseous state. Evaporation cooling is produced in this process. To ensure an optimal cooling capacity in the evaporator at all times, the expansion valve regulates the refrigerant flow as a function of temperature. For example, if the refrigerant temperature rises at the end of the evaporator, the refrigerant expands in the expansion valve, increasing the quantity injected into the evaporator. This prevents the refrigerant from changing back into a liquid state. If the refrigerant temperature drops at the end of the evaporator, the volume in the expansion valve decreases and the flow of refrigerant to the evaporator is reduced. The refrigerant then passes to the air conditioning compressor in gaseous form.

Liquids cannot be compressed like gases. Therefore, without expansion or throttle valves, there would be serious damage to the air conditioning compressor.

Important! Never change the factory calibration of the valve!

Function expansion valve

Function throttle valve

Common causes of failure

Expansion and throttle valves need to withstand major pressure and temperature differences. Yet, they have a very delicate structure on the inside. Their exposed position and function means that component failure will soon be noticeable from poor cooling performance or total failure of the air conditioning system. The most common reason for failure is contamination (foreign objects such as metallic abrasion particles) in the system. As a result, expansion and throttle valves become clogged and stop working. However, temperature problems due to overheating or icing, as well as leaks at the components or connection lines, can also result in failure. This usually affects the air conditioning compressor.

Comparison of new and clogged throttle valve
Throttle valve clogged with metallic abrasion particles

Preventing damage

To avoid damage to the entire air conditioning system or the expansion/throttle valve, use caution when working on the air conditioning system and keep a few important points in mind:

• In case of contamination, flush the air conditioning system and always replace the expansion/throttle valve, as they cannot be flushed!

• Apply correct assembly torques.

• Always replace the accompanying seals and never install them dry.

• Ensure general cleanliness during assembly.

If your customers experience problems with the air conditioning system, you should follow these steps:

• Visual inspection for external damage

• Acoustic test for noise development

• Check that the connecting lines are correctly and securely attached

• Check component and connections for leaks

• Temperature measurement on the pipework system

• Pressure measurement with air conditioning compressor switched on and engine running

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