Components
15. April 2021

Thermostatic expansion valves Structure and function

How does a thermostatic expansion valve work?

Thermostatic expansion valves

An alternative to thermostatic expansion valves are capillary tubes, although these are mainly used in refrigerator systems and simple, small refrigeration systems. The most important advantage of capillaries is the cost factor. The disadvantage is the inflexible regulation (or no regulation) of the injection into the evaporator.

Another alternative to thermostatic devices is the automatic expansion valve, which, however, hardly plays a role in today's refrigeration system construction. This is mainly because these valves aim to keep the evaporating pressure constant under all circumstances, even in the case of liquid return to the compressor.

Three-part expansion valve

 

 

functionality

Thermostatic expansion valves are designed in such a way that sufficient overheating at the evaporator outlet is guaranteed so that the compressor is not damaged. The overheating is the difference between the measured temperature value at the evaporator outlet and the wet vapor temperature of the pressure read on the pressure gauge (usually a corresponding temperature template is already placed over the pressure scale of the service pressure gauge to facilitate orientation, which varies from refrigerant to refrigerant). At the same time, this ensures that the entire exchange surface of the evaporator is used.

In this context, the refrigeration specialist should always keep an eye on the important role of the aggregate state change in compression refrigeration systems: in the evaporator, which absorbs the heat and in the condenser, which gives off the heat again (outside the room to be cooled). In this way, a significantly smaller volume flow can be used in the heat exchangers than would be possible with our usual refrigerants due to the heat transport through sensible temperature increase (measurable temperature increase). In addition, with this latent (hidden) heat absorption, we also benefit from the fact that there is no temperature change in the wet vapor zone (in the phase change) with single-component refrigerants and azeotropic refrigerant mixtures. This fact is very convenient in terms of heat transfer calculations.

 

But how does such a thermostatic expansion valve work?

an automatic mechanical controller without external energy. This means that it does not need an electrical connection or higher, externally generated pressure to function. In this context, it should be mentioned that there are also a large number of electronic expansion valves that work on the principle of overheating control. A separate article will be dedicated to this product family at a future point in this series in the KKA.

Expansion valve with external pressure equalization and evaporator

 

 

The components

The thermostatic expansion valve consists of a thermostatic element with a sensor, a sensor filling, a membrane, a connection by means of a tappet between the membrane and the valve seat and a housing, which usually offers the option of using power nozzles of different sizes. Coming from the receiver, liquid refrigerant from the liquid line is present at the expansion valve. This liquid is now controlled by the expansion valve and injected into the evaporator via the injection line and, if necessary, a refrigerant distributor. The sensor is attached to the evaporator outlet and it must be ensured that the evaporating pressure is present under the diaphragm in the expansion valve. If thermostatic expansion valves with internal pressure equalization are used – mostly in the case of very low refrigeration capacities and systems without a distributor in the injection line – the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the membrane. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the diaphragm. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the diaphragm. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold).

 

 

the membrane

Expansion valve on the evaporator with venturi distributor

 

For the following theoretical considerations, we assume parallel filling of the upper part of the expansion valve. Parallel filling means that there is the same refrigerant as a sensor filling in the expansion valve as in the system. In fact, parallel fillings are only used today for small series of special valves. In large series valves, refrigerant mixtures are used almost exclusively. We also assume that there is sufficient sensor charge in the expansion valve, so that we are always in the wet vapor range of the refrigerant in the sensor charge and never get into the overheated area. Various forces now act on the membrane in the expansion valve: from above, the pressure of the sensor charge and from below the evaporation pressure together with the superheat adjustment spring. If the diaphragm moves downwards, the valve will open and in the opposite direction, the expansion valve will tend to close. Let us now assume that the forces acting from above and below are in equilibrium. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled.

 

 

superheat setting

If you want to increase the superheat, turn the adjusting screw clockwise (this is the case with practically all expansion valves). With smaller compact valves, such as the "T2" or "TU", a fairly large change in superheat can be achieved with just a 360° turn.

 

P1 = bulb pressure P2 = evaporating pressure P3 = spring pressure

 

This can be up to 4 K per revolution, but varies depending on the evaporating temperature range and whether a MOP valve is used. With larger, three-piece valves, the change in superheat per revolution is significantly smaller. The rule of thumb is: 0.5 K per 360°. If in doubt, we recommend reading the relevant setting instructions. If we now start from our point of inertia with equilibrium of forces, the stronger tension of the spring changes the force that acts on the membrane from below. The valve moves towards the closed position. As a result, less refrigerant is injected into the evaporator, which leads to an increase in superheat. If the additional spring force is now equalized by the additional sensor force - due to the higher overheating -

MOP filling

 

 

MOP expansion valves

 

Standard filling (universal filling)

 

If an MOP valve is used instead of a standard expansion valve (MOP = maximum operating pressure or maximum working pressure), the valve aims to avoid excessive evaporation temperatures. Such MOP valves are particularly popular in deep-freeze systems, since deep-freeze compressors have a smaller electric motor than normal refrigeration or air-conditioning compressors and are protected against overload with this measure. The only technical change with a MOP valve compared to a "standard valve" is the limited sensor charge. If the complete sensor filling has evaporated at a certain temperature, the sensor pressure practically no longer increases (or only increases negligibly). This means that the force acting on the expansion valve diaphragm from above

 

Make superheat setting changes in small increments and never in haste

 

This results in an upper limit for the evaporation pressure. For example, at a room temperature of +10 °C, a MOP valve with an MOP point of -10 °C will continue to vaporize at -10 °C, whereas a "standard valve" would already vaporize at 0 °C. It is important for the practitioner to know that higher overheating can occur, especially with MOP valves, without there being a malfunction.

 

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