The Thermostatic Valve

For the understanding of the operation of the Thermostatic Expansion Valve, it is important to understand the concept of superheated refrigerant.

The Thermostatic Expansion Valve in the cooling cycle is perhaps one of the least understood components, that is why we will try to review its principle of operation. An air conditioning and refrigeration cycle basically consists of six elements so that it can work (Fig. 1): compressor, thermostatic valve (element regulating the amount of refrigerant or capillary tube), condenser, evaporator, pipe and refrigerant.

The operation of a thermal cycle is governed by nature or ambient pressure-temperature conditions. When these variations are minor or the environment is stable as in the case of a domestic refrigerator located within a place of little environmental variation, in small air conditioning packages, the use of a capillary tube is acceptable.

Unfortunately as we can see in Fig. 1, the cycle is subject to continuous variations. By the Thermostatic Valve is the condenser. In a place that is hot or cold it is subject to variations in ambient temperature during each day, and to that of the different seasons during the year. All these different temperatures cause different condensate pressures.

On the opposite side of the valve is the Evaporator, which is also subject to continuous changes such as variations in thermal load, relative humidity, product movements, cooling speed, thaws, etc. This also produces the corresponding evaporation pressure variations.

The amount or flow of refrigerant required in the cycle depends mainly on the thermal load and temperature conditions required in the cold room or chamber. The compressor is the machine that pumps this amount of refrigerant at condenser and evaporator pressures.

The pressure and temperature for a condenser are governed by environmental conditions.

The diameter of the hole of the Thermostatic Valve determines the pressure of the refrigerant in the evaporator that corresponds to the saturation temperature necessary for the required application.

Therefore, the minimum information required for the selection of an expansion valve is:

1 Compressor discharge pressure, which corresponds to the saturation temperature of the refrigerant in the condenser, and in turn directly proportional to the ambient temperature.

2 Pressure of the evaporator or suction of the compressor, which corresponds to the saturation temperature of the refrigerant in the evaporator, and in turn directly proportional to the temperature of the room or cold chamber.

3 Thermal capacity or load in Btu/h, tonnes, kcal/h, etc. under the conditions of points 1 and 2.

4 Since conditions 1, 2 and 3 are continuously changing, it is important that a device is available to adapt the cycle to these variations. The Thermostatic Expansion Valve, TXV, is able to regulate the flow of refrigerant under these different conditions.

For the understanding of the operation of the Thermostatic Expansion Valve, it is important to understand the concept of superheated refrigerant.

The condition in which the temperature and pressure for which a substance or refrigerant can exist simultaneously in liquid and vapor state is called the "SATURATION" point.

After a liquid has changed to vapor, keeping its pressure constant, any addition of heat (sensitive heat) causes an elevation in its temperature. The term "SUPERHEATED" vapor is used to describe a gas whose temperature is above its saturation point or boiling point. See Fig. 2.

The Thermostatic Expansion Valve is a device designed to regulate or dose the flow of liquid refrigerant to the evaporator. It does so in the same proportion in which the refrigerant is evaporated.

This is achieved by maintaining a predetermined overheating at the evaporator outlet (in the suction line towards the compressor).

This ensures that all the refrigerant liquid has evaporated inside the evaporator, so all the refrigerant that returns to the compressor is in a gaseous state. See Fig. 3, and Fig. 4.

That the overheating at the compressor inlet is correct (58 – 34 = 24ºF (13.3 ºC))

That the charge and refrigerant flow are correct

That the evaporator is used completely, (maximum efficiency)

The temperature is as desired

The Thermostatic Valve is installed in the liquid line (see Fig. 1) at the evaporator inlet, and separates the high and low pressure sides of the system. The temperature sensor bulb is placed at the outlet of the evaporator. The Thermostatic Valve is kept in its closed position until the predetermined overheating is reached.

Subsequently, the refrigerant flows through the valve hole, consistently maintaining the flow of refrigerant required for the thermal load and that of its overheating adjustment.

The mass amount of refrigerant passing through the VTX is the same as that to be pumped by the compressor, and they must be perfectly twinned.

The thermodynamic process in the expansion valve is Isoentalpic, in which there is no heat transfer, no energy or mechanical work. The cycle of Fig. 1 is a closed system, the work or energy given to the compressor plus the heat absorbed in the evaporator is equal to the heat rejected in the condenser.

There are three forces which govern the operation of the Thermostatic Valve which are (see Fig. 5 and 6): P1 corresponding to the power element due to the pressure of the remote sensing bulb, P2 corresponding to the pressure of the evaporator, and P3 corresponding to the force of the spring equivalent to overheating.

Among the types of Thermostatic Valves we can mention the Valve with Internal Equalizer, in which the pressure P2 of the evaporator or valve outlet, is transmitted to the lower part of the diaphragm through a hole inside the valve body (see Fig. 6). These valves are used when the pressure drop in the evaporator is less than 3 psi. The equalizer hole is the passage that connects the evaporator with the bottom of the diaphragm.

In applications where the pressure drop between the valve outlet and the evaporator outlet is negligible, valves with internal equalizer are effective.

The Valve with External Equalizer is used where the pressure drop in the evaporator is greater than 3 psi. The pressure P2 is that of the evaporator outlet by means of a tube connector to the valve.

As an example in Fig. 3, a pressure drop in the evaporator of 10 psi (68.5 – 58.5) is observed. When using R-22 refrigerant, the P2 pressure below the diagram is 58.5 psig (detected through the equalizer tube), which corresponds to a saturation temperature of 33°F (in table P-T) of the refrigerant R-22, therefore the overheating at the evaporator outlet is the measured evaporator output temperature of 40°F minus the saturation temperature of 33°F, and equal to 7°F (40°F – 33°F).

In this same example Fig. 3, in case of not using the equalizer tube; the P2 pressure below the diagram would be 68.5 psig, and a pressure in the sensor bulb of 78.5 psig would be required for proper pressure balance. This pressure corresponds to a bulb saturation temperature (equal to the evaporator outlet temperature) of 46.4°F, therefore the overheating at the evaporator outlet is the evaporator outlet temperature of 46.4°F minus the saturation temperature at the outlet of 33°F, and equal to 13.4°F (46.4°F – 33°F).

In conclusion, if the pressure drop is significant>3.0 psi, it is necessary to use valves with external equalizer, since excessive overheating is an indication of the low use of the evaporator that results in a low efficiency of heat transmission, with high operating costs.

The remote sensing bulb and the pressure of the power element P1 corresponding to the saturation pressure of the refrigerant gas leaving the evaporator, moves the valve needle or bolt in the direction of opening, (see Fig. 5).

Opposed to this opening force at the bottom of the diaphragm and acting in the direction of closure are two forces, the one due to the pressure of the P2 evaporator and the one due to the action of the P3 superheating spring. The first situation occurs when the valve is in a stable control position, and the three forces are balanced, (see Fig. 6 i.e. P1 = P2 + P3).

The following situation is due to an increase in thermal load on the evaporator. The overheating of the gas leaving the evaporator (remote sensing bulb site) is also increased above the saturation temperature corresponding to the evaporation pressure. The pressure generated inside the P1 sensor bulb, due to the increase in temperature, is increased above the two combined forces (from the P2 evaporator, and the P3 overheating spring), causing the valve needle to move in the direction of opening.

Conversely due to a decrease in thermal load on the evaporator, the overheating of the gas leaving the evaporator (remote sensing bulb site) is also reduced below the saturation temperature corresponding to the evaporation pressure. The pressure generated inside the P1 sensor bulb, due to the increase in temperature, is decreased below the two combined forces (from the P2 evaporator, and the P3 overheating spring), causing the valve needle to move in the direction of closing.

In both situations, when increasing or decreasing the flow of refrigerant, the valve resumes the balance of its pressures, always maintaining the overheating necessary for an efficient and safe operation of the evaporator and compressor.

Factory adjustments of Thermostatic Expansion Valves are made just as your needle begins to stop touching your seat (see Fig. 5) approximately 0.002 pg. The increase in overheating just as the needle begins to move is referred to in Fig. 5 as Static Overheating (SS) or Initial Opening.

Valves usually operate in such a way that an increase in the overheating of the gas leaving the evaporator is usually above 4 to 6°F of the value of the adjustment of its static value, this is necessary for the valve needle to open to its nominal operating position. This additional overheating is known as Gradient (OS). If a factory valve comes with an adjustment of 6°F of overheating it will operate at its nominal value when its needle is in its corresponding position, being of the order of 10 to 12 degrees of overheating.

Total overheating at compressor inlet of 20°F is recommended. The sensor bulb should be mounted firmly in a clean section of the tube and horizontally, and as close as possible to the evaporator outlet. Its recommended location is between the position of three and four of the hands of the clock. Isolate it from the ambient temperatures and drafts that surround it, to prevent erroneous readings of the suction line temperature.

It should not be installed where liquid coolant atroping exists or at the bottom of the suction line, as it will send an incorrect signal with a false valve response.

A proper installation of the sensor bulb ensures a good functioning of the Thermostatic Valve, and a good response to temperature changes of the gas leaving the evaporator.

See original.