Characteristics, principle of operation and installation of thermostatic expansion valve

In heating and air conditioning systems operating under variable environmental conditions, it is absolutely necessary to adjust the power of the existing installation. This allows you to maintain the required temperature and save energy consumption during its operation. In automatic mode, this task is handled by a thermostatic expansion valve. It controls the flow of the working medium, reacting to external temperature changes.

Design and principle of operation

In refrigeration units and air conditioners, a closed circuit is used, through which the refrigerant circulates, changing its state of aggregation in the evaporator. In heating systems, heating is carried out by pumping hot liquid to thermocouples. Despite the development of various alternative methods of cooling and heating, such a scheme of operation is the main one.

With a low power of the device, constant adjustment to external changes is not required. In low-power cooling systems, the role of the regulator is performed by a throttle from a capillary tube. Its operation does not depend on the performance of the evaporators and is not able to change the refrigerant level in the circuit.

Heating circuits are equipped with manual controllers. In them, the change in the flow of hot liquid is carried out by turning the handle, lowering or raising the restrictive rod.

In systems where constant adjustment to changing external conditions is required, the cooling or heating power is adjusted by changing the flow of the working medium.

The main flow regulator is an expansion valve, which stands for thermostatic expansion valve. This is a direct action device. It does not require any external energy to operate. The valve reacts to the overheating of the vapors leaving the evaporator. And it, in turn, depends on the load on the cooling system.

An additional advantage of using expansion valves is that the system is not critical to the exact amount of filling refrigerant.

The internal structure of the regulator is shown in the figure.

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The main elements of the TRV are:

• a membrane or diaphragm that controls the movement of the shutoff rod;
• a capillary tube with a thermal bulb that transmits to the device changes in the temperature of the vapors at the outlet of the evaporator,
• adjusting spring for adjusting the installation level,
• inlet and outlet fittings.

The combination of a diaphragm, a thermal bulb and a capillary tube is called a thermoelement. It is he who perceives the ambient temperature and regulates the supply of refrigerant.

The principle of operation of the valve is to move the membrane under the action of three forces:

• medium pressure from the bulb,
• evaporator equalizing pressure,
• spring action.

After reaching a balance between these three force components, the diaphragm sets the required amount of refrigerant flow.

Bulb pressure = equalizing pressure + spring pressure on diaphragm.

With a change in temperature and an increase in the thermal load in the evaporator, the heating of the bulb and the pressure of the liquid filling it increase. Through the capillary tube, it is transferred to the diaphragm, as a result of which the valve opens and the refrigerant supply to the evaporator increases.

The thermostatic valve of the heating radiator is arranged according to a similar principle.

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In it, the role of a thermal bulb is performed by a sensitive element (float) located in a cavity filled with liquid or gas. When the temperature changes, the volume of the medium decreases or increases. As a result, the float changes its position, shifting the stem, which changes the valve flow area.

Gas-filled thermocouples are considered the most sensitive. They react to temperature changes faster than liquid ones. But they also cost more.

Characteristics and types of expansion valves

When choosing a device, you need to pay attention to the following parameters:

• The maximum temperature at which the valve can operate. It can reach 200 °C.
• The pressure of the working environment. Usually in the range of 16 — 40 bar.
• Manufacturing material. The body is made of bronze or brass. But stainless steel valves have the best anti-corrosion properties.
• TRV performance. This is the maximum flow allowed by a fully open valve. It must correspond to the capacity of the refrigeration unit.
• The diameter of the inlet and outlet fittings must correspond to the pipelines of the entire regulated system.

Expansion valves for refrigeration and air-conditioning are distinguished by the type of supply of equalizing pressure from the evaporator.

Internal adjustment

The pressure is transferred under the lower edge of the diaphragm through the machined gaps around the stem. This type of valve is used only for single pass evaporators with low hydraulic resistance.

The pressure of the refrigerant on the membrane is carried out before it enters the evaporator.

External adjustment

In a more advanced control system, equalizing pressure enters the valve directly from the outlet of the evaporator. To supply this pressure, an additional inlet pipe is provided in the housing, which ensures the flow of refrigerant from the evaporator under the thermoelement membrane. In this case, the subdiaphragmatic cavity is isolated by a separate seal from the outlet pressure of the valve.

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Such regulators are applicable to work with any methods of cooling and with different types of refrigerant. But they cannot be used according to the scheme with internal adjustment. The equalization tube must be connected to the evaporator outlet. It cannot be silenced.

Methods for connecting valves to system pipes:

• using a threaded connection;
• through the flange
• permanent welded joint.

The thermostatic expansion valves of heating systems differ in shape depending on their location on the pipe. Straight or axial cut into a flat section of the pipeline. Corner options are installed in places where the pipe bends and change the direction of fluid movement.

Mounting Features

The installation of thermostatic expansion valves for heating and air conditioning should be considered separately, since the requirements and recommendations in these cases are different.

Installation in the air conditioning system

A general view of the inclusion of a temperature control device in the piping scheme for refrigeration units is shown in the figure.

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During installation, the following rules must be observed:

• The valve is installed on the line in the immediate vicinity of the evaporator. The part of the housing with the diaphragm must be vertical.
• The installation location of the bulb is as close as possible to the outlet of the evaporator. But it should be installed only on a horizontal section of the pipeline. The location of the cylinder on a vertical pipe will lead to malfunctions of the thermostat, especially at the time of starting the air conditioner.
• The bulb must fit snugly against the outlet pipe of the evaporator. Location — only on top of the pipe, it is unacceptable to install a bulb under the pipe or on the side.
• Fastening to the pipe must be carried out with a special clamp included in the thermostatic valve kit. Other methods do not provide reliable contact, which ultimately leads to a distortion of the pressure transmitted to the valve thermocouple.
• For devices with external pressure equalization, it is mandatory to connect the equalization pipe to the evaporator outlet. The outlet must be carried out from the top of the outlet pipe at a distance of at least 100 mm from the bulb and at the same distance from the oil lifting loop.

If it is not possible to install a thermal bulb on a horizontal section of the pipeline, then it is allowed to mount it on a vertical pipe. But the direction of the refrigerant must be from top to bottom, and the cylinder is fixed with a capillary tube upwards.

Installing a thermostatic valve in heating pipes

The main element of the centralized system is a heat radiator or convector. It is most convenient to adjust the amount of hot liquid flow in each device separately.

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For reliable regulation of heat flow, two devices are installed on each radiator — at the inlet and outlet. In single-pipe systems, where the movement of the working medium through the elements is sequential, it is necessary to install bypasses. These are bypass tubes that ensure the functioning of the line in case of blockage or clogging of one of the radiators.

Possible installation errors and malfunctions

The main problems in the operation of the expansion valve arise due to the incorrect installation location of the valve itself or the bulb. The accuracy of adjustment can also be affected by minor factors when fixing the elements of the device.

One of the common problems is the inaccurate transfer of the required pressure by the bulb to the thermocouple. The reason for this may be its poor contact with the outlet pipe of the evaporator. The installation site must be carefully cleaned and covered with heat-conducting paste. Do not place the bulb on the welds connecting the pipes.

The sensor itself must be insulated so that ambient air does not affect its temperature.

The complete expansion of the expansion valve is often due to the use of models with plastic internals.