Thermal expansion valve An important part of refrigeration system

also known as constant temperature expansion valve, is an automatic expansion valve,  is an important part of the refrigeration system,  it depends on the evaporator export gas refrigerant superheat to control the opening of the valve, to automatically adjust the refrigerant flow to the evaporator, and at the same time play a throttling role. Thermal expansion valve is composed of temperature sensing bag, capillary, diaphragm, fixed spring, throttle needle valve, adjusting screw and other parts. It is widely used in freon refrigeration system and is used in combination with non-full liquid evaporator. 

Thermal expansion valve is divided into internal balance type and external balance type according to the different extraction point of evaporation pressure under the diaphragm. The evaporation pressure of the former is drawn from the inside of the valve body, while the equilibrium pressure of the latter is drawn from the evaporator outlet. Therefore, for the evaporator with large resistance loss (and large cold capacity), the external balance type should be selected. 

Internal balance type

Figure 1 shows the working principle diagram of the internal balanced thermal expansion valve. It can be seen from the diagram that it is composed of a spool, a seat, an elastic metal diaphragm, a spring, a temperature sensing bag and adjusting screws. Among them, 1 - valve core; 2 - elastic metal diaphragm; 3 - spring; 4 - Adjusting screws; 5 - Temperature sensing bag. With the common liquid-filled thermal expansion valve analysis, the elastic metal diaphragm is subjected to three forces:
P1 - The pressure of the refrigerant behind the valve, acting on the lower part of the diaphragm, causing the valve to move in the direction of closing;
P2 - The spring force is also applied under the diaphragm to move the valve in the direction of closing, and the force can be adjusted by adjusting the screw;
p3 - The pressure of the refrigerant in the thermal envelope, acting on the upper part of the diaphragm, causing the valve to move in the open direction, depending on the nature of the refrigerant in the thermal envelope and the temperature felt by the thermal envelope.
For any operating condition, the three forces will reach a balance, that is, p1+p2=p3, at this time, the diaphragm does not move, the valve core position does not move, and the valve opening is certain.

hen the evaporation coil is thin or relatively long, or multiple coils share a thermal expansion valve, and the refrigerant flow resistance is large, if the internal balance thermal expansion valve is still used, the refrigerant superheat at the evaporator outlet will be very large, and the evaporator area can not be effectively utilized. At this time, the internal balanced thermal expansion valve should not be used.
Figure 2 shows the working principle diagram of the external balanced thermal expansion valve. Among them, 1 - valve core; 2 - elastic metal diaphragm; 3 - spring; 4 - Adjusting screws; 5 - temperature bag; 6 - Balance tube. As can be seen from the figure, the structure of the externally balanced thermal expansion valve is basically the same as that of the internally balanced thermal expansion valve, except that the lower space of the elastic metal diaphragm is not connected to the expansion outlet, but is connected to the evaporator outlet through a small diameter balance, so that the lower part of the diaphragm bears the pressure of the refrigerant at the evaporator outlet, thereby eliminating the influence of the refrigerant flow resistance in the evaporator.

Externally balanced type

When the evaporation coil is thin or relatively long, or multiple coils share a thermal expansion valve, and the refrigerant flow resistance is large, if the internal balance thermal expansion valve is still used, the refrigerant superheat at the evaporator outlet will be very large, and the evaporator area can not be effectively utilized. At this time, the internal balanced thermal expansion valve should not be used.
Figure 2 shows the working principle diagram of the external balanced thermal expansion valve. Among them, 1 - valve core; 2 - elastic metal diaphragm; 3 - spring; 4 - Adjusting screws; 5 - temperature bag; 6 - Balance tube. As can be seen from the figure, the structure of the externally balanced thermal expansion valve is basically the same as that of the internally balanced thermal expansion valve, except that the lower space of the elastic metal diaphragm is not connected to the expansion outlet, but is connected to the evaporator outlet through a small diameter balance, so that the lower part of the diaphragm bears the pressure of the refrigerant at the evaporator outlet, thereby eliminating the influence of the refrigerant flow resistance in the evaporator.

Install
The installation position of the thermal expansion valve should be close to the evaporator, and the valve body should be placed vertically, not tilted, and not installed upside down. Because the thermal expansion valve depends on the temperature sensed by the temperature packet to work, and the sensitivity of the temperature sensing system is relatively low, the time lag of the signal transmission is large, which is easy to cause the frequent opening and closing of the expansion valve and the fluctuation of the liquid supply, so the installation of the temperature sensing packet is very important.
(1) The installation method of the temperature sensing bag, the correct installation method aims to improve the heat transfer effect of the refrigerant in the temperature sensing bag and the suction tube, so as to reduce the time lag and improve the working stability of the thermal expansion valve.
Usually, the temperature sensing bag is wrapped around the suction tube, and the temperature sensing bag is close to the tube wall and wrapped tightly. The oxide should be removed at the contact point, and an anti-rust layer can be applied if necessary. When the outer diameter of the suction tube is less than 22mm, the influence of the temperature around the tube can be ignored, and the installation position can be arbitrary, generally wrapped in the upper part of the suction tube; When the outer diameter of the suction tube is greater than 22mm, if there is liquid refrigerant or lubricating oil flowing at the installation place of the temperature sensing bag, the temperature difference between the upper and lower sides of the horizontal tube may be large. Therefore, install the temperature sensing bag between 45° (generally 30°) below the horizontal axis of the suction tube, as shown in Figure 3. In order to prevent the temperature sensing bag from being affected by the external temperature, it is necessary to wrap the non-absorbent insulation material after tying.
(2) The installation position of the temperature sensing bag, the temperature sensing bag is installed on the evaporator outlet, the compressor suction pipe section, and as far as possible installed in the horizontal pipe section. However, it must be noted that it must not be placed in a place where there is fluid accumulation and oil accumulation. As shown in Figure 4, in order to prevent liquid accumulation in the horizontal pipe and incorrect operation of the expansion valve, when the suction pipe at the evaporator outlet needs to be raised, the liquid trap should be set at the elevation, otherwise, the temperature sensing bag must be installed on the riser. When the external balanced thermal expansion valve is used, the external balance tube is generally connected to the suction tube of the compressor after the evaporator outlet and the temperature sensing packet, and the connection port should be located at the top of the suction tube to prevent it from being blocked by lubricating oil. Of course, in order to suppress the fluctuation of the operation of the refrigeration system, the external balance tube can also be connected to the part of the evaporation tube with a large pressure drop.

Problems and improvements
All kinds of thermal expansion valves are used to adjust the refrigerant flow by sensing the temperature change of the refrigerant temperature at the evaporator outlet. When the temperature sensing bag leaks, the expansion valve will be closed, and the refrigerant flow to the evaporator will be zero, resulting in the system cannot work. To solve this problem, a bidirectional thermal expansion valve with safety structure is proposed, as shown in Figure 5. When the temperature sensing envelope does not leak, its principle is the same as that of the external balanced thermal expansion valve; When leakage occurs, the throttle channel between the valve core 5 and the valve seat hole 2-1 is closed, the limit block 1-6 and the diaphragm 1-4 move upward under the action of the refrigerant pressure at the evaporator outlet transmitted through the pressure transfer tube 3, and drive the valve pin 4 to move upward, so that the axial through hole in the valve core 5 is opened, becoming the throttle channel, and continue to supply liquid to the evaporator to ensure that the system continues to work.

Shanghai KUB Refrigeration Equipment Co., Ltd.

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