Selection and Installation of Thermal Expansion Valves

The thermal expansion valve is one of the four major components in a refrigeration unit. It plays a role in throttling and reducing pressure in the system and is also known as a throttle valve. In the systems of freezers and cold storages, the expansion valve is commonly used as the throttling element.

In a refrigeration system, the expansion valve is responsible for reducing the pressure of the high-pressure refrigerant liquid before it enters the evaporator, that is, reducing the condensing pressure to the evaporating pressure. According to the corresponding principle between saturated pressure and saturated temperature, the temperature of the refrigerant also decreases simultaneously. Meanwhile, the expansion valve also functions to control the flow rate of the refrigerant. The quality of the thermal expansion valve in the system will directly affect the operating performance of the entire system. Therefore, correctly selecting the thermal expansion valve will enable the evaporator to be fully utilized and always match the heat load, which is of great significance for the operating life, refrigeration effect and operating cost of the refrigeration system.

When the selection of the thermal expansion valve does not match the system, the refrigerant flow rate in the system will vary from time to time, resulting in the refrigeration capacity fluctuating. If the refrigeration capacity is too small, it will cause insufficient liquid supply to the evaporator and excessive superheat, which will have an adverse impact on the system performance. When the refrigeration capacity is too large, it will cause oscillations, intermittently resulting in excessive liquid supply to the evaporator, leading to violent fluctuations in the suction pressure of the compressor and even causing liquid refrigerant to enter the compressor, resulting in the phenomenon of liquid slugging (wet stroke) of the compressor.

When selecting a thermal expansion valve, factors such as the type of refrigerant, evaporation temperature, condensation temperature and evaporation load need to be considered. Generally, the selection is carried out through the following steps:

Connection mode between the expansion valve and the evaporator:
The thermal expansion valve must be installed on the liquid pipeline located in front of the evaporator.

The position should be close to the evaporator, and the valve body should be placed vertically. It should not be inclined or installed upside down. The sensing bulb of the expansion valve should be fixed on the suction pipeline of the evaporator as close as possible, and it must be installed before any U-shaped oil return bends. Installation position of the sensing bulb on the suction pipeline: The position of the sensing bulb installed on the horizontal suction pipeline should be between 12 o'clock and 4 o'clock. When the diameter of the horizontal return gas pipe is less than 22 mm, the sensing bulb should preferably be installed at the upper end of the return gas pipe, that is, at the "12 o'clock to 2 o'clock position" of the suction gas pipe. When the diameter of the horizontal return gas pipe is greater than 22 mm, the sensing bulb should be installed at an angle of approximately 30 degrees below the axis of the return gas pipe, that is, at the "3 o'clock to 4 o'clock position" of the suction gas pipe.

The thermal expansion valve senses the temperature at the outlet of the evaporator through the sensing bulb, converts this signal into the pressure of the vapor inside the sensing bulb, and then controls the opening degree of the valve needle of the expansion valve to achieve the purpose of feedback regulation. Therefore, the sensing bulb should be closely attached to the wall of the suction pipeline. Meanwhile, in order to prevent the sensing bulb of the expansion valve from being affected by the external temperature, after the sensing bulb is fixed, it needs to be insulated from the outside with a non-absorbent and heat-insulating material.

During the operation of the system, generally, the appropriateness of the adjustment size of the thermal expansion valve can be judged by observing the frosting situation on the outside of the valve body:
A. If the entire expansion valve body is frosted, it indicates that the flow rate is too small and should be adjusted appropriately larger. If the frosting shape remains unchanged after being adjusted larger, it may be that the throttling hole of the expansion valve is partially blocked and should be cleaned.

C. If only the outlet side of the expansion valve body is frosted, it indicates that the flow rate is too large and should be adjusted smaller.
D. If the outlet side and the lower part of the expansion valve body are frosted in a 45-degree inclined shape and the inlet side is not frosted, it indicates that the adjustment is accurate and appropriate.
E. If only the inlet side of the expansion valve body is frosted, it indicates that the filter screen at the inlet of the valve body is partially blocked and should be cleaned.
F. If the expansion valve body is completely frost-free, it indicates that there is no flow. It may be that the refrigerant in the system leaks, or the stop valve in the pipeline is not opened, or the capillary tube of the temperature sensing probe of the expansion valve leaks gas, or the throttling hole of the expansion valve is blocked, or the filter screen at the inlet of the valve body is blocked and should be cleaned.

The thermal expansion valve does not have as fast a response speed as the electronic expansion valve, so it needs to be operated and adjusted slowly during the adjustment process. When making adjustments, first unscrew the valve cap under the thermal expansion valve. Rotating the valve stem clockwise will reduce the opening degree of the valve's pinhole, that is, reduce the liquid supply flow rate. Rotating the valve stem counterclockwise will increase the opening degree of the pinhole, that is, increase the liquid supply flow rate. Due to the certain lag in the temperature sensing of the thermal expansion valve in actual work, it is necessary to wait until the operation is basically stable (generally it takes 15 - 30 minutes) before making another adjustment. During the operation of the refrigeration system, if frost forms and then melts from time to time at the suction valve of the compressor, it indicates that the expansion valve is functioning normally.