Thermal expansion valve is a key component in refrigeration systems, mainly used to regulate the flow of refrigerant, ensure that the refrigerant at the outlet of the evaporator is in a suitable superheat state, and thus improve the efficiency and stability of the refrigeration system.

1、 Opening and closing principle

The opening and closing of a thermal expansion valve depends on three basic pressures:

F1 (thermal pressure): acts on the surface of the diaphragm, located in the direction of valve opening. The temperature bulb is installed on the suction pipeline. When the temperature of the suction pipeline changes, the state of the medium inside the temperature bulb changes, and the pressure changes accordingly. When the temperature packet pressure is greater than the sum of evaporation pressure and spring force, the diaphragm moves upward, pushing the valve to have a tendency to open.

F2 (evaporation pressure): acts on the direction of valve closure. Evaporation pressure is the pressure at which the refrigerant evaporates inside the evaporator, which can hinder the opening of valves.

F3 (spring pressure): also acts on the bottom surface of the diaphragm, located in the direction of valve closure. Springs are used to set the degree of superheat, and the opening degree of the valve can be changed by adjusting the pre tension force of the spring.

When adjusting the thermal expansion valve, the temperature surge pressure on one side of the diaphragm and the evaporation pressure acting on the other side are balanced with the spring force, thereby achieving valve opening and closing control.

2、 Overheating degree

Definition: Overheating refers to the difference between the actual temperature of refrigerant vapor at the outlet of the evaporator and the saturation temperature at the corresponding pressure, measured at the position where a temperature bulb is installed on the suction pipe. Measurement unit: Kelvin (K).

Function: The degree of superheat is used as a signal to regulate liquid filling through an expansion valve. For example, when the degree of superheat is too high, it indicates that there is insufficient refrigerant in the evaporator, the effective utilization area of the evaporator decreases, and the cooling capacity decreases. At this time, the expansion valve will automatically increase its opening to increase the flow of refrigerant; On the contrary, when the degree of superheat is too low, the compressor may suck in wet steam, causing liquid hammer, and the expansion valve will decrease its opening, reducing the flow of refrigerant.

3、 Undercooling degree

Definition: Subcoling refers to the difference between the actual temperature of the refrigerant liquid at the outlet of the condenser and the saturation temperature at the corresponding pressure. Measurement unit: Kelvin (K).

Function: The subcooling of the refrigerant is essential to prevent the formation of steam bubbles in the refrigerant in front of the expansion valve. The vapor bubbles in the refrigerant will reduce the capacity of the expansion valve, thereby reducing the liquid supply to the evaporator. The degree of subcooling can be achieved by installing a subcooler at the outlet of the condenser or utilizing the natural subcooling inside the condenser. In most cases, a subcooling degree of 4-5K is sufficient.

4、 Thermal expansion valve MOP

Definition: MOP is a specialized design made on a thermal expansion valve to limit the suction pressure of a compressor from exceeding a certain value. The MOP set point is determined based on factors such as the rated suction pressure of the compressor and the safety operation requirements of the system.

working principle

Thermal expansion valve without MOP function: Most of the medium filled in the temperature sensing bulb exists in liquid form. When the temperature sensing bulb comes into contact with the compressor suction pipeline and is heated or cooled, the liquid medium inside will evaporate into gas or condense into liquid, causing pressure changes. Through capillary action on the diaphragm, the valve opening changes, regulating the flow rate of refrigerant and ultimately controlling superheat.

Thermal expansion valve with MOP function: The temperature sensing chamber is filled with a small amount of liquid, and most of the medium is in a gas state. When the compressor operates under conditions of high refrigerant flow rate and high suction pressure, the temperature of the compressor suction pipeline is very high,

When a certain set point is reached, all the liquid in the thermal expansion valve temperature sensing chamber can be evaporated into gas. As the temperature continues to rise, the pressure change of the single state gas in the temperature sensing chamber is very insignificant and almost unchanged, which can affect the valve opening and limit the suction pressure of the compressor from further increasing. The MOP set point mentioned refers to the maximum suction pressure that the applied system can accept. For example, under high temperature conditions, the MOP function can prevent the compressor suction pressure from being too high, protecting the compressor and refrigeration system.

5、 Installation of expansion valve

Installation position: The expansion valve must be installed in the liquid pipeline before the evaporator, and its temperature bulb should be fixed on the suction pipeline as close as possible to the evaporator. If there is external pressure balance, the balance pipe must be immediately connected to the suction pipe after the temperature bulb is released. When installing the expansion valve, the inlet and outlet directions should not be reversed, and severe vibration and collision should be avoided.

It is best to install the temperature pack on the horizontal suction pipe, with the position corresponding to the clock between 1-4 o'clock, depending on the outer diameter of the pipeline. [Source of this article: official account: refrigeration encyclopedia] This is because this position can better reflect the real temperature of the refrigerant in the suction pipe and avoid affecting the measurement accuracy due to liquid accumulation at the bottom of the pipe or abnormal temperature at the top.

6、 Expansion valve adjustment

Factory settings: The factory settings of the expansion valve can meet the requirements of most applications, and the factory static superheat is generally set to 4 ℃.

Adjustment method: Rotate the adjustment lever clockwise (turn it down) to increase the static superheat of the valve; Rotate the adjustment lever counterclockwise (increase opening) to reduce the static superheat of the valve. Generally speaking, when the adjusting rod rotates once, the static superheat changes by 1-2 ℃.

If the superheat in the evaporator is too high, it may be due to insufficient supply of liquid refrigerant. The setting rod of the expansion valve can be gradually rotated counterclockwise. After each adjustment, wait for a period of time for the system to stabilize and observe the change in superheat until fluctuations are observed to reduce the superheat. When adjusting, the adjustment lever should be slowly rotated to avoid excessive adjustment that may cause system instability. To determine whether the degree of superheat in the evaporator is appropriate, methods such as observing the refrigerant state at the outlet of the evaporator, measuring the temperature and pressure at the inlet and outlet of the evaporator, etc. can be used.