How Thermal Expansion Valve Works in Refrigeration

Thermal expansion valve is the indispensable component of refrigeration device and also one of the four basic parts in refrigeration system. It is capable of realizing the throttle from condensing pressure to evaporating pressure and controlling refrigerant flow running to evaporator; it guarantees s stable degree of superheat for compressor return air and offers evaporator the optimal refrigerant. The valve is small in size but tremendously powerful; therefore its operation situation influences directly the performance of whole system.

Control principle

Thermal expansion valve controls refrigerant flow into evaporator through adjusting degree of superheat in evaporator outlet. Shini water chillers adopt external balance thermal expansion valve to avoid excessive closing of valve so that normal feed liquid to evaporator can be ensured. Freon or other mediums are filled in sensing facilities; thermal bulb is placed at evaporator outlet. Return air should be without liquid, thus there is temperature difference between evaporator outlet and evaporating temperature, which is what we call Degree of Superheat. Thermal bulb senses evaporator outlet temperature and whole sensing system is under saturated pressure P1, which will be transmitted to ejector rob then to valve plug via diaphragm. Only P1 exists in the diaphragm located in the upper of pressure cavity; below diaphragm is there spring force P2 and evaporating pressure P3. if these three factors stay in balance, there is formula: P1=P2+P3. If P1>P2+P3, it means that thermal load of evaporator is over great and outlet degree of superheat is over high. Valve plug can be lowered to largen expansion valve via the pressure signal from diaphragm to ejector rob, refrigerant flow will increase in proportion and vice versa.

Thermal bulb

To accurately detect temperature of compressor return air, thermal bulb has to be in good contact with return air pipe and insulates heat to avoid the influences of ambient temperature. It should not be installed at the bottom of return air pipe to avoid influences of return oil temperature. The refrigerant similar to that of inside refrigeration system is filled into thermal bulb, thus by means of pressure feedback from thermal bulb (compressor air suction temperature corresponding to saturated pressure of refrigerant), expansion valve can realize the optimized feed fluid for evaporator when changes of operating situations (Such as thermal load alteration).

Regulation of the valve

It can be seen from above structure that spring force P2 can be adjustable by moving the D (valve body) up and down. Thus expansion valve can adjust spring force to control return air degree of superheat so that open size of valve can be controlled. For example, if cooing water temperature of standard models exceeds 28℃, air suction pressure would probably exceed 6.2bar and working point of compressor would be insecure. So we need to adjust expansion valve to increase return air degree of superheat to maintain suction pressure staying within the reasonable range. Keep in mind that return air degree of superheat can not exceed 12℃ for the sake of safe compressor operation. If both factors above are far from guarantee, a proper evaporator is able to achieve the normal operating condition. The regulation of expansion valve is that, clockwise turning is to increase degree of superheat and lower evaporating pressure while counterclockwise turning is to decrease degree of superheat and enhance evaporating pressure. With the large enough degree of superheat, counterclockwise turning can enhance refrigeration.

What is MOP

MOP stands for "maximum operating pressure", is the maximum allowable suction pressure/evaporating pressure in evaporator and suction pipeline. For most refrigeration system without pressure limit or ones with thermal bulb can be installed higher than temperature sensing element, or with a relatively high evaporating pressure and temperature, constantly-filled expansion valve is the primary equipment. Constantly-filled means that there is always liquid filled in sensing ball. There is always liquid inside the ball regardless of whether the position of sensing element is higher or lower than sensing ball due to the big liquid filling amount. Expansion valve filled with MOP is applicable for plant pre-cast devices that need to limit suction pressure when startup, such as those devices in transportation and AC system. A little amount liquid filled in MOP expansion valve within sensing ball, which means positions of valve or sensing element should be higher than that of sensing ball, otherwise liquid will pour into sensing element from sensing ball, and causes abnormal operation of expansion valve. When temperature reaches MOP point, filled liquid will be evaporated; as suction pressure elevates, expansion valve will stop opening slowly when it is lower than MOP by 1.3~1.4bar. When suction pressure equals to MOP point, opening of expansion valve will stop extending.

Model selection of expansion valve

A clear idea on refrigerant of system is critical for selecting the proper expansion valve. Firstly we need to have values of refrigerating output, condensing pressure, evaporating pressure and way pressure loss. Calculate valve pressure drop by subtracting evaporating pressure and way pressure from condensing pressure. Then based on the value of evaporating temperature and valve pressure drop, valve capacity would be drawn from the table below; here we define the capacity as Q1.

Table 1: Comparison of model selection for expansion valve and output (BAE series Unit: kW)

According to the correction factor of liquid refrigerant temperature, value of Q1 multiplying the corresponding factor would come to the actual capacity of expansion valve in system. The actual capacity should be larger than value of designed refrigeration capacity.

Table 2: Correction factor of liquid refrigerant temperature