Thermal expansion valve in air conditioning system

What is a thermal expansion valve?

Thermal expansion valve is an important automatic control component of refrigeration system. It can adjust the opening of the valve and change the refrigerant flow rate by using the temperature change of the air box head -LRB-temperature sensing packet) as a signal.

Refrigeration systems do not necessarily use thermal expansion valves as throttling elements

1. Capillaries and short throttle tubes (CCOT) can be used as throttling elements with low cost and good reliability

2. Electronic expansion valve can be used to make the control more accurate, but because of the use of sensors, control circuit board, with stepping motor actuator, manufacturing complex, high cost;

3. The new CO2 system also uses an ejector tube as the throttling element.

The control principle of expansion valve

Three-force balance principle

P1＝ P2 + F

P1: the pressure produced by the temperature of the filling medium in the gas box head acts on the diaphragm;

P2: evaporation pressure (internal balance for the valve outlet pressure, external balance for the evaporator outlet pressure) , the role of the film, the film below;

F: Spring Force, acting under the diaphragm.

When the gas box head temperature decreases P1＜P2+F the valve opening decreases;

When the temperature of the air box head increases P1＞P2+F the opening of valve increases;

Other forces: such as diaphragm stiffness, transmission parts friction resistance, fluid force on the spool and so on.

Inner balance expansion valve

1. The pressure under the diaphragm comes from the valve outlet. 

2. When the evaporator pressure drop is too large, will make the gas box head to feel the superheat decreases, so that the valve opening is insufficient.

External balanced expansion valve (copper)

1. The pressure under the diaphragm comes from the evaporator outlet. 

2. The influence of evaporator pressure drop on valve opening is avoided, and the disadvantage of internal balance valve is overcome. 

3. Because the valve outlet and the balance cavity under the diaphragm need to seal treatment, increased the manufacturing difficulty.

External balanced expansion valve (aluminum)

1. Another form of external balance valve, evaporator to compressor return gas directly into the valve, gas box head temperature and pressure signals are obtained from the valve internal return gas channel; 

2. The block structure makes the installation more convenient and reliable.

 3. The non-capillary structure makes the valve more resistant to vibration;

 4. The use of aluminum materials to reduce weight.