Thermostatic Expansion Valve

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What is Thermostatic Expansion Valve?

The thermostatic expansion valve keeps up a steady superheat of the liquid refrigerant towards the end of the evaporator. And it is done by controlling the flow of liquid refrigerant through the evaporator. It comprises of a needle valve and a seat, a metallic diaphragm, spring, and adjusting screw. The thermostatic expansion valve is also known as a constant superheat valve. It also has a feeler or thermal bulb which is mounted on the suction line near the outlet of the evaporator coil. The liquid refrigerant part is also present in the filler bulb. The opening and the end of the valve relies on the accompanying reasons 

  • The spring pressure acting on the bottom of the diaphragm
  • The evaporator pressure acting on the bottom of the diaphragm
  • The feeler bulb pressure acting on the top of the diaphragm.
Thermostatic Expansion Valve Diagram

Thermostatic Expansion Valve Diagram

Working of Thermostatic Expansion Valve

Lets discuss the thermostatic expansion valve working

  • The bulb is fit on the suction line, therefore the temperature of bulb and refrigerant will be the same. Any adjustment in the temperature of the refrigerant will cause an adjustment in pressure in the feeler bulb. Because of this, it will transmit to the highest point of the diaphragm under ordinary conditions. The feeler bulb pressure acting at the highest point of the diaphragm is adjusted by the spring pressure and the evaporator pressure acting at the base of the diaphragm.
  • The power tending to close the valve is reliant upon the spring pressure and the evaporator pressure. It relies on the immersion temperature of the refrigerant in the evaporator coil. The power tending to open the valve relies on the feeler bulb pressure which thus relies on the temperature of the refrigerant in the bulb. In this manner, the activity of the valve is controlled by the difference between the temperature which is at the superheat.   
  • The level of super warmth of the liquid refrigerant leaving the evaporator relies on the underlying set of the spring strain. It can be changed according to the assistance of spring modifying screw. At the point when the valve is set for a certain superheat, at that point, it keeps up that setting under all load conditions on the evaporator.
  • The liquid refrigerant in the evaporator will boil fast if the load on the evaporator increases. The temperature of the feeler bulb increment because of the early vapourisation of the fluid refrigerant. In this way, the feeler bulb pressure increments and this pressure transmits through the capillary tube. The diaphragm moves downwards and opens the valve to concede a greater amount of the liquid refrigerant to the evaporator. This continues until the pressure equilibrium of the diaphragm reaches. 
  • At the point when the load on the evaporator diminishes, less fluid refrigerant vanishes in the evaporator. The abundance liquid refrigerant streams towards the evaporator outlet which cools the feeler bulb with the outcome the feeler bulb pressure diminishes because of diminishes in temperature. The low feeler bulb pressure is transmitted through the capillary to the diaphragm and moves it upwards. 
  • This decreases the opening of the valve and reduces its flow. The evaporator pressure diminishes due to the decrease in the amount of fluid refrigerant flowing to the evaporator. This proceeds until the evaporator pressure and the spring pressure keep up the balance with the feeler bulb pressure.

Hence this is the complete working of the thermostatic expansion valve.


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