Thermostatic Expansion Valve ( TEV )

> The working of the thermostatic expansion valve can be explained with the help of the attached image of the valve. The valve comprises of external body inside which various parts as shown in the figure are enclosed.> There are three pressures acting inside the thermostatic expansion valve. P1 is the pressure at the top of the thermostatic expansion valve acting inside the power element above the diaphragm. Due to this pressure the diaphragm tends to move down due which the needle also moves down and the valve tends to open. When the evaporator temperature becomes higher the gas in the feeler bulb expands due to which the gas pressure inside the power element increases. This causes the downward movement of the needle to open the valve.
> The pressure P2 is the pressure acting on the lower side of the diaphragm due to the refrigerant pressure inside the evaporator. This pressure tends to move the diaphragm upwards and close the opening of the valve.
> The pressure P3 is the spring pressure that tends to close the opening of the valve. This pressure remains constant.
> The pressures P2 and P3 act against the pressure P1. The pressure P1 tends to open the valve while the pressure P2 and P3 tend to close the thermostatic expansion valve. Thus if the valve has to open P1 should be greater than combined forces of P2 and P3.

> During the normal working of the refrigeration plant the thermostatic expansion valve remains opened in certain position. When the refrigeration load increases, the temperature inside the evaporator also increases. In such cases there is need of the more refrigerant to take care of the increased load. The increased temperature in the evaporator is sensed by the feeler bulb of the thermostatic expansion valve. This leads to the expansion of the gas in the feeler bulb and also in the power element of the TEV leading to the increase in pressure P1. Due to this the diaphragm of the TEV moves down and tends to open the valve further to increase the flow of the refrigerant to the evaporator.
> At the same time the pressure P2 below the diaphragm also increases due to superheating of the refrigerant inside the evaporator. This pressure tends to close the valve. There is also spring pressure P3 below the diaphragm that opposes the opening of the valve. If the increase in the refrigeration load is much higher the pressure P1 overcomes pressure P2 and P3 leading to the further opening of the thermostatic expansion valve. This allows for the increased flow of the refrigerant to the evaporator to take care of the extra load.
> When the refrigeration load reduces, the magnitude of pressure P1 reduces and the combined pressures P2 and P3 overcome pressure P1 that allows for partial closing of the valve so the flow of the refrigerant to the evaporator reduces. Thus the TEV maintains the flow of the refrigerant inside the evaporator as per the refrigeration or air conditioning load. The TEV constantly modulates the flow to maintain the superheat for which it has been adjusted by the spring.