A thermostatic expansion valve ( TXV) is precision device used to meter the flow of liquid refrigerant entering the evaporator at a rate that matches the amount of refrigerant being boiled off in the evaporator, This is it's main purpose but like all the other metering devices it also provides a pressure drop in the system, separating the high pressure side of the system from the low pressure side. thus allowing low pressure refrigerant to absorb heat onto it's self.
The valve it self has 3 forces that act upon each other to accomplish this task. They are
Bulb Pressure P1
Evaporator Pressure P2
Spring Pressure P3
As you can see from figure 1 there are 2 closing forces P2 & P3 and one opening force P1. The valve in this figure is in equilibrium, P1 * P2+P3. If the evaporator pressure was to increase while the bulb pressure stayed that same the valve would close. P2 + P3 would be greater than P1. Now if the bulb pressure was to increase to the larger amount, the valve would open . P1 is greater than P2+P3.
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In this example we will be discussing the operation of the most basic type of TXV, the single outlet style. We will also be assuming there is no pressure drop in the evaporator as well as no requirement for super heat.
As the load across our evaporator increases the available refrigerant will boil off more rapidly, if it is completely evaporated prior to exiting the evaporator , the vapor it self will continue to absorb heat . This heat is referred to as super heat, Superheat is heat added to a substance above it's saturation temperature. The bulb will sense this increase in temperature exiting the evaporator and increase the pressure on P1. P1 now being greater than P2+P3 will allow the valve to open allowing more refrigerant to enter the evaporator. Now that more refrigerant is being introduced into the evaporator there is more availability to absorb heat, if there is insufficient heat to boil off all the refrigerant prior to it exiting the evaporator the temperature at the sensing bulb will decrease reducing the pressure at P1 and causing the valve to close. P1 is less than P2+ P3.
As previously mentioned super heat is heat added to a substance above its saturation temperature. As refrigeration mechanic's we must be concerned with the amount of super heat we have in a system. To little and we may have liquid refrigerant entering our compressor washing out the oil and at worst case, causing hydraulic pressure thus breaking mechanical parts. To much and we are wasting valuable evaporator space and possibly causing compressor overheating problems. From the factory TXV's come pre calibrated at a static superheat setting 6oF. Static superheat is the amount of superheat required to get the pin to begin travel, any increase of superheat over this point is referred to a gradient. The manufactures often calibrate for 4o to 6oF of gradient superheat, The combination of the 2 mean that at 10o to 12oF total superheat the valve will be opened to it's full rated capacity. Please note that this superheat setting is set at the exiting of the evaporator and not at the compressor so to see a 15oF superheat reading at the compressor is normal.
*-* EQUALIZED */-*
Externally equalized TXV's must be used when the pressure drop in the evaporator exceeds
3oF for Air Conditioning
2oF for Refrigeration
1oF for Low Temp applications
Looking at figure 3 you will notice that we are leaving our TXV with a refrigerant pressure of 69 PSIG @ 40oF this is saturation for R22. As the refrigerant passes through the evaporator it experiences a pressure drop of 10 PSIG. Now with a refrigerant pressure of 59 PSIG our saturation temperature is 33oF, once our refrigerant has completely evaporated we will start to add super heat to it and the condition leaving the evaporator will be a low temperature, low pressure superheated vapor 59 PSIG @ 50F . At 59 PSIG our saturation temperature is 33oF and leaving the evaporator the refrigerant temp is 50F this means there is 17oF superheat, Saturation temp - leaving temp * superheat. since our superheat setting is 10oF the 85 PSIG bulb pressure is greater than the evaporator pressure at the external equalized port and the spring pressure (85 > 59+16). there fore the valve will open and allow more refrigerant into the evaporator. If in this example we did not use a externally equalized TXV our valve would be at equilibrium and not allow any more refrigerant to enter (*+16).
One of the major problems with single ported valves is that they are susceptible to valve hunting. Valve hunting is defined as the alternate overfeeding and starving of refrigerant flow to the evaporator. This can be noticed buy the constant movement you may see in your refrigerant gauges. Hunting can be reduced buy either relocating the sensing bulb to a better location or by purchasing a valve designed for reduced hunting.
*-* Port */-*
Balanced port valves are designed for systems that experience fluctuating pressures. On conventional TXV's when the pressure drop across them changes so does the amount of superheat, Potentially you could have either flooding or overheating of the compressor due to this reason. The balanced port cancels this effect allowing proper superheat to be maintained.
*-* Ported Balanced Port */-*
When dealing with systems that have light load conditions, double ported valves should be used these valves have 2 ports for varying capacities and can run down to 15% of their rated tonnage.
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- September 22, 2002 .