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Expansion Valves

The cool thing about TXVs is that they only have a one-line job description. They are responsible for maintaining constant evaporator superheat... nothing more... nothing less.

In order to maintain constant superheat, the difference between the evaporator saturation temperature and the temperature at the outlet of the evaporator needs to be constant. We can use this fact when evaluating the valve's operation. There are a number of things that can be checked relatively easily to determine if the TXV is operating well or not.

The first is to disconnect the thermal bulb from the outlet of the evaporator and gently heat the bulb. Rubbing your hand over it will often do the trick. You should hear an inrush of refrigerant as the bulb pushes the valve open. I like to use that method as we don;t have to access the refrigerant circut. Be careful to apply gentle heat to avoid overheating the bulb, which can result in damage to the valve.

The second thing is to calculate the evaporator superheat and then turn the superheat adjustment screw out of the valve a couple of turns. Be sure to make note of exactly how many times the screw was turned so that you can put the screw back into its original setting. Then, allow the system to operate for 15 minutes and then measure the evaporator superheat again. If there is no change in the evaporator superheat, there is one other thing you can check before replacing the vale and/or power head assembly.

The inlet strainer can prevent the valve from feeding refrigerant even though the valve is opening. The system must be pumped down in order to access the inlet strainer. I have seen a number of TXVs that were replaced when the actual problem wa noting more than a strainer problem.

If the strainer is clear, check the transmission line on the power head for physical damage and signs that the bulb may have lost its charge. If the transmissionline looks good, I would replace the entire valve. I am not a big fan of replacing parts of the TXV... Just a personal thing I guess.

Hope this helps Jeff.

Comments

  • Jeff Lawrence_24
    Jeff Lawrence_24 Member Posts: 593
    Good way to diagnose?

    Professor,


    With the higher efficiency cooling systems coming in, there will be a lot more TXV's out there. Is there a good, reliable way to diagnose under-performing (or over-performing) TXV's? I have my own ways, but they leave something to be desired.

    The 2 bad ones I've had this year were simply slammed shut and would not let a drop of refrigerant through. One of them was part of a dual compressor system with an identical system, so I changed the power head between them and still nothing. I replaced the TXV and the system worked perfectly.

    Thanks.


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  • TimJ
    TimJ Member Posts: 13
    A/C for oil heat tech

    Mr. Silberstein.I'm trying to find out info on the class you have at SCCC.I'm sure you won't have another until next year,however,I'd like to find out more about it.

    TimJ
  • Jeff Lawrence_24
    Jeff Lawrence_24 Member Posts: 593
    That one

    The first one was slammed shut. Plain and simple, it wouldn't open. I had to use a line tap between the refrigerant filter and the expansion valve to make sure I was able to recover all the refrigerant.

    I also am not a big fan of swapping parts, but I had to make sure.

    Thanks, Prof.


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  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Give me a Call

    You can give me a call at the office at (631) 851-6897
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