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Thread: expansion valve

  1. #1
    farzam_f2005's Avatar
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    expansion valve



    why in thermostatic expansion valve with external equlaizer, the equlaizer line is after the sensing bulb. with best regards



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    Re: expansion valve

    The equalizer is after the sensor so that it does not affect the tev.

    If it was before, it would affect the temp of the bulb.

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    Re: expansion valve

    Correct me if I am wrong (As I am not a TEV expert), but isn't this because it is possible for the valve to leak liquid refrigerant past the rods that drive the valve open (in the case of Sporlan). I seem to remember that this will cause liquid to travel down the equalizing line and close the valve prematurely when it cools the bulb.

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    Re: expansion valve

    Quote Originally Posted by farzam_f2005 View Post
    why in thermostatic expansion valve with external equlaizer, the equlaizer line is after the sensing bulb. with best regards
    Hi there,

    The external equalizer line must be connected after the TEV's sensing bulb (towards the compressor). The reason is this :
    Experience showed that this kind of valve which is internally sealed (evaporator inlet temperature does not effect the diaphragm), after a while there is a possibility of internal leakage.
    When this happens , because of pressure differential , some liquid can get into the suction line. If the equalizer line is connected before the sensing bulb then TEV will start to close. But if it is connected after the sensing bulb then nothing happens.

    If internal leakage does not occur then there is no difference where you connect the equalizer line.
    You can find this fact in any refrigeration book, try Dossat.

    Hope this helps.

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    Re: expansion valve

    Quote Originally Posted by NH3LVR
    ...but isn't this because it is possible for the valve to leak liquid refrigerant past the rods that drive the valve open (in the case of Sporlan). I seem to remember that this will cause liquid to travel down the equalizing line and close the valve prematurely when it cools the bulb.
    I would agree.
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    Re: expansion valve

    One could also say that it is because the manufacturer has said so in his installation instructions.
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    Re: expansion valve

    Quote Originally Posted by Brian_UK View Post
    One could also say that it is because the manufacturer has said so in his installation instructions.
    Knew I had read that SOMEplace
    Perhaps reading the instuctions is not a bad thing after all!

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    Re: expansion valve

    Quote Originally Posted by Brian_IK
    One could also say that it is because the manufacturer has said so in his installation instructions.
    I thought you were not supposed to read the directions until you had a problem and had exhausted all other possibilities?
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    Re: expansion valve

    Some of the instructions I've read where the problem to start with!

    Translated from English to Chinese, then to German, then to Japanese then back to English with a grade school educated translator in a dark room!

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    Re: expansion valve

    Quote Originally Posted by The MG Pony View Post
    Translated from English to Chinese, then to German, then to Japanese then back to English with a grade school educated translator in a dark room!
    See, you have read them
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    Re: expansion valve

    Hey guys, I understand the theory and I follow directions, but if you have a tev that is flooding through badly, wouldn't you want the tev to sense this and in turn shut down?(as best it can)

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    Re: expansion valve

    Quote Originally Posted by goodguy
    ...but if you have a tev that is Flooding through badly, wouldn't you want the tev to sense this and in turn shut down?
    A reasonable question.

    If the valve is causing the flooding problem, the sensing bulb should sense the liquid and close off the valve anyway. This assumes the bulb is properly located in the first place on the suction line for the particular pipe diameter.

    Another question might be; why is it flooding in the first place regardless of the bulb location before or after the equalizer connection?
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    Re: expansion valve

    Quote Originally Posted by goodguy View Post
    Hey guys, I understand the theory and I follow directions, but if you have a tev that is flooding through badly, wouldn't you want the tev to sense this and in turn shut down?(as best it can)
    Hi goodguy,

    When internal leak in the valve happens then "couple of drop" of liquid will pass from the equalizer line to the suction, there will be no flooding.
    As I explained before, if internal leak does not happen then there is no difference for equalizer line connection before or after the bulb, but there is a high probability of internal leakage.

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    Re: expansion valve

    Thank you for the input, I guess the only application that a internal leak may cause problems is low temp.A normal 6-8 degF superheat may not be as forgiving compared to a A/C system with 10-15degF.
    A suction accumulator can be cheap insurance for the price of new compressor.

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    Re: expansion valve

    But, in this case we could have a problem, i really prefer that if i have an internal leakage of liquid the tev stop and not put the compressor into danger with a liquid propelled vapour. I think thats for the charge loose(i dont know how do u name it in english), it doesn means if the bulb is close after or before the equalization line, only if theres exists a large distance because this loose.

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    Re: expansion valve

    Let me ask a somewhat foolish question.

    How many valves has anyone seen on a system that flooded due to leaks around the push rods? This has to be as rare as a chicken having teeth.

    More often than not, the power element will loose it's charge is the most probable fault I can think of.

    TXV's in general do not go bad very often. A lot of them get changed for no good reason, but they seldom fail horribly.

    If the valve is selected properly and installed per the manufacturers directions, that is 99.9% of the problem.

    At least that's my experience.
    Last edited by US Iceman; 05-06-2007 at 03:54 AM. Reason: editing paragraph
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    Re: expansion valve

    I have 3 Tevs flooding badly into the suction in the last 3 years. Two medium temp(sporlan) one low temp (danfoss). The low temp valve was found after the new compressor was installed! As far as I could tell they were all installed correctly. Anybody eles?

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    Re: expansion valve

    You can't always blame the TXV for flooding, just like you can't blame it for hunting. They do this for certain reasons, which most of the time is not the valves fault.

    Someone has either selected the valve incorrectly or selected it improperly or adjusted it when they should not have.

    I have never seen the extent of valve flooding alluded to in this thread and I have worked on a lot of systems.

    The biggest problem I have ever seen with valve flooding was on a VAV DX system and that was not the valves fault. The valves were misapplied on a system with a widely varying load range.
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    Re: expansion valve

    Hi everybody,

    I have to agree with US Iceman. It is not always the TEV's fault.
    Expansion valves are very complicated piece of equipment which require exact understanding for proper usage.
    People who can not use these TEVs properly then almost always blame them for NOT WORKING.

    I give you a very new experience which I had two days ago. A simple split air conditioning unit had to be tested for my friend.
    He did evacuate the system and charged it with R22 then I arrived to do the running test. When I started the system TEV started hunting very badly. Suction pressure went to 100 Psi(g) and down to 40Psi(g). I did whatever I thought to stop the hunting.
    1- I changed the bulb location.
    2- I opened the valve.
    3- I closed the valve.
    4- I checked the TEV bulb by putting it into cold and hot water.
    5- I opened the TEV and checked the orifice size.
    6- I changed the whole TEV with a new one.
    7- AGAIN THE SAME HUNTING PROBLEM.

    Finally I said that this may be caused by incorrect refrigerant distributor. The day was over and the test was postponed. Yesterday I wend there again and did the test with another system which was exactly the same as the previous one (which did the hunting).
    Now I charged the system with something near 5 kg of R22. Then system started working without any problem.

    The previous system was charged (by my dear friend) with 13 kg of R22. And also not evacuated properly. The problem I had for not diagnosing the system as overcharged was that there were more than two faults present, then symptoms become very complicated to diagnose. I don't know what other faults were there and I don't want to know because it wasted my whole day.

    Now he has to evacuate the system and charge again properly. I am sure that the system will work well if charged correctly.
    SO it is not always the valve. It is almost always the People.
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    Re: expansion valve

    Quote Originally Posted by lana
    1- I changed the bulb location.
    2- I opened the valve.
    3- I closed the valve.
    4- I checked the TEV bulb by putting it into cold and hot water.
    5- I opened the TEV and checked the orifice size.
    6- I changed the whole TEV with a new one.
    7- AGAIN THE SAME HUNTING PROBLEM.
    These are things I used to see all of the time. Analyzing this sort of problem is difficult and I will be honest to mention I have done this before also.

    One of the first things someone needs to learn about TXV's is to not be in a hurry to solve the problem. And... do not rush to change the valve. My guess is about 90% of the time the problem is due to something else.

    Usually the last person who worked on the system.
    Last edited by US Iceman; 05-06-2007 at 01:09 PM. Reason: spelling
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    Re: expansion valve

    With all respect Iceman, the valves I had replaced were all in service 10-15 years and were working properly. Except of course the frosting quarter inch equailzer line dumping liquid back into the suction.

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    Re: expansion valve

    Quote Originally Posted by goodguy
    Except of course the frosting quarter inch equalizer line dumping liquid back into the suction.
    OK, anything is possible, but how much does this contribute to flooding?

    Maybe the pushrods were leaking, but how much liquid does actually pour back into the suction line through the equalizer? With the pushrods still in place any leak would be very, very small.

    More than likely the small volumes of liquid leaking caused the valve to start hunting and the valve became unstable and then the flooding occurred. At full load this is probably less of a concern. However, at part load it could create a mess.

    Frost on the line may not necessarily mean flooding. What was the suction line superheat at the compressor?

    TXV's do fail, just like compressors or anything else mechanical.

    What I'm saying is; I think more TXV's are changed than is necessary and most of the time for the wrong reasons.
    If all else fails, ask for help.


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    Re: expansion valve

    I will step on some toes now: in all the books and manuals you find, there's said allways that the bulb must installed before the equalising line.

    Well, we don't do it this way and the reason why was already mentioned by Church2k.

    The reason why they recommend this is indeed leakage around the inner stem inside the TEV of liquid passing through the equalising line which will affect the proper preset SH of the valve.

    I wrote some time ago (+/- 2 years) a mail to Danfoss asking them if the TEV's fabricated with the high-tech machines still leak. They said that they can make this leakfree for at least 10 years and even far more.
    So if a valve is leaking, then it's defective in some way.

    So i you install the bulb after the eq. line, then in case of a leak, the valve will close and protect the compressor.
    On the other hand, US Iceman already mentiond this, how much liquid do you think can pass around the stem of a TEV? Almost noting.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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