Conventional txv
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  1. #1
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    Conventional txv

    If I have a conventional txv in a system with fan cycling and head pressure control and I set the superheat at say 10 degrees when ambient is low (Winter) would I see a drop in superheat when the ambient rises in the summer?
    I believe you would due to an increase in head pressure which is a opening force. Am I right? I had this discussion with another tech who said the valve will maintain the same superheat throughout all seasons. To me a change in head pressure is no different than a change in spring pressure. Both are opening forces and changing either one will effect superheat.
    Thanks

  2. #2
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    Quote Originally Posted by slonergan82 View Post
    If I have a conventional txv in a system with fan cycling and head pressure control and I set the superheat at say 10 degrees when ambient is low (Winter) would I see a drop in superheat when the ambient rises in the summer?
    I believe you would due to an increase in head pressure which is a opening force. Am I right? I had this discussion with another tech who said the valve will maintain the same superheat throughout all seasons. To me a change in head pressure is no different than a change in spring pressure. Both are opening forces and changing either one will effect superheat.
    Thanks
    If it does change, it will be very, very slight...

    Yes the head will be a very slight opening force, but the bulb is charged with the same gas, so it would stand to reason that the forces will cancel each other out...


    The only thing that really matters is that you have 5+ degrees of subcooling against the liquid port of the valve, if that stays the same, so will the superheat, the suction pressure will rise because of the loss of efficiency of the compressor due to higher head pressure, so the valve wont have to work too terribly hard to keep the superheat in check...

    What'd a win?

    GT

  3. #3
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    Quote Originally Posted by slonergan82 View Post
    If I have a conventional txv in a system with fan cycling and head pressure control and I set the superheat at say 10 degrees when ambient is low (Winter) would I see a drop in superheat when the ambient rises in the summer?
    I believe you would due to an increase in head pressure which is a opening force. Am I right? I had this discussion with another tech who said the valve will maintain the same superheat throughout all seasons. To me a change in head pressure is no different than a change in spring pressure. Both are opening forces and changing either one will effect superheat.
    Thanks
    As long as your pressures stay within 'normal' ranges, then you should see no real measurable difference in superheat.

  4. #4
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    I dont believe the change will be drastic but there will be a change. In the spring and fall when utilizing fan cycling I think these valves would start to hunt due to the fluctuation in head. Hence the reason for balanced port valves, no?

  5. #5
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    Another way of looking at it....


    As long as the TXV's capacity does not get exceeded (min./max. tonnage) then you should be dead on balls accurate as far as superheat....

    GT

  6. #6
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    Balanced-Ported valves

    With a conventional TXV, the pressure differential across the valve results in a force that tends to “open” the valve. As operating conditions vary, this pressure differential changes and results in a variation of the original

    superheat. Engineers have developed the balance-ported TXV to compensate for this (Figure 4). In this design, the inlet pressure is applied across the valve pin as well as an undercut on the push-rod.

    Since these forces are in opposite directions, they cancel or balance one another resulting in no change in superheat, regardless of operating conditions. Balanced-ported valves are ideal for use in systems that operate over large changes in operating conditions. An example of this is a commercial a/c system that must operate both winter and summer, resulting in system operation under widely varying head pressures.

  7. #7
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    Quote Originally Posted by slonergan82 View Post
    I dont believe the change will be drastic but there will be a change. In the spring and fall when utilizing fan cycling I think these valves would start to hunt due to the fluctuation in head. Hence the reason for balanced port valves, no?

    Balanced port has more to do with the pressure differential of the coil than any thing else, it will help, but not the actual reason for it....

    I don't like to use the traditional fan cyclers just for this reason, I like the solid state variable speed temp controllers, but in reality, what would the difference be of a falsely raised head pressure vs. a hot day?

    Other than the variable up/down of the head on the cycler, not much...You would need to "average" the superheat if the head is not consistent anyways...I bet it works out to be stupid close...

    GT

  8. #8
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    Quote Originally Posted by slonergan82 View Post
    Balanced-Ported valves

    With a conventional TXV, the pressure differential across the valve results in a force that tends to “open” the valve. As operating conditions vary, this pressure differential changes and results in a variation of the original

    superheat. Engineers have developed the balance-ported TXV to compensate for this (Figure 4). In this design, the inlet pressure is applied across the valve pin as well as an undercut on the push-rod.

    Since these forces are in opposite directions, they cancel or balance one another resulting in no change in superheat, regardless of operating conditions. Balanced-ported valves are ideal for use in systems that operate over large changes in operating conditions. An example of this is a commercial a/c system that must operate both winter and summer, resulting in system operation under widely varying head pressures.
    Here is the crux of the issue as I'm seeing it.

    Widely varying head pressures = widely varying valve capacities = widely varying superheats.

    Since, in your first post, you said that we had head pressure control and fan cycling to keep the pressure up during low ambients, then I'm going to say that the head pressure isn't going to swing over a WIDE range, therefore, you aren't going to see a wide swing in valve operation.

  9. #9
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    404 in winter running at 190 psi up to say 250/ 260 psi in the summer is very possible

  10. #10
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    Thanks for the info on this topic guys. Ive just always been told that a balanced port txv should be used anytime you have fan cycling. I assumed that this was the reason.

  11. #11
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    Quote Originally Posted by slonergan82 View Post
    404 in winter running at 190 psi up to say 250/ 260 psi in the summer is very possible

    As I said before, as long as the valve does not experience much flash gas and has constant subcooling, I doubt seriously much if any fluctuation would be realized...

    You seem to be forgetting, as the head pressure rises, so does the suction....The compressor has to suck it before it can push it...

    If you put an amp clamp on the compressor, you will see what I mean....

    GT

  12. #12
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    Here's a link of how a balanced port txv works. http://www.ra.danfoss.com/TechnicalI...02%2802%29.pdf
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  13. #13
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    You said back in the beginning of the post that the discharge is an opening force for the valve.

    The valve (TEV) actually works on the following;

    BULB
    Spring|Evap

    The spring and evap are pushing against the bulb pressure.

    The discharge does not really do anything with the valve except push the refrigerant through it.

    JJ

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