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  1. #1
    Join Date
    Dec 2011
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    pensacola fl.
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    88

    do system pressures equalize with txv ?

    From what I have read some units with a txv will not allow pressures to equalize during the off cycle. With these types of units, I suppose, (I really don't know for sure) is if they use start capacitors along with potential magnetic relays due to the high starting torque. But what really gets me to thinking is how does any TXV remain closed during the off cycle when as soon as the unit cycles off, the bulb for the metering device starts to naturally warm up. If the bulb warms up because there is no more cold vapor in the line during the off cycle the TXV opens regardless if the unit is in the off cycle or run cycle. So it would seem that all TXV units equalize pressures during the off cycle. But I know this can't be so. I know I missing something in my logic but what?

  2. #2
    Join Date
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    Western PA
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  3. #3
    Join Date
    Feb 2012
    Location
    VA
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    109
    it dpends on the typ of txv there are some that have a spring in them that pushes against the diafram and the bulb pressure pushes it open, it has a stainless steel closeing soring and a stainless steel flange spring , if you dont have enough head pressure and enough heat load your closeing spring will push your needle shut and you will have to differnt pressures .

  4. #4
    Join Date
    Feb 2012
    Location
    VA
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    109
    oh yea and you may have a txv without a external pressure equalizer, or you equalizer is cloged , that will do it also ..

  5. #5
    Join Date
    Jul 2009
    Location
    Columbia, MD
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    4,408
    you valve works on 3 pressures...correct me if i am wrong.

    spring pressure, suction pressure, bulb pressure.

    if the unit stays off long enough it should eventually equalize.

    i found a manual on my computer about txv's....hopefully this helps you
    Attached Images Attached Images

  6. #6
    Join Date
    Dec 2011
    Location
    pensacola fl.
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    88
    I understand that txv tries to remain close with suction pressure and spring press. As you already know its opening force is generated by heat generated from the suction line to the bulb then to the valve by press. So, during the off cycle does the suction line get warmer and the suction press get lower ? So, how does a txv remain closed during the off cycle?

  7. #7
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    Jul 2009
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    Columbia, MD
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    read the article i attached to my post #5. it should sum it all up for you.

  8. #8
    Join Date
    Sep 2008
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    Western PA
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    Quote Originally Posted by pressordrop1 View Post
    oh yea and you may have a txv without a external pressure equalizer, or you equalizer is cloged , that will do it also ..
    The external equalizer tube has nothing to do with pressure equalization.

  9. #9
    Join Date
    Dec 2011
    Location
    pensacola fl.
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    88
    Thanks jpsmith1cm for helping me understand. My thinking that the suction pressure becomes lower when the unit cycles off is incorrect. I suppose that although the TXV bulb warms during the off cycles, it's effects are negligible.

  10. #10
    Join Date
    Nov 2011
    Location
    Charleston, Wv.
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    1,525
    Probably in that article.....but some TXV's have internal bleed ports...these are the ones that will allow equalization. the ones without, may bleed off over an extended period of time, but not during normal operation.

    on TXV's, spring pressure and evap. pressure close the valve.....the opening force is the diaphragm pressure. the goal is to operate the valve in equilibrium. Say your evap. pressure is 118psig, and your spring pressure is 24psig.......your bulb pressure would be need to 142psig.

    internally equalized valves allow evap. pressure to flow through the push rod holes to bottom of the diaphragm. Externally equalized valves, have a separate line out the side of the valve that runs to the outlet of the evap. after the sensing bulb. Externally equalized valves are used on evaps. with distributors. the drop through the distributor would cause an internally equalized valve to sense an incorrect evap. pressure.....as the true evap. pressure will be the pressure drop of the distributor, plus the pressure drop of the coil.

    sometimes the terms are confused......an internally equalized TXV is the not the same as a TXV with an internal bleed port.

  11. #11
    Join Date
    Dec 2011
    Location
    pensacola fl.
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    88
    This is a good subject. In addition to serving as a means to equalize pressures across the evap coil, does not all TXV's with external equalizing line serve as a bleed during the off cycle? So, are we saying, 71chops, that if a unit does not equalize during the off cycle the unit will probably use a internally equalized TXV ?

  12. #12
    Join Date
    Nov 2011
    Location
    USA
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    401
    Quote Originally Posted by bluecool View Post
    Thanks jpsmith1cm for helping me understand. My thinking that the suction pressure becomes lower when the unit cycles off is incorrect. I suppose that although the TXV bulb warms during the off cycles, it's effects are negligible.
    The sensing bulb temp/pressure isn't "negligible", just equal to or less than the evaporator pressure on shut down. In "theory" the bulb pressure and evaporator pressure balance (cancel each other) at some point as a result of being at the same temperature. The only remaining force is the spring pressure, which of course closes the valve.
    Last edited by Saturatedpsi; 03-07-2012 at 11:02 AM. Reason: correction

  13. #13
    Join Date
    Dec 2007
    Location
    north suburbs of Chicago
    Posts
    533
    I'm following this thread because I have noticed that txv coils made in the last few years seem to take forever to equalize.

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