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  1. #1
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    TXV equalizing system when unit is off

    I am a student in a commercial refrigeration program at my local community college, and I had a test tonight in my Electrical class that kind of has me stumped. I can't recall the question *exactly* (and of course I couldn't write down a test question during the test) but it went something like this:

    Customer has a refrigerated case containing a TXV that has no equalizer tube or balanced port. What type of motor start components would you expect the compressor to utilize?

    A. Capacitor Start, Induction run
    B. Permanent split capacitor
    C. Capacitor start, capacitor run
    D. Centrifugal switch

    I put "C", and asked the teacher after the test about this question. I said something like "I think I got it right, because the txv won't equalize and you need a large amount of torque to overcome the high pressure on the high side of the system (since it doesn't equalize). He said "yes, you're right". The thing is I don't understand *why* it doesn't equalize. I've seen units in the field that have been off for a while (down) and when I hook up my guages the pressures are equal, but I think that's due to most txvs I see having an equalizing tube or balanced port. But even if the txv didn't have these equalizing devices, wouldn't the suction line begin to get warm, which would cause the sensing bulb to heat up and exert pressure on the power head and actuate the piston in the txv...which would create a little orifice for the pressures to equalize???

    Sorry for the long post...

  2. #2
    Join Date
    Jun 2003
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    Chicago, IL
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    4,458
    It's a bad question. Neither an equalizer line or balance port affects whether the system equalizes. An equalizer line allows the txv to function better when used with a high pressure drop evaporator and a balanced port design allows the valve to maintain more accurate superheat across a wide range of operating conditions

    A bleed port will allow a system to equalize

    Both A and C are correct choices.

  3. #3
    Join Date
    Feb 2009
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    Tallahassee, FL
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    6,051
    Yes the suction does warm but with the higher suction pressure acting to close valve - it stays shut.

    However there are ways like if the bulb temp gets higher than pressure that it does start to equalize .

  4. #4
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    But, aren't I correct in assuming that when the unit is off the suction line should warm up and throttle the valve, causing the pressures to equalize even if there's no bleed port?

  5. #5
    Join Date
    Feb 2010
    Location
    Newnan Ga,
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    Quote Originally Posted by craig1 View Post
    It's a bad question. Neither an equalizer line or balance port affects whether the system equalizes. An equalizer line allows the txv to function better when used with a high pressure drop evaporator and a balanced port design allows the valve to maintain more accurate superheat across a wide range of operating conditions

    A bleed port will allow a system to equalize

    Both A and C are correct choices.
    Correct √ a system will equalize thru the metering device when cycled off for a long period (defrost) the external equalized port is for normal temp cycle on/cycle off where it does not have the time to equalize

  6. #6
    Join Date
    Sep 2008
    Location
    Western PA
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    25,714
    Quote Originally Posted by craig1 View Post
    It's a bad question. Neither an equalizer line or balance port affects whether the system equalizes. An equalizer line allows the txv to function better when used with a high pressure drop evaporator and a balanced port design allows the valve to maintain more accurate superheat across a wide range of operating conditions

    A bleed port will allow a system to equalize

    Both A and C are correct choices.

    ^THIS^

    ...and we wonder why tech school grads have a hard time understanding what is happening in the REAL world.


  7. #7
    Join Date
    Jul 2005
    Location
    FLORIDA
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    1,163
    answer C ........ txv/cap tubes equalize very slow and the start cap helps to start pumps under heavy loads and short off times and in most conventional freezer applications they have pump downs valves and never equalize most compressors are alike with piston size and stork, now the motor size may/may not be different but running at higher or lower pressure and starting torque, make the reason for amount of caps and size
    once you think you've seen it all
    (THINK AGAIN)
    I would rather work for free than be look upon as a thief!!!!!

  8. #8
    Join Date
    Feb 2006
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    Phoenix,AZ
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    I don't see too many small compressor's on reach-in's with a run cap. Most just a start cap and relay. So in the real world the answer is A. On larger compressor's the answer would be C!

  9. #9
    Join Date
    Jul 2005
    Location
    FLORIDA
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    1,163
    small systems = cap tube small compressors 1/3 hp or lower i agree................ with txv not many without run cap just seems to fit better
    once you think you've seen it all
    (THINK AGAIN)
    I would rather work for free than be look upon as a thief!!!!!

  10. #10
    Join Date
    Feb 2006
    Location
    Phoenix,AZ
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    delfield has txv w/ start cap on some of their smaller junk!

    http://www.delfield.com/asset/?id=lmsod

  11. #11
    Join Date
    Oct 2011
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    Quote Originally Posted by Servicerunner View Post
    But, aren't I correct in assuming that when the unit is off the suction line should warm up and throttle the valve, causing the pressures to equalize even if there's no bleed port?
    The pressures will eventually equalize in a non functioning unit assuming the box temp and ambient around the condenser are equal. On a functioning unit, during the off cycle, your refrigerant temperature will be equal or close, to that of the box temp. As the box gains heat so does the refrigerant. You valve will not throttle because the compressor is not running. The valve will be open until the thermostat or pressure control closes and energizes the pump
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

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  12. #12
    Join Date
    Feb 2010
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    Newnan Ga,
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    77
    [QUOTE=KB Cool;15717111]delfield has txv w/ start cap on some of their smaller junk!

    Junk? Delfield Is The Shizz nit.....

  13. #13
    Join Date
    Feb 2006
    Location
    Phoenix,AZ
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    [QUOTE=Grayline;15720361]
    Quote Originally Posted by KB Cool View Post
    delfield has txv w/ start cap on some of their smaller junk!

    Junk? Delfield Is The Shizz nit.....
    Just a term i use for all small refer units regardless of who makes it! Serious, if i had my way i would only do A/C, walk-ins and ice machine's! Period!

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