Does a TXV cancel the benefit of dehumidification on demand using a/c?
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  1. #1
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    Does a TXV cancel the benefit of dehumidification on demand using a/c?

    My understanding is that the TXV will reduce the amount of refrigerant entering the evaporator coil if there is little heat being transferred from the return air to the coil.

    If this is the case, would a TXV interfere with the dehumidification on demand feature available with a variable speed blower?

    My thinking is that reducing the air flow through the evaporator coil will reduce the load. The TXV will then automatically reduce the refrigerant flow to ensure all of it evaporates. If the refrigerant flow is reduced, the coil temperature will rise and will not provide as much dehumidification as desired.

    So does the TXV cancel the benefit of reducing the air flow over the coil or is the benefit of less air flow larger than the negative impact of reducing refrigerant flow?

  2. #2
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    I'm guessing that external equilized expansion valves wont allow severe starving of the evaporator as you describe. And also the fact that a call for reasonable dehumidification the return air should contain enough latent heat for a txv bulb pressure to react as well. HGBP may be required for super - agressive dehumify. All speculation on my part.

  3. #3
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    Quote Originally Posted by john_ertw View Post
    My understanding is that the TXV will reduce the amount of refrigerant entering the evaporator coil if there is little heat being transferred from the return air to the coil.

    If this is the case, would a TXV interfere with the dehumidification on demand feature available with a variable speed blower?

    My thinking is that reducing the air flow through the evaporator coil will reduce the load. The TXV will then automatically reduce the refrigerant flow to ensure all of it evaporates. If the refrigerant flow is reduced, the coil temperature will rise and will not provide as much dehumidification as desired.

    So does the TXV cancel the benefit of reducing the air flow over the coil or is the benefit of less air flow larger than the negative impact of reducing refrigerant flow?
    I don't understand that statement in bold. The mechanical TXV has a fixed SH curve that it must follow. SH will still decrease slightly as load decreases, but with a properly sized valve it will be minimal. Regardless, unless you are using a variable speed compressor, the coil temp will actually decrease, removing more moisture. The TXV is simply going to optimize refrigerant flow to the evap and in my mind, should have a positive effect on demand dehumidification by keeping the evap full of liquid. With a cap tube or orifice you would need a larger "cushion" to prevent floodback and thus use less of your coil surface for condensation.

  4. #4
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    Quote Originally Posted by man from trane View Post
    I don't understand that statement in bold.
    I probably made some mistakes and may be totally wrong in my thinking.

    Quote Originally Posted by man from trane View Post
    The TXV is simply going to optimize refrigerant flow to the evap and in my mind, should have a positive effect on demand dehumidification by keeping the evap full of liquid. With a cap tube or orifice you would need a larger "cushion" to prevent floodback and thus use less of your coil surface for condensation.
    Thanks for your explaination. This makes sense. If a TXV allows for a smaller SH than a fixed orface, you would get better dehumidification since the evap coil would have more "liquid" in it.

  5. #5
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    The TXV will reduce refrigerant flow. But it won't cause the coil to warm up. Since the air flow through the coil has decreased, the coil will either remain the same temp, or become colder.

    If it remains the same temp. The moisture removal ability is still increased because the reduced air flow has decreased the sensible capacity.

    With a TXV, the coil is a more even temp across it then a piston system. So more of the coil removes water.
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  6. #6
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    For all conditions, I'll take a 'good-working' TXV

    Quote Originally Posted by beenthere View Post
    The TXV will reduce refrigerant flow. But it won't cause the coil to warm up. Since the air flow through the coil has decreased, the coil will either remain the same temp, or become colder.

    If it remains the same temp. The moisture removal ability is still increased because the reduced air flow has decreased the sensible capacity.

    With a TXV, the coil is a more even temp across it then a piston system. So more of the coil removes water.
    A perfect explanation by beenthere.

    Additionally, the fixed piston orifice is sized so that under lower temps outdoors the lowered condenser pressure will cause a higher Superheat & thus in some situations a warmer indoor coil than a TXV that controls the superheat & coil temp rather irrespective of either, outdoor or indoor condition.

    There is a danger with a piston orifice & a high outdoor temp & somehow a very light load indoors' that there will be freezing of the coil & flood-back that could take-out the compressor, also combines with too much compressor oil & causes lubrication problems, & an insulation of the interior of the coils, problem.

    I'll take, a good working TXV every time for all conditions including high humidity latent loads - at low indoor &/or high or low temp outdoor conditions.
    - Darrell
    Last edited by udarrell; 06-02-2009 at 04:19 PM. Reason: fixed piston orifice

  7. #7
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    I wouldn't recommend running a VS air handler in any dehumidification mode if it doesn't have a TXV.

    A TXV is pretty much a required item if you want the compressor to chance at a long healthy life.
    If more government is the answer, then it's a really stupid question.

  8. #8
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    Thanks for all the replies. I didn't think about how a TXV allows the system to run closer to an optimal state under all conditions as opposed to a fixed orifice which has to be sized to protect the compressor in high load and low load situations, thus not providing optimal cooling capacity in all situations.

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