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  1. #27
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    It seems to me like if nothing else the compressor would run cooler due to lower superheat.

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  2. #28
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    Also just to clarify. I dont plan on doing any of these things in the field. Its just food for thought.

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  3. #29
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    Refrigeration cycle efficiency breakdown?

    Quote Originally Posted by R600a View Post
    I thought of another non conventional idea. What effect would it have if you put the txv outside?
    Just weld it in after the liquid line service port, weld an equlizer port in the vapor line to leave your service port open, strap the bulb to the vapor line and insulate the liquid and vopor lines really well. Also remove the piston of course.

    Do youall think it would work like normal or would it effect the efficiency?

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    Mini split units do this all the time to quiet down the noise at the indoor unit. Works well. Just insulate the lines properly and make sure it forms a good vapor barrier

    Edit: should also note that you are then sizing the outlet of the TXV to maintain your low pressure liquid /vapor mix as such until it gets to the distributor at the evaporator. I donít have a table for that but it would want to be within your considerations. You could probably get a line sizing chart from one of the install manuals out there. If you run a typical suction line size from the TXV to the evaporator then that suction line would become part of your evaporator and ice up pretty quickly. Keep the pipe small to reduce the room for expansion and preserve the liquid quality.


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  4. #30
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    That is where the thought came from. I just wondered what else it would effect.
    Quote Originally Posted by thatguy View Post
    Mini split units do this all the time to quiet down the noise at the indoor unit. Works well. Just insulate the lines properly and make sure it forms a good vapor barrier


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  5. #31
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    Quote Originally Posted by R600a View Post
    That is where the thought came from. I just wondered what else it would effect.

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    Iím guessing it reduces the efficiency slightly because it would pick up a small amount and f heat along its travels. But if the line size is short and the insulation is good itís probably negligible.


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  6. #32
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    I figured the same. Also the evaporator would be very efficient becsuse it would be plenty well fed. And the compressor would have less superheated suction vapor so it would run nice and cool.
    Quote Originally Posted by thatguy View Post
    Iím guessing it reduces the efficiency slightly because it would pick up a small amount and f heat along its travels. But if the line size is short and the insulation is good itís probably negligible.


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  7. #33
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    And i expect the txv would not hunt much at all.

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  8. #34
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    Refrigeration cycle efficiency breakdown?

    Quote Originally Posted by R600a View Post
    I figured the same. Also the evaporator would be very efficient becsuse it would be plenty well fed. And the compressor would have less superheated suction vapor so it would run nice and cool.

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    The problem I see with trying to run too low of a superheat is that you run the risk of turning part of your suction line into your evaporator. This could lead to icing up of your suction line, or at the least, preforming work where work didnít need to be done which would reduce your efficiency. Then there is also the manufactures specs for their compressors, I hear from other techs on here that Copland wants something like 20 degrees superheat to their compressors. I havenít seen the literature to support that but I also havenít looked for it either. If you happen to see those documented I would be curious to see them also. Iím not sure if their required superheat is a blanketed number for all models or if scroll, for instance, would be different from their reciprocating units.

    As for the stable hunting thing. You may see a reduction on that since the suction has a long time to mix all of the different distributors back into one slurry.

    Edit, corrected my autocorrects.

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  9. #35
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    I dont remember them mentioning a required superheat number when i went to Copeland compressor school. But they do mention that the suction line temp shouldn't be above 65* in normal operation or the compressor will run hot and its life will be shortened. It seems that 10* is good safety. But i have seen units that call for 0* at the service valves. Also minisplits run much lower than 10*.
    Quote Originally Posted by thatguy View Post
    The problem I see with trying to run too low of a superheat is that you run the risk of turning part of your suction line into your evaporator. This could lead to icing up of your suction line, or at the least, preforming work where work didnít need to be done which would reduce your efficiency. Then there is also the manufactures specs for their compressors, I hear from other techs on here that Copland wants something like 20 degrees superheat to their compressors. I havenít seen the literature to support that but I also havenít looked for it either. If you happen to see those documented I would be curious to see them also. Iím not sure if their required superheat is a blanketed number for all models or if scroll, for instance, would be different from their reciprocating units.

    As for the stable hunting thing. You may see a reduction on that since the suction has a long time to mix all of the different distributors back into one slurry.

    Edit, corrected my autocorrects.

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  10. #36
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    Yeah, that's always bugged me about mini splits. Just seems like it would not be efficient to have the metering device in the outdoor unit.


    Quote Originally Posted by thatguy View Post
    I’m guessing it reduces the efficiency slightly because it would pick up a small amount and f heat along its travels. But if the line size is short and the insulation is good it’s probably negligible.


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  11. #37
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    I have always wondered how it works so well. The dramatically lower power consumption compared to conventional systems seem to indicate that is must not be too bad.
    Quote Originally Posted by BBeerme View Post
    Yeah, that's always bugged me about mini splits. Just seems like it would not be efficient to have the metering device in the outdoor unit.
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  12. #38
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    Quote Originally Posted by R600a View Post
    I dont remember them mentioning a required superheat number when i went to Copeland compressor school. But they do mention that the suction line temp shouldn't be above 65* in normal operation or the compressor will run hot and its life will be shortened. It seems that 10* is good safety. But i have seen units that call for 0* at the service valves. Also minisplits run much lower than 10*.

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    Iíve seen those low requested superheats also, typically on mini splits. Those units that request that typically have EEVs with pressure transducers and thermistors all over the thing so they know exactly what the quality of refrigerant is exiting the evaporator, but I believe that number is to specify the superheat leaving the evaporator and not to specify the superheat entering the compressor. Those systems have always had an accumulator on them to protect against floodback if it were to occurring, from what Iíve seen.

    8-12 is typically what I see manufactures request in their literature for a typical system when equipped with a TXV but havenít had a need to look for anything mentioned by the compressor manufacture because the units installation instructions would supersede anything from the compressor manufacture.


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  13. #39
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    Quote Originally Posted by thatguy View Post
    Iíve seen those low requested superheats also, typically on mini splits. Those units that request that typically have EEVs with pressure transducers and thermistors all over the thing so they know exactly what the quality of refrigerant is exiting the evaporator, but I believe that number is to specify the superheat leaving the evaporator and not to specify the superheat entering the compressor. Those systems have always had an accumulator on them to protect against floodback if it were to occurring, from what Iíve seen.

    8-12 is typically what I see manufactures request in their literature for a typical system when equipped with a TXV but havenít had a need to look for anything mentioned by the compressor manufacture because the units installation instructions would supersede anything from the compressor manufacture.


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    The 0* superheat i referred to was a piston metered ac. It was quite a curiosity to me. It was only at 105* outdoor with a fairly low indoor wet bulb.

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