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  1. #1
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    Superheat question

    I’m having a brain fart right now.

    R-448a WIF
    Suction temp at evap 7deg
    Suction pressure at evap 11 psi = -18 deg dew
    7 minus -18 = 25. I have 25 deg SH at my evap. Does that sound right?

  2. #2
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    With 404 I come up with 34*f

  3. #3
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    Quote Originally Posted by VanMan812 View Post
    I’m having a brain fart right now.

    R-448a WIF
    Suction temp at evap 7deg
    Suction pressure at evap 11 psi = -18 deg dew
    7 minus -18 = 25. I have 25 deg SH at my evap. Does that sound right?
    Your math is correct (I didn't get my PT chart out to verify that part).

  4. #4
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    I try to imagine a number line when dealing with subtracting a negative number. Start at -18 and couynt how many numers there are to get to +7...ie, 25

    -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 - 4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7

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  6. #5
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    Thread Starter
    I was thinking I need to compensate for the glide. Yes?


    Sent from my iPhone using Tapatalk

  7. #6
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    25degree superheat is way too much on freezer. Is it new install?

  8. #7
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    Quote Originally Posted by VanMan812 View Post
    I was thinking I need to compensate for the glide. Yes?


    Sent from my iPhone using Tapatalk
    No, the dewpoint is the saturation temperature of the refrigerant gas leaving the evaporator, so by definition, the superheat is the measured temperature minus the dewpoint temperature.

    Some guys have been using the midpoint or average evap temperature to determine superheat, but that isn't the way to do it properly.

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  10. #8
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    Thread Starter
    Quote Originally Posted by icemeister View Post
    No, the dewpoint is the saturation temperature of the refrigerant gas leaving the evaporator, so by definition, the superheat is the measured temperature minus the dewpoint temperature.

    Some guys have been using the midpoint or average evap temperature to determine superheat, but that isn't the way to do it properly.
    That makes sense and that’s what I usually do. The p/t chart I was using gave the average temp (bubble and dew) and it made me think for a minute that I should be using that instead of the dew.

    Why are some guys using the midpoint? Wouldn’t that give a false SH that’s higher than it actually is? That’s a slippery slope.


    Sent from my iPhone using Tapatalk

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  12. #9
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    Quote Originally Posted by VanMan812 View Post
    I was thinking I need to compensate for the glide. Yes?


    Sent from my iPhone using Tapatalk
    https://www.honeywell-refrigerants.c...012&download=1

  13. #10
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    Quote Originally Posted by VanMan812 View Post
    That makes sense and that’s what I usually do. The p/t chart I was using gave the average temp (bubble and dew) and it made me think for a minute that I should be using that instead of the dew.

    Why are some guys using the midpoint? Wouldn’t that give a false SH that’s higher than it actually is? That’s a slippery slope.


    Sent from my iPhone using Tapatalk
    It's been mentioned a couple of times in recent threads by some market techs that the chain they do work for requires they use the midpoint for superheats with blends because the cases run better and are more able to meet the design temps. I don't agree with that method as it's not technically correct. It may work for them but not for the reason they think. The techs say they're just doing as they're told...and you can't knock that too much.

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