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  1. #1
    Join Date
    Apr 2008
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    Tucson, AZ
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    123

    Temp pressure chart error

    I found my discharge air colder than my coil! 407A at 18# = -6*f on the Sporlan pocket chart. I found it wrong on some of the iPad apps also from bittzer and danfoss. Oops. I'm sure they will be corrected soon. The Johnstone app seems to have it right.

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,915
    You're looking at the dew point chart, for the evaporator temperature, use the bubble point chart.
    Use dew point when the refrigerant is changing from a vapor to a liquid, bubble point when it is changing from a liquid to a vapor.

    With R407A, 18 psig suction pressure gives a saturated temperature of -16.6ºF in the evaporator.
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    May 2004
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    1,886
    I've always been taught to use the dew point when determining evaporator superheat.

  4. #4
    Join Date
    Oct 2004
    Location
    Central WA
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    1,650
    Bubble point is for Subcooling (BUB = SUB)

    Dewpoint is for checking superheat.

  5. #5
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    14,915
    Quote Originally Posted by cjpwalker View Post
    Bubble point is for Subcooling (BUB = SUB)

    Dewpoint is for checking superheat.
    Yeah, apparently I should refrain from posting when I'm drinking and watching football, or after midnight, or before my morning caffeine.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Sep 2008
    Location
    Western PA
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    Another thing that I'm finding with 407a, and icemeister explained it to me somewhere on these forums and I've found his explanation to be accurate.

    The evaporator will change in temp as the refrigerant goes through it.

    The inlet side of the evap will be at (or near) bubble point and the outlet will be at (or near) dew point.

    When trying to determine evap temperature, particularly for purposes of setting pressure controls used to control temperature and EPR valves, using an average of DP/BP is the best and most accurate method.

  7. #7
    Join Date
    Jul 2000
    Location
    Guayaquil EC
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    10,485
    Quote Originally Posted by jpsmith1cm View Post
    Another thing that I'm finding with 407a, and icemeister explained it to me somewhere on these forums and I've found his explanation to be accurate.

    The evaporator will change in temp as the refrigerant goes through it.

    The inlet side of the evap will be at (or near) bubble point and the outlet will be at (or near) dew point.

    When trying to determine evap temperature, particularly for purposes of setting pressure controls used to control temperature and EPR valves, using an average of DP/BP is the best and most accurate method.
    Here's that thread: http://hvac-talk.com/vbb/showthread....p-small-walkin

    Since mccann still isn't a Pro Member... ...I'll post a link to DuPont's article on temperature glide with 400 series refrigerants. It's likely the best explanation available.

    DuPont - Temperature Glide With 400 Series Refrigerants

  8. #8
    Join Date
    Apr 2008
    Location
    Tucson, AZ
    Posts
    123
    Quote Originally Posted by mark beiser View Post
    You're looking at the dew point chart, for the evaporator temperature, use the bubble point chart.
    Use dew point when the refrigerant is changing from a vapor to a liquid, bubble point when it is changing from a liquid to a vapor.

    With R407A, 18 psig suction pressure gives a saturated temperature of -16.6ºF in the evaporator.
    My point is the Sporlan chart is wrong. It shows 18# at -6*. I showed the chart to the local supply manager. He emailed the Sporlan rep. I just thought it was amazing.

  9. #9
    Join Date
    Sep 2008
    Location
    Western PA
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    Quote Originally Posted by mccann View Post
    My point is the Sporlan chart is wrong. It shows 18# at -6*. I showed the chart to the local supply manager. He emailed the Sporlan rep. I just thought it was amazing.
    My Bitzer App shows the same PT value for 18# on the dewpoint scale.

    At 18# on the bubble point scale, it is -16.6

    This would give you an average evap temp of about -11 which could, with low airflow could give you an air temp lower than the dew point.


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