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Thread: delta T ?

  1. #1
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    on a Residential A/C

    Between me an my fellow coleage we have that discussion.

    I say 20-25 F

    he says 10-15 F.

    My argument is: Temp. after Evap. is approx. 50 F - and in the Return is 75 F.

    Help me out - who is right ?

  2. #2
    20 to 25 is right ... if you live in the desert then lower delta t has some merit

  3. #3
    Join Date
    Jul 2001
    Location
    Anderson,SC
    Posts
    979
    If you subtract 50 from 75, your right.

    Now if you are asking what is the correct DT you need a chart and WB temp. Most will be happy with DT of 20. Your fan speed is the biggest factor on how much heat is being removed from the air.


    STUD

  4. #4
    Join Date
    Jul 2001
    Posts
    40
    I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

    Just my opinion.

  5. #5
    hey i know plenty of pros who arent

  6. #6
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    umd,

    what is your opinion - if you have one ?

    I am sure you are a Super Professional, that is why I humbly ask you for your opinion.

    Let's see what are you up to? C'mon be a man, say it ...

  7. #7
    Join Date
    Mar 2004
    Location
    Gone
    Posts
    5,340
    Originally posted by umd
    The very question indicates the author knows nothing about HVAC...

    Just my opinion.
    The man asked a specific question pertaining to Delta T, and showed where he was getting the temperatures to verify this. I think he knows a little more than just nothing, don’t you?

  8. #8
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    this guy is not a pro but reasonable question
    depending on blower speed
    18 to 25 is good but super heat and subcool is the best way to check

  9. #9
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    madeinusa,

    I took the readings right after the Evap. Coil, let's say 3-5 " on the Supply - and 12-24" before filter on the Return.

  10. #10
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    20 degree TD
    FILL OUT YOUR PROFILE!!

  11. #11
    Join Date
    Apr 2004
    Location
    Niantic, Illinois
    Posts
    545

    Smile

    Originally posted by umd
    I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

    Just my opinion.
    The guy is in the learning process. Wow, switch to decaf.

  12. #12
    Join Date
    Jul 2001
    Posts
    40
    Ok, I'll take the bait...The dry bulb delta T across the evap coil will be a function of the amount of sensible heat removal from the airstream and the amount of air going across the coil. Now if you were only doing sensible coiling (not removing any moisture) the formula would look like this: BTU/HR(sensible)=CFM X1.08 X DELTA T. Unfortunately in the real world you are usually removing some latent heat which is not measurable by dry bulb temperature. To accurately predict the delta T, you would need to know the wet bulb temperature of the entering and leaving air. You would also need to know the airflow across the coil. Using a psychrometric chart you could predict what your delta T would be by calculating the actual enthalpy per pound of heat in your entering and leaving air and use the formula: BTU/HR (total)= CFM X 4.5 X DELTA H (actual heat content of air).
    The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T.

    Sorry if you were offended.

  13. #13
    Join Date
    Nov 2004
    Location
    CA
    Posts
    238
    umd,

    I really thank you for your answer.

    "Offended" - not.

    a bit ticked-off by your first reply - yes !

    Not anymore.

    Thank you, again.

    See you around.

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