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Need help with ADP Temperature!!

Discussion in 'Commercial' started by Dijinero, Oct 4, 2017.

  1. Dijinero

    Dijinero New Member

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    6
    Hello Everyone!

    My question is regarding the Apparatus dew point (ADP) temperature.

    ADP definition from my understanding:
    "The temperature at which moisture begins to form on coil (saturation temperature of air)"
    or
    "The average cooling coil temperature"

    I am designing a basement using carrier E-20 estimation sheet. The Effective sensible heat ratio (ESHR) came out to be 0.78, and the consequent indicated ADP was 53.7 F. But at this ADP, the RH of off-coil air was 75%.

    From my understanding, in a good HVAC design, the RH of off-coil air should me 95% or more.

    So, my question is: Can I decrease the selected ADP to a value where I would get the 95% RH
    off-coil air? Say 50 F?

    Or am I making a blunder somewhere? :gah:

    Really looking forward to get an answer for my confusion.

    Thanks!!
     
  2. Chuck

    Chuck SSP

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    Yes. Supposed to introduce yourself and tell us what you do in the trade. Then there's guys here who know stuff and do those calculations in their heads
     
  3. flange

    flange Act like you care and do SOMETHING!!

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    I am curious. you say for a basement. are your return air conditions out of ordinary due to moisture infiltration or general conditions, whereby there is low sensible load due to being below grade, but high rh for the same reason.....

    your final room conditions will be a factor of room condition combined with conditioned air, therefore its possible you are doing more grain depression and less sensible work at the coil. bergeerk.
     
  4. doc havoc

    doc havoc A cunning linguist

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    Because of @flange, every time I read something beyond my understanding I hear this noise in my head.:loltard:

    That and the Purdeep Purdoop guy. Sometimes that just comes at me from nowhere.

    Thanks flange, for these random words/noises that may or may not affect my sanity someday.:D
     
    404eh, newtech and knave like this.
  5. flange

    flange Act like you care and do SOMETHING!!

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    which part did you not understand....and for the record I am a bit of an evil genius, minus the genius part
     
  6. doc havoc

    doc havoc A cunning linguist

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    grain depression
     
  7. Dijinero

    Dijinero New Member

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    Sorry about that :D
    My name is Hamza. I am a mechanical engineer from Pakistan and have been working in HVAC for about a year now.
    I have been doing cooling load calculations and HVAC system designing for residencial/commercial buildings.
     
  8. flange

    flange Act like you care and do SOMETHING!!

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    when you discuss humidity in engineering certain types of systems, you are really talking about grains of moisture in the air. so, a pound of water equals 7000 grains. at standard conditions, 70 degrees and 50% its like 110 grains in a pound of air. in an ice arena we might target 37-40 grains. in hvac it might be 50-80 for example.
     
    knave likes this.
  9. flange

    flange Act like you care and do SOMETHING!!

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  10. pecmsg

    pecmsg Junior my ASS

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  11. flange

    flange Act like you care and do SOMETHING!!

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    think of it this way........if you had a pound of water...or basically one eighth of a gallon, that one pound of water would equal 7000 grains. its just a smaller unit of measure. now, air can only hold so much water per pound. so, the chart shows you how many grains in a pound of air at a given dewpoint.

    when discussing dehumidification, you can talk about grain depression. so lets say you take air with a dewpoint of 72. from the chart it has 118.8 grains of moisture. you then drive It to a dewpoint of say 52, or 57.8 grains. you have removed 61 grains out of each pound of air. its just another way to look at life.
     
    doc havoc and knave like this.
  12. pecmsg

    pecmsg Junior my ASS

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    Put it in something we can relate to
    Beer
    Whiskey
    Vodka
     
    jays 65 and newtech like this.
  13. flange

    flange Act like you care and do SOMETHING!!

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    please provide the specific gravity of each, along with specific heat.
     
  14. Dijinero

    Dijinero New Member

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    Thanks for your response. I'll try to answer the best I can, and ask some more questions :)
    The design room conditions in the basement are 75 F (DB), 60% RH. So the return air conditions will also be 75 F (DB), 60% RH right?

    I'll share other details which may help..
    the floor area is 3,000 sqft. And the wall height is 25ft (so I think the general rule of thumb 350-400 cfm/ton won't apply here because the wall height is above 15ft?)

    The moisture infiltration from all internal surface area is about 15 lbs of water/hour, and sensible load is low. Total cooling load came out to be 78,000 btu/h.

    ESHR = 0.78, consequent indicated ADP from psych. chart = 53.7 F
    selected ADP = 53.7 F (meaning the effective cooling cooling temperature will be 53.7 F)
    Bypass factor is 0.2, so at these conditions, the calculated off-coil supply cfm came to be around 3,800 (600 cfm/ton), with 80% RH.

    The term 600 cfm/ton and 80% RH worries me a bit. Makes me wonder if the system will be an over design? is the off-coil RH inefficiently low?

    My solution (I want to know if it is the right or wrong to do it like this?)

    ESHR = 0.78, consequent indicated ADP from psych. chart = 53.7 F
    selected ADP = 50 F (meaning even though ADP is at 53.7 F, the effective cooling coil temperature is selected to be 50 F in order to achieve higher RH and reduce total supply cfm)
    Bypass factor is 0.2, so at these conditions, the calculated off-coil supply cfm are reduced to 3,100 (480 cfm/ton), with 97% RH.
    The DB/WB of supply air with this approach are = 54.7 DB / 54.2 WB (Still above the indicated ADP)

    Would you agree with my approach or is it the wrong way to do it?
     
  15. frozensolid

    frozensolid Psychologist's Dream

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    If you are in a 72 degree bar and have seven beers in an hour. Now you are sh!tfaced and pass out.

    So the bouncer throws you outside in 32 degrees weather. Suddenly the grain alcohol depression in your blood stream is reduced and you are no longer totally sh!tfaced. So, you can have another beer. :)
     
  16. flange

    flange Act like you care and do SOMETHING!!

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    how will the space be used is my first question. you are heading down a path I don't like. you see, you are calculating for full load. that is great, BUT when its not full load, your lower than usual leaving condition will overcool the space, resulting in the mechanical cooling cycling a bit in some cases, a lot in others. this will lead to a cool, damp space under certain conditions. Being only 53 F, the first thing I would do is use a draw through unit, verses a blow through style. it wont be much, but the added heat picked up over the motor could be a degree or two, resulting in drier air ( relative) and acting like a mild reheat. I would then consider full reheat so as to not overcool the space.

    note reheat in such a small unit is rather simple, a cheap electric duct heater either staged or with SCR control.
     
  17. newtech

    newtech 636

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    Last edited: Oct 4, 2017
  18. LemonWizard

    LemonWizard Blinky Lights Wizard

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    If the return air is at 75* with 60% rh and that is the desired conditions of the room then what are you trying to achieve?

    Is this purely for dehumidification? Do you have reheat to prevent lowering the space temp while allowing the unit to remove moisture? I don't know the calculations but I get the general idea and what you're describing to me sounds off.
     
  19. flange

    flange Act like you care and do SOMETHING!!

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    rawal is a great solution to maintaining desired leaving temperature in varying loads. if sized and setup correctly can allow for about a 65% reduction in load without frosting a coil. that doesn't fix the cold room though. we have used a lot of those for medical applications.
     
  20. pecmsg

    pecmsg Junior my ASS

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    See if this helps: (or gets more confusing)!
     

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