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Thread: Enthalpy

  1. #1

    Enthalpy

    i am going to try to use enthalpy for my ahu units. i would like advise as to what enthalpy value i should use to allow outside air in? i will be comparing outside enthalpy against return air enthalpy. after a certain dead band is reached the outside air will start to be allowed in for free cooling. the only problem is that i do not know what setting i should use to start free economizer cooling.

  2. #2
    Join Date
    Aug 2006
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    New England
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    520
    Since you're using comparative enthalpy, you don't need a setpoint, only a differential. I would enable free cooling when outside air is around 1 btu/lb below return air and disable when outside air = return air.

  3. #3
    Join Date
    Sep 2007
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    Michigan
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    Typically we don't use differentials for enthalpy comparison. Normally you won't see the enthalpy bouncing around. In engineers specs it's been worded "when OA is equal to, or less than, RA enthalpy..." Same goes for strait dry bulb compare.

    My guess would be if you run OA, until OA is greater than RA you provide fresh, non contaminated air, for a longer period of time. In that case we don't have to modulate the OA damper to maintain the min CFM, so were extending the life of the actuator(s).

    Why did you say, "i am going to try to use enthalpy for my ahu units"?

    In some cases it's a waste of labor, materials, programming, to install humidity sensors depending on your region, and the type of internal building load.

    My food for thought
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  4. #4
    Join Date
    Apr 2007
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    little rock ak
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    i never herd of contaminated air just because its humid doesnt mean it contaminated

  5. #5
    my question goes back to what setting i should use before i let outside air inside. i would use enthalpy to cool down the area with free cooling but i would have to have a setpoint to reference. in other words i would not want to let in 55 degree air in with 98% RH.(Raining).

  6. #6
    Join Date
    Nov 2007
    Location
    New Haven, CT
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    55
    We typically use less than 26 BTU/Lb outside air enthalpy for free cooling here in the North east.

  7. #7
    Join Date
    Jun 2004
    Location
    SC
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    139
    I use enthalpy a good bit and learned on air washers in cotton mills. I saw above
    where someone said use 26 btu/lb, that's a good number. Comparative enthalpy
    is better, but you need to put a temperature cap on it, say 60 degrees. Other wise the conditions may be right on a summer day. Also follow the psyc chart out to the grains of moisture. you'll find that at 58 degrees dry bulb that the grains of moisture is usable even if their's fog outside. On drybulb changeover I use 58 deg. Hope this helps.

    Later,
    Darryl

  8. #8

    Talking

    thanks guys. much appreciated.

  9. #9
    Join Date
    Sep 2007
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    Michigan
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    Quote Originally Posted by wellhung View Post
    thanks guys. much appreciated.
    You said your comparing return air enthalpy, to outside air enthalpy... there is no setpoint



    Do you understand how you calculate enthalpy?
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  10. #10
    yes. i see your point. but would i not have to have a reference point to prevent bringing in 95 degree air with 10% RH?

  11. #11
    Join Date
    Jun 2007
    Location
    Darwin, Australia
    Posts
    28

    Optimized start using enthalpy

    We had a heap of irate customers call us on Monday 3/12/07, complaining that their chillers time schedules were playing up, and they started around 03.30 - 04.00am. Didn't twig till on the radio it was announced that the temp/ humidity over night was the hottest ever recorded in Darwin. (29C @ 65%). Optimized start (looking at enthalpy) at these sites had to kick in 3 hours early to reach setpoint (23C) by 07.00am. Enthalpy was 17cal/kg (67 BTU?) just before dawn.
    Global warming's real good for business!

    Crankshaft

  12. #12
    Join Date
    Sep 2007
    Location
    Michigan
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    290
    Quote Originally Posted by wellhung View Post
    yes. i see your point. but would i not have to have a reference point to prevent bringing in 95 degree air with 10% RH?
    This would have been easier if you just said you don't understand enthalpy. Get out a Psychometric chart, I have Trane ones that are colored and have instructions on the back.(I don’t work for Trane) Most HVAC books have them, once you plot the numbers you'll understand. Your example is way out there since I'm in the Midwest, but it's null anyway. If you showhow have... 95/10 outside and 95/12 inside, yes, your going to bring in full outside air and cool it. It will take less mechanial energy to cool the OA.

    In the summer here, it's not uncommon for outside temperature, and humidity to suddenly drop during the night, and now the building return air has more Btu/Lb than the outside air. Even if the outside air may be dry bulb warmer than return air, it may have less humidity, and when you calculate those (total heat = enthalpy) one will have a lower number than the other. That’s the whole point of using enthalpy, there is no question at what dry bulb temperature should we economize.

    Had a commissioning agent that couldn’t understand why the OA dampers were full open on day, I said we were economizing and he had a confused look on his face because it was 60ish outside.

    This is commonly found in buildings that have high internal latent heat gains such as kitchens, auditoriums, industrial locations that have processes releasing high amounts of moisture.

    I hope it's becoming clear.
    "Yeah I can figure out whats wrong with it, but you were here first and there isn't room for two, plus it's cold up here, I'll be in the van"

  13. #13
    Join Date
    Jun 2001
    Location
    Louisville, KY
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    12,181
    Quote Originally Posted by wellhung View Post
    thanks guys. much appreciated.
    Order this book. It will help you understand enthalpy. Frankly, it's one of the easiest reads regarding psychrometrics.
    Perhaps you should have read the instructions before calling.

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