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  1. #1
    Join Date
    Nov 2005
    Location
    Sacramento, ca
    Posts
    19

    How to Calculate BTU's on CHW System

    Whats a good way to calculate BTU'S on a chilled water system.

  2. #2
    Join Date
    Dec 2007
    Location
    Oregon
    Posts
    525
    Quote Originally Posted by stopher9 View Post
    Whats a good way to calculate BTU'S on a chilled water system.
    Tons= (GPM*delta T)/24

  3. #3
    Join Date
    Mar 2008
    Location
    Baltimore
    Posts
    50

    BTU/hr Calculation

    You will need the mass flow rate and temperature differential.
    The formula is Q(BTU/hr) = (Mdot)(Cp)(delta-T).
    Mdot can be solved for if you gave a GPM value by assuming that the treated chilled water weighs about 8.33 lbm/gal. Converting gallons to mass and coverting the time to hours yields a constant of 499. The formula would then reduce to:

    Q(Btu/hr) = (GPM)(499)(delta-T)

    Also as chillerfreak pointed out:

    Q(tons) = [(GPM)(delta-T)]/24

    or

    Q(tons) = Q(BTU/hr)/12000

    Units analysis:

    Q(BTU/hr) = (Mdot)(Cp)(delta-T)
    BTU/hr = {(Gal/min)(60min/hr)(8.33lbm/gal)}{1 BTU/(lbm-degF)}{degF}

  4. #4
    Join Date
    Oct 2006
    Posts
    191

    Or, if you're not really good in math,

    get one of these;

    http://www.onicon.com/System10.shtml

    I have 3 tied into my ems system and bill the 3 users for their BTU's

  5. #5
    Join Date
    Dec 2007
    Location
    Oregon
    Posts
    525
    Quote Originally Posted by jdd67 View Post
    You will need the mass flow rate and temperature differential.
    The formula is Q(BTU/hr) = (Mdot)(Cp)(delta-T).
    Mdot can be solved for if you gave a GPM value by assuming that the treated chilled water weighs about 8.33 lbm/gal. Converting gallons to mass and coverting the time to hours yields a constant of 499. The formula would then reduce to:

    Q(Btu/hr) = (GPM)(499)(delta-T)

    Also as chillerfreak pointed out:

    Q(tons) = [(GPM)(delta-T)]/24

    or

    Q(tons) = Q(BTU/hr)/12000

    Units analysis:

    Q(BTU/hr) = (Mdot)(Cp)(delta-T)
    BTU/hr = {(Gal/min)(60min/hr)(8.33lbm/gal)}{1 BTU/(lbm-degF)}{degF}
    JDD67....I would have gone on past the 8th. grade if I knew I was going to have to know all this.

  6. #6

    Cqalculation

    I have a chilled water system that has the following information:

    285 GPM
    Supply Temp - 45 Deg F
    Return Temp - 48 Deg F

    What is my BTU's per hour for this system??

  7. #7
    Join Date
    Oct 2005
    Posts
    321
    BTU/HR = 285 x 499 x 3 = 426,645

  8. #8
    JDD67-

    Wouldn't it be more correct to say Qdot(BTU/hr) = (Mdot)(Cp)(delta-T)?

  9. #9
    Join Date
    Oct 2006
    Posts
    116
    The formula is fairly simple BTU/Hr = FlowRate(in USGPM)*500(Weight of Water Constant) * Delta T.

    The accuracy of the data is another thing.

    1. You need a highly accurate flow meter, installed with the correct number of up stream & down stream straight runs of pipe. Personal experience, I like Mag meters because of the low up/downstream pipe runs and they have a much better turn down ratio than most other meters.

    2. Matched(meaning calibrated and read the same value at the same time in the same medium) temperature probes. This is very important because it can make a large difference in the data. Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!

  10. #10
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by canadian_jci_guy View Post
    2. Matched(meaning calibrated and read the same value at the same time in the same medium) temperature probes. This is very important because it can make a large difference in the data. Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!
    Good point.
    When I commission a CHW or HW system, I always make sure to run the pump(s) for a while (longer the better) to see if my inlet and outlet sensors are close to each other. More than a half degree difference and I'm looking for the cause.
    Jogas
    Four wheel therapy, my 1968 Camaro is gone and will be missed

  11. #11
    You dont have to have a flow meter to determine GPM. What you must have is the factory startup sheets that show design GPM and FTWG. Here is the equation if you have these variables.

    GPM= dGPMx[ square root ( mFTWG/dFTWG)]

    dGPM is design GPM
    mFTWG is measured feet of head
    dFTWG is design feet of head

    To find the feet of head, measure preasure drop across cooler barrel in PSI and convert to feet of head.

    example: 4PSI to feet of head is 4x2.31 or 8.4 feet of head. Multiply pressure drop by 2.31 to get feet of head.

  12. #12
    Join Date
    Mar 2007
    Posts
    272
    Quote Originally Posted by canadian_jci_guy View Post
    Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!
    If the accuracy is 0.1 % wouldn't the range of error be +/- 0.3 Deg F .... instead of 3 Deg F ?????

  13. #13
    Join Date
    Sep 2009
    Location
    Midwest
    Posts
    33
    Here is a low cost BTU meter non intrusive. Uses clamp on flow sensors. No hot tapping
    or draining the system to pick up flow:

    http://www.shenitech.com/STUF-300R1B.htm

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