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Theory: Finding Unknown Cv

Mitch_4
Mitch_4 Member Posts: 955
in THE MAIN WALL
did Adam and Eve have belly buttons?

Comments

  • Mark_46
    Mark_46 Member Posts: 312
    Slide rules and Formulas

    It seems hydronic component manufacturers tend to publish pressure drop values for only a few GPM flow rates. Is it possible to find a component’s pressure drop value (solve for the unknown) at other GPM rates using the known published values? Is there an inverse formula? Similarly, does pressure drop increasing and decreasing by the square of the flow play a role in calculating something like this?

    For example:

    At 8 GPM = 6 head feet (known)

    At 7 GPM = ? head feet

    At 6 GPM = ? head feet

    At 5 GPM = ? head feet

    Or:

    At 6 GPM = 7.1 head feet (known)

    At 5 GPM = ? head feet

    At 4 GPM = 3.2 head feet (known)

    At 3 GPM = ? head feet

    At 2 GPM = .9 head feet (known)

    At 1 GPM = ? head feet

    Or:

    At 8 GPM = 16.1 head feet (known)

    At 7 GPM = ? head feet

    At 6 GPM = 9.3 head feet (known)

    At 5 GPM = ? head feet

    At 4 GPM = 4.3 head feet (known)

    At 3 GPM = ? head feet

    At 2 GPM = ? head feet

    At 1 GPM = ? head feet
  • John Barba
    John Barba Member Posts: 166
    Try this ...

    (Actual flow / Cv rating)2 (that's "sqaured") x 2.31 equals actual head loss through a piping component. Published Cv ratings indicated the flow rate needed to impart 1 psi of pressured drop, or 2.31 feed of head, on the fluid.

    Hope this helps...

    JMB
  • Brad White
    Brad White Member Posts: 2,399
    Exponentially Yours

    Hi Mark-

    Because I am ever learning, I have to offer an open issue in my own understanding, the exponent involved in fluid flow.

    My training always had the difference in head varying by the square of the flow (^2), while John Siegenthaler, to whom I would defer in so many cases, states it is an exponent of 1.75 (^1.75).

    I will have to head back to the books to compare the differences which float between the Darcy-Weisbach and the Hazen-Williams formulae. These have exponents of between 1.85 and 2.00.

    Yawn. :)

    I would, if I were you, set up a spreadsheet with these variables.

    The syntax would be (GPM-2 / GPM-1)^1.75 x h

    For example, say your starting GPM was 8 and imposed 6 feet of resistance (head h).

    Your next checkpoint would be 7 GPM (per your first string above). So this would work out as:

    (7/8)^1.75 x 6.0 feet = 4.7496 feet at 7.0 GPM (edited corrected.)

    (6/8)^1.75 x 6.0 feet = 3.6267 feet at 6.0 GPM

    (5/8)^1.75 x 6.0 feet = 2.6360 feet at 5.0 GPM

    (4/8)^1.75 x 6.0 feet = 1.7838 feet at 4.0 GPM

    Between these you should be able to interpolate but also check the tested assumptions of your lesser but known head pressure data.

    Does that help?
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • John Barba
    John Barba Member Posts: 166
    Wow...

    Boy, you engineers make me feel inadequate!

    JMB
  • Brad White
    Brad White Member Posts: 2,399
    At least not in

    the shower, big guy.

    :)
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Mark_46
    Mark_46 Member Posts: 312
    OK

    Brad

    Yes it does help.

    Is there no question you can’t answer or at least help with? By the way - what came first, the chicken or the egg? :o)

    While I know curiosity killed the cat, it also gets the best of me. I am a Technical Writer by trade (currently) and I get in knots when manufactures’ materials are vague. I got to know...and I love to learn! I guess this is all part of what seems to be the boundless world of hydronics!

    Of course I'm the guy in this conversation that doesnt know what he's talking about but with all due respect I'm confused by your first line computation. (7/8)^1.75 x 6.0 feet = 4.7496, not 5.5413. Or am I missing something? The others check fine.

    At any rate (no pun intended) I'd like to gnaw on these numbers for a while. OK if I come back with questions?
  • Joe Billow_6
    Joe Billow_6 Member Posts: 69


    Actually the egg. There where reptiles laying eggs millions of years before the chicken.
  • Brad White
    Brad White Member Posts: 2,399
    I did

    mess up that one when I was changing the post to reflect the entire range you requested. Thanks for checking my work. I make plenty of simple dumb mistakes. Come back any time.

    Oh, as for the chicken and the egg question?:

    The Rooster.

    :>
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • ken wickre_8
    ken wickre_8 Member Posts: 8


  • ken wickre_8
    ken wickre_8 Member Posts: 8


  • ken wickre_8
    ken wickre_8 Member Posts: 8
    Cv Number Crucher Online

    Here is an online Cv calculator that can crunch some numbers for you. Enjoy...

    http://www.butech-valve.com/Cv_calc_appl.htm
  • bob_46
    bob_46 Member Posts: 813
    There is a

    calculator on the back of B&G's Syzer.

    There was an error rendering this rich post.

  • JK_3
    JK_3 Member Posts: 240
    THE ANSWER

    Which came first the chicken or the egg? the answer lies in the question which was smoking first?

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

    There was an error rendering this rich post.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,858
    The ULTIMATE answer...

    WHO cares...:-)

    As for finding Cv's, get ahold of a Bell and Gossett SystemSyzer and you will know the answer to EVERY hydronic question..Grasshopper...(either the flying wheel of guaranteed hydronic death, or the software version)

    I own 3 of them...

    ME

    There was an error rendering this rich post.

  • Don Robertson_2
    Don Robertson_2 Member Posts: 5
    calculating

    Just a tip for those who don't know.. If you don't have a calculator, you can google
    X to the 1.75 power =
    and it will give you the answer.
  • Brad White
    Brad White Member Posts: 2,399
    Thanks, Don

    Good tip- especially when one is promoting helping others to help themselves.

    In my work, I use a Ti-30XA- best calculator for $15 anywhere, IMHO. (My Susan's 11 year-old uses the same for her 6th grade applications; not so sure how I feel about that... :)

    Anyway, the "Yx " key sets this up also. One of those things that one may not often use but sure is handy.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
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