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Needed Wet Head Engineer

bob_44 Member Posts: 112
Hot Rod but the answer isn't in that article. bob


  • bob_44
    bob_44 Member Posts: 112
    Please explain

    howcome almost everything I have read about hydronics says that the head varies as the square of the flow, except Siggy's book says it varies as the flow raised to the 1.75 power? What am I missing? bob
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 15,259
    Have you asked

    Retired and loving it.
  • Ron Schroeder
    Ron Schroeder Member Posts: 998

    Because the Darcy friction factor is a function of fluid velocity (f~v^(-0.2))
  • bob_44
    bob_44 Member Posts: 112
    Dan & Greg

    Dan, I wish I knew Siggy. Greg thanks for the reply I followed the link and read the page and you might as well have hit me up side the head with a 24. I drug out Crane 410 referenced on the web page read about Bernoulli and Darcy and looked at a lot of equations and everything is squared. With all due respect Greg I asked for an explaination by a"Wet Head Engineer" not a M.E. or P.E. you guys have a different vocabulary, math.

    If you read the system sizer instruction book or just about anything else by B&G or Crane or ASHRAE everything is squared. Can anybody explain? bob
  • hr
    hr Member Posts: 6,106
    This may help explain,2379,137319,00.html

    If not, drop Siggy an e-mail at I'm sure he would help you with an explanation in contractor-speak.

    hot rod

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  • tls_9
    tls_9 Member Posts: 89

    that DVW guy go.... Dave Van Wicker, or something like that?

    he used to explain things in a way even I could understand.

    I bet he could field this one.

  • Jerry Boulanger_2
    Jerry Boulanger_2 Member Posts: 111
    The assumption

    that the head loss varies as the square of the flow is based on the Darcy-Weisbach formula. This formula squares the average velocity, but also includes an empirically determined friction factor that takes other variables into consideration.

    John's formula using flow to the power of 1.75 is for use on smooth tubing only (copper and PEX), as stated on page 150 of his textbook. On page 154 of his text, he refers to flow coefficeints (Cv) and how Cv can be used to estimate pressure drop based on the square of the flow. He also says that using the square relationship will slightly overpredict the the pressure drop for smooth tubing.
  • bob_44
    bob_44 Member Posts: 112

    thanks, I'm still a little confused. Why wouldn't he change r which is the hydraulic resistance rather than the flow coefficient? I found a referance in Strock's Handbook page 4-152 where they list four different exponents depending on wall roughness from 1.75 to 2. I can understand why he is building a new pipe sizing chart and how he is doing it but when he constructs a system curve he uses the same exponent. That's where I get confused. That means that the system curves drawn useing a system syzer are too steep. The same with the nomographs in Crane 410. Based on the info in Siggy's book most of the systems I see posted on the Wall appear to be over piped and over pumped. bob
  • Jerry Boulanger_2
    Jerry Boulanger_2 Member Posts: 111
    The system curve

    is a compilation of all of the components in a system. The exponent for the smooth tubing may be 1.75 and for some other components like control valves it may be 2. The System Sizer and Crane TP410 use the square, as do most designers - it affords a slight margin of safety. Some designers get carried away with 'safety factors' - John suggests adding up to 15% to the calculated pump head (page 208 in his textbook).

    I use some fairly sophisticated software for analyzing piping systems and just for the fun of it I modelled a 100' length of 3/4" type L copper. At 3 USgpm of 160 F water the head loss was 0.939 psi. At 6 gpm, it was 3.227 psid. Using John's exponent of 1.75, it would be 3.158 psid - pretty close. I then added a valve to the line with a Cv of 3 (3 USgpm at 1 psid, water at 60 F)- at 3 gpm the head loss is 1.921 psid and at 6 gpm it is 7.155 psid, an exponet of about 1.9. Using John's exponent of 1.75, it would be 6.461 psid, appreciably lower. Using the square would give you 7.684 psid, high by about as much as John's is low. The type and amount of pipe and components in a system would determine the absolutely correct exponent to use.

    Most pumps are over-sized for head, for a lot of different reasons. I personally believe that it is a major source of wasted energy in many installations. We evaluated a water source heat pump system where the pump runs 8,760 hours per year and was conservatively estimated to be 30% over-sized for head. Our estimate of the potential electrical savings by trimming the pump was about 80,000 kwh per year.

  • hr
    hr Member Posts: 6,106
    I tried to model

    it on Siggys HDS, but it only accepts copper, pex, and PAP. And yes, there is a small difference even in these various "smooth" tubes.

    Seems more and more engineers are looking to squeeze hydraulics as a cost of operation savings. This months issue of PM talks about this topic.,2379,153104,00.html

    hot rod

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  • bob_44
    bob_44 Member Posts: 112
    Jerry& Hot Rod

    tanks to da boat ah ya. A guy could get lost for weeks in those PM magazine archives. I don't have any hydronic software so I'm going to draw up some system curves with my syzer and Siggy's method and see how much difference I come up with. It seems like every time you think you really know something somebody moves your cheese. bob
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Moving Cheese!

    "Who Moved My Cheese?" is a great book and should be required reading for all!
  • Bigugh!
    Bigugh! Member Posts: 2
    Cheeze again!

    After all this (above) I live at an altitude of 4700 feet (asl). now with that infromation there is less gravity, and the water is thinner. So what now? Do I have to redo all my calculations to get my pump exactly on! Less gravity, the pipes are larger? The water is thinner so should my pump be biggger? Seems to me to be picking apart the velocity of water in a system is looking for the grass to be greener all along the the other side of the fence. KISSMIF here! Keep it simple stupid make it functional! Oh a big job undoubtedly needs to tighten up the pumps and piping, but the Home is not worth the effort.
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