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

• Member Posts: 112

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
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Siggy?
Retired and loving it.
• Member Posts: 998

Because the Darcy friction factor is a function of fluid velocity (f~v^(-0.2))

http://www.coolit.co.za/pipeflow/tech01.htm
• 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
• Member Posts: 6,106
This may help explain

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,137319,00.html

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

hot rod

• Member Posts: 89
Where'd

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.

tom
• 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.
• Member Posts: 112
Jerry

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
• 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.

• 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.

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,153104,00.html

hot rod