Darcy - Who?

Darcy-Weisbach. It's a phrase you are probably hearing more and more of. What is it? It is a formula for calculating friction loss just like Hazen-Williams. But whereas Hazen-Williams is used for water in sprinkler pipe, Darcy-Weisbach can be used for any liquid (including water, but NFPA 13 does not permit its use).

Its recent popularity is due to new fluids that have been produced for use in antifreeze systems. In order to calculate these systems you will need to know some things about the fluid being used, specifically the Density and the Viscosity. The Density is in the units of lbs per cubic foot (kg per cubic meter) and the Viscosity is the units of centipoise. These need to be entered in the appropriate spots in the caclulation dialog box.

The C-Factor Roughness Equivalents also need to be entered. Darcy-Weisbach does not use the usual 100, 120, etc. Instead there is a whole new set of numbers, some numbers can be found in NFPA 750 Paragraph 6-2-2. There you will see copper at 0.000005 and Stainless Steel at 0.00015. You will need to contact the manufacturer of the pipe you are using and ask them what numbers to use. For our program, we ask that you assign 100, 120 and 140 like normal and then when you hit the Calc button, you type in the equivalent numbers that these relate to. Internally these equivalents will be used in place of the 100, 120, etc.

For more information on Darcy-Weisbach vs. Hazen-Williams see a great comparison article from Scott Martorano of The Viking Group in PDF form at: Darcy-Weisbach Article

I will be in Virginia most of this week for the biannual Virginia Chapter of the AFSA trade show. Talk to you later this week.