Darcy-Weisbach Calculations

Darcy-Weisbach is a formula for calculating friction loss like the Hazen-Williams formula is used for water-based fire sprinkler systems. Whereas Hazen-Williams is used for water in sprinkler pipe, Darcy-Weisbach can be used for other liquid based calculations (*note* NFPA 13 does not permit its use for standard sprinkler systems).

Its use is required in antifreeze systems over 40 gallons of capacity (but not in the non-anti-freeze portions of the system) and in water-mist systems that operate at more than 175 psi. These are, of course, subject to the approving authority.

In order to calculate these systems, you will need to know specific details about the fluid being used, specifically the Density and Viscosity. The Density value uses the units pounds per cubic foot (lb/ft³) or kilogram per cubic meter (kg/m³). The Viscosity uses centipoise and will be found in the datasheet of the liquid. These need to be entered in the appropriate spots in the calculation dialog box.

In most cases, two calculations are required. The first for the portion of the system containing antifreeze only and the second portion containing plain water. This is because Darcy-Weisbach cannot be used for water systems and Hazen-Williams cannot be used for antifreeze systems.

A two-part example job is available in the folder \HES\HydraCALC\Ver50\Data\Jobs\Example Jobs. It is named Darcy-Weisbach Antifreeze Loop Example - Part One * and * Part Two. For more information on Darcy-Weisbach vs. Hazen-Williams see a comparison article from Scott Martorano of The Viking Group in PDF format at: Darcy-Weisbach Article.