Fanning factors in isosceles-triangular ducts are examined. Data obtained in the literature were examined for deviations from the smooth circular tube line. It was found that the constant C in a form of the Blasius equation $4f Re0.25=C$ decreases as the apex angle does within the extent of experiments, and has 20 percent low deviation at 4 degree. For the apex angles greater than 60 degree, it was found that the constant C decreases as the apex angle increases. It is thus concluded that the hydraulic diameter is not the proper length dimension to use in the Reynolds number to insure similarity between the circular and triangular ducts. Instead, if an area equivalent round diameter is used in the Reynolds number, the deviations from the smooth circular tube line is within ∼6 percent. By using this area equivalent round diameter, it is demonstrated that circular tube methods may be readily applied to triangular ducts eliminating large errors in estimation of friction factors. [S0098-2202(00)00503-4]

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