VC-21-008 -- Sizing Ducts Based on a Prescribed Friction Rate

This paper presents an algorithm that can be used to sizea round duct for a given friction rate. This requires an iterativecalculation that uses the secant method. It was shown that thealgorithm yields predicted diameters equal to those obtainedfrom ASHRAE Duct Fitting Database (ASHRAE 2016) forfitting CD11-4. The algorithm is straightforward to program,making it well-suited for implementation in a standard spreadsheetapplication. In addition, a correlation in both I-P and SIunits was developed between the duct diameter, prescribedfriction rate, and volume flow rate, using the least squarescurve-fit method. The correlation is applicable to airflowsoccurring at standard conditions in ducts exhibiting an “average”absolute roughness. The correlation is suggested as analternative to friction charts or ductulators, as those approximatemethods can be difficult to read or interpret correctly.Furthermore, the proposed correlation can readily be implementedin numerical duct design models.