Abstract
An experimental program was conducted to measure the relative and absolute roughness of phenolic duct systems connected with a four-bolt flange and cleat joint. Ducts with seven distinct rectangular cross sections (internal aspect ratios ) were investigated. In every instance, the duct material thickness was 7/8 in. (22 mm), with smooth aluminum foil covering the internal surface between flanges. For unreinforced phenolic ducts with 10 ft (3.0 m) sections connected by four-bolt flanges, the relative roughness ε/Dh ranged from 0.0002 to 0.0005, with an average value of 0.0003. For similar ducts having 5 ft (1.5 m) duct sections (double the number of flanges per test section), the relative roughness range and average doubled. It appears that the flanges are the dominant effect on relative roughness and that the ducts tested have about 1.5 to 2.8 times more absolute roughness than the average value for galvanized steel ducts described in the ASHRAE Handbook.
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