Determining the Absolute Roughness of Phenolic Duct
From eSociety, April 2019
The results of an experimental program during an ASHRAE research project are available in Science and Technology for the Built Environment.
During the project, Stephen Idem, Ph.D., Member ASHRAE, and Avinash Paruchuri performed an experiment that measured the relative and absolute roughness of phenolic duct systems connected with a four-bolt flange and cleat joint. “Determine the Absolute Roughness of Phenolic Duct (RP-1764)” explores their experiment.
Idem, a member of ASHRAE’s Conferences and Expositions Committee and the program subcommittee chair for TC 1.2, Instruments & Measurements, explains the research’s significance and lessons.
1. What is the significance of this research?
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.
2. Why is it important to explore this topic now?
Phenolic ducts are a new air distribution product that is entering the North American market from Europe and China. SMACNA recently published the first edition of their “Phenolic Duct Construction Standards” (2015). Those standards describe structural construction requirements, but they do not include data to design phenolic duct systems.
Technical data such as the effects of surface roughness and flange connections on pressure drops are necessary to properly design duct systems.
3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?
For unreinforced phenolic ducts with 10 ft (3.0 m) sections connected by four-bolt flanges, the relative roughness e/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.
4. How can this research further the industry's knowledge on this topic?
It is recommended that roughness data resulting from this study should be included in the ASHRAE Duct Fitting Database (2017), and in future editions of the Duct Design chapter in the ASHRAE Handbook—Fundamentals.
However, many phenolic duct systems feature other section joining methods, and may utilize longer section lengths. There is a need for more research to further our understanding of how duct construction characteristics affect duct system pressure losses.
5. Were there any surprises or unforeseen challenges for you when preparing this research?
The measured absolute roughness of the phenolic duct system considered in this study was about 1.5 to 2.8 times greater than the “average” absolute roughness (0.0003 ft, 0.09 mm) described for galvanized steel ducts in Table 1 of the “Duct Design” chapter in the 2017 ASHRAE Handbook—Fundamentals.
However, that value is based on a previous investigation that reported absolute roughness values of 0.00027 to 0.0005 for “galvanized steel rectangular ducts with various types of joints, with 4 ft spacing.”
However, they did not test the effects of galvanized ducts with actual joints. Instead, the ducts were constructed from plywood, and wood strips placed inside the ducts were used to simulate joints.
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