I recently received a somewhat anxious call from a friend in the sheet metal industry. He won a nice duct order, but his submittals were being rejected because he needed to provide published performance data for his duct fittings. Now, one advantage of getting older is that you become more and more like those guys in the popular insurance commercials — "We know a thing or two because we have seen a thing or two." There was a time, 40 or 50 years ago, when only a small group of companies manufactured spiral and flat-oval ducts. Each company developed its own duct fittings catalog, and standard dimensions varied between them. Those duct systems operated at higher pressures and velocities than we use now so small differences in pressure drops could be a big deal.
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