Testing data for Spiral duct and flat-oval duct

If you make round and flat-oval duct, these two ASHRAE guides have the testing data you need.

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.

If you were a “serious” duct manufacturer back then, you paid for the very expensive testing of your product. To have any validity, your test for duct fittings has to be similar to someone else’s test, and to separate yourself from the hordes of new companies in the duct fabrication business, you “published” performance data.

Most duct manufacturers in business since the ’70s and ’80s (like my friend and me) may still have these old catalogs somewhere collecting dust with friction charts for spiral duct and a few loss coefficient graphs for standard duct fittings.

Where can duct manufacturers and contractors reliably find that data now? In the ASHRAE Duct Fitting Database (current version 6.00.05) and the BSR/ASHRAE Standard 120 (2016).

About 30 years ago, our industry had one of those rare moments of clarity where we changed paths in a way that benefited all of us. Computers began handling real duct design, running calculations instead of approximating with a slide wheel, and third-party companies developed the programs. Not duct manufactures.

Meantime, we all started buying plasma tables that came pre-loaded with patterns for common fittings. As a result, we no longer made our own hand patterns for duct fittings, and we all pretty much make duct the same way for dynamic performance (pressure drops and loss coefficients).

Instead of spending large amounts of money to build catalogs, our industry is best served by a common library of duct performance data that can be expanded as needed.

ASHRAE has a number of Technical Committees that produce data and guidelines for the common good of the industry. I am a member of ASHRAE TC5.2 “Duct Design,” and our scope is that we are concerned with the design, characteristics and construction of all types of ductwork for the handling of air and other gases.

We produce the ASHRAE Handbook chapters on duct design and duct construction. We also promote and monitor ASHRAE research relating to ductwork and develop standards and tools for the industry.

The BSR/ASHRAE Standard 120 (2016) “Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings,” first published in 1999, established uniform methods of laboratory testing of HVAC ducts and fittings to determine their resistance to airflow. This assures that all ducts are tested the same way so we have uniform performance data.

The ASHRAE Duct Fitting Database (current version 6.00.05), initially offered in 1994, is an online database with flow resistance data for over 200 spiral duct, rectangular and flat oval fittings. Rather than a collection of graphs, it is a calculation tool for determining precise pressure drops given sizes and flow conditions. It also includes industry research going back 80 years. When a particular fitting is on screen, the database references the published research that provided the data. Plug in size and flow conditions, and it will determine pressure drop.

Both BSR/ASHRAE Standard 120 and the **ASHRAE Duct Fitting Database are available for purchase in the ASHRAE Bookstore. I encourage anyone that fabricates ductwork to own a copy of both of them.

There is no need to spend thousands of dollars on researching how your ducts perform when that information is already published and available for use. As an industry, we can now start to focus on more important matters involving ducts, such as making them leak less and designing duct systems that are less expensive to install and operate.

**Although the ASHRAE Duct Fitting Database has performance on a wide range of duct fittings, the resource is intended to be an ongoing project. Any ducts or fittings not in the database can be added by contacting Bob Reid at bobtheductman@gmail.com. This story originally appeared in the February 2020 issue of SNIPS magazine.

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Bob Reid has 40 years of experience in ductwork fabrication, HVAC sales and design. He's a SNIPS Sheet Metal Icon and he works for Spiral Pipe of Texas in Business Development & Engineering.