“While the calculator resembles a wheel similar to those used during the days of slide rules, it incorporates three new fields for equivalent duct sizing,” said Chris Van Rite, developer of the calculator. “These new fields help demonstrate the significant loss of airflow due to improper installation of flexible ducts.”

The calculator includes fields for 4, 15, and 30 percent compression in flexible ducts. Van Rite notes that the calculations used to create these size references are based on straight line compression as performed in the laboratory on a flat surface. Field installed flexible ducts with bends, kinks and excessive lengths will have additional resistance, which will result in diminished airflow. 

“The use of this tool allows duct designers to account for less than optimum installation and gives a more accurate design to installed performance correlation,” Van Rite said. 

ASHRAE research has quantified the effects of compression (not stretching) flexible duct, which increases the roughness and, therefore, the friction loss inside a flexible duct. Airflow testing follows protocols prescribed by ANSI/ASHRAE Standard 120-2008 — Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings. 

Testing at Lawrence Berkeley National Laboratory and at Texas A&M University along with data analysis by Tennessee Technical University has quantified the adverse effects of compression on airflow. Those correlations agreed closely with the equations published in Chapter 21 of the 2013 ASHRAE Handbook, Fundamentals, so the equations were used to create the new calculator, according to Van Riteaid.

The Duct Size Calculator features inch-pound (I-P) units on one side and the International System of Units (SI) on the other.

For more information, visit www.ashrae.org/news.

To order, visit www.ashrae.org/bookstore. 

Publication date: 1/9/2017

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