Flexible versus metal duct sizer

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A new duct sizing calculator from ASHRAE and the Air Distribution Institute (ADI) is designed to allow HVAC air distribution system designers to more accurately size ducts, especially flex ducts under varying amounts of compression.

The Duct Size Calculator is a quick reference tool for approximating duct sizes and equivalent sizes of sheet metal duct versus flexible duct. It was developed with information from the ASHRAE HVAC Duct Efficiency Measures research project.

“While the calculator resembles a wheel type ductulator 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 four, 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, resulting 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,” said Van Rite. 

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, he said.

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

Visit www.ashrae.org/bookstore to order the calculator.

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