A new duct-sizing calculator from ASHRAE and the Air Distribution Institute (ADI) allows HVAC air-distribution-system designers to size ducts more accurately, especially flex ducts under varying amounts of compression, based on research results.

The Duct Size Calculator is a quick reference tool for approximating duct size and equivalent sizes of sheet-metal duct and flexible duct. The calculator uses information from ASHRAE Research Project 1333, HVAC Duct Efficiency Measures, and was developed with funding support from ASHRAE and ADI. ASHRAE Technical Committee 5.2, Duct Design, sponsored the 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,” Chris Van Rite, developer of the calculator, said. “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 noted 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 2013 ASHRAE Handbook—Fundamentals, so the equations were used to create the new calculator, Van Rite said.

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

The cost of the calculator is $34 for ASHRAE members and $40 for non-members. To order, visit www.ashrae.org/bookstore, or contact ASHRAE Customer Contact Center by phone at 1-800-527-4723 (United States and Canada) or 404-636-8400 or by fax at 678-539-2129.