ASHRAE, ADI Release New Duct Sizing Calculator

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The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Air Distribution Institute (ADI) have issued a new duct sizing calculator as a guide for HVAC air distribution system designers and manufacturers. The new duct sizing calculator aims to provide more precise size ducts, specifically flex ducts under changing amounts of compression, based on research results.

According to an article by ACHR News, the new duct sizing calculator, called the Duct Size Calculator, is a reliable reference tool for measuring duct sizes and similar sizes of sheet metal duct versus flexible duct. Details from the ASHRAE Research Project 1333: HVAC Duct Efficiency Measures are used by the Duct Size Calculator. It was created with funding support from ASHRAE and ADI and was also sponsored by the ASHRAE Technical Committee 5.2 Duct Design.

Chris Van Rite, the developer of the Duct Size Calculator, said, “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. These new fields help demonstrate the significant loss of airflow due to improper installation of flexible ducts.”

The calculator features fields for varying percentage compressions in flexible ducts. According to Van Rite, the calculations utilized to create the size references are based on straight line compression as done in the laboratory on a flat surface. Additional resistance to field-installed flexible ducts with bends, kinks, and excessive lengths will lead to diminished airflow. It also has an inch-pound unit on one side and the International System of Units (SI) on the other.

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

Airflow testing complies to decorum prescribed by ANSI/ASHRAE Standard 120-2008: Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings.

The testing for the Duct Size Calculator at Lawrence Berkeley National Laboratory and Texas A&M University has determined the detrimental effects of compression on airflow. Van Rite said that the results matched closely with the equations stated in Chapter 21 of the 2013 ASHRAE Handbook Fundamentals, so the said equations were used to develop the new calculator.

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