This note show how to determine the rectangular duct size by using Equivalent Diameter (De).
Figure 1 illustrates the procedure for sizing a rectangular duct from a circular duct using the equal friction method. The sizing is based on Table 3 from Chapter 21: Duct Design of the 2017 ASHRAE Handbook—Fundamentals (SI edition).
Figure 2 presents the circular and equivalent rectangular duct sizes obtained using the HVAC duct size calculator v2.6_20250706 software, available from somjinnotes.com.
Related notes:
HVAC duct sizing by Equal Friction Method
HVAC duct size calculator – Try it Now!
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