Equivalent Round Diameter (ERD) is a calculated measurement that converts rectangular or irregular ductwork into a circular diameter value for pressure drop calculations. This conversion allows engineers to use round duct formulas and tables for non-circular duct systems. ERD enables consistent analysis across different duct shapes.
Calculation Method
ERD is calculated using the formula: ERD = 1.3 × [(A × B)/(A + B)]^0.625, where A and B are the rectangular duct dimensions in inches. For example, a 12 × 8 inch rectangular duct has an ERD of approximately 10.2 inches. This formula accounts for the friction characteristics of various duct geometries.
Practical Applications
HVAC designers use ERD when analyzing airflow resistance in non-standard ductwork configurations. It simplifies pressure drop calculations for residential and commercial systems. ERD helps ensure accurate sizing of fans and blowers to maintain required airflow rates of 400-450 CFM per ton of cooling capacity.