The equal friction method is a standardized ductwork design technique that maintains consistent friction loss per unit length throughout the entire duct system. This approach simplifies calculations and ensures balanced air distribution with minimal damper adjustment. Engineers select duct sizes so that all sections maintain approximately 0.10 inches of water column friction loss per 100 feet of equivalent length.
Application Process
Designers begin with main ductwork friction loss targets, typically 0.10 inches per 100 feet, then apply the same friction loss rate to all branch ducts and transition fittings. Friction loss charts or software quickly identify appropriate duct diameters for each section based on required CFM and desired friction rate. This method works well for systems with relatively balanced load distribution and simple layouts. Equal friction design typically requires less field balancing than alternative methods.
Advantages and Limitations
The equal friction method provides reasonable results with faster calculations compared to velocity reduction methods. However, this approach may not optimize systems with significantly unbalanced zones or complex layouts. Initial cost savings from simplified design must be weighed against potential balancing requirements. Most residential and light commercial systems use equal friction design due to its simplicity and proven performance record.