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This is the most common approach for commercial supply and return systems. You select a constant friction loss (usually 0.1 inches of water column per 100 feet) for the entire system. Easy to calculate; ensures consistent pressure drop. duct sizer mcquay
The McQuay Duct Sizer relies on the Darcy-Weisbach equation adapted for air: [ h_f = f \cdot \fracLD \cdot \fracV^22g ] Where: He typed the numbers into the input fields