Air Ducts  Friction Loss Diagram
A major friction loss diagram for air ducts  Imperial units ranging 10  100 000 cfm.
Friction loss (head loss) in standard air ducts:
Download and print Air Ducts  Friction Loss vs. Air Flow and Duct Size Chart
The diagram is based on standard air 0.075 lb/ft^{3} in clean round galvanized metal ducts.
 1 inch water = 248.8 N/m^{2 }(Pa)= 0.0361 lb/in^{2} (psi) = 25.4 kg/m^{2} = 0.0739 in mercury
 1 ft^{3}/min (cfm) = 1.7 m^{3}/h = 0.47 l/s
 1 ft/min = 5.08x10^{3} m/s
 1 inch = 25.4 mm = 2.54 cm = 0.0254 m = 0.08333 ft
Example  Friction Loss in Air Duct
The friction loss in a 20 inches duct with air flow 4000 cfm can be estimated to approximately 0.23 inches water per 100 feet duct as shown in the diagram above. The air velocity can be estimated to approximately 1850 feet per minute.
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