Friction loss (head loss) in standard air ducts are indicated in the diagram below:
The diagram is based on standard air 0.075 lb/ft3 in clean round galvanized metal ducts.
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 below. The air velocity can be estimated to approximately 1850 feet per minute.
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