The following example shows how Revit calculates the pressure drop for a 100 foot segment of 36” x 24" rectangular duct carrying air flow of 12,000 CFM.
- Air Density = 0.0751 lbs/cu ft.
- Air Dynamic Viscosity = 0.01805 cP (standard atmospheric air at 66 degrees F)
- Roughness = 0.0003' (medium smooth galvanized steel duct as defined by 2005 ASHRAE Handbook: Fundamentals, page 35.7)
Pressure drop is defined as:
This value matches the value for the Hydraulic Diameter parameter in the properties of the duct.
The velocity is based on the cross sectional area, where:
The value for the calculated pressure drop matches the value found in the duct’s properties in Revit.