Wednesday, June 9, 2010

Pressure Drop Calculation

From: Revit MEP

I got an call from a client last week asking about some of the calculations that Revit MEP does.

I am trying to get results for Duct Pressure drop in Revit MEP.

Can you please share your ideas, as how can I achieve this?

This post has some additional information but is based on the Revit MEP calculation white paper from Autodesk. Revit MEP Duct Sizing calculations

Revit MEP computes pressure losses in ductwork based on the geometry and roughness of the ductwork, air

density, and air viscosity. Values for Air Density and Air Viscosity are specified in the Mechanical Settings.

Roughness is specified in the type properties for duct/duct fitting component families.

The following example shows how Revit MEP calculates the pressure drop for a 100 foot segment of 36"x24" duct carrying air flow of 12,000 CFM. Pressure drop is defined as:

This values checks with the Hydraulic Diameter parameter shown in the Properties of the Duct in Revit:

The velocity is based on the cross sectional area:

After determining the friction factor, the pressure drop can be calculated:

The value for the calculated pressure drop matches the value found in the duct’s properties in Revit MEP.

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