Air Duct Calculator
by
Michael J. Rocchetti PE
Mechanical Engineer

This program is designed for the use of Mechanical Engineers who design air distribution or exhaust systems for Heating Ventilating and Air Conditioning systems.

The calculator, in either english or metric, allows users to input a desired flow rate, a limiting velocity, air pressure, air temperature, the length of the duct, and the duct material. The program then calculates the duct cross sectional area, the circular duct diameter, rectangular duct equivalents (with aspect ratios of 1.0 through 4.0), air density, fluid viscosity, Reynold's number, relative roughness, the Moody friction factor, the Colebrook friction factor, the pressure loss, and the duct friction loss per unit of duct length.

The program uses the Moody equation, the Colebrook equation, the Darcy-Weisbach equation, the perfect gas laws, and the Huebscher equation.

Slide-rule type duct calculators designed for standard air and galvanized steel ductwork do not allow users to correct for non-standard air conditions, or for other types of duct material. The Air Duct Calculator allows users to input any type of duct material, and can input any air temperature and pressure. The program will then calculate air density and air viscosity for precise pressure loss calculations.

References:

  • Fluid Mechanics and its applications by James W Murdock, PE
    Houghton Mifflin Co.
  • Heating Ventilating and Air Conditioning Analysis and Design
    by Faye C. McQuiston and Jerald D. Parker
    John Wiley & Sons.
  • Flow of Fluids Through Valves, Fittings, and Pipe
    Technical Paper No. 410
    The Crane Company
  • ASHRAE Fundamentals 1993
    The American Society of Heating Refrigerating and Air Conditioning Engineers

Inch-Pound Units

Flow rate CFM:

Velocity FPM:

Pressure PSI:

Temperature F:

Length FT:

Duct Material:
Uncoated Carbon Steel (smooth)
PVC Plastic Pipe (smooth)
Aluminum (smooth)
galvanized steel (medium smooth)
galvanized steel (average smooth)
fiberglass duct (medium rough)
fiberglass duct (rough)
flexible metallic duct (rough)
flexible duct fabric and wire (rough)
concrete (rough)

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Metric Units

Flow rate M^3/S:

Velocity M/S:

Pressure Pascals:

Temperature C:

Length Meter:

Duct Material:
Uncoated Carbon Steel (smooth)
PVC Plastic Pipe (smooth)
Aluminum (smooth)
galvanized steel (medium smooth)
galvanized steel (average smooth)
fiberglass duct (medium rough)
fiberglass duct (rough)
flexible metallic duct (rough)
flexible duct fabric and wire (rough)
concrete (rough)

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