The Velocity Reduction Method can be used when sizing air ducts. The method can be summarized to
- Select suitable velocities for main and branch ducts from the table below
- Find the sizes of main and branch ducts from the air flow rates and the velocities by using eq. 1 and the charts below
- From velocities and duct dimensions - find the frictional pressure loss in the main and branch ducts using the friction chart below
- Add minor dynamic loss
A proper velocity depends on application and environment. The table below indicate commonly used velocities:
|Type of Duct||Comfort Systems||Industrial Systems||High Speed Systems|
|Main ducts||4 - 7||780 - 1380||8 - 12||1575 - 2360||10 - 18||1670 - 3540|
|Main branch ducts||3 - 5||590 - 985||5 - 8||985 - 1575||6 - 12||1180 - 2360|
|Branch ducts||1 - 3||200 - 590||3 - 5||590 - 985||5 - 8||985 - 1575|
Be aware that high velocity close to outlets and inlets may generate unacceptable noise.
Velocities commonly used for different applications:
- upstream medium pressure VAV boxes: 2000 to 2500 fpm (10 - 13 m/s)
- transport of fumes, mist or very light particulates: 2400 fpm (12 m/s)
- dust collection systems with small particulate: 3500 fpm (18 m/s)
- dust collection systems with heavy particulate like metals: 5000 fpm (25 m/s)
Sizes of ducts are then given by the continuity equation like:
A = q / v (1)
A = duct cross sectional area (m2)
q = air flow rate (m3/s)
v= air speed (m/s)
Alternatively in Imperial units
Ai = 144 qi / vi (1b)
A = duct cross sectional area (sq.in.)
q = air flow rate (cfm)
v= air speed (fpm)
Frictional Pressure Loss
Estimate friction loss in main and branch ducts from the charts below:
Air Ducts Sizing Spreadsheet Template
Air Ducts Spreadsheet Template - The Velocity Method
The velocity method can be done manual or more or less semi automatic with the spreadsheet template below.
This template is based on the in the spreadsheet. Customize sections, airflows, duct sizes and minor dynamic loss coefficients - add pressure loss paths and evaluate and reconfigure the system to fit your criteria. Summarize pressure loss for each path and add damper pressure losses manually to balance the system.
The Google Docs spreadsheet template can be opened and copied here! The spreadsheet can also be downloaded as an excel file. Use the Google Docs "File" menu on the top of the template.
Insert air ducts to your Sketchup model with the Engineering ToolBox Sketchup Extension
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Typical air duct velocities in applications like ventilation systems or compressed air systems.
A major friction loss diagram for air ducts - Imperial units ranging 10 - 100 000 cfm.
Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.
Required duct area vs. air flow.
Air flow volume, duct size, velocity and dynamic pressure.
Design procedure for ventilation systems - air flow rates, heat and cooling loads, air shifts according occupants, air supply principles.
The equal friction method for sizing air ducts is easy and straightforward to use.
Duct systems are commonly divided into three pressure classifications.
Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator.
Circular ducts sross-sectional areas.
Recommended distance between ductwork support hangers.
Dynamic pressure is the kinetic energy per unit volume of a fluid in movement.
Rectangular equivalent diameter for air flows between 100 - 50000 cfm.
The Equation of Continuity is a statement of mass conservation.
Commonly used HVAC abbreviations.
Estimate noise generated by air flow in ducts.
A rough guide to maximum air volume capacity of circular ducts in comfort, industrial and high speed ventilation systems.
Standard dimensions of spiral ducts - Imperial units.
Recommended air velocities in ventilation ducts