Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Ducts Sizing - Velocity Reduction Method

The velocity reduction method can be used for sizing air ducts

Sponsored Links

The Velocity Reduction Method can be used when sizing air ducts. The method can be summarized to

  1. Select suitable velocities for main and branch ducts from the table below
  2. Find the sizes of main and branch ducts from the air flow rates and the velocities by using eq. 1 and the charts below
  3. From velocities and duct dimensions - find the frictional pressure loss in the main and branch ducts using the friction chart below
  4. Add minor dynamic loss

Proper Velocities

A proper velocity depends on application and environment. The table below indicate commonly used velocities:

Type of DuctComfort SystemsIndustrial SystemsHigh Speed Systems
m/sftmm/sftmm/sftm
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)

Sizing Ducts

Sizes of ducts are then given by the continuity equation like:

A = q / v                                      (1)

where

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)

where

A = duct cross sectional area (sq.in.)

q = air flow rate (cfm)

v= air speed (fpm)

air duct velocity diagram

Frictional Pressure Loss

Estimate friction loss in main and branch ducts from the charts below:

Air Ducts Friction loss - Imperial units

Air Ducts - friction loss - SI units

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.

Air Duct Sizing

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

Engineering ToolBox Sketchup Extension - Insert ducts

Sponsored Links

Related Topics

Related Documents

Tag Search

  • en: duct sizing
  • es: dimensionamiento de conductos
  • de: Kanal Dimensionierung
Sponsored Links

Search the Engineering ToolBox

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro .Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse!

Translate this page to
About the Engineering ToolBox!

Privacy

We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.

Citation

This page can be cited as

  • Engineering ToolBox, (2003). Ducts Sizing - Velocity Reduction Method. [online] Available at: https://www.engineeringtoolbox.com/sizing-ducts-d_207.html [Accessed Day Mo. Year].

Modify access date.

. .

close

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Scientific Online Calculator

Scientific Calculator

11 15

Sponsored Links
.