Ducts Sizing - the Velocity Reduction Method

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:

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 

A_i = 144 q_i / v_i                                    (1b)

where

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.

• open air duct sizing spreadsheet!

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

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm