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# Ducts Sizing - the Velocity Reduction Method

## The velocity reduction method can be used when sizing air ducts.

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

### Proper Velocities

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

Ducts - Proper 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)

### Frictional Pressure Loss

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

### 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.

## Related Topics

• ### Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

## Related Documents

• ### Air Duct Velocities

Typical air duct velocities in applications like ventilation systems or compressed air systems.
• ### Air Ducts - Friction Loss Diagram

A major friction loss diagram for air ducts - Imperial units ranging 10 - 100 000 cfm.
• ### Air Ducts - Major Friction Head Loss, Online Calculator

Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.
• ### Air Ducts - Sizing

Required duct area vs. air flow.
• ### Air Ducts - Velocity Diagram

Air flow volume, duct size, velocity and dynamic pressure.
• ### Design of Ventilation Systems

Design procedure for ventilation systems - air flow rates, heat and cooling loads, air shifts according occupants, air supply principles.
• ### Duct Sizing - the Equal Friction Method

The equal friction method for sizing air ducts is easy and straightforward to use.
• ### Duct Systems - Pressure Classifications

Duct systems are commonly divided into three pressure classifications.
• ### Duct Velocity

Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator.
• ### Ducts - Cross-Sectional Area vs. Diameter

Circular ducts sross-sectional areas.
• ### Ductwork - Supports

Recommended distance between ductwork support hangers.
• ### Dynamic Pressure

Dynamic pressure is the kinetic energy per unit volume of a fluid in movement.
• ### Equivalent Diameter - Rectangular vs. Circular HVAC Ducts

Rectangular equivalent diameter for air flows between 100 - 50000 cfm.
• ### Fluid Flow - Equation of Continuity

The Equation of Continuity is a statement of mass conservation.
• ### HVAC Abbreviations

Commonly used HVAC abbreviations.
• ### Noise generated in Air Ducts

Estimate noise generated by air flow in ducts.
• ### Sizing Circular Ducts

A rough guide to maximum air volume capacity of circular ducts in comfort, industrial and high speed ventilation systems.
• ### Spiral Ducts - Dimensions

Standard dimensions of spiral ducts - Imperial units.
• ### Velocity Classification of Ventilation Ducts

Recommended air velocities in ventilation ducts

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## Citation

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

Modify the access date according your visit.

7.25.9

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