Duct sizing with (Equal Friction)

Equal Friction is a duct-sizing method that keeps the pressure loss

from friction per unit duct length approximately constant throughout the system. For example,

Friction Rate = 0.8 Pa/m or approximately 0.1 in.wg / 100 ft

As airflow decreases at each branch, duct sizes gradually decrease

to keep the Friction Rate close to the specified value.Why do engineers use this method?

1. Simple, fast calculations
With the CFM and Friction Rate known, a Ductulator or software can determine the duct size.
2. Suitable for many general applicationsFor example, offices, hotels, shopping centres, shops, and AHU/FCU systems with relatively simple duct routes.
3. Easy to size each duct section For example, as air branches from a 4,000 CFM system:
4,000 → 3,500 → 3,000 → 2,500 CFM The Main Duct size decreases with the airflow.
4. Helps control air velocity Reducing duct size as Flow decreases helps avoid unnecessarily large ducts.

5. Convenient estimation of fan ESPThe critical duct route, or Critical Path, can be used to sum the Pressure Loss

of ducts, bends, Dampers, Filters, Coils and other components when selecting the fan ESP.


A simple example
Assume an AHU supplies 4,000 CFM and Equal Friction is set to 0.8 Pa/m.
Initial duct section: 4,000 CFM → larger duct
After supplying 500 CFM → 3,500 CFM remains → reduce the duct size
After supplying another 500 CFM → 3,000 CFM remains → reduce the size again

The principle is:

Airflow decreases → Duct Size decreases → Friction Rate remains approximately constantHowever, Equal Friction

does not mean that Static Pressure is equal at every outlet. A real system still requires

Volume Dampers and TAB (Testing, Adjusting & Balancing) to achieve the design airflow.

For very large or long duct systems, such as airports, factories or large buildings,

Static Regain may be considered to improve pressure distribution compared with Equal Friction.

Equal Friction duct-sizing concept

Diagram showing airflow, main-duct sizing, duct-size reduction after branching, and approximately constant Friction Rate.
Diagram summarising Equal Friction and the example Friction Rate stated in the article.
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