Equal Friction Method

Reference data and engineering information about equal friction method for hvac systems applications.

equalfrictionmethod

Overview

Engineering reference data for Equal Friction Method in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

Recommended Air Velocity Ranges

The selection of maximum duct velocity is a critical initial step in the equal friction method, primarily to control noise levels in the finished space.

3 rows

Common airflow velocity ranges for duct design.
System Type	Minimum Velocity(m/s)	Maximum Velocity(m/s)	Minimum Velocity(ft/s)	Maximum Velocity(ft/s)
Comfort Systems	4	7	13	23
Industrial Systems	8	12	26	40
High-Speed Systems	10	18	33	60

Source: Engineering ToolBox (adapted)

Typical Friction Loss Values

A common starting point for the constant friction rate used in the method is:

Supply Ducts: $\Delta p / L = 0.1 \text{ in H}_2\text{O} / 100 \text{ ft}$ or $0.85 \text{ Pa/m}$
Return Ducts: $\Delta p / L = 0.08 \text{ in H}_2\text{O} / 100 \text{ ft}$ or $0.65 \text{ Pa/m}$

These values are used as the constant pressure drop per unit length ( $\Delta p / L$ ) in Step 4 and 5 to size all duct branches, ensuring the friction loss is "equal" throughout the system.