Sizing Ducts

Overview

Engineering reference data for Sizing Ducts 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

Velocity Reduction Method

The Velocity Reduction Method is a common approach for sizing air ducts. The process involves:

1. Select suitable velocities for main and branch ducts from the table above.
2. Calculate duct sizes from the air flow rates and selected velocities using the continuity equation.
3. Determine the frictional pressure loss in the ducts from velocity and duct dimensions (using a friction chart).
4. Add minor dynamic losses for fittings and transitions.

Application-Specific Velocities

For specialized systems, appropriate velocities are:

• Medium pressure VAV boxes (upstream): 10 - 13 m/s (2000 - 2500 fpm)
• Transport of fumes, mist, or light particulates: 12 m/s (2400 fpm)
• Dust collection (small particulates): 18 m/s (3500 fpm)
• Dust collection (heavy particulates like metals): 25 m/s (5000 fpm)

Note: High velocities near inlets and outlets can generate unacceptable noise.

Interactive Charts

References

• Original Source