Duct Design Fundamentals for HVAC Engineers

Duct system design balances air delivery requirements, pressure drop, noise control, and cost. Proper sizing ensures adequate airflow to all zones while minimizing fan energy and construction costs.

Duct Design Methods

Equal Friction Method

Most common method: maintains constant pressure drop per unit length throughout system.

Procedure:

Select friction rate: typically 0.08-0.15 “w.g./100 ft
Size main duct for total CFM at selected friction rate
Size branch ducts at same friction rate for their CFM
Calculate total pressure drop = friction rate × equivalent length

Advantages: Simple, widely used, duct sizing charts available Disadvantages: Doesn’t optimize for energy; all paths have different static pressures

Static Regain Method

Sizes ducts to maintain constant static pressure at each branch takeoff.

Principle: Velocity reduction in downstream duct converts velocity pressure back to static pressure, offsetting friction losses.

$$P_{s,downstream} = P_{s,upstream} - \Delta P_{friction} + \Delta P_{regain}$$

Velocity regain:

$$\Delta P_{regain} = \frac{\rho}{2}(v_1^2 - v_2^2)$$

Advantages: Balanced system, eliminates balancing dampers, saves energy Disadvantages: Complex calculations, larger duct sizes, higher first cost

Velocity Method

Sizes ducts based on maximum allowable velocities.

Typical velocity limits:

Main ducts: 1,200-2,000 FPM
Branch ducts: 800-1,200 FPM
Final runouts: 600-900 FPM

Advantages: Simple noise control Disadvantages: No pressure optimization

Duct Sizing Calculations

Circular duct area:

$$A = \frac{\pi D^2}{4}$$

Velocity:

$$v = \frac{Q}{A} = \frac{CFM \times 144}{A_{in^2}}$$

Reynolds number:

$$Re = \frac{v D}{\nu}$$

For standard air: $\nu = 1.63 \times 10^{-4}$ ft²/s

Friction factor (turbulent flow, smooth ducts):

$$f = \frac{0.25}{[\log_{10}(Re/7)]^2}$$

Pressure drop:

$$\Delta P = f \frac{L}{D} \frac{v^2}{(4005)^2 \times 12.96} \text{ in. w.g.}$$

Fan Selection

Total system pressure:

$$\Delta P_{total} = \Delta P_{supply} + \Delta P_{return}$$

Fan static pressure:

$$FSP = TSP - P_{v,discharge}$$

Fan power:

$$hp = \frac{CFM \times FSP}{6,356 \times \eta}$$

Typical fan efficiencies:

Forward-curved: 50-65%
Backward-inclined: 70-80%
Airfoil: 80-85%

Practical Applications

Residential: Equal friction, 0.10 “w.g./100 ft
Commercial low-rise: Equal friction, 0.08-0.12 “w.g./100 ft
High-rise: Static regain for tall shafts
VAV systems: Static regain or equal friction with pressure control

Related Technical Guides:

References:

ASHRAE Handbook of Fundamentals, Chapter 21: Duct Design
SMACNA HVAC Systems Duct Design, 4th Edition
ASHRAE Duct Fitting Database