Air Flow & Duct Sizing Calculator (HVAC & Industrial)

This industrial-grade calculator solves complex air distribution problems. It features Constant Friction Sizing (Equal Friction Method), Velocity Sizing, and System Analysis modes. It includes an Atmospheric Physics Engine to correct air density for Altitude and Temperature (Ideal Gas Law), ensuring accuracy for any location worldwide.

1. System Configuration

Mode
Geometry

2. Flow & Constraints

Design Data
Atmosphere
Length & Fittings

Engineering Insights: HVAC Duct Design Fundamentals

1. The Friction Rate vs. Velocity Method

Duct design is a balance between initial cost (duct size) and operating cost (fan energy). Two main criteria are used:

  • Constant Friction (Equal Friction): Designing the entire system for a specific pressure drop per unit length. Typical commercial values are 0.08 to 0.1 in.wg/100ft (0.65 to 0.8 Pa/m). This ensures balanced pressure distribution without complex balancing dampers.
  • Velocity Reduction: Starting with a high velocity at the fan (e.g., 2000 fpm) and reducing it in branches. This helps with noise control.

2. Altitude & Temperature Correction

Standard Air is defined at sea level (29.92 inHg) and 70°F, with a density of 0.075 lb/ft³. However, at 5000 ft elevation (Denver), air is thinner ($\rho \approx 0.062$).

Why does this matter? Fans are constant volume machines. They move the same CFM regardless of density. But the Pressure (Static Head) and Horsepower required are directly proportional to density.
$\Delta P_{act} = \Delta P_{std} \times (\rho_{act} / \rho_{std})$.
If you design for sea level but install at 5000ft, your pressure drop will be lower, but your mass flow (heating/cooling capacity) will also be lower by ~17%.

3. Rectangular to Round Conversion ($D_e$)

Air friction charts are based on round ducts. To size a rectangular duct, we calculate the Equivalent Diameter ($D_e$). This is the diameter of a round duct that would have the same friction loss and capacity as the rectangular one.

$$ D_e = 1.30 \times \frac{(a \cdot b)^{0.625}}{(a + b)^{0.25}} $$

Note: This is different from the Hydraulic Diameter ($4A/P$). $D_e$ is for friction equivalence (Huebscher equation), while hydraulic diameter is for heat transfer or Reynolds number calculations.

4. Velocity Pressure ($P_v$)

Moving air has kinetic energy. This manifests as Velocity Pressure. It is always positive and exerted in the direction of flow.

$$ P_v = \left( \frac{V}{4005} \right)^2 \quad (\text{Imperial, V in fpm}) $$

When air slows down (e.g., in an expansion fitting), some Velocity Pressure converts back into Static Pressure. This is called Static Regain. In high-velocity systems, ignoring this can lead to oversized fans or noisy ducts.

5. Dynamic Losses ($C$-factors)

Elbows, transitions, and entries cause turbulence, dissipating energy. This "Dynamic Loss" is calculated as a multiple of the Velocity Pressure:

$$ \Delta P_{dynamic} = C \times P_v $$

Where $C$ is the loss coefficient. A standard sharp 90° elbow has $C \approx 1.3$. Adding turning vanes reduces $C$ to $\approx 0.3$, saving significant fan energy.