Duct Design and Airflow Formulas

 

Introduction

Duct design ensures that conditioned air reaches all areas efficiently, without excessive noise or energy loss. Correct sizing also prevents high static pressure, which can damage fans and reduce performance.

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Key Formulas

1. Airflow Velocity:

$$V = \frac{Q}{A}$$

• V: Velocity (ft/min)

• Q: Airflow (CFM)

• A: Duct cross-sectional area (ft²)

2. Friction Loss (Approx.):

$$FL = \frac{P \times L}{100}$$

• FL: Friction loss (in.wg/100 ft)

• P: Pressure drop

• L: Duct length

3. Equivalent Diameter for Rectangular Ducts:

$$D_e = 1.3 \times \frac{(a \times b)^{0.625}}{(a + b)^{0.25}}$$

• a, b: Width and height of duct

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Example Calculation

• Airflow: 1,200 CFM

• Duct: 2 ft × 1.5 ft

• Velocity: $V = 1200 / (2 × 1.5) = 400 ft/min$

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Practical Tips

• Use the velocity method for small duct systems.

• Use the friction loss method for large systems with long duct runs.

• Always round up duct sizes to standard sheet metal dimensions.

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Suggested Images

• Duct cross-section diagram

• Velocity vs. CFM chart

Internal Linking:

• Link back to Load Calculation page: HVAC Load Calculation Formulas

• Link forward to Fan/Pump page: Fan and Pump Horsepower Formulas

• Link back to Master Page: HVAC Formulas Master Guide

Tags:
duct design formulas, airflow velocity formula, HVAC duct sizing, friction loss formula, MEP duct calculation