1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of HVAC ductwork based on airflow requirements and desired air velocity. Proper duct sizing is essential for efficient HVAC system operation.

2. How Does the Calculator Work?

The calculator uses the fundamental duct sizing equation:

Where:

• \( A \) — Duct cross-sectional area (square feet)
• \( CFM \) — Airflow in cubic feet per minute
• \( V \) — Air velocity in feet per minute

Explanation: The equation balances airflow requirements with velocity to determine the appropriate duct size that will minimize pressure drop while maintaining proper air distribution.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures optimal system performance, energy efficiency, proper air distribution, and noise control. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to poor air distribution and higher installation costs.

4. Using the Calculator

Tips: Enter the required airflow in CFM and desired air velocity in FPM. Typical velocities:

• Main ducts: 1000-2000 FPM
• Branch ducts: 600-900 FPM
• Residential systems: 500-700 FPM

5. Frequently Asked Questions (FAQ)

Q1: What is the recommended air velocity for different applications?
A: It varies by application - 700-900 FPM for low-velocity systems, 1200-1800 FPM for medium-velocity, and 1800-2500 FPM for high-velocity systems.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular ones. When using rectangular ducts, aim for an aspect ratio (width:height) no greater than 4:1.

Q3: What about friction loss?
A: This calculator provides basic sizing. For complete design, friction loss calculations using the Darcy-Weisbach equation or equivalent length method should be performed.

Q4: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM remains constant but mass flow decreases. Duct sizing may need adjustment for very high altitudes.

Q5: What's the relationship between duct size and static pressure?
A: Smaller ducts increase velocity and static pressure drop, requiring more fan power. There's always a balance between duct size and energy consumption.