1. What is the Duct Diameter Equation?

The duct diameter equation calculates the required diameter of a round duct based on airflow (CFM) and velocity (FPM). It ensures proper airflow distribution in HVAC systems while maintaining optimal velocity.

2. How Does the Calculator Work?

The calculator uses the duct diameter equation:

Where:

• \( CFM \) — Cubic feet per minute (airflow rate)
• \( FPM \) — Feet per minute (air velocity)
• \( \pi \) — Mathematical constant Pi (approximately 3.1416)

Explanation: The equation calculates the cross-sectional area needed for the given airflow and velocity, then converts it to diameter in inches.

3. Importance of Duct Sizing

Details: Proper duct sizing is crucial for efficient HVAC system operation. Undersized ducts increase resistance and noise, while oversized ducts reduce velocity and may cause poor air distribution.

4. Using the Calculator

Tips: Enter CFM (cubic feet per minute) and FPM (feet per minute) values. Both values must be positive numbers greater than zero for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical FPM range for HVAC ducts?
A: For residential systems: 600-900 FPM for supply, 500-700 FPM for return. For commercial: 1000-2000 FPM for supply, 800-1500 FPM for return.

Q2: Why multiply by 12 in the formula?
A: The multiplication by 12 converts the diameter from feet to inches, which is the standard unit for duct sizing.

Q3: Can this formula be used for rectangular ducts?
A: No, this formula is specifically for round ducts. For rectangular ducts, equivalent diameter calculations are needed.

Q4: What factors affect duct sizing besides CFM and FPM?
A: Duct material, layout complexity, number of bends, and static pressure requirements also influence duct sizing decisions.

Q5: How accurate is this calculation for real-world applications?
A: This provides a theoretical minimum diameter. Actual duct sizing should consider system losses, fittings, and safety factors per HVAC design standards.