Circular Duct Size Calculator

Circular Duct Diameter Formula:

\[ D = 2 \times \sqrt{\frac{CFM / V}{\pi}} \times 12 \]

Duct Diameter (D):

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1. What is the Circular Duct Size Formula?

The circular duct size formula calculates the diameter of a circular duct based on airflow rate (CFM) and air velocity. This is essential for proper HVAC system design to ensure efficient airflow and system performance.

2. How Does the Calculator Work?

The calculator uses the circular duct diameter formula:

\[ D = 2 \times \sqrt{\frac{CFM / V}{\pi}} \times 12 \]

Where:

\( D \) — Duct diameter in inches
\( CFM \) — Airflow rate in cubic feet per minute
\( V \) — Air velocity in feet per minute
\( \pi \) — Mathematical constant Pi (approximately 3.14159)

Explanation: The formula calculates the cross-sectional area needed for the given airflow at the specified velocity, then converts that area to a circular diameter.

3. Importance of Duct Sizing

Details: Proper duct sizing is crucial for HVAC system efficiency. Undersized ducts create excessive pressure drop and noise, while oversized ducts are inefficient and costly.

4. Using the Calculator

Tips: Enter CFM (cubic feet per minute) and velocity (feet per minute). Both values must be positive numbers. Typical velocity ranges are 600-900 ft/min for supply ducts and 500-700 ft/min for return ducts.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical velocity range for HVAC ducts?
A: Supply ducts typically range 600-900 ft/min, return ducts 500-700 ft/min, and low-velocity systems may use 400-500 ft/min.

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: How does duct shape affect the calculation?
A: This formula is specifically for circular ducts. Rectangular ducts require different calculations based on aspect ratio.

Q4: What factors influence optimal duct velocity?
A: Noise limitations, space constraints, energy efficiency, and system static pressure all influence optimal velocity selection.

Q5: Can this calculator be used for exhaust systems?
A: Yes, the same formula applies, though velocity requirements may differ based on the type of exhaust system and materials being conveyed.