FPM to CFM Round Duct Calculator

FPM to CFM Formula:

\[ CFM = FPM \times \pi \times \left(\frac{Diameter}{24}\right)^2 \]

CFM (cubic feet per minute):

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1. What is FPM to CFM Conversion?

The FPM to CFM conversion calculates the volume of air flow (in cubic feet per minute) through a round duct based on the velocity of air (in feet per minute) and the diameter of the duct. This is essential in HVAC system design and analysis.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ CFM = FPM \times \pi \times \left(\frac{Diameter}{24}\right)^2 \]

Where:

\( FPM \) — Feet per minute (air velocity)
\( Diameter \) — Duct diameter in inches
\( \pi \) — Mathematical constant (approximately 3.1416)
The division by 24 converts diameter from inches to feet (since 24 inches = 2 feet, and area calculation requires radius in feet)

Explanation: The formula calculates the cross-sectional area of the duct in square feet, then multiplies by the air velocity to get volumetric flow rate.

3. Importance of CFM Calculation

Details: Accurate CFM calculation is crucial for proper HVAC system design, ensuring adequate air flow for ventilation, heating, and cooling requirements in residential and commercial buildings.

4. Using the Calculator

Tips: Enter air velocity in feet per minute and duct diameter in inches. Both values must be positive numbers. The calculator will provide the CFM result.

5. Frequently Asked Questions (FAQ)

Q1: Why divide diameter by 24 in the formula?
A: The division by 24 converts diameter from inches to feet (since 2 feet = 24 inches) and also incorporates the radius calculation (radius = diameter/2).

Q2: What are typical FPM values in HVAC systems?
A: Typical values range from 600-1200 FPM for supply ducts and 500-900 FPM for return ducts, depending on the application and noise considerations.

Q3: Can this calculator be used for rectangular ducts?
A: No, this calculator is specifically for round ducts. Rectangular ducts require a different formula using width and height dimensions.

Q4: What is the relationship between CFM and air pressure?
A: Higher CFM values generally require more powerful fans and may result in higher system pressure drops, which affects energy consumption.

Q5: How does duct size affect CFM?
A: Larger diameter ducts can handle higher CFM values at lower velocities, which typically results in quieter operation but requires more space.