1. What is the Rectangular Duct CFM Formula?

The Rectangular Duct CFM formula calculates the airflow in cubic feet per minute through a rectangular duct based on velocity and cross-sectional dimensions. This calculation is essential for HVAC system design and analysis.

2. How Does the Calculator Work?

The calculator uses the rectangular duct CFM formula:

Where:

• \( CFM \) — Cubic Feet per Minute (airflow)
• \( V \) — Velocity in Feet Per Minute (FPM)
• \( L \) — Length of duct in inches
• \( W \) — Width of duct in inches

Explanation: The formula converts the rectangular cross-sectional area from square inches to square feet (by dividing by 144) and multiplies by velocity to determine volumetric flow rate.

3. Importance of CFM Calculation

Details: Accurate CFM calculation is crucial for proper HVAC system design, ensuring adequate airflow for heating and cooling, maintaining indoor air quality, and optimizing energy efficiency.

4. Using the Calculator

Tips: Enter velocity in FPM, length and width in inches. All values must be valid positive numbers. The calculator will compute the CFM based on these inputs.

5. Frequently Asked Questions (FAQ)

Q1: Why divide by 144 in the formula?
A: We divide by 144 to convert the cross-sectional area from square inches to square feet (since 1 square foot = 144 square inches).

Q2: What is a typical velocity range for HVAC ducts?
A: Typical velocities range from 600-900 FPM for low-pressure systems and up to 2000 FPM for high-pressure systems.

Q3: How does this differ from round duct CFM calculation?
A: For round ducts, the formula is CFM = V × (π × D² / 4 / 144), where D is the diameter in inches.

Q4: What factors affect airflow in ducts?
A: Duct size, shape, material, length, number of bends, and static pressure all affect airflow in duct systems.

Q5: When should I use this calculation?
A: This calculation is useful for HVAC designers, technicians, and engineers when sizing ducts, selecting fans, or troubleshooting airflow issues.