How To Calculate CFM For Rectangular Duct

CFM Formula For Rectangular Duct:

\[ CFM = V \times \frac{Length \times Width}{144} \]

CFM (Cubic Feet Per Minute):

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1. What Is CFM For Rectangular Duct?

CFM (Cubic Feet per Minute) is a measurement of airflow volume. For rectangular ducts, CFM calculation determines how much air flows through a duct of specific dimensions at a given velocity, which is essential for HVAC system design and analysis.

2. How Does The Calculator Work?

The calculator uses the CFM formula for rectangular ducts:

\[ CFM = V \times \frac{Length \times Width}{144} \]

Where:

\( CFM \) — Cubic Feet per Minute (ft³/min)
\( V \) — Air Velocity (ft/min)
\( Length \) — Duct length dimension (inches)
\( Width \) — Duct width dimension (inches)
\( 144 \) — Conversion factor (144 square inches = 1 square foot)

Explanation: The formula calculates the cross-sectional area of the duct in square feet, then multiplies by velocity to determine the volume of air moving through the duct per minute.

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 in ventilation systems.

4. Using The Calculator

Tips: Enter velocity in feet per minute, and duct dimensions in inches. All values must be positive numbers. The calculator will automatically compute the CFM for your rectangular duct system.

5. Frequently Asked Questions (FAQ)

Q1: Why is the 144 conversion factor used?
A: The factor 144 converts square inches to square feet (since 12×12 = 144), allowing us to calculate duct area in square feet for consistent CFM measurement.

Q2: What are typical velocity values for HVAC ducts?
A: Residential systems typically use 600-900 ft/min, while commercial systems may use 1000-2000 ft/min, depending on application and noise considerations.

Q3: How does duct shape affect CFM calculations?
A: Rectangular ducts require this specific calculation. Circular ducts use a different formula based on diameter. The cross-sectional area calculation differs between shapes.

Q4: Can I use this for duct sizing?
A: Yes, this calculation helps determine either the required duct size for a given CFM or the resulting CFM for an existing duct, aiding in proper HVAC system design.

Q5: How does air density affect CFM calculations?
A: CFM measures volumetric flow. For mass flow calculations (which consider air density), additional factors like temperature and pressure must be accounted for.