1. What is a Duct Air Flow Calculator CFM?

Definition: This calculator determines the air flow rate in cubic feet per minute (CFM) through a duct based on air velocity and duct cross-sectional area.

Purpose: It helps HVAC professionals and engineers design and evaluate ventilation systems by calculating air flow rates.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( Q_{cfm} \) — Air flow rate (cubic feet per minute)
• \( V \) — Air velocity (feet per second)
• \( A \) — Duct cross-sectional area (square feet)
• 60 — Conversion factor (seconds to minutes)

Explanation: The formula converts velocity in feet per second to volume flow in cubic feet per minute by multiplying by the duct area and time conversion factor.

3. Importance of Air Flow Calculation

Details: Proper air flow calculation ensures adequate ventilation, efficient system operation, and compliance with building codes and standards.

4. Using the Calculator

Tips: Measure air velocity with an anemometer and duct dimensions to calculate cross-sectional area. Enter both values (must be > 0) to get CFM.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical air velocity in ducts?
A: Residential systems typically use 600-900 ft/min (10-15 ft/s), while commercial systems may use higher velocities.

Q2: How do I calculate duct area for rectangular ducts?
A: Multiply width by height (in feet). For round ducts: \( \pi \times r^2 \) where r is radius in feet.

Q3: Why multiply by 60 in the formula?
A: This converts seconds to minutes since CFM is cubic feet per minute while velocity is in feet per second.

Q4: What's a good CFM range for residential systems?
A: Typically 350-400 CFM per ton of cooling capacity, but varies based on system design.

Q5: How does air flow relate to duct size?
A: For constant CFM, smaller ducts require higher velocity, which increases noise and pressure drop.