1. What is the CFM Formula?

The CFM (Cubic Feet per Minute) formula calculates airflow using diameter and velocity pressure. It is a standard air formula used in HVAC and ventilation systems to determine airflow rates.

2. How Does the Calculator Work?

The calculator uses the CFM formula:

Where:

• \( D \) — Diameter (inches)
• \( VP \) — Velocity Pressure (inches water gauge)
• \( \pi \) — Mathematical constant Pi (approximately 3.1416)
• 4005 — Constant for standard air conditions

Explanation: The formula converts diameter to radius in feet, calculates cross-sectional area, then multiplies by velocity (derived from pressure) to get volumetric flow rate.

3. Importance of CFM Calculation

Details: Accurate CFM calculation is essential for proper HVAC system design, ventilation requirements, air balancing, and ensuring adequate air exchange rates in buildings and industrial applications.

4. Using the Calculator

Tips: Enter diameter in inches and velocity pressure in inches water gauge. Both values must be positive numbers greater than zero for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is standard air in this context?
A: Standard air refers to air at 70°F (21°C) and 29.92 inches Hg (101.325 kPa) with a density of 0.075 lb/ft³.

Q2: Why is the constant 4005 used?
A: The constant 4005 comes from the conversion factors needed to derive velocity from pressure for standard air conditions.

Q3: Can this formula be used for non-circular ducts?
A: No, this formula is specifically for circular ducts. For rectangular ducts, equivalent diameter calculations or different formulas are needed.

Q4: What are typical VP values in HVAC systems?
A: Typical velocity pressure values range from 0.1 to 1.0 inches wg, depending on system design and airflow requirements.

Q5: How accurate is this calculation?
A: The calculation provides theoretical values. Actual airflow may vary due to factors like duct roughness, fittings, and system losses.