Air Flow Through An Orifice Calculator CFM

Orifice Airflow Equation:

\[ CFM = 29.92 \times C_d \times \left(\frac{D}{12}\right)^2 \times \sqrt{\frac{\Delta P}{SG}} \]

Discharge Coefficient (C_d):

dimensionless

Orifice Diameter (D):

inches

Pressure Difference (ΔP):

in H₂O

Specific Gravity (SG):

dimensionless

Air Flow Rate (CFM):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Orifice Airflow Equation?

The orifice airflow equation calculates the volumetric flow rate of air through an orifice plate. It is commonly used in HVAC systems, pneumatic controls, and industrial ventilation to determine air flow rates based on pressure differentials.

2. How Does the Calculator Work?

The calculator uses the orifice airflow equation:

\[ CFM = 29.92 \times C_d \times \left(\frac{D}{12}\right)^2 \times \sqrt{\frac{\Delta P}{SG}} \]

Where:

\( CFM \) — Cubic Feet per Minute (air flow rate)
\( C_d \) — Discharge coefficient (dimensionless)
\( D \) — Orifice diameter (inches)
\( \Delta P \) — Pressure difference across orifice (inches H₂O)
\( SG \) — Specific gravity of fluid (dimensionless)

Explanation: The equation relates air flow rate to the square root of pressure differential and the square of orifice diameter, adjusted by discharge coefficient and specific gravity.

3. Importance of CFM Calculation

Details: Accurate CFM calculation is essential for proper HVAC system design, ventilation requirements, pneumatic system sizing, and ensuring adequate air flow for industrial processes and environmental control.

4. Using the Calculator

Tips: Enter discharge coefficient (typically 0.6-0.8 for sharp-edged orifices), orifice diameter in inches, pressure difference in inches of water column, and specific gravity (1.0 for air at standard conditions). All values must be positive.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical discharge coefficient value?
A: For sharp-edged orifices, C_d is typically 0.60-0.65. For rounded orifices, it can be 0.95-0.98. The exact value depends on orifice geometry and Reynolds number.

Q2: Why convert diameter from inches to feet?
A: The equation uses diameter in feet for consistent units with CFM (cubic feet per minute), while orifice diameters are commonly measured in inches.

Q3: What is specific gravity for air?
A: For dry air at standard conditions (68°F, 14.7 psia), specific gravity is 1.0 relative to air. For other gases or conditions, adjust accordingly.

Q4: When is this equation most accurate?
A: The equation works best for turbulent flow conditions and when the orifice diameter is small compared to the pipe diameter (typically D_orifice/D_pipe < 0.7).

Q5: Can this be used for liquids?
A: While the basic principle applies, liquid flow calculations may require different constants and considerations for viscosity and density variations.