Stack Gas Flow Rate Calculator
Calculation Results
Understanding Stack Gas Flow Rate Calculations
Accurately calculating the volumetric flow rate of gas exiting a stack or flue is critical for environmental compliance, process control, and emission monitoring (CEMS). This calculation converts the raw velocity and physical dimensions of a stack into volumetric data (Actual Cubic Meters) and then corrects that volume to Standard Conditions (Standard Cubic Meters) for regulatory reporting.
Key Metrics Explained
- Actual Flow Rate ($Q_a$): The volume of gas moving through the stack at the current operating temperature and pressure conditions.
- Standard Flow Rate ($Q_s$): The volume of the gas corrected to standard reference conditions (typically 20°C or 0°C and 101.325 kPa). This allows for consistent comparison of emissions regardless of operating temperature.
- Dry Standard Flow Rate ($Q_{sd}$): The standard flow rate after removing the volume contributed by water vapor (moisture). Many emission limits are based on dry standard volume.
The Formulas
The calculation follows a three-step process based on fluid dynamics and the Ideal Gas Law.
1. Calculate Cross-Sectional Area ($A$)
For a circular stack:
$$A = \pi \times \left(\frac{D}{2}\right)^2$$
Where $D$ is the inner diameter of the stack in meters.
2. Calculate Actual Flow Rate ($Q_a$)
$$Q_a = v \times A$$
Where $v$ is the average gas velocity in m/s. Multiply by 3600 to convert to m³/hr.
3. Convert to Standard Conditions ($Q_s$)
We correct the actual volume using the ratio of absolute temperatures and pressures:
$$Q_s = Q_a \times \left(\frac{T_{std}}{T_{stack}}\right) \times \left(\frac{P_{stack}}{P_{std}}\right)$$
Where:
$T$ is absolute temperature in Kelvin ($C + 273.15$).
$P$ is absolute pressure.
$P_{std}$ is usually 101.325 kPa.
Why Temperature Correction Matters
Hot gases expand. A stack operating at 200°C will have a much higher "Actual" flow rate than the same mass of gas at 20°C. To calculate the mass of pollutants emitted (e.g., kg/hr of NOx or SO2), regulators require the volume to be normalized to a standard temperature to eliminate the expansion effect of heat.