There are two main ways to calculate the airflow of a fan in an industrial plant or mine: by direct measurement of velocity and area, and by using fan curves together with system pressure measurements. In both cases, the goal is to determine the volume flow rate, expressed in m³/s or CFM, that the fan is actually delivering at its operating point.
The most straightforward method is based on the simple relationship Q = A × V, where Q is the airflow, A is the cross-sectional area of the duct or airway, and V is the average air velocity through that area. To apply this method:
- Measure the cross-sectional area of the duct, airway or opening where the air passes. For a circular duct, A = πD²/4. For a rectangular duct or roadway, A = width × height (adjusted for obstructions).
- Use a suitable instrument, such as a Pitot tube, vane anemometer or hot-wire anemometer, to measure air velocity at several points across the section. In mines, handheld vane anemometers are often used during roadway traverses.
- Calculate the average velocity from these readings. Do not rely on a single point in the centre; follow recognised traversing methods so the average is representative.
- Multiply area by average velocity to obtain the volume flow rate Q in m³/s (or convert to CFM using standard factors).
The second method uses fan curves and system pressure measurements. If you know the fan model, speed and air density, the manufacturer’s curve shows the relationship between airflow and pressure. By measuring the pressure rise across the fan or the pressure at specific points in the system, you can locate the approximate operating point on the curve and read off the corresponding airflow. This approach is useful when accurate velocity measurements are difficult but you can access pressure tappings and have reliable fan data.
In many mining and industrial applications, engineers combine both approaches. They may use Pitot traverses or roadway airflow surveys to confirm the air quantity, while also checking that the measured fan pressure and power match the expected duty point from the curve. Discrepancies can reveal issues such as higher-than-expected system resistance, duct leakage, reversed rotation or build-up on fan blades.
Remember also to consider air density. If conditions differ from standard (for example, high temperature or altitude in a mine), you may need to correct measured volumes to standard conditions for comparison with design values, and then convert back to actual conditions for fan power and gas dilution calculations.
In summary, to calculate the airflow of a fan you either measure area and average velocity and use Q = A × V, or you use fan curves together with pressure data to find the duty point. Both methods, correctly applied, give you the air quantity you need for reliable industrial and mining ventilation design.
