Ventilation rate describes how much fresh air is supplied to or exhausted from a space over time, and it is calculated by combining airflow and room or airway volume. In offices, factories and underground mines, getting this calculation right is essential for air quality, temperature control and safety.
The simplest way to express ventilation rate is as air changes per hour (ACH). ACH tells you how many times per hour the entire volume of air in a room is theoretically replaced. The basic relationship is:
ACH = (Supply or exhaust airflow) / (Space volume)
where airflow is in m3/h (or CFM converted to m3/h) and volume is in m3. Rearranging this formula, the airflow required for a target ACH is:
Airflow (m3/h) = ACH × Room volume (m3)
For example, if a workshop has a volume of 2,000 m3 and a guideline recommends 6 air changes per hour, the required ventilation rate is 12,000 m3/h. This airflow can be supplied by one large fan or several smaller fans, as long as the total matches the calculated value and is properly distributed.
Another method uses airflow per person. Many standards specify a minimum volume of outdoor air per occupant, such as a certain number of litres per second per person. To calculate the ventilation rate, you multiply the required flow per person by the number of people expected in the space. This approach is common in offices and public buildings, but it is also used in some industrial settings where occupancy is the main driver of fresh air needs.
In industrial and mining ventilation, ventilation rate is often calculated from heat and contaminant loads. Engineers estimate how much heat is released by machinery, processes and lighting, and how much dust, fumes or gases are generated. They then determine the airflow required to dilute these contaminants below occupational exposure limits and to keep temperatures within acceptable limits. In underground mines, regulations may set minimum ventilation rates per kilowatt of diesel power, per worker or per tonne of production, and the total mine ventilation rate must satisfy the highest of these requirements.
In all cases, the calculated ventilation rate is only the starting point. Engineers must also check air distribution to ensure that the supplied air actually reaches occupied zones, work faces and process areas rather than short-circuiting from supply to exhaust. Ductwork, main and auxiliary fans, diffusers and regulators are then designed so that the calculated rate is delivered effectively where it is needed.
In summary, ventilation rate is calculated by relating airflow to space volume, occupancy and contaminant or heat loads, using tools such as air changes per hour, airflow per person and regulatory formulas. These calculations underpin the selection and control of fans in industrial plants and mines.
