Air Changes per Hour

What is this formula?

Air Changes per Hour (ACH) measures how many times the air within a mine opening, chamber, refuge station, workshop, or enclosed underground space is theoretically replaced in one hour.

It is a widely used ventilation performance indicator in mining, industrial facilities, tunnels, and underground infrastructure.

The factor 3600 converts airflow from cubic meters per second to cubic meters per hour.

When to use it

Use this formula when:

- Evaluating ventilation effectiveness.

- Designing underground workshops.

- Assessing refuge chambers.

- Ventilating maintenance areas.

- Comparing ventilation performance between mine sections.

Example

An underground maintenance chamber has a volume of 900 m³ and receives an airflow of 2.5 m³/s.

Data:

Q = 2.5 m³/s

V = 900 m³

Formula:

ACH = (3600 × Q) / V

Substitution:

ACH = (3600 × 2.5) / 900

ACH = 9000 / 900

Result:

ACH = 10 h⁻¹

The air inside the chamber is theoretically replaced 10 times per hour.

Applications

- Underground mine ventilation

- Refuge chamber ventilation

- Mine workshops

- Equipment maintenance bays

- Underground infrastructure

- Ventilation performance monitoring

Note

Air Changes per Hour is a simplified ventilation indicator that assumes perfect air mixing within the space. Actual airflow patterns may contain dead zones, recirculation regions, short-circuiting, and stagnant areas. Therefore, ACH should be considered an overall ventilation performance metric rather than a direct measure of contaminant removal effectiveness.