Air Changes per Hour (ACH)

What Are Air Changes per Hour?

Air changes per hour, abbreviated as ACH, indicate how many times the volume of air inside an enclosed space is theoretically replaced during one hour.

For example, a ventilation rate of 6 ACH means that the volume of air supplied to or exhausted from the space each hour is equivalent to approximately six times the total volume of the room.

ACH is commonly used to evaluate:

• General industrial ventilation
• Workshop and warehouse air exchange
• Heat and odor removal
• Dilution of low-concentration fumes or gases
• Indoor air quality
• Make-up air requirements
• Overall ventilation capacity

ACH is a theoretical average based on airflow and room volume.

A room with 6 ACH does not necessarily have every part of its air completely replaced six times per hour. Actual ventilation performance also depends on the locations of supply and exhaust openings, airflow patterns, room geometry, equipment obstructions and short-circuiting between air inlets and outlets.

How Do You Calculate ACH?

The basic metric formula is:

ACH = Ventilation Airflow ÷ Room Volume

When airflow is measured in cubic meters per hour and room volume is measured in cubic meters:

ACH = Airflow (m³/h) ÷ Room Volume (m³)

Room volume is calculated as:

Room Volume = Length × Width × Height

For example, consider a workshop with the following conditions:

• Length: 20 meters
• Width: 10 meters
• Height: 5 meters
• Ventilation airflow: 6,000 m³/h

The room volume is:

20 × 10 × 5 = 1,000 m³

The air change rate is:

ACH = 6,000 ÷ 1,000 = 6

The workshop therefore has a theoretical ventilation rate of:

6 ACH

When imperial units are used, airflow is normally expressed in cubic feet per minute and room volume in cubic feet:

ACH = CFM × 60 ÷ Room Volume (ft³)

The airflow is multiplied by 60 to convert cubic feet per minute into cubic feet per hour.

When both supply and exhaust airflow values are available, they should not automatically be added together. The airflow used in the calculation should represent the effective ventilation or clean-air delivery for the space.

Why Is ACH Important in Industrial Ventilation?

ACH provides a convenient way to compare the general ventilation capacity of spaces with different sizes.

Appropriate general ventilation may help:

• Remove accumulated heat
• Dilute low-concentration fumes and gases
• Reduce odors
• Supply fresh or make-up air
• Control background contaminant concentrations
• Support local exhaust ventilation systems

A larger room requires more airflow to achieve the same number of air changes per hour.

For example:

• A 500 m³ room requires 3,000 m³/h to achieve 6 ACH.
• A 2,000 m³ workshop requires 12,000 m³/h to achieve 6 ACH.

For this reason, fan airflow alone does not describe the ventilation performance of a workshop. The volume of the space must also be considered.

However, ACH does not indicate whether a contaminant is being effectively captured at its source. Two workshops with the same ACH may have very different air quality because their contaminant types, emission rates, ventilation layouts and airflow patterns are different.

Can ACH Be Used to Size a Dust Collection System?

ACH can be used for an initial assessment of general workshop ventilation, but it should not normally be the only method used to determine industrial dust collector airflow.

Industrial dust is usually generated at specific locations, such as:

• Welding stations
• Grinding machines
• Material feeding points
• Crushers
• Conveyor transfer points
• Mixing equipment
• Packaging machines

For these localized dust sources, the primary objective is to capture the particles before they spread into the surrounding workplace.

The required airflow for a local dust collection system generally depends on:

• Dust hood type and dimensions
• Distance between the hood and dust source
• Required capture velocity
• Open-face area
• Direction and velocity of dust generation
• Number of collection points operating simultaneously
• Duct transport velocity
• System pressure losses

For example, a large workshop may require tens of thousands of cubic meters per hour to achieve a specified general ACH. However, when dust is generated by only one grinding machine, a properly designed local exhaust hood may capture the dust at its source with a much smaller and more concentrated airflow.

By contrast, relying only on roof exhaust fans to increase ACH may allow dust to pass through workers’ breathing zones before it is removed from the building.

The difference can be summarized as follows:

• ACH evaluates general ventilation throughout a space.
• Capture velocity and hood airflow are used to control a specific dust source.
• Industrial dust should normally be captured as close to the source as practical.

ACH may form part of an overall workshop ventilation strategy, but it should not be treated as the sole basis for selecting a dust collector, hood, duct system or industrial fan.