What you should know
- ACH is a room-airflow rate, not an ISO cleanliness classification.
- Required airflow equals room volume multiplied by the selected air-change rate.
- The design rate must respond to particle generation, recovery, heat, moisture, exhaust and pressure balance.
- Supply location, return location and airflow path can matter as much as the ACH number.
What ACH measures
Air changes per hour describes how many room volumes of supply air are delivered in one hour. It is convenient for comparing ventilation intensity, but it does not show whether the air reaches the critical zone, short-circuits to a return or removes particles at the required rate.
Rearranged for design airflow: supply airflow (m³/h) = room volume (m³) × selected ACH.
ISO class does not prescribe a single ACH
ISO 14644-1 classifies air cleanliness by airborne particle concentration. It does not assign one mandatory ACH to each ISO class. Two rooms with the same class can need different airflow because their people, process, layout, release rate, recovery target and operating state are different.
Published rules of thumb can help with an early concept, but they are not proof of performance. The selected design must be tested in the stated condition and supported by a monitoring and control strategy appropriate to the process.
Worked airflow example
Consider a room 6 m long, 4 m wide and 3 m high. Its gross volume is 72 m³. If the design study selects 30 ACH, the gross supply airflow is 2,160 m³/h, approximately 1,271 CFM.
Then check exhaust, transfer air, room leakage, pressure offset, equipment heat, moisture loads and diversity. The final supply may be governed by a larger requirement.
| Selected ACH | Supply airflow | Approximate CFM | One theoretical air change |
|---|---|---|---|
| 15 | 1,080 m³/h | 636 CFM | 4 minutes |
| 30 | 2,160 m³/h | 1,271 CFM | 2 minutes |
| 45 | 3,240 m³/h | 1,907 CFM | 1.33 minutes |
What should determine the design air-change rate?
| Design factor | Engineering question | Possible airflow impact |
|---|---|---|
| People and process | How many sources operate, where, and for how long? | More or stronger sources may need higher removal capacity or better local control |
| Critical zone | Where must clean air protect the product or operation? | May favour directed or unidirectional flow rather than simply increasing room ACH |
| Recovery target | How fast must the room recover after a defined disturbance? | Faster verified recovery can require more effective clean-air delivery |
| Heat and moisture | What sensible and latent loads must be removed? | Thermal or moisture duty may govern supply quantity and condition |
| Exhaust | How much air is removed by process or safety exhaust? | Supply and makeup air must maintain mass balance and pressure |
| Room geometry | Are there obstructions, tall equipment or enclosed zones? | Distribution and return placement may need adjustment |
Supply and return layout makes ACH effective
- Place supply air so clean air reaches the critical work zone before it reaches contamination sources where practical.
- Use return or exhaust locations that pull contamination away without creating harmful crossflow.
- Check high-level heat plumes, low-level particle sources, equipment wakes and obstructions.
- Avoid direct short-circuit paths from a diffuser to a nearby return.
- Review door operation, transfer openings and pressure-control airflow together with room recirculation.
- Confirm performance with balancing, airflow visualization, particle testing and recovery testing where required.
A practical ACH design sequence
Define the controlled state
State the required cleanliness class, occupancy and process condition.
List contamination sources
Map people, process, materials, equipment and adjacent-space risks.
Calculate competing airflow duties
Compare contamination control, heat, humidity, exhaust replacement and pressure balance.
Lay out supply and returns
Develop an airflow path that protects critical zones and avoids dead areas.
Model or prototype when risk warrants
Use airflow visualization, computational study or mock-up evidence appropriate to the consequence of failure.
Balance, test and trend
Verify actual airflow and performance, then adjust setpoints from evidence rather than rule of thumb alone.
Frequently asked questions
What ACH is required for an ISO 7 cleanroom?
ISO 14644-1 does not prescribe one ACH for ISO 7. The project must select and verify airflow based on contamination sources, operating state, recovery, heat and moisture loads, exhaust, pressure control and the applicable industry requirements.
Should return air be included in ACH?
ACH is commonly based on total supply airflow delivered to the room. The return, exhaust and leakage paths are then checked through a separate mass balance so the intended pressure and flow direction are maintained.
Does doubling ACH halve particle concentration?
Not automatically. The result depends on source strength, filtration, mixing, airflow path, leakage and ongoing activity. Testing or a validated model is needed for the actual room response.
Technical sources
Confirm the current controlled edition of every standard and the requirements that apply to your facility.
- ISO 14644-1:2015: Classification of air cleanliness by particle concentrationInternational Organization for Standardization
- ISO 14644-3:2019: Test methodsInternational Organization for Standardization
- ISO 14644-4:2022: Design, construction and start-upInternational Organization for Standardization
Prepared by 360S Projects as a practical learning reference. Final design, validation and compliance decisions should be made by competent professionals using current project requirements, standards and manufacturer data.