Cleanroom Air Changes per Hour (ACH): How to Calculate and Select the Right Airflow

Air Changes per Hour (ACH) is one of the most commonly used airflow parameters in cleanroom HVAC design.

In simple terms, ACH describes how many times the total room air volume is supplied and circulated through the cleanroom in one hour.

The basic calculation is:

ACH = Total Supply Airflow ÷ Room Volume

For example, if a cleanroom has a volume of 300 m³ and the AHU supplies 6,000 m³/h of conditioned air:

ACH = 6,000 ÷ 300 = 20 air changes per hour.

Why ACH Matters in Cleanroom HVAC Design

Cleanrooms continuously generate airborne contamination from personnel, equipment, processes and materials.

Clean supply air helps dilute and remove these contaminants through the return-air system.

Increasing airflow can improve contamination removal and reduce cleanroom recovery time. However, excessive airflow also increases:

✔ Fan energy consumption
✔ HEPA filter pressure drop
✔ Cooling and heating loads
✔ HVAC equipment size
✔ Operating cost

The engineering target is therefore not the highest possible ACH, but the lowest practical airflow that consistently meets cleanliness and process requirements.

Does Each Cleanroom Class Have a Fixed ACH?

No.

ISO 14644-1 classifies cleanrooms primarily by airborne particle concentration. It does not assign one mandatory ACH value to every ISO cleanroom class.

Actual airflow requirements depend on:

✔ ISO cleanliness class
✔ Process contamination generation
✔ Number of operators
✔ Equipment heat and particle loads
✔ Air distribution pattern
✔ HEPA / ULPA filter coverage
✔ Recovery time requirements
✔ Ceiling height

For this reason, ACH values should be treated as an engineering design parameter rather than a universal cleanliness requirement.

ACH and Cleanroom Ceiling Height

Room height has a direct impact on ACH because ACH is calculated using total room volume.

For the same airflow velocity, increasing ceiling height increases room volume and therefore reduces the calculated ACH.

High-bay cleanrooms should therefore be evaluated using airflow velocity, contamination source location and air distribution—not simply by applying a standard ACH value used for conventional rooms.

How to Calculate Cleanroom Supply Airflow

If the target ACH is known:

Supply Airflow = Floor Area × Ceiling Height × ACH

Example:

Floor area: 200 m²
Ceiling height: 3 m
Design ACH: 20

Room volume:

200 × 3 = 600 m³

Required supply airflow:

600 × 20 = 12,000 m³/h

This airflow becomes an important basis for selecting:

✔ Air Handling Unit (AHU)
✔ HEPA filter quantity
✔ Supply and return duct sizes
✔ Diffuser layout
✔ Fan external static pressure

ACH Is Only One Part of Cleanroom Design

A properly designed cleanroom HVAC system must also consider:

Cleanliness Classification

The required particle concentration determines the overall contamination control strategy.

Fresh Air and Exhaust Balance

Outdoor air must compensate for exhaust airflow, maintain required pressure relationships and provide adequate ventilation.

Room Pressure Control

Controlled pressure differentials help reduce uncontrolled contamination migration between clean and less-clean spaces.

Airflow Pattern

Non-unidirectional, unidirectional and mixed airflow systems require different airflow strategies.

Filtration

HEPA and ULPA filter efficiency, face velocity and pressure drop affect both cleanliness performance and HVAC energy consumption.

Recovery Time

Some applications require the cleanroom to return to its specified cleanliness level quickly after contamination events.

YUJU Cleanroom Air Handling Solutions

YUJU provides customized air handling and cleanroom HVAC solutions for laboratories, controlled production spaces, healthcare environments and specialized industrial applications.

Our engineering approach considers:

✔ Cleanliness requirements
✔ Airflow and ACH calculations
✔ AHU selection
✔ HEPA filtration
✔ Temperature and humidity control
✔ Pressure control
✔ Supply and return air design
✔ HVAC automation and monitoring

Rather than simply increasing airflow, YUJU focuses on balancing cleanliness performance, system stability and long-term energy efficiency.

FAQ

Does higher ACH always mean a cleaner cleanroom?

Not necessarily. Higher airflow can improve particle dilution, but cleanroom performance also depends on filtration, contamination sources, airflow distribution, pressure control and operating procedures.

How is cleanroom ACH measured?

Supply airflow is measured at the terminal filters or supply outlets. Total measured airflow is then divided by the cleanroom volume to calculate ACH.

Does ISO 14644 specify required ACH values?

ISO 14644-1 classifies cleanrooms by airborne particle concentration rather than assigning a fixed ACH to each ISO class.

How should high-bay cleanrooms be designed?

High-bay cleanrooms should be evaluated using airflow velocity, process height, contamination sources and supply/return air distribution instead of relying only on conventional ACH rules of thumb.