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

Cleanroom Airflow Calculation, AHU Selection & Contamination Control

ACH is an important cleanroom HVAC design parameter, but it should be evaluated together with cleanliness class, contamination load, airflow pattern, filtration and pressure control.

What Is Cleanroom ACH?

ACH represents the relationship between total cleanroom supply airflow and room volume.

For example, if a cleanroom has a volume of 300 m³ and the HVAC system supplies 6,000 m³/h:

The cleanroom therefore receives an equivalent supply-air volume of approximately 20 room volumes per hour.

Why ACH Matters in Cleanroom HVAC Design

Cleanrooms continuously receive contamination from people, processes, equipment and materials.

Filtered supply air helps dilute and remove airborne contaminants through the room airflow and return-air system.

However, higher airflow also increases the energy and equipment requirements of the HVAC system.

Does Every Cleanroom Class Have a Fixed ACH?

No.

ISO 14644-1 primarily classifies cleanrooms according to airborne particle concentration. It does not provide one mandatory ACH value for every ISO cleanliness class.

Actual airflow should be determined according to the complete contamination-control requirement.

How to Calculate Cleanroom Supply Airflow

If the target ACH has been established through the cleanroom design process, required airflow can be calculated from the room volume.

Calculation Example

Design Parameter	Value
Floor Area	200 m²
Ceiling Height	3 m
Room Volume	600 m³
Design ACH	20
Required Supply Airflow	12,000 m³/h

From ACH Calculation to AHU Selection

Once the required supply airflow has been determined, it becomes one of the major inputs for HVAC equipment selection.

ACH and Cleanroom Ceiling Height

Ceiling height directly affects calculated ACH because room volume increases as height increases.

Two cleanrooms with the same floor area and the same supply airflow can therefore have different ACH values if their ceiling heights are different.

This becomes especially important in high-bay cleanrooms.

ACH Is Only One Part of Cleanroom Design

Cleanliness Classification

The required airborne particle concentration determines the overall contamination-control strategy.

Fresh Air & Exhaust Balance

Outdoor air is required to compensate for exhaust air, provide ventilation and support room pressure relationships.

Room Pressure Control

Controlled pressure differentials help limit unwanted contamination migration between spaces with different cleanliness requirements.

Airflow Pattern

Non-unidirectional, unidirectional and mixed airflow systems use different contamination-control strategies and should not be designed using ACH alone.

HEPA / ULPA Filtration

Filter efficiency, filter area, face velocity and pressure drop directly affect cleanroom performance and fan selection.

Recovery Time

Some cleanrooms require rapid restoration of the specified cleanliness condition after personnel activity or other contamination events.

HEPA Filter Quantity and Airflow

After the total airflow is established, the designer must determine how that airflow will be distributed through the terminal filtration system.

HEPA filter selection should consider:

Static Pressure Is Critical for Cleanroom AHU Selection

High airflow alone is not enough to select an AHU.

The fan must also overcome the resistance of the complete cleanroom air-distribution system.

The selected fan should therefore be verified at the required airflow and required external static pressure, rather than using maximum airflow alone.

Cleanroom Pressure Relationships

Cleanroom airflow design also determines how much air enters and leaves each space.

The balance between supply, return and exhaust airflow influences room pressure.

Why Excessive ACH Is Not Always Better

Using substantially more airflow than required can increase operating cost without providing a proportional improvement in cleanliness.

Excessive airflow may also increase:

Cleanroom optimization therefore requires balancing contamination control with energy and system performance.

YUJU Cleanroom Air Handling Solutions

YUJU EnvironTech supports laboratories, controlled production areas and specialized clean environments with customized air-handling and HVAC system solutions.

Our approach evaluates the complete air system rather than selecting an AHU from nominal airflow alone.

What Information Should You Provide?

Frequently Asked Questions

Does higher ACH always mean a cleaner cleanroom?

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

How is cleanroom ACH measured?

Total supply airflow can be measured at the terminal filters or supply outlets and divided by the cleanroom volume to calculate ACH.

Does ISO 14644-1 specify a fixed ACH for every cleanroom class?

No. ISO 14644-1 classifies cleanrooms primarily according to airborne particle concentration rather than assigning one fixed ACH value to every cleanliness class.

How should high-bay cleanrooms be designed?

High-bay spaces should consider airflow velocity, process height, contamination-source location and supply/return airflow distribution rather than relying only on conventional ACH rules of thumb.

Can YUJU help select a cleanroom AHU?

YUJU can support airflow, static-pressure, filtration and temperature/humidity requirements to coordinate suitable cleanroom air-handling equipment for the project.