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Air Changes Per Hour (ACH) is the number of times the entire volume of air in a room is replaced with filtered air within 60 minutes. It is the most critical calculation for achieving and maintaining ISO cleanliness levels. If your ACH is too low, you won't meet your particle count targets. If it's too high, you are wasting energy and money. Here is how to get it right.

The ACH Formula

To calculate the ACH of an existing room:
ACH = (Total Airflow in m³/h) ÷ (Room Volume in m³)

To calculate the required airflow for a new design:
Required Airflow (m³/h) = Room Volume (m³) × Target ACH

Recommended ACH by ISO Class

Real-World Calculation Examples

Example 1: Pharmaceutical ISO 7 Fill Room

• Room Size: 10m (L) × 8m (W) × 3m (H) = 240 m³
• Target ACH: 100
• Calculation: 240 m³ × 100 ACH = 24,000 m³/h
• FFU Requirement: If using FFUs with 1,200 m³/h capacity, you need 20 FFUs.

Example 2: Electronics ISO 8 Assembly Area

• Room Size: 20m (L) × 15m (W) × 3m (H) = 900 m³
• Target ACH: 30
• Calculation: 900 m³ × 30 ACH = 27,000 m³/h
• FFU Requirement: If using FFUs with 2,000 m³/h capacity, you need 14 FFUs.

3 Factors That Influence ACH Selection

1. Personnel Load: More people = more particles = higher ACH needed.

2. Process Activity: High-speed machinery generates more dust than manual assembly.

3. Recovery Time: How fast must the room "clean itself" after a door opening? Higher ACH improves recovery time.

FAQ: Frequently Asked Questions

1. What is cleanroom air changes per hour?
It is a measure of how quickly the HVAC system replaces the room's air with fresh, filtered air. It is expressed as the number of cycles per hour.

2. How do I calculate ACH for my cleanroom?
Multiply the room's length, width, and height to get the volume, then divide your total measured supply airflow (from all diffusers/FFUs) by that volume.

3. Is higher ACH always better?
No. Once you meet your ISO class requirements, additional ACH only increases energy consumption and filter wear without adding value.

4. What is the minimum ACH for an ISO 8 cleanroom?
Most standards recommend a minimum of 20 ACH for ISO 8, though some light-duty electronics labs can operate at 15 ACH.

5. How does FFU quantity relate to ACH?
FFUs provide the airflow. The more FFUs you have, the higher the total airflow, and thus the higher the ACH.

6. Do return air vents affect the ACH calculation?
The calculation is based on supply air. However, return vents must be sized to handle the same volume to prevent over-pressurization.

7. Does the height of the ceiling matter?
Yes. A higher ceiling increases the room volume, requiring more total airflow to achieve the same ACH compared to a lower ceiling.

8. Can I reduce ACH during the night?
Yes. This is called "setback" or "night mode." Many labs drop ACH by 50% when unoccupied to save energy while maintaining positive pressure.

Get your cleanroom math right. Use KLC’s Online ACH Calculator or Request a custom design proposal.