Airvelocity & Air Change Calculation

  • Air change rates are important factors in determining the design and performance criteria for a cleanroom HVAC system. The total rate, flow pattern, and exchange efficiency have far-reaching implications on cleanroom performance and cost. The performance and cost eventually determine the rate-of-return on investment for a cleanroom.
  • Airflow engineering is the primary method of contamination control for both viable (living) and non-viable particles or microbes. However, it’s also a primary requirement for the comfort and safety of a cleanroom’s most prized producers: its operators.

ISO Definition of Cleanroom Air Exchange Rate

As defined by ISO 146144-4 standards: rate of air exchange is expressed as the number of air changes per unit of time and calculated by dividing the volume of air delivered in the unit of time by the volume of the cleanroom or clean zone.

How to Choose the Proper Air Change Calculation Rate for Your Cleanroom

As defined by ISO 146144-4 standards: rate of air exchange is expressed as the number of air changes per unit of time and calculated by dividing the volume of air delivered in the unit of time by the volume of the cleanroom or clean zone.

How to Choose the Proper Air Change Calculation Rate for Your Cleanroom

First, designers must construct the required classification of the room based on ISO standards for particulate control.

1. Air Changes Per Hour (ACH)
2. Average Airflow Velocity
3. Percentage of fan filter unit coverage

ISO standards outline unique air change calculation methods for unidirectional vs non-unidirectional cleanrooms.

  • In non-unidirectional and mixed flow cleanrooms, use the air changes per hour (ACH) calculation method.
  • In unidirectional airflow cleanrooms, calculate air change with average airflow velocity

The formula for calculating cleanroom ACH:

Air changes / hr = CFM X 60min / Volume of Room

The rate of cubic feet per minute is recalculated into cubic feet per hour, which is then divided by the volume of the room (height X width X length).

Below we’ll outline the basic principles of air change rates, but also provide a more rational outline of standardized air change tables for specific cleanrooms. Most importantly, we’ll help you understand the proper air change calculation rate based on the industrial standards of your facility’s cleanroom.

1. Most non-unidirectional or mixed airflow cleanrooms fall within ISO 5 – ISO 8 cleanrooms
2. ACH calculations must adjust the calculation for ceiling height
3. Applications: Pharmaceutical, health, biosciences, packaging, isolation, handling