www . refrigerationandaircon . co . za RACA Journal I August 2023 47
Contributor in the ISPE Good Practice Guide for Heating , Ventilation and Air
Conditioning on Page 280 Appendix 13 : “ Air Change Rate The number of times the total volume of a defined space is replaced in a given unit of time . This is computed by dividing the total volume of the subject space ( in cubic volume ) into the total volume of air exhausted ( or supplied to ) the space per unit of time .”
Final cleanroom ACH requirements are largely determined by contamination levels . A cleanroom built at the low end of air change benchmarks may be adequate when the cleanroom requires few operators during relatively clean processes .
Facilities account for particulate expectations by extending air change rates , or sometimes with built-in , variable airflow systems . Some cleanroom facilities opt for demand-controlled filtration systems which optimise air recirculation based on real-time particle counts . Likewise , variable drive-controlled fans are programmed with parameters for preset adjustment in the case that a cleanroom is vacant for extended periods . Because cleanrooms generally remain powered 24 / 7 , throttling back fan drives reduces overall energy cost .
In nearly every example , the air change rate is established by a low and high range for a simple reason … it ’ s not an exact science . The high end ( 48 ACH ) of an air exchange rate in an ISO 8 cleanroom is nearly nine times cleaner than the low end ( 5 ACH ) of 146144-4 standards . This doesn ’ t provide a lot of clarity for contractors or project leads when budget , product safety , or operating cost is of paramount importance .
Air change tables suggest a high and low range because these tables do not account for complexities of achieving a final cleanliness level in a specific cleanroom . Further , some facilities may over-spec their air system for more favourable temperature and humidity control . However , in most cases , facilities build toward the lower end of the threshold to maximise air efficiency . In South Africa we work on 20 ACH for ISO 8 cleanrooms with almost no further consideration . Nonetheless , air change tables allow a gross estimate which is a significant factor in the overall cost per square metre .
The cleanroom airflow pattern and HVAC design will determine which air change rate calculation is appropriate . Consideration of unidirectional or non-unidirectional airflow , turbulence , eddies , processes equipment , and pressure
differentials all influence air exchange rates as exiting and returning air interact throughout the cleanroom .
An aggressively designed airflow rate requires a more powerful HVAC system , more air distribution components and extends overhead operating costs . However , an oversized air handling system easily mediates air demand when production scales and the facility require a cleaner classification or involves expanding the number of cleanroom suites .
A cleanroom design incorporates the analysis and deliberation at every intersection between manufacturers , architects , engineers , HVAC specialists , and facility operations staff . Air change tables often provide points of inference and consideration but have no traceable technical basis for contamination control . Initial estimates of air exchange rates must reflect final conditions and adjust for less obvious parameters such as the offset of heat gains , air leakage , and recovery rate .
Air change tables are important tools during initial discussions . However , efficiency and cleanliness have far more elusive parameters than square metres , average airflow velocity , or hourly air change rates . Experienced cleanroom designers understand that other factors such as proper fan size selection , area coverage , and room design have serious consequences if not considered early . A cleanroom system requires many co-dependent evaluations and preliminary decisions for a strong foundation .
CONCLUSIONS The fundamental goal of each cleanroom is unique to its process , and so is the science behind its design . For most facilities , speaking with a controlled environment specialist is the fastest path from clean idea to a clean build .
We are reminded that the guidelines are just that : guidelines that are there to assist designers to achieve results that are predetermined to manage the air quality risks for the facility . A wellconsidered design that is cost effective and practical to implement and maintain is the desired result for any designer .
The intention of this article is not to provide a detailed process for selecting or designing required air changes for a cleanroom but to highlight the confusions in the industry . And certainly , to conclude , despite the reference to the ISPE Good Practice document above , that it is our finding to base the air changes on the air supplied to a room ( which should be equal to the air exhausted from the room + air lost ( or gained ) due to leakage between rooms ( pressure cascade ). Calculating ACH using air exhausted from a room may give one a lower rate of change than air supplied to a room . In our opinion , ACH using air exhausted from is not the correct reflection of ‘ actual ’ ACH . Others , of course , may have a different opinion . RACA
www . refrigerationandaircon . co . za RACA Journal I August 2023 47