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Contributor
AIR CHANGE RATES IN CLEANROOM HVAC DESIGN
By Ryan Rennie , from the Spada-Rennie Group
“ The reality is that there is no simple way to relate a cleanliness class level to a specific cleanroom air velocity or air change rate because of complex factors to be considered in design and operations . It has been common in practice and in ASHRAE and IEST ’ s publications , however , to use cleanroom air velocities and / or air change rates to quantify the amount of airflow requirements .” - Lawrence Berkeley National Laboratory
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Cleanrooms are specialised environments designed to maintain strict control over particulate contamination , temperature , humidity , and other parameters for various industries such as pharmaceuticals , microelectronics , biotechnology , and healthcare . HVAC systems are the backbone of cleanroom design , responsible for controlling temperature , humidity , filtration , and air change rates . In this article , we focus on air change rates and their critical role in maintaining cleanroom conditions .
Air change rates are the key part of any cleanroom HVAC design . They impact everything . They govern air classification , they impact the air pressure cascade and really importantly they directly proportionally impact the plant capacity and resulting energy usage . One thus understands how important it is to practically minimise them particularly in full fresh air systems . Along with flow pattern , and exchange efficiency , they have farreaching implications on cleanroom performance and cost . The performance and cost eventually determine the rate-of-return on investment for a cleanroom .
Air changes are the primary method of contamination control for both viable ( living ) and non-viable particles or microbes . But they are also a primary requirement for the comfort and safety of a cleanroom ’ s most prized producers : its operators . We do not delve into the testing methods in this article .
There are a number of best practice guides and benchmarks for cleanroom air change rates ( ACH ), many of which reference charts dating back as far as 20 + years . Most commonly referenced are ISO 146144-4 for hourly air changes and air velocity rates . Many of these charts found on the internet reflect a grey interpretation of industry benchmarks , or sometimes data found in ‘ outdated ’ standards . Many of these charts combine results and graphics from the following documentation .
RACA Journal I August 2023
1 . IEST-RP-CC012 2 . ISO 14644-4 : Design , Construction , & Start Up 3 . Fed . Standard 209E ( Outdated ) 4 . ASHRAE
Here is such a chart :
Typically , the design process is based on selecting an air change rate ( a range having been specified during the User Requirement Specification compilation ) and basing the supply air quantity on that value . In static systems , basing the air cascade on an assumed leakage rate through the facility and in a dynamic system , utilising a pressure controlled damper system on the exhaust or return air ( or combination of both ). With the supply air change rate set , the heat and cooling load calculations can be concluded , and the equipment selected and finally the ducting and air registers design concluded .
Practically , we have always found good reason to have a return / exhaust and supply air register in all areas . There has been a tendency for designers to omit return / exhaust air registers when the cascade does not require it ( and save cost in the process ) but we have found that at some point in the future a change to the cascade will be required and having to add return / exhaust air registers can be highly disruptive to the operations of the facility .
In our experience with design , qualification activities and audits carried out by various regulators , the supply ( versus the return / exhaust ) air change has always been the basis of the required air changes . So it was with great interest that the following was noted
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