By Morgan Polen
A disturbing trend has been noted for several years, enough that I would like to ask contamination control users worldwide about high-efficiency particulate air (HEPA) and ultralow-penetration air (ULPA) filter life duration and replacement frequency.
Companies report replacing HEPA filters frequently. One company recently reported filter failure after one year. And in one country a local regulation requires filter replacement every three years.
Are you or anyone you know experiencing short HEPA filter life?
When I first got involved in cleanrooms in the 1980s, terminal HEPA/ULPA filters lasted for the life of the facility. Some of the facilities I was involved with in the 1980s are in operation today – some with the original HEPA filters. In fact, an aerospace cleanroom with unidirectional airflow built in the 1960s and still in operation has replaced filters only twice. (Upstream air handling systems with good filtration are really important). It’s always been my understanding that you replace filters if they are damaged, leak, and you don’t want to fix them. For instance, the pressure differential or the uniformity of velocity is too great across the filter or the filter can’t deliver the desired airflow. What, if anything, has changed?
Next month, IEST will provide HEPA filter and testing training. Please consider joining a panel of experts on Nov. 16 as they discuss real-world HEPA and ULPA filter testing and problem scenarios. Many of the panelists worked on IEST-RP-CC034.5: HEPA and ULPA Filter Leak Tests, a recommended practice developed and first published in 1999 by IEST – and just revised in July. The panelists have more than 100 years of combined experience with filter leak testing. Learn more about attending IEST’s HEPA Filter and Testing Workshop in person or virtually.
Morgan Polen is the principal consultant for Microrite, where he uses his 30 years of experience working in cleanroom design, construction, validation, monitoring program development, particle counter design and product management for cleanroom-related products and systems.
Thank you mark for you input. We hope to have a good session on HEPA filters!
Testing filters esp too long using fine PTFE filters incl with too much aerosols based on oils or PSL particles, can lead to partial plugging and lower airflow.
Also wanted to add: We do not do filter challenges/testing with PSL spheres. This can be an overload to filters and definitely shortens their life. We have found using the ambient air (incoming background) to be a sufficient challenge to testing the filters for leaks after installation.
Samuel, thank you for your reply. When you use the ambient air upstream of the HEPA filters, do you remove the pre filters? Do you measure the challenge aerosol concentration? I have heard of people doing this so my question is do you have enough of a challenge upstream of the filter to detect any leaks?
Hi, Morgan. In most cases we find that we don’t have to remove the pre-filters (golf ball filters) to ensure an adequate challenge. We do measure the concentration of the ambient to confirm that it meets a minimum. We have tested this on filters with known leaks and filters known to not have leaks, to ensure that it is an effective test. We’ve been very happy with the results.
Hi, Morgan! I have not experienced any HEPA or ULPA filter failures in our facility in recent years, some of which are over 25 years old. Our experience has been similar to what you mention – lasting the lifetime of the cleanroom. We do have upstream filtration in the make-up air handling units, so I know that helps. The only new HEPA’s we have installed in recent years have been with fan/filter units that are being used in modular labs. We have not had any failures in these, which have been in place 3-10 years.
HEPA and ULPA filters can fail for many reasons incl plugging if poor prefilters used, cracking of potting compounds, separation of potting compounds from frame, leading to leakarounds, HF (hydrofluoric acid) attack on borosilicate glass ULPA filters, swelling of gel seals due to solvents in air, depolymerization of gel seals incl silicones incl due to acid exposure. For semiconductors, organo phosphorus compounds incl from potting compound fire retardants, gel seals, can outgas and lead to wafer doping by P and HF attack of borosilicate can put Boron in air as BF3 that dopes wafers. Silicone sealants can outgas and ruin optics, esp UV optics and corona ionizers can get coated with SiO2. Warped grids incl due to building settling can have leak arounds develop, FFU motors can fail or outgas too much. Just a few failure modes I have heard of or encountered over 30 years working with Semiconductor, disk drive, laser, aerospace fabs, labs, etc. If amines are ever used, they can condense with acids and plug filters with ammonium salts. Some urethane catalyst (Sn, Bi…) might have volatility that could be of concern in rare applications. IEST RP12 on cleanroom design might also cover some of these considerations, and IEST RP35 on Cleanroom design for AMC control covers some of the Airborne Molecular Contamination issues.