R. Vijayakumar, Ph.D., Director of Market Development, TSI Incorporated, and Chair, IEST Working Group CC001, HEPA and ULPA Filters
In most cleanrooms, the final step in removing airborne particles occurs in the HEPA or ULPA filter that’s delivering air into a controlled space. Whether the cleanroom attains and maintains its designed cleanliness class depends largely on the performance of these filters. Hence, it is a common and good practice to test the performance of all filters installed in cleanrooms to ensure that they meet the designed specification.
Filters are typically tested as manufactured for overall efficiency and leaks. Furthermore, in many cleanrooms within regulated industries, such as the pharmaceutical or nuclear industries, these filters are also required to be certified periodically, to ensure acceptable performance during their service life. IEST publishes three recommended practices (RPs)-IEST-RP-CC001.4, HEPA and ULPA Filters; IEST-RP-CC007.1, Testing ULPA Filters; and IEST-RP-CC034.2, HEPA and ULPA Filter Leak Tests-that provide guidelines for testing and classifying these filters.
RPs 001.4 and 034.2 have been revised, and new versions were released in 2005. IEST-RP-CC001.4 is devoted to classification of filters based on their efficiency and test method. Highlights and revisions in this version of the RP are addressed in this month’s column. The other, IEST-RP-CC034.2, is devoted to leak or integrity testing of filters, and will be discussed next month. The third RP deals with testing ULPA filters and was addressed in an article written by Phil Winters titled “IEST updates and improves its recommended practice for testing ULPA filters” (CleanRooms, June 2005).
First published in 1986, IEST-RP-CC001 has been the most widely used document in the HEPA filter industry. It has been revised regularly to keep it relevant to current industry practice. It was also one of the first RPs from IEST to be considered as a Canadian standard. Currently, several agencies, including the FDA and the National Science Foundation (NSF), rely on the filter classifications in this RP as part of their operating guidelines and recommendations. In its fourth and current revision, this RP remains current and compatible with the state of the art in the industry.
The main goal of this revision is to make the filter classification compatible with the common test methods used by the filter manufacturers. Further ambiguities and errors in the definitions have been addressed, and significant information on several related topics has been added in the appendices.
What’s new (and improved)
The number of filter types has been increased to 11 from the seven in the older version. The new filter types (Types H, I, J and K) are based on filter testing using a particle counter, and complement the original filter types (A, B, C, D and E) that rely on photometric test methods. Although no equivalence between the methods has been established, the new filter types should assist the manufacturers that test filters with one or the other method. The requirements have been set stringent enough that the filter types in the RP based on either of the test instrumentations should be acceptable for the end user seeking a filter of a defined performance. The different filter types and their test requirements are shown in Table 1.
Table 1: Recommended test and minimum rating for filter Types A through K (from IEST-RP-CC001.4).
Most technical documents and standards use special terms and abbreviations; this RP is no different. Over the years, familiarity with many of these common terms has meant that errors in their definitions have gone unnoticed. This revision has taken great pains to address many of these errors, which include:
- The 0.3 μm mean particle commonly assumed for HEPA filter testing refers to the mass median diameter measured by photometers. The size of these aerosols intuitively understood (i.e., count mean diameter) is really quite a bit smaller and closer to the most penetrating particle size (MPPS) for HEPA filters. Since the efficiency of filters varies greatly with particle size, these new terms have been defined in an attempt to elaborate and clarify the differences. Count mean diameter (CMD) is the average particle size of the number distribution of the aerosol. Mass mean diameter (MMD) of the aerosol is the average particle size of the mass distribution of the aerosol. Typically, since the mass of a particle varies with the cube of its diameter, most of the mass of an aerosol tends to be in the larger sizes, resulting in the mass mean being larger than its count or number mean. Understanding the difference is significant since the filter efficiency is size-dependent (i.e., a filter will have a higher efficiency when tested with 0.3 μm particles than when tested with a 0.3 μm MMD aerosol, which has a CMD under 0.2 μm). The filter types listed in the RP explicitly take this difference into account, thereby eliminating the confusion surrounding the subject.
- Polyalphaolefin (PAO) is used as an alternative to dioctylphthalate (DOP) in filter testing. It should be noted, however, that not all PAOs or DOPs are acceptable. Only PAO with CAS # 68649-12-7 and DOP with CAS # 117-81-7 will result in the appropriate particle sizes and, hence, are the only ones allowed.
- Particle size distribution of aerosols from common generators is noted in an effort to eliminate any confusion regarding the merits of using one or the other type of generator.
- Although historically MIL-STD-282 defined the methods for HEPA filter testing and, by implication, its classification, several alternate methods for testing HEPA filters have come into common practice in recent years. Among these are the HFATS (High Flow Alternative Test System) and ASME AG-1 test methods, as well as the methods described in the European standard EN1822.
- HFATS was developed by the Los Alamos National Labs under contract to DOE. The HFATS provides a test method that is equivalent to and somewhat less restrictive than MIL-STD-282. The HFATS took advantage of particle-sizing instruments, which are capable of providing information regarding particle-size-specific filter performance. It also allowed the use of liquids other than DOP and at a much lower concentration to test the filter. The major drawback was the increased amount of time to test a filter, as well as the increased inherent variability associated with the particle-sizing instruments.
- In 1971, the ANSI N45.8 committee was formed to develop standards for high-reliability air-cleaning equipment used in nuclear facilities, as well as the corresponding tests to confirm performance of the equipment. This committee was subsequently reorganized as the American Society of Mechanical Engineers (ASME) Committee on Nuclear Air and Gas Treatment (CONAGT). The committee developed the ASME AG-1 Section FC, Code on Nuclear Air and Gas Treatment, and includes codes and standards for design, fabrication, inspection and testing of air-cleaning and air-conditioning components and appurtenances, as well as air-cleaning components used in engineering safety systems in nuclear facilities. In particular, it covers the design, operation and testing of all HVAC system components. As a result-and under the mandate from DOD to eliminate Mil-Standards-these were transferred to AG-1. The previous relevant filter standards for nuclear applications-MIL-F-51068, Filters, Particulate (High-Efficient Fire Resistant) and MIL-F-51079, Filter Medium, Fire Resistant, High-Efficiency-were integrated into the sixth edition of ASME AG-1 Section FC.
- EN1822 is the European standard that combines filter classification, testing of filter media, overall filter efficiency testing, and leak testing into one document. Although no direct equivalence between the standards is possible, they have many similarities as well as several key differences. Chief among these differences are:
- Filters with efficiencies as low as 85 percent (H10) at MPPS are nominally termed HEPA filters, as opposed to IEST’s traditional definition of HEPA with 99.97 percent efficiency at 0.3 μm MMD thermally generated DOP.
- Filters are tested at the MPPS, while the IEST allows tests to count mean size well under 0.2 μm (0.3 μm MMD) or close to the MPPS of typical HEPA filters.
- EN1822 utilizes single or discrete particle-counting instruments for all aspects of testing filters with efficiencies of 99.95 percent at MPPS and greater, while IEST RPs allow the user the choice of either photometric (multiple-particle detection) measurement methods or discrete particle-counting methods for efficiency and, in most cases, for leak testing.
- Unlike IEST, EN1822 allows for the computation of filter collection efficiency based on the particle count data collected during the scan test. The IEST RPs require two separate tests.
This RP, much like other standards, strives to keep abreast of current practices. How well it delivers on this promise depends to no small measure on the input of the many professionals in the contamination-control industry. Suggestions and input on current practices are always welcome by the author and IEST. Better yet, come join the working group (WG) deliberations and share your insight into this and other key topics addressed by the many RPs published by IEST.
Without the input, urgency, and lively debate among the members of the working group, neither the timeliness nor the quality of this RP would be possible. As chair of the WG, I thank them all for their support. Jim Wagner’s quick review of this article is also much appreciated.
Dr. Vijayakumar holds a Ph.D. in particle technology from the University of Minnesota and has over 20 years of experience in filtration, filter testing and air quality. He has held senior technical and marketing positions in leading filter, filter media and particle instrument companies, where he has assisted customers worldwide with their filtration and filter-testing problems. He is an instructor at the University of Minnesota’s Short Course on Filtration and has also lectured worldwide on the subject. Currently, he is the director of market development at TSI Incorporated and is responsible for developing new markets and applications for the company’s test instruments. Dr. Vijayakumar chairs IEST’s working groups 001 and 034, and is acting chair of several other working groups. He also held the position of Chair of Standards and Practices from 1997 to 2001. He can be reached at [email protected]