New filtration media, technologies, and airflow design strategies are helping users of critical environments improve filtration efficiency and control costs in the face of tightening air-quality tolerances.
By Bruce Flickinger
There was a time that, if critical environment operators wanted to ensure the air in the facility met or exceeded accepted standards of cleanliness, they simply increased filtration efficiency and pumped up the volume of air being exchanged in the room-an approach, in other words, akin to using a baseball bat to dispose of a fly. And though it worked, some wondered whether there might be better ways of getting the job done. The scrutiny of air quality standards and practices that has ensued during the past several years has culminated with the conclusion that cleanroom designers and owners need not overspecify filter performance or airflow rates beyond what is necessary-indeed, often recommended-to achieve high performance cleanroom operation.
Of course, the impetus for and great leveler in these discussions is cost. Air treatment and handling are by far the cleanroom’s biggest cost center. The evidence for this is ample: In a cleanroom benchmarking plan published in 2000, researchers with the High-Tech Buildings Program at Lawrence Berkeley Laboratories (Berkeley, CA) found that up to 40 percent of the operating and initial costs of cleanrooms are associated with air changes per hour in the room. Other published research shows that an additional 10 to 30 percent in airflow supply-that is, increasing the air’s “sweeping” effect in the cleanroom-requires a related increase in fan power of 30 to 120 percent, which would yield some interesting utility bills, to be sure.
As for the filters themselves, the crux of the matter is that the higher the filtration efficiency-the more particles the filters can trap-the more power is required to push air through them. This is referred to as a filter’s “alpha efficiency,” which expresses the relationship between filter pressure drops and particle removal efficiencies. The idea is to maintain or boost efficiency levels of the filter material while lowering its resistance to airflow and lowering pressure drops to the filter. In layman’s parlance, one had better ascertain for sure whether a difference in filtration efficiency of 99.97 percent at 0.3 μm vs. 99.9995 percent at 0.12 μm is worth the air-handling horsepower required to achieve it.
“In many applications, filter manufacturers are being pushed to increase filtration efficiency beyond what was once considered acceptable. At the same time, more end users are becoming aware of the energy costs associated with running their clean air system,” says Aaron Frost, marketing manager for air filtration at Lydall Filtration/Separation, Inc. (Rochester, NH), which designs and manufactures microfiberglass high-efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filter media. “The challenge is finding the right balance.”
How much is too much?
In seeking to strike this balance, researchers are not only bringing technical innovation to the table but also are revisiting established industry and regulatory standards that address airflow and air exchange rates. In some cases, they are finding that the core science might warrant revisiting. A good example comes from pharmaceuticals, where one of the myriad issues being addressed in FDA’s efforts to instill risk-based approaches in its good manufacturing practices is the validation of HEPA filtration and airflow rates. The topic has prompted industry comment, with a key issue being that the requirement that terminal HEPA filters operate or be tested at 90 feet per minute (fpm) is not based on any valid technical basis and in fact could be detrimental.
One observer is Raj Jaisinghani, president and CEO of Technovation Systems, Inc. (Midlothian, VA). In an April 2003 comment submitted to the agency, he notes that requiring 90 fpm through the filter means that in an ISO 5 (Fed. Std. 209E Class 100) environment (typically found in pharmaceutical manufacturing) there will be fewer points of discharge of air in the cleanroom and therefore less ceiling coverage. Having the same air changes per hour with 100 percent ceiling HEPA supply, while resulting in better cleanroom performance by elimination of dead zones in the cleanroom, requires either lower filter velocity or higher air changes per hour, the latter of which entails much higher operating costs and initial costs for cleanroom facilities.
Jaisinghani notes that the ISO 1 or 2 cleanrooms typically found in the semiconductor industry are designed at 75 fpm because higher average room velocities result in turbulence and eddy formation; maintaining rooms or filters at 90 fpm results in much higher turbulence and higher potential for contaminants to be kicked up and distributed throughout the clean space. He further cites published research that states, based on dilution analysis alone, a point of diminishing returns is reached at about 65 fpm. So why should pharmaceutical ISO 5 rooms have to operate at a velocity higher than ISO 1 rooms?
“Filters actually work better at lower velocity than at higher velocity,” Jaisinghani writes. “Filter face velocity has no bearing to cleanroom performance. What is really important is that the room particulate concentrations and airborne bioburden levels are maintained during operation. How one gets there should be up to the cleanroom designer.” As such, validation should be done at the design conditions that have been demonstrated to be effective under operational and performance qualification procedures in the cleanroom. Continuing to base filter testing and performance qualification on what Jaisinghani calls the “fallacy of 90 fpm” is “baseless.”
Frank Austin, president of Clean Air Technology, Inc. (Canton, MI), agrees it is important to design a system that achieves “proper” airflow and rates of exchange, “but also important is the higher initial cost of the filter and the long-term operating cost in electrical power,” he says. “More air pressure means larger motor horsepower and electricity consumption, and a higher electrical bill.” Some semiconductor facilities operate at 50-60 fpm for energy savings but are turned up to 90 fpm when using VFD motor drives when a customer wants to meet an old standard.
“I’ve seen many cleanrooms that have been built without consideration of the larger electrical consumption and the added cost to manufacture a product,” Austin continues. “If you are installing just a few ULPA filters, it will not result in a large increase in electricity. But if you are installing 150 or more ULPA HEPA filters then there should be consideration as to whether these are really needed and will the profit margin accept both the increased initial cost and the higher operating cost.” Energy efficiency is a big-ticket item in Clean Air Technology’s “Design & Build” cleanroom concept.
Filtration experts say that cleanroom users would be wise to re-evaluate existing methods of airflow design. Most notably, “properly designed airflow can reduce the amount of clean air by allowing for clean zones, which in turn allow for lower first cost and lower operating costs,” says Richard Matthews, founder and chairman of Filtration Technology, Inc. (Greensboro, NC).
Furthermore, even as HEPA and ULPA filtration efficiencies continue to rise, some observers are seeing a decrease in the levels of filtration efficiencies that users are requesting. Several years ago, users were interested in filters that offered the highest possible levels of particulate filtration-99.9999 percent or higher. Today there is a much greater focus on understanding the levels of filtration that are actually required in different areas within a cleanroom. Companies are analyzing the levels of filtration that are necessary to improve product quality, yield, and costs-without overspecifying efficiency.
Figure 1. HEPA filter and fan units in a ceiling grid system. Photo courtesy of Clean Air Technology.
A key trend is the move to clean zones in a facility, or “spot cleanliness,” as Matthews calls it, instead of the ballroom approach, where air quality levels are maintained throughout the entire environment. Simply, this reduces the amount of filtration that is necessary. “Especially as we move into nanotech manufacturing, production spaces and production equipment are getting smaller, so the clean spaces are smaller. And cleaner, too,” Matthews says. “This presents a lot of opportunity for flexibility and cost savings.” That is, while the total amount of air in a particular area remains the same, proper use of flexible airflow patterns allows for easy redesign as process layouts change over time.
Beyond airflow design, equipment and filter manufacturers constantly work to extract more performance with less electricity and smaller blowers. Filters create pressure drop in the flow of air supplied to a room, which must be compensated for with blowers and handling equipment. Even a slight decrease in pressure drop pays dividends in energy cost reduction, particularly in reducing fan electricity costs, which is a major expense for most cleanroom operators. Selecting a filter with the required efficiency and least pressure drop is a high priority.
Lydall, for example, offers high-alpha versions of its HEPA and ULPA media, which it says deliver up to 20 percent lower pressure drops than standard media. One example is a newly developed line of filter media called AlphaMaxTM, which is a wet-laid microfiberglass medium that requires reduced force to move air through the filter, according to Frost.
Clean Air Technology’s Austin states that the cost of cleanrooms is being driven by arbitrarily expecting to meet the old, “carryover” standards based on overkill, which were written back when electricity was cheaper, he notes.
“Companies are demanding more efficient cleanrooms and are looking for assistance in lowering operating costs by reducing power consumption,” says Nelson Werkema, president of Clean Rooms International (Grand Rapids, MI), which specializes in designing and providing cleanrooms. “We’ve placed a priority on high-efficiency motors and have been able to reduce amp draw about 50 percent for our standard SAM [supplied air module] units.” The company also offers the Airlink control package, which decreases power consumption by tightly controlling SAM units for specific time periods or workstations within a cleanroom.
Clean Rooms International builds its SAM group of fan filter units inside an ISO 7 cleanroom, thereby preventing outside airborne particles from contaminating the HEPA or ULPA filters and other components used in those units. The company requires its HEPA and ULPA filter suppliers to certify the filters are leak-free and have been tested in accordance with the Institute of Environmental Science and Technology (IEST) recommended test procedures. Each SAM unit is also tested according to certain UL standards before the UL mark is applied.
Efficiency and pressure drop are the two key cost and performance measures of a filter. Several factors determine overall HEPA and ULPA filter collection efficiency. These include gas filtration, velocity, particle characteristics and filter media characteristics. Generally, collection efficiency increases with decreasing filtration velocity and increasing particle size. In addition, the collection efficiency increases as the dust cake thickness and density increase on the filter.
A HEPA filter has a minimum efficiency of 99.97 percent in removing 0.3-μm particles; that is, only three out of 10,000 particles 0.3 μm in size penetrate the filter. The filter achieves this efficiency when the velocity of the air passing through it is 3 to 5 fpm. ULPA filters provide a minimum of 99.9995 percent efficiency on 0.12-μm challenge-testing aerosols.
The standard 99.99+ percent on 0.5 μm or larger particle size-rated HEPA filters is used throughout the world in electronics, aerospace, optics, pharmaceutical, medical, and other manufacturing applications, and is usually acceptable to produce down to an ISO 14644 Class 5. Most filter media now are actually rated at 0.3 μm, Austin notes, “but the older Federal Standard 209E tested filters at 0.5 μm. The older automatic particle counters had statistical variances due to the interference with the wavelengths of light at the 0.3-μm particle size settings on these instruments. The newer laser particle counters are more statistically accurate.”
For the microelectronics industry, the ULPA filter 99.9995+ percent efficient on 0.12-μm or larger particle size is normally used below ISO 5. Austin says ULPA filters should also be used in very strict pharma-ceutical and medical applications and certain biohazard level P3 and P4 biocontainment research facilities.
“Although the manufacturers add additional pleats and filter media, these HEPA filters, under actual usage, are more restrictive for airflow and cfm [cubic feet per minute] volume at same air pressures as the standard 99.99+ percent HEPA filters,” says Austin, who has been involved in cleanroom design and build for nearly 40 years. “In actual design of the cleanroom system, we have found that the initial starting air pressure of the standard 99.99+ percent HEPA filter is approximately 0.5 in. H2O and the ULPA 99.9995 percent initial pressure is approximately 1.0 in. H2O,” plus losses due to duct work and the like. The ULPA filter operates at about twice the pressure drop of the standard HEPA filter.
Depending on their size, particulates are trapped or stick to HEPA and ULPA filters by one of four mechanisms. Sieving, the most common mechanism of filtration, simply stops large particles that are too big to fit through the open areas of the filter. This includes all particles above 5 μm. Inertial impaction works on large, suspended particles that cannot avoid fibers by following the curving contours of the airstream, and thus impact a fiber and are captured. This effect is dominant from the 0.5-μm size region to around 5 μm and increases with diminishing fiber separation and higher airflow velocity.
Direct interception works on mid-size particles (about 0.1 to 1 μm), which follow the airstream as it bends through the fiber spaces and are intercepted, or captured, when they touch a fiber. With interception, particles following a line of flow in the airstream come within one radius of a fiber and adhere to it; particles that are farther than one particle diameter will not be removed by this process. So the denser the media, the higher the probability of particle capture. Finally, very small particles less than 0.1 μm in size are trapped via Brownian diffusion. In the airstream, these particles collide with gas molecules and create a random path through the media. The smaller the particle, the longer it will “zigzag” around in a random motion, increasing the probability of the particle contacting a fiber.
Fibers and frames
The media creating these trapping and holding effects typically are constructed from a non-woven paper or microfiber material. Media makers are researching and incorporating nonwoven components in their products, and nanotechnology is affording new types of materials that either produce smaller fibers or a material with grid-like properties at the submicron scale. New approaches in binder chemicals and processing methods are also being explored in order to bring higher-performance products to the cleanroom community. “We continually monitor new and existing raw materials and implement physical process improvements and sophisticated process controls,” Frost says of Lydall.
Filter media are made in a process and supplied to filter manufacturers in roll form. A polymeric binder material is used to hold the fibers in place. A mixed slurry of these materials is fed onto a web former; drum formers or rotoformers are also commonly used. The water in the slurry is then drained by gravity and vacuum before the wet fiber sheet is dried by passing it over heated drums. The drying not only removes the water in the media but also helps cure the binder so that the resulting sheet of media is suitable for use in filters.
At filter manufacturing companies such Lydall, samples of filter media are tested periodically during production to ensure that the entire production lot meets specifications. “Depending on the application, some of these rolls of media may undergo further performance testing, such as particle collection efficiency at specified flow rate and particle size,” Frost says. The methods followed for these tests are prescribed in IEST-CC-RP-021.2; other recognized test procedures have been established by the American Society for Testing and Materials (ASTM) and the Technical Association of Pulp and Paper Industry (TAPPI). IEST’s Handbook on Air Filtration contains seven recommended practices relating to filtration in cleanrooms and other controlled environments.
Along with the filter media, HEPA and ULPA filters typically comprise a frame, separator, adhesive, and gasket. Most HEPA filters are constructed from a mat of randomly arranged special glass fiber sheet pleated in a “V” pattern like a folded paper fan with corrugated aluminum separators between the folds. Close pleating, however, can cause particulate matter to bridge the pleat bottom, reducing the surface area, so corrugated aluminum separators are often employed to prevent the media from collapsing. This assembly is attached to a base, forming the core of the filter.
The most common designs are a box filter cell and a cylindrical filter cell. In a box cell the pleated media is placed in a rigid square frame constructed of wood or metal. The media in a cylindrical filter cell is supported by inner and outer wire frameworks, with a metal cap sealing the media at one end. Air flows from the outside to the inside of the filter, so a higher airflow rate than a box cell can be achieved because more surface area is exposed. Both box and cylindrical cell assemblies can be mounted directly in the duct or in a separate housing, and both require pre-filtering to remove large-diameter particulate matter from the airstream.
“As a cleanroom design and build operation, we see the finished HEPA/ULPA filter at the time of installation,” Matthews says. “The issue of quality is very important because if you’re this far down the pipeline, a filter failure means it was poorly constructed, or hidden damage occurred during transport or installation.” Matthews’ experience is that faulty filters usually do not indicate a flaw in the media but rather a problem in the filter manufacture, typically where the media is attached to the frame.
Companies such as Filtration Technology deal with a wide variety of end users. “What everyone is striving for is less filter usage,” Matthews says. “With better media, you generally don’t need as many filters as you used to achieve ISO 5 air quality. That used to require around 70 to 100 percent ceiling coverage, and now the same air quality can be achieved with less ceiling coverage. We’re also seeing newer filters that have lower pressure actuals, so you can get a much longer life from them, often beyond 10 years. This is dictated by other design criteria such as exhaust needs, room pressurization needs, cleanroom size and shape, and airflow pattern, as well as the idiosyncrasies of the manufacturing process in the cleanroom.”
Matthews adds, “We’re also seeing newer filters that have lower pressure actuals, so you can get a much longer life from them-often beyond 10 years.”
As indicated earlier, extending filter life is an important consideration on several fronts. Electrostatic charge is one technology being applied to existing filter media to increase the capabilities and life of the particulate filter. Typically, when dust collects on a fiber, it tends to deposit on to the leading edge of the fiber facing the air stream. Applying an electric field can effect a greater loading of dust on the downstream side of the fiber. This approach is still largely in development, although there is cautious optimism about its practical application. Clean Air Technology’s Austin, for one, says that while it would be an advantage to be able to put an electrostatic charge on the HEPA filter in special applications, at minimum cost it would have to enhance the performance by some significant, statistically measurable amount, and the filter manufacturer would need to provide the actual operating cost besides the cost of the enhanced feature.
“There are many electrostatic filter systems that use grids rather than a direct interception method of trapping airborne particles using filter media, but in the cleanroom industry they are used primarily for a pre-filter system because of the open grid technology,” Austin says. “The efficiency range from 35 to 90 percent ASHRAE would be a possibility. The advantage would also be realized in more efficient air pre-filtration without adding more static air pressure and resulting electrical energy cost, assuming that the electrostatic feature does not add significant electrical energy usage.”
Along with dust-load capability, static precipitation technology is being advanced as a means of advancing filter life and reducing filter maintenance. This is an important gain, particularly for pharmaceutical, medical device, and other regulated companies that do not relish having to change their HEPA filters until they are sufficiently loaded and airflow is restricted to below their operating criteria. “Besides cost, this disrupts production so a scheduled downtime is required, followed by recleaning, recertification, and revalidation,” Austin says.
Air ionization, which already plays an important role in applications such as removing static electricity and preventing the spread of viruses and microorganisms in health-care settings, could also have use in critical environment manufacturing. By creating a large supply of balanced positive and negative ions, proponents say ionized air helps to reduce air contamination: Ions clinging to dust particles will weigh down the particles, slow their speed in the airflow, and cause them to stick to the filter media.
While such technologies are intriguing, some note that “end users understand and place a high value on guaranteed performance,” Frost says. “Microfiberglass filter media use the time-tested, proven performance of mechanical filtration. Unlike filters produced with media that rely on an electrostatic charge to remove particles from an airstream, there is no danger that filters constructed with wet-laid microfiberglass will lose collection efficiency as the result of charge decay.”
Right tool for the job
Technology and design improvements are continuously being made by suppliers of filter media, filter assemblies, air handling systems, and even turn-key cleanrooms. The focus of these efforts has shifted in recent years to providing efficient, “right-sized” air management solutions that account for both escalating energy costs and more stringent air-quality standards. Demand is coming from traditional operators of critical environments as well as other manufacturing sectors that are recognizing the impact that proper air filtration can have on their productivity and profitability.
“We have experienced growth beyond our core markets in pharmaceuticals, semiconductors, and electronics, and I think that will continue as those industries see the value of cleanroom technology to control airborne contaminants in their operations,” Werkema of Clean Rooms International says, naming aeronautics, health care, automotive, and plastics manufacturing as growth industries. “Their ultimate goals are to increase product yield and project to their customers a level of confidence in processes and supplied products. HEPA and ULPA filtration within a cleanroom accomplishes both goals.”
Resources and contacts
Clean Air Technology, Inc.
41105 Capital Canton, MI 48187
Clean Rooms International 3718 Buchanan Ave. SW
Grand Rapids, MI 49548
Filtration Technology, Inc. P.O. Box 18168 Greensboro, NC 27419
Lydall Filtration/Separation, Inc. 134 Chestnut Hill Rd.
Rochester, NH 03866-1960
Technovation Systems, Inc. 13511 East Boundery Rd. Suite D/E
Midlothian, VA 23112-3941