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Air Filtration Systems Introduction In critically controlled environments the sources of product contamination are: 1. Personnel 2. Air 3. Equipment and materials used in manufacturing Air is the second largest source of contamination. Contaminants in the air can be divided into two main groups they are those in solid phase and those in liquid phase. Solid phase contaminants are soils, minerals, metals, fibers, synthetic materials, biological materials like skin cells, hair, bacteria. Contaminants in liquid phase can include sprays of all kinds, condensed vapors and chemical vapors. The main method for prevention of air borne contamination in production areas is air filtration. FILTER CLASIFICATION: Air filters retain particles by various collection methods. According to the type of collection method used air filter can be classified as sieving or dynamic. Sieving method: The sieving filter use fibrous media with a porosity smaller than the retained contaminant which is considered the most elementary type of Air filtration. The retentive action also occurs in all the filters operating under the dynamic principle. Because of the high operational cost and limited applicability of Sieving filters, new filters use this collection method exclusively. Page 1  Air Filtration Systems Dynamic method: The filtration mechanism of dynamic filter involves a combination of various effects including the following:  Inertial Impaction  Direct Impaction  Diffusion  Electro Static Forces Page 2  Air Filtration Systems Inertial Impaction: When the air flows and encounters randomly oriented obstacles, particles travelling in the air stream must change direction if they are to follow the stream. Because of inertia forces this can’t happen quickly enough and the particle impacts a fiber in the fiber matrix. The effectiveness of the inertial impaction is increased by increasing the air stream’s velocity and by decreasing the fiber’s diameter. This effect is more evident on particle larger than 1 mm. Direct impaction: The particles of negligible mass are likely to be trapped not by inertial impaction but by the action of Electrostatic forces or direct impact. The efficiency of this method varies inversely on particles with diameters in the range of 0.5 to 1 micro meters. Diffusion: Retention by diffusion takes place with very small particles in the range of 0.2 to 0.3 micro meters. The particles are captured by taking advantage of their disorganized motion (Brownian motion), and retention effectiveness is a function of Avogadro’s number, the air velocity and the particle diameter. As the particle move in a disorganized fashion, they are trapped by the randomly oriented micro fibers that constitute the filter. Electrostatic Forces: Particle can be retained by controlling the polarity of the filter medium using the action of electrostatic forces. These attractive forces vary inversely with the air velocity and the dimension of the particle. To select a filtration system using the appropriate collection methods, it is necessary to evaluate the level of air borne contamination surrounding the manufacturing facility as this will have a direct Page 3  Air Filtration Systems effect on the particle burden to be controlled by the filtration system. It is also necessary to determine the operative characteristics of each filter. EVALUATION OF AIR FILTRATION SYSTEMS To select the proper filter system the following operative characteristics of each filter must be evaluated. 1. Airflow resistance 2. Collection efficiency and rasting method 3. Service life 4. Arrestance 1. Evaluating Airflow Resistance: The optimum filter should provide the highest efficiency with the lowest resistance to the airflow. This is a fundamental consideration in times when energy costs are high, since at higher airflow resistance, more power is required to move air through the filter and the system. Airflow resistance is measured in inches of water column height by using a water manometer. The instrument has two ports, one located upstream from the filter to be tested and the other downstream. The reading will show the difference in static pressure from one side to the other indicating the total resistance to the flow of air. This is also known as pressure drop. The pressure drop is proportional to the efficiency of the filter and to the airflow pressure. The smaller the particle i.e, intended to be filtered out, the higher is the airflow resistance. The same occurs with airflow volume, as the airflow volume increases for a given filter, the pressure loss increases. The airflow volume and the filter efficiency together determine resistance to air flow. This is important for the selection of the blowers and air handling equipment. 2. Evaluating filter efficiency: Page 4  Air Filtration Systems Air filter efficiency is described by the percentage of particles that are retained by the filter, called the filter’s collection efficiency. Different tests and methods for filter efficiency evaluation have been developed. No one test is applicable to all filters. It is important to know which filter is being used before selecting the method for evaluation, since the methods of testing efficiency are directly related to the type of filter media used. The method can be divided into three basic groups, depending on the type of challenge, presented to the filter. These methods are as follows:  Granulometric Method: Methods that use synthetic dust composed of precise mixtures of different types of particles of different sizes, known as Granulometric Methods. They are used for filter employing the sieving effect, with the capability of retaining particles in the range of 15 – 20 microns. These filters are used as a primary filtration stage in commercial and industrial heating, ventilation and air-conditioning. An example of this method is the AFI test.  Colorimetric Method: Method that use atmospheric air without any artificial addition of dust such as the ASHRAE test, which is a colorimetric test used to determine the efficiency of intermediate air filters with capability of retaining particle in the range of 2 – 3 microns. These are the filters used as an intermediate filtration stage, typically as pre-filters for high efficiency filters.  Photometric method: Method employing an aerosol with particles of uniform size and weight, such as dioctylphthalat (DOP) test, which uses a photometric detection device. This test is used for high efficiency filters that are capable of retaining particles in the submicrometer range (0.3µm and larger). These filters are used as a final filtration stage in aseptically controlled environments. Filter efficiency evaluation tests: Test Challenge Filter type Filter material AFI (granulometric) Dust Low efficiency Natural fibers, metal, Page 5  Air Filtration Systems nonwoven fibers ASHRAE (calorimetric) Atmospheric Medium efficiency Fiber glass, synthetic fibers air DOP (photometric) DOP aerosol High efficiency Microfiber glass 3. Evaluating service life: The life of a filter is directly proportional to its capacity to retain contaminants. At the time a filter is selected, there is no way to forecast accurately how long it will be useful. Most filtration systems are set up in stages to extend the life of the higher efficiency filters. For example, environments with no stringent air quality requirements, such a offices and laboratories, commonly use a two stage filtration system. This consists of one rough filter (metallic or similar) capable of retaining large particles 15 micrometer and larger (90% AFI efficiency) and a second filter capable of retaining particles 4 micrometer and larger (85-90% ASHRAE efficiency). Nonaseptic controlled environments typically have three stages: a primary filter (90% AFI), an intermediate filter (90-95% ASHRAE), and a final stage high efficiency filter (95-99.97% DOP efficiency). Aseptic controlled environments usually have three stages consisting of a rough (90% AFI), intermediate (90-95% ASHRAE), and final HEPA filter 99.97% DOP minimum and up) discharging directly into the controlled environment. As a filter become dirtier, the passage of air becomes more difficult, or the pressure drop increases. The remaining service life of a filter is predicted by monitoring this pressure drop increase. With adequate prefiltration (ASHRAE 85-95%), the service life of a final or high efficiency filters can be extended to 3 years of continuous use. Filter service life will vary depending on the source of incoming air. The typical life expectancy for an intermediate bag filter with 100% new air in an urban metropolis is approximately 3-6 months. Primary filters are normally washable so they can be used indefinitely. 4. Evaluating arrestance: Arrestance is defined as the capacity of the filter to retain dust. The filter design is the factor that determines the arrestance of the filter, and this generally is a function of the amount of filter Page 6  Air Filtration Systems medium used and the way the medium is pleated. The more medium a filter has the higher its dust loading capacity, or arrestance. The higher the arrestance, the more dust the filter can retain before the pressure drop buildup is unacceptable, and the longer the service life of the filter. High arrestance is the typical design objective for intermediate bag filters and for accordion pleated intermediate and final, or high efficiency filters. PREFILTERS Fresh outside or recycled air must first be filtered to remove gross particulate matter. A spun glass, cloth, or shredded polyethylene filter may be used for this preliminary cleaning operation. HEPA and ULPA filters require pre filtering to remove large particulate matter or for dust concentrations greater than 0.03 gram/cm2. Pre filtering may be performed in several stages. Mechanical collector such as cyclone or venturi scrubbers may be required to reduce large diameters particulate matter. Standard baghouse or catridge are required to filter out particulate matter greater than 2.5 µm in diameter. Some times more than one pre filter may be required in series, the first of quite large and next Page 7  Air Filtration Systems of somewhat smaller pore size to provide a gradation of particle size removal from heavily contaminated air. Pre filter are manufactured by using synthetic non woven poly ester fabric material. These filters are also manufactured using Aluminum or Stainless steel frame and high density washable poly ethylene mesh. HIGH EFFICIENCY FILTERS HEPA filters are defined as 99.97% or more efficient in removing from the air 0.3 µm particles size and larger. To achieve class 100 conditions HEPA filters are required for the incoming air, with the effluent air sweeping the downstream environment at a uniform velocity, normally 90-100 ft/min ± 20% along parallel lines (laminar air flow). A class 100 clean room is defined as the room in which the particle count in the air is not more than 100 per cubic foot of 0.5 µm and larger in size. HEPA filter design The HEPA filter is a disposable, extended media, dry type filter in a rigid frame. It has the capacity of retaining particles as small as 0.3 microns. The filter medium consists of extremely fine (0.1 µm diameter) glass fiber. The mediums glass fiber content is 99%, leaving 1%for binders. Most of the micro glass fiber media on the market have fire retardant and water proof properties. This superfine glass medium gives the HEPA filter distinctive filtration capabilities. It allows the retention of small particles due to the principle of Brownian diffusion, and provides the interception by inertial effect of particles of intermediate size (impacted against the fiber by means of sudden changes of air directionality). Finally it provides a sieving effect for large particles, the most elementary form of air filtration. Page 8  Air Filtration Systems New generation of ultrahigh efficiency filters are now available with 99.99% retention of particle at the 0.1µm level. The current designation of those filters is ultra retention particle filters (UPLA) and very efficient particle air filters (VEPA). HEPA FILTER CONSTRUCTION To form a HEPA filters, the filter medium is pleated in an accordion-like fashion. Every pleat is spaced, by use of corrugated aluminum or paper separators, paper ribbons, strings (of glass or urethane foams or hot melt adhesives), or paper dimples. Medium and separators are then bonded to a frame with an adhesive. The separator prevents the narrowing of the air passage due to the expansion or other movement of the filter medium. This allows the maximum air flow with a minimum of pressure. Thus, the basic components of a HEPA filter are: 1. Frame 2. Filter medium 3. Separators 4. Adhesive 5. Gasket Page 9  Air Filtration Systems HEPA filter  Frame: Frame selection is based on filter application. Possible frame materials are particle board, plastic and metal (galvanized or stainless steel or anodized aluminum). Particle board has been widely used by the industry with good results, although there is some concern about the possibility of particle release from this material. Generally, if the filter passes the DOP test and, once installed, provides class100 air, it can be assumed that the filter will operate safely during its service life. Normally, only defective frames, regardless of the construction material, represent risk of contamination metallic frames are typically free from particle emission. However, mechanical stresses induced in the metal frame by changes in temperature or by mechanical flexing could stress the filter medium, the adhesives, and rest of the components to react, with potential loss of integrity. Plastic frames have only been introduced recently. Some of the electrostatic properties of plastic materials need to be considered due to the attraction, retention, and possible quick release of settled particles along the frame .Fire resistance is a consideration. Frame selection, in general, is based on chemical or fire resistance, since in normal operation any one of the above mentioned materials offers satisfactory results.  Filter medium: Most HEPA filter media in use today are manufactured with glass microfibers. Air velocities through the media are in the range of 5-12ft/min. Media can be made to sustain temperatures from 4 to 2500c.  Separators: HEPA filter corrugated separators can be made of heavy Kraft paper, aluminum alloy, plastic, or fiber glass. Because of their mechanical resistance, aluminum separators show surface particles contamination more easily than do Kraft paper separators. Recently, mini pleat and “separator less” filters have been introduced the latter uses the glass media molded to produce a corrugated effect, or achieves this effect by molding a dimple. The others use glass paper ribbons (Astrocel II) or urethane Page 10  Air Filtration Systems foam (vecopleat 90) or a glass string (filtra). The elimination or reduction in the number of separators increases the amount of medium, thereby increasing the amount of airflow per ft 2 of filter. This reduces the amount of space required to process a given air volume.  Adhesives: Several types of adhesives are used in HEPA filters to bond the frame to the glass medium. The ideal adhesive is one that has a high solid, low solvent content. Any adhesive that has the potential of leaving solvent or air bubble trapped could in time create a leakage problem not evident at the time of the factory testing. Before filter testing, it is necessary to verify if the adhesive has cured, meaning that the solvent has evaporated. Hot melt adhesives represent one of the best alternatives for filters operating at temperatures below 1500 F. Polyurethane foams also represent a good alternative, since their solid content is high. In foam, cell control and the control of mechanical resistance are important. Some foam will dry with time, reducing their mechanical resistance and releasing particles due to impact and air erosion. Silicate adhesives, generally used in high temperature filters, should be carefully selected to assure no particle release with time.  Gaskets: Most of the gaskets currently in use with HEPA filter are made of closed cell neoprene foam. Alternate gasket materials, such as Teflon or molded urethanes, should assure resilience that is equivalent to or better than that of closed neoprene. Substitutes for gasketing materials have developed. They consists of use of channels filled with viscous, jelly like materials that are penetrated by a protruding extension foot of filter frame. The nature and shelf life of the gel are an important consideration. Improper mixing ratios of gel components could lead to dimensional and viscosity changes that compromise the seal integrity. HEPA FILTER TESTING Page 11  Air Filtration Systems HEPA filter testing method uses a challenge of mono disperse aerosol of DOP of 0.3 µm diameter particles. To assure a uniform aerosol particle size and concentration, special devices are used to generate the aerosol. To determine the efficiency of the filter, aerosol uniformity is required. To achieve this uniformity the aerosol must be generated by vaporizing DOP and then condensing it to the desired size in a quenching chamber. For determining the integrity of the HEPA filter in situ, cold aerosol generation, or polydisperse aerosol, is used. The intent of this method is to show points of leakage, but not efficiency, since no constant particle challenge is provided.  Efficiency testing (Hot DOP test): The hot DOP test assesses the efficiency of the HEPA filter in retaining articles of 0.3 µm. HEPA filter efficiency testing is used mostly by manufacturers of the filter media and by manufacturers of the finished filter in determining medium and filter efficiency. Efficiency is tested by controlling size 3 and concentration of the challenge aerosol at the rated air flow ( ft /min ; liters/min ). An aerosol photometer is then used to measure aerosol concentrations upstream and downstream to obtain the difference. The photometer principle of operation, used for hot DOP and integrity testing, consists of a chamber containing a photocell and a light source (with the photocell shielded from the light source). Once the filtrated aerosol is introduced into the chamber, usually at a rate of 1 ft 3 /¿ min, particles present in the aerosol retract the light and are detected by the photocell. The unit is calibrated to a voltage representing 100% concentration of the aerosol (80-120µg/liter in the case of the filter integrity test). The change in electrical impulse (from the 100% concentration) is registered and the difference expresses the percentage of penetration. For example, a HEPA filter with a collection efficiency of 99.97% has a penetration equal to 100%-99.97%, or 0.03%. Page 12  Air Filtration Systems  Integrity testing: A similar test is used for verification of the filter integrity (leak testing or pinhole detection), but this test differs from the efficiency test by the size, distribution, and volume of the challenge aerosol, thus reducing the complexity of the aerosol generator an increasing the portability of the photometer employed. The aerosol in this case is generated by mechanical means; pressurized air is pumped into a vessel containing room temperature DOP, and the pressurized mixture is then released through a nozzle (laskin nozzle) with a calibrated orifice. The concentration and particle size of the DOP aerosol will vary. The content of 0.3 µm particles could range from 20% to 30%, and the rest of the particles could be larger or smaller. This type of test is used to assess the integrity of the filter medium, the filter housing, the adhesive used in framing the filter, and the gasketing of sealing devices. ULPA FILTERS ULPA stands for ultra low particulate air. ULPA filter are defined as 99.9995% efficiency for the removal of 0.12µm diameter or larger particulate matter. ULPA filters are available for class 10 particle requirement. ULPA filters even have higher rating and filtration effect than HEPA filters and are some of the highest grade filters around for removal of ultra fine particulate matter from the air. However, because the ULPA material is denser than that of a HEPA filter, air flow is more restricted means less air circulation. ULPA units do not necessarily perform as well over all as HEPA units in providing particle free air as they may not change the air in a room as frequently as a HEPA unit, due to their restricted air flow. A better ULPA unit would compensate for the lower air flow by providing a large surface area of filter media. It is also critical for ULPA filter to be well sealed as leaks can compromise the draw of air through the filer media. The ultimate ULPA filters are known as SULPA filters- super ULPA. These have a higher efficiency rating. Page 13  Air Filtration Systems CONCLUSION HEPA and ULPA filters are best applied in situations where high collection efficiency of sub micron particulate matter is required, where toxic and hazardous particulate matter cannot be cleaned from the filter, or where the particulate matter is difficult to clean from the filter. HEPA and ULPA filters are typically utilized for application involving chemical, biological and radioactive particulate matter. Filters are used in a number of commercial applications and manufacturing processes such as clean rooms, laboratories, food processing and the manufacture of pharmaceuticals. HEPA and ULPA filters are installed as the final component in particulate matter collection system downstream from other particulate matter collection devices such as electrostatic precipitators or bag houses References: 1. Lachman.L, Lieberman HA, Avis KE. Pharmaceutical Dosage Forms; Parenteral Medications, Vol 2, 2nd edition., Marcel Dekker, INC, New York, 2005. Page no: 415-416, 421-427. 2. Lachman.L, Lieberman HA, Kanig JL, The Theory and Practice of Industrial Pharmacy, 3 rd edition., Varghese Publications, Bombay.1987. Page no. 660. 3. Michael JA, Remington. The Science and Practice of Pharmacy. Vol 1, 21st edition., Lippinccott Williams & Wilkins, USA, 2007. Page no: 815. 4. http://www.airpurifiers-online.com Page 14