AIR PURIFIERSAir filtration is a topic affecting everyones lives, whether it is for occupational safety requirements, environmental or home health concerns. Control of airborne particulates in indoor environments is critical to develop quality products, protect employees from contact with hazardous materials, or prevent health problems from prolonged exposure to allergens. How airborne particulates are controlled varies from industry to industry and from an occupational setting to a home environment. Air purifiers are appliances that aid in removing pollutants from air. Air purifiers serve residential as well as commercial needs. Commercially air purifiers serve various industrial, medical and commercial industries.HEPA FILTERSAir purifiers have HEPA (high efficiency particulate air) filters that aid in cleaning the air around that is circulated. A HEPA filter is a type of mechanical air filter; it works by forcing air through a fine mesh that traps harmful particles such as pollen, pet dander, dust mites, and tobacco smoke. They help get rid of contaminants and impurities from the air. It is possible for the HEPA filter to remove about 99.9% of dust particles bigger than 0.3 microns (the standard measure for microns) that is, that for every 10,000 air particles that are 0.3 microns, the HEPA filter will only allow 3 of them to pass through (Choudhary, 2014).Human Hair(70 - 100 microns)

Human Sneeze(10 - 100 microns)

Pet Dander(0.5 - 100 microns)

Pollen(5 - 100 microns)

Spores from Plants(6 - 100 microns)

Mold(2 - 20 microns)

Tobacco Smoke(.01 - 1 micron)

Dust Mite Debris(0.5 - 50 microns)

Household Dust(.05 - 100 microns)

Skin Flakes(0.4 - 10 microns)

Bacteria(0.35 - 10 microns)

Fungi(0.5 - 5 microns)

Viruses(.002 to 0.3 microns)

*A micron is 1/25,000 of an inch

Table: Showing examples of some common air contaminants and their size in microns (Achoo Allergy, 2014).HEPA FILTER CONFIGURATIONHEPA filter can be used in numerous configurations, including the following: in booths or enclosures recirculating air back into the room, within a room either in freestanding (portable) air-cleaning units or in wall-mounted recirculation units, in HVAC ductwork prior to recirculation to other areas, or in exhaust ducts prior to exhaust to the outside (Flannigan et al., 2003).INSTRUMENTATIONHEPA filter is constructed of borosilicate microfibers in the form of pleated sheet. Sheet is pleated to increase the overall filtration surface area. The pleats are separated by serrated aluminum baffles or aluminum seperators, which direct airflow through the filter. This combination of pleated sheets and baffles acts as filtration medium. It is installed into an outer frame made of fire-rated particle board, aluminum, or stainless steel. The frame-media junctions are permanently glued or sealed to ensure a leak proof. HEPA filters are typically manufactured with a mat of randomly arranged fibers and of various materials. The air space (effective density) between HEPA filter fibers is usually larger than 0.3 microns. The techniques by which HEPA filters trap pollutants and particles are discussed next.

Figure: Instrumentation of HEPA filter (Das, 2012).FILTRATION MECHANISM

Figure: Mechanism of HEPA filters to capture airborne particles (Kelly, 2007).HEPA filters are composed of a mat of randomly arranged fibers. Key factors affecting function are fiber diameter, filter thickness, and face velocity. The fibers are typically composed of fiberglass and possess diameters between 0.5 and 2.0 micrometers. The air space between HEPA filter fibers is much greater than 0.3m. HEPA filters are designed to target smaller pollutants and particles. These particles are trapped (they stick to a fiber) through a combination of the following four mechanisms:1. Straining/sieving is defined as when a particle is too large and becomes trapped between two filter fibers. These particles are too large to pass between the fiber spaces.

Figure: Straining process2. Impaction or inertia impaction is when a particle of relatively greater mass than the fluid surrounding them and as the fluid changes direction to enter the fiber space, the particle continues in a straight line because of inertia (which is a function of the particles mass and velocity) and collides with the media fiber where it is trapped and held. Inertia works on large, heavy particles suspended in the flow stream (Donaldson Company, 2011).

Figure: Inertia impaction process3. Diffusion works on smallest particles. Small particles are not held in place by the viscous fluid within the flow stream. As the particles traverse the flow stream, in a random manner known as Brownian motion, they collide with the fiber and are collected.

Figure: Diffusion process4. Interception occurs when a section of a particle runs into a filter fiber. These particles follow the flow stream as it bends through the fiber spaces. Particles get intercepted or captured when they touch the fiber (Kelly, 2007).

Figure: Interception processThe larger the particle, the greater the role of interception and impaction in capturing particles, which is a way of saying that these mechanisms are more efficient in capturing larger particles than smaller ones. Diffusion works in the opposite direction: smaller particles act more like the gas molecules we associate with Brownian motion, so it is a more efficient particle capture mechanism for smaller particles.HEPA filters work by sucking air through a filter that removes particles as small as 3 microns. HEPA air cleaners come with outer filters to preserve the life of the HEPA filter, which can be almost as expensive to replace as buying the HEPA filter originally. The outer filters can be carbon or foam and are washable. The HEPA filter uses a fan to pull air through it and it puts the filtered air back into the room to reduce dust, pollen and dander (Johnson, 2014).TYPES OF HEPA FILTERTypesGradeApplicationPerformance

A industrialNon-critical areas, first stage filtration, hospitals, food processing areas.99.97 % at 0.3 m

B nuclear Nuclear R & D, commercial reactors.99.97% at 0.3m

Claminar flowLaminar flow work stations and cleanrooms, semiconductor and parental drug manufacturing areas.99.99% at 0.3m

Dvery-large-scale integration (VLSI)Semiconductor and parental drug manufacturing areas.99.999% at 0.3m

Ebiological Hazardous biological containment facilities for stopping toxic, nuclear, chemical and biological threats.99.97% efficiency 0.3m

Fultra-low penetration air (ULPA) Semiconductor manufacturing areas.99.999% at 0.1 to 0.3m

Table: The different grades of HEPA filter and their applications (Flanders Corporation, 2004).

OVERALL APPLICATIONS OF HEPA FILTERHEPA filters are used in many industries including, pharmaceutical manufacturing, Bio-Tech, Bio-Electronics, Aerospace, Hospitals, and more. Overall applications are discussed below:1. HEPA filters are used in a clean room, where the goal is to remove as much particulate from the air as possible. Clean rooms are used in the manufacturing of electrical components where a single spec of dust can ruin the entire circuit board. Computer chips, computer hard drives, motherboards, cellular phone components, and a vast array of electronic components are a few examples of the type of manufacturing where HEPA filters are a must. 2. These are also used in the healthcare industry, where the goal is to have a sterile environment. A spec of dust will not do as much damage as long as it is sterile, but it easier to remove the particulate than it is to sterilize it. Also, working with hazardous items such as the AIDS virus, the TB virus, E. coli bacteria, and many other deadly bacteria and viruses for the sake of research, requires HEPA filtration. Most pharmaceutical companies have vast operations requiring HEPA filtration from their research departments to their drug production line and finally their packaging departments. Some hospitals use HEPA filters over surgical tables and in their TB isolation areas.3. A HEPA Filter of efficiency of 99.97% is usually used in aseptic manufacturing filing and packing, parental dosage manufacturing. These filters are mounted in Laminar Air Flow Bench which are then used as clean area for actual aseptic manipulations or aseptic filling where usually a clean area of class 100 are required, such laminar air flow bench are typically found within a clean area of class 1000, which again are maintained by HEPA filters installed in a HVAC system (Das, 2012).4. Many vacuum cleaners also use HEPA filters as part of their filtration systems. It is used in industrial vacuum cleaners.5. Modern airliners use HEPA filters to reduce the spread of airborne pathogens in recirculated air (Boeing, 2014).6. Other areas where HEPA filters are used include for nuclear contamination and lead and asbestos abatement. They are even used in universities and food processing industries.

ADVANTAGES OF HEPA FILTERSHEPA filters are used in clean rooms, operating rooms, airplanes, and homes where people need relief from allergies or have lung problems. Highly efficient: The HEPA filters are highly effective at removing air borne pollutants. Air purifiers actually clean the air around us purifying it by removing microns of 0.3 and larger size to render it 99.9 percent dust free. The HEPA filters are rated according to their ability to remove particulates measuring 0.3 microns, an industry standard. Because of the way these filters are designed, their efficiency actually increases for particles both smaller and larger than the most penetrating particle size, which is about 0.1 to 0.2 microns. The efficiency of HEPA filters to remove bacteria and viruses (.01 to .1 microns) is greater than 99 percent (Boeing, 2014).No microbial growth: Bacteria and viruses do not breed in true HEPA filters. HEPA filters provide a dry environment and without moisture there is nothing to keep bacteria alive, much less breed. These trapped contaminants eventually die from lack of moisture. This is why only true HEPA filters are used where health is critical as in hospital operating rooms and ICU wards. Bacteria can breed within the filter only if the air coming into the filter is moist. If such is the case, methods for killing bacteria within a HEPA filter include antimicrobial treatment of the media or a germicidal ultraviolet light. Removal of a wide range of particulate matter from the air: Because of their design the HEPA filters used in cleanrooms remove a broad range of airborne contaminants including fine dusts, smoke, pollen, soot, bacteria, viruses, asbestos and radioactive particles.Chemical Resistance: All materials used in the filter have good resistance to most organic solvents and are resistant to many weak organic and inorganic alkalies and acids (Donaldson Company, 2011).DISADVANTAGES OF HEPA FILTERSFilter Replacement: HEPA filters cannot be cleaned repeatedly to remove particulate and dust loading and must be replaced. As the particle load increases, so will the resistance to flow and hence the pressure drop across the filter. HEPA filters quickly clog and become ineffective, so they must be replaced regularly. Exposure to contaminants during filter replacement: These filters may tend to hold moisture in, which actually breeds mold and releases it back into the room. It is true that they can remove mold spores and bacteria from the air, but unless the filter is treated with an anti-microbial substance the still-living microbials continue to grow and multiply on the HEPA filter. When one changes the HEPA filters, one can expose himself to extremely high concentrations of the harmful substances the HEPA is filtering from indoor air (James, 2012).Cost of Operation: The HEPA filers are very expensive. HEPA filters must use electricity to operate. HEPA filters are extremely fragile and can easily be damaged during installation, cleaning, and even when the unit is moved. HEPA filters should be tested at least annually (James, 2012). Calculating the initial cost of the HEPA units, plus the maintenance and replacement costs, as well as the energy cost of running the unit, it will cost as much as a major appliance to operate.Noise generation: Most HEPA air cleaners on high fan speed, which is needed for full efficiency, sound like a train going through the room.Filter limitations: HEPA air purifiers are not very good at capturing ultra-fine particles like viruses or eliminating foul odors, organic compounds, or chemical fumes (Achoo Allergy, 2014). HEPA filter media will not tolerate excessive amounts of moisture, either from airborne droplets or condensation as this can plug the filter and result in failure by over-pressure (James, 2012).CHARACTERISTICS AND EXTRA FEATURES OF A HEPA AIR PURIFIERCharacteristics that need to be taken into account when buying a HEPA air purifier are as follows:1. Proper air flow rate: Air flow rate is the amount of air that the filter pulls through and processes from the air around it. An HVAC technician and some home inspectors have equipment that can easily measure the air flow rate at a given point in the system. The air flow rate or velocity should be proper so that a balance is maintained between the room requirements for a sterile environment with the cost and energy consumption.Class NamesAir Changes Rate per Hour

1>750

10500 - 600

100150 - 400

1,00060 - 100

10,00025 - 40

100,00010 - 15

Table: Air flow rate for cleanroom design2. High quality, effective activated carbon filters: A low quality activated carbon filters will not be able to pull VOCs (volatile organic compounds) and other gases and vapors from the air. So HEPA air purifiers with high quality, effective activated carbon filters should be chosen for the best results.3. Long term performance: Top HEPA air purifier models, that is, those that have proven long term performance should be used. This means that they clean the air just as efficiently over a long period of time as they do right at the start.Some special and useful features that may be present are as follows:1. A layer of activated charcoal: This removes odors and some chemicals the HEPA layers do not.2. A built-in ionizer: It helps remove all kinds of airborne pollutants and bad odor.3. LED indicator: This reminds one regularly when the right time is to change the filter.4. UV protection: It helps to neutralize and kill harmful germs, viruses and bacteria present in the air passing through the filters (Clark, 2014).

REFERANCEKelly, J. (2007) Tutorial on HEPA Filtration Controlled Environments. Available from: http://www.cemag.us/articles/2007/03/tutorial-hepa-filtration [Accessed at 30th May 2014]Choudhary, A. (2014) HEPA Filters used in Pharmaceuticals Pharmaceutical Guidelines. Available from: http://www.pharmaguideline.com/2011/02/hepa-filters.html [Accessed at 30th May 2014]Clark, G (2014) Things to Consider When Buying an Air Purifier Clean Air Plus. Available from: http://www.cleanairplus.com/blog/when-buying-an-air-purifier.html [Accessed at 30th May 2014]Das, P. (2012) About HEPA filters used in HVAC system USCGMP Available from: http://www.uscgmp.com/2012/11/using-hepa-filters-in-industry.html [Accessed at 30th May 2014] Donaldson Company (2011) High Efficiency Particulate Air: HEPA Filtration Facts Available from: https://www.donaldson.com/en/aircraft/support/datalibrary/042665.pdf [Accessed at 30th May 2014]Flanders Corporation (2004) HEPA Filters and Filter Testing Available from: http://www.flanders-csc.com/Downloads/hepa%20filters%20and%20filter%20testing.pdf [Accessed at 30th May 2014]Flannigan, B., Samson, R. A. and Miller, R. A. (2003) Microorganisms in Home and Indoor Work Environments: Diversity, Health Impacts, Investigation and Control. 2nd ed. Florida: CRC Press.James, C. A. (2012) HEPA Filter: What It Is, What It Does and What It Doesn't Do Inspired Living. Available from: http://www.inspiredliving.com/airpurifiers/hepa-filters.htm [Accessed at 30th May 2014]Johnson, J. (2014) HEPA Vs. Ionic eHow. Available from: http://www.ehow.com/about_6656809_hepa-vs_-ionic.html [Accessed at 30th May 2014]Achoo Allergy (2014) Air Purifier Buying Guide Available from: http://www.achooallergy.com/air-purifier-buying-guide.asp [Accessed at 30th May 2014] Boeing (2014) Commercial Airplanes: Frequently asked Questions about Cabin Air Systems Available from: http://www.boeing.com/boeing/commercial/cabinair/facts.page [Accessed at 30th May 2014] 3