What is the difference of HEPA filter and membrane filter?

21.6.1 HEPA and filtration problems

HEPA filters are made of polyolefin and glass [60,61]. The majority of fibrous filters are non-uniform in mass and thickness generally, which affects their collection efficiency and pressure drop [61]. Permeability, filtration performance, and attaining uniformity in structure should be considered during development. The existing high HEPA filters filter particles of more than 0.3 μm with 99.97 % efficiency fibers, and could filter particles greater than 0.3 μm by impaction and interception, but they are not sufficient for filtration of smaller pathogenic agents like viruses [61].

8.3.2.4 Round filters

Round HEPA filters are finding utility in room fresheners because of the stand alone nature of the unit. In the United Kingdom circular filters have been adopted for nuclear containment systems. A circular HEPA filter is depicted in Figure 8.14. What is not shown is a circular lip seal which allows the filter to be pressure sealed in place without clamping. ter Kuile and Doig(177) indicate the following advantages to circular filters over square or rectangular HEPA filters:

Higher airflow and lower pressure drop

Easier installation

Less maintenance

Improved operation

Easier disposal – the spent filters can be crushed to disc-like dimensions.

8.7.6 Manual versus remote filter change

Circular HEPA filters, which are typically around 600 mm long and 500 mm in diameter, do not last forever. Gradually they become clogged up with tiny particles and need changing, or simply exceed their shelf life. When developing designs for new facilities and looking ahead to the time when spent filters will need to be replaced, quite often an interesting problem arises when considering how that change will be effected.

It all comes down to a debate on whether filters will be located in an easily accessible area and replaced manually, or located behind thick concrete shielding and changed remotely. Everything hinges on how much contamination may be airborne, and therefore drawn towards filter banks, along with what isotopes may be present, and from that determining the levels of radiation emitted by particles of contamination as they build up on filters.

At the extremes of the debate on whether to opt for manual or remote change, calculations are straightforward and the answer is obvious. For example, if particles trapped on filters originate from materials at the low end of the intermediate level waste (ILW) spectrum and from an environment where little airborne contamination is present, then even those that are quite heavily loaded can be managed to remain within low level waste (LLW) limits. So in this case HEPA filters can be approached and changed by hand. Having said that, safety considerations always dictate that any exposure to radiation is minimized, so a filter change team will not stand around discussing the previous evening's TV. The exchange will be carried out and they will be on their way.

On the other hand, if contamination originates from highly radioactive sources that also generate appreciable airborne contamination, then the solution is equally unambiguous; here even small accumulations will emit significant radiation, so these filters need to be located behind hefty shielding and out of harm's way. Let's look at manual change first.

If you think of changing the air filter in your car you can unclip its cover, take it out, hold it up to the light and have a good look at it, then if you do decide to change it you can leave the old one on the driveway until you get around to tidying up. Changing HEPA filters which are loaded with contaminated particles is not such a casual affair. Happily, the technique used to change them, without breaking containment, is another good example of a simple low-tech solution to a tricky problem.

In essence, it is accomplished by withdrawing a spent filter into a plastic bag (Fig. 8.19) which is then heat sealed to ensure any loose contamination is contained. In addition, the procedure for fitting a new bag includes wrapping and heat sealing the remainder of the previous bag, so that it too can be safely disposed of.

We do not need a calculator to figure that the costs, both capital and operational, associated with remotely changing filters in a shielded cave (Fig. 8.20) will be exponentially higher than those of a simple hands-on approach. Sometimes costs can be minimized considerably by locating highly active filters within a cave which is needed for some other purpose, rather than building a dedicated filter cave. That way we get most of the filters remote handling equipment for free, so to speak. Of course the cave itself would need to be stretched somewhat to accommodate a bank of HEPA filters, but that is a relatively small price to pay.

Quite often, other factors come into play that scupper plans to locate HEPA filters within a convenient cave. There are obvious issues such as imposing potentially unwelcome changes to the way an operational cave is configured, and other more subtle ones such as diverting remote handling equipment from important production duties, something that is unlikely to go down too well. The upshot is that some very detailed analysis will be needed to evaluate the pros and cons of providing a dedicated filter cave, versus forcing an operational cave to take on a certain amount of multitasking.

So we have considered both extremes, where on the one hand it is crystal clear that filters can be changed manually and on the other there is no doubt they must be shielded and changed remotely, either in a dedicated cave or by using space in one provided for other operational purposes. Unsurprisingly, many cases sit in that awkward gray area between the two. The favored solution is always going to be manual change, so the challenge is striving to achieve it, while at the same time complying with the nuclear industry's myriad of regulations. In this particular case, the main objective is to ensure any exposure to operators is, as low as reasonably practicable, a subject we shall discuss in Chapter 14.

There are some fairly simple techniques that can make a contribution, such as constructing shield walls between banks of filters (Fig. 8.21) so that operators exchanging a filter in one bank are not needlessly subjected to radiation shining from others nearby. I must stress, however, that such shield walls are certainly not the primary form of radiation protection for a workforce, but are rather a prudent measure that can be utilized to keep operator exposure to the lowest possible level.

In striving to achieve manual change there is one design approach that, in the right circumstances, can work very well indeed. If we step back from the usual assumption that filters should be fully loaded or close to it before they are changed, then there can be some room for maneuver in avoiding potentially dangerous levels of radiation accumulating on them. With this approach filters are exchanged before high levels of contamination, and therefore radiation, get a chance to build up. The downside is easy enough to see; filters must be changed more often.

Problem encountered; impacts

During floor decontamination, a high-efficiency particulate air (HEPA) filter system used to vacuum debris and dust failed. This work was being carried out in a containment tent with a portable radioactive air emissions unit that discharged the air to the environment through HEPA filtration. Based on survey and air monitoring results, the vacuum failure caused no environmental release.

The decontamination tool was a scabbler associated with a HEPA vacuum. The debris and dust from the scabbler operation was pulled through a knockout pot that collected the large particles and then into a HEPA vacuum which discharged into the Operations Room of the tent. After the fifth pass of the scabbler, a mist spread in the Operations Room. Work stopped, and the area was secured. Radiological surveys on exiting workers showed no contamination. Radiological surveys carried out on the outside of the containment tent indicated no loss of containment. Likewise, the air samples taken at the discharge point indicated no loss of containment. Therefore, the containment system as a whole operated as designed to control contamination releases.

17.3.1 Air and aerosol filtration

In air filtration applications, materials such as high efficiency particulate air (HEPA) filters are used to remove particulates and other debris from the air. The recent advances in nanomaterials have driven the study on the use of nanofibers in combination with materials such as textiles or fiberglass. The use of nanofibers on filters can be expected to significantly improve their performance, with increased filtration efficiency accompanied by only a slight rise in the pressure drop (Barhate and Ramakrishna, 2007; Qin and Wang, 2006). Indeed, most air filtration media are made of nonwoven microfibers, which have a low initial filtration efficiency due to their large pore size. The use of nanofibers in these filter media can greatly improve the filtration of submicron particles, and the nanofibers can be incorporated in a composite or layered design (Podgórski et al., 2006) with existing microfibers to take full advantage of differently sized fibers in order to obtain the best combination of properties to efficiently trap nanoparticles along with other dispersed micrometer-sized aerosol particles (Fig. 17.3).

Studies on nylon-6 nanofibrous membranes applied on top of support fabrics (Yin et al., 2013) show that the addition of nanofibers increases their filtration efficiency in the ultrafiltration range, and their high porosity also increases the dirt loading of the filter. Such a composite membrane was made through a continuous roll-to-roll process, signifying the readiness of this technology for industrial production. Another recent study also shows that further addition of ultrathin nanofibers (Kuo et al., 2014) (with diameters of 20 nm) may further increase the filtration efficiency without increasing the low basis weight of electrospun material. However, it was also found that a filter made with only very fine nanofibers also has the tendency to compact, and thus may result in a high pressure drop, offsetting the benefits of the high filtration efficiency. As such, it is likely that such ultrafine nanofibers would be most efficiently used in a composite structure with thicker support fibers, or a method needs to be found to improve their solidity and structural integrity. Such multilayered design was demonstrated by Wang et al. (2014), who incorporated electrospun poly(acrylonitrile) (PAN) and SiO2 nanoparticles in a combined electrospinning/electrospraying method. The SiO2 nanoparticles increased the roughness and surface area of the fibers, and it was shown that multiple layers of stacked, thin membranes exhibit improved filtration efficiency and pressure drop compared with monolayer membranes of equal basis weight (Fig. 17.4).

Yeom et al. (2010) studied the use of electrostatic charging to prepare filter media with a more open structure, resulting in lower pressure drops. In conventional HEPA filters, fine fiber diameters have been used to increase filtration efficiencies, but this comes at the expense of higher pressure drops. By incorporating boehmite nanoparticles into the filters as a new electret filter media, significant improvements can be achieved in the electrostatic surface potential and aerosol capture efficiency without significant changes in air flow resistance. Cho et al. (2013) achieved a similar result using TiO2 nanoparticles where their inclusion improved the filtration efficiency of PAN nanofibers, especially for finer particles in the range of 100–500 nm.

Electrospun nanofibers have also been studied for the adsorption of volatile organic compounds (VOC) present in the air, a role traditionally given to materials such as activated carbon. Electrospun polyurethane (PU) (Scholten et al., 2011) from suitable building blocks have been shown to display rapid absorption of VOCs such as toluene and acetone. The electrospun PU shows comparable absorption capacity to activated carbon despite their lower surface area, indicating that the absorption occurs not only on the surface but also in the polymer matrix of the fibers themselves. These fibers can also be readily regenerated by desorption under ambient conditions. Another study investigated electrospun nanofibrous membranes functionalized with α-, β-, and γ-cyclodextrins (CD) for this application (Kayaci and Uyar, 2014; Uyar et al., 2010). CDs are cyclic oligosaccharides with a toroid-shaped molecular structures that are able to form non-covalent host-guest complexes with most metal ions. Electropsinning of CD-containing polymer solutions results in the CD being distributed evenly throughout the fibers without signs of crystallisation, and their incorporation increases the entrapment of molecules such as phenolphthalein and aniline due to inclusion complex formation. Their filtration capability also depends on the CD cavity size and the strength of interaction between the CD and the molecule of interest, showing a potential for tunable selectivity.

The incorporation of nanoparticles such as TiO2, MgO, and Al2O3 into nanofibers (Sundarrajan and Ramakrishna, 2007; Sundarrajan et al., 2014) has been investigated due to their ability to decontaminate a wide range of toxic gases and biological contaminants. In order to ensure that the nanoparticles are on the surface and are not covered by the polymer (and thus maximizing their active surface area), approaches such as a combined electrospinning-electrospraying technique (Baji et al., 2014) can be used where the inorganic nanoparticles are electrosprayed onto the electrospun polymer nanofiber support. The stability of the composite and the nanofiber–nanoparticle adhesion can be further increased through the use of surface functionalities on the nanofibers that allows a specific interaction or binding with the nanoparticles.

Although most studies on electrospun nanofibers have focused on polymeric nanofibers, some have developed inorganic nanofibers for high temperature applications, such as the filtration of hot waste gases. Although various ceramic nanofibers can be fabricated by electrospinning, their brittleness may become a barrier for practical use. Some studies have developed silica nanofibers with remarkable flexibility and robustness through a combination of electrospinning and sol-gel methods (Guo et al., 2010; Mao et al., 2012; Yang et al., 2012; Zhao et al., 2011). Such nanofibrous mat is able to withstand bending and show excellent thermal stability up to 900°C. They have also been shown to achieve better filtration performance than the baseline criteria for HEPA filters (removal of 99.97% of particles that have a size of 0.3 μm) (Mao et al., 2012)

6.4.3.1 Heating, ventilation, air-conditioning

The air filters are rated by a minimum efficiency reporting value (MERV) standard204 and European standard,205 which rates filters from 1 to 20 in terms of their degree of efficiency. At the high end, MERV 17–20-rated HEPA filters are typically used in situations that require absolute cleanliness for the manufacture of microchips, liquid crystal display screens, pharmaceutical production, and microsurgery in hospital operating rooms. HEPA filters are primarily constructed from wetlaid glass nonwoven filtration media, with a smaller portion of the market serviced by polytetrafluoroethylene membranes laminated to a polyester base substrate for support. MERV 1–16, considered heating, ventilation, air-conditioning (HVAC)-grade filters, are principally constructed of synthetic melt blown, spunbond, or glass fabrics. Overall, 75% of synthetic nonwoven media go into commercial markets, such as manufacturing facilities, offices, theatres, hospitals, cruise ships, casinos, and other such markets, with about 25% found in residential and general consumer air filters.

A series of standards, BS EN 1822, High Efficiency Air Filters (EPA, HEPA, and ULPA), describes the factory testing of the filtration properties of air filters, while another series of standard, BS EN ISO 14644, describes the requirements in relation to clean rooms and associated controlled environments. BS EN ISO 14644-3:2005 establishes the in situ testing of air filters under installed conditions.

BS EN 13142:2004: Ventilation for buildings. Components/products for residential ventilation. Required and optional performance characteristics

BS EN 13053:2006: Ventilation for buildings. Air handling units. Ratings and performance for units, components and sections

BS EN 779:2012: Particulate air filters for general ventilation. Determination of the filtration performance

BS EN 1822-1:2010 High efficiency air filters (EPA, HEPA, and ULPA). Classification, performance testing, marking.

BS EN 1822-2:2010, High efficiency air filters (EPA, HEPA, and ULPA). Aerosol production, measuring equipment, particle-counting statistics

BS EN 1822-3:2010 High efficiency air filters (EPA, HEPA, and ULPA). Testing method for flat sheet filter media.

BS EN 1822-4:2010 High efficiency air filters (EPA, HEPA, and ULPA). Describes determination of the leakage of a filter element (scan method)

BS EN 1822-5:2010 High efficiency air filters (EPA, HEPA, and ULPA). Describes determination of the efficiency of a filter element

ASHRAE 52.2 Method of Testing General Ventilation Air-cleaning Devices for Removal Efficiency by Particle Size

Mil F-51,068F Filters, Particulate (High-Efficiency, Fire Resistant)

IES RP-CC021.1 HEPA and ULPA Filter Media

IES RP-CC001.3 HEPA and ULPA Filters

10.3.1 Health Issues

Nanotechnology has been becoming a global technology for over two decades, and the energy industry has been taking advantage of this technology since the beginning. More than 50 different nanomaterials in the forms of metals and alloys, ceramics, polymers, and composites have already been developed and used in the energy industry, some of them include Au, Pt, TiO2, IrO2, SnO2, ZnO, SiO2, VO2, SrTiO3, LiNbO3, PbWO4, NiO, GaAs, ITO, ZnS, CdSe, CNT, CNFs, C60, and graphene [1–3]. Workplaces, such as manufacturing facilities and laboratories where nanomaterials are engineered, processed, recycled, and disposed, are the major places of safety concern [19]. Engineered nanomaterials are generally classified as very fine particulate matter measuring between 1 and 100 nm in one dimension. These nanomaterials can translocate from the location of deposit in the respiratory tract to pulmonary organs, such as brain, liver, heart, and bone marrow. Numerous studies have demonstrated the adverse health effects of some engineered nanomaterials [19–23].

Recent research studies have shown that CNTs and some other nanomaterials are harmful to animal cells and many other living organisms [19]. The ultrafine nanoparticulates released into the atmosphere remain airborne for a few hours to several days and even weeks, travel several kilometers, and can be inhaled repeatedly, then collected in respiratory systems, with some parts deposited in lungs. This study also stated that the nanoparticles could interact with cells, cross blood-brain barriers, and alter cell functions [19]. Inhalation is the most common form of exposure to airborne nanoparticles in the workplace.

Strict control of airborne nanoparticles can be achieved using fume hoods and other vacuum systems. High-efficiency particulate air (HEPA) filters can provide protection against the possible release of nanoparticles into the atmosphere. Preventing inhalation, skin exposure, and ingestion of nanoparticles are common in the workplace. Using respirators and gloves can minimize these kinds of risks and hazards. Good work practices can minimize exposure to nanomaterials [19,23]:

Avoid direct contact with nanomaterials as much as possible.

Wear a respirator with a helmet equipped with HEPA filters.

Use an efficient exhaust system with a particle filtration and ventilation system.

Wear safety goggles (eye protection), protective shoes, and protective clothes.

Avoid consuming or storing food in areas where nanoparticles are handled.

Avoid using cosmetics in areas where nanomaterials are used.

Remove laboratory coats and wash hands before leaving the laboratory.

Avoid touching the face or any other part of the body before washing the hands.

Label all containers with necessary information.

Clean the areas where nanomaterials are handled by wet-wiping or using a HEPA filter vacuum.

Dispose of contaminated materials correctly and obey all hazardous waste disposal policies and procedures.

Use two pairs of disposable gloves if possible.

Make sure dusts formed in manufacturing do not cause fires and explosions at higher concentrations.

4.3 High Efficiency Particulate Air Filters

The main component of the heating, ventilating and air conditioning (HVAC) system is the HEPA filter. The HEPA filter was designed as a filter for gas masks by the United States Chemical Warfare Service. With the development of atomic energy, they were seen to be of value. In 1955, fire retardant HEPA filters were developed for the Atomic Energy Commission. They became available commercially 2 years later. For use with the AEC, it became necessary to have a noncombustible filter that would remain in tact and continue filtering after the fire. In 1959, Underwriters Laboratories issued the UL 586 Standard for the High Efficiency Particulate Air Filter. The first tested filters were available in 1965. The HEPA filters are capable of removing 99.97% of the airborne particles 3 microns or larger. They must be able to operate at rated flow with an air temperature of 700°F(371°C) and not contribute to a fire within the ventilation system. They must also act as a fire stop to prevent a fire from being transmitted through the ventilation system. While this paragraph addresses HEPA filters, the testing applies to the more efficient filters and filter systems used in modern cleanrooms.

The Ultra filter or Ultra Low penetrating air filter would be listed under the HEPA filter listing. These filters are designed for 99.997% efficiency where particles are .01 microns or larger.

The filters are available in various listings from recognized testing laboratories. Underwriters Laboratory will list the filters in many ways. Any or all of the components may be listed separately when tested in accordance with the UL test for surface burning characteristics of Building Materials (UL 723). This is the least desirable listing because it does not test all the components in the manner in which they are being used.

The test performance of air filter units (UL 900) tests air filters designed for filters for heating, air conditioning, and ventilation systems other than residential type. They are to be used in accordance with NFPA 90A or 90B. The better filters tested in this manner are given a Class I rating. The Class I filters are those which, when clean, do not contribute fuel and only would emit a negligible amount of smoke. The Class II filters will burn moderately and emit smoke. While this is an adequate test for the pre-filters, it is not representative of the HEPA filters. It is not tested in the manner in which the filters are being used. The UL 900 test would represent a single filter in the ducting system. It uses an oxidizing flame passing over the filtering material and the frame for 30 seconds. The filter cannot ignite or burn for the Class I listing. The low BTU flame and short duration of time is not indicative of real-life situations in a semiconductor facility.

The most representative test is the standard of Underwriters Laboratories Inc. for High Efficiency Particulate Air Filter Units (UL 586). These units will generally consist of a filter medium of glass fiber or other inorganic material and a frame of metal construction. It is possible to utilize wood but it will have a flame spread of 25 or less. When tested in accordance with UL 723, test method for surface burning characteristics of building materials, UL 586 tests the filter, frame and supports as a unit. It utilizes a 4” blue flame with direct impingement for 3 minutes. It must remain intact. The UL 586 test apparatus units are up to 2′ × 2'. The typical filter used in the modern cleanroom are 2′ × 4′. These units are available in this size from major manufacturers constructed in an identical fashion to those tested. They will not be labeled since they are beyond the scope of the test. These are the most appropriate units for use in cleanrooms for the semiconductor industry.

Air-filtration System

Particle filters are critical to creating and maintaining clean environments. Most modern cell-processing facilities have ventilation systems with filtered air—using high-efficiency particulate air (HEPA) filters, a technology developed in the 1940s. HEPA filters have a minimum particle collective efficiency of 99.97% for particles of 0.3 µm diameter or more. A HEPA filter typically lasts three to five years, depending on hours of operation, cleanliness of the laboratory, and type of work being performed. Most are inspected and certified on an annual or semiannual basis, depending on the standard of the facility.