Air filtration

The increased purity demands of high precision, complex and fully automatic pneumatic systems, and chemical, biochemical, biomedical, electronic, pharmaceutical, and food processing operations, are the reasons for the continuously increasing needs for better quality compressed air. 

A typical air sample is contaminated to the extent of 140 million dust particles per cubic metre (and more in heavily industrialized areas). As many as 80% of these particles are smaller than 2 jm - 25 times smaller than the smallest particle that the human eye can see - and most of them pass straight through compressor intake filters. These are supplemented by water vapour, along with unbumed hydrocarbons from aviation, heating and vehicle fuels. When this poor quality air is compressed to 8 bar, the particle content increases to 1.1 billion particles per cubic metre. A typical analysis of the size distribution of attnospheric dust is shown in Table 6.8. 

The degree of treatment required for lanoving contaminants from a compressed air system depends to a great extent on the intended application. For general industrial 

1 The quality of the air delivered by non-lubricated compressors is influenced by the quality of the intake air and the compressor design. 

2 The minimum accuracy of the measuring method used is 20% of the limiting value of the class. 

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These fibers may find use in controUed release of dmgs, bactericides, and corrosion prevention chemicals (103). Fibers with different active groups have been made for sorption of chemicals. These fibers are designed to replace granular sorbents for air purification, for example, in air filtration masks (104). 

FIG. 24-18 Sketch of housing and membrane cartridges for air filtration. Typical cartridges are 76 cm long and 7.36 cm in diameter of polyvinylidene difluoride with 0.22- im pores. 

The prime design objeetive of the filter system is to proteet the gas turbine. The performanee of the gas turbine inlet-air filter system has important and far-reaehing influenees on overall maintenanee eosts, reliability, and availability of gas turbines. There are three major results of improper air filtration (1) erosion, (2) fouling of the axial-flow eompressor, and (3) eorrosion of the gas turbine hot-gas path inlets. The importanee of the inlet-air filter, as it relates to eaeh of these three phenomena, ean be appreeiated if... 

Clearly the largest application for filtration is air pollution control. On a worldwide basis the annual sales revenues for all types of pollution control equipment is around 150 billion dollars, with air pollution control devices being a substantial part of tliese equipment sales. The following is a summary of Web sites recommended to be consulted for specific vendor information on air filtration devices. A brief description of each site is provided. Many of these sites will link the reader to other sites containing additional information on product information. 

TMS Air Filtration Systems Limited Specialist in air filtration, extraction and ventilation systems. http //www.westmids.co.uk/tmsairfiltration... 

Coppys Provider of portable ventilators which help fume and vapor removal, compressed air filtration, personnel cooling confined space ventilation with propane heaters, cooling mist ventilation blowers, http //www.coppus.com... 

Air Cleaning Systems, Inc. Sells, services and provides equipment and parts for air filtration systems. http // v m.airdeaninssystems.com... 

The airflows on the wall surfaces and the air filtration through the walls significantly influence contaminant accumulation, and therefore it is essential to carry out the calculation also for the cold season. 

Effective inlet air filtration is required to ensure satisfactory operation of the engine. The location of the unit determines the most appropriate filter system to use. Desert environments where a large amount of sand particles could be expected in the ambient air may use an automatic roll type of filter that allows new filter material to be rolled in front of the inlet without frequent shut-downs to change filters. Arctic or extremely cold locations may use pad type filters, snow hoods to prevent blockage, and exhaust recirculation to prevent icing. Filter assemblies for offshore marine environments may include weather louvers, demister pads, and barrier elements for salt and dirt removal. Screens may be u.sed for insect removal prior to filtration in areas with bug problems. 

Exposure to dust or other pollutants that may exist in certain industrial processes will dictate requirements for such aspects as equipment enclosures and air filtration. For the same reasons, it may be prudent to avoid air-cooled heat transfer surfaces. 

Ambient air contains many solid impurities, ranging from visible grit down to fine dusts, smokes and fumes [10]. An air-conditioning system will aim to remove a proportion of these, depending on the application. There are three reasons for air filtration ... 

Dust-free air is required for many process applications. The requirements of air filtration differ from those of process gas filtration mainly in that the quantity of dust to be removed will be lower, typically less than 10 mg/m3 ( 5 grains per 1000 ft3) and also in that there is no requirement to recover the material collected. 

Three basic types of air filter are used viscous, dry and continuous. Viscous and dry units are similar in construction, but the filter medium of the viscous type is coated with a viscous material, such as a mineral oil, to retain the dust. The filters are made up from standard, preformed, sections, supported on a frame in a filter housing. The sections are removed periodically for cleaning or replacement. Various designs of continuous filtration equipment are also available, employing either viscous or dry filter elements, but in which the filter is cleaned continuously. A comprehensive description of air-filtration equipment is given by Strauss (1975). 

K. R. Goddard, Air filtration of microbial particles, Publication 953, U.S. Public Health Service, Washington, DC, 1967. 

A laboratory where the measurement takes place must be free from odor and is typically air-conditioned with air filtration. The odor sample is placed in an olfactometer that basically is a device for dilution of the sample. Typically, the meter has two outlet ports diluted odorous air flows from one, and clean odor-free air flows from the other. In dynamic olfactometry, panel members assess the two ports of the olfactometer. The assessors indicate from which of the ports the diluted sample is flowing. The measurement starts with a dilution that is large enough to make the odor concentration beyond the panelists threshold. This concentration is normally increased by a factor of two in each successive presentation. Only when the correct port is chosen and the assessor is certain that the choice is correct and not just a guess, is the response considered a true value. 

Airborne precautions using correct ventilation including negative air-pressure rooms with high-efficiency particulate air filtration should be initiated for hospitalized confirmed or suspected smallpox patients, unless the entire facility has been restricted to smallpox patients and recently vaccinated persons. 

Table 6.1 presents recommended and optional emergency preparedness measures for a variety of residential and urban settings. The recommended measures should be seriously considered. Optional measures are not absolutely essential they will provide added protection. In some settings, certain types of preparedness measures are not required. For example, a HEPA air filtration device is not required for a sports stadium. 

Using a HEPA air filtration unit (fitted with appropriate filters) to remove contaminants from the outside air before it is pulled into the building... 

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