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Galina RAYEVSKAYA, Director, FRACTAL R&D Center (Tver),
Fyodor CHAUSOV, FRACTAL Chief Engineer,
Yuri GERMANOV, Cand. Sc. (Tech.).Tver State University
Until recently the search for an optimal filter design amounted to finding a compromise between what looked like mutually contradictory requirements, such as combining high performance with low resistance and having reasonably good technical parameters at moderate price. And it has now been possible to reconcile these contradictions with the help of what we call fractional geometry methods.
Practical experience proves the best purification efficiency can be achieved by using fine fiber filtering materials with optimal technological dimensions. Most such materials, however, are too expensive and not durable enough, which hinders their practical application on any large scale. This being so, fibers of several fractions are usually applied; but until recently there have been no sufficiently substantiated recommendations as to the optimal structure of filtering materials.
At the start of the 1990s a team of researchers, including the authors of this article, were on the point of solving the problem with the help of special synthetic polymer materials (fibrides) and synthetic fibers which provided for high filtration efficiency In filters of this kind the thickest fibers (diameter of 20- 50 u,) were used as the framework with the space between them filled in with finer fibers (10-15 и). The latter, in their turn, performed the same framework function relative to the very finest fibers (down to 5 u) while also acting as a binder which ensured the strength of the whole structure. The size, number and distribution of pores within such a filter were the same throughout its thickness-they were of non-gradient structure, as we say. The sifting factor of grains of standard polysterene latex 0.4 u, in diameter reached about 99 percent.
Structures of this kind, however, had numerous flaws one of which was their relatively high resistance which involved increased electricity consumption for pumping through the purified liquids. Secondly, the fine-grain structure was quickly clogged down by impurities that put the whole filter out of operation after which it could not be regenerated. And, finally, these materials were produced from synthetic polymers with all their drawbacks, such as high cost (tens of US dollars per kilo), ecological hazards (initial components include some highly poisonous substances) and the complexity of manufacture.
Essentially, materials on the basis of fibrides were of rather theoretical interest, demonstrating the effectiveness of what is known as the "nest-of- dolls" principle: building the structure of a filter from a number
of fractions with grain size diminishing in a certain progression and with the pores between the fibers being filled with ever finer fibers. It should be stressed, however, that until now all technical decisions aimed at reducing filter resistance and increasing the service life amounted to trying to build a structure with pore size diminishing from layer to layer in the direction of the liquid flow. In this arrangement the input section or layer possessed low resistance and captured the bulk of large-grain impurities, thus "facilitating" the work of the fine-pore filler. At the same time this multilayer structure made the filter manufacture more complex and more costly. And as for the fine- porous layers, they had all the drawbacks mentioned above. That is why while working on relatively inexpensive and high-performance filters our FRACTAL experts pinned their hopes on one of the budding areas of mathematics- fractional geometry, which supplies engineers with a powerful instrument of analysis and synthesis of systems with an intricate inner structure.
In general, the term fractals applies to a broad class of objects possessing some uncommon topological parameters. These include, among others, systems consisting of separate elements, each with a geometry similar to that of the whole object. This also includes filtering systems build on the "nest-of- dolls" principle.
We investigated those of them in which grain, or fiber size diminishes from fraction to fraction in a geometric progression, while their numbers in each successive section grow in the same proportion. Under certain conditions filters with this structure happen to possess one important advantage: with the growing number of fiber fractions their particle capturing
efficiency approaches unity (100 percent), and the resistance tends to some finite (and small) value. And that means that this approach provides for a combination of mutually contradictory characteristics: the high efficiency possessed by filters developed on the basis of fibrides, and the low resistance demonstrated by their multilayer analogues.
In practice materials produced from only three fiber fractions provided for a more than 99 percent rate of filtration, or capture, of polysterene latex particles of 0.4 in diameter with a resistance close to that of the ordinary filter paper used in labs. Materials of this kind have been given a trademark name of TEFMA (which stands in Russian for technological filtering materials).
As the raw materials for their manufacture one can use, in principle, fibers of any kind, fractionated by size. The most practical in our experience are certain industrial wastes, such as textile wastes (cotton fluff and wool floss-fibers from 3 to 5 mm in diameter) and even finer fibers such as can be "provided" by pulp and paper mills (say cellulose, or pulp "savings"-very fine-grain fractions which have no other uses). And that means that the manufacture of new filters causes no environmental pollution, and helps protect the environment from harmful industrial discharges.
TEFMA filters can be manufactured in a technological process which has been developed for the production of filters on the basis of fibrides. It is relatively simple and does not require any special equipment.
TEFMA filters cost 50 to 30 percent as much as their traditional analogues; they are safe for the environment, and their low resistance helps save electricity for pumping.
The technological solutions with respect to the structure of the new materials, production techniques and the manufacture of filters on their basis have all been patented in the Russian Federation.
TEFMA filters are now used in this country for systems of purification of a whole range of technological media, such, for example, as industrial systems of water supply and heating. In water the filters capture as good as all the particles responsible for turbidity and color, and also colloidal suspensions of trivalent iron (purification efficiency in both cases approaches 95 percent). The use of these filters in closed central heating circuits rules out boiler scale formation; and when used in combination
with a comprehensive (with the help of special purification complexes) and magnetic treatment of water such filters make it possible to use as the heat carrier virgin and uncleaned artesian water. They can also be used for the purification of water in the production of building materials (solutions, concrete mixtures), when it is necessary to remove silt particles and/or iron admixtures.
Systems of water preparation on the basis of TEFMA filters are compact in size because a 6 mm layer of this material is more efficient than a one-meter layer of quartz sand or anthracite. Besides, systems of this kind can be easily back-washed, or regenerated with a reverse now of water and, if need be, they can be cleaned of some substances (non-ferrous metals, for example) by means of reagent treatment of the filtering material or its incineration with the subsequent reprocessing of the ash. The latter factor provides for TEFMA applications in hydrometallurgy.
There is a bright future for TEFMA-based systems of air purification in compressor units, pneumatic industrial systems, at power stations and substations and in air conditioning. They will replace the complicated and expensive analogues of the KLINAR type (made in Yekaterinburg) which are produced on the basis of porous non-ferrous metals. What is more, the cleaning of air from mechanical particles by the new filters (99 percent efficiency) will make it possible to rule out abrasive mechanical damage to pneumatic tools caused by solid inclusions (the main cause of such damage).
TEFMA-based filters for capturing moisture and oil aerosols working on the principle of aerosols coalescence and fitted with settlers can be installed at textile mills where they clean the air from solid particles and also from oil and water aerosols. These contaminants are sedimented on the fibers and gradually drip down into the casing of a filter (cleaning efficiency up to 4-5 mg/m 3 ); the sediments can later be removed by "blowing through".
Finally, at automobile gas stations TEFMA filters can clean gasoline from solid particles of 5 pi and more (purification efficiency of no less that 90 percent) and also from products of oxidation and polymerization (60 percent efficiency), which cause car engine clogging with oily residue and poor combustion (discharge of harmful substances into the environment is reduced by 20 to 40 percent).
All the above systems are manufactured by the FRACTAL Plant in the town of Tver.
The ongoing research in this field holds out the promise of some new and even more impressive results in the future.
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