Filter Fabrics and Their Application

1. Introduction: -

                                    Filter fabrics are a class of fabrics used in the process of filtration i.e. removal of solid particles from a fluid. Despite growing demands for apparel & household textile owing to exponential growth in population, filter fabric have gained significant importance in various fields. Besides separation of water & land, filtration of polluted water will no doubt be most important task of future generations. Filter fabrics taken major role in improving scarcity of water as sophisticated filtration help to meet demand for drinking water.

                        In area of Industrial textile, filtration range from simple straining to difficult separation conducted in complicated equipment .The filter media are included in most promising market sector: INDA, the US association for non woven filtration purpose alone, to be billion square yards with demand increasing at steady pace.

 

 

2. Scope of the Filter fabrics

                                 Current emphasis on environmental protection by government & ecologists has increased tremendously scope for filter fabrics. The separation of solids from liquids or gases by textile filter media is the essential part of the countless industrial processes contributing to purity of products, improvement in process efficiency, saving in energy, recovery of precious material & general improvement in pollution control. Woven filter fabrics are commonly used for liquid & dry filtration, distillation, brewing industry, dairy, pharmacy &cosmetic industry, sewage disposal industry, printing industry, carburetor screen etc.woven glass fibre is used for removal of fly ash in utilities.

 

 

3. Abbreviation: -

                                   Filter fabrics are generally

1.      Woven: - Plain, Satin, Basket weaves

2.      Knitted

3.      Non- woven

            The above fabrics include the fibres like cotton, glass, polypropylene, polyester, wool, acrylic, etc The demand for these fabrics are enormous, but the criteria is that these fabrics should meet the specifications such as pore size, porosity, chemical inertness, tolerance for flow velocity, temperature etc. depending upon its applications.

 

 

 

4.  Classification of filtration: -

                                 

                           The filtration is classified in two headings viz.

 

  1. Dry filtration –

                             The dry filtration is a process of removing solid particles from a gas. Cleaning of dusty air and recovering extremely valuable carbon black from gaseous medium are two examples of dry filtration. In more modern application of dry filtration, cloth filters follow electrostatic precipitators or cyclones. Although these electrical or mechanical precipitators remove 95% of the solids in most cases the properly engineered fibre is up to 99.99% efficient in removing the remaining 5%

 

 

  1. Wet filtration: -

                              The purpose of wet filtration is either the recovery of solids from solutions in the form of a filter cakes as in wet process cement manufacture, or the production of a clear liquid or solutions such as wine, beer. Wet filtration further subdivided in to following three main categories.

 

i)                    Pressure filtration: -

                                           In this method, the slurry is forced into the medium under pressure and the cloth cades and filters the solution .At the completion, the cloth is scrapped to collect the cake. Threfore, these are quite heavy cloths. Hence, the fabric required is to be tough and durable to withstand the pressure brought about in this operation.

 

ii)                   Vacuum filtration: -

                                          This is similar to the pressure filtration .The cycle is: Each segment of a rotating drum is subjected to vacuum and other changes in pressure which successively suck out the filtrate, dry out the cake and discharge the before returning to the next cycle. Dimension stability is the prime importance in this process.

 

iii)                 Gravity filtration: -

This method is slow in operation and depends primarily on proper               fibre and fabric selection.

 

5. Selection of Materials for Filter fabrics

                  Textile fabrics are the best of all filter media due to their fine diameters & flexibility. In selection of the fibre, many thins have to be given considerations, the most important, however is the economics. Therefore, cotton on economical ground at times may turn out to be a false satisfaction, as a properly engineered synthetic cloth, costing twice the price of cotton, may give ten times longer operating life.  Besides cost, other important characteristics, which must be given, due consideration while selecting fibres for filter fabrics are: -

 

                                                1. Type of fibre (spun or filament) and its staple length (if spun)

2. Its swelling characteristics                

3. Electrostatic charge generation.

4 .fibre cross section

5. Strength & elongation

6. Chemical resistance

7. Heat resistance etc.

 

Technical requirements: -

                       

                        #  The filter fabric formation process requires fault free weaving of a dense sett.

# The filter fabric must with stand the temperature of filtration process.

#  This is followed by shrinking, calendaring etc to improve its dimensional                          stability

#  Special finishes like corona discharge, micro porous coating, chemical   protection finish may be required in specific cases

#  Air permeability to avoid the rise in pressure effect

 

 

6. Use of different fabrics as a filter media: -

               

                   The use of different types of fabrics as a filter media are:-

 

6.1 Woven fabrics as Filter Media.

 

                        Influence of fabric weave on fabric filter media is explained in the following

table: -

 

Parameter                        *********************Fabric Weave************************

                                               

Plain                             Twill                             Satin

 

Rigidity                                     Excellent                       Regular                         Little

Voluminosity                             Less                             Excellent                       Excellent          

Initial Permeability                 Less                              Good                               Excellent         

Retention Efficiency               Excellent                            Regular                        Less

Cake Loosening                       Good                             Less                            Excellent         

Blidning Resistance                   less                                Good                           Excellent                           ---------------------------------------------------------------------------------------------------------------------       

 

6.2 Non – woven as filter Media:               -

                                            

                                                     Up to the late 60s Filter media were almost exclusively made from woven man made, natural, mineral fibres. The greatest progress in the field of dust control in the past thirty years was the introduction of non-woven.  Mechanical reinforced non-woven has led to fundamental changes in dust control techniques.

They provide :-

a)      better dust removal,

b)      lower emission figures,

c)      smaller filter plants and d

d)      lower maintenance cost.

                                           They have displaced standard woven filter media in a relatively short time & were expected to reach 80% of the entire filter media market in Germany and Western Europe. In the areas of wet filtration, non-woven is extensively used in the roller filtration plants meant for cleaning lubricants, emulsion, thin oils or fully synthetic liquids. These fabrics are potentially better filters. They are cheaper to produce & are successful to capture fine particles.

 

6.3 Knitted fabrics as filter media: -

                                              

                                                             The knitted fabrics are used in the filtration purpose. The loop size & the yarn count decide the filtration efficiency. Thus according to the end use the knitted fabrics are woven & the required target is achieved.                                                                                                                        

 

 

 

6.4 Use of pile fabrics as filter media

                                              

                                                   Use of pile fabrics as filter media represents promising alternative to filter media used today. By targeted adjustment of woven or knitted pile fabric construction, effective filtration levels of convential filters in dead-end filtration can be attained & surpassed.

               The characteristics of the pile filter fabrics are: -

  1. Very high retention capacity with low pressure loss
  2. Especially satisfactory dedusting behavior due to markedly effective dedusting shape change in dynamic pile layer.

In wet filtration, as pile fabrics are used, calcium carbonate dispersion (mean particle diameter 0.075 mm, density 2.7 to 2.8 gm/cm2) with concentration of 10 gpl was used .At the filter  plant the effective filter area is 20 cm2.                   

 

 

7. Applications

 

7.1  Protection Masks :-

                                         Masks have been introduced for protection of respiratory fracts against pollutants. Mask is composed of bonded non wovens material as filter support , filtering element .Filter support is made from 50:50 blend of polyester fibres of same length (65 mm)bonded with latex(SKN-4014P)

 

7.2 High temperature filteration :-

                                                Aromatic polyamide fibre permit use of temperature in excess of 2000C and Teflon fibre use in excess of 2500C .

 

7.3 Needled Non woven fabric :-

                                                 Needling non woven batt of PTFE staple fibre in to woven scrim of textured continuous filament PTFE yarns useful in high temperature acidic environment found in coal fired boilers.     

 

7.4 Aerosol Filtration: -

                                                 Non Woven filter fabrics have relatively porous open packing of fibres that offer small aerodynamic resistance to air flow. Woven fabrics on other hand usually have compact packing of fibres & most generally operate at much lower velocities. Dust cake form slower with non-woven fabric than woven fabric because flow is relatively uniform across fabric surface rather than concentrated through a discrete no. of openings in weave.

 

7.5  Dust Cake filtration: -

                                                     Role of fabric is primarily that supporting for substrate for initially collected dust cake what then becomes primarily porous media for subsequent filtration .Ideal fabric is one which rapidly promotes formation of dust cake & then supports it uniformaly & through out filtration period. For given dust, fabric finish & texture can be varied to optimize cake formation.

                                                    In non dust cake filteration , fabric plays a more active filteration role. In this method cleaning takes place before formation of dust cake.True dust cake does not form in sense that properties of porous media depend on both dust & fabric through out filteration media.

 

7.6  Gas  filtration: -

                                                     Two US companies have jointly developed media for gas filtration. PurafilInc of Atlanta & AQF Techniques says that their media suspends particles of potassium per magnet        

 

7.7 Air filtration: -

                                                     Air filtration is improved with air filter that any of two fibrous layers of different bulk densities adhesively bonded together& made up of several this layer of spun bonded non-woven fabric. Today air filteration is done by the two methods :

a)      Low pressure drop filter :- One or more layer of micro fibres of average diameters less than 0.3 micrometers sandwitched between porous non woven webs of polyester fibres.

b)      New Pectecon PC 200 dust filter :- This is used for continous air pollution control & in foundries & chemical  plants.In this the air blows at 60,000 mt/hr. & operating temperature up to 1350C.                                                                                                         

                                                                                                                        

7.8 Antistatic filter media for dust removal :-

                                                                Electrostatically antistatic filetr element is used for filtering explosive dust .Surface of this filter media are provided with complete antistatic finish.       

 

By

Jitesh Oza & Bhavandekar , DKTE.

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