Chapter 2

Literature Review

23

CHAPTER 2

LITERATURE REVIEW

This chapter deals with the various works carried out by different research workers on

quality aspects of filter fabrics for dust control in particular relation to cement industries,

the evaluation of filtration parameters i.e., development of filtration apparatus by various

scientists and the research findings by various researchers on woven and non-woven needle-

punched filter fabrics for dust control.

2.1 Introduction

For controlling air pollutants, typically in the range of 0.1-0.25 µm or higher, they have to

be collected by several techniques [1]. Mechanical types of filters are, in general, effective

for the removal of coarser particulate matter, these can be used to reduce the burden of the

filter unit. For collecting small particulate matter, electrostatic precipitator, wet scrubber and

fabric filters are the only options. Among all the filters, the most efficient and the versatile

is the fabric collector, especially when processing very fine particles, which are very slow to

settle. The overall collection efficiency of the existing devices is high for fabric filters,

followed by electrostatic precipitator, wet scrubber. In selecting air pollution control

equipment, both technical and economical considerations should be made. The selection

should primarily concentrate on technical merits. As the technical selection is over, it is

essential that economic factors, capital and operating costs, should play an important role.

The specific reasons for wider acceptability of fabric filters can be summarized as follows

[2-6].

Fabric filters possess extremely high collection efficiency on both coarse and fine

particulates. Fabric filters can be designed to collect particles to the sub-micrometer

range with 99.9% collection efficiency. Removal of very small particles (PM2.5) at a

very high level of efficiency is becoming increasingly important as more stringent

emission controls are required.

The fabric filter is quite versatile as it handle large varieties of dust differing in

physical and chemical properties. It can capture all particles including those that can

be charged electrically. Performance of fabric filters is effective compared to

electrostatic precipitators when the electrical resistivity of dust particles is very high.

24

Fabric filtration is successful even under very high temperature and under different

chemical condition. A fabric filter system is very effective for the collection of fine

particulates and metals. Collected material is dry for easy subsequent handling and

treatment. Many filter configurations are available to fit wide range of specifications.

Fabric filters are simple in operation. ESP is believed to give less running cost in

terms of power consumption than a fabric filter, but a closer look may give a

different perception.

Needle-punched filter media permits higher filtration efficiency at relatively lower pressure

drop. Although there is some growing interest on spun-bonded and hydro entangled fabrics,

now, in many ways, needle felts would appear to be the ideal alternative for filtration,

combining the possibility of greater flexibility and versatility in construction. The

performance of needle-felted filter fabrics as filter medium has been strongly influenced by

their structural features. Synthetic fibres, particularly polyester fibres, are predominantly

used for filter bags. Proper selection and designing of filters involves the understanding of

the following parameters:

Physical characteristics of the dust: Particle size distribution, particle shape

Chemical composition of the dust: Flammability, alkaline, acidic, corrosive

Chemical composition of the gas: Oxygen content, moisture, corrosive

Operating temperature: Ambient, high temperature, fluctuating temperature

Mode of operation: Process

Conventional needle-punched nonwoven fabrics had a limitation in filtration as these failed

to satisfy the recent strict environmental standards. The fabric is generally acceptable for

collecting large particles (>5µm); however, it is not equally effective for smaller,

submicron-sized particles. The peg holes created in the needling process might allow the

particle penetration. On the other hand, fine particles with a diameter smaller than 2.5 µm

(PM 2.5) are known to have the highest impact on human health because they can penetrate

deeply into the human respiratory system. Problems associated with conventional filters

include higher emissions, higher pressure differential (∆P) across the filter due to clogging

of the felt, and `puffing‟ just after the cleaning cycle. Puffing occurs when particles are

dislodged from the felt during cleaning, resulting in a temporary increase in emissions.

Adequate surface finishing may highly influence filtration, the type of filter cake, cleaning

behavior, and finally the lifetime of the filter. Finishing may be based on thermal (heat

setting, singeing, calendaring, and condensing), chemical (anti-adhesive, hydrophobe/

25

oleophobe, antistatic, antiflammable, chemical resistant, etc.), and physical process as well

as the combination of the above. Use of coated finish and application of membranes, in

particular, have become common in the manufacturing of filter fabric.

A large number of publications in this area highlight the research on characterization of

fabric filters and properties in relation with process variable, machine variables and raw

material parameters. In the present review, in addition to general information regarding

filter fabrics, existing methods of measurement of filtration parameters, the factors affecting

filtration and mechanical properties and a comparative study of cross-sectional shapes, fibre

fineness and machine parameters of needle-punched nonwoven filter fabrics have also been

reviewed.

2.1.1 Concept and mechanisms of filtration

The main objective of the filter medium is to maximize the possibility of collision and

the subsequent retention of the suspended particles in the fluid (air/gas or liquid)

stream with the media‟s fibrous structure while minimizing the energy loss of the

stream of the fluid.

Filtration is a process of separating solid particle from a liquid or gas (air) in which

they are suspended by passing liquid or gas through a barrier, which is filter medium.

Other closely associated techniques are ultrafiltration and reverse osmosis, but these are

distinguished from conventional filtration by the much smaller size of particle

removed, approximately 0.003 to1 micron and 0.0003 to 0.3 micron respectively, while

conventional filtration normally deals only with particles size larger than 1 micron [7].

Conventionally filtration has been divided into two areas, wet filtration, that is the

filtration of liquids, most commonly water, and dry filtration, which is the filtering of

gases, mainly air. The obvious differences between the two areas are the enormous

difference in density between water and air and the fact that in one case the filtered

solids are sludge and in the other case a dry powder. These differences have led to

corresponding differences in the design of filter installations, but in spite of these

differences, it has been realized that the basic concepts of filtration are the same in both

areas. It is possible to imagine a filter medium consisting of a thin membrane perforated

with pores of a uniform size. In theory the fluid, either gas or liquid, would flow through

these pores, together with any particles smaller than the given size, particles greater

26

than the pore size would obviously be stopped by the membrane [8]. Still thinking of the

same model membrane one can imagine that some particles will be almost exactly the

same size as the pores, so that they become firmly wedged in the pores and cannot be

removed, even for instance by a reverse flow of the fluid. This situation is known as

blinding, which must be avoided by correct design of filter medium.

2.1.1.1 Particle collection theory

Much has been written on the various mechanisms by which particles are arrested by

unused filter media. These are normally explained in terms of the effect of a spherical

particle on a single fibre and may be summarized as:

1) Gravitational, 2) Impaction, 3) Interception, 4) Diffusion (Brownian motion),

5) Electrostatic.

Figure 2.1 Particle collection mechanisms

These mechanisms are shown diagrammatically in Fig. 2.1.The theories behind these

mechanisms notwithstanding, it has been argued that although they may be valid for

certain air filtration applications where particle capture total is vital, for the purposes of

industrial dust collection, they are of limited value [8].

27

A sieving mechanism is probably more appropriate wherein the size of the apertures in

the medium assumes a more dominant role, at least until the fibres have

accumulated a layer of dust which then takes over the sieving action.

2.1.1.2 Air microfiltration

Although practically all dry filtration is the filtration of air, has conventionally been split

into two divisions gas filtration and air filtration. Gas filtration using textile filter media,

deals with air with high solid loads and often at very high temperatures, for instance,

collecting fly ash from coal fired boilers, or collecting carbon black as a part of the

production process [9].

Air filtration is always at or near room temperature and deals with air carrying only very

low loads of solids, e.g. less than 5gm/m3. Filter media are used in the air circulation

system of public buildings and to supply dust free air to paint spray booths and computer

rooms. Nonwoven materials, produced for instance by carding and cross lapping, and

bonded either by resin or thermally to produce a thick, open and high relatively light

weight structure [7]. When a high purity of air is required, as in clean rooms and spray

booths, a structured medium can be used, giving better retention but also having

better pressure drop across the filter.

2.1.1.3 Permeability of fibrous filters during filtration

Fluid flow through fibrous filters is a well-established topic. For a fluid moving with a slow

steady velocity, the pressure drop (∆p) through the filter and the velocity (v) are related

through the Darcy law (Eqn. 2.1)

µf z

∆p = -------- v …………(2.1)

K

where K is the filter permeability, µf the dynamic viscosity of the fluid and „z‟ the

thickness of the filter.

2.1.2 Flow porometry

Use of modern technology has made flow porometry a very powerful and versatile tool

for determination of a wide variety of pore structure characteristics of filtration media.

Test results have been presented to demonstrate the use of the technique for

28

measurement of pore structure characteristics [10]. Measurable characteristics include the

constricted pore diameter, the largest pore diameter, the mean flow pore diameter, pore

distribution, gas permeability, liquid permeability, envelope surface area and effects of

operational variables such as temperature, pressure, chemical environment and stress.

Applications of the technique including pore characteristics in the thickness

direction, pore characteristics in the x-y plane, properties of individual layers of multi-

layered products determined in-situ without separating the layers and evaluation of

properties without cutting samples and damaging the products. 2.1.2.1 Types of pores Textile filtration media contains three kinds of pores

1) Closed pores, 2) Blind pores and 3) Through pores (Fig.2a)

Closed pores are not accessible. Blind pores do not permit flow. Through pores permit

flow.

Figure 2.2 a) Three possible kinds of pores. b) Constricted pore diameter

The diameter of the through pore at its most constricted part determines (fig.2b) flow and

is equal to size of the smallest particle that would be prevented from passing through

the filtration media [10]. The largest through pore constricted diameter, the mean flow

pore diameter and the pore distribution determine the efficiency of separation by the

filtration media. Similarly, liquid permeability determines the rate of the filtration

process. For separation of small solids from gases, external surface area (surface area of

through pores) is important because small particles tend to stick to the surface. The

rates of such filtration processes are determined by gas permeability. All these

important characteristics and the effects of operational variables can be determined by

a single instrument based on flow porometry. No other instrument is capable of such

29

versatility.

2.1.3 Textile filter medium

Since early year of 1950, the textiles used in filtration were uniquely based on woven

fabrics of wool, cotton and glass fibres. After development of synthetic fibres and

nonwoven fabric technology substantially modified the use of textiles in filtration.

Therefore, textile filter media can be divided broadly into two groups woven textile

filter media and nonwoven textile filter media [11].

2.1.3.1 Woven textile filter media The woven filter media dominates in certain cases due to easy estimation of pore size

distribution, easy to construct to obtain desire filtration efficiency and easy cleaning of

chocked filter medium. The woven textile filter medium can be constructed

according to a particular size and desirable filtration efficiency by simply changing

weave parameters and yarn characteristics which provide an indication of saving of cost

and best results suitable for different industries as their requirements. Woven textile

filter media can be divided into three groups woven monofilaments, woven

multifilament and woven staple fibre fabrics [7].

2.1.3.2 Nonwoven textile filter media Types of Nonwovens Used In Filtration:

The processes for manufacturing nonwoven fabric can be grouped into four general

technology bases: textile, paper, extrusion, and hybrid/composite. The textile

technology base forms textile fibers into selectively oriented webs in the dry state, and

fabrics produced by this method are commonly referred to as "Dry laid nonwovens". The

paper technology base includes dry laid pulp and wet laid systems, each

manufactured using synthetic fibers and wood pulp, and then suspended in either air (dry

laid pulp) or water (wet laid). The extrusion technology base forms a sheet structure by

extruding molten polymer through a die or spinneret and laying filaments onto a moving

belt. The hybrid technology base marries two or more fabric substrates using at least one

nonwoven web formation technology. A good example of a hybrid sheet is "SMS" which

represents a composite of spunbond/melt blown/spunbond web structures.

30

2.1.3.2.1 Spunbonded (or Meltspun) nonwovens Spunbonded is relatively strong with modest dust holding capacity, but lacks the

consistent pore size distribution of wet-laid and melt-blown webs [12]. The fibers are

relatively coarse (15-20 µm), resulting in media that, although strong, can present

difficulties in cake release and “residual media weight gain.”

2.1.3.2.2 Needle felts Needle felt fabrics are common when strength and durability are necessary such as in bag-

house applications. Manufacturing of needled non-woven fabrics using a mixture of

fibres without antistatic spin finish results in obtaining the filtering material, whose

properties depend strongly on the proper settings of technological parameters. The

most important parameters significantly influencing the particle capture efficiency are

linear density of constituent fibres, gauge of needle, depth of needle punching and mass

per unit area of a nonwoven sample [13].

Air-laid are popular because of its high-loft, bulk and large dirt holding capability in air

filtration, including pre-filters capable of capturing larger particles. These fabrics

typically have high void volumes. Wet-laid synthetics are especially used when a

thinner web and consistent pore size is important.

2.1.3.2.3 Meltblown nonwovens

For meltblown nonwoven, fiber diameters have been determined by analyzing fabric

images obtained through scanning electron microscopy (SEM). Under the specific

conditions, the fraction of fibers of diameter smaller than 10 microns (µm) can

increase by 72% with a 7.9 x 10-2

g/min/hole (82%) reduction in throughput. A 54%

increase of the same can be observed with a 2.8 bar (400%) increase in attenuating air

pressure. A change of 45ºC (16 %) in air temperature is shown not to significantly

affect fiber diameters produced, while an increase of 67ºC (26%) in die temperatures can

result in an increase of 17% in the fraction of fibers of diameter smaller than 10µm.

All fiber diameter distributions are shown to be unique to the condition evaluated

as no overlap across distributions for changes in a given parameter is observed.

Further fiber fraction smaller than 10 µm data is also shown to be unique to each

parameter evaluated [12].

Meltblown nonwovens produced by extruding molten polymer especially polypropylene

through spinneret orifices. The resultant discontinuous fibers are finer (2-7µm) than

31

those that are spun bonded and bulkiness for excellent dirt holding capacity. They

also have significantly less strength and often must be used in combination with

other strong fiber webs. Melt blown nonwoven fabric continues to be the rising star with

rapid penetration into many liquid and air filtration applications [14].

2.1.3.2.4 Advantages of nonwoven fabric filtration

Advantages of nonwoven fabrics over woven fabric in filtration are high permeability,

higher filtration efficiency, less blinding tendency, no yarn slippage, good gasketing

characteristics and good cake discharge. There are many other advantages of

nonwoven fabrics, including their versatility, low cost and diverse functionality. The

price-performance ratio is outstanding. Nonwoven fabrics are made from standard and

many specialty inorganic and organic fibers including common wood pulp, cotton or

rayon. Fine glass fibers are traditional in air filtration from HVAC to HEPA filters.

Resin bonded glass fiber liquid filter cartridges also provide many excellent

properties. Another advantage of nonwovens is the wide number of diverse processes

fibers can be incorporated including needlefelt, air and wet laid, resin bonded. The direct

web manufacturing method offers a cost advantage and very popular [13]. These direct web

processes produce fine and sometimes continuous filaments and in case of spun bond, a

strong and non-shedding web which cannot be achieved by any other means for a

comparable cost. 2.1.3.2.5 Disadvantages of nonwoven fabric filtration

Nonwoven fabrics like most filtration media have disadvantages compared to other

media choices, such as polymeric membranes, woven fabrics, metal media etc. No

single media can or will ever satisfy every filtration requirement. In the case of

nonwoven fabrics, the disadvantages are not so much the shortcomings of nonwovens, but

the advantages other media. For example, membranes provide narrow pore size

distribution, particularly below 1 micron mean flow pore [13]. Monofilament fabrics and

wire cloth offer strength and straight through holes for use in sifting and excellent sieving

capabilities. All three of these are surface filters, a feature not easily achieved for

nonwoven fabrics, because of the nature of the manufacturing processes and resultant

constructions; at least, not yet. Many filtration and separation applications require

stiffness, minimal flex, and rigidity or even low stretch as is the case of dewatering

belts, which by their nature are less favorable to the use of nonwovens.

32

2.1.3.3 Importance of textile filter media

Main importance of textile filter media in air filtration is to control air pollution. Air

filtration plays an important factor in improving air quality and hygiene at work. The

demands on air quality and hygiene at workplaces have increased greatly due to new

regulations, new scientific knowledge and a change in health consciousness. Apart from

temperature and relative humidity, primarily the concentration of gaseous and solid

contaminants is an important parameter to evaluate the air at workplaces. There exists a

multitude of possibilities to improve the air quality[15-16] . Filters made of nonwovens are

suitable to effectively collect particles (dust) from intake or exhaust air and have therefore

been used for this purpose.

2.1.3.4 Applications of textile filter media Fabric filters find intensive industrial application in metallurgical industries foundries,

cement industries chalk & lime plants, brick works, ceramic industries flour mills,

medical, pharmaceutical, acoustics and screen printing. The multitude of different

workplaces ranging from workshops, manufacturing halls, control stands to offices,

schools etc. Main application areas for textile filter media are aerospace, automotives,

bioclean, disk drives, flat panels, food, hospitals, medical devices, pharmaceuticals and

other electronics [16].

2.1.3.5 Filter configurations and function Filters, made from nonwoven fabrics, are extremely wide in their configuration and

applications. Perhaps, the simplest application of nonwoven media is found in coolant

filtration. One popular method is to unroll and index nonwoven media over a tank and flow

coolant containing metal chip and grindings through the media. Other common filtration

configurations for nonwovens include cut & sewn bags which are attached to pipe outlets

used as strainers for milk, paint and chemical products. Needle felted sewn bags are fitted

with metal or plastic rings at the open end, which seal into housings for liquid filtration and

long bags that hang in baghouses, which are flexed, or back pulsed to remove large

volumes of particulate in air filtration. Other times pleated filter cartridge manufacturing

processes use nonwoven fabrics involving spun or point bonded and wetlaid fabrics

that serve as drain layers and media separators in cartridges or as a membrane

supports. Melt blown and wet-laid media are frequently used in these cartridges as the

main filter media. Overall spun bonded fabrics are common as the main medium in

33

pool and spa filters providing a septum for diatomaceous earth. High-loft media has

bulk and traditional for air filtration [17]. These structures rely on and are effective,

largely because their thickness and an open structure. Thicker media allows for longer

residence time for the capture of particulate across a wide size range within the random

size voids. Special fabric post-treatments, such as corona-discharge and the co-mingling

of select fibers can yield constructions having electric properties or triboelectric potential

which will improve air filtration efficiencies and/or permit greater air flow at lower

differential pressure depending upon the construction.

In filtration, nonwoven fabrics can be generally described as a random fibrous web,

formed by either mechanical, wet or air laid means and having interconnecting open area

throughout the cross-section and able to remove a percentage of particulate from liquid or

gaseous fluids streams flowing through it [18]. Typically, nonwoven fabric filtration

media have 1 to 500 micron mean flow pore (MFP) ratings. Below 10-15 micron, the

fabrics must be calendared in order to achieve the finer micron ratings. Nonwoven

fabrics have been manufactured, in multiple forms, from many grades of cellulose and

most natural and synthetic fibers. The most popular fibers used being polyester,

polypropylene and glass. Following are acrylics, rayon, nylon, cotton, fluoropolymers

and a host of others that fill niche applications because of their special material

compatibility for particular applications. Melt blown technology allows for fibers in the

1-10 micron diameter size range and bulkiness for excellent dirt holding capacity.

2.1.4 Pneumatic conveying of particles Pneumatic conveying, in which solids are transported in a pipe or channel by gas flow

through blowing or suction [19].

2.1.4.1 Vertical pneumatic transport

In vertical pneumatic transport, particles are always suspended in gas stream mainly

because the direction of gravity is in line with that of the gas flow. Hence to transport the

same solid mass flow rate in a suspended flow, it takes lower gas velocity for upward

vertical conveying than for horizontal conveying. For most cases in vertical pneumatic

conveying, the radial particle concentration distributions near uniform and thus, gas and

solids can be reasonable treated as being in one dimensional flow. The drag force, wall

friction, and gravity combine to produce a pressure drop in upward vertical conveying

for given gas and solids flow rates [19].

34

2.1.5 Cake formation In accordance with classical filtration theory, porosity determines the residual

moisture and the flow resistance of a filter cake. A reliable estimation of porosity is

possible only on the basis of filtration experiments. It is well known that the influence of

particle-particle interactions increases with particle sizes of less than approximately 100

µm. In that case the particle-particle interactions become decisive and lead to

agglomeration and loosening up of the filter cake [20].

2.1.5.1 Criteria for cake formation The model calculates the distances between a new falling particle and all other

particles deposited on the filter media. It also finds the minimum distance among other

particles, and compares this distance with the summation of the radii of the new particle

and closest particle. If the minimum distance is smaller than this summation, it makes the

new particle adhere to the surface of the closest particle. Depending on the X-Y

coordinates of both particles and using geometrical analysis, a new height will be

calculated. This new height corresponds to a new cake thickness. The model also

calculates the average cake porosity (Ek) from the eqn.2.2 by summation of the total

volume of the particles deposited on the filter [21].

……………..(2.2)

2.1.5.2 Filter cake as a random packing Under consideration is a filter cake formed by monodisperse, non-deformable

spherical particles of diameter dp. Through the random joining of the particles, pore

sizes of between dk,min and dk,max may be formed (Fig. 2.3).

35

a) highest density packing b) lowest density packing

Figure 2.3 Minimum and maximum pore size of a random packing without

particle-particle interactions

A simulation of the pore size distribution results in a normal distribution within the

limits

d k ,min

= 0.223 d k, d

d

k ,min

p

,………………(2.3)

= 1.1 d k

d

36

p

The porosity of a random packing of monodisperse spheres (eqn. 2.3) is predicted to be ε0

=0.43. Porosity under this value is only possible if orderly packed particles are present

and porosity above 0.43 is possible if significant interactions exist [20].

2.1.6 Air filtration parameters

2.1.6.1 Air permeability

Permeability is rate of flow of fluid under a given pressure differential through an open

area of fabric while the porosity „h‟ of a fabric is defined as the ratio of open space to

the total volume of porous material calculated from the measured fabric thickness and

weight per unit area of fabric [22].

2.1.6.2 Face velocity

The face velocity can be expressed by the ratio of the volumetric flow through the filter

(Q) to the area of filter medium [23] The expression is given by the eqn.2.4.

V = Volumetric flow rate through the filter (Q)

= Q

(2.4)

Area of filter ( A) A

2.1.6.3 Pressure drop

Drop in pressure through a filter is defined by following expression,

∆P = P1 − P2 …………………… …(2.5)

where

P1 =Pressure on the face side of fabric i.e. the side facing the air stream.

P2 = Pressure on the reverse side of fabric.

Initially the difference depends uniquely on the fluid properties of the pours medium as

the filtration progresses, this loss also depends on the properties of particles retained

by the filter.

2.1.6.4 Filtration efficiency

The filtration efficiency is defined as the ratio of the number of particles arrested by the

filter fabric to the particles number of contaminated gas fed. It expressed as

percentage. (eqn.2.6)

37

Filtration efficiency (%) = Number of particles arrested by the filter fabric(N) X 100

Inlet particles number (n1)

= Inlet particles number (n1) – Outlet particles number (n2) X 100

Inlet particles number (n1) ………………………………(2.6)

2.1.6.5 Outlet concentration

From the eqn.2.7, the outlet concentration C0 can be expressed by the ratio of the mass of

particles passed through the filter to the volume of air passed during a given filtration

time

m p

C0 =

Q t f (2.7)

38

where

mp = mass of particles in given filtration time

tf = filtration time

Q = volumetric flow rate through the filter [23].

2.1.6.6 Filter drag

Filter drag is the filter resistance across the fabric-dust layer. The equation for filter drag

essentially gives the pressure drop occurring per unit velocity. It is a function of the

quantity of dust accumulated on the fabric and is given in eqn. 2.8 as:

S = ∆p V f

.

……………………….(2.8)

Where: S = filter drag, in. H2O/(ft/min) [cm H2O/(cm/sec)]

∆p = pressure drop across the fabric and dust cake, in.H2O (cmH2O)

vf = filtration velocity, ft/min (cm/sec)

The true filtering surface of a woven filter is not the bag itself, but the dust layer. Dust

bridges the pores or openings in the weave, plugging the openings with particles,

increasing the drag rapidly [24].

2.2 Filter media for dust control

Emerging technologies of nonwoven formation give opportunities to obtain novel filters

leading to improved effectiveness of dust filtration. The market for filter media is growing

together with new filter applications [25]. The use of nonwoven filter fabrics is growing

steadily in applications like separating industrial dust in cement, coal mines, purifying air.

Regardless of the final applications, non-woven filters possess the following features [26]:

High air permeability

High filtration efficiency

Low airflow resistance

It is rather difficult to combine all of these features in practice. Generally there are two main

groups of tests used for the characterization of nonwoven filter fabric. The first group takes

into account the structural measurements of filters like the mean porosity of filtration layers

39

and air permeability. The second one comprises of measurements of the dynamic changes in

filtration efficiency and airflow resistance during filter loading as well as the determination

of retained capacity [27].

Overall filtration performance of filter media is influenced by void volume within

nonwoven filter materials . In most applications both the void volume and its accessibility to

the passage of air are important. [28].

The effect of process parameters on filtration efficiency has been discussed in detail in

many papers. Numerous studies have been conducted on needle-punched nonwoven as well

as melt-blown, spun-laced and composite structures [29-30]. Kothari and Newton assigned

the effect of the concentration of the binding agent used for padding and the surface weight

of nonwoven to the air permeability of structures obtained [31].

2.2.1 Material type

The selection of proper fabric is one of the primary factors for the proper functioning of the

fabric filters [32]. A variety of common fibres like natural cotton, wool, etc. or man-made

fibres like polyester, acrylic, polypropylene, polyamides, glass, aramide,

polytetrafluoroethylene (PTFE), etc., are being used for the manufacturing filter bags. In the

market, many manufacturers offer various types of fibre materials, either named after used

chemicals or in their own trade name. Properties of various common fibres and their uses in

the industry are given in Table 2.1:

Table 2.1: Properties of fibre material used in filter bags

Fiber Continuous

Operating

Temperature

(0F)

Acid

Resistance

Alkali

Resistance

Abrasion

Resistance

Tensile

Strength

Cotton 180 Poor Good Very good Good

Wool 200 Very good Poor Fair to Good Poor

Nylon 200 Poor-fair Excellent Excellent Excellent

Polyester 275 Good Good Very Good Excellent

Polypropylene 190 Excellent Excellent Excellent Excellent

Glass 500 Fair-Good Fair-Good Fair Excellent

Nomex 400 Fair Very good Very good Very good

Teflon 450 Excellent Excellent Below

Average

Average

Selection of a particular fibre material in a specific application depends on various

properties of fibre like heat resistance, chemical resistance, resistance to moisture, physical

properties, relative price, etc. If the fibre material is not properly selected, there could be

many consequences, such as severe shrinkage of material, fibre degradation following

40

embrittlement structure, accelerated pulse-flex fatigue, etc. However, the worst affected

bags do show severe flex fatigue at the top of the bag with apparent but less significant flex

fatigue at the bottom of the bag. High- temperature filtration is one of the most promising

developments in particle-collection technology as only on the basis of the selection of types

of materials as filter media.

2.2.2 Fibre fineness and cross-sectional shape

Fibre fineness in a filter element varies widely. The fibres are normally in the range of 1.66-

3.33 decitex, though trends of considerably finer `microfibres‟ (e.g. less than 1 decitex) have

gained some prominence. Using microdenier fibre can bring down the pore size from 35-66

µm to 12-25 µm. Higher filtration efficiency can be obtained due to larger surface area and

smaller size of pores. In a study, use of coarser fibre (7 denier) instead of 2.7 denier/3 denier

fibre in PPS/felt has been found to improve the performance of a hybrid fibre unit (Compact

Hybrid Particulate Collector-COHPAC) as permeability increases substantially without

compromising particulate collection efficiency. The general trend is to use finer fibre for

achieving higher filtration efficiency and through the selection of the fineness of fibre of

filter media, one can optimize the pressure drop during filtration.

Different fibre shapes are available now-a-days for filtration purposes like Hollow, Trilobal,

etc.

2.2.3 Filter media types and specifications

Media types and specifications mainly depend on equipment requirement, application, and

filtration specific operations. Most of the cases, fabrics with distinct surface characteristics

are used in both scrim-supported and self-supported and with both sewn seams and fused

seams. Fabric specifications could vary depending on the intensity of pulse-jet pressure, its

frequency, and duration in the equipment.

Various requirements of fabric filter can be enlisted as follows:

Filtration requirement, which encompasses smallest particle retained, overall

filtration efficiency, resistance to flow, tendency to blind and cake discharge

characteristics

Physical properties, such as dimensional stability, elongation at break, bursting

strength, resistance to creep/stretch, resistance to wear, absorption of moisture,

electrostatic charge, flexing strength

Heat resistance

Chemical resistance, such as alkali, acid, reducing agent, organic solvent, etc.

Ability to be fabricated, sealing, and gasketing function are also important

41

A needle punched fabric with weight (GSM) of 350-500 g/sq.m is very commonly used,

which provides adequate strength and life to filter element. Material of higher weight

usually leads to greater thickness. A thick material with small equivalent pore size gives

better efficiency. On the other hand, material with higher thickness decreases the flex,

making the cleaning difficult. Higher fabric thickness also leads to higher pressure drop

across the fabric. Therefore, fabric weight should be judiciously selected depending on the

filtration requirement.

In designing the nonwoven fabric, material consolidation and hence the pore size and

porosity can be regulated through:

Needle design, Needle fineness, and Needle orientation

Needle board pattern, Needling program (punch rate and penetration)

With the increase in material consolidation, filtration efficiency can also be increased but at

the cost of higher pressure drop.

2.2.4 Filter fabric finish

Through the application of finish, basic fabric characteristics can be improved for efficient

filtration. These are designed essentially to improve the following characteristics:

Fabric stability

Surface characteristics

Permeability of the fabric

Filtration collection efficiency

Cake release characteristics

Resistance to damage from moisture and chemical agents

To restrict the dust particles to the surface of the fabric so as to reduce the

blinding tendency

A number of finishing processes are employed to achieve these goals, e.g., heat setting,

singeing, raising, calendaring, special surface treatments (membrane lamination and

coating), and chemical treatments (hydrophobic finishing, flame retardant finish, and

antistatic treatment). Most of the finishing processes are very common for many textile

applications except the special surface treatment through incorporation of a more efficient

membrane in a lamination operation or by coating of filter media. Apart from enhancing

filtration efficiency, membrane and coated fabric offer the following advantages:

Reduced emission of finer particles

42

Better cake release property through imbibing smooth surface and moisture-

repellent property of the fabric

Enhances the life of filter due to reduced pore blocking by the dust particles

2.3 Evaluation of filtration parameters

Filtration parameters are being generally evaluated by different research workers in terms of

Air permeability, Pressure drop, Filtration efficiency, Cleaning efficiency, etc. In order to

evaluate the above filtration parameters, different scientists had developed various kinds of

filtration apparatus according to their requirement and uses. The details of such

developments are discussed in Chapter-III.

2.4 Early investigations on dust filtration

Lamb et al [33] have reported that in a non-woven, single fibre characteristics assume

dominant role, since the effect of weave pattern, yarn twist, weave density etc. are absent.

When single fibre, rather than the yarn is filtering element of the structure, single fibre may

affect and control filtration performance through their geometrical properties, surface finish,

electrical properties, hardness and mechanical properties. They concluded in their study on

the influence of fibre properties of model non-woven fabrics as follows:

i) the use of trilobal instead of round cross-section fibres increased the

efficiency

considerably, the beneficial effects of trilobal cross-sections being found

more in less efficient fabrics: the performance was better with fibres having

three or four lobes but the performance improved with the lobe depth.

ii) the use of 3 den (3.3 dtex) instead of 6 den (6.6 dtex) fibres improved the

efficiency but at the cost of increased drag.

iii) the use of crimped fibres rather than uncrimped ones reduced drag, with an

improvement in filtration efficiency for the particles.

According to Igwe [34], if the pore size is large, a higher dust mass is expected to pass

through the filter fabrics, while there will be higher dust retention and a low mass of dust

will pass through if the pore is small. He also stated that the filtration efficiency depends on

the needle dimensions and the needling density.

Sayers and Barlow [35] have reported that when dust bearing gas is passed uni-directionally

through a permeable textile medium, the dust particles are arrested on the dirty side of the

43

fabric, while the cleaned gas passed through the cloth and out of the collectors to be either

vented to atmosphere or returned to some part of the processing operations. As the dust

accumulates on the cloth, a “cake” is formed which aids filtration by improving particle

capture. This results in a gradual improvement in capture of fine particles, which in turn

raise the fabric filtration efficiency. If the cloth, following a cleaning cycle, does not return

on each occasion to the same acceptable pressure drop level, the particulate residues are

building up within the fabric structures and ultimately blinding will occur. The dust particles

are of different sizes and shapes. The particles, which are larger in size than the pore size of

the filter media, are easily prevented from penetrating the fabric and are arrested on the

surface of the filter fabric. This type of filtration is known as the “surface filtration”. The

cakes produced in this process are easily removed by simple “Shaking” type of cleaning

processes.

On the other hand, as reported by Rodman [36], the fibrous filter media under consideration

can be either water laid or air laid and usually comprises a blend of coarse and fine fibres

with an increasing gradient density of fibre packing from the dirty side to the cleaner filter

side. The filtration mechanism is one of sorption of fine particulate on the fibre surface by

impaction-impingement followed by depth mechanical sieving of coarse particulates, which

pass through the initial open fibrous network. This type of filtration is commonly known as

“depth filtration”.

Filtration performance criteria can be well illustrated by the filtration theory presented by

Rothwell [37] which covers both cake filtration and depth filtration. Essentially for general

filtration, the relationship between flow rate, dv/dt, pressure drop ( ∆P ) and the parameters

of a packed bed, may be expressed using Darcy‟s law [38] as given in equation (2.1)

1/A x dv/dt = K ( ∆P/ µL ) …………………………….. (2.1)

Kozeny and Carman expanded the above expression to include beds of uniform spherical

particles, thus

1/A x dv/dt = E³/K1(1-E)²Sp² x ( ∆P/ µL ) ………………….... (2.2)

Where, E is the porosity of the bed of particles, K1 is the Kozeny‟s constant and Sp is the

specific surface of the particle comprising of bed, i.e., total surface area of a unit volume.

The above equation basically applies only to spherical particles and does not take changes in

filter permeability due to cake filtration into account and assumes Newtonian flow. Some

basic filtration equations have been derived from the Kozney‟s equation for various

44

filtration operations like constant pressure filtration, constant rate filtration, constant rate

followed by constant pressure operation and variable pressure variable rate operation.

Experiment carried out by Sievert and Loeffler [39] with different cleaning methods for

non-woven filter fabrics have shown that effective cleaning of the surface of a filter medium

is possible not only by acceleration, but also with the aid of reverse flow. In order to remove

a dust cake from a flexible polyester needle felt, typical reverse flow rates of minimum 550

cubic meter/sq. meter/hour (15.3 cc/sq. cm/sec) were used. It was found that the dust cake

release from a nonwoven fabric is improved when thicker dust layer are deposited. Thus for

bag filter installations, it is advantageous to reduce the cleaning efficiency, i.e., to clean

after longer filtration intervals. In addition to this the outlet dust concentration is also

reduced.

Dietrich [40] has suggested the following probable or combination of mechanisms

responsible for filtration :

(a) Sieve effect, where the particles bigger than the pores are separated

(b) Trapping effect, where if the shape of the pore is different from that of a particle

the latter will be separated even if it is smaller than the pore. The mechanisms of

sieving and trapping can be classified together since in both cases the same result

is achieved, i.e., the particles remain on the surface.

(c) Inertia effect, where a particle due to own inertia travels towards a fibre although

the air stream bend around it. The mechanism operates better for heavy particle

and at higher speeds so that one would expect from this theory higher filtration

efficiency at higher gas speeds. However, it is well known that it does not

happen and the filtration efficiency actually goes down. It is always assumed that

the reason for this is that while at low gas speeds a particle striking a fibre will

be captured, at higher gas speeds the particles will have sufficient energy to

bounce off the fibre increasing its chance of completely penetrating the filter.

(d) Diffusion effect: This mechanism can be neglected as it is very small with needle

felts. This is based on Brownian motion of the gas particle. In needled

nonwoven, face velocities are so high that diffusion effect rarely takes place.

Particle size would be below 0.2-0.5 microns.

(e) Electrostatic Effect, where depending on the electrostatic charge the particles are

either attracted or repelled by the filter media. As far as the needled felts are

concerned, the particles and fibres used are heavily charged.

45

The experiment of Rothwell [41] also relate the mechanism of filtration for needled fabrics

to sieving through the developing filter cake after the initial pore blocking period and

“fibrous filtration period” i.e., inertia, diffusion, electrostatics. It is necessary to mention

that with needled fabrics, the mechanism of sieving and trapping which takes place before

cake formation, does not necessarily takes place on the surface alone, could also include

some “depth filtration”.

In another study, Igwe and Smith [42] stated that, like most filters, needled fabrics do not

operate purely by a mechanism, and the general mechanism which would predominate will

very frequently dictated by various factors such as the size of the particles being filtered out,

fabric density, air permeability, fineness of fibre, type and size of felting needles, depth of

needle penetration and needle density.

According to Smith et al [43], the fibre properties are crucial in filtration. They concluded

that finer fibres give higher filtration efficiency owing to the larger surface area of the fibres

available to absorb the dust in depth filtration. They found that the finer fibres produce a

lower permeability than the coarse fibres, owing to the greater surface area of the fibres but

it is probably significant that in the heavy needled fabric, the difference is less due to the

higher density of this fabric.

Atwal [44] stated that air resistance is a result of friction (drag) occurring between the fibres

of the fabric, the magnitude of the air resistance would be expected to increase with the

increase of number of fibres i.e., the increase in the total exposed surface area of the fibres.

Due to this reason air permeability of fabric will be higher, which is composed of coarser

fibre than of finer fibre. Atwal in his paper on needle-punched nonwoven also stated that the

air permeability or air resistance depends on fabric weight per unit area and thickness. The

result of step-wise multiplication analysis based on his data is as follows:

r = 15.73 + 141.1 x m – 0.012 x h³/(1-h)² x d + 29034 t/d ………….(2.3)

Where m = weight per unit area of fabric (kg/sq.m.)

h = porosity of fabric

d = fibre fineness (dtex)

t = thickness of fabric (mm)

r = air resistance

The filtration properties and the resistance to the flow of air is greatly influenced by fibre

fineness. As reported by Lamb and Costanza [45] if fabric weight and density are kept

constant, air permeability varies linearly with fibre fineness. When filtering a very dilute

46

aerosol of submicron particles they observed that the capturing efficiency varies linearly

with the d/s, when “d” is the effective particle diameter and “s” is the inter fibre space.

The three variables investigated by Kothari and Newton were web weight, needle

penetration and needling density. They stated that needling process cause a fibre web to

increase in density and decrease in thickness. These changes in web tend to be greater with

increase in either the needle penetration or needling density but they are affected by the

spreading action of needling, which lessens the weight per unit area of the web being

processed. Air permeability was found to be proportional to the reciprocal of the fabric

weight per unit area. They established a relationship between air permeability(Pa) and

weight per unit area (w) as follows:

Pa = K1 /w + K2 ……………….(2.4)

Where K1 and K2 are constants and for needle punched fabrics :

K1 = 1.75 x 10 , K2= -0.66

Dent [46], however, did not agree that fabric weight alone controlled the air permeability of

needled fabrics, but that fabric density and thickness affect air permeability just as such as

fabric weight.

Hearle and Sultan [47] also investigated the effect of web weight on the properties of

needle-punched fabrics. They reported that the fabric weight as expected, increases with an

increase in web weight. There is, however, a reduction of weight during the needling action

gives an increase in length, as the web is dragged through the gap between the bed and

stripper plate. It is, also, partly caused by recovery of fibres pulled down by the needles into

the holes in bed plate. When the needles are withdrawn, they will tend to pull up the fibres

again, and the recovery force will lead to spreading of web. Fabric thickness increases

linearly, though not proportionately with the weight of fabric. Fabric density also increases

with the increase of fabric weight. These can be attributed to two causes: firstly the

increased web weight and hence, the thickness, increases with both the effective distance of

barb penetration and the effective number of barbs penetrating through the web, and tends to

lead greater fibre entanglement. Secondly, the heavy weight web provides a higher

frictional resistance to the movement of punched fibres and thus increases the forces

compacting the fibres. Air permeability is lower in the heavier and denser fabric while their

abrasion resistance is higher.

Clayton [48] suggested the use of “Sectional Permeability” Ps, is given by:

47

Ps = Pa x t …………………..(2.5)

Where t is the fabric thickness

The parameter Ps was defined so as to enable the effect of fabric density to be studied

independently of that of fabric thickness.

From the previous equation,

Ps = Pa x t = K1 x t/w + K2 x t,

Or Ps = K1 /D + K2 x t ,

Where, D = Fabric density and given by D = w/t

If, K2 is small then Ps is linearly related with the reciprocal of the density.

Gardmark and Martensoon [49] found that the thickness decreases with more needle

penetration per square inch, the density, however, increases for the first phase, and then

decreases. This is both due to fibre damage and to a weight decreases per square inch of

about 5% for each pass through the needle machine, as there is some loss of fibres and an

extension of the felt when being needled. Air permeability also decreases with the increase

of needling density. They also stated that with the increase of depth of needle penetration,

the consolidation of fibre in the web increases.

There are two aspects of needle penetration :

(a) It can consolidate the fibre mass (which also depends on the fibre

elasticity) which restricts the air flow,

(b) It can make peg holes, which allow relatively easy flow of air and dust

particles through the fabric and hence, this leads to reduced value of

pressure differential and filtration efficiency.

Types of needle barbs has also an effect on the filtration characteristics.

Hearle et al [50] in their study concluded that increase in the amount of needling decreases

the fabric weight produced from a particular web weight. This is due to drafting and

spreading of fibres during punching.

In doing research on 2.2 dtex polypropylene needle-punched filter fabric, Igwe and Smith

[51] found that the existence of needling density as another parameter affecting both

collection capacity and filtration efficiency. An increase in needling density leads to a

reduced collection capacity as might be expected, but it was unusual to find that it also

cause reduced filtration efficiency.

In another study, Igwe [52] concluded that the fibrous „pegs‟ produced by needling form

easy paths for dust to pass through the filter. Consequently, increased levels of needling and

48

increased needle penetration reduce the efficiency of the filter even though the density of

the filter is increased.

In another book, Igwe [53] stated that at low level of needling, e.g., 100 P/sq.cm for both

Nylon and Nomex fibre, the pressure differential increases with the depth of penetration. At

the second and third level, i.e., 200 P/sq.cm and 300 P/sq.cm, the rise in pressure differential

for both types of fibre at increasing depth of needle penetration tend to decrease when

compared with previous result, indicating large flow channels. The effect of depth of needle

penetration on pressure differential at 400 P/sq.cm shows a complete change for Nomex

fabrics with the resistance steadily decreasing with depth of penetration from15 mm

onwards. This phenomenon became more prominent at a needling density of 500 P/sq.cm

for Nomex and Nylon fabrics. Two explanations can be advanced for this reduction in

pressure differential at increased needling intensity and depth of penetration: (a) that due to

the pegs (b) the reduction of fabric weight. It is thought that the pegs made by the felting

needles tended to be more open ended with larger penetrations, allowing relatively easy

flow of air and dust particles through the fabric and hence this leads to reduced value of

pressure differential and filtration efficiency.

Hearle and Purdy [54] found that at constant needling density an increase in depth of

penetration increases the reduction of the mean fibre length from that of the web. It is due to

the breakage of fibres due to extensive needling action.

Subramaniam et al [55] studied the individual and interactive effects of web weight, needle

density and needle penetration by using Box and Hunter central compound rotatable design

of three variables. They concluded that at lower web weight, an increase in depth of needle

penetration or needle density causes an increase in tenacity, but with higher webs, at any

needle density, an increase in depth of needle penetration first increases and then decreases

the tenacity. The web weight at which the maximum tenacity occurs reduces, as the depth of

needle penetration increases. With the increase in web weight, needle density or depth of

needle penetration, the fabric breaking elongation decreases while the initial modulus

increases, the Poisson‟s ratio first increases and then decreases. At higher levels of needle

density, at lower web weight, an increase in needle penetration depth does not cause much

change in air permeability value, but at higher web weight, an increase in depth of needle

penetration causes n increase in air permeability value. At lower levels of needle density, at

49

any level of web weight, an increase in needle penetration depth lowers the air permeability

of the fabric. The increase in web weight at all levels of depth of needle penetration and

needle density results in a decrease in air permeability.

Hearle and Sultan [56] found that by using a woven backing material the anisotropic with

regard to strength and extension can be reduced compared to needled fabrics without a

backing.

Gupta stated in a book [57] that Needled fabrics that have not been reinforced are of low

dimensional stability i.e., have a low elasticity and suffer from a high degree of permanent

distortion after being subjected to strain. The initial modulus of the reinforced needled

nonwovens is also considerable higher if a backing is needed.

Debnath [58] in his series of paper on “Needle-punching of jute webs” concluded that the

fabric weight and thickness gradually decrease with an increase in the needling density and

depth of needle penetration. The fabric density increased with an increase in the fabric

weight, needling density and depth of needle penetration. But the fabric tenacity decreases

beyond the optimum weight of the web irrespective needling density and depth of needle

penetration. Elongation of the fabric increases with a reduction in the fabric weight,

needling density and depth of needle penetration.

According to Sengupta et al [59], Needle-punched jute web has poor strength and stability

but inclusion of a suitable reinforcing material improves the strength and dimensional

stability of the nonwovens remarkably. In continuation of their study on the abrasion

resistance of nonwovens, they observed that an increase in the ratio of the reinforcing to

web weight increases the abrasion resistance of the fabric. The structural rigidity and

openness of the structure of reinforcing material play an important role in increasing the

abrasion resistance.

It has been reported by Midha and Mukhopadhyay [60] that Hollow fibres have tubular

cross-section which results in increased bulk followed by trilobal and normal round fibres.

Greater surface area is also responsible for lower effective density, thus providing a higher

cover power. The fabric thickness is minimum for hollow fibre fabrics and maximum for

trilobal fibre fabrics, which is due to low crimp frequency of hollow fibres. It was also

observed that the hollow fibre fabrics have higher air resistance owing to their closer

packing, effectively reducing the air gap in the fabric. But in case of trilobal fibre fabric, the

higher bulk and protrusion of lobes in the fibre prevent closer packing of fibres in the fabric,

resulting in higher air space and reduced pressure drop. Hollow fibre fabrics show higher

50

tenacity at each level of fabric weight followed by trilobal and normal round fibre fabrics

respectively. This is due to the higher bulk of hollow and trilobal fibres which provides

higher surface area, which in turn, increases the fibre cohesion, and the higher tenacity of

hollow and trilobal fibres compared to round fibres which gives less breakage of fibres

during needling, resulting in corresponding increase in strength. Apart from that, the

stronger fibre is expected to produce stronger fabric.

According to Midha et al [61], bulk and physical properties of needle-punched nonwoven

fabrics depend on the nature of component fibre, the manner in which the fibres are

arranged in the structure and the degree of consolidation. A proper understanding of the role

of different parameters on fabric parameters on fabric properties is important in designing

the fabric suitable for its use. Normally, the longer and finer fibre in the web leads to greater

fabric strength, provided the fibre breakage is controlled. The increase in needle density and

needle penetration improves the fibre consolidation, but a beyond a certain limit the fibre

damage becomes greater, leading to deterioration in fabric characteristics. Higher fabric

weight and introduction of seam generally improve the functional properties of fabric.

Finishing operation is opted in the cases where some special requirements are to be fulfilled.

The filtration behavior of two types of spun-laid nonwoven fabrics, namely thermobonded

and needle-punched, with wide range of physical properties has been studied [62]. A

computerized air filtration apparatus has been designed and developed for measuring the air

filtration characteristics of different types of filter fabrics. The developed apparatus

measures the filtration parameters following the principles of dry filtration mechanisms.

Needle-punched nonwovens show good filtration efficiency with lower pressure drop than

the corresponding thermobonded nonwovens. Overall, the needle-punched filter fabrics

perform better as a filter fabric in comparison to thermobonded nonwovens.

According to Balasubramanian et al [63], a good filter should have high compactness, low

air permeability, reasonable strength and low elongation.

Debnath etal [64] in their study used normal round, circular hollow and trilobal cross-

sectional shaped polyester fibres used to prepare needle-punched nonwoven fabrics for

technical textile application. Effects of fabric weight and fibre cross-sectional shapes on

fabric thickness, density, percentage compression, air permeability and sectional air

permeability (SAP) have been studied. The thickness, density, air permeability and SAP

51

fall under different sub-cluster but all these parameters are dependent on fabric weight.

Trilobal fabric sample showed highest thickness followed by regular and hollow polyester

needle-punched fabrics. Thickness and density of the fabric increase but air permeability

and SAP decrease with the increase in fabric weight. The fabric thickness is significantly

correlated with fabric weight. Fabric weight versus air permeability and fabric density

versus SAP are negatively correlated with significant correlation coefficient.

2.5 Mechanism and factors governing filtration

2.5.1 Mechanism of particle capture

In terms of the way in which a particle is retained by a filtrate, and so removed from the

air/gas, a number of mechanisms can be distinguished. As a particle-laden gas stream

approaches a fibre, particles suspended in the stream move towards the fibre surface by a

number of forces. Capture of particles by a filter can be explained on the basis of different

theories, and final effect depends on a combination of several mechanisms/factors classified

as follows:

Single fibre theory

Capture by fibrous assembly

Capture based on the mode of filtration

Capture influenced by design of filter unit

Capture governed by operating parameters

The mechanism behind particle puffs emitted during the stages of pulse cleaning as the

combination of the following mechanisms [65]:

Partial disappearance of the cake of particles

Migration of particles in the medium

Re-suspension of the upstream particles due to the rupture of cake

Re-suspension of the deposited particles on the walls of the duct

The contributions of these different mechanisms strongly depend on the association of the

medium/aerosol and to a lesser unit, on the operating conditions of filtration and

unclogging.

The nature and size of the particles remain important, the effectiveness of the medium

remaining decisive at the beginning of the operation of an installation and after many cycles.

However, difficulties persist, including the inability to assess the actual contribution of the

mechanisms, such as the migration of the particles (phenomena of seepage), or re-

suspension of the cake particles when unclogging. It was observed that the mechanism

52

mainly responsible for particle puff for alumina particles (<5 µm) is different from wood

dust (<10 µm). Regarding the nature of the medium, tests have shown that the addition of

poly tetra fluoro ethylene (PTFE) membrane on the filter surface could significantly reduce

these puffs [66].

2.5.1.1 Particle capturing based on single fibre theory

Fabric filter often captures particles much smaller than the fabric pore size, which shows

that the mechanism of capture goes beyond simple sieving. Capture of particles on a fibre

element follows one or combinations of the following mechanisms: diffusion, inertial

impaction, direct interception, gravitational settling, and electrostatic attraction [67]. In

general, the theory considers that the separation of particulate matter from the gas flow by

fibrous filters happens by the combination of a number of collection mechanisms and the

effect of Brownian diffusion is predominant at the sub-micrometric size range [68]. The

effectiveness of the collection mechanism will finally depend on particle size and its mass,

velocity, density and viscosity of the gas, electrostatic forces, and the filter unit. Moreover,

the different mechanisms are not independent but operate simultaneously.

Wang [69] reviewed both theoretical and experimental studies on the application of

electrostatic forces in filtration. In the absence of electrostatic forces, movement of a small

particle in gas is governed by thermal forces and particle inertia, and a fibrous filter

efficiently captures particles by inertial impaction, interception, and corrective, and

convective Brownian diffusion. The relative contributions of thermal forces and particle

inertia to deposition are mainly the functions of particle diameter, gas velocity, and fibre

diameter. In general, particle inertia makes a greater contribution for particles larger than 1

µm, while Brownian motion caused by thermal forces plays a greater role for particles

smaller than 0.1 µm. For particles in the size range of 0.05-0.5 µm, both particle inertia and

thermal forces are relatively weak. As a consequence, the collection efficiency of a fibrous

filter has a minimum effect in this size range. Application of electrostatic forces can

significantly augment the collection efficiency of a fibrous filter. The process of aerosol

filtration in the presence of electrostatic forces is complicated and is particularly useful for

improving the collection of particles in the size range of 0.15-0.5 µm, which are difficult to

capture by other mechanisms. The variables that influence the collection efficiency of a

filter in the presence of electrostatic forces include chemical composition of particles and

53

fibres, charges on particles, surface charge density of fibres, and the intensity of the

externally applied electric field.

Most of this theory, however, was developed and validated for micron-size particles, and

their extension to the nano-size range needs attention, both experimental and theoretical.

The filtration of nano particles in a polyester filter had shown that particle capture efficiency

decreases with increasing particle diameter and increasing gas velocity. The results were

compared to well-known theoretical predictions based on the classical collection

mechanisms: diffusion and direct interception. The comparison of the calculated collector

efficiency with the experimental results showed that the predictions underestimate the

results. A correction was proposed to the existing correlation for direct interception where

an `effective‟ diameter accounting for the Brownian motion of the particle in the vicinity of

the collector surface was accounted for. The results showed a considerable improvement in

the prediction correlation [70].

.

2.5.1.2 Mechanism governed by structure of filter material

In the case of the fibrous assembly, in addition to single fibre theory, size and shape of pores

defined by the fibrous assembly play a vital role in particle filtration. The orientation of

fibre can also influence the particle filtration. Surface type media are not perfectly smooth

on their surfaces nor are their pores perfectly uniform in shape and direction. Broadly,

particles capturing by filter media can be distinguished as surface and depth filtration. The

small particles, which are unable to retain over the surface of filter media, penetrate inside

the fibrous assembly and are likely to get trapped due to the tortuosity and confined region

in the pore structure. The phenomena can be distinguished as depth filtration. However, in

surface filtration, even the particles which are smaller than pores can be retained over the

surface through the bridges over the pores.

Most of the cake formation occurs by a combination of blocking and bridging

In surface filtration, two different mechanisms – sieving and bridging filtration exist .

Sieving: When the pores of the fabric are smaller than the incoming dust, then

the particle is completely captured through screening process. Sieving is usually

predominant for particles above 10 µm for felt fabric. Through the enhancement

of surface filtration by using membrane at the upstream side of the fabric,

filtration of particle size even below 2.5 µm is possible without the formation of

dust/cake layer.

54

Bridging filtration: This mechanism is prominent if the particle concentration is

relatively high for the particles smaller than the pores (down even to about 1/8th

of the pore diameter). The bridging of the particles across the entrance to a pore

forms a base upon which a stable and permeable cake will grow. In reverse-jet

system, filtration occurs through cake formation. On the other hand, in pulse-jet

system, both cake and non-cake filtration can be distinguished; the latter being

more common in industrial practice

In the depth filtration, the mechanism of filtration may result in the trapping of far smaller

particles that might be expected from the size of the pores in the medium. In aerosol

filtration, both depth straining and depth retaining are common .

Depth straining : For a filter media, particles will travel along the pore until they

reach a point when the pore is confined to a size too small for the particle to go

any further so that it becomes trapped. Particles are also trapped while passing

through blind pores.

Depth retaining : A particle can also be retained in the depth of the medium,

even though it is smaller in diameter than the pore at that point. Such behavior

involves a complex mixture of physical mechanisms. In a tortuous pore, the

particle loses its velocity and becomes attached to the pore wall, or to another

particle already held, by means of Van Der Waals and other surface forces.

In industrial filtration, as particle concentration is much above 5 mg/cu.m., surface filtration

is the only option where the fabric filter is needed to be regenerated to regain the

effectiveness of the filtration process [71]. In case of depth filtration, the dust particles are

retained at the certain depth of filter medium, which are difficult to clean. All the industrial

filter fabrics are therefore, predominantly surface filters, although some amount of depth

filtration is inevitable, which results in the blinding of pores. Depth filtration can have a

profound influence on the filtration performance and life of the filter fabric.

2.5.1.3 Mechanism governed by mode of filtration

2.5.1.3.1 Dust penetration and seepage theory

Through a series of experiment [72], it is established `seepage‟ as a dominant mechanism of

particle penetration in conventional fabric filtration. Dust can penetrate through the fabric

filter in two ways:

It can fail to be collected and penetrated straight through the fabric, or

It can be collected at first but `seep‟ through later [73]

55

According to straight-through theory, particles should be captured with greater efficiency as

dust collects upon the fabric. However, it was reported [74] that gradual seepage of

collected dust through the fabric into the cleaned gas stream is more important than straight-

through penetration. Seepage is a failure of the fabric to retain collected dust rather than a

failure to collect that dust in the first place. Seepage implies a mechanism in which particles

already captured by the fabric are subsequently released and eventually re-entrained in the

exit gas. Furthermore, the primary driving force for this release and re-entrainment

mechanism comes from the rapid deceleration of the particle-laden fabric as it collapses

back on the cage at the end of the cleaning pulse.

In addition to straight-through and seepage mechanisms, the dust penetration through a filter

medium can also be accomplished by pinhole bypass. Pinholes are small holes, which may

be formed during cleaning or filtration [75]. During filtration of hot particles or particles of

abrasive nature, pinholes can be produced. Also, during the needling punchingprocess on a

synthetic felt, small ruptures caused by mechanical stress might lead to pinholes.The

pinhole bypass mechanism is of particular importance when straight-through is not possible

any more due to filter cake. Pin holed surface filters show a clear impact on the separation

efficiencies of the test filters in experiments, such as different particle-size distributions and

much higher integral clean gas side concentrations than for faultless filters. The observed

pinholes in surface filters are required to be rated critically concerning actual emission limit

values. Bigger pinholes decrease the collection efficiency, and higher filter face velocities

increase the collection efficiency of pinholed filter media [76].

Considering `seepage‟ as a dominant mechanism for emission, a model is presented for

seepage penetration that uses impulse and momentum principles and based on the

assumption that all seepage occurs as the bag strikes its cage [77].

The proposed equation is as follows:

N = kw² v/t,

Where,

N = mass outlet flux from the fabric in kg/m² /s,

w = the areal density of dust on the fabric in kg/m²,

v = superficial filtration velocity in m/s,

t = the time between cleaning pulses in s,

k = a proportionately constant that was reported to depend on the dust and fabric used

The above equation shows that outlet flux should be zero with a clean fabric, but should

increase as the fabric becomes conditioned and dust areal density increases. If most of the

56

dust that penetrates passes straight through the filter, outlet flux from the filter should

decrease over time, whereas if most of the dust passes through by seepage, outlet flux from

the filter increase over time [78]. Immediately after installing new bags, this flux was nearly

zero. Initially, outlet flux increased rapidly with time, but later the increase was much more

gradual. This may occur because, over time, more dust works its way below the fabric

surface and is thereby able to penetrate by seepage. These results support earlier supposition

that seepage accounts for virtually all the dust that penetrates a filter. A series of tests were

performed to determine outlet flux for a filter operated at three filtration velocities, equipped

with three types of bags and dusts. The amount of dust carried by the bags, and the outlet

flux from the bags, varied greatly in these tests. However, all outlet flux values, regardless

of velocity, fabric, or dust type, correlate well with a single parameter, i.e., w² v/t. Fabric

type and dust type affect the amount of dust (w) carried by a bag, which in turn largely

determines outlet mass flux [78].

It has been established by the early works cited above that emissions from a pulse-cleaned

filter peak immediately after each regeneration pulse [79] then decrease rapidly, while the

filter cake builds up and takes over as a more efficient filtration layer. Such data have been

obtained repeatedly for different types of filter media ranging from rigid ceramics to needle

felts [80]. The emissions spike filter immediately after each cleaning pulse and are

consistent with the hypothesis that seepage accounts for virtually all the dust that passes

through these filters, as under this hypothesis, emissions occur as each bag strikes its

supporting cage. However, decrease in filtration efficiency with time is contradictory to the

earlier findings backed by theoretical proposition. These facts create some suspicion that the

`re-entrainment hypothesis‟ may not apply universally as often presumed, or perhaps only to

the kind of very open and porous media used widely during the 1970s and early 1980s. With

time, there is a significant improvement in filter media through enhancing surface filtration

amidst many other functional requirements. It has been pointed out that the emission

processes in surface filter media are difficult for assessment due to their rather transient

nature and the experimental facilities required [81].

Bining et al. [81] re-examined the contribution of each of the above-mentioned factors to the

emissions from pulse-cleaned needle felt media. The theory applies to two types of media

(singular and layered fabric) using two different types of dust (agglomerating and free-

flowing) and under all aging conditions used during the experiments. Considering the effect

of different dust types and filter aging, experiments were performed on flat circular samples

of calendared needle felts made out of polyphenylenesulphidefibres. At different stages of

57

aging, it was found that 96-99% of recorded emissions were caused by direct particle

penetration, which is by far the dominant emission mechanism as compared to re-

entrainment. With increasing filter service life, simulated by accelerated aging up to 20,000

cycles, the emission level decreases by factors of about 10 to 20 due to the progression of

clogging and a corresponding increase in filter efficiency. The observed behavior is in

support of direct penetration of dust particles and is assumed to be valid for the entire

operating life of a surface filter, except when mechanical failure occurs and disregarding the

effect of holes caused by stitches.

As and when dust loading is stopped, penetration is likely due to re-entrainment wherein

emitted dust mass per cycle was of the order of 10 of the mass stored inside the medium. Its

impact on overall emission is much lower than the combined impact of direct penetration

and re-entrainment. The prevalence of direct penetration is further confirmed by the size

distribution of the emitted particles, which is centered narrowly around the most penetrating

particle size between roughly 0.5 µm and 1 µm. Contrary to expectations, a significant

fraction of re-entrained dust should have led to a noticeable coarse size distribution with

aging; instead, the emitted particle size tended toward finer particles with aging due to an

increase in filter efficiency. Cross- sectional analysis of a few media by electron microprobe

indicates that the support scrim may act as an effective barrier to particle seepage more than

to direct penetration [81].

Re-deposition of parts of a filter cake immediately after the cleaning pulse is shown to have

a very significant effect on emissions. In addition, the well-known increase in emission level

with cleaning pulse intensity can be attributed to a slowing down or prevention of the

clogging process rather than enhanced re-entrainment of stored dust. Earlier reports of

increased emissions at higher filter face velocities are probably due to the re-deposition of

dust from the filter cake immediately after a cleaning pulse. Seams in the filter medium can

have an increasingly strong effect on emissions and filter service life [81]. It may be noted

that the above hypothesis is based on flat circular samples, and therefore the influence of

dust cake re-deposition of seams or the rebounding of the filter on the supporting cage may

mismatch the prediction in a filter test uniy (VDI 3926, Type-1) with small coupons.

2.5.1.3.2 Cake vs non-dust cake filtration

Most pulse-jet operations eliminate the need of primary dust cake [82] but not all. Failure to

make the distinction between these two pulse-jet operating modes causes confusion, as one

group working on the dust-cake pulse filters reports results in conflict with those of the

58

other group dealing with non-dust cake pulse filters. In non-dust cake filtration, the layer of

dust is cleaned before the cake is formed. In the said mode, the fabric plays a more active

filtration role and the properties of porous filter media depend on both the dust and the

fabric throughout the filtration period. Non-dust cake filtration also implies that the cleaning

action itself is adequate to remove sufficient dust before reaching the dust cake threshold

and that a steady state will be reached without the formation of homogeneous dust cake.

However, if filtration is allowed for a longer time without cleaning, a surface dust cake

forms on the fabrics. In case of cake filtration, once the primary dust cake is formed over the

fabric, the role of fabric is secondary in the outgoing emission of dust particles.

The performance characteristics of these two operating modes differ, as in ideal dust cake

filtration the drag characteristics depend only on the dust itself and the dust cake it forms; in

non-dust cake filtration, the drag characteristics depend mainly on the interaction of the dust

with the fabric substrate. Drag is often linear with areal mass density in dust cake filtration,

it is seldom linear in non-dust cake filtration. [83]. In the process of dust deposition over

filter material, the phenomenon of filter clogging and/or particle dendrite growth has been

observed in number of studies [ 84-85]. The operational characteristics during filtration are

strongly influenced by the structure of the dust cake.

2.6 Conclusion

It can be summarized from the literature survey made that in the conventional types of

needled filter materials, separation does not only take place on the surface, but also on the

porous inside as a result of the three-dimensional structure. This, however, results in the

filter medium clogging sooner and causes problems in cleaning. More recent types of

needled filter materials have been so modified (reinforced) that filtration takes place

predominantly in the upper layer on the side exposed to the dusty air/gas. This method,

known as surface filtration, results in less loss of power, higher specific surface loads, and

significantly longer service life. Because of the high pressure to which they are subjected

during the separation process and the shearing stress experienced when the filter cake is

removed, the filter media have to be very strong, extensible, and have a suitable surface.

Close structured needle-punched fabrics with a smooth surface and reinforced with woven

fabrics provide a preferable alternative to heavy textile fabrics. The random arrangement of

the fibres of such needle punched fabrics nearly always ensures shorter turbidity times and

59

more rapid build up of the filter cake. The GSMs of suitable needle punched fabrics is

between 300 and 800 gms/sq.m, including the woven fabric worked in to strengthen it.

While designing the filter elements, a thorough understanding of environmental regulations,

nature of particulates and state of aerosol conditions is needed. Four factors related to the

design of filter elements are:

Material of the fabric

Fabric type and specification

Fabric finish and structural modification, and

Filter element dimension and fabrication

Other key design parameters are the choice of filter medium, operating differential pressure

and baghouse footprint.

Therefore, a systematic study could entail a proper designing of the system and filter media.

There is limited information available on cement dust collection by nonwoven fabrics.

Hence, in the present study, an effort has been made to develop an instrument for measuring

filtration properties of filter fabrics and to study the quality aspects of woven and needle-

punched nonwoven filter fabrics using cement dust.

60