Filtration Textiles

Dr. Harshvardhan Saraswat

Assistant Professor
(M.Tech, PhD, IIT Delhi)

MLV Textile and Engineering College

Bhilwara, Rajasthan
 Contents
• Introduction
• Applications
• Types of filtration
• Dust collection
• Practical implications
• Cleaning mechanism
• Design criteria for filter fabrics
• Fabric construction for filter fabrics
• Finishing treatment
• Fabric design and selection consideration
• Filtration requirement
Introduction
The separation of solids from liquids or gases by
textile filter media is an essential part of countless
industrial processes, contributing to purity of
product, savings in energy, improvements in
process efficiency, recovery of precious materials
and general improvements in pollution control.
Applications
Types of Filtration

• Solid-gas (Dust Collection)

• Solid-liquid
Dust Collection

Gas-borne dust particles arise wherever solid materials are

handled. Examples include conveyors, smelting processes, hopper
filling, pulverising processes, combustion processes, milling
operations, bag filling and so on. The dusts may create
environmental pollution problems or other control difficulties
caused by their toxicity, flammability and possibly risk of
explosion.
Dust Collection Theories and Principle
For the purposes of industrial dust
collection, they are of limited value. 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.
Practical Implications
In operation, fabric dust collectors work
by drawing dust laden gas through a
permeable fabric, usually constructed in
the form of tubular sleeves, longitudinal
envelopes or pleated elements. As the gas
passes through the fabric, the particles in
the gas stream are retained, leading to the
formation of a layer of dust on the
surface. This is normally referred to as a
‘dust cake’.
Practical Implications
Cleaning Mechanism

• Shake Cleaning
• Reverse Air Cleaning
• Pulsejet Cleaning
Shake Cleaning Dust Collector
Design Criteria of Filter Fabrics

• Thermal and Chemical Conditions

• Filtration Requirements

• Equipment Considerations

• Cost
Design Criteria of Filter Fabrics
• Thermal and Chemical Conditions:
1. Thermal and chemical nature of the gas
2. Duration of exposure
3. Presence of moisture
Design Criteria of Filter Fabrics
• Filtration Requirements
1. Particles size and size distribution
2. Abrasive nature of particles
3. Electrostatic charge generation
4. Extremely hot particles
5. Moisture in the gas
Design Criteria of Filter Fabrics
• Equipment Considerations
1. Cleaning mechanism
2. Abrasion resistance
3. Flexibility
4. Elasticity
5. Style of filter
Design Criteria of Filter Fabrics
• Cost
In spite of all the design considerations and performance guarantees that
are frequently required of the media manufacturer, this is still a highly
competitive industry. As a consequence every effort is made to reduce
media manufacturing costs, either by judicious sourcing of raw materials,
or more efficient manufacturing (including fabrication) techniques
Fabric Construction for Filter Fabrics

• Woven fabrics

• Needlefelts (Nonwoven)

• Knitted fabrics
Woven Filter Fabrics
• Shake Collectors
• Resistance to stretch
• Resistance to flex fatigue
• Efficient dust release
• Maximum particle capture
• Minimum resistance to gas flow
Woven Filter Fabrics
• Twisted continuous filament yarns
• Superior cake release
• Staple fibre yarns
• Higher filtration velocity
• Combination of both filament and staple fibre yarn
• Weave pattern: Elementary twill or simple satin
• Smoother surface and greater flexibility
• Mechanical raising
Nonwoven Filter Fabrics
• Most wide spread in dust collectors
• Reverse air and Pulse cleaning mechanism
• Larger number of pores
• Stability and necessary tensile characteristics (Woven scrim)
• Needling parameter
• Fibre fineness
• Minimum resistance to gas flow
Knitted Filter Fabrics
• Seamless tubular form
• Economic in production
• Inferior filtration efficiency
Finishing Treatments
• Fabric stability
• Fltration collection efficiency
• Dust release
• Resistance to damage from moisture and chemical agents
Heat Setting
• Dimensional stability
• To prevent shrinkage during use
As heat is the primary cause of shrinkage, it is logical that
fabric stability should be achieved by thermal means. Such an
operation is normally referred to as heat setting, and may be
carried out by surface contact techniques, ‘through air’
equipment, or by stentering, the latter two being preferred
because they enable greater penetration of heat into the body
of the structure
Singeing
• Singeing is a process in which the fabric is passed, at
relatively high speed, over a naked gas flame or, in another
technique, over a heated copper plate

• Filter fabrics, especially needlefelts, which are produced from

short staple fibres, invariably possess surfaces with
protruding fibre ends. Since such protrusions may inhibit
cake release by clinging to the dust, it is common practice to
remove them
Raising
• The raising process is designed to create a fibrous surface,
normally on the outlet side of the filter sleeve, to enhance
the fabric’s dust collection capability
• Raised fabrics may comprise 100% staple-fibre yarns or a
combination of multifilament and staple-fibre yarns,the latter
being woven in satin style in which the face side is
predominantly multifilament and the reverse side
predominantly staple. The smooth surface provided by the
multifilaments will aid cake release whilst the raised staple
yarns on the reverse side will enhance particle collection
efficiency
Calendaring
• The calendaring operation fulfils two objectives,viz.to
improve the fabric’s surface smoothness and hence aid dust
release, and to increase the fabric’s filtration efficiency by
regulation of its density and permeability
• Most calenders in the industry consist of at least two bowls,
one manufactured from chrome-plated steel and the other
from a more resilient material such as nylon or highly
compressed cotton or wool fibres. The steel bowl is
equipped with a heat source, for example gas, electric
elements, superheated steam, or circulating hot oil.
Chemical treatments
• Chemical treatments are applied for following reasons:
 To assist in dust release, especially where moist sticky dusts,
possibly containing oil or water vapour are encountered
 To provide protection from chemically aggressive gases such
as SO2 and SO3 referred to earlier.
 Proprietary treatments, usually involving silicone or PTFE,
enhance yarn to-yarn or fibre-to-fibre ‘lubricity’ during pulse
or flex cleaning
 Where flammability is a potential hazard, padding through
commercially available flame retardant compounds may be
necessary
Special surface treatments
This category of treatments is devoted to improving still
further the fabric’s filtration efficiency and cake release
characteristics
 Attachment of a more efficient membrane, for example
PTFE in a lamination operation
 The application of a low-density micro-porous foam
Both these treatments are designed to restrict the dust particles, as
far as possible, to the surface of the fabric, thereby reducing the
tendency for blinding.
Solid-liquid separation
Mechanisms of solid/liquid separation
• Settling
• Floatation
• Hydro cyclones
• Evaporation
• Magnetic
• Electrostatic
• Gravity
• Centrifuge
• Vacuum and pressure
The mechanisms that consume the largest volume of textile filter media and on
which this section will concentrate, are those of pressure and vacuum.
Solid-liquid separation
Media Type Particle size retained (mm)
Flat wedge screens 100
Woven wire 100
Sintered metal sheets 3
Ceramic elements 1
Porous plastic sheets 0.1
Yarn (cheese wound) cartridges 2
Compressed, fibre sheets 0.5
Filter aids (powders/fibres) 1
Membranes 0.1
Woven monofilaments <10
Other woven fabrics <5
Needlefelts 5
‘Link’ fabrics 200
Fabric design/selection considerations
There are many factors which confront the technologist when
choosing or designing a fabric for a particular application.
These may conveniently be grouped under the following
general headings:

1 Thermal and chemical conditions

2 Filtration requirements
3 Filtration equipment considerations and
4 Cost
Fabric design/selection considerations
There are many factors which confront the technologist when
choosing or designing a fabric for a particular application.
These may conveniently be grouped under the following
general headings:

1 Thermal and chemical conditions

2 Filtration requirements
3 Filtration equipment considerations and
4 Cost
Thermal and chemical conditions
Before the advent of synthetic materials, the only fibres available for industrial purposes
were those of natural origin such as flax, wool and cotton.

Cotton is still used in one or two applications even today; the tendency of this fibre to
swell when wet facilitates the production of potentially highly efficient filter fabrics.

Polyamide – nylon 6.6 – arguably the first and most widely used true synthetic material is
notoriously sensitive to strong acidic conditions and, conversely, polyester is similarly
degraded by strong alkaline conditions.

By comparison, polypropylene is generally inert to both strong acids and alkalis and,
primarily for these reasons, is the most widely used polymer in liquid filtration.
Thermal and chemical conditions
Filtration requirements
• Filtrate clarity
• Filtrate throughput
• Low cake moisture content
• Resistance to blinding
• Good cake release
• Resistance to abrasive forces
• Filter aids and body feed
Filtrate clarity
The mechanisms by which particles are removed by fabric
media may be identified as
• Screening or straining
• Depth filtration
• Cake filtration
Screening or straining
This is a simple mechanism in which particles are retained by the
medium only as and when they are confronted with an aperture
which is smaller than the particles themselves.
Depth filtration
In this mechanism the particles are captured through attachment to
the fibres within the body of the filter medium, e.g. because of Van
der Waal or electrostatic forces,even though they may be smaller
than the apertures that are formed.This is particularly relevant to
nonwoven media.
Cake filtration
It involves the accumulation of particles that ‘bridge’ together in a
porous structure on the surface of the fabric. The cake effectively
becomes the filter medium with the fabric thereafter acting simply as
a support. In cases where it is difficult for the particles to form a
naturally porous cake, the use of a special precoat or body feed may
be employed to assist in this task.
Filtrate throughput
Although largely dictated by the equipment, restrictions to flow
imposed by the unused filter fabric could pose serious pressure
losses for a plant engineer and, in some applications, additional
problems in forming a satisfactory filter cake. In practice therefore,
were it possible to tolerate the presence of a measure of solids in
filtrate, some compromise is normally accommodated between
throughput and clarity.
Low cake moisture content
As it is necessary for filter cakes to be dried before moving to the next
process and because drying by thermal means is energy intensive, it is
important that as much liquid as possible is removed by mechanical means
prior to the actual drying operation.

A similar situation applies in effluent treatment operations. If the processed

effluent is transported to landfill sites, it is important to reduce the moisture
content, first in order to meet local statutory regulations and second,
because it is simply uneconomic to transport water.
Resistance to blinding
Blinding is a term which is commonly applied to filter fabrics which, after
normal cleaning operations, are so contaminated with embedded solids that
the resistance to filtrate flow is unacceptably high.
The blinding may be temporary or permanent; temporary in the sense that
the cloth may be partly or completely rejuvenated by special laundering or in
situ cleaning and permanent in the sense that the solids are irretrievably
trapped within the body of the fabric.
Good cake release
At the end of the filtration cycle the dewatered filter cake must be removed
from the fabric in preparation for the next cycle. It is important that the
cake is effectively discharged at this point since any delays will lead to
extended filtration cycle times and therefore reduced process efficiency.
To some extent this topic may be linked to cake moisture content because,
broadly speaking, wetter cakes will adhere more tenaciously to the cloth.
Resistance to abrasive forces
The abrasive forces in this context arise from the shape and nature of the
particles in the slurry. Materials with hard sharp quartz-like edges may lead
to internal abrasion, the breakage of fibres and filaments and ultimately a
weak point and possibly a pinhole in the fabric. Being the point of lowest
resistance to flow, enlargement of such a hole The filter fabric should
therefore be designed, as far as possible, to withstand the impact of such
forces.
Filter aids and body feed
In identifying filtration requirements, it is recognized that, in some cases, the
filter fabric may require additional assistance, for example, by way of filter
aids, body feeds or even filter papers. The use of filter aids, is designed to
precoat the fabric with a layer of powder, such as diatomaceous earth. This
is carried out in order (i) to protect it from blinding, (ii) to assist in the
collection of particularly fine particles, or (iii) to enable more efficient cake
release. In special circumstances filter papers may also be used for similar
reasons, especially where absolute clarity is essential. Body feeds are added
to the slurry to be filtered to enable the formation of a more porous cake
than would otherwise be the case, thereby enhancing the rate of filtration
flow.
Filtration equipment considerations
Having determined the preferred type of polymer and
identified the various filtration requirements, of equal
importance is the need to ensure that the fabric is capable of
providing trouble-free performance on the equipment itself.
1 Resistance to stretch
2 Resistance to flex fatigue
3 Resistance to abrasive forces and
4 Cost
Resistance to stretch
The propensity for stretch is evident on most types of filter
and may arise as a result of cloth tensioning mechanisms,
internal pressures or other forces such as the mass of the filter
cake and the gravitational pull that it exerts on the fabric.
Resistance to flex fatigue
In Rotary vacuum drum and
rotary vacuum disc filters, the
filter fabric, will operate in
both vacuum and pressure
modes. The constant flexing
that the fabric receives in
moving from vacuum to
pressure can lead to a measure
of fatigue and possible loss in
filtration efficiency.
Resistance to flex fatigue
During the initial phase, dewatering commences as the slurry is drawn by
vacuum on to the surface of the immersed fabric and, as the equipment
rotates, this continues until completion of approximately two-thirds of a
revolution. At this point the vacuum is replaced with compressed air,
which causes the fabric to expand. This in turn causes the dewatered filter
cake to crack and fall from the fabric under force of gravity.
Resistance to abrasive forces
Abrasive forces arising from the design and/or construction of
the filter itself are found in many forms. In such
circumstances, if the ideal fabric in purely filtration terms is
incapable of withstanding abrasive forces special fabrication
techniques or the use of a backing cloth may be necessary. The
latter, being of a more robust construction, will also be
designed to facilitate the free flow of filtrate which passes
through the primary filter cloth.
Influence of yarn type on filtration properties
Yarn and fabric constructions

Monofilament Yarn Multifilament Yarn Staple fibre Yarn

1. Resistance to blinding 1. High strength and resistance 1. High dimensional stability
2. High filtrate throughput to stretch 2. High abrasion resistance
3. Efficient cake release 2. Moderate filtrate throughput 3. High filtration efficiency
3. High filtration efficiency
Thank You