Purchas D. Handbook of Filter Media

Screens and Meshes 2 5 5

counter-rotating heads, each extruding a set of filaments around its perimeter;

the two sets of filaments overlay each other to form a continuous tube of netting,

which is slit as required to make fiat strips. The net pattern, square, diamond,

etc., depends upon the angle between the heads, while post-treatment, such as

stretching, can produce other aperture shapes.

The Netlon process was invented in 1955 by the British textile technologist

Brian Mercer and is now exploited in various forms by manufacturers in more

than 40 countries throughout the world. The resultant diversity of products

UNDERNEATH PLAN VIEW

OF DIE LIPS

Die slots

in

DIAGRAMS SHOWING

FORMATION OF STRANDS

FROM A UNITARY INTEGRAL

INTERSECTION

Slots in register f~

Inner ~ Outer

d,e r,j , d,e

i J

Unitary

integral intersection

Inner

die

Outer

die

Division ~ ,,~,,,,I

taking place

u,u ~,ots out

of register

J

~'~\ \~/, Strand

.-tO2- - ~ \ , \

Intersection

Figure 6.33. Basic principles of the Netlon coz~nter-rotating dies and the formation of strands of extruded

polymer.

256 Handbook of Filter Media

have many applications, perhaps the most significant being as geotextiles in civil

engineering, agriculture, horticulture and gardening, as well as in packaging.

The crucial component of the Netlon extrusion machine is the die head that, in

the simplest version shown schematically in Figure 6.33, comprises two

concentric counter-rotating dies, with a series of slots cut into the two edges or

lips which are in contact with each other. When the slots are in register, the

polymer melt is extruded as streams of double thickness: but rotation to bring them

out of register causes each stream to be divided until they reunite as a slot in the

inner die registers with the next one in the outer die. The result is the formation of a

continuous mesh structure in tubular form, which is then drawn over a mandrel

and subjected to stretching, slitting and quenching operations as required.

If the slots are regular and both dies are counter-rotated at the same speed, the

mesh is of regular diamond pattern. Many variations are possible on this basic

form, some of which are indicated in Figures 6.34. The characteristics of any

particular mesh are largely determined by the profile and position of the slots (the

precision of which is of crucial importance), the speed of rotation and the nature

of the movement (which need not be constant).

A flat square mesh sheet is made as follows. One die is kept stationary, initially

producing a diamond mesh with right-angled intersections. The tension of the

sheet, as it is hauled off the extruder and passed under a roller set at 45 ~ to the

material path, causes the mesh to rotate as it is drawn down the mandrel. A

cutter is set against this mandrel in such a position that it slits the material

helically between a pair of adjacent strands, to form the desired flat square-

meshed sheet.

The strands forming the mesh need not be of equal cross-section. Further

variations, such as oscillation of one or both dies, permit a very wide range of

figured pattern effects to be obtained. It is also possible to make a three-strand

mesh by inserting a stationary die between the usual two rotating dies.

A highly important method of improving the strength, flexibility and lightness

of Netlon mesh is by stretching the material, using rollers of varying speed and

hot water as necessary; the stretching may be either longitudinally, transversely

or both (i.e. biaxially), at production speeds up to 100 linear metres per minute.

Figure O. 34. Typical Netlon mesh patterns resulting from process variables.

Screens and Meshes 2 5 7

The effect is to increase the mesh area and also to strengthen the material very

considerably by molecular orientation.

Simple methods of orientation stretch the mesh strands but leave the

intersections unorientated. However, it is possible to make the intersections

with a cross-section that ensures that they also orientate under suitable stress.

Although polyolefins are the most common materials used, the Netlon process

is applicable to numerous other polymers, including nylon, vinyls, polystyrene

and elastomers. An indication of the diversity of products possible is given in

Figure 6.35. The range extends from very fine and flexible meshes containing as

many as 1500 strands per linear metre and weighing only 10 g/m 2, to rigid

tubular or sheet structures with 7 mm thick strands.

Figure 6.3 5. Two examples of products of the Netlon process.

,.....

X

,....

,...,.

,%

~>~~,,~

,

;.++++~ ~o

0 0 0 0

~ o~ ~, ~,, +

I

......

v~ d;- ~ ~ "-! ~ "-1 "-! "-1 "-1 OC

I+ I+ I+ I+ I+ I+ I+ I+ I+ I+ I+

A

--~

io

@

mo

f~

I

i

@

.,o

~.~~~ ~'~

-.o ~ ~" ~- ~

-~ ..~ ~o

~ ~'~ ~-~

~~' ~ ~-

~'~ ~=~

~-~< ~ ~ u ~ ~

0

~~ ~'~-~ ~

,....

Screens and Meshes 259

6.7 Selecting Screens and Meshes

The prime characteristics of screens and meshes from the point of view of

filtration lie in their accuracy of separation, and in their operating features of

resistance to high temperatures, corrosion and abrasion- these latter features

referring essentially to metal media, although polymeric media can now operate

at quite high temperatures. Thus they should be chosen where accuracy of size

separation is an important requirement - the most obvious example being their

use in sets of test sieves, used for analysing the particle size composition of a

mixture of solids.

The presence in the feed slurry of chemically corrosive liquids or abrasive

solids indicates that the first place to look for a suitable medium is among the

materials covered in this chapter.

A high proportion of coarse filtration is done by woven mesh screens - in such

devices as in-line strainers and inlet screens. There is a strong element of

unattended operation in many of these applications, especially where the screens

are automatically cleaned.

By virtue of the fact that these media largely operate by surface filtration, they

are prone to the risk of plugging, where a particle lodges in a pore, thereby

blocking it. This means that prolonged operation requires some form of

backflushing to clear the plugged holes. However, the nature of these materials

means that this cleaning operation is relatively easy - by shaking or rapping, by

brushing, by pressurized water or by chemical cleaning.

The heaviest duties in separation in terms of abrasion, such as mineral

dressing operations, are satisfied with bar screens and punched or milled

perforated sheets, while, at the other extreme, in, say, the sifting of flour, fine

wire or plastic meshes are required.

Apart from the finest levels of separation, which might require membranes,

and situations where very high degrees of solid removal are required, then

woven meshes and screens now offer a good choice to the filter designer. They

include some of the strongest constructions, such as bar screens or laminated

sintered wire mesh, and are, of course, widely used also as supports for other

media.

CHAPTER 7

Coarse Porous Sheets and Tubes

Considerable attention has been given to natural and synthetic organic fibres as

the main materials for dry- and wet-laid filter media in the earlier chapters of this

Handbook. A quite different group of media is based on inorganic materials, with

the use of granules or fibres bonded together, usually by the process of sintering.

They are covered here, together with similar materials made from plastics, where

the granules are also sintered.

7.1 Introduction

A group of porous media that provide filtration of coarser particles, from

suspension in gases or liquids, is made by the aggregation of small particles

(granules or fibres) of the basic material into useful shapes, either between rolls

or in a mould. The aggregate is then heated to a temperature close to the melting

point of the material, under pressure, so that there is localized melting at the

points of contact among the particles (and any binder used in the aggregation is

driven off or incinerated).

This sintering process confers an element of rigidity upon the resulting

materials, so that they are used, for filtration purposes, either as sheets

(including sheets cut into appropriately shaped pieces), or as tubes (open ended

or closed at one end). This is a very useful group of media, with the inorganic

nature of some of the materials enabling their use at quite high temperatures.

Included here are media made from the same basic materials (plastics, metals

and ceramics), but now from the molten bulk material as foams - still rigid and

strong in their solid form. For the sake of completion, tubes made from sintered

glass fibres are also included in this chapter, even though the bulk of glass fibre

media is covered in detail in Chapters 4 and 5 (as wet-laid glass paper and pads).

There is also an overlap, in application terms, between the sintered metal fibre

media discussed here, and the sintered metal meshes covered in Chapter 6.

As already mentioned, a major feature of the inorganic materials included

here is their ability to operate at high temperatures. The importance of this

262 Handbook of Filter Media

particular set of applications is continually growing, although Bergmann's

review of high-temperature gas cleaning requirements ~1~ still has a great deal of

relevance. The topic is reviewed at a series of symposia, arranged at

approximately three-yearly intervals.

7.2 Porous Plastic Media

By virtue of their organic nature, the media made from plastics stand apart from

the other media discussed in this chapter, which are all inorganic. Nevertheless,

their filtration characteristics merge seamlessly into those of the inorganic

materials, the only significant difference being the reduced applicability in

temperature terms.

The plastic media covered here are those made by the sintering of polymeric

powders or granules, and those made from the molten state as foams. Not covered

are sintered plastic fibre materials - because most of the non-woven media of

Chapter 3 could be classified as sintered plastic fibre, since most synthetic fibre

needlefelts and spun media have an element of sintering in their manufacture.

7.2.1 Sintered granular plastics

Thermoplastic powders may be moulded and sintered to produce fiat porous

sheets or a wide variety of three-dimensional shapes, as indicated in Figure 7.1.

The most commonly used materials are high-density polyethylene and

polypropylene, others being PTFE and PVDF. Table 7.1 summarizes the

properties of the 1 m 2 sheets of one supplier. Examples of moulded discs and

cylinders are given in Table 7.2, while a good view of the structure of such

materials is shown in Figure 7.2.

Figure 7.1. Examples of sintered porous plastic mouldings.

0

m~

0 9 0 0 0 0 0

0 0 m~

m~

* -ii- -ll- -~ ~ * * .~ * -~ -~ -~ * -)l- * * -)(- * .~ -i(- -,x- -)(- -~ -i(-

~, 0 0 0

I+ I+ I+ I+

0 0 0 0

V V V

~l O 0 0

~-i ~'~ ~rl ~,a |~.,~ |~.~ .... ~I~ ~.~a |,,.a ~.,~ [%,~ |,,..a ~"~ i.~ ~,~ l%a l,,.~ ~ ~.-i ~c) ~,~ .~] ~,,~ |,~ I .... ~ ~.~ ~i~ ~.,~ ~ ~o ~.,~ .~.~] ~.~ a

O 0 0 0 0 O 0 0 ~ 0 0 C 0

~a,

0 O ~ O 0 O C ~, C ~ 0 oc O O

~a,

0 ~, ~ ~ ,....., ,._., 'J, C

14- 14- 14- Is IF 14- 14 14- 14 14- IF IF 14- l-F 14- 14- I-s 14- II IF I-I- 11L Is IF 14- IF 14- 14- 14- I-s I-I- I-s 14- 14- 14- 14 14-

V V V

I I I l~a ~.~ ~.,~ I ~-' ~-~ ~-" I I '~ ~ ~-~ I I I I

I I I ~,~ b,a b,a I ~ ~ ~-~ ~-~ I

~ b.a ~.,.~ b.a I I I I I I

I I I I I I

I I I I I I I I I

I I

I I I

t~ ~ ~ ~ t~ ~ ~

~, ~'~

~I~ I I I I I I

I I I I I I I I I I I I ~--~ l~ t~

d~ I~ Qo

I I I I I

I I I I I ~ ~ ~ ~ ~

~-~ ~-~ ~--~ l~a Ixa ~ ~--,

l,,a l,a ~ ~.~ ~ ~

HI)PE

PP

[IHMWPE

~a

:%

I,,a

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0

l,a

T

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r

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p'l

i

m~

.-o

264 Handbook of Filter Media

Table 7.1

(continued)

Trade name

Material type Thickness

(mm)

Pore size (l~m)

Typical air flow Removal

(m S/min/m2 at efficiencies

various pressures) (pm)

(mbar)

Max

Mean Min 1.27 2.45 5.00 10 20 Air Water

flow nomi- nomi-

nal nal

* 6.00+0.40

Vyton T * 1.00+0.07

* 1.50+0.07

* 2.00+0.10

* 2.50+0.12

* 3.20+0.16

* 5.00+0.16

Vyton RT * 3.20+0.25

* 6.35+0.40

* 10.0+0.40

- 1354:/-- 11 19 34 - - 40 60

40 18 10 - - 4 8 20 2 10

30 12 8 - - 3 4 13 2 8

15 10 6 - - 2 4 8 1 5

15 9 6 - - 1 3 6 1 5

20 9 4 - - 1 3 6 1 5

28 11 3 - - 1 6 9 6 10

- 25~- - - 1 3 5 6

10

- 21-;q: .... 1 3 6 10

Table 7.2 Properties of moulded HDPE discs and cylinders a

Grade Pore size distribution

Min Mean Max

(lam) (l~m) (~tm)

Permeability Density Minimum Porosity

(darcies (g/cm s) thickness (%)

x 10 -s) (mm)

Removal

rating from

liquid b (btm)

P05 4 15 35 30 0.4-0.6 3.0 45 15

PIO 7 30 75 40 0.4-0.6 3.0 45 30

P20 10 60 100 70 0.4-0.6 3.0 45 60

P30 15 75 175 70 0.4-0.6 4.0 45 75

P40 20 90 275 280 0.4-0.6 5.0 45 90

P50 30 125 350 440 0.4-0.6 6.0 45 125

a Porvair Technology Ltd.

b @ 99.9% efficiency.

7.2.2 Plastic foams

Polyurethanes are a conglomerate family of polymers in which the formation of

the urethane group,-NH-CO-O-, by reaction between hydroxyl and isocyanate

groups, is an important step in polymerization. This provides a linkage

mechanism that may involve a variety of aromatic or aliphatic groups; aliphatic

isocyanates tend to form the more flexible polyurethanes, such as the polyether

and polyester types, which are used as filter media. Polyester foams are stiffer and

less resilient than the polyether type.

Coarse Porous Sheets and Tubes 265

Urethane foam formulations consist of low viscosity liquids, which, when

mixed in appropriate proportions, react to form a foam and then cure into a

cellular mass. Typically formulations contain isocyanates and polyols, together

with catalysts, surfactants for stabilizing the foam structure, and blowing agents

to generate gas and expand the mass. Most urethane foams are produced by one-

shot processes in which all the raw materials are combined in a single step. Foam

is produced in blocks that can be sliced to give thicknesses from 3 to 200 mm, and

form sheets 2 m • 1 m.

An important feature of these foams is that the cells are reticulated, which

means that they are open and interconnected, with a porosity of some 9 7%. This

results from thermal chemical treatment, which causes shrinkage of material

enclosing cells to leave the very open skeletal structure illustrated in Figure 7.3;

in so doing, the thickness of the residual contracted walls is increased, with a

corresponding increase in tensile strength and in resistance to heat, abrasion

and chemical attack.

Appropriate regulation of the manufacturing process enables foams to be

produced with pores of predetermined sizes. It is usual practice to characterize the

pores in terms of the number per linear inch (e.g. 30 ppi); the average pore

diameter corresponding to foams graded on this basis is indicated in Figure 7.4.

An alternative basis for classifying reticulated foams for use as filter media is in

terms of the resistance of a 25 mm thick pad to the flow of air at 175 m/min, foam

grades being expressed in nominal forms such as PPI-60. The relationship

between these two grading scales is given in Figure 7.5.

Both polyester and polyether urethanes have good chemical resistance

excepting against strong acids, alkalis and solvents: soaps, detergents, mineral

Figure 7.2. Micrograph of the fracture surface of 'Filtroplast'porous sintered plastic media.

Purchas D. Handbook of Filter Media

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