Purchas D. Handbook of Filter Media
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Air and Gas Filter Media 19 5
5.7.2 Hybrid mists
Hybrid droplets in the size range 2-5 l~m can be agglomerated into greater than
5 ~m droplets by prefiltration through a coalescer pad, the resultant larger
droplets then being trapped by a conventional coarse demister. For example,
Begg Cousland Ltd use this two-stage technique by combining their Becoil
demister and Becone coalescer in series, to achieve an overall 100% removal of
droplets greater than 5 l~m and 98% removal of those greater than 2 ~m, the
corresponding overall pressure drop being approximately 120 mm WG.
The Becone coalescer pad is itself a modified form of the demister pad
illustrated in Figure 5.2 3, being fabricated from a composite fabric that consists
partly of monofilaments and partly of a staple fibre yarn. The very fine staple
fibres, having a correspondingly higher filtration efficiency, serve as sites for
collection of fine droplets: these coalesce into larger droplets that are then re-
entrained into the discharging gas. The pressure drop across a coalescer pad is
typically some three times that across a demister.
5.7.3 Fine mists
Candle-type demisters comprising annular pads of very fine fibres were
developed simultaneously by Fairs ~171 of ICI in the UK and by Brink ~18~ of
Monsanto in the USA in the late 19 50s. Separate accounts of both are included
in the 1964 book edited by Nonhebel ~19.2o~.
I000
i
I
Superficial gas velocity (m/see)
Figure 5.26. Effect of face velocity and water loading on the pressure drop of KnitMesh Type 9030 SL/SS
demister. Air~water @ 20 ~ C. Water loading curves: 1,40 m ~/m 2
h" 2.35 m 3/m 3 h" 3.30 m
3/m2 h" 4, 25
m3/m2 h 9
5, 20 m3/m2 h: 6, 15m3/m2h:
7. lOm3/m2 h: 8, dry pad.
196
Handbook of Filter Media
While the general forms of these demisters are understandably very similar,
comprising a multiplicity of long vertical candles (Figure 5.2 7) suspended in a
suitable housing, there was a point of crucial difference in respect of the fibres:
Fairs insisted on the importance of the fibres being hydrophobic, whereas Brink
regarded this as an unnecessary limitation.
The Fairs thesis was based on the observations that hitherto the filtration
efficiency of candle demisters was significantly less than that theoretically
predicted, and that they tended to become waterlogged with collected liquid. By
contrast, demisters of hydrophobic fibres were found to achieve the high
efficiencies theoretically predicted, and to be free from water logging. This was
explained by differences in the mode of collection of droplets on fibres, which
could be ascribed to the one being hydrophilic and the other hydrophobic, as
revealed in the photographs reproduced as Figure 5.28. The wetting of the
surface of the hydrophilic untreated glass fibre (Figure 5.28(a)) causes its
diameter to increase and therefore its filtration efficiency to decrease, a
deterioration not suffered by the unwetted hydrophobic silicone-treated glass
fibre (Figure 5.28(b)).
This erstwhile ICI technology is the basis of the Becofil range of candle
demisters summarized in Table 5.24 and produced by Begg Cousland, utilizing
fibres of different sizes, materials and packing densities. The fibres need to be as
fine as possible, and typically are less than 15 l~m. In practice, however, it is
customary to incorporate a proportion of coarser fibres of about 30-40 lam, so as
to provide stability and strength: experience showed that, with aqueous mists,
Figure 5.27. Candle type demister filter cartridges. (Photograph: Begg Cousland Ltd )
Air and Gas Filter Media 19 7
fine fibres alone tended to become saturated and not to drain due to partial
collapse of part of the bed.
The carded mixture of fibres is compressed into a mould at about 110-160 kg/
m 2, to form sections up to O. 6 m long, 5 cm thick and with an outside diameter up
to 60 cm. These are thermally stress relieved and made up into candles up to
about 5 m long, with internal and external mesh screens; the preformed media
can be replaced in the field. Flow may be either out-to-in or in-to-out, the
relationship between velocity through the exit phase (at equilibrium, with
continuous drainage) and pressure drop being shown for two styles of cartridge
in Figure 5.29.
5.8 Selection of Gas Cleaning Equipment
Four very different kinds of filtration equipment have been described in this
chapter:
9 ventilation and other atmospheric air filters;
9 industrialdustcollectors;
9 compressed air systems; and
9 demisters.
Of these four, the choices in compressed air treatment and demisters are
relatively simple: standard or special quality for the delivered air, and size of
liquid droplet in demisting.
(a) (b)
Figure 5.28. The mechanisms of mist collection depends on the wettability offibres: (a) film-wise collection on
hydrophilic glass fibres, (b) drop-wise collection on silicone-treated hydrophobic glass fibres.
198 Handbook of Filter Media
The choices in industrial dust collection have a major divider, the level of
operating temperature: a high temperature (certainly above 150~ needs
inorganic media- ceramics or possibly glass fibre. For ambient temperature
conditions, then the choice lies basically between achieving solids recovery
(using fabric filters) or an adequately clean air stream (the two together are not
Table 5.24 Examples of "Becofil" non-wettable fibre demisters a
Style Removal efficiency Pressure loss (mm WG) Typical service
FFG 25 FF
TGW 15
B.12
P.P.12
P.T.12
H.T.P.
High throughput
100% above 3 la
90% above 1 ~t
70% above 0.5 p
High efficiency
100% above 1 la
98% above 0.5 p
High efficiency
100% above 3 la
95% above 1
80% above 0.5 p
High efficiency
100% above 3 p
98% above 0.5 p
High efficiency
100% above 3
95 % below 3
High throughput
85%+ on 1-3 !~
70%+ on 0.5-1 ~t
100-300 Acid plants
150-4 50
Acid plants
High pressure systems
Plasticizer mists
100-250 Acid plants
Soluble fume
100-350 Corrosive service
Soluble fume
100-300 Wet chlorine systems
100-300
Restricted space
Lower efficiency below 3
a Begg Cousland Ltd.
b Types of fibre used: glass wools, polypropylene, polyester.
c Support cage materials: mild steel, stainless steels, titanium, high nickel alloys, etc., polypropylene,
pvc, pvdf, grp.
/
01~ 02 OT3 014 05 016 0-7 08' 'C)19
FACE VELOCITY (MEIRES/$ECOND)
Figure 5.2 9. Pressure drop versus exit face velocity through 'Becofil'candle type demisters.
9
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9
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200
Handbook of Filter Media
16. Foundry Trade Journal (1992) December
17. G L Fairs ( 1958) 'High efficiency fibre filters for the treatment of fine mists',
Trans. Inst. Chem Eng., 3 6, 4 77
18. J A Brink (1959) 'New fiber mist eliminator', Chemical Engineering, 66,
183
19. G L Fairs (1964) 'Hydrophobic fibre filters', Gas Purification Processes (ed. G
Nonhebel), Geo Newnes Ltd, London, pp. 711-15
20. J A Brink (1964:) 'Fibre mist eliminators', Gas Purification Processes (ed. G
Nonhebel), Geo Newnes Ltd, London, pp. 731-6
CHAPTER 6
Screens and Meshes
The several very different types of filter media gathered together in this chapter,
woven meshes, perforated sheets and structures of shaped wires, have one main
common feature - an accuracy of aperture size. Another common feature is that
they are all made from metal, largely because abrasion is a normal problem in
their application, although many forms are now available in plastic as well.
All the dry screening (sieving, sifting) operations are covered by the media in
this chapter, as are almost all of the straining and coarse filtration applications.
6.1 Introduction
The prime feature of media made from meshes or screens is that of aperture
shape - the size and shape of the apertures in the medium is critical for the
intended application. The material of construction is less critical, although its
high tensile strength may be vital as well.
There are three broad classes of media covered under this heading: woven
meshes, sheets perforated with a variety of holes, and elements made up from
preformed materials. Some overlap exists between the woven meshes of this
chapter and the woven monofilament materials of Chapter 2.
6.2 Woven Wire Mesh
The weaving of wire is no different, in principle, from the weaving of any other
yarn- as described in Chapter 2. The product is a roll of woven material, which
then is processed in a variety of ways, to produce the components of a filter
medium. The term wirecloth is frequently used to refer to meshes woven from
finer grades of wire, while the term
bolting cloth
refers to lightweight versions of
square mesh cloths, comprising those based on the finest wires.
A wide variety of wire meshes is produced by weaving monofilaments of either
ferrous or non-ferrous metals in widths of about 1 m up to 2 m. Two main
202
Handbook of Filter Media
categories can be distinguished, in terms of weave and of the shape of the
apertures, as in Figure 6.1. One category utilizes plain weave with single wires of
the same diameter for the warp and weft, to form rectangular apertures (the
great majority being square): many of these are the screens typically used for
sieving and sizing operations. The other category is 'zero aperture filter cloths',
with the wires pressed closely together. These embrace a number of more
complex weaves, such as dutch twills, which are commonly used in pressure and
vacuum process filters.
Information on the metals used in wire mesh is given in Tables 6.1-6.7
(provided by Haver and Boecker). Each includes some guidance on resistance to
corrosion, in terms of numerical values extending from 1 (= very good) to 5 (=
poor); an added asterisk (*) indicates danger of localized corrosion.
Square Rectangular Rectangular Zero Aperture
Aperture "Oblong .... Broad .... Filter Cloth"
Figure 6.1. Some types of apertures in woven wire.
Table 6.1 Metals for woven wire cloth: steel
Material Material Trade name
no.
Max service temp. Finest weaving Resistance against:
wire diameter
:C :F mm inch
Plain steel 1.0010 Carbon steel 500
Galvanized 200
steel
Tinned steel 150
Spring steel 1.0500 NIA-Steel 500
High carbon steel
9 30
390
30O
930
0.08 0.0030 5 5 2-4 4-5
0.16 0.0065 3 5 2-4 4-5
0.10 0.0040 5 5
2-4 4-5
0.125 0.0050 5 5 3-4 4-5
Screens and Meshes 203
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77
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X M M X X M X X M
204
Handbook of Filter Media
6.2.1
Square
mesh
Listed in Table 6.9 is the range of square mesh wire meshes produced by one
leading manufacturer, indicating which grades are available in specific metals.
The tolerance of the aperture sizes specified varies according to the fineness of the
cloth, as summarized in Table 6.8; a crucial factor in determining this is the
tolerance of the diameter (including the extent to which it deviates from being
truly round) of the wires from which the mesh is woven.
Crimping of the wires happens automatically as part of the weaving process,
provided that the wires are sufficiently fine and ductile. With heavier and more
rigid wires, however, such as those of high tensile steel for heavy-duty screens, a
separate pre-crimping operation is necessary, both to form the desired apertures
and to ensure appropriate stability during extended use. Various types of crimp
are used, as outlined in Table 6.10.
Table 6.3 Metals for woven wire cloth: special steels
Material AISI Symbols Composition
no.
Max service Finest weaving Resistance
temp. wire dia. against:
Cr Ni ~ ~ mm inch
~D
00 I-,
03
HB253 HITHERM 21 11 1200 2190 0.025 0.0010 1 2* 1-2 2-3*
HB 165 Corresist 20 25 900 1650 0.10 0.0040 1 2 2 2
1.4841 310 X15CrNiSi2520 25 20 Si2 1200 2190 0.05 0.0020 1 3* 2-3 2-4*
314
1.4742 XIOCrAll8 18 All 1050 1920 0.05 0.0020 1 4* 2-3 2-4*
Table 6.4 Metals for woven wire cloth: copper and alloys
Material Material Composition Max. service Finest weaving Resistance against:
no. temp. wire dia.
Cu Zn ~ ~ mm inch
03
L
03 I-,
.s 03
..a
03
.a <
Copper E-Cu 2.0060 99.9
Brass CuZn 37 2.0321 63 37
Ms63
Low CuZn 20 2.0250 80 20
brass Ms 80
Common CuZn 10 2.0320 90 10
bronze Ms 90
Phosphor CuSn 6 2.1020 94
bronze
150 300
200 390
200 390
200 390
Sn 6 200
P
390
0.050 0.0020 2 3 3 2-5
0.050 0.0020 5 5 3 4-5
0.050 0.0020 4 4 2 2-5
0.050 0.0020 2 3 2 2-5
0.025 0.0010 1 2 3 2-5