Key Points• Fresh and waste water
filtration is now the largest and fastest growing segment of the filter media
market.
• The growing demand for cleaner water is attracting major new players into the water treatment market.
• H&V's Nanowave™ technology yields a filter with a wavy surface which provides half the pressure drop of the equivalent glass filter with 3 times the dust holding capacity and a higher filtration efficiency when discharged.
• Irema Filter is co-forming PP nanofibres with a standard PP meltblown to give a gradient structure which has higher capacity, lower pressure drop and longer charge retention than the usual two-layer structure.
• CamilFarr use a new form of active carbon with “Rapid Adsorption Dynamics” in an F7 filter to absorb low concentrations of pollutants quickly.
• Norafin's 420 gsm hydroentangled fabrics with a split bico microfiber surface can match the performance of a 500 gsm needlefelt with an ePTFE membrane surface.
• Europlasma use plasma to polymerise a monomer onto PP to render it permanently hydrophilic. The monomers used are secret and the process needs a vacuum.
• With air handling fans consuming 15% of all electricity consumption, the EU are expected to require energy efficiency labelling of air filters in about 3 years.
• The nanoalumina whiskers used in Ahlstrom's Dispruptor™ papers can immobilize 8 micron active carbon powder without the need for binders which can reduce efficiency. The resulting filters are excellent for cleaning up water for drinking.
• Dupont's HMT nonwoven uses direct-spun continuous nanofibers to achieve membrane-like filtration at lower pressure drop.
• The growing demand for cleaner water is attracting major new players into the water treatment market.
• H&V's Nanowave™ technology yields a filter with a wavy surface which provides half the pressure drop of the equivalent glass filter with 3 times the dust holding capacity and a higher filtration efficiency when discharged.
• Irema Filter is co-forming PP nanofibres with a standard PP meltblown to give a gradient structure which has higher capacity, lower pressure drop and longer charge retention than the usual two-layer structure.
• CamilFarr use a new form of active carbon with “Rapid Adsorption Dynamics” in an F7 filter to absorb low concentrations of pollutants quickly.
• Norafin's 420 gsm hydroentangled fabrics with a split bico microfiber surface can match the performance of a 500 gsm needlefelt with an ePTFE membrane surface.
• Europlasma use plasma to polymerise a monomer onto PP to render it permanently hydrophilic. The monomers used are secret and the process needs a vacuum.
• With air handling fans consuming 15% of all electricity consumption, the EU are expected to require energy efficiency labelling of air filters in about 3 years.
• The nanoalumina whiskers used in Ahlstrom's Dispruptor™ papers can immobilize 8 micron active carbon powder without the need for binders which can reduce efficiency. The resulting filters are excellent for cleaning up water for drinking.
• Dupont's HMT nonwoven uses direct-spun continuous nanofibers to achieve membrane-like filtration at lower pressure drop.
Keynote: Filter Media Markets
Ken Sutherland of Northdoe UK defined
the filtration market to include deep-bed sand filtration, coarse porous
materials, wound yarns, woven fabrics, felts, papers and nonwovens including
glass and carbon materials, sintered metal and ceramic media and polymeric
membranes. Strictly speaking membranes were not filters because they separated
molecules by rate of diffusion, and were not intended simply to remove
particulates.
His $ values were at the point of sale to the filter
equipment maker or replacement user and included all industrial, commercial and
residential filters, whether fitted as original equipment or sold as
replacements. Data of most interest to the nonwovens industry – tonnes or square
metres of roll-goods - was not available:
• The world market for filter media in 2007
was $21.7 billion.
• 38% of this was sold with the filter hardware, and 62% in the aftermarket.
• Nonwovens accounted for 27% of this and Paper (surprisingly) another 2.9%.
• Membranes were the biggest sector (35.5%) with spunmelt nonwovens (14.3%), metallics (10.4%), felts (9.9%) and constructed cartridges, i.e. filter media plus frame and end caps as in most automotive air and oil filters, at 7.1%, making up the top-five filter types.
• USA (24.4%), China (12.1%) and Japan (7.5%) were the main countries of use. China was growing much faster than the others and because of the difficulty of getting good information could already be much larger than this. (The USA had fallen from 50% a few years ago.)
• Fresh and waste water filtration (14.3%) is the largest individual sector, followed by Bulk Chemicals and Petrochemicals (13.6%), Fine Chemicals and Pharmaceuticals (9.6%), Food & Beverage (8.3%), Power Generation (7.4%), Medical and Health (6.3%) and Mining and Metals Processing (6.0%)
• The global market was growing at 5.6% annually, China at 10.1%. Water treatment was growing at 6.8%.
• Major growth drivers were:
• End user demands for cleaner and purer water and air. (“Everyone, all the major companies, are getting into water treatment”)
• Environmental regulations tightening
• Growth in China and India and other developing regions.
• “Most of the areas the World now needs to improve involve using better filtration.”
• 38% of this was sold with the filter hardware, and 62% in the aftermarket.
• Nonwovens accounted for 27% of this and Paper (surprisingly) another 2.9%.
• Membranes were the biggest sector (35.5%) with spunmelt nonwovens (14.3%), metallics (10.4%), felts (9.9%) and constructed cartridges, i.e. filter media plus frame and end caps as in most automotive air and oil filters, at 7.1%, making up the top-five filter types.
• USA (24.4%), China (12.1%) and Japan (7.5%) were the main countries of use. China was growing much faster than the others and because of the difficulty of getting good information could already be much larger than this. (The USA had fallen from 50% a few years ago.)
• Fresh and waste water filtration (14.3%) is the largest individual sector, followed by Bulk Chemicals and Petrochemicals (13.6%), Fine Chemicals and Pharmaceuticals (9.6%), Food & Beverage (8.3%), Power Generation (7.4%), Medical and Health (6.3%) and Mining and Metals Processing (6.0%)
• The global market was growing at 5.6% annually, China at 10.1%. Water treatment was growing at 6.8%.
• Major growth drivers were:
• End user demands for cleaner and purer water and air. (“Everyone, all the major companies, are getting into water treatment”)
• Environmental regulations tightening
• Growth in China and India and other developing regions.
• “Most of the areas the World now needs to improve involve using better filtration.”
Technologically, the development of ever
finer fibres, and materials with better heat/chemical resistance was leading to
innovative new filters. Heat/chemical resistance was getting more important as
companies tried to save energy by extracting heat from effluent: the heat
exchangers now requiring pre-filters to keep them working properly.
Asked for more detail about the $ values, Mr
Sutherland said the total filtration market (equipment plus media) was $44bn,
and all his numbers were based on the exchange rate on July 1 st 2007.
Nanowave™
Andre Boni, Director of Marketing for
Hollingsworth and Vose described their proprietary new technology to
make an “extended surface synthetic media”. Nanowave™ was a development of
Nanomelt™ introduced at Index in 2005, the latter having had problems due to its
higher initial pressure drop, lower test dust capacity and premature blinding by
fine dusts.
The unique feature of Nanowave™ was its wavy surface,
and this gave the following benefits:
• Double the alpha value of conventional discharged melt-blown media (alpha=Efficiency/Pressure drop)
• Double to treble the dust holding capacity (wavy surface, like crepeing, gives twice the surface area per square metre of filter.)
• The nanofibres give a high discharged efficiency.
• The structure can also be charged for even higher efficiencies
• Wave height, density and polymer type, as well as fibre dimensions can be adjusted for specific applications.
• It is stiffer and more resistant to compression.
• Double the alpha value of conventional discharged melt-blown media (alpha=Efficiency/Pressure drop)
• Double to treble the dust holding capacity (wavy surface, like crepeing, gives twice the surface area per square metre of filter.)
• The nanofibres give a high discharged efficiency.
• The structure can also be charged for even higher efficiencies
• Wave height, density and polymer type, as well as fibre dimensions can be adjusted for specific applications.
• It is stiffer and more resistant to compression.
Initial focus is on an HVAC bag filter to meet the F5 to
F9 classes of EN779. Here synthetics, usually charged and so liable to lose
efficiency over time, are battling with glass fibre which is more consistent but
might shed fibres. Nanowave™ gives half the pressure drop of the equivalent
glass filter, with 3 times the dust holding capacity and a higher efficiency
even when discharged.
Asked about price, Mr Boni said it would be a premium product, but H&V would position it competitively. It is not commercial but test quantities can be made available.
Asked about price, Mr Boni said it would be a premium product, but H&V would position it competitively. It is not commercial but test quantities can be made available.
Integrating Nanofibres
Wolfgang Rupertseder, Plant Manager, Irema
Filter ( Germany ) reminded us that diffusion is the main mechanism for
fine particle capture, and then the particle must stick to the fibre. The force
of adhesion is highest when the fibre diameter equals the particle size, and in
a real world gradient filter you can see that the coarse fibres catch the big
particles and the fine fibres catch the small ones. Once fibres get smaller than
about half a micron, a “slip-flow effect” (the air velocity at the surface is no
longer zero) occurs and this reduces the pressure drop to about 70% of that
calculated using theory which works for larger fibres.
Rather than create a layered structure with nanofibres
laid on top of coarser melt-blown fibres, Irema co-mingle the fibres by spraying
the two types into one another before they land on the conveyor. A bulky
gradient structure is obtained with the top surface being richer in nanofibres
than the bottom. For some applications a third head will apply a protective
microfiber layer to the top side. The heads look like conventional melt-blow
heads, and the nanofibre size (0.1 to 0.5 micron claimed) is probably obtained
by running the nanofibre head at very low productivity, and the whole line at
low speed.
The product is commercial in automotive cabin air
filters where it gives a 70% NaCl capture efficiency and longer filter life.
Media for HEPA filters is under development.
Results shown were for charged media, but in response to
questions Mr Rupertseder said the blend structure discharges more slowly than a
layered structure. PP is the main polymer used but the process has been shown to
work with PET.
Nanogate Technology
Yvonne Klattwitter, Product Manager for Nanogate
( Germany ) said their core competence was formulating products using
nanomaterials and processes based on nanotechnology. Their N-Charge® coating had
been designed to improve the ability of surfaces to store an electric charge and
works as a dielectric which prevents discharging due to particle capture.
Realising that this may have applications on fibres and fabrics, they developed
Permastatic® as a spray-on or paddable water-based coating.
Nonwovens coated with Permastatic® have:
• The same permeability as untreated nonwovens. The coat is 100nm thick.
• Increased fractional filtration efficiency. Retention of submicron particles improves from 70% to 90% in vacuum cleaner bags as tested by Filz Fabric Fulda.
• Longer lasting electrostatic properties. Charge retention was up from 61% to 83% in a 5 months test. In a 2 year test 100% PP and 100% PET retained their initial charge of 3-4kV almost unchanged. (Were these fibres or films?)
• Resistance to water washing and IPA extraction
• The same permeability as untreated nonwovens. The coat is 100nm thick.
• Increased fractional filtration efficiency. Retention of submicron particles improves from 70% to 90% in vacuum cleaner bags as tested by Filz Fabric Fulda.
• Longer lasting electrostatic properties. Charge retention was up from 61% to 83% in a 5 months test. In a 2 year test 100% PP and 100% PET retained their initial charge of 3-4kV almost unchanged. (Were these fibres or films?)
• Resistance to water washing and IPA extraction
Permastatic® can be combined with other treatments to
give improved cleanability, hydrophobicity or hydrophilicity, and is said to be
easily incorporated into existing processes as a low-cost upgrade. Ms
Klattwitter could not answer questions about cost.
Nano-pollution
Thore Bertilsson and Jörg Scharnberg of
CamilFarr ( Sweden ) defined air pollution as “matter found in the
atmosphere not belonging to its natural consistency, or matter found in indoor
air with harmful effects on people and materials”. Examples included dust, soot,
mist, fog, gases and vapours and radioactivity. They can be natural (forest
fires, volcanos, dust storms) or man-made (incineration, fossil fuel combustion,
factory effluent). Very small particles and gases presented particular problems.
Normally polluted town air had a bimodal distribution with particle size peaks
at around 0.2 and 8 microns. Modern diesel engine exhausts emitted particles
below 0.3 microns and these could pass through the lungs into the bloodstream.
However an F7 air filter with >50% discharged efficiency at 0.4 micron would
give acceptable air quality, reducing PM10s by 87% and sub PM1s by 75%. To
improve on this and get noticeably better air quality a filter meeting
EN13779:2007 would be required.
Atmospheric polluting gases - NOX, SOX, NH3, HCl, VOCs – and indoor pollutants, formaldehyde from wood and carpets, ozone from photocopiers and various unpleasant odours from food and waste can only be removed by adsorbtion on active carbon, active alumina or ion exchange resins. Existing granular forms of active carbon are too slow to adsorb low concentrations to be useful in HVAC systems but an new form with Rapid Adsorption Dynamics is now available. RAD deals with really low concentrations of pollutant quickly by virtue of being a really small particle of active carbon, and it can be integrated into an F7 particle filter.
Atmospheric polluting gases - NOX, SOX, NH3, HCl, VOCs – and indoor pollutants, formaldehyde from wood and carpets, ozone from photocopiers and various unpleasant odours from food and waste can only be removed by adsorbtion on active carbon, active alumina or ion exchange resins. Existing granular forms of active carbon are too slow to adsorb low concentrations to be useful in HVAC systems but an new form with Rapid Adsorption Dynamics is now available. RAD deals with really low concentrations of pollutant quickly by virtue of being a really small particle of active carbon, and it can be integrated into an F7 particle filter.
Other pollutants which are currently untreatable include
CO 2 , CO, CH 4 , NO, He and Ra.
Ahlstrom Disruptor™ in Reverse Osmosis
Domingo Sanchez (R&D) of Ahlstrom Filtration
(USA) replaced Rodney Komlenic, VP Business Development, to deliver the
paper on the Dispruptor™ fabrics made under licence from the Argonide
Corporation. Here, naturally occurring 2nm by 250nm boehmite whiskers are
attached to submicron microglass fibres which can then be easily processed by
Ahlstrom into wet-laid nonwovens. These nonwovens have a 2 micron average pore
size and a 42,000 m 2 nanofibre surface area per square metre of nonwoven. At pH
7.2 they have a >50millivolt zeta potential which is effective for up to a
micron from the whisker surface, this allowing them to capture particles below
0.5 micron electrostatically, while the large particles are retained
mechanically. In some tests they have been shown to retain 6 times their own
weight of contaminant. They can be used at operating temperatures above 400 o F,
yet have filtration efficiencies similar to the UF/MF polymer membranes while
incurring a fraction of their pressure drop. Examples of use covered products
normally associated with reverse osmosis, nanofiltration, ultrafiltration and
microfiltration:
• Compared with membranes having a range of
pore sizes (0.025 to 1.2 microns) Disruptor™ media removed B. diminutia, MS2
virus, and an endotoxin with higher efficiency at lower pressure drop and higher
flow rates.
• T4 phage was adsorbed from a concentrated solution by simply immersing Disruptor™ in the solution.
• Iron, tin, lead and copper particles, problematic in drinking water and waste from metal finishing, were all removed easily by Disruptor™. Dissolved iron was also removed.
• The nanoalumina whiskers will immobilise 8 micron powdered active carbon (PAC) powder without the need for binders which can reduce the PAC adsorption efficiency.
• The resulting Disruptor™PAC paper can remove 1gm of chlorine or iodine from water per gram of PAC.
• Disruptor™PAC successfully removed all the humic acid from drinking water spiked with 20 FTU and reduced the turbidity of the water to zero. Even standard Disruptor™ removed more of the humic acid than the tightest (0.22 micron) Millipore membrane.
• Disruptor™ remove trace amounts of organic matter from lake water at high flow rates and very low pressure drop.
• They effectively reduce water contaminants missed by GAC beds, flocculation, microfiltration and ultrafiltration processes.
• As prefilters on reverse osmosis processes, the membrane cleaning routines can be increased from monthly to six-monthly due to their ability to reduce the silt density index (SDI).
• Biofouling of membranes by proteins, lipids, carbohydrates and amino acids can be significantly reduced.
• T4 phage was adsorbed from a concentrated solution by simply immersing Disruptor™ in the solution.
• Iron, tin, lead and copper particles, problematic in drinking water and waste from metal finishing, were all removed easily by Disruptor™. Dissolved iron was also removed.
• The nanoalumina whiskers will immobilise 8 micron powdered active carbon (PAC) powder without the need for binders which can reduce the PAC adsorption efficiency.
• The resulting Disruptor™PAC paper can remove 1gm of chlorine or iodine from water per gram of PAC.
• Disruptor™PAC successfully removed all the humic acid from drinking water spiked with 20 FTU and reduced the turbidity of the water to zero. Even standard Disruptor™ removed more of the humic acid than the tightest (0.22 micron) Millipore membrane.
• Disruptor™ remove trace amounts of organic matter from lake water at high flow rates and very low pressure drop.
• They effectively reduce water contaminants missed by GAC beds, flocculation, microfiltration and ultrafiltration processes.
• As prefilters on reverse osmosis processes, the membrane cleaning routines can be increased from monthly to six-monthly due to their ability to reduce the silt density index (SDI).
• Biofouling of membranes by proteins, lipids, carbohydrates and amino acids can be significantly reduced.
Disruptor™ nanoalumina whiskers have been
used in medicine since 1926 and have FDA approval for use in human vaccines.
They have now passed the NSF/ANSI standard 42 for water contact use and standard
53 for water contact and cyst reduction, USP class VI and endotoxin testing. In
conclusion Ahlstrom believe this is a real breakthrough in reducing a wide range
of reverse osmosis biofoulants via electroadhesion and mechanical
entrapment.
Asked about the cost of the papers, Mr Sanchez simply said $20/m 2 for an average basis weight.
Asked about the cost of the papers, Mr Sanchez simply said $20/m 2 for an average basis weight.
Microglass and Spunbond
Dr Werner Groh of Johns Manville ( Germany )
introduced their new CombiFil® Premium air filter medium made by
flame-blowing 1-3 micron glass fibres between two layers of spunbond polyester.
The process can be used to make filter efficiencies up to HEPA with high
stiffness and pleatablility without the need for wire supports. It holds more
dust at lower pressure drops and maintains a stable filtration performance over
longer times than the charged synthetic media with which it competes.
Specifically, an F8 Combifil®Premium filter holds more dust at a lower pressure
drop than an F7 synthetic.
Price? Dr Groh said it would be ~€3/m2 or about twice the cost of a standard material. The microglass was spray bonded with phenolic resin, which also served to bond the polyesters in place.
Price? Dr Groh said it would be ~€3/m2 or about twice the cost of a standard material. The microglass was spray bonded with phenolic resin, which also served to bond the polyesters in place.
Spunlaced for Membranes?
Victor Lorentz, Sales Manager of Norafin (
Germany ) described the versatility of their core technology,
hydroentanglement, pointing out that it was capable of making filters to compete
with needlefelts in all applications. Furthermore it gave filters with a more
uniform pore structure over a wider range of basis weights (40-700gsm). For
demanding applications, metal or glass scrims were used as reinforcement. A 320
gsm hydroentangled filter gave efficiencies comparable with a 500gsm needlefelt
at a much lower pressure drop when the same fibre were used.
If splittable bicomponents were used, much finer filters
could be made, but only over an 80-120 gsm range because lighter fabrics were
too thin to withstand the splitting pressures, and heavier ones were too costly
to split sufficiently. Because the splitting occurred mainly at the surface, a
gradient structure with a membrane-like surface was obtained in a one-pass
operation without the need for lamination and binders. The fibre splitting
increases the filtration efficiency from 65 to 95% for 1 micron particles on 1.3
dtex fibres. If a 100 gsm “microsplit” fabric was laminated to the 320 gsm
standard HE fabric the resulting filtration performance had been shown to match
a 500 gsm needlefelt with an ePTFE membrane surface.
Asked about the mechanical properties of the spunlace,
Mr Lorentz said they were more robust than the equivalent needlefelts and the
membrane surface more durable than the ePTFE. Could such materials be used in
HVAC? Yes, if the split-fibre layer was laminated to a more bulky base.
Pleatable PET for Depth Filters
Dr Ulrich Hornfleck Sales Director of Sandler AG
( Germany ) observed that carded polyester, chemically bonded in
thicknesses up to 3mm can be easily pleated to make an excellent self-supporting
coarse dust filter for use in filter classes from G3 to F5 depending on the
fibre decitex. However the finest cardable fibre diameter (i.e. about 10
microns) prevented this construction from entering F6 and above.
A new 3-layer structure with a highloft base, an
ordinary fine fibre carded polyester centre and a melt-blown polyester surface
layer has been shown to maintain its pleatability, offer temperatures of up to
120 o C at constant load with 200 o C peaks, have high mechanical stability and
a very high crack resistance. Air permeability and dust holding capacity was
superior to F7 microglass media.
A questioner was concerned that the comparisons between
microglass and such a thick polyester filter would be invalid, because the use
of flat sheet test methods rather than pleated media tests would bias the
conclusions in favour of the thicker material. In real filters the glass media
would allow a higher pleat density.
Characterisation of Needlefelt Surfaces
Joseph Schuberth of the Vienna University of
Technology ( Austria ) has developed a method for comparing the
particle penetration and particle holding capacity of filter media. A light
microscope and image analysis software was used to determine the surface
porosity and the pore depth distribution of a range of felts. Combining these
with the mean clean gas concentration and dust loading obtained from a VDI 3926
Type 2 tester, correlations between the optical and physical tests were
obtained. These calibration graphs now allow the clean gas concentrations and
dust holding capacities of other needelefelts to be estimated simply by using
the microscope and image analysis software. The optical measurements can also be
used to calculate the relative standard deviation of the pore-size and the
average porosity on both clean and dust-loaded filters, and this will allow
investigation of clogging mechanisms.
Engine Oil Filters
Claes Frennfelt of Volvo Powertrain ( Sweden )
underlined the importance of oil filtration in reducing the costs of
operating a fleet of trucks and complying with the ever-tightening regulations
on exhaust emissions. Oil lubricates, cleans and cools a truck diesel and is
typically filtered in a bank of three large cartridges. Two of these operate at
60-80 litres/hour on the whole oil circuit, while the third, a finer “by-pass”
filter operates at 8 litres min. The 40 liters of oil in the circuit is at 400
to 600 kPa.
Standard cellulose filters last 60,000 km while long
life versions can do 100,000 km. For 150,000 km service intervals a synthetic
blend filter is needed. These synthetic filter cartridges are much more
expensive because they are not self-supporting. Other factors affecting oil
service life are fuel quality (>500 ppm sulphur shortens the filter-life),
exhaust gas recycling (reduces it by a third), biofuel use (a 30% biofuel blend
halves it), and topography (hilly worse than flat). Compared with the 3 times
more expensive synthetic oils, mineral oil works well up to 100,000km in normal
climates. The low viscosity synthetics are needed in cold climates.
Plasma Coatings for Engine Air Filters
Marc Pauwels of Europlasma NV ( Belgium )
described how plasma could be used to polymerise a monomer onto the
fibre surface to give a permanent coating. Filter media could be processed in a
vacuum chamber on a roll-roll system, or complete cartridges could be processed
batchwise. Hydrocarbon monomers give permanently hydrophilic coatings when
treated in the presence of oxygen and fluorocarbon mixtures give oleophilic and
hydrophobic coatings. The process is used to improve air filters for diesel
engines by increasing the hydrophobicity of a PBT nonwoven filter so that it can
pass the water decantation and repellency tests. The benefit is achieved without
altering the dust holding capacity or filtration efficiency. It is also used on
facemasks, air conditioner filters, battery separators and blood filters. The
roll to roll process runs at 5 m/min and the current kit can take 60cm diameter
rolls. Variable costs are €0.02/m 2 , said to be cheaper than a wet-chemistry
approach and better for the environment. Asked what monomers were used to
achieve permanent hydrophilicity on polypropylene, Mr Pauwels said this was a
secret. Atmospheric plasma treatment did not work because the monomer loss
proved too costly.
Predicting Filter Performance
Martin Lehmann, Head of Simulation Filter
Elements at Mann and Hummel ( Germany ) said measuring the distribution
of fibre diameters in a filter is the key to predicting filtration performance
and Mann and Hummel had developed a new way of doing it. Using a single average
diameter was clearly wrong and SEM or optical microscopes coupled with image
analysis were poor. He used the new Matlab image analysis software but provided
no details. Furthermore his work with cellulose filters assumed that the clearly
flattened fibres in the filter were round with a diameter equivalent to their
width. Asked about this, he felt the microscope “saw” the fibres as a dust
particle would, so the assumption of roundness was OK. The technique could be
used in QC.
Testing Single Pleats
Dr Harmut Sauter of Mahle Filtersystems (
Germany ) described the three scales of filter evaluation: micro, meso
and macro where macro considers the whole filter in its housing, meso considers
the filter alone and micro considers filtration at the interfibre level. Most
automotive air filters are pleated so at the meso level, understanding the
behaviour of a single pleat becomes fundamental to filter design. Furthermore,
the single pleat lends itself to computational fluid dynamics (CFD) simulation
of air velocities and hence comparisons of experimental with simulation
data.
Air velocities were measured by particle image
velocimetry (PIV) where the particles were too small to be separated by the
filter. Static pressure measurements at one air velocity showed that the
simulation technique provided roughly comparable data, the differences being
attributed to the pleat changing shape under pressure. In fact in a real filter
they were observed to stretch and vibrate and change the way the dust
distributes itself. Maybe the use of a V shaped pleat was too simple. The N
shape might represent the smallest filter element more exactly.
Classifying Air Filters
Dr Thomas Caesar, Manager Filter Technics,
Freudenberg Nonwovens ( Germany ) observed that as energy prices and CO
2 emission concerns increased, the power consumption of air filters was being
scrutinized. In the UK and Sweden where the data is available, 15% of all
electricity consumption is by the fans used to handle air, and a third of this
power was lost in pressure drops across the filters used to clean the air. So
energy efficient air filters would help to save the planet and they therefore
had to be classified according to their energy efficiency as well as their
filtration efficiency. The target is to identify the types of filter which
provide the highest possible particle removal efficiency at the lowest possible
pressure drop, the complication being that both of these parameters varies with
the life and dust loading of the filter.
Dr Caesar proposes the derivation of a “key energy
performance” or kep number which is a function of the average efficiency
from EN 779 using a 0.4 micron particle at 3400 m 3 /hr, and the
average pressure drop derived from a loading of 800g of ISO A2 dust
also at 3400 m 3 /hr. Kep numbers less than 1 are given “Energy Class 1” as the
most efficient now possible, and kep numbers below 0.6 are given “Energy Class
5” being the least efficient. These 1 through 5 Energy Ratings would be used
like refridgerator energy classifications so that filter buyers could choose the
efficiency they could afford.
Asked if this was just for Freudenberg or would it
become an official standard, Dr Caesar said he was working with ISO and would
expect it to become an international standard in about 3 years. Freudenberg
would be introducing energy efficiency labelling on their range by the end of
next year. Is the market demanding this? “Basically yes”.
Nexylene PPS fibres
Dr Jens Neumann-Roderkich, Development Manager,
Nexis Fibres (Germany) ran through their PPS brochure, reminding us
that poly phenylene sulphide fibres offered exceptional thermal stability up to
200 o C (melting at 290 o C) and were therefore positioned, both from a
performance and price viewpoint, between nylon 6 and the meta-aramids. Unlike
the meta-aramids, they were melt spinnable and could be produced in crystalline
and amorphous forms with differing properties. The stronger crystalline form
exhibited a shrinkage of up to 8% on heating to 220 o C, and the amorphous form
could be used as a stretchy binder fibre with identical thermal stability. They
could be made into all forms of nonwovens, the needlefelts with 25% Nexylene
woven scrim being offered as recyclable hot gas filters. There were several
questions and one comment from the audience:
• The comment pointed out that this fibre
could not be sold as a hot gas filter because it did not meet the required
thermal stability at 250 o C.
• How did Nexylene compare with Japanese and Chinese sources of PPS? It was as good as the best Japanese and better than the Chinese, one supplier in China being poor for needlefelts. Furthermore neither the Chinese nor the US producers offer 1.3dtex
• Were special extruders, piping and spinpacks needed to handle the PPS melt? Yes, the chlorine residues necessitated this.
• Were there any problems with the gas in the workplace? No, it was within German limits without special ventilation.
• What were the wet-laid PPS nonwovens used for? Electrical insulation – in combination with other fibres.
• Was there still a PPS shortage. No, the market is now in balance and two or three producers have expansions underway.
• How did Nexylene compare with Japanese and Chinese sources of PPS? It was as good as the best Japanese and better than the Chinese, one supplier in China being poor for needlefelts. Furthermore neither the Chinese nor the US producers offer 1.3dtex
• Were special extruders, piping and spinpacks needed to handle the PPS melt? Yes, the chlorine residues necessitated this.
• Were there any problems with the gas in the workplace? No, it was within German limits without special ventilation.
• What were the wet-laid PPS nonwovens used for? Electrical insulation – in combination with other fibres.
• Was there still a PPS shortage. No, the market is now in balance and two or three producers have expansions underway.
Air Filtration Composite
Dr Carsten Heldmann, Global R&D Director of
Fiberweb (Germany) is developing high performance air filtration media
with a low pressure drop by combining fine-fibre meltblown (FFMB), spunbond and
carded webs to form graduated filters. The FFMB has diameters between 0.6 and
1.4 micron and thus straddles the currently used boundary between micro and nano
fibres. In one example of a 4-layer “TopFlow”composite, a top layer of coarse
meltblown protected a layer of standard meltblown on top of a FFMB which had
been laid on to a spunbond reinforcing layer.
Carded prefilters could be made from fibres of different
denier on the same line to get an even coarser prefilter for use in HVAC. These
could be laminated to the spunmelt structures using sonic bonding to minimise
the loss of open area. The main message was that Fiberweb were now willing to
combine their carding and spunmelt expertise to offer a wide range of air
filtration products.
Needlefelts for Liquid Filtration
Heribert Kurt of Heimbach Filtration ( Germany )
said that scrim-reinforced needlefelts are generally better than woven
fabrics in liquid filtration because they offer good capacity at lower pressure
drop due to their bulk and permeability. In industrial process filters they can
be designed to survive the often harsh chemical environments and have
established themselves in vacuum belts filters, rotary drum filters and filter
presses. Their main uses are in the production of:
• Minerals and zeolites, including ores, lime, diatomite, and active carbon.
• Chemicals, e.g. fertilisers
• Dyestuffs and pigments
• Pharmaceuticals and antibiotics
• Non-ferrous metals including copper, nickel and zinc and its oxides.
• Food additives
• Minerals and zeolites, including ores, lime, diatomite, and active carbon.
• Chemicals, e.g. fertilisers
• Dyestuffs and pigments
• Pharmaceuticals and antibiotics
• Non-ferrous metals including copper, nickel and zinc and its oxides.
• Food additives
One example used a 300gsm woven monofil PET scrim with a
microfiber needlefelt surface which had been densified to remove 2 micron
particles of zeolite from the process liquor.
Asked how Heimbach deal with the “smile” and “frown”
variations in basis weight across the width (that needlefelts are prone to and
wovens not), Mr Kurt said they could compensate for these in careful calendering
and heat setting to get uniform permeability. Fibre loss from needlefelts was
another issue, and here again the surfaces could be stabilised in the calendar.
Liquid Filtration for a better society
Professor Richard Wakeman, a consulting engineer
(UK) described the variety of filtration media now available and
wondered whether they could help reduce the 2 million deaths that occur from
preventable water borne diseases in regions of poor water quality. Delivering
large quantities of water free from viruses and bacteria requires
membrane-quality filtration, probably in the shape of membrane bioreactors
(MBR). These are hybrid processes which combine active sludge anaerobic
biotreatment with membrane filtration. Because the sludge fouls the membrane,
bubble “aeration” is used to create turbulence and clean the membranes. Reuse of
water in heavily populated regions is increasingly complicated by drug,
herbicide and pesticide residues persisting through purification processes, and
these require adsorbents like active carbon for their removal.
So where do the filters come in? Ahlstrom's Disruptor™
with active carbon was mentioned along with the industrialisation of nanofibre
nonwovens.
Hybrid Membrane Technology
Dr Antoine Schelling of Dupont ( Switzerland )
described their new HMT filters designed to fill the gap between
traditional filters and microporous films. HMT uses continuous filaments in the
0.2 to 0.6 micron range to achieve filtration efficiencies close to membranes
with lower pressure drop. The filament diameters are, unlike meltblown or
electrospun, controllable, and this yields controllable pore-size.
When tested according to ASTM-F795-88 with ISO ultrafine
test dust in water against a cellulose/meltblown composite which targets removal
of ~1 micron particles, HMT showed:
• An initial pressure drop between 0.25 and 1.2 kPa (for 5gsm and 31 gsm respectively) compared with 1.7 kPa for the composite.
• The 31 gsm HMT had an initial ability to remove 100% of 1 micron particles compared with 87% for the composite.
• The dust holding capacity of the 31 gsm HMT was 1.9 gms compared to 1.1 gms for the composite.
• An initial pressure drop between 0.25 and 1.2 kPa (for 5gsm and 31 gsm respectively) compared with 1.7 kPa for the composite.
• The 31 gsm HMT had an initial ability to remove 100% of 1 micron particles compared with 87% for the composite.
• The dust holding capacity of the 31 gsm HMT was 1.9 gms compared to 1.1 gms for the composite.
Applications in HVAC, bag, cabin air and cartridge
filters are foreseen, as well as in battery separators. The technology is
currently being used with a nylon 66 polymer on a 1.8m wide line and is
commercially available in the 10 to 30+ gsm range as roll-goods.
Elmarco questions related to whether or not the process
used electrospinning, what the productivity was, and what Dupont meant by high
throughput did not yield any useful answers. Dr Schelling did however add that
extra capacity would be added in the next year. The EDANA conference programme
did describe HMT as “nanofibre from electrospinning”. The main diameter produced
was 0.4 micron to match microglass.
Calvin Woodings
14 th October 2008