Here Thomas Carlsson of Sea&filter AB, Sweden, reminds us all

why synthetic fibres remain the

develop innovative media for

materials of choice when looking to

indoor airfiltration applications.

ver a ten-year period

there has been a

considerable change in

what filter media are

used in ventilation

applications. Up until about 30 years ago,

almost all such filters were based on glass

fibre. Today, glass fibre only accounts for

40% of the market. Polymer based

materials now hold the major market

share. To find the reason(s) behind this

change, the requirements that filter media

for ventillation applications need to satisfy

must be considered.

The performance specification of a

filter medium suitable for use as a class

G 1 -F9 filter is based upon the following

parameters:

l filtration efficiency;

l pressure drop;

l mechanical strength;

l environmental impact;

l fire resistance;

l process considerations; and

l resistance to microorganisms.

How well do present day polymer

based materials satisfy these performance

specifications?

High Filtration Efficiency The efficiency with which particles are

separated in a filter is primarily

determined by fibre size. Polymer fibres

can now be produced in a wide range of

sizes. The most commonly used method is

the meltblow-n process, which involves

extruding polymers under high pressure

and temperature into a high velocity air-

stream. By varying the process parameters,

it is possible to produce fibres that range

in diameter from 0.1 pm-100 pm [ 11. Conversely, the average fibre diameter

for a class F7 glass fibre material is

1 I-‘m VI. -

methods: the triboelectric effect and the

corona treatment, and the choice of

method is dependent on the type of

polymer material being used.

Low Pressure Drop A low pressure drop is important in order

to achieve a low life cycle cost (LCC), an

effective way of optimising the total cost

of an installation. Several methods of

calculation have been developed for

variety of equipment including filters,

fans, pumps, etc.

In the case of a filter, the life cycle cost

can be calculated as follows:

LCC = Investment + LCCtncrgv +

’ LCCIvlaintrnance + LCCDispoaal

As an example, consider a class F7 filter

with the following cost breakdown:

Investment + LCCMaintcnsnrp = 18.5% LCC

Energ? = 81%

LCCDispo\al = 0.5% [4].

n Investment n Energy E Disposal

Polymer fibres

are also ideally

suited for

electrostatic

charging, a

technnique which

can further increase

filter efficiency.

Thus, this ability

makes them

preferable to

conventional fibres

that cannot be

charged. Several

methods of charging

materials have been

developed and

refined over recent

years. There are

essentially two

A low pressure drop is an important

element in keeping total running costs

down because the pressure drop directly

affects energy running costs. From

Figure 1 it can be seen that energy

running costs account for 8 1% of the total

cost.

Using polymer fibres, the structure of

the material can be varied and controlled

(Figure 2). By using different fibre sizes, a

filter web can be constructed with an

open structure on the inlet face, to catch

the larger particles, followed by a pro-

gressively finer structure as you move

towards the centre of the web to capture

the smaller particles. This means that

the increase in pressure drop with time

can be kept low, which in turn lowers the

LCC.

30 March 2OOl Filtration+Separation

Mechanical Strength

An important parameter for an)' f i l t e r

material is its mechanical strength. Pulvmer based fibres possess a combination of high tensile strength and good strain resistance, even under high humidity conditions.

High tensile strength and good elongation performance are important, not

only when the filter is being fitted but also during operation. Materials, with a low strength can, for example, be torn by sharp metal edges when being fitted. The elongation performance of polymer based fibres give the material good resistance to tearing, and therefore reduces the risk of it becoming damaged during installation.

I t can be seen from Table 1 that the tensile strength of the polymer based material is nearly twice that of a glass fibre material. The combination of high tensile strength and good elongation to break guards against the polymer based fibres breaking during use.

Environmental Impact A life cycle assessment (LCA) evaluates the environmental impact of a product by taking into consideration the total

environlnental impact of a product during Inanufacturc, operation and disposal, i.e. a cradle to grave approach. The energy requirement of the various processes provides an important indicator of the environmental impact of a

product. In terms of energy use during the

manufacture and disposal, a polymer based

filter material, with a plastic frame, is said to have the least environmental impact of any commercial filter currently available on the market [3]. If the energy used hv or in connection with a polymer based filter, with a plastic frame is given the index 1, then other filters have indices in the range 2-4.5.

Polymer based mater ia l

Ultimate tensile strength* (N) 99

* m e a s u r e d in a c c o r d a n c e w i t h ISO 5081:1977

Glass f ibre mater ia l

58

The greatest energy demand of a filter occurs when overcoming the pressure drop that happens {luring normal operations [4]. In order to achieve a low pressure drop, and therefore a low energy demand, the filter needs to have a high dust loading capacity.

a common error when assessing dust loading capacity is to compare the surface areas of the filters. However, a filter having a large surface area does not necessarily have a high dust loading capacity. Using

polymer fibres, it is possible to vary the fibre sizes throughout the thickness of the medium (Figure 2), so that dust is loaded in the interior of the medium, rather than on the surface. This gives a low pressure drop across the filter, resulting in good running costs.

In addition, ~ hen manutacturing polymer based filter media, it is possible to some extent (up to 30 %) to use recycled fibres, e.g. from polyethylene terephthalate (PET) bottles, giving these fibres a further environmental edge.

Microorganism Resistance Outdoor air contains thousands of naturally occurring microorganisms (bacteria, mould spores, fungi, ctc). They are separated in the filter and, together with the other })articles, form a dust cake on the filter. Unfortunately, this {lust cake can provide a source of nutrients for the

microorganisms. The Freie Univ~irsit~it Berlin, Germany,

carried out an investigation [5] to determine the di[Ii~rences in bacteria anti mould growth patterns between a polymer based anti a glass fibre filter. The two

filters were installed in the same plant and exposed to outdoor air. The biological activity on them was measured once a week. The results of the study are shown in Figure 3.

% ~D

Mou lds

6000000

4000000

2000000 .... J - ~ ......

i

i 0 2

• Micro glass . . . . . Polymer fibres

!

4 W e e k

4000000

3000000

"E 2000000

0

1000000

Bacteria

• M i c ro glass ......... P o l y m e r fibres

_ . . . . . . [

i I

4 6 W e e k

Filtration+Separation March 2001 31

Output Heating Effect Smoke Emission

- Micro glass -- Polymer fibres

20 40 60 80 100 1

Set

300

250

200

150

100

50

50

Air

in air @ration &at of%rs a wide r&e of glass and synthetic media for prim&,

secondary and final. WEPA filtration levels, as we11 as filters for special apphahions. Over

the course of the past year Russel Mcleod has been focussing its business on clean air

filtration applications through several acqusitions. Early in 2000 it purchasedvokes from

Invensys plc, creatingvokes Air Filtration, which is a well known name within the fields

of high technology air filtration, i.e. cleanrooms and the pharmaceutical industry. It is

based at Interfilta’s site in Burnley (FiZtrution+Separotion, January/February 2000, p.4).

More recently, Russel Mcleod acquired Eurpoean filter technology company, Luwa,

which is headquartered in Switzerland (Filtrotion+Separation, January/February 2001).

The recent acquistions have had the result of significantly strengthening Mcleod Russel’s

position in the Euopean clean air filtration market.

It was found that the growth of both

bacteria and mould in the polymer based

material was less than the corresponding

growth in the glass fibre material. The

report [5] concludes that ‘the new

generation of air filtration material (multi-

layered polymer) appears more suitable for

use in heating, ventilation & air

conditioning (HVAC) systems because the

material itself appears able to reduce the

growth or survival of microorganisms.’

It can bc seen that the glass fibre filter

releases considerably more heat and fire

gases. The high heat output of the glass fibre

filter not only incrcascs the risk of damage

to the rest of the ventilation system, but also

of spreading the fire. In addition, the

amount of smoke released by the glass tibre

filter was sufficiently high that any electrical

component exposed to it would have to be

completely replaced.

References 1.

2.

CiachT. 2000. Deep bed filtration

structures: Properties and

manufacturing, ProceedInS 8th World

Filtration Congress, Brighton, UK.

Gustafsson J, Camfil. Energy &Wjb’,

10/93

3.

Fire Resistance From a safety point of view the fire

resistance of a filter medium is very

important. The factors that need to be

taken into consideration are the possible

generation of toxic gases and how much

heat is released. These characteristics were

investigated for a polymer based and a

glass fibre filter. The filters were installed

in the same ventilation plant for a period

of five months, after which they were

tested at the Fire Testing Laboratory of the

Swedish National Testing and Research

Institute, Boris, Sweden. The heat and fire

gas release results obtained in the study are

shown in Figure 4.

Process Considerations One particular benefit of polymer based

materials is their ability to be welded, which

ensures airtight joints, and produces a filter

that is less prone to distortion. In addition,

there is no need to add any binder to the

product, as the fibres are bound together

either thermally or by friction. Therefore,

concerns regarding potentially toxic binders

being dissolved by moisture and distributed

through the ventilation system by the supply

air are eliminated.

Maria Eriksson, Chalmers University of

Technology, Luftfilter: tekniska

kvaliteter, miljiibediimning och

innemiljii’ [Alrjilters: technical qualities,

environmental assessment and the indoor

environment].

4.

5.

6.

Eurovent/Cecomaj: 1999,

Recommendation concerning

calculation of life cycle cost for air

filters.

Kemp P C. 1999. Comparison of

microorganism loading from two air

filter materials. Indoor Air,

Gustafsson J, Camfil. Energi &MiJjc,

2/00

It is also possible, when using polymer For more mformatmn contact: Thomas Carlson,

based filter media, to incorporate fibres R&D Director, Scan$Jter AB. 512 85 SvenJjup,

with varying characteristics (flame Sweden. Tel: f46 325 66 16 00;

resistant, bactericide-containing, high Fax: f46 325 6 1 14 90;

tensile strength, good elongation to break, Webnte: www.scandfiJter.com

32 March 2OOl Filtration+Separation

See

etc) to produce a tailor-made filter media

for a specific application.

Conclusions The development of new polymer based

fibrcs is progressing rapidly, making it

possible to customisc filter with different

performance levels to satisfy a variety of

environments. In fact, even manufacturers

which previously concentrated on the use

of glass fibre materials are now beginning

to appreciate the advantages of these

materials: ‘fine electrostatically charged

fibres, with the mechanical strength and

process potentials of svnthetic fibres have

excellent future prospects’ 161.