02-02-2013, 11:08 AM
Comparative Methods for the Pore Size Distribution of Woven and Metal Filter Media
[attachment=49338]
Abstract
This paper investigates the permeability of several multi-layered, woven filter media using air, water and
bubble point methods. Estimated filter efficiencies from the bubble point measurements were then
compared to a new sonic challenge test method where precision microspheres are fluidised through the
pores. Inconsistencies in the bubble point filter efficiencies were found to be dependent on the openness
or permeability of the weave. The filter efficiency of a non woven metal filter medium was then related
to the pore size distribution measured by microscopy.
Introduction
In plain or semi plain filter /mesh materials the pore
structure is clearly defined so when viewed under a
microscope an aperture can be observed with minimal
interference from the warp and weft filaments, figure
1(a). In most cases it is possible to measure across the
opening (in each direction) to get an indication of the
pore width/size.
Microscopic aperture measurement is not possible
however on composite or double-layered weave (DLW)
structures similar to those in figure 1 (b). When viewed
from the top surface it is difficult to see through the
fabric (unless it is extremely open). In most cases the cross sectional path through a composite or DLW
is a torturous one with interweaving between the layers. Furthermore increasingly tighter weaves in the
more recent plain fabrics severely limit the use of microscopy as an analytical tool to measure aperture
size.
In complex, 3-dimensional filter media, porometry via capillary flow from wetted media has long been
used for assessing the relative pore size distribution (PSD). Theoretically it is based on the LaPlace
equation for cylindrical capillary pores (ASTM F316-80 and SAE ARP901). However, most filter media
have irregular pores, which makes the theoretical assumptions suspect and this is especially the case in
multi-layered structures. Nevertheless, porometry has been firmly established as a key test procedure
for filter media characterization, and now can even be reasonably used to assess efficiency.
Experimental
The challenge test method
Preparing microsphere standards
In the challenge method, particles of known size distribution are presented
to a filter and any changes down stream measured by a particle size
analyser. Traditionally test dusts have been used but the accuracy of the
method is limited by the shape of the irregular particles, figure 2. Elongated
particles can pass through smaller pores than their equivalent spherical
diameter would suggest. Although the situation can be improved by using
spherical particles, the accuracy of the method can be compromised by
using broad particle size distributions.
The most accurate method of challenge testing is to use narrow size
distribution spherical particles, figure 3.
Furthermore to simulate the way in which the particles pass through a filter
medium, any particle sizing method should measure their width, for
example a sieve dimension. However, sieve dimensions in wire woven
sieves have an unacceptable wide distribution. For highest accuracy,
precision electroformed sieves should be used for analysis, figure 4.
PMI Porometer
The latest version, 1100AEX Capillary Flow Porometer, see figure 9, was used in the work. The same
parameters were measured as in the Coulter analysis except that the measurements were restricted to
the Gatwick wetting agent. The more accurate Dry up/Wet down mode of
operation was again employed.
The operational principle is similar to that of the Coulter Porometer 1
instrument. A fully wetted sample is placed in the sample chamber and the
chamber is sealed. Gas is then allowed to flow into the chamber behind
the sample. When the pressure reaches a point that can overcome the
capillary action of the fluid within the pore (largest pore), the bubble point
has been found. After determination of the bubble point, the pressure is
increased and the flow is measured until all pores are empty, and the
sample is considered dry.
To calculate the ‘Porometer efficiency’, the average bubble point was
divided by 1.7 (a typical screen tortuosity factor), which was equivalent to
approximately a 98% filtration (or removal) efficiency. The data could then
be compared with the Whitehouse micron rating as measured by the
challenge test, which corresponds approximately to 97% of the largest
pore size.
Results and discussion
This work has endeavoured to comprehensively evaluate all the
current methods of analysing filter efficiency using a wide
range of double-layered weave filter media. When determining
the suitability of a particular filter medium, potential users will
usually examine filtration efficiency from a combination of air and/or water permeability, mean flow pore
size and bubble point pore size from which a porometer efficiency can be calculated. Bubble point and
flow testing methods of assessing pore sizes have therefore been compared with a new challenge
method based on precision, narrow particle size distribution glass microspheres. For completion, the
secondary methods of air and water permeability have been included. The results summarised in table 1
below.
Conclusions
This study on comparative methods for measuring pore sizes between 20 - 200μm has shown good
agreement between theoretical porometer measurements and a new challenge test involving the
permeation of precision microspheres through the media. Furthermore, when the data is normalised,
both Frazier air permeabilities and water flux tests are remarkably consistent over a wide range of
samples.
One significant observation was that, when unusually high permeabilities were observed, there were
higher than expected results from the theoretical pore size measurements compared to the challenge
test results. For high through flow media therefore, some modifications to the tortuosity factor may be
required for converting bubble points to filter efficiencies to bring them more in line with the challenge
tests.
In applications involving the clarification of particle suspensions, mean pore size is not a meaningful
parameter especially when the filter medium has a wide range of pore sizes. The Sonic challenge test is
an unambiguous and high speed method of measuring the performance of filter media.
Having more accurate methods of measuring filter cut point is already leading to benefits, both from the
manufacturer who has a greater confidence in specifying filter media and from the process operators
who are reaping the rewards from improved performance.
[attachment=49338]
Abstract
This paper investigates the permeability of several multi-layered, woven filter media using air, water and
bubble point methods. Estimated filter efficiencies from the bubble point measurements were then
compared to a new sonic challenge test method where precision microspheres are fluidised through the
pores. Inconsistencies in the bubble point filter efficiencies were found to be dependent on the openness
or permeability of the weave. The filter efficiency of a non woven metal filter medium was then related
to the pore size distribution measured by microscopy.
Introduction
In plain or semi plain filter /mesh materials the pore
structure is clearly defined so when viewed under a
microscope an aperture can be observed with minimal
interference from the warp and weft filaments, figure
1(a). In most cases it is possible to measure across the
opening (in each direction) to get an indication of the
pore width/size.
Microscopic aperture measurement is not possible
however on composite or double-layered weave (DLW)
structures similar to those in figure 1 (b). When viewed
from the top surface it is difficult to see through the
fabric (unless it is extremely open). In most cases the cross sectional path through a composite or DLW
is a torturous one with interweaving between the layers. Furthermore increasingly tighter weaves in the
more recent plain fabrics severely limit the use of microscopy as an analytical tool to measure aperture
size.
In complex, 3-dimensional filter media, porometry via capillary flow from wetted media has long been
used for assessing the relative pore size distribution (PSD). Theoretically it is based on the LaPlace
equation for cylindrical capillary pores (ASTM F316-80 and SAE ARP901). However, most filter media
have irregular pores, which makes the theoretical assumptions suspect and this is especially the case in
multi-layered structures. Nevertheless, porometry has been firmly established as a key test procedure
for filter media characterization, and now can even be reasonably used to assess efficiency.
Experimental
The challenge test method
Preparing microsphere standards
In the challenge method, particles of known size distribution are presented
to a filter and any changes down stream measured by a particle size
analyser. Traditionally test dusts have been used but the accuracy of the
method is limited by the shape of the irregular particles, figure 2. Elongated
particles can pass through smaller pores than their equivalent spherical
diameter would suggest. Although the situation can be improved by using
spherical particles, the accuracy of the method can be compromised by
using broad particle size distributions.
The most accurate method of challenge testing is to use narrow size
distribution spherical particles, figure 3.
Furthermore to simulate the way in which the particles pass through a filter
medium, any particle sizing method should measure their width, for
example a sieve dimension. However, sieve dimensions in wire woven
sieves have an unacceptable wide distribution. For highest accuracy,
precision electroformed sieves should be used for analysis, figure 4.
PMI Porometer
The latest version, 1100AEX Capillary Flow Porometer, see figure 9, was used in the work. The same
parameters were measured as in the Coulter analysis except that the measurements were restricted to
the Gatwick wetting agent. The more accurate Dry up/Wet down mode of
operation was again employed.
The operational principle is similar to that of the Coulter Porometer 1
instrument. A fully wetted sample is placed in the sample chamber and the
chamber is sealed. Gas is then allowed to flow into the chamber behind
the sample. When the pressure reaches a point that can overcome the
capillary action of the fluid within the pore (largest pore), the bubble point
has been found. After determination of the bubble point, the pressure is
increased and the flow is measured until all pores are empty, and the
sample is considered dry.
To calculate the ‘Porometer efficiency’, the average bubble point was
divided by 1.7 (a typical screen tortuosity factor), which was equivalent to
approximately a 98% filtration (or removal) efficiency. The data could then
be compared with the Whitehouse micron rating as measured by the
challenge test, which corresponds approximately to 97% of the largest
pore size.
Results and discussion
This work has endeavoured to comprehensively evaluate all the
current methods of analysing filter efficiency using a wide
range of double-layered weave filter media. When determining
the suitability of a particular filter medium, potential users will
usually examine filtration efficiency from a combination of air and/or water permeability, mean flow pore
size and bubble point pore size from which a porometer efficiency can be calculated. Bubble point and
flow testing methods of assessing pore sizes have therefore been compared with a new challenge
method based on precision, narrow particle size distribution glass microspheres. For completion, the
secondary methods of air and water permeability have been included. The results summarised in table 1
below.
Conclusions
This study on comparative methods for measuring pore sizes between 20 - 200μm has shown good
agreement between theoretical porometer measurements and a new challenge test involving the
permeation of precision microspheres through the media. Furthermore, when the data is normalised,
both Frazier air permeabilities and water flux tests are remarkably consistent over a wide range of
samples.
One significant observation was that, when unusually high permeabilities were observed, there were
higher than expected results from the theoretical pore size measurements compared to the challenge
test results. For high through flow media therefore, some modifications to the tortuosity factor may be
required for converting bubble points to filter efficiencies to bring them more in line with the challenge
tests.
In applications involving the clarification of particle suspensions, mean pore size is not a meaningful
parameter especially when the filter medium has a wide range of pore sizes. The Sonic challenge test is
an unambiguous and high speed method of measuring the performance of filter media.
Having more accurate methods of measuring filter cut point is already leading to benefits, both from the
manufacturer who has a greater confidence in specifying filter media and from the process operators
who are reaping the rewards from improved performance.