Aerosols and Dust.....

In my blog on filtration testing, I alluded to the use of challenge dusts and aerosols for testing. In this blog I'd like to address this issue a little further. 

All filtration testing requires a predefined standardised material of known particle size to effectively determine the efficiency of the filter whether by arrestance (gravimetric efficiency) or by fractional efficiency using optical particle counters or Scanning Mobility ParticleSizer.

The selection of test aerosol or dust is determined by the ISO standard selected but also depends upon the nature of the filter being tested. Finally the selection of the aerosol has to align with the environment that the final filter will operate in. For instance, having a good filter for sand and dust will not give a good prediction of lifetime in a city where soot in the atmosphere will shorten the lifetime of the filter. 

The testing protocols used widely in the filtration world generally, not perfectly, reflect this. ISO committees all over the world spend many days drinking coffee and debating the detail of how filters should be tested. The resultant standards reflect the real life status and the standards continue to alter over time. 


Aerosols

….in automotive air

For lower efficiency media such as automotive air and heavy duty air, a dedicated, controlled particle size aerosol would not have the level of efficiency to make the measurement of fractional efficiency effective.  Consequently it is much more effective to measure the fractional efficiency of the challenge test dust.

...in lube, fuel and hydraulic applications

The major limitation in liquid filtration testing currently is the resolution of the OPC (Optical particle counter) technology. In air these go down to nanometer scale but in liquid filtration the technology only has a resolution to the micron scale. The current limit specified by all the ISO standards for liquid multipass filtration is 4 microns. More recent technology from Pamas in Germany has taken the limit down to 1.5 microns. This relatively poor resolution favours a relatively coarse test dust and the ISO standards all state the use of SAE ISO Medium test dust both for loading and as a measure of fractional efficiency. 

…in HVAC and industrial applications

For higher efficiency filters such as HVAC air cleaners, the selection of an aerosol is essential to measure the filtration effectiveness as measuring the fractional efficiency of the contamination dust would be ineffective. Consequently dedicated aerosols based on DEHS for EN779 and KCl are used to measure the fractional efficiency whilst a separate contaminant dust is used for loading. The particle size range is dictated by the test standard used and typically range from 0.3 microns to 10 microns.

The aerosols are statically discharged to remove the effect of static build on the aerosol particles up that can artificially raise the efficiency of the media.

…in HEPA and ULPA

For the highest level of filter efficiency the choice of aerosol is driven by the need to determine not simply the efficiency but the penetration of the particles. In a filter with efficiencies >99.95%, this means that <5 particles out of 10,000 are penetrating the media. Consequently the tests are carried out at a particle size known as MPPS (Most Penetrating Particle Size). The definition of this comes from the fundamental theories of filtration.

In larger particle sizes the key drivers of filtration mechanisms are driven by the fluid flow this leads to a linear particle flow into the filter where particle capture through inertia and interception dominate. As the particle size decreases, the probability of these events happening decreases.

At a point of a particle size of about 0.13-0.18 microns, the particles are so light and small that the macro effects of the fluid flow are overtaken by molecular buffeting and the particle starts to be moved sideways as opposed to forwards, this creates diffusion filtration mechanisms and below these particle sizes the efficiency effectively starts to rise (see figure below).

MPPS Efficiency Curve

The inflexion point is known as the MPPS, most penetrating particle size. Typically this lies between 0.13 and 0.18 microns and at this point the maximum aerosol penetration occurs.

Measurement of efficiency at this point focuses on a single particle size leading to a different type of particle counter which is range specific and typically a SMPS type design. The aerosol for HEPA and ULPA testing are typically oil based DOP (Dioctyl phthalate) or DEHS (Di ethyl hexyl sebacate).

Contaminant Dusts

The measurement of a filter lifetime is determined by the time to load up the filter with a contaminant to a known pressure drop. In order to achieve this, the filter has to be challenged with a realistic test that represents the environment in which the filter will operate. The will depend upon a whole host of environmental issues related to the location. Some examples are:

•         Deserts and arid areas: Dust and sand
•         Major conurbations, China and India: Hydrocarbons, soot and dust
•         Sea: salt spray
•         Mines: silicate and coal dust
•         Building Interiors: Household dust

Each environment has a major challenge associated with it. The nature of the contaminants, particle size and the chemical properties associated with it. Each has an ability to significantly shorten the lifetime of the filter element and impact the overall performance through pore blocking and rapid pressure rise.

Each of these topics is a major subject in itself but in essence the ISO tests, to a degree try to reflect the environmental challenges in which the final filter will operate.

SAE ISO Test Dusts

The most common standard contaminants used for loading studies are based on the SAE ISO test dusts. There are 4 common dusts from Arizona in the US specified to ISO 12103-1. Details of the compositions of these dusts by volume (not weight) are shown in the Table below.

SAE ISO Test dust compositions by volume

Of these, the most commonly used test dusts are:

• A2 ISO fine test dust is widely used for automotive air filtration with some limited use of A1 ultrafine test dust for higher efficiency filter media.
• A3 ISO medium test dust is specified for liquid filtration where the particle counters are unable to operate effectively below 4 microns.

ASHRAE Test Dust

For higher efficiency HVAC applications, the specially formulated ASHRAE 52.1 test dust is specified. This is a blend of:

• 72% SAE ISO A2 Test Dust
• 23% Carbon black powder
• 5% Milled cotton linters

The cotton linters can limit the ability of standard dust feeders to cope with the long fibres and the standard feeder for ASHRAE 52.2 and EN779 is in fact a conveyor belt with a vacuum slot to suck the dust in pre-cut segments into the airflow. For large elements this is suitable however for smaller flatsheet stands this is inadequate as it is too crude for the much lower airflow. Consequently the flatsheet testing in H&V has to be undertaken with SAE ISO A2 medium test dust which makes a suitable alternative.

KCl or NaCl loading

For most HEPA and ULPA applications testing, the loading capacity is not a measured outcome of filtration testing. However for some applications such as face mask, or higher efficiency air applications, NaCl or KCl loading can be seen as a viable test of element lifetime. However the testing time is extremely long and therefore the test requirements look at loading to a limited pressure drop increase or to a limited mass (Facemasks)

Soot

The growth in pollution in developing countries and conurbations in Western cities has led to a significant growth in trying to understand the impact of soot. Unlike a standard dry, hard particle such as Arizona test dust, soot is a complex organic material that doesn’t have a specific, controllable particle size.  Typically starting at about 50nm the hot soot particle from incomplete combustion will grow over an interval of time to around 100-120nm cooling from a hot, sticky particle to a cool, resinous material which is much harder. This inherent instability makes standardising the testing protocols very challenging and this is, as of 2014, still in development. Use of KCl as a viable alternative in terms of particle size distribution has been validated but not fully accepted in the market. There are commercially available soot generators from Matter Aerosol and some excellent new ideas from Palas but many test systems use home made soot generators using kerosene as a basis. 
As with KCl/NaCl loading, the real challenge is to be able to effectively load with soot in a realistic test timeframe. 

As usual, if you have any comments about this or any other blogs, feel free to comment and contact me.