Filtration Testing

In this, my second blog, I want to focus on a general overview of filtration testing. I will, in later blogs, expand on specifics of each testing protocol and the key issues of specific tests and get into the deep and very dirty world of filter testing. 

I have, over the years, been amazed by the lack of understanding of this critical subject within the industry. Often there is a belief that an air filter can be tested using whatever test stand is lying about. This is a very wrong assumption as the test protocols used in filter testing document not only the protocol to be used but the test stand design, it's calibration and how it should operate. 

EN779 HVAC Element Test stand from TOPAS GmbH

The design and layout of a test stand makes a significant impact on measured performance and given that the test is a destructive test on a material that can't be exactly replicated with an absolute level of precision, then it is impossible to align filter test stands. What we therefore have to rely on, is a standard protocol on each test stand with the design being as constant as possible. These protocols are outlined in a range of ISO standards covering all markets and whilst they differ in a range of factors they have a lot in common that allows a new user to be able to understand the fundamentals of each standard based on previous experience. All the test standards, air or liquid, follow some, if not all of the following measurement steps:

• Measurement of filter lifetime under loading of a ISO standard contaminant (e.g. SAE ISO Standard Test dusts) at a known flow rate (or face velocity) to a pre-defined increase in pressure drop, known as the terminal pressure drop.
• Determination of the dust holding capacity (DHC) of an element or media during the lifetime of the test when challenged with a specified contaminant. This is a calculated measurement based on knowing the concentration of the contaminant in the test fluid and the test time to the end, or terminal, pressure drop.
• Measurement of the gravimetric efficiency of the media to hold contaminant. This is also known as “arrestance” and should not be confused with fractional efficiency.
• Measurement of fractional efficiency (often also the inverse of the “penetration” or “transmission”) of the media either as an initial or an overall efficiency when challenged with an aerosol of predefined particle size or particle size range. In some cases this can be the challenge dust and in others a secondary aerosol can be used.

Test Stand Operation

A typical test stand layout for a flatsheet automotive air filter manufactured by PALAS is shown in the Figure below. This illustrates all 4 principles listed above.

Filter Lifetime

• The sample is placed in a holder between two pressure sensors measuring the pressure drop of the media during the test
• Air is pulled by an air ventilator at a controlled flow rate through the media creating a pressure drop across the media.
• Dust is fed from a piston feeding dust against a rotating brush contaminating the air with a known concentration of contaminant (typically a SAE ISO standard test dust). 

• This is sucked through the test filter and material which is not captured is captured on a second final filter downstream of the test filter.

• Pressure increases as the filter is loaded with contaminant. At a given pressure drop the test is terminated. The lifetime is effectively the run time of the test. As we already know the concentration of the dust and flow rate we can extrapolate this to the Dust Holding Capacity (DHC) of the media or element- The longer the test, the higher the DHC.

Illustration of the Principle of a Flatsheet Filter Test Stand for Automotive filtration measurements. The test stand is a PALAS MFP 3000 test stand with a air humidity control unit manufactured by TOPAS.  

Dust Holding Capacity and Gravimetric Efficiency (arrestance)

Whilst measuring the feed rate can and does give a good indication of media or element lifetime the simplest way to determine dust holding capacity of a media or an element is to weigh the test material and final filter. This determines the actual dust reaching the test material and how much passed through to the final filter. From this the arrestance can be simply determined as a percentage

Typically this will be determined at several points during the test.

Measurement of Fractional Efficiency

The fractional efficiency is considered the most critical performance measure in a filtration test and is determined by a measurement of the number of particles of a given size from a standard aerosol. 

The choice of aerosol for such a test differs from standard to standard. For automotive air filtration and in liquid filtration for hydraulic, fuel and lube, the aerosol is effectively the contaminant dust. For higher efficiency air media such as HVAC and HEPA, the aerosol chosen is a specific narrow particle size liquid material such as DEHS (diethylhexyl succinate), DOP (dioctyl phthalate) or a solid aerosol dried from a solution sprayed out of a solution such as sodium chloride (NaCl) or potassium chloride KCl). I will talk about this much more in a later blog. 

This process is undertaken by classifying the particles into sizes and counting them. By the simple expedient of counting the number of particles that pass through the test filter at any given particle size and comparing them to the number of particles on the upstream side of the filter, a determination of fractional efficiency can be determined using the following equation:

E_i is the efficiency at particle size i, n_i is the number of particles in the size range i downstream of the filter and N_i is the number of particles of particle size i upstream of the filter.

Most aerosols are distributions of particles over a wide range and not unimodal.  In a typical KCl aerosol the particle size distribution looks like the plot below:

KCl Aerosol distribution from a typical spray solution

As can be seen the distribution of particle counts is strongest at about 0.1 micron with a rapid drop off to higher particle sizes. Consequently whilst particle size can be measured across a wide range, larger particle sizes often have very few particles and therefore the ability to measure efficiency accurately diminishes rapidly with particle size.

Particle Size Classification and Measurement

The classification can be undertaken in a number of ways.

• Optical particle counting (OPC). In this a sample of the diluted contaminant is pulled out of the fluid flow and through the path of a laser, where the particle either bocks or scatters the light and the particle size is determined.
• Typically blocking type particle counters are used widely in liquid filter testing where the detection limit is theoretically around 1 micron.
• Light scattering particle counters are effective to a much finer particle size (>0.05 micron) and are used widely in air filter testing.

Schematic of an Optical Particle Counter

Scanning Mobility Particle Sizer (SMPS). This allows much smaller particle sizes to be measured than can be measured optically. It measures the size distribution and concentration of particles in the size range of 2 nm to 1 μm using differential mobility analysis. This method is based on the physical principle that the ability of a particle to traverse an electric field (electrical mobility) is fundamentally related to particle size. The principle of operation is shown in the figure below:

Schematic of a SMPS Particle Counter

Testing Standards

Testing is industry specific and each industry sets testing requirements based on National or International Standards such as ISO, EN, NIOSH, DIN and ASHRAE. These are in place to ensure standards of performance are met and enforced. In a market where the performance of the final product is often extremely subjective, these standards are critical. 

The table below shows some of the most commonly used global standards, the challenge dust used and any aerosol used to measure fractional efficiency. These represent an overview only and will be discussed in much more detail later. 

In summary the fundamentals of filtration testing are complex but they are similar across the technologies. My next blog will be on aerosols and contaminant dusts and how they relate to reality. 

Have fun.