Face velocity significantly impacts the measured performance of a filter in terms of lifetime and, to a lesser extent, efficiency.
Increasing face velocity in any filter media increases pressure drop in a near linear relationship, as was discussed in the initial blog in this series. If you increase the initial pressure drop and your test (such as EN779 or ASHRAE 52.2 in particular) has a fixed terminal pressure drop then this will significantly reduce the flatsheet or even element capacity.
Even where tests are run to a differential terminal pressure drop (i.e. initial dP + xPa) the impact of the face velocity can be significant. The simplest examples of these are in air filtration to ISO 5011. An example is shown below. A typical Heavy Duty Air filter was tested on the PALAS MFP 3000 flatsheet gravimetric test stand to a final pressure drop of initial dP + 2000Pa. The test protocol was replicated at 4.2, 11.1 and 16.7 cm/s.
The test measured also the initial gravimetric efficiency at the initial pressure drop of Initial + 500Pa. As this grade is a typical Heavy Duty Air grade, we didn't expect to see a significant drop in gravimetric efficiency.
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| Effects of face velocity on DHC and gravimetric efficiency to ISO 5011 |
The results show a clear drop in both efficiency and DHC. Why?
The fundamentals of dust capture for most materials is via interception of the dust by the fibres. Secondary mechanisms exist such as diffusion but in essence for large particles such as ISO fine dust the interception rules.
Higher face velocities add kinetic energy to the particles using the well known equation:
The higher the velocity the more the particles will penetrate the media decreasing the efficiency. The limited loss in efficiency comes down to the weight distribution of the test dust which in itself limits the ability to penetrate the media.
The impact on the lifetime is also related to the additional kinetic energy of the particles. The dust forms a filter cake both on the surface and in the depth of the filter media. Normally this cake build up is relatively loose and particles (and airflow) can still penetrate through the filter. At higher face velocities the pressure drop increases creating a suction effect on the dust cake plus the dust cake is much more compact due to higher speed impacts reducing the ability of the media to allow airflow, through, increasing the pressure drop rapidly and leading to an early termination of the test.
A third factor is compression due to airflow increases. As the pressure drop increases the pressure drop causes media compression and thinning increasing the pressure drop further. In depth filters such as HVAC filters this can be quite significant and causes a non lienar pressure drop response.
A third factor is compression due to airflow increases. As the pressure drop increases the pressure drop causes media compression and thinning increasing the pressure drop further. In depth filters such as HVAC filters this can be quite significant and causes a non lienar pressure drop response.
A similar trend is seen with liquid filtration. The multipass test for fuel ISO19438 was used to demonstrate the impact of face velocity on efficiency and DHC of a next generation composite synthetic fuel media earlier this year.
Here the settings were for a 200 sq. cm sample with the multipass flowing at 0.5 l/min and 1.5 l/min. The results are very similar to the air sample with a significant drop in lifetime and also a drop in both initial and overall efficiency.
The face velocity effects are very general in all filtration media and test standards and therefore the face velocity should be stated on all tests. The problems often arise not in the flatsheet testing but in elements.
With cost pressures on elements, there is always pressure to reduce the amount of media in an element. Decreases in the area of media in an element will always lead to a loss in performance.
Another element issue is poor element design which leads to pleats or bags pressing together under flow, restricting the available filtration area, increasing the face velocity and pressure drop with the same effects.
In summary, face velocity is a variable that you ignore at your peril in filtration. It leads to lower performance both in terms of efficiency and lifetime.
Have fun....
Tony
The face velocity effects are very general in all filtration media and test standards and therefore the face velocity should be stated on all tests. The problems often arise not in the flatsheet testing but in elements.
With cost pressures on elements, there is always pressure to reduce the amount of media in an element. Decreases in the area of media in an element will always lead to a loss in performance.
Another element issue is poor element design which leads to pleats or bags pressing together under flow, restricting the available filtration area, increasing the face velocity and pressure drop with the same effects.
In summary, face velocity is a variable that you ignore at your peril in filtration. It leads to lower performance both in terms of efficiency and lifetime.
Have fun....
Tony
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