We are always so keen to learn more about disease to improve our health, study failures to be more successful and research cancer to live longer. In fact, the history of humanity is full of great lessons to learn from.

In October 1707, the English Admiral, Clowdisely Schovell, miscalculated his position in the Atlantic Ocean, and his fleet of ships collided into the rocks of Scilly Isles, a series of islands off the southwest coast of England. A fleet of four warships following blindly behind crashed, claiming 2,000 lives. Given the crude measurements professional seamen had to rely on at the time to estimate their average speed and, consequently, their position, the admiral’s miscalculation can be understood. The methods available at the time to measure longitude lacked precision, and in the case of Clowdisely Schovell, perhaps the underlying reasons of the disaster could be identified [1].

There is much food for thought if we superimpose the Admiral’s incident on the field of air filtration and ask: what do you know that is critically important but we are unable to measure? Further, what do you know that is critically important but we seem to overlook?

It is beyond any reasonable doubt that the performance of dust-loaded filters is not well-established and there is yet so much to be discovered. Frankly, no one can claim today that we know everything about the air filtration field. Future discoveries will help to rectify our past and current activities in filter design, manufacturing and operation of air filters.

OVERALL FILTER EFFICIENCY

Let’s go back to the most- addressed performance characteristic of air filters, which is filter efficiency. The overall efficiency of a filter is a function of the single-fibre efficiency of each capture mechanism [2]. When evaluating the overall efficiency of a filter, the most important mechanisms considered are diffusion, interception and inertial impaction [3]. Illustrations of interception and diffusion capture mechanism in air filtration are shown in Figure 1 (see below).

In gas filtration, it is common to assume that each mechanism acts independently of the rest. However, when absolute filters are assessed, the effect of the impaction mechanism is usually omitted as diffusion and interception are the two dominant mechanisms around the Most Penetrating Particle Size range (MPPS). Filtration efficiency, in general, is a function of particle size, particle shape, filtration velocity and frequently particle charge, which makes the comprehensive evaluation of air filter efficiency a tedious task.

To better understand the dominance regions, the MPPS has to be defined. For particles of 1 µm and smaller, complex interactions between Brownian diffusion and inertial effects results in the so-called MPPS. The MPPS is a characteristic of depth filters where minimum capture efficiency is observed, typically in the size range of 0.05 to 0 0.6 µm. The MPPS arises because particles smaller than 0.05 micron are captured efficiently via a diffusion mechanism, whereas particles larger than 0.6 micron are captured more efficiently by a combination of interception and inertial impaction [4]. Absolute filters are rated at MPPS since the filter efficiency scores its lowest value. The diffusion mechanism is dominant in the sub-MPPS and the interception and inertial impaction mechanisms are dominant and larger than MPPS as shown in Figure 2 (see below). MPPS is only significant when dealing with absolute filters, such as HEPA and ULPA filters. The MPPS is a function of air velocity and filter media characteristics. Therefore, the air velocity is of a critical importance to the filtration process highly as it influences the filter performance.

ABSOLUTE FILTERS DESIGNS

Increasing the velocity reduces the single-fibre efficiency due to diffusion. Conversely, the collection efficiency of absolute filters increases when the velocity of the air stream flowing through the filtration medium is reduced. This can be done by virtue of increasing the filter surface area by means of pleating. The oldest HEPA filter design introduces higher surface area by pleating the filtration media and inserting corrugated aluminum foil spacers between the pleats to minimize contact between the pleated filter medium. This sort of design limits the increase of the surface area since the spacer occupies a volume on their own. Further, there is a considerable risk that the spacers might damage the pleats either during transport, installation and/or during operation. If excessive flow rate is used, the pleats might deform, distort or dislocate from their original position leading to fluctuation in permeability and eventually possible ruptures in pleated panels.

Another technique used to extend the surface area is the minipleat technology using hot melt to separate the pleats, as shown in Figure 3 (see below). Higher surface area can be introduced via this technique and the filter exhibits lower pressure drop when compared to the aluminum spacer technique for the same efficiency.

Extending filter surface area by pleating the flat media, if done appropriately and professionally, can reduce the air velocity and extend the particle residence time inside the filter media and, therefore, enhances the overall filter efficiency. Absolute filters are manufactured in different designs to accommodate various applications. There are pleated panel filters, as shown in Figure 4 and the cartridge type, as shown in Figure 5 (see below).

Appropriate selection of the filter should carefully consider the particle concentration and the flow rate. Using lower surface area filters with relatively high flow rates leads to pleat distortion, pleated panel deformation and possible disintegration of the filter elements. Figure 6 (see below) highlights distortion of the pleats due to high flow rates used which will lead to permeability reduction and flucation in the local velocities. According to Darcy’s Law, permeability is inversely proportional to pressure, which in turn means, any reduction in the filter’s permeability will be translated into increase in pressure drop. If permeability continues to decrease, the pressure drop will start to rise in a non-linear manner causing more distortion of the media and eventutally causing filter material to rupture, as shown in Figure 7 (see below) and pleated panels to deform as shown in Figure 8 (see below).

The writer is Regional Director, Middle East, and International Consultant, EMW Filtertechnik, Germany. He can be contacted at iyad.al-attar@emw.de

References:
1 Marcus Buckingham and Curt Coffman. 1999. “First, break all the rules” Simon and Schuster New York.
2 Hinds W.C., 1998. “Aerosol Technology”, Wiley, New York.
3 Davies C.N., 1973. “Air Filtration”, Academic Press, New York.
4 Tarleton E.S. and Wakeman R.J. 2008. “Dictionary of Filtration Separation” Filtration Solutions, Filtration Solutions, Exeter.