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Given that a filter may not be able to remove all of a suspended solid, there will, nevertheless, be a particle size cut-off point above which no particle should be able to pass through the filter. The cut-off point thus refers to the diameter, usually expressed in micrometres (m), of the largest particle that will pass through the filter, although this is not the smallest particle retained by the filter, because smaller particles are quite likely to be retained, by the adsorptive mechanisms described in the previous chapter.

If the filter medium has an exact and consistent pore size or opening, then this cut-off point can be termed an absolute rating. Most real media, of course, do not have exactly consistent pore sizes, while tests to assess filter ratings are normally
undertaken with spherical particles (because these are easiest to size accurately), whereas very few real suspensions contain spherical particles.

The actual shape of the particle may, in fact, have a marked effect on the effectiveness of the filter. An acicular (needle-like) or plate-like particle can pass through a pore of size considerably less than the particle ’ s nominal diameter, as shown in
Figure 1.8 . The figure shows an acicular particle, but the illustration could as easily represent a plate-like particle passing through a slot in a metal-edge filter. This example shows the care that must be exercised in selecting a protection filter to
ensure that adequate protection is, in fact, given.

The chance of such a lining up of particle and pore as illustrated in Figure 1.8 may be rare, but if the potential damage that could be done by such a particle having got through the filter is serious, then steps must be taken to ensure that it can
never happen. With depth filtration the chances of passage of this kind of rogue particle is minimized. Another insurance against this risk is achieved by using two separate layers of filter medium in series, so minimizing the existence of a continuous pore through both layers.

The occurrence of large continuous pores is also reduced because most real filtration systems do create a layer of filter cake, even if quite thin. These solids decrease the permeability of the medium and increase the filtration efficiency. This explains why the performance of a filter can often exceed its given rating, based on the performance of a clean element, and also why test figures for identical elements can differ widely with different test conditions.
Some types of filter media, such as paper, felt and woven cloth, have a variable pore size, and so do not have an absolute rating. The effective cut-off is largely determined by the random arrangement of pores and the thickness of the medium. The
performance may then be described in terms of a nominal cut-off or nominal rating.

It may be argued that the term ‘absolute rating’ is not, in most cases, a realistic description. Strictly speaking, an absolute rating is, as its name infers, absolute, and no particle larger than that rating can pass through the filter. This limits the type of medium that can have an absolute rating to those of consistent pore size, capable of retaining 100% of particles. It is also likely that the absolute rating would, from the point of view of being sure, need to be higher than a practically observed mean or nominal rating. Even with consistent pore sizes or openings, an absolute rating is not realistic if based on the smallest dimension of a non-circular opening such as a square, triangle or rectangle.

Considerable differences between actual performance and quoted ratings may also occur because of the differences between service and test conditions. Practical tests to establish ratings are normally conducted with high concentrations of suspended particles, which will tend to yield a higher filter efficiency because of the filter cake effect. Many tests, in fact, may be conducted under near clogged conditions of the filter, whereas, in practice, the filter may be operating for long periods with relatively clean fluids and in a lightly loaded state, when its efficiency is that much lower. A true absolute rating is necessary to enable prediction of filtration performance under these conditions.

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