Filter particle capture efficiencies

Figure 2. Particle capture efficiencies of 8.0-fim and 0.4-fim Nude pore filters for a face velocity of 5 cm/s.

Most models currently available for blood filtration are based on empirical models/ Bruil proposed a mathematical model for leucocyte filtration process and could explain the filtration law in the plain membrane filter. However, the effect of direct interception in blood filtration is not clearly understood, and the particle capture efficiency may be modelled based on an empirical model proposed by Khilar and Fogler"" for Newtonian liquid flow. With the consideration of the further particle capture due to the reduction of the pore sizes and the porosity of filter fabric by particle bridging, pore blockage, and pore closure, a modified Khilar—Fogler model of the particle capture efficiency by Gruesbeck and Collins" may be applied in blood filtration. 

The quantity of interest from a utilitarian point of view is the extent of removal of particles from the air stream in the depth filter. A predictive approach to this usually involves a number of steps. The first step is to calculate the particle capture efficiency of a single fiber element in a filter via a given mechanism, say Ejs due to inertial impaction. In the second step, add up appropriately the corresponding single-fiber capture efficiencies due to the different mechanisms to obtain EtSi the total efficiency for a single fiber, and apply the result to the whole filter bed/depth filter. 

An alternative approach calculates the particle capture efficiency of a single fiber element due to the simultaneous action of different capture mechanisms to obtain Ets and then applies such a result to the whole bed. We will consider these approaches in Section 7.2.2 along with the other capture mechanisms primarily for hydrosol removal by granular filters. 

As we have seen in Section 6.3.1.4 on the removal of particles from air by a fibrous bed via the mechanism of inertial deposition, if one can locate the limiting trajectory (dimension b in Figure 6.3.9A), the particle capture efficiency can be determined (e.g. definition (6.3.42a)). Determination of the limiting trajectory is achieved via particle trajectory analysis in the porous medium, i.e. the granular filter medium. The governing equation for particle motion in the inter-particle space is equation (6.2.45) ... 

The capture efficiency of sticky surfaces for dry particles, and of dry surfaces for moist or sticky particles (ascospores or dye particles), agrees reasonably with theory, but dry surfaces are less efficient for dry particles. Theory and experiments on filter efficiency (Dahneke, 1971 Esmen et al., 1978 Ellenbecker et al., 1980) have shown that the critical parameter is the kinetic energy of the particle. Figure 6.4 shows Cp/C, as given by the ratio of catch on non-sticky compared with sticky pine needles (Chamberlain Little, 1980). Also shown are the results of experiments by Ellenbecker et al. (1980) (0.2-//m fly ash particles on 8-fim steel fibres) and Aylor Ferrandino (1985) (Lycopodium particles... 

The effectiveness of deep-bed filters in removing suspended particles is measured by die value of die filter coefficient which in turn is related to the capture efficiency of a single characteristic grain of the bed. Capture efficiencies are evaluated in the present paper for nil cases of practical importance in which London forces and convective-diffusion serve to transport particles to the surface of a spherical collector immersed in a creeping How field. Gravitational forces are considered in some cases, but the general results apply mainly to submicron or neutrally buoyant particles suspended in a viscous fluid such as water. Results obtained by linearly superimposing the in-... 

The effectiveness of the filter element has been evaluated in terms of capture performance. For a cross-sectional area normal to the bulk flow direction and upstream from the filter element, a "capture window" exists in this plane through which all particles that eventually collide with the cylinder pass. The width of the capture window can be calculated from the cylinder radius and acoustic energy density, and used as an experimental measure of capture efficiency. 

Capture efficiency within the filter varies with su ended particle size and flow rate, owing to the variety of mechanisms that exist vrithin the bed. Suspended solid outlet (effluent) concentrations of < 0.1 mg f are posable at low flows. It has been discovered [Yao et al, 1971] that filter efficiency can di Iay a minimiim at a particle aze of 1pm, at constant flow rate, as the particles are too large to diffiise onto the bed solids and too small for the capture mechanisms of gravity and interception (Figure 6.2). 

In the event that the sohds in the suspension are either too fine, even for the most efficient of filter fabrics, pre-coating the filter cloth with filter aids such as diatomaceous earth or expanded perlite will significantly enhance particle capture. In similar vein, where filtration conditions are difficult due to slimy suspensions, such agents are often added as body feed... 

As with dust collection media, surface treatments routinely feature singeing and calendering operations to enhance cake release and/or particle capture. On the other hand, were the suspension to contain extremely small particles, a more efficient approach may be needed. Filter aids have already been mentioned, but if for practical reasons they cannot be used, the application of a PTFE membrane may seem a logical choice. Whilst these are used in some applications (e.g. on candle filters in sodium hypochlorite... 

Filtration of particles through nonwoven filters is depth filtration rather than surface filtration. The particle capture capacities of a nonwoven filter are based on the interactions between targeted particles, individual fibres of the filter, and the fluid molecules, and the dust particles captured by nonwoven filters are described by using single fibre particle collection theory. The filter efficiency of nonwoven filter, which is composed of many individual fibres, depends on the particle collection efficiency of a single fibre. The equation defining overall filter efficiency of a nonwoven fabric in dry air filtrations, Y(d(), for any particle size, (L, and the set of conditions is as follows ... 

Membranes and separation cJiaracteristics in mlcroflltration Whereas filter clotbs/woven fabrics are useful for filtration of larger particles, microfiltration membranes are widely used in deadend filtration mode to remove effectively suspended matter in the size range 0.1 to 10 pm. The objective can be purification, clarification, sterilization, concentration and analysis. The suspended particle capture/retention efficiency Et may be described by... 

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