Filter ratings nominal rating

Another use for woven wire is for cleanable cartridge filters with nominal ratings from 300 to 1 rm. Although the use of metal media is expensive, the cartridge can be chemically cleaned and re-used. The aircraft and aerospace industries have been big users of these types of elements. 

Note that this is a gravimetric test rather than a particle count test. Counting particles upstream and downstream is a more meaningful way to measure filter effectiveness. The various tests used to give nonfixed-pore stmcture filters a nominal rating yield results that are misleading. Typical problems are as follows ... 

In recent years the collective wisdom of producers and regulators has been that sterility will be achieved using filters with mean pore diameters from 200-250 nm. This was a step up from the situation 20 years ago or so when it was believed that filters with nominal diameters of 450 nm would achieve sterility. As noted by Jomitz and Meltzer (2004), there may be situations that require 100 nm porous filters but this causes problems with the flow rates which may become very low on a large scale. 

Graded-density submicron depth filters (Fig. 18.13) are suitable for broader PSD slurry global distribution loop filtration. These filters with large surface area and low face velocity are suitable for high flow POU and POD filtration applications, are typically disposable in nature, and may have nominal ratings of 0.2, 0.3, 0.5, and 1.0 pm. 

FIGURE 18.18 LPC for 0.5-pm (Planargard CS05) and 1.0-pm (Planargard CMPl) nominal rating (1-1/2" length) depth media filters in single-pass filtration test. 

FIGURE 18.20 LPC for 1.0-gm (Planargard CMPI) nominal rating depth media (1-1/ 2" length) filters in single-pass filtration experiments, (a) silica-1, (b) ceria-1, and (c) alumina-1 slurry. 

This study shows the significant benefits of an MLP pump in handling shear-sensitive CMP slurries in single-pass applications and under normal turnovers ( 100) expected in a typical fab operation. Since, the MLC pump generated far fewer >lpm particles in shear-sensitive slurries, the filter lifetime for MLC pump-based slurry delivery systems should be longer than the other AOD or bellows pump-based systems, especially when relatively open (> 1 pm nominal rating) filters are used in the global loop and point-of-use (POU) locations. 

There are two main types of barriers depth and screen. A depth filter retains particles both on its surface and within its matrix. Figure 37-6, p. 433, shows a depth filter made of fiberglass bonded with acrylic resin to prevent pieces from getting into the filtrate. Depth filters have a random matrix of fibers. The pore structure is irregular so they retain a variety of particle sizes. These filters are given a nominal rating, and they will retain 98% of all particles larger than that size. 

Filtration. Baths should be continuously filtered to remove impurities. Most manufacturers utilize spiral-wound cylindrical polypropylene-type filters with nominal pore-size ratings. Some manufacturers have decreased or actually eUminated the need to dump selected process baths, such as the brown oxide bath, by using a more expensive absolute-type... 

Cartridge filters are typically designed to remove at least 90% of particles larger than their nominal rated size. For all but the smallest capacity SWRO systems, many filter cartridges, up to 100 cm (40 inches) long, are installed in stainless-steel or fiberglass-reinforced plastic (FRP) pressure housing vessels (see Fig. 3.5). 

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. 

A nominal rating is an arbitrary value for the performance of a filter, determined by the filter manufacturer, and expressed in terms of percentage retention of a specified contaminant (usually spherical glass beads) of a given size. It also represents a nominal efficiency figure for the filter. Figures typically quoted are at the level of 90,95 or 98% retention of the specified particle size. Many filter manufacturers use such tests, but the lack of uniformity and reproducibility has caused this measure to fall into disfavour. 

A mean filter rating is a measurement of the mean pore size of a filter element. It is far more meaningful than a nominal filter rating, and, in the case of filter elements... 

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