Engineering Fifth Edition

Efficiency

(a) Synthetic test dusts. The staining of walls and fabrics in rooms is not a function solely of the mass of dust present in the air, it also depends on the size of the particle, small particles of small mass stain surfaces just as much as larger and heavier particles. Straightforward gravimetric tests according to […]

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Classification according to efficiency

The filters in common use have typical efficiencies and arrestances as listed in Table 17.1, according to Eurovent standards. Table 17.1 Typical classification of arrestance and efficiency Filter type Face velocity m s“1 Pressure Drop Pa Arrestance % Atmospheric dust spot efficiency % Automatic Viscous 1.87-2.50 100 80 Dry 2.54-3.58 125 70-80 — Panel Cleanable, […]

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Viscous filters

Essentially, these filters have a large dust holding capacity but a low efficiency, and this defines their sphere of application; for example, they are more suitable for use in industrial areas where a high degree of atmospheric pollution prevails. Their drawback is usually the expense, particularly in automatic versions. The principle of the viscous filter […]

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Dry filters

Cotton-wool, glass-fibre fabric, pleated paper of various types, foamed polyurethane, cellular polythene and other materials are used for the construction of dry filters. As with viscous filters, there are cell-type and automatic roll-type filters available. The construction of these, in broad outline, is very similar to that of viscous filters. A certain amount of viscous, […]

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Electric filters

An electric filter is illustrated in Figure 17.8. The principle of operation is that when air is passed between a pair of oppositely charged conductors it becomes ionised if the voltage difference between the conductors is sufficiently large. Both negative ions and positive ions are formed, the latter being in the larger quantity. By contact […]

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Wet filters

Washers and scrubbers of various sorts are used extensively throughout industry, largely for the absorption of soluble gases. They are not very commonly used for cleansing the air of solid dust particles. The effectiveness of a washer in removing a dust depends on the ‘wettability’ of the dust by water. This is a function of […]

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Centrifugal collectors

If air is made to travel in a circular path, centrifugal force acts both on the molecules of the air and on the associated dust particles. The dust particles being the heavier, the force on them is the greater and so they are forced to the outer boundary of the curved airstream. This is the […]

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Adsorption filters

The process of absorption, which is a chemical process, is to be distinguished from that of adsorption, which is a purely physical process. A detailed explanation of adsorption is beyond the scope of this book, but Robinson and Quellet (1999) have provided a useful account of the process, which is sometimes termed gas-phase filtration and […]

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Safety

With the passage of time, the filtration medium used in air filters may tend to erode under the influence of airflow and some concern has been expressed11 about this. Glass fibre, rock wool and slag wool, used in filters, shed minute particles into the airstream and thence into the conditioned space, where they may be […]

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Power-volume and efficiency-volume characteristics

Two further characteristic curves, power-volume and efficiency-volume, are also used to express the performance of a fan at a given speed and these are shown in Figure 15.20(a), (b) and (c), for three different types of fan. In Figure 15.20(a), for a centrifugal fan with backward-curved blades on its impeller, we see that peak efficiency […]

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