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 is that if the mixture of dust and air is forced to follow a tortuous path in negotiating a passage through the filtering medium, inertial separation of the more massive dust from the lighter air will occur. If the filtering medium is coated with a suitable oily fluid, the particles of dust will be trapped by the oil and retained, the air passing on. To effect this, the oil must have a surface tension low enough to permit easy entry for the dust. Once within the oil, it is desirable that the dust should disperse, not remaining in one place but being retained in depth throughout the filter (hence its large dust-holding capacity). It is thus necessary that the oil used should have a high enough capillarity to encourage dust to flow away from its point of entry to the oil. It is also essential that the oil should be non-inflammable, non-toxic, germicidal and comparatively non-evaporative. The oil should not deteriorate during its life. There are two types of viscous filter: the cell-type and the automatic type.
The cell-type consists of a cheap retaining box, open front and rear, which contains the filtering medium. The medium (often some form of industrial waste such as swarf, brass turnings, etc.) is coated with the oil, and the cells are assembled into a battery of convenient shape and placed across the airstream for filtration purposes. After use, when dirty, the cells are either thrown away and replaced with new cells or, if they have been selected with this in mind, they may be treated with a cold wash, drained, and recoated with fresh oil for further use.
The automatic viscous filter takes the form of a continuous roll of material coated with the oil and is motor-driven across the airstream. It is arranged so that the roll is drawn through a trough of oil at the bottom of the assembly. The trough serves the dual purpose of washing off the dirt and recoating the fabric of the roll with relatively clean oil. The material of the fabric takes several forms: one is an array of hinged metal plates which, dangling downwards, force the dirty airstream to change direction several times, thus achieving the inertial separation of the dust into the oil, as desired; other versions of automatic viscous filter are elaborated by the use of oil sprays and pumps. Figure 17.4 illustrates the cell and automatic types.
Pressure drops across these filters vary from 40 to 140 Pa, and recommended air velocities over the face are from 1.7 to 2.5 m s-1. Viscous filters are not very effective in removing particles of size less than 0.5 |j. m.
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