Pad and Panel Filters

The proper management of dust collection follows a basic rule of any pollution control technology, namely that it is best to deal with a dust suspension as near to its source as possible, rather than after it has been diluted with gas from other sources. Thus, fume from a smelter should be treated in hoods immediately over the smelter, rather than in the general exhaust from the smelter building. 

Some indication of the size ranges of various classes of particulate material is given in Table 3.4, from which it can be seen that dust collection is primarily concerned with particle sizes of 1 am and above. 

As far as fine dust separation is concerned, it is the range of filters that is largely concerned, both the two types covered here, and those of Sections 3K and 3L. 

Dust collection mechanisms include aU of the entrapment processes described in Section 1C, with the effect of electrostatic forces also being very important in many systems. Filtration of dusts is achieved by depth filtration - for which the filter pad is very effective - and by the combination of surface and cake filtration that the pleated sheet media do so well. The two materials have vied with one another over the years as to which can achieve the highest degree of separation, a history of success that is largely determined by the development of new materials of one kind or the other. 

The pad filter (often called a cassette) is a special case of the range of panel filters, all of which are made in standard sizes, to fit air-conditioning installations. The pad, as its name implies, is a thick flat sheet of fibrous filter medium, made either by wet-laying (as in paper) or by dry-laying (as in a felt). Felt pads are the most common, and they can be either as-laid, which would normally be the case for natural fibres such as cotton or wool, or needle-punched, for synthetic fibres. 

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A. Introduction 3B. Strainers 3C. Screens 3D. Nutsche filters 3E. Tipping pan and table filters 3F. Rotary drum filters 3G. Rotary disc filters 3H. Horizontal belt filters Centrifugal filters Pad and panel filters 3K. Bag, pocket and candle filters 3L. Cartridge filters... 

Although perfectly usable on the scale, and in the sizes of, liquid bag filters, the corresponding units for gas filtration are usually much larger, and are used in the multiple element installations, because they usually have to deal with very high flow rates. For the small-scale treatment of gas streams, the pad and panel filters of Section 3J are most commonly employed. 

Figure 3.40 shows a variety of filter pads and panels with their methods of eonstruction. 

Air-Filter Types Air filters may be broadly divided into two classes (1) panel, or unit, filters and (2) automatic, or continuous, filters. Panel filters are constructed in units of convenient size (commonly 20- by 20-in or 24- by 24-in face area) to facilitate installation, maintenance, and cleaning. Each unit consists of a cleanable or replaceable cell or filter pad in a substantial frame that may be bolted to the frames of similar units to form an airtight partition between the source of the dusty air and the destination of the cleaned air. 

Panel filters may use either viscous or dry filter media. Viscous filters are so called because the filter medium is coated with a tacky liquid of high viscosity (e.g., mineral oil and adhesives) to retain the dust. The filter pad consists of an assembly of coarse fibers (now usually metal, glass, or plastic). Because the fibers are coarse and the media are highlv porous, resistance to air flow is low and high filtration velocities can be used. 

Immobilon membrane 1 Whatman filter 1 filter pad, and (+) top (clear panel). 

In the dry filtration processes that use pads or panels to capture dust, the filter surface is basically flat - one side of the filter sheet in a pad filter, or the upstream surface of a pleated medium panel. The group of filter types to be described next capture the dust inside or on the outer surface of a generally bag-shaped element, of circular or elongated oval cross-section. These three types - the filter bag, the filter pocket and the filter candle - are very important components of the dust collection equipment spectrum, but bags are also used in liquid clarification. 

Various media, woven and nonwoven fabrics, paper, wire mesh and even membranes may be employed for dry panel filters. The most favoured materials are synthetic fibres and glass fibre, in a pad or mat, in multi-layer or pleated form. It is becoming increasingly common practice to coat the media with a viscous agent to ensure high dust retention. 

By far the largest number of ventilation filters is of the flat panel type, square or rectangular in shape, and of a standard size to be accepted as a push fit into appropriately sized spaces in the dividing wall separating the ventilated area from the outside atmosphere. The simplest form has a flat panel of thick, depth filtration medium (Figure 6.3) held in a cardboard or thin metal frame. This pad filter has a relatively low surface area available for filtration. 

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