|De-dusting a gaseous current is usually done to purify the gaseous phase of powders which is dragged along for some reason (e.g. controlling dust emissions to limit environmental pollution) or to recover powders intentionally carried along by the gas (e.g. picking up solid materials in pneumatic transport systems).
Different types of devices can be used for collecting the dusts, such as scrubbers, electrostatic separators, centrifugal separators and filtering systems. This document describs the principles underlying filtering systems made up of modular elements which, because of their manufacturing and operating characteristics, provide maximum efficiency in terms of cost/benefits ratio.
Basically, a filtering system comprises an inlet chamber of the gaseous phase to be purified, containing the filtering elements, and a chamber through which the filtered gas passes, housing the cleaning systems for the filtering elements.
The retention of solid particles by the filter may occur in two main
1) surface filtration: mainly based on depositing of the collision or screening particles. Reduction of porosity may be caused by:
- retention of very fine particles in the pores;
- formation of a cake of particles on the filter surface.
2) deep filtration: achieved by thicker filtering means, where the particles are forced to follow a longer path, thus increasing the probability of interaction with the fibres, in terms of direct interception or screening effect.
The main properties of filtering means are:
- separation efficiency
- pressure drop
- capacity for accumulating dusts
Generally, the filtering element requires frequent cleaning to guarantee that it remains effective throughout its life.
The cleaning operation may be achieved in different ways:
- by mechanically shaking the filtering element;
- by inverting the flow, e.g. conveying compressed air on the filtering means in counter-current relative to the gaseous current to be purified;
- using reverse pulse jets, which create vibrations in the fabric that cause the particles to be detached.
Replacement of a filtering element is necessary when its separation capacity can no longer be guaranteed (laceration of filtering medium or alteration in its porosity), or when - even after a prolonged cleaning cycle - passage of the gaseous current causes an excessively high pressure drop for working with the nominal flow rate required (blockage of the filtering medium).
Once the characteristics of the gaseous current to be subjected to filtration, in terms of properties of the solid particulate, and solid content, density, viscosity, flow rate, temperature and pressure of the gas stream, are known, it is possible to choose the most suitable filtering medium and configuration - in terms of efficiency, ease of cleaning, space required and economy - for setting up the filtering system.
The materials most commonly used as filtering media are:
The materials used to prepare these filtering media, generally polypropylene and polyester, may be subjected to treatment with PTFE by impregnation or for membrane application, thus obtaining a surface layer that impacts with the product that is extremely smooth and free of supporting points for adhesion.
It is also possible to carry out anti-static treatments to prevent the formation of discharges or sparks due to electrostatic phenomena between the product conveyed and the filter; basically, stainless steel fibres incorporated in the weft of the media are used, to achieve a continuous conductor connected to the entire structure of the purifying filter, suitably earthed.
The configurations that can be adopted to prepare the filtering element
Choice of the material and the filtering element configuration, which represent the fulcrum of such systems for dusts collection, is almost always the result of a compromise between economy of the filtering system - in terms of the costs of purchase and machine running - and the filtration efficiency that it can provide.