Air Cleaning Devices

The air cleaning devices are designed for removing and purifying polluted air from the source of chemical emissions and harmful pollutants.

Generally, filtration and ventilation devices can be divided into wet and dry dust collectors. In addition to the general division, devices can be divided according to the phenomena used in them:

• • inertia force (settling chambers),
• • centrifugal force (cyclones),
• • electrostatic force (electrostatic precipitators),
• • materials used for filtration (fabric filters).

Settling chambers are one of the simplest dust collectors used at the beginning of the technological process of air purification. As a rule, settling chambers serve as preliminary dust collectors for cleaning air from particles with a falling velocity above 0.5 m/s. They are used in multistage air cleaning systems. Their operation is based on the use of the phenomenon of gravity. A condition for separating all grains in the chamber with a falling velocity in the purified air greater than the limiting value of the falling velocity in the air is to select the air velocity and the dimensions of the chamber so that the air flow is laminar. The process parameters of the settling chambers are:

• • low flow resistance in the range of 20-50 Pa,
• • low energy consumption,
• • application for purification of hot gases without prior cooling.

Cyclones are the most common type of dust collectors. The principle of centrifugal force to separate grains from a swirled gas flow is used there. During a spiral movement, the dust particles are subjected to a centrifugal force causing them to move towards the walls. After touching the w'alls, dust grains lose their velocity and fall under the influence of gravity. The minimum particle size that can be separated in the cyclone depends on its design parameters and the physicochemical properties of the gas being purified, such as:

• • volume air flow in the cyclone,
• • number of revolutions of the gas flow in the cyclone,
• • inner tube radius, cylindrical part, cylindrical layer height,
• • dynamic gas viscosity,
• • volumetric weight of dust particles.

Cyclones can work in single systems or can be combined into batteries. Cyclone and multi cyclone batteries are used to obtain the highest possible efficiency of grain cleaning, while cleaning large amounts of gases. Multi cyclones are a parallel connection of several dozen cyclones with small diameters placed in a common chamber. The multi cyclone design uses the phenomenon of increasing efficiency of purification while reducing the diameter of the cyclone. Cyclones are characterized by simple design, small size, low investment costs, significant flow resistances from 300 to 1,300 Pa and relatively fast wear of structural elements due to erosion.

Due to their high cleaning efficiency, electrostatic filters are commonly used for the separation of large amounts of gases. They work by the principle of electrostatic field interaction on solid or liquid particles suspended in gas. In a strongly uneven electric field created in a properly shaped system there are two types of electrodes that are electrically separated from each other into two opposite poles. Negative electrodes are shaped as thin rods, and at the positive pole has electrodes in the form of plates. The most commonly used electrode system is a system made up of a series of parallel plates at the same distance from each other and between them there are rows of thin rods. The application of high voltage to the emission electrodes causes the release of electrons, which move towards positive collecting electrodes, which results in the precipitation of further electrons from gas particles (corona discharge). Outside the corona discharge sphere, electrons flowing towards the collecting electrode negatively charge the gas particles, which in turn transfer the charge to dust grains, which are attracted to the collecting electrodes. They settle thereon and discharge. When the collecting electrodes are shaken, the dust grains fall to the bottom of the tank due to gravity. The cleaned gas is extracted through a pipe in the upper part of the electrostatic filter. The effectiveness of work of electrostatic filters depends on the strength of the electrostatic field and the physicochemical properties of dust and gas carrier. Electrostatic filters are characterized by a low pressure drop of 30-150 Pa, relatively low power consumption and high cleaning efficiency.

Fabric filters are among the most effective ways of cleaning air. The flow of polluted air through porous materials deposits particles in the filters. Fabric filters require large surfaces, but they are characterized by very high efficiencies of air purification depending on the dimensional distribution of particles.

Air cleaning devices use filtering fabrics manufactured using mainly three methods:

• • needling,
• • melt-blown,
• • paper.

The spun-lace method is a technique used to produce fabrics by swirling the fibers in the fleece by means of geometrically defined water or air flows. This method belongs to the group of advanced techniques for producing fabrics. In the most general sense, the principle of fabric production is as follows: a fleece (fiber board with appropriate structure) is produced by a wet or dry method. This fleece is laid on a metal, supporting, metal sieve with an appropriate perforation. To interlink the fibers and obtain an appropriate fabric structure determining its functional characteristics, the fleece then passes through the three zones of the nozzle battery located above the surface of the fleece. In each zone, water jets of defined geometrical shape are pressed under high pressure towards the surface of the fleece. The water pressure in the first zone is about 294 Pa, and in the third from 539 to 882 Pa. In the middle zone it is an intermediate value between the pressures of the first and third zones. The distance between the top of the fleece and the surface of the nozzles is 5-7.5 cm. Fleece travel velocity is up to 30 m/min. After passing through all the spray passages, the fleece is inverted and the same operation takes place on the inverted fleece surface. At such high pressures, the extruded flows of water could cause uncontrolled movement of the fibers in the fleece, and as a result give a random fabric structure. To avoid this undesirable phenomenon, the top surface of the fleece is covered with a mesh of appropriate perforation and geometry of mesh arrangement.

The spatial orientation of fiber fragments as a consequence of the operation of their entanglement by water flows, increase of material density and friction forces between the fibers, structure of the fabric and the resulting physical and mechanical properties of the product depends on the apparatus and technological solutions of the production line, and the applied technological parameters. The spun-lace method has several equipment and technology variants that are used depending on which basic type of product is to be manufactured in the aspect of its application in a given machine park.

A characteristic feature of fabrics obtained by the spun-lace method is that they are made of staple fibers, without the use of additional binders, such as thermoplastic polymers, synthetic rubbers, acrylic polymers and polyurethanes. The presence of the binder phase always complicates the product structure, which is particularly undesirable for filtering fabrics. Spun-lace fabrics achieve cohesion as a result of friction forces between the fibers during the fiber entanglement operation by water flows. The structure of fabrics and their rheological properties depend on the type of fibers used (physicochemical and geometrical properties) and technological parameters of the production of fleece and fabrics.

Needled filtering fabrics are manufactured, among others, using polyester fibers (PES) with the following structural parameters:

• • average fiber diameters in the range from 10 up to 400 pm,
• • surface mass in the range from 150 up to 500 g/m²,
• • thicknesses of fabrics in the range from 5 up to 15 mm,
• • mass of 0.003 g/cm³.

The spun-lace techniques used in the country and in the world also allow producing layered composite fabrics, in which individual layers are made of fibers of different average diameter, using different intensities of needling. The above actions lead to the formation of fiber composites with various structures and porosities.

Pneumothermic fabrics obtained by the melt-blown method, also known as the technique of molten polymer blowing, allow producing filter materials from fibers with submicron diameters. If it is assumed that they have a circular cross section, then the average diameter of fibers, in the case of fabrics with high aerosol filtration efficiency, is in the range of 0.3-0.7 pm. Fabrics are also produced from fibers with a diameter distribution from 1 to several micrometers. The greater the differences in fiber sizes, the less uniform the structure of these fabrics. The melt-blown method allows producing fabrics with very high filtration properties.

In the melt-blown method, a polymer in the form of granules is moved from the hopper to the heated extruder cylinder. This way it is adjusted to the right viscosity before extrusion from the fiber-forming head. Compressed air passes from the regulator to the heater, in which it is dried and heated to the correct temperature. Then, it is directed to the fiber-forming head, and when it is lowered, the extruded polymer flows are blown into elementary fibers. Settling on the receiving device, they form a compact porous fleece.

The melt-blown method is a technologically flexible process. By selecting appropriate process conditions, fibers with a given diameter are obtained, and by choosing the operating conditions of the receiver, porosity values of the obtained fabric can be adjusted.

Fabrics produced by the melt-blown method are characterized by a compact structure, which provides them with a higher original packing density compared to fabrics made using the spun-lace method, without the need for additional process and technological operations.

Paper fabrics are primarily filter paper and different types of papers. These fabrics are made of thin glass, cellulose or chemical fibers and mixtures thereof. Paper fabrics have a compact spatial structure and are not mechanically strong. They are used for the production of special application filters, e.g. for the filtration of radioactive and biologically polluted air. Due to the high flow resistance of fabrics, a strong development of its surface is used already at the production stage. The method of obtaining paper fabrics determines their properties.

Purification of air in the aforementioned air cleaning devices allows moving it into the atmosphere or reintroducing into a room, while meeting the requirements of local regulations and standards concerning emissions and environmental protection. The advantages of air cleaning devices are the possibility of placing them in the immediate vicinity of the source of air pollution and operational savings that result from the use of recirculation air (especially in winter). Air recirculation should not be used in work rooms w'here it is possible to suddenly increase the concentration of harmful substances [Dz. U. 1997, No. 129, poz 844 z pozn. zm.]. Technical parameters and actions that have an effect on reducing the risk of air pollution are:

• • type of harmful substances (solid, liquid and gas),
• • value of the volume of air flow extracted from the emission source area,
• • size and shape of the inlet adapted to the characteristics of the emission area,
• • method of connecting ventilation ducts,
• • particle filtration efficiency.