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DEFIL | dust removal and air filtration

The concept of filtration

Filtering means eliminating from a fluid, partially or completely, the particles and aerosols contained in it. Filtration by porous separator doesn’t produce only a sieve-like effect, as we’re generally led to believe: if so, it would be impossible to explain how a one-micrometer sieve can hold even particles with a diameter of 0.3 μ.

When a gaseous stream passes through a porous separator, the stream splits into a lot of microstreams that pass through the fibres forming the separator; the particles transported by these microstreams sustain a lot of changes in direction until they are captured.

This happens when a particle passes through any kind of porous separator regardless of the material of which it is made. The repeated changes in direction make the particle cover a distance up to three hundred times larger than its diameter.

During these changes of path the particle can be captured mainly by three types of forces: by inertia, by diffusion and by interception.

The first capture mechanism is by “inertia” (fig.1-a) it occurs especially with a large fibre (15-30 micrometers) separator and with speed from 1 to 5 m/s.
In these conditions the coarsest particles have such an amount of kinetic energy that a rectilinear trajectory can be maintained even where the air stream tends to bend around the fibres of the separator This determines the collision of the particles against the fibres and the subsequent adhesion by mechanic effect or by adhesive coverings.
This mechanism has no effect on small particles that, having low kinetic energy, tend to follow the curves of the stream lines and pass between the fibres.
Therefore this type of capture is effective only for low and medium performance filters and its efficiency diminishes as the diameter of the particles decreases and as their speed increases.

The second capture mechanism is by “diffusion” (fig.1-c) it occurs when the fibres of the filtering separator have a diameter of about 0.1 micrometers and are arranged in more layers parallels to the direction of the air stream.
With a separator like this the smallest particles of dust tend to follow the stream lines of the air stream, which is laminar inside the thickness of the filtering separator. In these conditions the particles tend to behave as if they were air molecules, and when the stream is diverted by the fibres of the separator they move away in a zig-zag from the direction of the stream itself, they are attracted by the fibres and tend to be fixed permanently because of electrostatic attraction.
This mechanism for the capture of dust is typical of fibre microfilters.

The third capture mechanism is by “interception” (fig.1-d), it depends on the mass and on the electric charge of the particles, whose diameter from 1 to 3 micrometers allows some of them to be attracted by the fibres of the filtering means.
If a particle passes at a distance from the fibre lower than the diameter of the particle, it is attracted and stopped by the fibre itself.

The fourth capture mechanism is by “sieve” (fig.1-b) it happens when the distance between the fibres is lower than the diameter of the particle, that therefore can’t pass through the separator itself. This mechanism is of minor importance for high and ultra-high efficiency filters, while it produces some effect on the prefilters that work on large-sized dust.

The effect of interception is proportional to the diameter of the particle and inversely proportional to the diameter of the fibres and to their distance. The interaction between these three capture mechanisms varies depending on the particles (fig. 2).

DEFIL | dust removal and air filtration

Fig.1 - Schematic representation of the capture mechanisms.
Fig.2 - Graphic representation of the influence exerted by these mechanisms when the speed and the size of the particles vary.

These capture mechanisms affect on various accounts the global efficiency of the filter which, under certain conditions, has a well-defined limit.
Since the effect of interception and inertia mechanisms grows as the particles increase while at the same time the force of diffusion decreases, there will be an amount of particles that are in equilibrium with these three capture mechanisms and that will therefore be the most difficult to hold back.
This amount is around 0.25 micrometers as you can see from the table (Fig.3), in which on the vertical axis there is the permeability of an absolute filter and on the horizontal axis the diameter of the particles.

DEFIL | dust removal and air filtration

Fig.3 - Graphic representation of how 0.25 micrometers particles are the most difficult to be intercepted by a microfilter.

Characteristics of the main pollutants of atmospheric air

Atmospheric air is a mixture of gases that includes a lot of suspended substances in the form of dust or fogs both of animal and vegetal origins.

Under normal conditions air is made up of 21% oxygen, 78% nitrogen , 1% argon and 0.03% carbon dioxide. In addition to this there are minor percentages of hydrogen and rare gases such as neon, helium, etc.., as well as variable amounts of water vapor.
The proportion between oxygen and nitrogen is basically constant while the amount of water vapor is very variable. The first one varies according to the climatic regions, higher in the equatorial regions and lower in the desert regions. The second one varies cyclically according to day and night, during which it reaches its highest concentration. The concentrations are variable from rural areas to urban areas and they go from 0.015% to 0.09% in volume.

The solid and liquid particles dispersed in the air are called dust particles: they remain in suspension because of their extreme smallness (from 0.3 to 1.3 micrometers) and they can be of meteoric, vegetal (spores, plant pollen), volcanic, desert (raised by the wind) or marine (salt coming from the evaporation of little drops) origin or they can even be formed by micro-organisms.
All these particles are usually classified as hygroscopic and non-hygroscopic. The first group includes soil particles originated in dry regions, as well as compounds of biological origin and residual combustion products.
Hygroscopic particles are made of easily soluble compounds such as sea salt, compounds of continental origin such as ammonia, and other inorganic compounds for example chlorides, sulphates, nitrates etc..

When the humidity in the air increases, little drops of water form around these particles, these impurities work as nuclei for the formation of drops and are therefore called condensation nuclei. Depending on their diameter the particles are divided into three classes; Aitken nuclei, large nuclei and giant nuclei (Fig.4).

DEFIL | dust removal and air filtration

Fig.4 - Denomination of the particles existing in the atmosphere according to their size, and importance of every class of particles in various fields of meteorology.

All these impurities dispersed in the air have a sensible influence on human health because, adding to the ones produced by industrial or agricultural processes, they can cause serious disorders for the organism. Among these we can recall silicosis, asbestosis and chronic bronchitis due to toxic gases.

Even biologically originated components can cause serious disorders such as a variety of allergies, among which we recall hay fever. Air pollutants have very variable size from less than 0.1 micrometers for the smallest viruses to over 100 micrometers for pollens.

DEFIL | dust removal and air filtration

Fig.8 - Graphic representation of the granulometric distribution of the main industrial pollutants in the atmosphere.

Even concentration is very variable from area to area. Generally speaking we can consider the following values:

  • Rural area 0.4 – 0.9 mg/m3
  • Urban area 0.9 – 1.8 mg/m3
  • Industrial area 1.8 – 3.7 mg/m3

DEFIL | dust removal and air filtration

Fig.9 - Graphic representation of the concentration of dust according to the transparency of air.

In atmospheric air there are particles of basically every size under the form of dust, fumes, fogs and aerosols. The size of dust is measured in micrometers (μm); 1 μm equals 10-6 m, that is to say one thousandth of a millimeter.

  • By “dust” we mean solid particles dispersed in the air with a size of at least 100 micron.
  • Aerosols” are formed by much smaller particles both solid and liquid generated by sublimation, condensation or combustion.
  • Fumes” are mixtures of solid particles and fluid and gaseous products. They are very small particles both in a solid and a liquid state generated by the incomplete combustion of organic substances (coal, wood, petroleum products, tobacco etc..) with a diameter from 0.1 to 0.3 micrometers for the most part.
  • Clouds” are constituted by water particles generated around condensation nuclei.
  • Fogs and mists” are formed by little drops suspended in the air, generated by condensation or by nebulization of liquids.
  • Viruses” have a size variable from 0.005 to 0.1 micrometers and they are often reunited in larger particles, formed by colonies or by accumulation with other materials.
  • Vegetal spores” have a size from 10 to 30 micrometers, in pollens from 10 to 100 micrometers.

The concentration of particles in the air is, as we have seen, vary variable and apart from the ones produces by specific processes it has not yet been possible to evaluate a relation between the particles deposited because of rain, solar irradiation etc.. and those generated again by natural phenomena.

During the last decades the increase in industrial processes has determined an increase in dustiness. Air purification occurs basically through rain.
Concentration depends also on the ability of the particles to remain in suspension in the air; the time of suspension is essentially linked to the size of the particle itself.

When the particles have a size of less than 0.1 micrometer, they behave as gaseous molecules and thus they tend to remain in suspension, which makes it difficult to measure their time of fallout.
Particles with a size from 0.1 to 1 micrometers already have a measurable fallout speed, but it is so low that normal air streams interfere with their sedimentation.
Particles from 1 to 10 micrometers have a constant and exactly measurable fallout speed but even in this case normal air streams tend to keep them in suspension. Particles with a size of over 10 micrometer, which can only be found in the air very close to the place of emission or when there are very high streams, precipitate very easily and it’s difficult to keep them in suspension.

These particles are visible to the naked eye in good lighting conditions, the smallest ones only if in high concentration. This is the case of cigarette smoke which has particles with an average size of 0.5 micrometers.

DEFIL | dust removal and air filtration

Fig.10 - graphically represents the percentage distribution of dust dispersed in the air according to its number, size and area. As we can see from curve (a), the particles with a diameter of less than 1 micrometer represent in figures 99% of the total amount of the particles composing the dust.
From curve (b) we notice that assembling the particles according to the projected area, the ones smaller than 1 micrometer represent 80% of the total.
With regard to the size we can see from curve (c) that the particles with a diameter of less than 1 micrometer represent approximately 30% of the total weight of the dust.
We immediately notice the differences between these values: only 0,1% of the dispersed particles has a size of more than 1 micrometer, but their weight represents even 70% of their total weight.

The opportunity of an effective filtration is pointed out by this statement: a normal ventilation system with an air flow of 100 m3/h, could introduce over 100 kg of dust in the environment in one year.
Besides keeping the rooms clean, filtration eliminates from the air all the pollutants which are harmful or irritant and can easily reach the lungs because of their size.

DEFIL | dust removal and air filtration

Fig.11 - As we can see from the following table (Fig.11) the maximum concentration of particles able to reach the pulmonary alveoli occurs with diameters of less than 0.1 micrometers (fumes) and around 1 micrometers (bacteria, soots).

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