Filter technology

Atmospheric Air

The air around us is a mixture of various gases. The natural atmospheric air is composed of approximately 21% of oxygen, 78% of nitrogen and 1% of argon, carbon dioxide and hydrogen. In addition to these gases there are also small amounts of rare gasses, such as neon, helium, krypton and xenon, as well as water vapour.

Atmospheric dust

Air contains a lot of various foreign materials originating partly as a result of natural processes, such as climatic erosion,

Filtering composite - Minipleat Technik

earthquakes or volcano eruptions, and partly are caused by human activities, e.g. fumes from combustion engines, industrial production processes or waste gases originating during technological processes of chemical industry.
The dust mixture in the atmospheric air consists of solid, granular particles, of smoke gasses, fog and different fibres. Chemical and physical examinations generally prove the presence of silicates of different origin: smoke, soot, or traces of weathering organic and inorganic materials, e.g. cotton or other plant fibres, and metal dust. Besides that the air contains also natural organisms such as bacteria, fungi, pollens and germs.
The particles floating in the air that we breathe are called aerosols.

Air filters

General knowledge of harmful concentrations and of the impacts of thereof has made for the use of air filters in many branches. Air filters are used for cleaning the incoming air in ventilating and air-conditioning equipment. Nowadays filters are used to clean the air which is supplied to many industrial processes, to combustion engines, turbo-generators, incoming and waste air in laboratories, waste air from nuclear power stations and cleaning plants. In short, the technical progress leads to the development of ever increasing quality of filters.
Filters can be divided into different categories: filters for coarse dust, filters for fine dust, standard and high effective aerosol filters. This division takes into consideration the materials of which the filters are made, respectively different physical effects of the procedures of separating particles. Besides, there exist also adsorption filters, which are used for separating gases and odours, electric filters based on the electrostatic principle. These filters are on minor importance due to higher demands on the operation safety and costs.

Filters for coarse dust

Filters for coarse dust are made of organic fibres, synthetic fibres, glass fibres, and metal materials. Their function is based on the principle of separating the coarse dust using the "impact effect". The dust particles do not follow the flow of air molecules in the closeness of the filter fibres since the inertial force of the particles causes their impinging on the fibres. The filter fibres can be treated with an adhesive agent, eventually the particles are caught on the fibre surface due to their special properties. In case of slight contamination the filters for coarse dust are less effective, and besides, the contamination is by a greater deal caused by fine dust, therefore these filters are gradually replaced by other filter types.

Filters for fine dust and aerosol filters (HEPA)

Filter layers used in these filters consist of micro-fibres with the cross section within the range 1 - 10 μm.

Filet medium - clean / covered with the dust particles.

The speed of airflow through the filter layers is usually 2-12 cm p. sec. Separation is based on the block and diffusion effects.
Due to their small weight the particles in the closeness of the filter fibres do not follow the direction of airflow, but they touch the fibres to which they are attracted and due to the adhesive power the particles are tightly caught. We call this procedure the block effect.
Particles smaller than 1 μm are influenced on the level of the movement of molecules and they start vibrating. Consequently the particles divert from the direction of airflow and they get close to the filter fibres and are separated. This procedure is called diffusion effect. In case of the block and diffusion effects the particles are fixed to the fibre by the influence of the Van der Waals forces.

Testing methods

On the base of different separating abilities of filters and different branches of use there have been developed different testing methods of the filer media. Filters for fine dust and filters for coarse dust are tested by means of testing procedures required by ČSN EN 779. Tests of filters for coarse dust (filtration classes G1 - G4) are performed with synthetic dust, which enables detection of the geometric level of separation.
Filters for fine dust (filtration classes M5 - F9) use natural aerosols, i.e. non-processed atmospheric air for testing the efficiency level. The filter efficiency is determined by examining the same amount of air before and behind the tested filter using filter papers. To determine the level of efficiency we compare the amounts of air needed for the same blackening of the filter papers.
The testing method for the aerosol filters (HEPA) is described in ČSN EN 1822. These filters are exposed to a test to determine the level of efficiency or permeability of the filter for particles of the diameter about 0,3 μm, where the separation is the most difficult. Larger or smaller particles are separated more easily.

Filter capacity

Efficiency parameters of filters determined by means of costly testing methods can be achieved only at prescribed operating values. Especially with filters for coarse dust the speed if the inlet airflow has a significant influence on the separation efficiency. The filter capacity depends on the set speed of airflow through the filtrating material. Filters for fine dust and aerosol filters maintain their efficiency level without any significant changes even at greater variations from the rated load. These physical characteristics must be taken into consideration for the technical design of the multilevel filter equipment with changeable volumes of airflow.