Products
Air filtration
FILTER CLASSIFICATION ACCORDING TO"EN779"- "EN1822" (rough draft)
AND FILTER COMPARISON ACCORDING TO EUROVENT 4/5 E 4/4"
|
MOD
PREFILTER |
EN 779 G 1 G 2 G 3 G 4 |
ARRESTANCE (%) 60÷65 70÷80 80÷85 85÷95 |
EUROVENT 4/5 EU 1 EU 2 EU 3 EU 4 |
|
INTERMEDIATE FILTERS |
F 5 F 6 F 7 F 8 F 9 |
EFFIC. COLOR. (%) 50÷55 60÷65 80÷85 90÷95 > 95 |
EU 5 EU 6 EU 7 EU 8 EU 9 |
|
ABSOLUTE FILTERS HEPA AND ULPA |
EN 1822
E 10 E 11 E 12 H 13 H 14 U 15 U 16 U 17 |
EFFIC. ON PARTIC. DA 0,3 m m(%) > 95 > 98 > 99,99 > 99,997 > 99,999 EFFIC. ON PARTIC. DA 0,12 m m (%) >99,9995 >99,99995 >99,999995 |
EUROVENT 4/4 |
IMPORTANCE OF FILTRATION
Filtration is important not only for the greater degree of confort it provides to those who make use of it, but also for the resulting economy in the proper maintenance of premises and machinery. The economic results are not immediately quantifiable, but a drop in contamination level can be immediately noted. The particles that are mostly responsible for contamination are extra fine which can only be filtered by high efficiency filters.
All the components of ventilation plants, such as duct parts, diffusors, false ceilings, are enormous dust collectors and their cleaning entails considerable time and money. When a plant has been operating for several years without an efficient filtration unit, all the components are blocked by particles and fibers. Heating and cooling units efficiency is consequently greatly decreased while diffusors and false ceilings look dirty and neglected.
It is well known that in the air there are particles of biological origin which cause many allergic deseases, such as hay fever. Cigarette smoke can also be highly irritating, especially to non-smokers who are forced to breath impure air against their will. All these problems will be eliminated with CORRECT FILTRATION.
Medical research has proved that the particles dispersed in the air and reaching the lungs are 0.3 mm in size while the larger particles remain in the upper passages.
Since roughing filters are not effective against particles of approimately 1 mm diameter, high efficiency or absolute filters must consequently be employed.
In hospitals, especially in operating theaters, filtration has been proving its importance by limiting the spread of hospital infections.
Food and pharmaceutical industries also require an high level of filtration in order to increase the shelf life of their products. This can be achieved by entrapping the germs responsible for fermentation with filtration, rather than by using antifermentation products.
Many steel products improve in quality if made in cleaner environments and the same applies to machinery construction.
A field that requires the maximun degree of filtration is that of electronic industry, where components whit such infinitesimal tollerance are processed, that contaminant particles may even be larger than components themselves.
CONCEPT OF FILTRATION
Atmospheric pollution is caused by manifold factors: heating plants in urban areas, air and motor traffic, industrial pollution, pollen, smokes, bacteria and virus.
Air filtration process is based, in addition to what above-mentioned, also on:
1)
- dimensions, shape and weight of particles;
- electric property of particles;
- air flow velocity
2) construction of the filter:
- filter dimensions;
- technical/economic aspect of the filter;
- type, diameter and density of fiber.
All this excludes the existence of only one filtration mechanism. The most important filtration mechanism are: sieve mechanism, inertia mechanism, interception mechanism, diffusion mechanism.
SIEVE MECHANISM
"Sieve" mechanism occours when distance between the filters is inferior to the diameter of particle which cannot consequently pass through the media. In filters of high and very high efficiency the mechanism is of very little consequence, but it becomes important in pre-filters working on thick grain dust.
INERTIA MECHANISM
"Inertia" mechanism occurs especially with heavy-fiber media. The grosser particles have such kinetic energy as to maintain a retilinear trajectory even when air flow tends to curve round the media fibers. Thus particles are caused to collide against the fibers to which they adhere either through mechanical force or by means of adhesive coatings.
This mechanism does not take place with small particles as they have a low kinetic energy and tend to follow the flow line curves thus passing between the fibers. This kind of mechanism is valid also for inertial filters and for send filters; its effectiveness increases with the increase of speed.
INTERCEPTION MECHANISM
Thin and light particles tend to follow the outline of fiber filter. Particles will be fixed to the fiber if their distance (to the fiber) will be inferior to their radius. If a particle passes at a distance from the fiber which is less than the particle diameter, it is attracted and stopped by the filter itself.
DIFFUSION MECHANISM
Dust particles of smaller dimensions tend to follow the flow lines of the air current.
The probability that particles meet fibres increases with the diameter decrease both of particles and fibers and with the air velocity decrease. Dust particles, endowed with a "brownian" molecular motion, tend to remain permanently attached to fibers owing to the electrostatic effect.
Diffusion mechanism is typical of very high efficiency filters.
CRITERIUS OF CHOICE
a) Choosing a filter the cleanliness air degree we want to obtain is very important. To this depends the choice of filter efficiency degree.
There is two standard methods to determine a filter efficiency:
- the method according to EN 779 to classify pre-filters and medium-filters;
- the method according to EN 1822 (rough draft) to classify absolute filters.
b) Filter life time is very important. A filter have an initial/final pressure drop due to its resistance to an air flow. At the reaching of the final recommanded pressure drop the filter must be remplaced.
c) Examples of applications:
| APPLICATION | FILTER TYPE |
|
Clean rooms Sterile areas for hospitals and pharmaceutical laboratories Electronic and photographic industries Nuclear industries - pollution prevention |
ABSOLUTE FILTERS |
|
Computer areas Air condictioning - paint cabins Chemichal industry - precision machining industry Filters for exothermic motors |
VERY HIGH EFFICIENCY FILTERS |
|
Ventilation for public areas and workshops Prefilters for "absolute filters" Filters for compressors |
MEDIUM EFFICIENCY FILTERS |