The molecular filtracion
The fume cupboard molecular filtration are equipped with filter units that allow to recycle the filtered air directly into the room, without the need to channel it outside the building. Several models of recirculating fume cupboard are designed for channeling outside air. The aspirated anatomical tables and filtered using this technology to avoid the channeling towards the outside, offering the advantage of freedom of installation.
Whereas the three main types of contaminants by the airborne - inert, chemical and biological - in the laboratory will primarily use two technologies: the absolute filtration of the air for the removal of dust and microorganisms, and molecular filtration to harmful vapors.
They are made up of solid particles suspended in the air with dimensions in the order of microns. There is talk of particle contamination and using filters so-called "absolute".
They are aereosol of microorganisms and viruses. The size of the droplets are in most cases not below 0.5 micron. The behavior of aerosols is similar to that of the solid particles and therefore you use HEPA filters.
Are molecules of chemical substances diffused in the air. The size of these molecules are in the order of Amstrong and then you must use specific filters (eg. Activated carbon filters, molecular sieves).
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The removal of chemical contaminants in the air is in many cases possible and affordable, especially in laboratories using chemicals defined (es. Histopathological Laboratory of Anatomy).
The technology used is that of the molecular filtration of the air through activated carbon which is based on physical adsorption and, in specific cases, on what the chemical neutralization (chemisorpzione).
Activated carbon is a natural amorphous microporous product from selected raw materials. The best filters using fine coal product with coconuts, whose exchange surface, after activation by means of high temperature steam, may exceed 1200 square meters per gram of active carbon.
The ability to attach molecules to the surface of the charcoal is known as adsorption and is influenced by several factors such as the contact time (or residence time) resulting from the speed of the air passing through the thickness of the filter, the pressure, the temperature and the humidity of the air, the concentration and chemical nature of the element contaminant, the dustiness of the air (amount of inert dust in the ambient air), the type particle size and the compactness of the coal used.
For substances with low molecular weight are used activated carbons impregnated with substances capable of chemically neutralizing the molecule (chemisorption) (es. Aldehyde, ammonia, etc.).
A characteristic of the active carbon is the continuous, incessant activity of adsorption of the molecules present in the air with which it comes in contact even when the hood is not in operation, the fact to be considered to evaluate the durability of the filter.
The type of filter to use in the chemical fume cupboard (recirculation) is critical for the safety in the laboratory and reference standards (eg. AFNOR NF X15-211 and BSI 7989), give the user and the supplier together, the responsibility for selecting the most appropriate filter for the intended use.
It's good to remember, however, the limits of this technology that shall not be used in the presence of
- large amounts of volatile substances
- very toxic substances
- highly corrosive, explosive or highly flammable
- chemically incompatible with each other
- substances incompatible with the filter installed, or with substances previously adsorbed by the filter
- substances that give rise to explosive or exothermic reactions when placed in contact with each other
- substances that produce reactions with uncertain outcome
- biohazardous material
When the contaminated particulate present in the air is of type (dust and aerosols) should be applied, an absolute filter (HEPA, High Efficiency Particulate Air), constituted by a thin sheet of glass fiber finely pleated, capable of retaining more than 99 , 99% of particles with a diameter equal to or greater than 0.5 microns.
Unlike the molecular filter that over time exhausts the adsorption capacity without varying the pressure drop (that is the resistance to passage of air), the absolute filter with time increases its efficiency in a manner proportional to the pressure drop and the noise of the hood.
The filters are classified by the technical standard reference EN 1822 according to their efficiency filtration HEPA H14 (minimum initial EI Efficiency 99.995%) and U15 ULPA (Ultra Low Penetration Air, Efficiency minimum initial EI 99.9995%) . For this reason they are used mainly in biological and microbiological safety cabinets (biohazard).
It's important to remember that the filters HEPA and ULPA acting as "sieves" have no effect on chemical contaminants in the air as vapors or gases. Also a chemical hood with HEPA filters can not and must not be used for handling pathogenic material, for which it is mandatory to use microbiological safety cabinets (biohazard) specially designed.
The recirculating fume cupboard require specific training of staff and the use of the one in charge of the periodical maintenance, and how to periodically check the efficiency of filtration.