RF DRYING TECHNOLOGY

When a wet product is submitted to a radio frequency field, the electromagnetic energy is absorbed by the product due to the so called “dielectric losses”, and its internal temperature increases. If a sufficient amount of RF energy is supplied, the water is converted into steam, which leaves the product; that is to say, the product gets dried.

The radio frequency drying process is:

endogenous: this means that the thermal energy is not transferred into the product through its surface from an external heat source, but rather it is generated directly throughout its mass, in all (wet) points at the same time;

selective: in the sense that the energy of the electromagnetic field is absorbed mainly by the water and not by the product itself, so that the heat is generated only where there is water;

controlled: in the sense that the electromagnetic energy is absorbed in proportion to the water content of the product, so that by controlling the RF power it is possible to bring the moisture content down to the desired residual value.

Thanks to these features, the RF drying of technical textiles and non-wovens after various wet treatments like washing, padding, dyeing, wet spinning, wet casting, vulcanising, felting, impregnating, coating, etc. has many advantages compared to the conventional hot air drying technologies, the most important ones being:

the high energy efficiency and the high drying speed: the electromagnetic energy is transferred directly to the whole of the wet product without losses to the surroundings, instantaneously, and it is entirely exploited for the drying process; furthermore, the energy transfer is not negatively affected by the intrinsic product parameters such as its dimensions and density;

the outstanding quality of the dried products: all drawbacks (mainly due to temperature and moisture gradients) normally caused by the heat transmission phenomena are totally eliminated, so the product dries quickly and uniformly, down to the desired residual moisture level, with a beneficial steaming and bulking effect and without the typical surface degradation phenomena (colour and physical-chemical properties) that normally occur in conventional drying processes.

STALAM RADIO FREQUENCY DRYERS

STALAM radio frequency dryers have some specific design and construction features which allow their users to obtain the maximum benefits from the RF technology in terms of quality of the dried products, reduced operating costs, operational flexibility and equipment reliability.

The RF generators are entirely designed and manufactured by STALAM; they are of the “lumped components” type, having high efficiency (Q factor) and outstanding reliability; all components and circuits are easily and quickly accessible.

The cooling system of the triodes is made up of a double water circuit; it is designed to allow the longest possible life of the triodes and does not require periodic maintenance operations. Upon request, and as standard in some specific applications, STALAM also supplies generators fitted with a forced-air cooling system.

The RF power adjustment is accomplished by means of a semi-automatic circuit which controls the power supplied to the product being dried through a variable capacitor, located in the generator. No continuous raising or lowering of the upper electrode for RF power adjustment is required in STALAM dryers: the electrode is fixed or automatically positioned at pre-set heights.

The power density is as low as 5 to 15 kW(RF)/m2 of electrode surface. Therefore the heating/evaporation process is extremely gentle and the quality of the dried product is greatly enhanced.

The construction is modular, in the sense that high capacity dryers are made up of two or more drying modules, connected in series; this construction principle allows STALAM dryers to work with reduced RF power densities in all circumstances. Further modules can be added to the first at any time, so that considerable investment and floor space savings are possible.

The construction materials are mainly stainless steel and aluminium, to ensure a long life of the dryers even in difficult working environments, and all electrical and electronic components used are of the best international makes, other than amply oversized, to guarantee the highest level of reliability.

STALAM dryers are also equipped with electromagnetic field leakage preventing and RF interference suppressing systems, in order to comply with all safety and electromagnetic compatibility regulations in force in the different countries.