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Direct vs indirect heating


Many industrial processes involve heating of a certain product.

Heating technologies can be classified on how the energy is transferred from the heat source to the product mass:

- the indirect (or conventional) methods are those in which heat is generated externally to the product and is then transferred to it through its external surface by conduction (the product is put in contact with the heat source), by convection (the product receives heat through hot air, steam or another intermediate heated fluid conveyed to it) or by radiation (the heat is transferred to the product surface by electromagnetic waves having a suitable frequency, such as Infrared (IR) rays);

- the direct (or endogenous) methods are those in which heat is generated within the product itself. These include the Radio Frequency (RF) and Microwave (MW) techniques.

The indirect methods apply heat to the surface of the product and rely on its thermal conductivity to move the heat to the centre. The speed of heat transfer from the surface to the centre depends both on the thermal conductivity of the product and on the temperature gradient between the surface and the centre. Often, due to the poor thermal conductivity of the product, it is necessary to use high temperatures to get a reasonably quick heat transfer to the centre, which consequently cause overheating of the surface and deterioration of the product. For the same reason, indirect methods, and especially the IR radiation, are suitable and effective in applications that require surface heating.

Differently from the indirect methods, direct methods "generate" heat within the product rather than transferring it from the outside, and rely upon the fact that high frequency electromagnetic fields (typically in the range of RF or MW) when applied to many materials, penetrate inside them and are converted into thermal energy throughout their mass. The amount of heat generated is related to the specific chemical-physical characteristics of the product and to the frequency employed. This heating method is referred to as a "volumetric" or "endogenous" heating, meaning that heat is actually generated within the entire mass of the product and not brought in from an outside heat source

Although the heating mechanisms by RF and MW are somewhat similar and both techniques are equally effective in certain application fields, some specific and unique features make of the RF technology the only or the best alternative in a large variety of industrial applications.

Benefits and applications of direct heating methods


Although their specific characteristics and application fields can be different, the direct heating methods achieve, in general, many technological, operational and economical advantages, the major ones being:

- high energy transfer efficiency
- uniform heating within the product
- short process time
- reduced equipment dimension
- instant and accurate process control
- better preservation of sensorial, chemical and physical characteristics of the product
- negligible effect of weather / environmental conditions on the heating process
- reduced thermal losses to the environment
- no noise or fumes released by the energy source
- improved working environment
- low maintenance costs

Thanks to the above benefits, the direct heating technologies are used extensively and profitably in the industry in the following application fields:

- drying of textile fibres, yarns and fabrics after dyeing or other wet treatments
- drying of leather and technical textiles (glass fibres, foams, felts, non-wovens, etc.)
- conditioning and gluing of wood materials (laminates, profiles, parquet, cork, MDF, etc.) and papery substrates (paper, corrugated cardboard, carton tubes, etc.)
- partial dehumidification before baking or sintering of ceramics and refractories (sanitary ware, pottery, catalytic supports, etc.)
- pre-heating of thermosetting powders and welding of thermoplastics
- drying of tablets and powders in the food and pharmaceutical industries
- drying of foundry sands, moulds, etc.
- drying and thermosetting of resins, lacquers, paints and inks on different substrates (fabrics, non-wovens, plastic films, leather, wood, glass, etc.)
- final drying and moisture content levelling of biscuits, crackers, sponge products, cereals, etc. after baking
- drying and sanitisation of cereals, flours, spices, tobacco, etc.
- thermal stabilisation and bacterial inactivation (pasteurisation or sterilisation) of packaged food (fresh pasta, gnocchi, baked products, ready meals) and liquid / fluidised products (milk, juices, fruit preparations, etc.);
- tempering/defrosting, softening and pre-heating of raw materials and intermediates for the food industry (meat, fish, vegetables in blocks or IQF, butter, honey, etc.)
- cooking, roasting, blanching, etc.

and many others.

A state-of-the-art R&D facility equipped with various RF, MW (and IR) equipment is available at STALAM’s premises to carry out lab- and pilot-scale tests, to develop new applications or to simulate the industrial processes mentioned above, according to the customers’ requirements.

STALAM S.p.A. - Via dell'Olmo, 7 - Z.I. 36055 Nove (Vicenza) Italy - tel. + 39 0424 597400 - stalam@stalam.com

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