Foam-mat drying: Energy and cost analyses
T. Kudra1* and C. Ratti2
1CANMET Energy Technology Centre –Varennes, Varennes, Quebec J3X 1S6, Canada; and 2Department of Soils and Agri-Food Engineering, Laval University, Quebec, Quebec G1K 7P4, Canada. *Email: tkudra@nrcan.gc.ca
Excerpt:
Foaming of liquid and semi-liquid materials has long been recognized as one of the methods to shorten drying time. Over the past decade, this relatively old technology, known as foam mat drying, received renewed attention because of its added ability to process hard-to-dry materials, obtain products of desired properties (e.g., favourable rehydration, controlled density), and retain volatiles that otherwise would be lost during the drying of non-foamed materials.
Energy consumption
It is commonly assumed that in conventional dryers heat supplied to the dryer is used for moisture evaporation, heating of the wet material and the dryer, as well as for compensation of heat losses. However, drying of a large number of materials, especially food products, involves energy not only for free water evaporation but also for capillary-bound water removal, heating of the wet material as its temperature changes in the course of drying, local superheating of the vapor, and overheating of the already dried layers of the material otherwise necessary to maintain the required temperature gradient and thus sufficient heat transfer rates. Therefore, the energy efficiency defined as the ratio of heat for moisture evaporation to total heat supplied to the dryer is a function of the material properties (e.g. porosity, hygroscopicity, size and shape, moisture bonding), dryer design (type, configuration, mode of heating, heat recovery, etc.), operating parameters (drying temperature, recycle ratio, fractional saturation, gas flow rate), and initial/final moisture content.
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Source: http://engrwww.usask.ca/oldsite/societies/csae/protectedpapers/c0621.pdf 3/2007;