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Article | REF: J2453 V1

Industrial drying: principles and calculation of machines

Author: Jean VASSEUR

Publication date: March 10, 2011

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ABSTRACT

This article presents two methods of drying other than by hot air. Drying by conduction is a process where the heat is brought to the product by conduction through a heated wall and then by conduction into the product. This technique offers many opportunities to save energy. Drying by overheated water vapour replaces the water circulating around the product by overheated quasi-pure water. This dry gas brings to the product the vaporization energy of drying. The article focuses on the fundamental difference between these two methods of drying "by boiling" and "convective" drying by taking the example of conduction drying. For each type of drying, this article presents the laws of transfer, the various hydro-thermal stages to which the product is subjected during the drying process as well as the ways to pilot fluxes.

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AUTHOR

  • Jean VASSEUR: GREF engineer - Doctor Engineer - Professor at AgroParisTech-MASSY (formerly ENSIA) - UMR 1145 – Food Process Engineering

 INTRODUCTION

The previous articles [J 2 451] and [J 2 452] presented "hot-air drying", its industrial applications and energy-saving techniques, for the common case where hot air provides the product with the energy to vaporize water, via convective air/product transfers. The following two articles [J 2 453] and [J 2 454] describe "other drying modes than hot air", where the air circulating around the product is generally colder than the product, and where it is not the air that provides the heat useful for vaporizing the water, but other heating modes, with specific advantages and disadvantages that will be described. In particular, it will be demonstrated that, in certain cases, energy consumption can be reduced by a factor of 2 to 15 compared with hot-air drying, by changing the drying mode, which is of the utmost industrial interest. Articles [J 2 453] and [J 2 454] are classified according to the type of heat input.

In [J 2 453] (this article):

  • conduction drying: the product is brought into contact with a wall heated to the hot-source temperature θsch (°C) at the surface, and heat is transferred to the product by conduction through this wall, then by conduction into the product. Depending on the conditions, in particular the plate temperature in relation to the pressure p ...

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