Abstract
The mathematical investigation presented in this paper concerns the conveyor-belt dryer with tangential flow operating in co-current. This dryer is bigger than the continuous through-circulation conveyor dryer but has the advantage of better preserving the organoleptic and nutritional qualities of the dried product. In a previous work a mathematical modeling of the conveyor-belt dryer with tangential flow was carried out to offer guidelines for its optimized design. The last of those design guidelines indicated the need for an optimized adjustment of the dryer to ensure the constant maintenance of the final moisture content of the product. The fast and very precise measurement of the moisture content as the first step in the feedback chain was therefore necessary. Considering the difficulty of this type of measurement, two specific ordinary differential equations (ODEs) were obtained with the mathematical investigation of this work. Their solution became a relationship between the final moisture content of the product, the outlet air temperature, and other quantities that could be easily implemented in an automatic dryer control system. Therefore, the fast and accurate and much less expensive measurement of the temperature of the air leaving the dryer, owing to the relationship found, replaces the measurement of moisture content for the adjustment system. The experimental verification of the relationship highlighted the need to introduce a modification by which the relationship was finally validated.
Highlights
Food products are dried to reduce water activity [1,2] within safe limits
Its mass flow rate GAI was independent of the air temperature at the input of the dryer, but the air speed vAI was instead different as the air density was different
Equation (11) is satisfied with the values of XF calculated with (10), but it is no longer satisfied if we introduce the experimental values of Table 3
Summary
Food products are dried to reduce water activity [1,2] within safe limits. In this way, food can be stored at room temperature, reducing storage costs [3,4,5,6]. The contact of the product with hot and dry air leads to an increase in temperature with the risk of loss of nutritional qualities [7,8]. This loss of quality is acceptable because during drying, the product assumes the wet bulb temperature which is much lower than the dry bulb temperature of the air If this is 120 ◦C, the wet bulb temperature is about 38 ◦C as shown in a psychrometric chart. This phenomenon is possible if the product remains above the critical moisture content XC until the end of dryer where the moisture content is XF. This is an assumption imposed in this work, that is XF > XC
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