Studies on the Bulk Curing of Tobacco Leaves (I)
On Application to Bulk Curing of Tobacco Leaves for the Solid Drying Theory
1965 Volume 27 Issue 2 Pages 116-120
Details
1965 Volume 27 Issue 2 Pages 116-120
The authors analyzed the changes of moisture content of tobacco leaves in practical bulk curing unit making use of the solid drying theory, and found the curing process of tobacco leaves are formed of the material heating, the constant rate and the first and the second falling rate period in turn. As the value of drying rate, critical moisture contents, coefficient of surface evaporation, film coefficient of air in the above-mentioned four periods are certainly requried to theoritical analysis on this process, the authors constructed experimental curing burn in small scale adjusting temperature, humidity and velocity of air, and investigated the above-mentioned values. Results obtained were as follows.
(1) The period of constant rate drying mixes with the material heating period, and the value of drying rate in these periods is 1-2×10-3kg/hr·m2 (where kg: moisture evaporated from leaves, m2: leaf areaes of two sides). When this value increased to 6×10-3kg/hr, m2, these periods finish. These periods are just equall to the middle of color fixed stage of tobacco leaves. Then after that, the first (the value of drying rate in this period is 2-3×10-3kg/hr·m2) and the second falling rate period go on to the end of drying without a break.
(2) Critical moisture content is generally consist between the period of constant rate' drying and falling rate drying. On tobacco leaves, this value is about 200-300% of dry basis moisture content.
(3) In the semi-constant rate period, the value of coefficient of surface evaporation (α) is 0.3-0.4×10-3kg/hr·m2. mmHg (where mmHg: difference between vapoure of air and saturated vapour pressure for leaf temperature). This period is just equall from the yellowing stage to the early color fixed stage of tobacco leaves. But this value gradually increases to 0.6×10-3kg/hr·m2. mmHg, and decreases again to 0.3-0.4×10-3kg/hr·m2. mmHg in the falling rate period.
(4) In the early stage of first falling rate, the film coefficient of air (α′) is about 1kg*/hr·m2 (where kg*: weight of dry air), but in the end of the stage this value decreases to 0.1-0.3kg*/hr·m2. This period is just equall after the middle of color fixed stage of tobacco leaves.
