Process for the temperature control of a drying apparatus for tobacco leaves

In a cut tobacco leaves drying apparatus having a rotary hollow cylinder around which a plurality of heater means are mounted and in which the cut tobacco leaves are dried while they are carried along the rotational axis of said cylinder toward the exit of said cylinder during the rotation of the cylinder, the process for the temperature control of the drying apparatus comprises the steps of

• (A)
• (a) disposing the heater means along the direction of movement of said cut tobacco leaves, in order beginning with a first heater means at a position closest to the exit of the cylinder, each heater means heating said cylinder at a position around which said each heater means is mounted thereby defining a drying-space in which the cut tobacco leaves are dried while they are carried and the amount of heat medium supplied to each heater means being adjusted individually by a heat medium adjusting means;
• (b) mounting, at a place forward to the entrance of said cylinder, a flow rate meter for measuring the flow rate (Fo) of the cut tobacco leaves and a first moisture meter for measuring the moisture rate (.omega.1) of the cut tobacco leaves before drying;
• (c) mounting a second moisture meter at the exit of said cylinder for measuring the moisture rate (.omega.2) of the cut tobacco leaves after they are dried;
• (d) mounting a thermometer in each one of said drying-spaces for measuring the temperature therein;
• (B) heating said drying-spaces to a first predetermined temperature (To), when operating the drying apparatus, prior to the drying process of the cut tobacco leaves;
• (C) defining said process for the temperature control of each one of said drying-space by three consecutive states, state I, state II, and state III;
• said state I being a period between the detection of the cut tobacco leaves flowing toward the first drying-space and a start of heating each drying-space to second predetermined temperatures (Tci), said state II being a period between said start of heating each drying-space to said second predetermined temperature and change in temperature in response to it, and said state III being a period during which the cut tobacco leaves are dried in each drying-space;
• said state III being further subdivided into two consecutive periods, an unsteady period during which the flow rate of the cut tobacco leaves at the exit of the drying apparatus has not reached its steady value yet, and a steady period during which the flow rate of the tobacco leaves at the exit of the drying apparatus has reached its steady value;
• and said state II starting in order beginning from the first drying-space followed by succeeding drying-spaces with a predetermined waiting time allowed before subsequent drying-space is started.
• (D) heating each one of said drying-spaces toward its corresponding said second predetermined temperatures (Tci) by allowing said heater means to begin heating at the end of said state I;
• (E) determining temperatures (Tao) of each one of said drying-spaces in said steady period on the basis of
• (a) the measured flow rate (Fo) and the moisture rate (.omega.1) of the cut tobacco leaves flowing into a first drying-space of said drying-spaces;
• and
• (b) a temperature (.alpha.) required per unit flow rate and a temperature (.beta.) required per unit moisture rate, both of which being proper to the cut tobacco leaves to be dried;
• said temperature (Tao) being selected such that said temperature (Tao) decreasesin discrete steps toward the exit of said hollow cylinder;
• (F) determining target temperatures (Tseti) of each one of said drying-spaces required at time t in said unsteady period on the basis of
• (a) said Tao;
• (b) the time constant (Tfi) of flow rate characteristics of each one of said drying-spaces;
• and
• (c) the time constant (Thi) of heat transfer characteristics of each one of said drying-spaces;
• said target temperature being a temperature toward which actual temperature (Ti) of each drying-space at said time t will rise;
• (G) outputting first temperature-adjusting signals (Mfi) to the corresponding heat medium adjusting means after computing said first temperature-adjusting signals (Mfi) through proportional, integral and derivative (PID) operation on the basis of
• (a) said target temperatures (Tseti);
• and
• (b) said actual temperatures (Ti) of each one of said drying-spaces at said time t measured by said thermometers;
• (H) applying, during said state III, a feed-foward control in which the temperature is controlled by adjusting the amount of heat medium supplied to each one of said heater means with said first temperature-adjusting signal (Mfi) corresponding to each one of said drying-spaces;
• (I) applying, beginning at a predetermined time (t8) during said unsteady period, a feed-back control in which the temperature is controlled by adjusting the amount of heat medium supplied to said heater means with second temperature-adjusting signals (Mbi) determined through proportional, integral and derivative (PID) operation on the basis of the measured moisture rate (.omega.2) of the dried cut tobacco leaves coming out of said cylinder and a predetermined target moisture rate (.omega.*);
• and said feed-back control being applied to at least final drying-space subsequently to said-feed forward control.