The present invention relates to a method and apparatus for conditioning tobacco, especially burley or greenleaf tobacco. More particularly, the invention relates to improvements in methods and apparatus for reducing the moisture content of tobacco to a desired value which is best suited for further processing. Still more particularly, the invention relates to improvements in methods and apparatus for reducing the moisture content from a relatively high value to a small fraction of such value while the material to be conditioned is conveyed in the form of a continuous stream.
It is already known to regulate the conditioning action upon burley tobacco by ascertaining its temperature immediately downstream of a zone where the tobacco is subjected to a very pronounced heating action and by altering the rate of admission of heat energy in dependency on deviations of ascertained temperature from a desired value. The temperature of tobacco is ascertained indirectly, namely by comparing the temperature of a stream of hot air prior and subsequent to exchange of heat energy with tobacco. The apparatus for carrying out such measurements includes a first detector serving to monitor the temperature of hot air which is about to contact the tobacco particles and a second detector which monitors the temperature of such air subsequent to heat energy exchange with tobacco. The just described apparatus exhibits the advantage that deviations, if any, from desired moisture content are detected practically immediately so that the percentage of improperly dried burley tobacco is low. A drawback of the apparatus is that the temperature of tobacco is ascertained in a roundabout way which is evidently less reliable than direct measurements. It can be said that such conventional apparatus is sufficiently reliable to insure that the final moisture content will not deviate appreciably from the desired moisture content; however, the apparatus cannot always insure that the final moisture content will be within a rather narrow optimum range. Such accurate drying is desirable because heated tobacco is thereupon cooled and is admitted into a moisturizing unit wherein the moisture content rises to a value which is best suited for further processing. Accurate and predictable changes of moisture content from the relatively low moisture content after drying to the optimum moisture content can be effected (or are less complex and expensive) only if the heating action has resulted in a reduction of moisture content to a predetermined value or is within a narrow range including such predetermined value.