US3760816A

US3760816A - Method and apparatus for conditioning tobacco

Info

Publication number: US3760816A
Application number: US00134004A
Authority: US
Inventors: W Wochnowski
Original assignee: Hauni Werke Koerber and Co KG
Current assignee: Koerber AG
Priority date: 1970-04-22
Filing date: 1971-04-14
Publication date: 1973-09-25
Anticipated expiration: 1990-09-25
Also published as: DE2019369C3; CA947607A; JPS5722544B1; GB1345175A; DE2019369A1; DE2019369B2

Abstract

The moisture content of a tobacco stream which contains a fixed quantity of tobacco per unit length is reduced during flow through a revolving drum which is heated by steam, oil or gaseous combustion products. At the same time, the tobacco stream exchanges heat with hot air which is conveyed through the drum counter to the direction of tobacco movement. The tobacco is thereupon cooled and its moisture content is measured to automatically adjust the temperature of heated air and the temperature of the drum when the measured moisture content deviates from a desired moisture content. The heating device for the drum is indirectly adjusted by a thermometer which measures the temperature of heated air. The regulating unit for the air heating device is directly connected with the unit which measures the moisture content of conditioned tobacco.

Description

United States Patent Wochnowski METHOD AND APPARATUS FOR CONDITIONING TOBACCO [75] Inventor: Waldemar Wochnowski, Hamburg,

Germany [73] Assignee: Hauni-Werke Korber & C0., KG,

Hamburg, Germany [22] Filed: Apr. 14, 1971 [21] Appl. No.: 134,004

[30] Foreign Application Priority Data Apr. 22, 1970 Germany P 20 19 369.5

[52] US. Cl 131/140 R, 34/31, 34/48, 131/135 [51] Int. Cl A24 b 09/00, F26b 03/04 [58] Field of Search 131/140 R, 135, 136, 131/137; 34/46, 31, 48

[56] References Cited UNITED STATES PATENTS 3,386,447 6/1968 Wochnowski.... l3l/l40 R X 3,429,317 2/1969 Koch et a1 1 131/140 R 2,827,058 3/1958 Bogaty 131/108 Primary Examiner-Robert W. Michell Assistant Examiner-.Iohn F. Pitrelli Att0rneyMichael S. Striker [57] ABSTRACT The moisture content of a tobacco stream which contains a fixed quantity of tobacco per unit length is re duced during flow through a revolving drum which is heated by steam, oil or gaseous combustion products. At the same time, the tobacco stream exchanges heat with hot air which is conveyed through the drum counter to the direction of tobacco movement, The tobacco is thereupon cooled and its moisture content is measured to automatically adjust the temperature of heated air and the temperature of the drum when the measured moisture content deviates from a desired moisture content. The heating device for the drum is indirectly adjusted by a thermometer which measures the temperature of heated air. The regulating unit for the air heating device is directly connected with the unit which measures the moisture content of conditioned tobacco.

20 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR CONDITIONING TOBACCO BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for conditioning tobacco, and more particularly to a method and apparatus for changing the moisture content of a tobacco stream. Still more particularly, the invention relates to improvements in a method and apparatus for reducing the moisture content of whole or comminuted tobacco leaves.

It is already known to reduce the moisture content of tobacco by conveying a tobacco stream through a revolving drum which is heated by steam and wherein the tobacco stream is heated by indirect exchange of heat with the medium which heats the drum. The inertia of the heating system for the drum is rather high; therefore, such drying apparatus normally further comprise means for conveying through the rotating drum a current of hot air which is thus caused to exchange heat with tobacco and whose temperature can be changed much more rapidly than the temperature of the drum. As a rule, the current of heated air is caused to flow through the drum in the same direction as the tobacco stream. Reference may be had to U.S. Pat. No. 3,372,488 to Koch et al., or to US. Pat. No. 3,386,448 to Wochnowski. In a conditioning apparatus of the type known as KLK and produced by I-Iauni-Werke, Kc'irber & Co. K.G., of Hamburg-Bergedorf, Western Germany, the moisture content of tobacco at the inlet of the drum is measured by a detector, preferably of the type disclosed in U.S,. Pat. No. 3,320,528 to Esenwein. Signals from the detector are transmitted to a regulating unit which can rapidly change the temperature of air as a function of the measured moisture content and as a function of the quantity of tobacco which is being fed into the drum per unit of time. A second detector measures the moisture content of dried tobacco and adjusts the temperature of the drum when the measured moisture content deviates from a desired moisture content. The air which flows concurrent with the tobacco stream can rapidly change the moisture content whereas the adjustment of heating device for the drum compensates for longer-lasting deviations of moisture content from the desired value.

Problems arise when the current of heated air is to be conveyed countercurrent to the movement of tobacco in the conditioning zone. Adjustments in air temperature as a function of the moisture content of incoming tobacco would be of little avail because the flow of air Additional problems'which arise in connection with 1 drying of tobacco by air which is conveyed counter to the direction of tobacco flow are attributable to the fact that (due to the nature of treated material) the temperature of air cannot be increased at will and also that the air cannot be conveyed at a relatively high rate such as would enable the air to entrain the particles counter to the desired direction of tobacco movement. Thus, the extent to which the heated air can influence the moisture content of tobacco in the conditioning zone is rather limited. On the other hand, drying of tobacco in a zone wherein the air flows counter to the direction of tobacco movement presents several advantages, such as elimination of a detector and continuous contacting of tobacco at the discharge end of the conditioning zone with freshly admitted hot air.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved method of reducing the moisture content of tobacco in a conditioning zone wherein a stream of hot gaseous fluid is caused to flow counter to the direction of tobacco movement. 7

Another object of the invention is to provide a method of reducing the moisture content of a tobacco stream to a desired value without necessitating a measurement of the moisture content upstream of the conditioning zone.

A further object of the invention is to provide an improved method of reducing the moisture content of tobacco in a conditioning zone wherein the treated material is heated indirectly by a first medium which heats the conveyor for tobacco and directly by a gaseous fluid which flows counter to the direction of tobacco movement.

An additional object of the invention is to provide a novel and improved tobacco drying apparatus which is capable of automatically adjusting its moisture expelling action when the moisture content of treated material deviates from a desired moisture content.

The method of the present invention comprises the.

steps of conveying a stream of moist tobacco in a predetermined direction through an elongated conditioning zone, subjecting tobacco in the conditioning zone to the indirect heating action of a first fluid (preferably by heating with steam or the like a conveyor which transports tobacco through the conditioning zone), subjecting tobacco in the conditioning zone to the direct heating action of a second fluid (preferably air) by conveying such fluid through the conditioning zone counter to the direction of tobacco transport whereby the moisture content of tobacco in such zone is reduced as a consequence of the direct and indirect heating actions, measuring the moisture content of tobacco downstream of the conditioning zone, comparing the result of the measurement with a predetermined (desired) moisture content, and adjusting the direct and indirect heating actions in response to deviations of the result of measurement from the predetermined value. The indirect heating action is preferably at least substantially uniform throughout the entire conditioning zone; this can be achieved by employing a tubular conveyor which surrounds the conditioning zone and is heated by coils for circulation of steam, oil or gaseous combustion products.

The adjusting step preferably comprises changing the temperature of the second fluid and changing the pres sure or the rate of circulation of the first fluid. In accordance with one presently preferred embodiment of the method, the adjustingstep comprises measuring the temperature of the second fluid, at least while such temperature changes as a function of deviations of the result of measurement of moisture content from the predetermined value, and changing the indirect heating thermometer which determines the temperature of the aforementioned conveyor), comparing the thus detected intensity of indirect heating action with a predetermined value, and changing the indirect heating action in response to deviations of the detected intensity from a desired intensity. Thus, the indirect heating action can be changed in dependency on changes in temperature of the second fluid as well as in dependency on deviations of the intensity of indirect heating action from a desired intensity.

In accordance with a more specific feature of the invention, the temperature of the second fluid can be changed in response to deviations of the detected intensity of indirect heating action from the desired intensity to normally maintain the second fluid at an average temperature which can be rapidly raised to compensate for sudden increases in moisture content or rapidly reduced to compensate for abrupt reductions of moisture content.

The indirect heating action can be changed or regulated in dependency on the changes in temperature of the second fluid and in dependency on at least one other factor, such as the pressure of the first fluid which is used to heat the aforementioned conveyor or the speed at which the temperature of the second fluid changes.

The method preferably further comprises the step of cooling the tobacco subsequent to heating and prior to measurement of its moisture content.

In accordance with still another feature of the improved method, moist tobacco is supplied to the conditioning zone at a rate which is at least substantially constant. This can be achieved by employing a weighing device which weighs successive unit lengths of moist tobacco and controls a drive which determines the rate at which moist tobacco is being withdrawn from a magazine or the like.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved conditioning apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a partly diagrammatic and partly elevational view of a conditioning apparatus which embodies the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus which is illustrated in the drawing comprises an adjustable tobacco feeding unit 1 which can supply a stream S of moist tobacco wherein each unit length contains the same quantity of material. The apparatus further comprises a drying or conditioning unit 2 wherein the stream S is subjected to a drying or moisture-reducing treatment, a cooling unit 3 wherein the treated (and thus heated) tobacco is cooled, and a measuring unit 4 which determines the moisture content of successive increments of the treated and cooled stream.

The adjustable feeding unit 1 comprises a magazine or hopper 7 which contains a supply of moist tobacco leaves and/or shreds and one side wall of which is formed by the upwardly moving stretch of an endless tobacco withdrawing conveyor here shown as a feed apron 6 which showers tobacco onto the upper stretch of an endless conveyor 8a forming part of a weighing device 8. The weighing device 8 produces signals indicating the weight of successive increments of the tobacco stream which is formed on the upper stretch of the conveyor 80, and such signals are utilized to regulate the speed of the apron 7 by means of a d-c motor 13. The operative connection between the weighing device 8 and the motor 13 comprises a transducer 9 which converts mechanical or pneumatic signals furnished by the weighing device 8 into electrical signals which are transmitted to a conventional signal comparing junction 10. At this junction, the signal from the transducer 9 is compared with a reference signal furnished by an adjustable potentiometer 14, and thejunction 10 produces a positive or a negative signal when the intensity of the signal furnished by the transducer 9 is less than or exceeds the intensity of signal from the potentiometer 14. The latter furnishes a signal which represents the desired quantity of tobacco per unit length of the stream S. The output signal from the junction 10 is amplified, first by a preamplifier l1 and thereupon by an operational amplifier 12, and is transmitted to the motor 13 which drives the lower pulley for the apron 6 at a greater or lesser speed, depending on the sign of the signal from the junction 10.

The drying or conditioning unit 2 comprises a hollow rotary tubular conveyor here shown as a drum 16 whose interior defines an elongated conditioning zone and which is mounted for rotation on guide rollers 17 and 18. An electric motor 19 drives a pinion 19a which meshes with a ring gear 19b of the drum 16 so that the latter is rotated when the circuit of the motor 19 is completed. The motor 19 is assumed to rotate the drum 16 at a constant speed. The axis of the drum 16 is slightly inclined in a downward direction, as considered in the direction of travel of the tobacco stream S, so as to insure that the stream automatically advances from the left-hand end or intake end toward and beyond the right-hand end or discharge end of the drum 16. The endless conveyor 8a of the weighing device 8 discharges tobacco into an inclined chute 20 which feeds such tobacco into the intake end of the'drum 16. This intake end is otherwise sealed by a hood 24a which is connected with an exhaust fan 24 serving to collect vapors and spent gaseous heating medium and to withdraw such fluids at the upstream end of the conditioning zone in the drum 16.

The tobacco which passes through the conditioning zone in the drum 16 is treated (heated) directly by a hot gaseous fluid (preferably air) which is admitted at the discharge end of the drum. The current of heated air flows countercurrent to the tobacco stream S and contacts all or nearly all particles of tobacco because the particles in the conditioning zone are subjected to an intensive agitating action by heating coils 26 which are mounted in and orbit about the axis of the drum 16 when the circuit of the motor 19 is completed. The heating coils 26 act not unlike blades or paddles by repeatedly lifting tobacco from the bottom part of the conditioning zone and by showering the lifted particles in the upper part of such zone. The source of hot gaseous fluid is a blower 22 whose intake end is connected with a conduit 22a containing at its inlet an electric resistance heater 23. The outlet of the blower 22 is connected with the discharge end of the drum 16 by means of a supply pipe 21.

A second hot gaseous fluid (preferably steam) is stored in compressed condition in a tank 27 or a similar source and is fed to and circulated through the heating coils 26 by means of a supply pipe 28 containing an adjustable valve 64 and discharging steam into a distributor 29. It is clear that the source 27 of steam can be replaced with a source of another fluid, such as hot gaseous combustion products of an internal combustion engine or air or oil which is heated by one or more burners or the like. The fluid (hereinafter called steam) which circulates through the coils 26 and is widthdrawn by way of the distributor 29 is caused to indirectly exchange heat with tobacco in the conditioning zone within the rotating drum 16. On the other hand, the hot gas'(hereinafter called air) which flows countercurrent to the tobacco stream S and is furnished by the supply pipe 21 is in direct heat exchanging contact with tobacco.

The apparatus further comprises a regulating unit 31 which serves to regulate the temperature of air admitted' by way of the supply pipe 21, and a regulating unit 51 which serves to regulate the intensity of heating action of steam furnished by the distributor 29, i.e., the temperature of the drum 16.

The regulating unit 31 comprises a potentiometer which constitutes a source of reference signals representing the desired moisture content of treated and cooled tobacco, a conventional junction 32 which receives signals from a detector (thermometer) 34 mounted in the supply pipe 21 and serving to produce signals which are indicative of the temperature of air flowing into the drum 16, and from another detector (thermometer) 55 which measures the temperature of the drum 16 (Le, the intensity of the indirect heating action of steam) and transmits signals which are indicative of such temperature. The

detectors

34, 55 respectively transmit signals to the corresponding inputs of the junction 32 by way of

amplifiers

36, 60. The junction 32 transmits signals to a second junction 33 of the regulating unit 31; the junction 33 further receives signals from the potentiometer 30 (by way of a third junction 35) and transmits signals, by way of conductor means 48, to two signal generating or energizing devices 37a, 37b of the regulating unit 31. The devices 37a, 37b respectively control relays 38a, 38b for a servomotor 41 which can pivot an adjustable flap or valve 42 in the aforementioned conduit 22a to thus bring about rapid changes in the temperature of air which enters the supply pipe 21. The flap 42 controls the admission of relatively cool atmospheric air into a mixing zone 43 in the interior of the conduit 22a. Such mixing zone receives a constant stream of air which enters the inlet of the conduit 22a and is heated by the electric heater 23. The parts. 21, 22, 22a, 23 constitute an adjustable heating device for air and the

parts

41, 42 constitute an adjusting device 39 for the heating device and hence a means for determining and changing the temperature of air which enters the supply pipe 21. The servomotor 41 is preferably a d-c motor which can rotate its output shaft in a first direction (in response to energization of one of the relays 38a, 38b) to thereby change the position of the flap 42 for the purpose of admitting more cool air into the mixing zone 43 and in a second direction (in response to energization of the other relay) to thereby move the valve 42 in a direction to reduce the inflow of cool air into the mixing zone 43.

The signal generating devices 37a, 37b respectively comprise voltage sources 44a, 44b for periodic transmission of sawtooth-shaped voltage pulses, pairs of resistors 46a, 46b, junctions a, 45b and operational amplifiers 47a, 47b. Depending on the polarity of voltage signals transmitted by the conductor means 48, the device 37a or 37b transmits to the respective relay 38a or 38b a signal which results in energization of the respective relay and in appropriate adjustment of the flap 42 by the servomotor 41. The duration of signals furnished by the devices 37a, 37b depends on the intensity of signals from the junction 33. The servomotor 41 remains in operation for a longer or shorter period of time, depending on the extent to which the position of the flap 42 must be adjusted in order to insure that the temperature of hot air in the supply pipe 21 will correspond to a desired temperature. The length of the aforementioned periods depends on the intensity of signal which is trnasmitted by the conductor means 48. The intensity of sawtooth-shaped signal furnished-by the source 44a or 44b rises gradually and the energization of relay 38a or 3812 is terminated when the intensity of the signal from 44a or 44b matches the intensity of signal from the junction 33. The signal from the junction 33 to the

junction

45a or 45b is compared with the signal from the source 44a or 44b, and the signal which represents the difference between the intensities of signals furnished to the inputs of the

junction

45a or 45b is thereupon amplified at 47a or 47b prior to transmission to the relay 38a or 38b. The intensity of signals furnished by the amplifiers 47a, 47b to the respective relays 38a, 38b is constant and the transmission of such signals is terminated when the signal from junction 33 to

junction

45a or 45b is matched by the signal from the source 44a or 44b. Thus, the servomotor 41 is automatically arrested when the junction 33 ceases to furnish a signal to the associated resistor 46a or 46b, namely, when the junction 32 furnishes to the corresponding input of the junction 33 a signal which matches the signal from the junction 35.

The regulating unit 51 for the temperature of the drum 16 comprises a junction 52 which is connected with the aforementioned detector 55 by way of the amplifier and with the detector 34 by way of the amplifier 36. The'junction 52 transmits signals to a second junction 53 which is further connected with a source of reference signals here shown as an adjustable potentiometer 54. The output of the junction 53 is connected with a junction 56 which is further connected with a signal-generating pressure measuring gauge 57 in the supply pipe 28 by means of an amplifier 58. The output of the junction 56 (which is analogous to the junction 33 of the regulating unit 31) is connected by conductor means 71 with two signal generating or energizing devices 59a, 59b. The gauge 57 transmits signals which are indicative of the pressure of steam in the supply pipe 28. The signal generating devices 59a, 59b respectively serve to energize two relays 61a, 61b for a d-c servomotor 63 (corresponding to the motor 41 of the regulating unit 31) and serving to adjust the aforementioned valve 64 in the supply pipe 28 which admits steam from the source 27 to the distributor 29. The

parts

26, 27, 28, 29 constitute an adjustable heating device for the drum l6 and the motor 63 and the valve 64 constitute an adjusting device 62 which controls the heating device for the drum 16 by determining the rate of admisssion of steam into the coils 26 in dependency on the (positive or negative) sign of signals which are transmitted by the conductor means 71. As explained in connection with the relays 38a, 38b and servomotor 41, the relays 61a, 61b in the regulating unit 51 respectively serve to cause rotation of the motor 63 in opposite directions to thereby adjust the valve 64 in a direction to either increase or reduce the flow of steam into the distributor 29.

The signal generating devices 59a, 59b respectively comprise sources 66a, 66b of sawtooth-shaped voltage pulses, junctions 68a, 68b, pairs of resistors 67a, 67b and operational amplifiers 69a, 69b. The sources 66a, 66b transmit sawtooth-shaped pulses depending on the sign of the voltage signal which is transmitted by the conductor means 71. The relay 61a or 61b is energized when the intensity of signal furnished to the junction 68a or 68b by the conductor means 71 exceeds the intensity of the gradually rising signal from the soruce 660 or 66b. The sources 44a, 44b and 66a, 66b transmit signals periodically. The servomotor 63 adjusts the valve 64 as long as one of the relays 61a, 61b remains energized, namely, as long as the intensity of signal from the junction 56 to the junction 68a or 68b exceeds the intensity of signal from the source 660 or 66b.

The cooling unit 3 comprises an elongated pneumatic conveyor 77 which receives tobacco by way of a chute 76 at the discharge end of the drum 16. The discharge end of the conveyor 77 is. connected with a suction fan 78 which withdraws the gaseous carrier for tobacco. The tobacco is separated from air in a separator 79a and is withdrawn from the penumatic conveyor 77 by way of an air lock 79. The latter discharges the treated and cooled tobacco into a vibrating trough 82 which contains a moisture detector 81, preferably of the type disclosed in US. Pat. No. 3,320,528 to Esenwein. The detector 81 forms part of the measuring unit 4 which is preferably of the type known as HWK and is furnished by Hauni-Werke, Kr'a'rber & Co. KG, of Hamburg-Bergedorf, Western Germany. Reference may be had to US. Pat. No. 3,372,488 to Koch et al. The detector 81 is a capacitor having two electrodes which are installed in the trough 82 and are connected with an oscillator circuit 83. The latter is loosely coupled to a high frequency generator 84. The amplitudes of oscillations of the circuit 83 are indicative of the moisture content of tobacco issuing from the air lock 79. The signals from the oscillator circuit 83 are amplified by an amplifier 86 and are transmitted to the junction 35 in the regulating unit 31.

An endless conveyor 87 is provided to convey tobacco from the trough 82 to a further processing station.

A conductor 88 connects the regulating unit 31 with the regulating unit 51 and contains a differential circuit 89 and a threshold circuit 91. The latter is connected with the junction 56 by a conductor 92. The circuit 89 transmits to the junction 56 a signal which is indicative of the changes in intensity of signal transmitted from the detector 34 by the conductor 89. The circuit 91 permits such signal to reach the junction 56 only if the intensity of the signal exceeds a predetermined threshold value.

THE OPERATION The motor 13 is assumed to drive the apron 6 so that the latter withdraws from the magazine 7 a continuous stream S of moist tobacco which is showered onto the endless conveyor 8a of the weighing device 8. The moisture content of tobacco in the magazine 7 exceeds the desired moisture content as indicated by the signal from the potentiometer 30. The device 8 weighs the thus withdrawn moist tobacco and transmits signals to the transducer 9. The tobacco stream S enters the rotating heated drum 16 by way of the chute 20 and is agitated by the coils 26. The transducer 9 transmits an electric signal which is indicative of the weight of tobacco on the conveyor and the junction 10 compares such signal with the reference signal furnished by the potentiometer 14. If the weight of tobacco on the conveyor 8a exceeds or is less than the desired weight (reference signal from the potentiometer 14), the amplifiers 11, 12 receive a signal and cause the motor 13 to rotate at a different speed to thus cause the apron to withdraw a greater or lesser quantity of tobacco from the magazine 7. Thus, the drum 16 is assumed to receive equal quantities of tobacco per unit of time so that the conditioning unit 2 must perform the sole function of changing the moisture content of tobacco during travel through the conditioning zone.

That portion of the stream S which passes through the drum 16 is indirectly heated by steam which circulates in the coils 26. Such steam heats not only the coils 26 but also the cylindrical wall of the drum 16. In addition, tobacco in the conditioning zone is heated by air which is admitted by the supply pipe 21 and flows counter to the direction of tobacoo transport from the chute 20 toward the chute 76. The thus treated portions of the stream S are then admitted into the pneumatic conveyor 77 and are cooled during transport toward the air lock 79. The transport of tobacco in the conveyor 77 takes place in response to generation of an air stream which is produced by the suction fan 78.

The tobacco which is separated from air in the device 790 descends into the vibrating trough 82 and its moisture content is determined by the measuring unit 4, namely, by the capacitor which includes the electrodes 81 in the trough 82. The tobacco is then transferred onto the endless conveyor 87.

The measuring unit 4 produces an electric signal which is amplified at 86 and is transmitted to the corresponding input of the junction 35 in the regulating unit 31. The other input of the junction 35 receives a reference signal from the adjustable potentiometer 30; such reference signal is indicative of the desired moisture content of tobacco which leaves the conditioning apparatus.

If the intensity of signal furnished by the amplifier 86 does not match the intensity of reference signal from the potentiometer 30, the junction 35 transmits an appropriate signal to the junction 33. If the measured moisture content of tobacco in the trough 82 is too high, the regulating unit 31 causes the adjusting device 39 to reduce the inflow of cool air by way of the flap 42 so that the temperature of air in the supply conduit 21 rises and the tobacco in the conditioning zone is subjected to a more intensive heating action. The transmission of signal from the junction 33 to the signal generating or energizing device 37a or 37b is terminated when the intensity of signal from the junction 35 is matched by the intensity of signal from the detector 34, Le, when the detector 34 indicates that the temperature of air in the supply conduit has been increased or reduced to the extent which is necessary to conform the moisture content of tobacco in the trough 82 to the predetermined moisture content (reference signal from the potentiometer 30). The junction 33 receives from the junction 32 a signal which is indicative of the temperature of air in the supply pipe 21 as well as of the temperature of the drum 16 so that the adjustment of servomotor 41 for the flap 42 is terminated when the signal from the junction 35 to the junction 33 is matched by the signal from junction 32 which is indicative of the temperature of hot air as well as of temperature of the drum 16. As mentioned before, the temperature of the drum 16 is measured by the detector 55. The energization of relay 38a or 38b in the regulating unit 31 in response to a positive or negative signal from the junction 33 (by way of the conductor means 48) takes place in the aforedescribed manner. The adjustment is completed within a period determined by the source 44a or 44b which transmits signals periodically and for certain intervals of time. The length of the interval during which the relay 38a or 38b remains energized in order to cause the servomotor 41 to move the flap 42 toward a closed or toward a fully open position depends on the intensity of signal from the junction 33 to the device 37a or 37b. If the moisture content of tobacco in the trough 82 is excessive, the signal from the junction 33 causes one of the relays 38a, 38b to complete the circuit of the servomotor 41 in such a way that the flap.42 is moved toward the closed position to thus reduce the quantity of cool air which enters the mixing zone 43 downstream of the heater 23. The fully or nearly fully closed position of the flap 42 is indicated by a phantom line; in such position of the flap 42, the regulating unit 31 insures that the temperature of air in the supply pipe 21 is rather high to thus effect the expulsion of greater quantities of moisture from tobacco which is conveyed through the conditioning zone. The regulating unit 31 reacts very rapidly so that the deviation of moisture content of tobacco leaving the conditioning apparatus from a desired moisture content is of short duration.

The signal which is furnished by the detector 34 is further transmitted to the junction 52 by way of the conductor 88. The junction 52 computes such signal with the signal from the detector 55 and transmits to the junction 53 a signal which is indicative of the sum of signals from the

detectors

34 and 55. If such signal does not match the signal from the adjustable potentiometer 54, the junction 53 transmits an appropriate signal to the junction 56. The latter further receives a signal from the gauge 57 which measures the pressure of steam in the supply pipe 28. Depending on the positive or negative sign of the signal from the junction 56, the signal generating device 59a or 59b energizes the relay 61a or 61b to thus enable the servomotor 63 to increase or to reduce the admission of steam from the source 27 into the coils 26 and to thereby raise or reduce the temperature of the drum 16. Thus, the adjusting device 62 determines the pressure of steam which is circulated in thecoils 26. The arrangement is such that, when the conductor 88 transmits from the detector 34 a signal which indicates that the temperature of air in the supply pipe 21 is on the increase, the regulating unit 51 raises the temperature of the drum 16 to thus insure a rapid reaction of the conditioning apparatus in the event that the moisture content of tobacco in the trough 82 is too high. Analogously, when the signal from the detector 34 indicates that the temperature of air in the supply pipe 21 is on the decrease, the regulating unit 51 reduces the pressure of steam in the coils 26 to thus reduce the temperature of the drum 16. The servomotor 63 can be designed to adjust the valve 64 in stepwise fashion or continuously during the interval of energization of the relay 610 or 61b.

The inertia of the heating device which includes the coils 26 is greater than the inertia of the heating device which includes the blower 22, i.e., an adjustment in the position of the flap 42 by the servomotor 41 is followed by a change in temperature of air issuing from the supply pipe 21 practically without any delay. The reaction of the heating device 26-29 is slower. Therefore, once the regulating unit 51 has started the adjustment of the valve 64 in a direction to either increase or reduce the temperature of the drum 16, the flap 42 is preferably returned to the illustrated intermediate or neutral position so that it can again cause a practically instantaneous reaction of the air heating device 21,22, 22a 23 if the moisture content of tobacco in the trough 82 deviates from the desired moisture content. In the neutral position of the flap 42, the regulating unit 31 is in a condition to rapidly shut off the inflow of cool air into the mixing zone 43 or to increase the inflow of cool air to a maximum value.

The return movement of flap 42 to the illustrated neutral position is effected by a signal from the detector 55 to the junction 32. When the temperature of the drum 16 reaches the desired value, i.e., when the heating action of the drum l6'and coils 26 can insure that the moisture content of tobacco leaving the pneumatic conveyor 77 is within the desired range, the junction 33 transmits a signal which causes the device 370 or 37b of the regulating unit 31 to energize the relay 38a or 38b so as to return the flap 42 to the illustrated neutral position. The detector 34 then furnishes a different signal (because the temperature of air in the supply pipe 21 either rises or decreases, depending upon whether the flap 42 has been moved to the neutral position from an open position or from a closed position) which is transmitted to the junction 52 and thence to the junction 53. The junction 53 furnishes a signal which is a reference signal for the desired'position of the valve 64 (junction 56).

The circuits 89 and 91 are provided for the purpose of enabling the apparatus to rapidly react to sudden and substantial changes in moisture content of tobacco which is caused to advance in the trough 82. The signal from the detector 34 is then transmitted to the junction 56 (by way of the conductors 88, 92 and circuits 89, 91) so that the signal from the junction 56 to the signal generating device 59a or 59b of the regulating unit 51 is intensified with the attendant rapid change in the position of valve 64. The signal from the threshold circuit 91 disappears as soon as its intensity decreases below a predetermined value.

As mentioned before, the steam is but one of heating fluids which can be circulated in the coils 26 to heat the drum 16 and to indirectly heat tobacco in the conditioning zone. The coils 26 can be used for circulation of gaseous combustion products, oil or another suitable gaseous or liquid fluid.

The

heating device

21, 22, 22a, 23 can react, almost without any delay, to signals from the junction 33 of the regulating unit 31 to rapidly change the heating action of air in response to actuation of the adjusting means 39. The heating device 26-29 for the drum 16 is slower (i.e., its inertia is higher); therefore, the rapid change in heating action of air which flows countercurrent to the tobacco stream in the conditioning zone is accompanied by a longer-lasting change in the heating action of the drum. Once the heating action of the drum 16 has been altered sufficiently to insure that tobacco issuing from the conditioning zone will haVe a desired moisture content while the valve 42 dwells in the illustrated intermediate (half open) position, the signal from the junction 32 to junction 33 causes the regulating unit 31 to actuate its adjusting means 39 in order to reset the flap 42 to the position which is shown by the solid line.

The circuit 89 insures that the valve 64 is adjusted to a greater extent in response to abrupt changes in the temperature of air flowing through the supply pipe 21, i.e., in response to a substantial adjustment of the flap 42 when the measuring unit 4 indicates that the moisture content of tobacco leaving the drum 16 has suddenly changed and deviates considerably from the desired moisture content as indicated by the signal from the potentiometer 30. This can take place when the magazine 7 receives a fresh batch of tobacco with a moisture content which is much higher than or well below the moisture content of the preceding batch. When the fluctuations of the measured moisture content are less pronounced, signals which reach the junction 56 from the junction 53 and gauge 57 (without a signal from the circuit 89) suffice to effect appropriate adjustments of the heating device 26-29.

It will be noted that the measuring unit 4 is directly connected with the regulating unit 31 and that such measuring unit is indirectly connected with the regulating unit 51 by way of the detector 34 of the unit 31. Thus, the

heating device

21, 22, 22a, 23 responds almost immediately to all detected changes in moisutre content of the treated and cooled tobacco stream whereas the heating device 26-29 responds indirectly by changing the heating action upon the drum 16 in response to changes in the temperature of air flowing through the supply pipe 21. Such response is intensified by differentiating the signal from the detector 34 to the junction 56 for the signal generating devices 59a, 59b of the regulating unit 51.

The signal from the detector 55 to the junction 32 insures that the adjustment of the flap 42 (and hence the temperature of air in the supply pipe 21) is influenced by the temperature of the drum 16. Such adjustment is terminated when the drum 16, together with air whose temperature corresponds to that at the intermediate setting of the flap 42, can insure that the moisture content of treated tobacco is within the desired range. In such position of the flap 42, the adjusting device 39 is ready to bring about substantial changes in the temperature of air in the supply pipe 21 (by moving the flap 42 to the fully open or to the fully closed position). Cooling of tobacco subsequent to conditioning insures that signals furnished by the measuring unit 4 accurately reflect the moisture content of conditioned tobacco without such distortions which would occur if the unit 4 were to measure the moisture content of tobacco at different temperatures. The weighing device 8 contributes to a more uniform conditioning action by insuring that each unit length of the stream S contains the same quantity of tobacco. Thus, as a rule, any fluctuations in the moisture content of treated tobacco are attributable to a single factor, namely, to fluctuations in moisture content of tobacco in the magazine 7.

An important advantage of the improved conditioning apparatus is that it insures rapid changes in moisture content even though the air is caused to flow countercurrent to movement of tobacco in the conditioning zone.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended 1. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to indirect heating action by heating the conveyor; subjecting tobacco in said conditioning zone to the direct heating action ofa fluid by conveying said fluid through said conditioning zone counter to said direction with attendant reduction of the moisture content of tobacco as a consequence of said direct and indirect heating actions; measuring the moisture content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value; and adjusting one of said heating actions in dependency upon the other of said heating actions.

2. A method as defined in claim 1, wherein said indirect heating action is substantially uniform throughout said conditioning zone.

3. A method as defined in claim 1, wherein said first mentioned adjusting step comprises changing the temperature of said fluid.

4. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to the indirect heating action of a first fluid by utilizing the first fluid to heat the conveyor; subjecting tobacco in said conditioning zone to the direct heating action ofa second fluid by conveying the second fluid through said conditioning zone counter to said direction with attendant reduction of the moisture content of tobacco as a consequence of said direct and indirect heating actions; measuring the moisture'content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; and automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value, said adjusting step comprising changing the temperature of said second fluid, measuring the temperature of said second fluid, and changing said indirect heating action in response to changes in the temperature of said second fluid.

5. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to the indirect heating action of a first fluid by utilizing the first fluid to heat the conveyor; subjecting tobacco in said conditioning zone to the direct heating action of a second fluid by conveying the second fluid through said conditioning zone counter to said direction with attendant reduction of themoisture content of tobacco as a consequence of said direct and indirect heating actions; measuring the moisture content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value, including changing the temperature of said second fluid; measuring the intensity of said indirect heating action; producing first and second signals respectively indicating the temperature of said second fluid and the intensity of said indirect heating action; and changing the intensity of said indirect heating action in accordance with said signals.

6. A method as defined in claim 5, wherein said adjusting step further comprises changing the temperature of said second fluid in accordance with said second signal.

7. A method as defined in claim 4, wherein said indirect heating action is changed in dependency on changes in temperature of said second fluid as a function of time.

8. A method as defined in claim 1, further comprising the step of cooling the tobacco which issues from said conditioning zone prior to said measuring step.

9. A method as defined in claim 1, further comprising the step of supplying to said conditioning zone moist tobacco at an at least substantially constant rate.

10. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby such stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value and for adjusting the heating action of one of said heating means in dependency upon the heating action of the other of said heating means.

11. Apparatus as defined in claim 10, wherein said conveyor comprises a tubular member which surrounds said conditioning zone and said first heating means is arranged to heat said tubular member, said tubular member having an intake end for admission of tobacco and a discharge end for evacuation of tobacco and for admission of said hot fluid.

12. Apparatus as defined in claim 10, wherein said regulating means comprises discrete first and second regulating units for said first and second heating means and said second regulating unit comprises adjusting means for changing the'temperature of said fluid as a function of said deviations.

13. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone;

feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone;

and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means, said measuring means being connected with said second regulating unit to effect changes in the temperature of said fluid in response to said deviations and said second regulating unit comprising detector means for producing signals indicating the temperature of said fluid, said first regulating unit being responsive to said signals to adjust said first heating means as a function of changes in the temperature of said fluid.

14. Apparatus for conditioning tobacco, comprising a conveyor defining a elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moistureexpelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said conditioning zone to a second heating and moistureexpelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from saidconditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means and said first and second regulating units respectively comprising first and second detector means for producing first and second signals respectively indicating the temperature of said conveyor and the temperature of said fluid, said first regulating unit further comprising adjusting means for adjusting said first heating means to thereby change the temperature of said conveyor in response to said signals, said second regulating unit including adjusting means for changing the temperature of said fluid in response to said deviations.

15. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone; feeding means for delivering to saidconveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moistureexpelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said conditioning zone to a second heating and moistureexpelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means and said first and second regulating units respectively comprising first and second detectors for producing signals respectively indicating the temperature of said conveyor and the temperature of said fluid, said second regulating unit further comprising adjusting means for changing the temperature of said fluid as a function of signals from said first detector and in response to said deviations.

16. Apparatus as defined in claim 10, wherein said feeding means includes a measuring device for admitting to said conveyor tobacco at a constant rate.

17. Apparatus as defined in claim 16, wherein said measuring device comprises means for weighing unit lengths of said tobacco stream, an adjustable second conveyor for supplying tobacco to said weighing means, and means for adjusting said second conveyor in response to deviations of the measured quantity of tobacco from a predetermined quantity.

18. Apparatus as defined in claim 10, further comprising means for cooling the tobacco which issues from said conditioning zone.

19. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone;

feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moistureexpelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating menas and said second unit comprising detector means arranged to produce signals indicating the temperature of said fluid, said second regulating unit further comprising adjusting means for changing the temperature of said fluid in response to said deviations; and means including a differential circuit for transmitting to said first regulating unit signals from said detector means for adjustment of said first heating means in dependency on changes in temperature of said fluid as a function of time.

20. Apparatus as defined in claim 19, further comprising means for controlling the transmission of signals from said circuit to said first regulating means so that the latter receives signals only when such signals indicate sudden changes in the temperature of said fluid.

Claims

1. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to indirect heating action by heating the conveyor; subjecting tobacco in said conditioning zone to the direct heating action of a fluid by conveying said fluid through said conditioning zone counter to said direction with attendant reduction of the moisture content of tobacco as a consequence of said direct and indirect heating actions; measuring the moisture content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value; and adjusting one of said heating actions in dependency upon the other of said heating actions.

4. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to the indirect heating action of a first fluid by utilizing the first fluid to heat the conveyor; subjecting tobacco in said conditioning zone to the direct heating action of a second fluid by conveying the second fluid through said conditioning zone counter to said direction with attendant reduction of the moisture content of tobacco as a consequence of said direct and indirect heating actions; measuring the moisture content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; and automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value, said adjusting step comprising changing the temperature of said second fluid, measuring the temperature of said second fluid, and changing said indirect heating action in response to changes in the temperature of said second fluid.

5. A method of conditioning tobacco, comprising the steps of conveying a stream of moist tobacco on a conveyor in a predetermined direction through an elongated conditioning zone; subjecting tobacco in said conditioning zone to the indirect heating action of a first fluid by utilizing the first fluid to heat the conveyor; subjecting tobacco in said conditioning zone to the direct heating action of a second fluid by conveying the second fluid through said conditioning zone counter to said direction with attendant reduction of the moisture content of tobacco as a consequence of said direct and indireCt heating actions; measuring the moisture content of tobacco downstream of said conditioning zone; comparing the result of such measurement with a predetermined value; automatically adjusting said direct and indirect heating actions in response to deviations of said result from said predetermined value, including changing the temperature of said second fluid; measuring the intensity of said indirect heating action; producing first and second signals respectively indicating the temperature of said second fluid and the intensity of said indirect heating action; and changing the intensity of said indirect heating action in accordance with said signals.

12. Apparatus as defined in claim 10, wherein said regulating means comprises discrete first and second regulating units for said first and second heating means and said second regulating unit comprises adjusting means for changing the temperature of said fluid as a function of said deviations.

13. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said meAsuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means, said measuring means being connected with said second regulating unit to effect changes in the temperature of said fluid in response to said deviations and said second regulating unit comprising detector means for producing signals indicating the temperature of said fluid, said first regulating unit being responsive to said signals to adjust said first heating means as a function of changes in the temperature of said fluid.

14. Apparatus for conditioning tobacco, comprising a conveyor defining a elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said conditioning zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means and said first and second regulating units respectively comprising first and second detector means for producing first and second signals respectively indicating the temperature of said conveyor and the temperature of said fluid, said first regulating unit further comprising adjusting means for adjusting said first heating means to thereby change the temperature of said conveyor in response to said signals, said second regulating unit including adjusting means for changing the temperature of said fluid in response to said deviations.

15. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said conditioning zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating means and said first and second regulating units respectively comprising first and second detectors for producing signals respectively indicating the temperature of said conveyor and the temperature of said fluid, said second regulating unit further comprising adjusting means for changing the temperature of said fluid as a function of signals from said first detector and in response to said deviations.

19. Apparatus for conditioning tobacco, comprising a conveyor defining an elongated conditioning zone; feeding means for delivering to said conveyor a stream of moist tobacco whereby said stream advances in a predetermined direction through and beyond said conditioning zone; first adjustable heating means for heating said conveyor to thereby subject tobacco in said conditioning zone to a first heating and moisture-expelling action; second adjustable heating means for conveying through said conditioning zone a hot fluid counter to said direction to thus subject tobacco in said zone to a second heating and moisture-expelling action; measuring means for determining the moisture content of tobacco after the tobacco issues from said conditioning zone; and regulating means connected with said measuring means and operative to adjust said first and second heating means in response to deviations of that moisture content which is detected by said measuring means from a predetermined value, said regulating means comprising discrete first and second regulating units for said first and second heating menas and said second unit comprising detector means arranged to produce signals indicating the temperature of said fluid, said second regulating unit further comprising adjusting means for changing the temperature of said fluid in response to said deviations; and means including a differential circuit for transmitting to said first regulating unit signals from said detector means for adjustment of said first heating means in dependency on changes in temperature of said fluid as a function of time.