Process for the temperature control of a drying apparatus

A process for controlling the temperature of a drying apparatus including a cylindrical rotor having a plurality of heating means which are independent of each other and arrayed in an advance direction of raw material, the process comprises controlling the heating means in response to measurements of the flow rate and moisture content of the raw material charged into the rotor and measurements of temperatures of respective sections of the rotor, each section corresponding to each heating means so that the temperature of each section is changed in accordance with a flow rate characteristics curve of each section and controlling the heating means so that a bias temperature for compensating for a lag in thermal response dead time of each heating means is applied prior to changing the temperature of each section in accordance with a flow rate characteristics curve by a feed back controlling at least a final heating means in response to the measurement of the moisture content of the dried raw material discharged from the rotor.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for temperature control, and in 
particular to a process for the temperature control of a drying apparatus 
in which the raw material which has been charged into the entrance thereof 
is dried so that the moisture rate of the raw material is kept constant 
and is discharged from the exit thereof. 
For example, when cut tobacco leaves are dried, it is desirable that the 
finished product possess a predetermined moisture. 
The period of time after the raw material cut tobacco leaves is charged 
into the drying apparatus until the amount of the raw material held at 
each part of the drying apparatus is stabilized to a substantially 
constant state, that is, the flow rate of the raw material at the exit of 
the drying apparatus is stabilized is referred to rise-up time or unsteady 
time, which differs from the subsequent period referred to as stable time 
or steady time. 
If a similer temperature control is carried out during both periods, drying 
would be excessive at rise-up time and final product would not possess the 
desired moisture. For example if the period of the rise-up time is 10 to 
15 minutes in a drying apparatus into which raw material is supplied at a 
flow rate of 6000 kg/h, there is a possibility of production of 50 to 100 
kg of undesired product. 
SUMMARY OF THE INVENTION 
The present invention was made for overcoming the problem of the prior art. 
It is an object of the present invention to provide a process for 
controlling the temperature of a drying apparatus. 
In one aspect of the present invention there is provided a process for 
controlling the temperature of a drying apparatus including a cylindrical 
rotor having a plurality of heating means which are independent of each 
other and arrayed in an advance direction of raw material, the process 
comprising controlling the heating means in response to the measurements 
of a flow rate and a moisture content of the raw material charged into the 
rotor and measurements of the temperatures of respective sections of the 
rotor, each section corresponding to each heating means so that the 
temperature of each section is changed in accordance with a flow rate 
characteristics curve of each section and controlling the heating means so 
that a bias temperature for compensating for a lag in thermal response of 
each heating means is applied prior to changing the temperature of each 
section in accordance with a flow rate characteristics curve by a feed 
back controlling at least a final heating means in response to the 
measurement of the moisture rate of the dried raw material discharged from 
the rotor. 
The flow rate characteristics curve is determined by the time of passage of 
the raw material and its effect on the initial temperature in each 
section. A lag in thermal response of the heater means is that the heater 
means does not start to heat the raw material immediately after the heat 
control is started upon arrival of the raw material. A bias temperature 
for compensating a lag in thermal response is set to preheat the raw 
material in a predetermined period prior to the arrival of raw material at 
each section of the drying apparatus. 
In another aspect of the present invention there is provided a process for 
controlling the temperature of a drying apparatus including a cylindrical 
rotor having a plurality of heating means which are independent of each 
other and arrayed in an advance direction of raw material, the process 
comprising keeping the temperature of the drying apparatus at a 
predetermined temperature prior to the drying operation of the raw 
material and then controlling the heating means in response to 
measurements of the flow rate and moisture content of the raw material 
charged into said rotor and in response to measurements of the temperature 
of respective sections of the rotor, each section corresponding to each 
heating means so that prior to changing the temperature of each section in 
accordance with a flow rate characteristics curve by feed back controlling 
at least a final heating means in response to the measurement of the 
moisture rate of the dried raw material discharged from the rotor.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The present invention will be described by way of embodiments with 
reference to the drawings. 
Referring to FIG. 1, there is shown a schematic structure of the system for 
accomplishing a process of the present invention. Reference numeral 10 
represents a drying apparatus comprising a cylindrical rotor having a 
plurality of heater means (not shown) which are independent of each other 
and arranged in a raw material feeding direction. The rotor of the drying 
apparatus may be deemed as being divided into a plurality of dryhing 
sections 1 to N corresponding to respective heating means. Reference 
numerals 12 and 14 represent a raw materials flow rate meter and a first 
moisture meter respectively. The flow rate meter 12 and the first moisture 
meter 14 are disposed outside the entrance of the drying machine 10 for 
determining the flow rate and the moisture content of the raw material 
charged into the drying apparatus 10. A second moisture meter 16 is 
disposed outside the exit of the drying apparatus 10 for determining the 
moisture content of the raw material which has been dried by the drying 
apparatus 10. Thermometers 18-1 to 18-N are provided at the drying 
sections 1 to N for determining the temperature thereof. Reference numeral 
20 represents means supply for supplying heat medium for the purpose of 
drying which supply means are connected with the heater means in each 
section of the drying apparatus. The heat medium is supplied in the form 
of steam in this embodiment. Heat medium adjusting means 22-1 to 22-N 
which are disposed between the heat medium supplying means 20 and the 
heater means in each section are adapted to adjust the supply of the heat 
medium to each heater means in the drying sections 1 to N from the heat 
medium supply means 20 under the control of the control means 24 which 
will be described hereafter. 
The heater means comprises heating pipes and the heat medium adjusting 
means 22-1 to 22-N comprise diaphragm valves if the heat medium is steam 
as described above. 
The cylindrical rotor which forms the drying apparatus is tilted so that 
the entrance is slightly higher. When the rotor is driven to rotate by 
means of rollers (not shown) the rotor serves to move the raw material 
which has been charged into the entrance thereof toward the exit and to 
dry the raw material into a given moisture content and to discharge it 
from the exit. 
The control means 24 comprises an electronic computer such as 
microcomputer. The control means 24 receives signals from the raw material 
flow rate meter 12, the first moisture meter 14, the second moisture meter 
16 and thermometers 18-1 to 18-N. The control means 24 controls the heat 
medium adjusting means 22-1 to 22-N by arithmetically processing the 
signals in accordance with a predetermined program. In other words, the 
control means 24 generates control signals for opening or closing the 
diaphragm valves. The outline of the structure will be described with 
reference to FIG. 2. 
In FIG. 2 reference numeral 241 represents a central processing unit 
(hereinafter referred to as CPU) which carries out control of jobs which 
are executed in accordance with a program, arithmetic processing which is 
necessary in the execution of jobs and control of other devices and 
management of reception and feeding of the data required for this control. 
A memory device 242 comprises a read only memory 242a (hereafter referred 
to as ROM) which stores a program for fixed jobs which the computer 
executes and a read and write memory 242b (hereafter referred to as RAM) 
which stores constants required for program, operation results and input 
information. 
A process input/output device 243 comprises a multiplexer 243a (hereinafter 
referred to as MX) which subsequently switches the analog input signals 
from the raw material flow rate meter 12, the first moisture meter 14, the 
second moisture meter 16 and the thermometers 22-1 to 22-N, an analog to 
digital converter 243b (hereafter referred to as A/D C) which converts the 
signals from the multiplexer 243a into analog signals which may be 
processed by the computer and digital to analog converter 243c (hereafter 
referred to as D/A C) which converts the digital information obtained by 
arithmetic processing in the computer into an analog output for actuating 
the diaphragm valves 22-1 to 22-N. 
An input/output device 244 comprises a serial interface 244a which provides 
video information and input data to a CRT display 26 and receives and 
feeds the data from and to the computer when the data is printed out by a 
printer 27 and a keyboard input device 244b which transforms the data from 
keyboard 28 operated for storing constants by an operator and transmits 
them to CPC 241. 
Reference numeral 245 represents an data but through which various data are 
received and fed among the aforementioned devices. 
The temperature control by the control device 24 will be described in 
detail with reference to FIG. 3 and the following figures. 
When flow rate of the raw material cut tobacco leaves at the entrance rises 
up to F.sub.0 as shown in FIG. 4 in the drying apparatus 10 which are 
divided into four drying sections 1 to 4 as shown in FIG. 3, the flow 
rates F.sub.1, F.sub.2, F.sub.3 and F.sub.4 at each drying section on raw 
material charging change as shown in FIG. 5. 
In FIG. 5, L.sub.1, L.sub.2 and L.sub.3 represent the time it takes for the 
raw material to pass the length between the drying apparatus entrance and 
the section 2, the length between the drying apparatus entrance and the 
section 3 and the length between the drying apparatus entrance and the 
section 4 respectively. Ts represents a time until the flow rate at each 
section reaches at the steady flow rate F.sub.0 which is referred to as 
setting time. The flow rate curves F.sub.1, F.sub.2, F.sub.3 and F.sub.4 
are approximated by omitting L.sub.1, L.sub.2 and L.sub.3 as follows; 
##EQU1## 
In the formula, i represents 1 to 4, T.alpha.i represents flow rate 
characteristics constant and s a Laplacian operator. The temperature 
T.sub.A0 at each section for making the moisture at the exit of the 
driving apparatus to a constant value under the condition at which F.sub.1 
to F.sub.4 reach at a constant flow rate F.sub.0 after the passage of the 
period T.sub.s may be represented as follows; 
EQU T.sub.A0 =.alpha.F.sub.0 +.beta..multidot..omega..sub.1 -.delta.(2) 
wherein .omega..sub.1 represents a moisture content of the raw material 
which is obtained from the first moisture meter 14 in FIG. 1. The constant 
flow rate F.sub.0 is obtained by the raw material flow rate meter 12. 
.alpha., .beta. and .delta. represent operation parameters. 
If the temperature at each section immediately before charging of the raw 
material is assumed as T.sub.0, a target moisture content may be obtained 
at the exit of the drying apparatus immediately after rise-up of the raw 
material by raising the temperature at each section to T.sub.A0 
represented by the formula (2) by tracking the curves in FIG. 6 which are 
similar to those in FIG. 5. 
If the optimum drying temperature curve T.sub.Ai (t) until reaching at 
T.sub.A0 at each section is deemed as .DELTA.T.sub.Ai (s) by omitting 
L.sub.1, L.sub.2 and L.sub.3, the .DELTA.T.sub.Ai (s) is represented as 
follows; 
##EQU2## 
wherein represents a Laplacian transformation operation. 
The temperature response curves at each section change as shown in FIG. 7 
when the target value of the temperature at each drying section is 
stepwise changed. If the target value, thermal transfer characteristics of 
temperature response among sections and the temperature of the section and 
represented as T.sub.sv (s), G(s) and T.sub.A (s) respectively by using 
Laplacian operator the following relation is established. 
##EQU3## 
The transfer characteristics G.sub.i (s) of each section is represented 
from the FIG. 7 as follows: 
##EQU4## 
wherein T.beta.i represents a constant of the thermal response 
characteristics at each sections. Lag in thermal response is omitted from 
the formula (5). 
From the formulae (3) to (5) the present temperature T*SETi for providing 
the optimum drying temperature T.sub.A at each drying section is 
represented by the formulae (6), (7) and (8). 
##EQU5## 
The formula (8) may be obtained by reverse-transforming T.sub.sv (s) which 
is obtained by putting the above formulae (3) and (5) into the formula 
(4). 
Since the raw material flow rate meter 12 which is disposed together with 
the first moisture meter 14 at the entrance side of the drying apparatus 
is positioned upstream of the entrance by a length L* as shown in FIG. 8, 
it takes time for the raw material detected by the flow rate meter 12 to 
reach the exit of the drying apparatus. The length L* corresponding to 
this time is known. Accordingly, a bias temperature T.sub.cl is 
preliminarily preset at an interval t.sub.0 to t.sub.1 before the reaching 
of the raw material as shown in FIG. 9 in order to raise the temperature 
of the driving section 1 at the time then the raw material reaches at the 
entrance of the drying apparatus 10 by correcting a lag in thermal 
response T in rise-up of the temperature at the drying section, which has 
been described hereabove. As similarly, bias temperatures T.sub.c2, 
T.sub.c3 and T.sub.c4 are initially preset between intervals t.sub.2 to 
t.sub.3, t.sub.4 to t.sub.5, t.sub.6 to t.sub.7 with respect to the 
sections 2 to 4 respectively. 
In connection with the sections 1 to 3, preset temperatures T*SET.sub.1, 
T*SET.sub.2 and T*SET.sub.3 which are obtained by the above-mentioned 
formula 8 are preset for the intervals t.sub.1 to t.sub.9, t.sub.3 to 
t.sub.9, and t.sub.5 to t.sub.9 respectively in FIG. 9. A preset 
temperature T*SET.sub.4 by the formula 8 is preset only the interval 
t.sub.7 to t.sub.8 in connection with the section 4. Other temperature 
presetting is accomplished for the time T.sub.8 and following time. 
In operation the moisture rate of the dried raw material is sequentially 
measured by the second moisture meter 16 at the output side of the drying 
apparatus 10. The drying temperature is controlled so that the measured 
signal .omega..sub.2 becomes a target moisture rate .omega.*. Such control 
is a feedback control. Since the control is carried out while measuring a 
true moisture rate, the target moisture rate may be assured. 
Since the temperature presetting at each section depends upon the forecast 
method in which a target moisture rate may be obtained upon basis of a 
model formula in which the flow rate time constant characteristics and 
then thermal response characteristics etc. are approximated. The errors in 
the model formula and other disturbance are of course involved so that 
there is a possibility that the moisture content of the dried raw material 
becomes a target moisture content. It is therefore an object of such 
control to correct the errors. 
Temperature T.sub.A0 is preset after a time t.sub.9 in accordance with the 
formula (2) in connection with the sections 1 to 3. This control is 
carried out in a steady state and referred to as feed forward control. 
Feed back control is continued in the section 4. 
Since the actual temperature adjustment is carried out by opening and 
closing the diaphragm valves even if the temperature is preset by the 
aforementioned preset temperature T*SET.sub.1 to T*SET.sub.4, a valve 
opening signal m.sub.i is obtained by carrying out the adjustment 
operation of the following formula (9), that is, proportion, integration 
and differential (PID) operation 
##EQU6## 
wherein K.sub.p, T.sub.1 and T.sub.0 represent operation parameters 
referred to as proportional gain, differential time and integration time 
repectively and T.sub.i represents temperature measuring signals from the 
thermometers 18-1 to 18-4. For the feed back control period, a target 
temperature signal m.sub.5 of the heating pipe corresponding to the 
section 4 is obtained by the PID operation of a following formula (10). 
##EQU7## 
The valves corresponding to the sections 1 to 4 is opened or closed at an 
opening which is obtained by the above formula (9) and the valve 
corresponding to the section 4 is opened or closed at an opening obtained 
in accordance with the formula (9) by a cascade control in which Tsv.sub.i 
is preset by a target temperature signal obtained by the above formula 
(10). By doing so, the moisture content at the rise-up of the raw material 
may be quickly changed to a target value soon. 
The constants T.alpha..sub.1, T.alpha..sub.2, T.alpha..sub.3 and 
T.alpha..sub.4 of the flow rate characteristics are determined by 
assumption of the results of a fundamental experiment upon basis of the 
constant T.alpha..sub.4 of the flow rate characteristics F.sub.4 of FIG. 
5. In practice, T.alpha..sub.1, T.alpha..sub.2 and T.alpha..sub.3 are 
obtained by multiplying T.alpha..sub.4 with a factor. 
If the temperature T.sub.0 of the drying apparatus just before when the raw 
material is charged into the drying machine is various depending upon the 
working beginning time and the environmental conditions, the condition 
becomes complicated and it is difficult to provide a good reproduction for 
controlling the moisture rate on raw material charging. 
FIG. 10 is a flow chart showing a program for the aforementioned control 
which the control means 24 executes. 
When the program is started in response to the detection of the raw 
material by the flow rate meter 12 in the shown chart, the heating means 
No. is set to 1 at step S1. That is, this setting appoints the control 
corresponding to the section 1. Following this, data are read out by 
addressing the RAM (represented as 242b in FIG. 2) which stores the 
constants relating to the control of the heating means No. 1 at step S2. 
The program then goes to step 3 at which it determines what control state 
is. 
The control state used herein includes three controls I to III which begin 
with the detection of the raw material as shown in FIG. 11. The term 
T.sub.R until a bias temperature T.sub.ci is preset since the detection of 
the raw material is defined as state I. A bias temperature preset term 
T.sub.s to T.sub.R is defined as state II and a term after the completion 
of the state II is defined as state III. Since the determination at step 
S3 just after start is state I, the program then proceeds to step S4. At 
step S4, it is determined whether or not the time after start is larger 
than T.sub.R. The time T.sub.1 is represented by the content of the 
counter which counts 1 per one second since the detection of the raw 
material. 
Since the time is just after the program start, of course, T&lt;T.sub.R. The 
result of determination is no and the program goes to step S5. 
The temperature preset value T*SET is set to 0 at step S5. The program then 
goes to step S6 at which the heating means No. is added with 1 so that the 
heating means No is changed to 2. It is determined whether or not the 
heating means No is larger than 5 at next step S7. Since the result of 
determination is no, the program returns to step S2. Data is read out by 
addressing the RAM which stores the constants relating to the control of 
the heating means No. 2 at step S2. The program goes to step S6 through 
the steps S3, S4 and S5. The heating means No. is changed to 3 at step S6. 
The program then goes to step S6 again through the steps S7, S2, S3, S4 
and S5. The heating means No. is changed to 4 at step S6. The program 
returns to step S6 again through steps S7, S2, S3, S4 and S5. The heating 
means No. is changed to 5. The program goes to step S7. The result of the 
determination at step S7 is yes, the program returns to start. However, 
the restart is waited until one second has passed since the previous 
start. 
The program is restarted after the passage of one second and goes to step 
S7 through the aforementioned steps S1, S2, S3, S4, S5 and S6. The jobs of 
steps S2 to S6 are repeated as is similar to aforementioned case until the 
heating means No. becomes 5. When the heating means No. becomes 5 the 
program returns to start. 
If the T.sub.R1 of the heating means No. 1 is assumed to be 8 seconds the 
above-mentioned jobs would repeated 8 times. When the determination at 
step S4 is yes, the program goes to step S8. The control state of heating 
means No. 1 is set to state II. Then the program goes to step S6 at which 
the heating means No. is set to 2. Thereafter the program goes to step S4 
through steps S2 and S3. 
Even if the heating No. 1 is 8, the determination at step S4 is No since 
the T.sub.R of the heating means No. 2, No. 3 and No. 4 is the times which 
are added with L.sub.1, L.sub.2 and L.sub.3 (refer to FIG. 9) 
respectively. Thereafter the heating means No. is 5 and the jobs are 
executed via steps S4, S5 etc. until the program is restarted. 
The program is then restarted and the heating means No. is set to 1 at step 
S1. The determination on the control state is carried out at next step S2. 
Since the result of determination is started II, the program will go to 
step S9 at which determination whether T.gtoreq.T.sub.S or not is carried 
out. Since the determination result is No, the temperature preset value 
T*SET.sub.1 is set to a bias temperature T.sub.c at next step S10. 
Thereafter the heating means No. is set to 2 at step S6. The program will 
return to step S6 through steps S7, S2, S3, S4 and S5 until the heating 
means No. is changed to 5. If the determination results is yes at next 
step 7, the program will return to start. 
Until the period T.sub.s has passed, loop job is carried out via the steps 
S1, S2, S3, S9, S10, S6 and S7 as to the heating means NO. and the loop 
job is carried out via the steps S2, S3, S4, S6 and S7 as to the heating 
means Nos. 2, 3 and 4. 
If the period T.sub.s has passed, the determination result would be No at 
step S9 and the program will go to step S11 at which the control state of 
the heating means No. 1 is set to state III. Thereafter the program will 
go to step S12 at which actuation of RAM which stores data is carried out 
so that the data on the raw material flow rate F.sub.0 and the moisture 
control .omega..sub.1 collected before by a dead time T.sub.s become 
initial data for control. Then the program will go to the step S7 via the 
step S6. The loop job of steps S2 to S7 as to heating means Nos. 2 to 4 
until the heating means No. becomes 5. When the heating means No. becomes 
5, the program will return to START. 
The heating means No. is set to 1 at step S1 again. The program will then 
go to step S3 via step S2. Determination on control state is carried out 
at step S2. Since the determiantion result is state III, the program will 
go to step 13 at which feed forward operation shown in the formula (2) is 
carried out upon basis of the data which have been initialized at the step 
12 and constants so that the final desired or target value T.sub.A0 is 
calculated. The program then goes to step 14 at which pattern operation 
shown in the formula (8) is carried out so that T*SET.sub.1 is set. The 
preset temperature T*SET at time t=0 corresponds to T in FIG. 11. The 
program will go to step S7 via step S6 after the operation at step S14. 
Following heating means Nos. 2 to 4 will be described. As apparent from 
FIG. 9, jobs of steps S2 to S7 are sequentially carried out as described 
above since the control of the heating means Nos. 2 to 4 is still in state 
I when the control of the heating means No. 1 is rendered into state III. 
The heating means Nos. 1, 2 and 3 are rendered into states II and III 
after periods of time L.sub.1, L.sub.2 and L.sub.3 have passed since the 
heating means No. 1 is rendered into states II and III. 
Steps S15 and S17 represented by dotted line in FIG. 10 are provided for 
carrying out feed back control of the heating means No. 4. Determination 
whether or not the heating means No. is equal to 4 is carried out at step 
S15. Determination whether or not T.sub.1 .gtoreq.T.sub.B at step S16 
wherein T.sub.B is a time when feed back control begins. Feed back control 
is accomplished at step S17. 
When the process of the present invention is carried out at a cut tobacco 
leaves drying appratus under conditions of 12.5% wB of target moisture 
rate at the exit and not higher than 11.5% wB of abnormal moisture rate, 
the cut tobacco having an abnormal moisture content can be suppressed to a 
remarkably low yield as 5 kg at a total amount at 6000 kg/h of flow rate 
of the raw material. Furthermore the control of moisture content may be 
stably carried out. 
Although feed back control is carried out at only final section in the 
above-mentioned embodiment, the same effect may be obtained by carrying 
out feed back control at other desired sections. 
In accordance with the above-mentioned process of present invention the 
temperature of the drying apparatus when the raw material is charged into 
the drying apparatus is controlled according to the raw material flow rate 
characteristics and the compensation for a lag in thermal response by 
application of bias temperature and feed back control based on the 
moisture content of the dried tobacco is carried out. The production of 
undesired product may be minimized by changing the moisture content of the 
dried product at the rise-up time of drying operation of the drying 
apparatus to a target value as soon as possible.