DE2141466A1 - Method of controlling a paper drying machine - Google Patents

Description

Westinghouse Electric Corporation Erlangen

Pittsburgh, Pa. USA Werner-von-Siemens-Str.

2UU66

our sign

VPA 71/8320 Chr / Fa

Method for controlling a paper dry machine

The invention relates to a method for controlling a paper drying machine, in which a belt goes through a pre-dryer and then through at least two further dryers with several heated cylinders.

In the manufacture of paper, the pulp from a container passes through a slot onto a fine fabric. | A vibration of the tissue distributes the material in a thin even layer. Then the layer dewatered and pressed into a firm, thin sheet as it passes through heavy rolls. From here the Paper web to a series of heated drying cylinders.

It has been found that there is a certain relationship between the caliber or thickness of the web and the speed the machine or its production volume. If the strength of the train increases, then the speed must the machine can be reduced, since with increasing thickness the mass and the water content of the paper layer grow. j |

With higher paper thicknesses, the production rate can be increased as a result be that the temperature of the steam is increased, which gets into the drying cylinder. If there is a change of brand however, the change in steam temperature is not entirely satisfactory, because thermal delays occur when heating or cooling the cylinders.

Furthermore, it has been found that there are limits to the maximum and minimum permissible paper machine speeds

2098 10/1218

- 2 - VPA 71/8320

as a function of the paper thickness, regardless of the temperature the drying cylinder. The * reasons for this are not entirely clear; maybe the upper speed limit of the paper dryer depends on the centrifugal forces that lift the web from the cylinders at higher speeds. The lower speed limit is harder to understand. One might assume that drying is done at low speeds would be very effective, but the fact is that there is a lower speed and production rate here regardless of the steam temperature.

In summary, one can say that with every paper thickness the speed of the machine and the production quantity can be changed between an upper and a lower limit However, if the paper thicknesses change, that can also change change the lower and upper speed limits.

It is theoretically possible to change the paper thickness and maintain constant production rate by changing the temperature of the steam; however, it has pointed out that this is a basically slow process which can lead to the production of inferior goods.

The object of the present invention is to provide a method for optimally controlling a paper dryer the paper thickness and production quantity are specified.

This object is achieved according to the invention in that the required temperature of the cylinders of the last dryer group from a consideration of the temperature of the web at the exit of the pre-dryer, the temperature of the web at the exit of the last dryer group, the thickness of the dried web and the production volume of the dryer are used as the advance signal it is calculated that the moisture content of the web leaving the dryer is determined as the feedback signal and that, depending on the difference in the signals, at least one regulation of the temperature of the drying cylinders in the last dryer group is performed.

209810/1218. " ³ "

2UU66

- 3 - VPA 71/8320

is taken. In this way, from a consideration of the Relationship between paper thickness and production volume Control signal are generated which is proportional to the desired steam temperature in the drying cylinders. The same relationship can then also be used to "predict the production quantity" for a new paper thickness while initially the steam temperature remains constant. In this way a change of type can be calculated, which avoids the need to change the drying conditions. The speed, 'with which the types can be changed is then only a function of the mechanical data of the machine and is completely coma from the thermal behavior with a lot larger time constants. After the new paper thickness has been introduced, the steam temperature can be can still be changed within a certain period in order to increase the production volume.

The invention will be explained in more detail with the aid of a drawing; show it:

Pig. 1 is a schematic representation of a paper dryer with its connections to the computer,

Fig. 2 is a graphic representation of paper thickness and allowable Machine speed,

3 shows the paper thickness with regard to various control parameters including steam per ton of paper, total steam flow and temperature of the last drying group,

4 shows a logarithmic representation of the paper thickness, and while assuming that the steam temperature and the inlet and outlet temperatures of the paper web are constant

Fig. 5 change of paper thickness and machine speed,

6 shows the paper thickness as a function of the production quantity and

Figure 7 is a flow chart showing the operation of the calculator of the present invention.

_ 4 _ 209810/1218

2UU66

- 4 - VPA 71/8320

The container 10 shown in Fig. 1 is closed at the top and instructs. its bottom an inlet for the introduction of fresh paper pulp. A slot 12 is provided between the bottom of the container 10 and a vertically standing plate 14. The plate can be displaced and thus the amount of paper pulp flowing onto the fabric 16 can be changed. The pulp is distributed on the running fabric 16, where the water by gravity and by mechanical suction is largely removed. The thickness or caliber of the web formed on fabric 16 is a function of height of the slot and depending on the ratio of the speed of the outflowing pulp to the speed of the fabric 16. The outflow speed is in turn a function of the pressure in the container 10. The material passes from the tissue 16 to another dewatering and pressing device in which the paper is pressed into a thin web · This device is only indicated by the number 18. Got from here then the paper web 50 to a series of heated drying cylinders.

In Fig. 1, three sets of drying cylinders, i.e. three dryers, are indicated, namely the pre-dryer 20, the second and third dryer 22 and fourth dryer 24. Each dryer consists of hollow drying cylinders 26 filled with superheated steam be applied. Between the dryers 20 and 23 is a press 28, between the dryers 22 and 24 one Press 30 and at the exit of the fourth drying group 24, a further press 32 is provided.

The main function of the pre-dryer 20 is to heat the material to a certain temperature. It has It has been shown that the remaining dryer can be used to the best of its ability when the one leaving the pre-dryer Track 50 has a constant temperature. By means not shown, the temperature at the outlet of the Pre-dryer 20 kept constant.

- 5 -209810/1218

- 5 - VPA 71/8320

The entire drying process is controlled by a computer 34, who works with a program 36. The computer 34 also has an input space 38 via which the Signal f *, which is proportional to the desired production volume in tons per hour and the signal t *, which is the desired Paper thickness is proportional, can be entered into the computer 34.

A signal Tp, which is proportional to the temperature of the web 50 at the output of the pre-dryer 20, is also fed to the computer 34 on the line 40. In addition, a further signal T 1 is present on the line 43, which signal is proportional to the temperature of the web 50 at the output of the fourth dryer 24. Furthermore, the computer 34 is fed a signal V on the line 42, which is proportional to the web speed. The signals T ₂ and T 1 are taken from temperature measuring devices 44 and 46, while the signal V on line 42 comes from a tachometer generator 48 which is connected to one of the heated drying cylinders 26. After the web 50 has passed the press 32, it arrives at a thickness gauge 52 which supplies an electrical signal D on the line 54 which is proportional to the density of the web. This signal is used in conjunction with the speed signal V to determine the production rate F and the thickness t of the paper leaving the machine.

The computer 34 supplies a signal sn on the line 56 to a control circuit 58 for a drive motor SO, which in turn is coupled to one of the drying cylinders 26. In a complete drying system, a large number of such motors are regulated for the individual cylinders by a single motor control circuit. For the sake of simplicity, however, only one of the motors 60 is shown in the present Pail. An additional® output signal from the computer 34 arrives on line 62 to iron steam regulator 64 ₉ which controls the steam valve for the drying cylinder of the fourth dryer 24. Am

209810/1218 -6-

- 6 - VPA 71/8320

Signal on line 63, which is connected to a steam regulator 65 for the steam valve 66 of the drying group 22, acts in a similar way. The supply of steam to the cylinders in dryer groups 22 and 24 determines the temperature on their surfaces. The signal from computer 34 on line 70 goes to a slot controller 72, which determines the position of the plate 14. This slit controller 72 also controls the speed of the tissue 16 via a Drive motor 74, since - as explained above - the thickness of the web is not only a function of the slot width, but also the speed of the fabric 16 is.

In this way, the caliber or strength of the Hechner 34 of the paper via a signal on line 70, the temperature of dryer groups 22 and 24 via signals on the lines 62 and 63 and the speed of the drying cylinder and thus the production speed of the dryer about a Signal on line 56 controlled.

The permissible caliber or thickness t of the paper web as a function of the machine speed V is plotted in FIG. As can be seen, the speed ranges Draw an envelope for different paper thicknesses, i.e. between 10 and 30 points. Why there is one such an envelope is difficult to explain. The top line 76 of the envelope could be due to the effect of centrifugal force be based; i.e. above a certain speed the web may tend to close against the drying cylinders leaving. This reduces the drying effect and thus the m@id.male speed and production rate are limited. Ss la!} Remarkable that when the strength and weight the paper web is increased, the maximum speed decreases; this could be an indication of the centrifugal force theory be. The lower line 78 of the envelope is more difficult to understand because in and of itself the drying process would have to be "9i will be more effective at low speeds. It remains οeh the fact exist * that there is a lower limit

- 7 203810 / 121S

- 7 - VPA 71/8320

speed below which drying is no longer possible is possible.

In Fig. 3 are with reference to the strength t (or daa caliber). the most common working conditions are indicated, namely the Steam pressure a in the fourth dryer, the steam pressure b in the second and third dryer, the steam temperature T- the fourth Drying group, the total steam flow c and the ratio of kg steam to ton of paper produced - curve d -.

It is noticeable that curve d is straight and falls slightly with increasing paper thickness t. This inclination of the curve could _{be explained by the longer contact side v,} ie lower machine speed, which is associated with greater paper thicknesses.

As can be seen, the steam pressure b in the second and third dryer, as well as the temperature, is always lower than the pressure and the temperature in the fourth dryer. If T ₃ ^ and T _{fl are used} to denote the steam temperatures in the third and fourth dryer and t denotes the thickness of the paper web, then there is a linear relationship in the form:

^T s3 ^{= T} s4- ^a 1 ^{+ b} 1 '< ^Μ >

where a., and b _{1 are} empirically determined coefficients, which can be referred to as the difference factor and distribution factor, and k represents a constant.

Since there is a well-defined relationship between T _fl , and T ₈ ^, a relationship between T ₈ ^, the rate of production, and the strength t can also be derived. In the present invention, the paper _{temperature T 2} at the exit of the pre-dryer is measured together with the temperature T 1 at the press 32. Because of the apparent relationship between T- and T _fl , a model can be designed based solely on T. is based

- 8 -, 209810/1218

2U1466

- 8 - VPA 71/8320 -

and in which the entire drying system is viewed as a heat exchanger. By applying the heat exchanger concept in simplified form and on the assumption that the production rate in tons per hour is denoted by F that the heat absorbed by the web as a function of

- T ₂ ) 'F (IJ)

varies.

The heat transported through the wall of the drying cylinders varies as a function of

T, - T ₉

-2 ^ - K (III)

In

¹ BA ^l 2

A utilization of the two above relationships results in an approximate relationship according to the form:

K = F in

n £

~ ^X 3

Here, the factor K varies inversely depending on the Strength t. In other words, the heat transfer coefficient between steam and paper decreases with increasing Paper thickness t. This means that it is much more difficult with increasing caliber, i.e. paper thickness, to transfer heat to the same paper weight. This is true with experience and results in part from the reduction in surface area per kg of paper weight that is in contact with the drying cylinders. A curve for the factor K is shown in FIG. 4, where T2 and T- are assumed to be constant. this gives the empirical relationship

TT
F In. W ^ wr * "hr - V ^nt "

- 9 209810/1218

2UU66

- 9 - VPA 71/8320

where a ^ and b. Are constants.

The above equation can also be arranged that the relationship arises:

- T

-in j (a _k - \ In t) = C, (VI)

where C is a constant based on the fact that all temperatures have been taken as constants. Therefore, the current value of 0 can be calculated from the desired or measured production quantity F and the paper thickness t, and with known T « ^UIL ^ ^ 3 the predicted value of T i can then be obtained from the equation

C T, + T "

^T "4 °

be calculated. The factor G is changed by the temperature T i actually measured. This is a feedback signal and changes the predicted value of T. so that the humidity at the exit of the dryer is kept constant. T ₈ , then T _{becomes Q.} calculated in accordance with equation. The distribution factor b .. can - if desired, be changed as a function of a calculated error between the actual and the desired distribution. The distribution is defined as the ratio of the steam in the condensate of the second and third dryer divided by the total heat in the condensate that comes from the second, third and fourth dryer.

As explained above, one of the most significant problems associated with the grade change is the problem of how the slow time constant of the dryer with the much faster changeability of the NaQ part of the system and which can link the mechanical parts of the machine. According to the present invention, varieties are changed in the manner made that one behaved the behavior of the dryer of Sem separates the mechanical parts of the machine in such a way that the paper type can be changed quickly from one to the other

209810/1218 - 10 -

- 10 - -VPA 71/8320

strength can be made, even if this results in the Production temporarily suffers. The grade changes are therefore carried out with constant steam pressure in the drying cylinders performed. A subsequent change in the production rate can then be carried out at your discretion, namely at a rate that corresponds to the reaction rate of the! dryer. The fact that steam temperatures and pressures being held constant during the first part of the braid change does not mean that either the steam flow remains constant during this time. Br will actually change automatically if the heat transfer changes. That is, an increase in condensation calls a pressure drop, which in turn automatically causes the steam valve to open. Since, as can be seen from equation VI, all temperatures are kept constant, there can be a transition from one variety to another through subsequent ones Equation can be defined:

*) ■ ^G · (VIII)

Assuming that one wishes to switch from one paper thickness t and one production speed V to the other, the current value of C is calculated from the above equation. Assuming that C is now kept constant, the value of F can be calculated for the paper thickness t and the production quantity can be adjusted accordingly. After a brief pause F "^^ ^7, the paper thickness t 'calculated and set the machine according to these conditions and has been so on, carried out until the desired change to the new paper thickness. A typical family of such transition functions is given in FIG. 5, in which the speed V is plotted as a function of the paper thickness t, and in FIG. 6, in which the production quantity F is given as a function of the paper thickness t.

Referring to Fig. 5, assume that it is desired to change the paper thickness from 15 to 18 points. Under

209810/1218

- 11 - VPA 71/8320

under these circumstances the calculated value is C = 0.636. To the Brand changes are now allowed to decrease the production quantity slowly in steps, from point 80 along the curve 82 to point 84 while the value of C constant at 0.636 is held. As soon as the point 84 is reached, the machine speed V and production quantity F can be at one Caliber of 18 points can be increased again by increasing the steam temperature in the drying cylinder. This shifts the working point along the dashed line 86 to point 88, which then represents the new working point. Of the The transition from point 80 to point 84 can take place relatively quickly because it only depends on the speed of operation of the Haschine off, during the transition from point 84 to point 88 only relatively because of the thermal time constants of the system will go slowly.

A program for calculator 34 is shown in FIG. the Paper density D, measured with the thickness gauge 52, is converted into production quantity F at 90 according to the equation ".

F = K ₁ * (D »W'V),

where K _{1 is} a constant, D is the density, W is the width and V is the measured speed of the web. At the same time, the density D becomes the strength t at 92 after

converted to the following equation:

where the basis weight B ^ is a known quantity.

The program begins by entering the desired paper thickness t * and the desired production quantity F * at 94. As mentioned above, this is done by an operator. The current value of 0 then becomes calculated from equation VIII at 96 and the program then tests at 98 whether the current strength t is equal to the desired strength t *. This happens every two minutes,

209810/1218

- 12 -

2U1466

- 12 - VPA 71/8320

and if the answer is "no", the paper thickness is changed at 100 in every two dot increments. After feathers In a two-point increment, a new production quantity F is calculated at 102 from equation VIII. This will then be the case 104 converted into a speed according to the equation

where B ™ is the basic weight in kg per square meter and C, a constant is.

A signal which is proportional to the new speed is then sent via line 56 to the motor control circuit 58 for the motor 60 to change the speed of the drying cylinders, either up or down, and although this depends on whether the paper thickness t is to be increased or decreased. Every time the paper thickness is changed at 100, a signal on line 70 xu is given to the slot controller 72 to open the slot and to change the speed of the motor 74. After two minutes, the above process is repeated, namely with the same value of C. You new production quantity F becomes then calculated at 102 and the new machine speed V at 104. This process is continued until t * »t; ■ then the program is over.

If now t * t *, a “yes” signal is taken from 98. This controls 106 to determine whether F = F *. If this is the case, the "yes" signal closes the program at 108. This would happen if, with reference to FIG. 5, for example, the production quantity is defined by point 84 at a C value of 0.636. If, however, the answer is "no", the new program is started, the speed being increased in steps at 110 so that the motor control circuit 58 increases the speed of the motor 60. After each increase in speed, a new value of T ₈ J at 111 is calculated from equation VII until the actual

- 13 -209810/1218

214U66

- 13 - VPA 71/8320

neuter production amount P corresponds to the desired production amount F *. The program is then ended. At the same time, the value of T i is calculated from equation I at 114 and this value is used to generate a signal on line 62 for controlling steam regulator 65 for valve 66. At the same time, depending on the calculated T. a signal on line 62 is generated. This signal is compared at a summation point 116 with a further signal which represents the integral - formed by integrator 118 - of the comparison of the actual output temperature T 1 with the desired output temperature I ~ _# at summation point 119. The resulting value is the feedback signal mentioned above. The signal is fed to the steam regulator 64 for the valve 68.

10 claims
7 figures

- 14 209810/1218

Claims (10)

'" ^: ' 2H1466 - 14 - VPA 71/8320 Claims M Λ Method of controlling a paper drying machine in which ^ ■ ^ a web through a pre-dryer and then over at least two more dryers with several heated drying cylinders is running, characterized in that the advance signal is the necessary temperature of the cylinder (26) of the last dryer (24) from a consideration of the temperature (T «) of the Web (50) at the exit of the pre-dryer (20), the temperature (T 1) of the web at the exit of the last dryer (24), the thickness (t) of the dried web and the production amount (F) of the drying machine is calculated as Feedback signal the moisture content of the web leaving the drying machine is determined and that depending on the difference in the signals, at least one control of the temperature of the drying cylinder of the last one Dryer (24) is made. 2. The method according to claim 1, characterized in that the temperatures of the drying cylinders (26) in all dryers (22, 24) can be regulated as a function of the difference between the signals. 3. The method according to claim 2, characterized in that the temperature (Tp) at the outlet of the pre-dryer (20) is constant is held. 4. The method according to claim 1, characterized in that the The feedback signal changed as a function of the temperature (T i) of the web (50) at the exit of the last dryer (24) will. 5. The method according to claim 1, characterized in that the signal proportional to 1 - G igt, where T, the temperature of the orbit at the exit of the 209810/1218 - 15 2UH66 - 15 - VPA 71/8320 last dryer, T _{2 is} the temperature of the web at the exit of the pre-dryer and C is proportional to ^ t) where J is the output of the dryer, t is the thickness of the web and a, and b, are constants. 6. The method according to claim 1, characterized in that the drying cylinders (26) are heated by steam and that the supply of steam to these cylinders is dependent is controlled by the difference between the two signals. 7. The method according to claim 1, characterized in that for Brand change first the machine with a certain specified paper thickness (t *) and production volume (P *), the temperature of the drying cylinder (26) constant is held and a transfer constant (C) is calculated therefrom, which is then used to calculate the thickness of the web in steps is changed to a new desired value, each time from a consideration of the transfer constant the new production rate is calculated after each incremental change in paper thickness, and that furthermore the speed, with which the web runs through the dryer, is changed every time a new production quantity is added is calculated. 8. The method according to claim 7 »characterized in that the transfer constant is proportional to ^{0 =} ¥ ' ^(a k " ^b k ^ln where 7 is the production volume, t is the paper thickness or represent the caliber and a ^ and bj_ constants. 9 * Method according to claim 7 »characterized in that the Paper thickness (t) by changing the slot width in '- 16 209810/1218 2U1466 - 16 - VPA 71/8320 Reservoir (10) and the speed of the tissue (16) is made. 10. The method according to claim 7, characterized in that the temperature of the drying cylinder (26) is then changed, if the measured paper thickness with the new desired Paper thickness matches. 209810/1218 Blank page