US 3748224 A
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July 24, 1973 W. ROBERTUS J. TlLLlE ETAL CONTROL SYSTEM FOR THE PRE-DRYER SECTION OF A PAPER MACHINE Filed Nov. 24, 1970 T35 T2 T =T T P 2 F I ,3
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United States Patent 3,748,224 CONTROL SYSTEM FOR THE PRE-DRYER SECTION OF A PAPER MACHINE W. Robertus J. Tillie, New York, N.Y., and James T.
Carleton, Pittsburgh, and Richard E. J. Putman, Penn Hills, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.
Filed Nov. 24, 1970, Ser. No. 92,347
Int. Cl. D21f 5/06; G06f 15/46 U.S. Cl. 162-252 5 Claims ABSTRACT OF THE DISCLOSURE System for controlling the pressure of steam introduced into a paper drying drum in such a way that the temperature of the paper web leaving the drum will be constant. This is accomplished by deriving a predictive feed-forward signal from consideration of a heat balance around the drum and by varying the predictive signal with a feedback signal proportional to actual exit temperature. The modified feed-forward signal is then used as an error signal for controlling the steam pressure within the drum, and, hence, its temperature.
BACKGROUND OF THE INVENTION In the manufacture of paper from wood pulp, the pulp in a head box passes through a slice gap or slit onto a wire comprising a fine wire screen in the form of an endless belt, usually several feet wide. Vibration of the screen spreads the material in a thin uniform layer or web. As it is carried along on the wire, much of the water in the soft mass drains out. To prevent the material from running over the edges of the wire, rubber belts are usually placed on either side. Mechanical suction removes much of the water from the wet pulp mass, after which the web passes between rollers to a thick woolen felt which is carried between a series of heaw rollers. Passing between these, the paper is pressed into a firm thin sheet, and thereafter passes over a series of heated drying rolls. Usually, the Web passes over the initial rolls while it is still on the felt mat; however, after suflicient drying has taken place, the web may be separated from the felt mat and passed over the remaining drying rolls to a sizing press without any external support.
The initial drying roll or rolls over which the web passes, are ordinarily referred to as the pre-dryer. The main function of the pre-dryer is to heat the pulp web or sheet to a predetermined temperature. It has been found from actual operating experience that if the web, upon leaving the pre-dryer, is at a certain desired temperature (which in one case is 160 F.), then the remaining dryers can be operated much more efiiciently. There is no theoretical justification for this; it is something that must be found out from experience.
Thus, it is of primary importance to hold the temperature of the sheet, as it emerges from the pre-dryer, at a constant, predetermined temperature. Introducing a set amount of steam into the pre-dryer drum or drums Will not give a constant output temperature for the reason that the moisture content of the sheet, its density, its velocity, the amount of air blowing over the drum, and other factors will vary, meaning that the amount of heat and, hence, the amount of steam needed to maintain a fixed output temperature will vary. Furthermore, a system which attempts to control output temperature by measuring the temperature of the issuing web is highly unsatisfactory since, among other thin-gs, it constitutes control after the fact rather than before the fact. That is, if an attempt is made to control the steam input to the drying drum as a function of exit temperature only, the
Patented July 24, 1973 "Ice measuring instrument, which is spaced from the roll, will measure the temperature after the strip has traveled from the drum; but during this time the conditions of pulp consistency, velocity and the like may have changed entirely.
SUMMARY OF THE INVENTION In accordance with the present invention, the temperature of pulp issuing from the pre-drying drum of a papermaking machine is controlled by conducting a heat balance around the drum to generate a feed-forward signal indicative of the pressure and temperature of the steam necessary to raise the temperature of the web from a known input temperature to a desired output temperature. This feed-forward signal is then modified by a feedback signal derived by comparison of desired output temperature with the actual output temperature of the web to derive an error signal for controlling the pressure of the steam in the drying drum.
The feed-forward signal is generated from a consideration of the fact that the heat, Q required to heat up the paper web from an incoming temperature, T to an outgoing temperature, T equals the heat, Q transferred from the inside of the dryer drum or drums to the web. The heat, Q transferred from the inside of the drum to the Web can be calculated from a consideration of the heat transfer characteristics of the drum, the temperature of the steam within the drum, and the sheet temperatures, T and T at the input and output of the drying drum or drums, respectively. Q the heat required to heat up the paper web from temperature T to temperature T can be calculated from the mass flow rates of pulp and water flowing onto the drying drum, the specific heats of the fiber and water, and the temperatures T and T Since Q must always be equal to Q an equation can be derived for determining the temperature, T,,, of the steam. This temperature then comprises a feed-forward signal which is modified by a comparison of the desired output temperature with actual output temperature to vary the steam flow into the pre-dryer drum.
The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying single figure drawing which schematically illustrates one embodiment of the invention.
With reference now to the drawing, the embodiment of the invention shown includes a felt belt 10 which carries, on its upper surface, a moist Web of pulp 12 which is to be dried and pressed into paper sheet. The felt, with the moist pulp thereon, passes over a pre-dryer 14 comprising a rotatable drum having a hollow interior into which steam from line 16 is introduced. The exhausted steam, in turn, passes out of the hollow interior of the drum 14 through conduit 18 to a condenser or separator 20.
As was mentioned above, the pre-dryer drum 14 is the first drying drum in the process of drying and rolling pulp into paper and may comprise a series of drums rather than the single drum shown herein. The drum normally comprises a cast iron cylinder with a wall thickness of about 1% inches, the heat of the steam within the hollow interior of the drum being transferred through the wall of the drum to the felt 10 and pulp 12. In order to obtain maximum efiiciency in subsequent drying operations, it is desired that the pulp issuing from the pre-dryer 14 have a predetermined temperature which, in one specific example, is F.
The temperature of the pulp issuing from the predryer is measured by a temperature measuring device 22 which produces an electrical signal on lead 24 proportional to that temperature. Similarly, the temperature of the incoming pulp is measured by a temperature measuring device 26 which produces anelectrical signal on lead 28 proportional to the measured temperature. In addition, the density of the pulp on felt is measured and converted into a proportional electrical signal by a beta gauge 30. This electrical signal is multiplied by a constant, K at 32 to produce a signal M which is proportional to the pounds of dry fiber pulp per minute passing along the felt belt 10. This, of course, assumes a constant speed of the belt 10. If the speed of the belt should vary, then a tachometer generator and compensating circuitry will have to be employed for variations in speed.
As was mentioned above, it is desired to maintain the output temperature, T at a desired specific value. Furthermore, the heat, Q required to heat up the pulp from the incoming temperature T; to the outgoing temperature T equals the heat, Q transferred from the inside of the dryer drum to the web. Mathematically, this can be expressed as follows:
N=nurnber of rolls in the pre-dryer (one in the example given);
U=overall heat transfer coefiicient (B.t.u./ F. -ft. -min.);
A=contact area of the sheet and roll (ft?) T =calculated desired temperature of steam in drum T =actual input temperature of pulp F.); and
T =desired output temperature of pulp F.).
Q can be calculated from: Q2=( w w+ nb nb)X( a* 2) where M =moisture in incoming sheet (lbs./min.); C =specific heat of water (B.t.u./lb.- F.); M =bone dry pulp rate (lbs/min); and C =specific heat of dry pulp (B.t.u./lb.- F.).
Thus, the desired steam temperature T,,* can be calculated from Equation 14; and this is performed in computer circuitry identified by the reference numeral 34 in the drawing to produce an electrical quantity comprising the feed-forward signal. The computer circuitry 34, as well as the other circuit blocks shown herein, may comprise part of a general purpose computer which is programmed to perform the calculation of Equation 14 and the other control functions described herein to produce an electrical quantity comprising the feed-forward signal. However, it is shown as hard-wired logic in the drawing to facilitate explanation.
The quantity M is derived in block 36 by multiplying M times a constant K. That is, it is assumed that a certain rate of dry pulp flowing to the drum 14 will contain a fixed percentage of water. The quantity T is entered into the computer circuitry 34 from block 38 and is selected by an operator.
Since the parameter to be controlled is the actual exit temperature T this temperature is compared with the desired temperature T at 40 and the quotient or ratio subtracted from 1 in block 42 to derive a corrective feedback signal which is integrated in integrating controller 44 and multiplied with the quantity T,,* at the output of circuit 34 at 46. This corrected value T is then applied to circuit 48 where it is multiplied by a factor 1 from a steam table 50 stored in the computer memory to derive P*. P* is the desired pressure of the steam within the drum 14 to achieve an exit temperature equal to T The actual steam fed into the drum 14 is controlled by means of valve 52 connected to a pressure controller 54 which receives a set point signal from integrating controller 56. The set point of controller 56, in turn, is derived by comparing in comparator 60 P* from block 60 with the actual steam pressure P as derived from a pressure sensing device 58. Thus, a servo loop is established in which any change in the value of P* will cause an adjustment in the setting of valve 52 which, in turn, will vary the output of pressure sensing device 58 until the two signals, P* and P, are equal, at which time the pressure in the drum 14 is equal to the calculated desired pressure P*.
In the foregoing Equation 2, the quantity U, the overall heat transfer coeflicient, includes losses to the air blowing across the pulp and perhaps the tightness of the felt and other factors. For this reason, the quantity U is not a fixed value but changes and, consequently, it is desirable to update the quantity U. This is achieved by measuring the amount of condensate coming out of the dryer at 60 and converting this at 62 into the amount of heat actually given up by the steam by multiplying pounds of condensate by a constant K This, then, can be compared with calculated heat transferred, Q in circuitry 64 and the difference derived at 66 integrated at 68 and used to multiply U*, the predicted heat transfer coefficient, at 70. This gives a corrected value of U which is fed back to circuit 34.
Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.
We claim as our invention:
1. In papermaking apparatus of the type wherein a web of moist pulp is heated by passing it over a hollow drying drum charged with steam through a steam supply line having a control valve therein whereby the heat of the steam is transferred through the wall of the drum to the pulp, the improvement comprising a system for maintaining the temperature of the pulp issuing from the drum at a preset value, said system including:
means for computing the desired temperature of the steam Within said drum from a consideration of the heat required to heat up the pulp from a known input temperature to a desired output temperature and the heat transferred from the interior of the drum to the pulp, said means for computing the desired temperature of the steam within said drum including a device for measuring the actual temperature of pulp passing tothe drying drum and means for determining the weight per unit of time of dry pulp and moisture passing to the drying drum, means responsive to the desired steam temperature thus computed for generating a feed-forward error signal for controlling the admission of steam into said drum,
means for comparing the actual output temperature of said pulp with said desired output temperature to derive a feedback error signal which modifies said feed-forward signal until the desired and actual temperatures are the same, and
valve control means responsive to said feed-forward signal as and if modified for controlling said control valve and, hence, the admission of steam into said drying drum.
2. The improvement of claim 1 wherein said computed desired temperature of the steam within said drum is compared with temperatures stored in a steam table to derive a feed-forward error signal proportional to the desired pressure of the steam within said drum.
3. The improvement of claim 1 wherein the means for computing the desired temperature of the steam within said drum computes said temperature from the equation:
where T,,* is the desired temperature of the steam within said drum, T is the temperature of the pulp passing to the drying drum; T is the desired output temperature of the pulp as determined by an operator; N is the number of rolls over which the pulp passes, U is the overall heat transfer coefiicient of the drying drum, A is the contact area of the pulp with the periphery of the drying drum, M is the bone dry pulp rate, C is the specific heat of the dry pulp, M is the moisture in the incoming sheet, and C is the specific heat of water and F is N -U-A w w+ fib+ ib 4. The improvement of claim 3 wherein said means for computing the desired temperature T,,* produces an electrical signal proportional to T,,* and includes means for comparing an electrical signal proportional to T with an electrical signal T proportional to actual output temperature to derive an error signal for modifying the electrical signal proportional to T,,*.
5. The improvement of claim 4 wherein said electrical signal proportional to T,,* is compared with known information stored in a steam table for deriving an electrical signal proportional to P which is the desired pressure of the steam within said drying drum.
References Cited UNITED STATES PATENTS 3,564,724 2/ 1971 Keyes et al 34-48 3,518,775 7/1970 Bartles et a1 162-198 X 3,295,842 1/ 1967 Stelling et a1 34-48 X 3,216,241 11/1965 Hansen 73-75 3,363,325 1/1968 Schie 34-41 2,309,993 2/ 1943 Skagerberg 34-41 X 2,922,475 1/1960 Alexander 162-252 S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R.