US3822179A

US3822179A - Method and apparatus for optimizing steam consumption of continuous digester

Info

Publication number: US3822179A
Application number: US00223331A
Authority: US
Inventors: N Chari; W Marteny
Original assignee: Individual
Current assignee: Georgia Pacific Corrugated LLC
Priority date: 1969-02-27
Filing date: 1972-02-03
Publication date: 1974-07-02
Anticipated expiration: 1991-07-02

Abstract

METHOD AND APPARATUS FOR OPTIMIZING STEAM CONSUMPTION AND YIELD OF A CONTINUOUS DIGESTER BY MEANS OF A CONPUTER. A COMPUTER RECEIVES INPUT SINGNALS REPRESENTATIVE OF DIGESTER SCREW FEED SPEED; FEEDS LIQUOR FLOW, REACTION CHAMBER CREW SPEED, STEAM FLOW AND PRESSURE; AND PRODUCES A SINGAL WHICH IS USED TO ADJUST THE OPENING OF BLOW VALUE MEANS BETWEEN THE DEFIBRATOR AND THE BLOW TANK OF THE DIGESTER. THE COMPUTER ALSO COMPARES THE ELECTRICAL POWER CONSUMPTION (CURRENT) FOR THE REACTION CHAMBER CREWS WITH A SELECTED MAXIMUM PERMISSIBLE VALUE IN ANTICIPATION OF PLUGGING CONDITIONS TO PRODUCE SAFETY SIGNALS TO CONTROL THE BLOW VALUE MEANS. CONSULT THE SPECIFICATION FOR FURTHER DETAILS.

Description

y 197l N. c. s. CHAR] AL 3,

METHOD AND APPARATUS FOR OPTIMIZING STEAM CONSUMPTION 0F CONTINUOUS DIGESTER Original Filed Feb. 27, 1969 2 Sheets-Shdet x, SCREW FEED RPM 0 COMPUTER 3 x REACTION CHAMBER RPM x STEAM FLOW MLR/hr Y=f{a W} e STEAM PRESSURE V ":1

2 LIQUOR G.P.M. 4 REACHON CHAMBER CURRENT PLUGGING ANTICIPATION {j- LIQUOR SUPPLY 20 STEAM SUPPLY l7 555" 22-0 5 m: |3M 2I-0 2|E g""- INVENTORS NOS. CHARI WILLIAM w. MARTENY ATTORNEYS July 2, 1974 c, 5, cHARl ETAL 3,822,1Yg

METHOD AND APPARATUS FOR OPTIMIZING STEAM OONSUIPTION 0F CONTINUOUS DIGEST!!! Original Filed Feb. 27,.1969

2 Sheets-Sheet 2 1| l l l I l l l NM 2 8T0 F EB: Av W dmmzis mosfim O9 2 Int. Cl. D21c 7/08, 7/12 US. Cl. 162-17 10 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for optimizing steam consumption and yield of a continuous digester by means of a computer. A computer receives input signals representative of digester screw feed speed; feeds liquor flow, reaction chamber screw speed, steam flow and pressure; and produces a signal which is used to adjust the opening of blow valve means between the defibrator and the blow tank of the digester. The computer also compares the electrical power consumption (current) for the reaction chamber screws with a selected maximum permissible value in anticipation of plugging conditions to produce safety signals to control the blow valve means. Consult the specification for further details.

This is a continuation of application-Ser. No. 802,856, filed Feb. 27, 1969, now abandoned.

The present invention relates to method and apparatus for optimizing steam consumption and yield in a continuous digester, and more particularly, relates to the use of a computer for computing the optimum blow valve setting of a continuous wood chip digester.

Objects of the invention include the provision of a method and apparatus for continuously controlling the blow valves in a continuous digester so as to optimize steam consumption, provision of such a control system for obtaining a more uniform yield from the digester, and the provision of control means for anticipating plugging conditions to avoid plugging of blow valves to thereby reduce down time of digesters.

In achievement of the foregoing objectives, it has been determined that the opening of the blow valves necessary for optimizing steam consumption and yield can be computed on the basis of heat balance around the digester; namely, from a computation based on the rate of chip flow, liquor flow, reaction chamber material flow speeds, and actual steam flow and pressure. From a comparison of a theoretical determination of the steam requirements with actual steam usage under normal operating conditions having manual control of digester blow valves, it was shown that the operators operated the blow valves to permit excessive amounts of steam to flow. The present invention is an improvement over prior art control systems in providing more accurate continuous control of steam consumption by controlling the blow valves primarily as a function of the rate of flow of chips and liquor to the digester, as well as speed of screw conveyors in the reaction chamber and the actual steam pressure and flow rate to the digester.

The present invention utilizes an analog computer which, for the most part, is essentially conventional to effect the necessary computation for producing signals for precision control of the degree of blow valve opening for optimizing steam consumption in the digester. The invention also incorporates safety features which will set the blow valve opening to a safe value in anticipation of an abnormal condition, and if the abnormal condition persists, the blow valve is opened fully or to 100 percent.

United States Patent Ofice 3,822,179 Patented July 2, 1974 DESCRIPTION OF THE DRAWINGS The foregoing objectives, advantages, and features of the invention will be described in detail in the following specification, wherein:

FIG. 1 is a schematic diagram of a continuous digester as manufactured by the Black-Clawson Company (Pandia Division), Hamilton, Ohio, and circuit diagram showing points of measuring variables, computer and control of digester blow valves thereby, and

FIG. 2 is a block diagram of the computer circuit as incorporated in the invention.

THE CONTINUOUS DIGESTER The continuous digester shown in FIG. 1 is of the type manufactured by the Black-Clawson Company (Pandia Division) and is used for the continuous pulping of wood chips, of semi-chemical pulp, etc., for paper corrugating medium; only the portion thereof to which the present invention pertains being shown. Chips, such as wood chips from chipping apparatus, pretreated if desired, are fed by a conveyor 10 to an overhead hopper 11 which continuously feeds variable speed volumetric screw feed conveyor 12 having drive motor 12M. Screw conveyor 12 compacts the chips to form a moving plug to seal the pressure zone at the input end 12E of the digester, and at the same time, this action of the screw feeder 12 opens up the chips which makes rapid impregnation possible. The digester itself consists of a plurality of horizontal tubes 13-1 136 and, within each of these tubes, is a screw conveyor 138 which are used for conveying the chips and liquor through the tubes, respectively. Digester tubes 13-1 13-6 are connected in cascade or serial order by vertical necks 14 which transport the chips from one tube to the next in succession. As the chips leave the throat of the screw feed conveyor 12, steam and cooking liquor are sprayed onto the chips. The steam is applied by way of a high-pressure steam line 14 at relatively constant temperature and pressure from a regulated supply 17.

Likewise, cooking liquor is supplied through line 19 to be sprayed upon the chips at the throat of screw conveyor 12 and is delivered from a liquor supply 20. After being sprayed with cooking liquor and steam, the chips enter the reaction chamber constituted by tubes 13-1 13-6. The speed of the reaction chamber screws 13S determine the retention time of the chips in the digester, and hence, the degree of cooking thereof. Reaction chamber screws 138 are commonly driven by motor 13M by a common drive mechanism 13D, but they may be driven by individual drive motors with separate speed and power monitoring. The cooked mass is discharged from the reaction chamber into a defibrator 15 from which the cooked mass is discharged continuously under pressure through a pair of

blow valves

21 and 22. Steam from the steam line 16 may be passed through a manually operated valve line (not shown) to the disc refiner 15D of the defibrator. Defibrator 15 includes a screw conveyor 15S driven by a motor 15M.

Blow valves

21 and 22 are mounted on the housing of disc refiner 15D but may be positioned in

pipes

23 and 24. The products passed through

blow valves

21 and 22 and

pipes

23 and 24 are fed through a cyclone unit (not shown) to a blow tank (not shown), which blow tank is normally at atmospheric pressure.

Blow valves

21 and 22 have valve elements 21E and 22B which vary the size of blow valve openings 21-0 and 22-0 and are under control of pneumatic controller 32B. Although one pneumatic controller is shown controlling

blow valves

21 and 22, a separate controller may be used for each blow valve and is preferred, only one being shown for purposes of simplifying the description.

'In the past, the

digester blow valves

21 and 22 were manually operated and set which, as will now be shown, resulted in excessive steam consumption.

EXCESS STEAM FLOW CALCULATION Oven dry chip flow rate, lb./hr.

C 60 C d I Heat required to raise the temperature of wet chips to digester temperature, B.t.u./hr.

z.=0. [v.+ (T..T.) (2) Heat required to raise the temperature of cooking liquor to digester temperature, B.t.u./hr.

The free convection heat losses (2 are computed in accordance with the treatment in Chapter X of Process Heat Transfer by D. Q. Kern (McGraw-Hill). The radiation losses (2 are computed in accordance with the equations presented in Chapter IV of the same reference source.

Theoretical steam required, lb./hr.

Typical computer output for excess steam flow tabulation is presented in the following table.

Inasmuch as optimum steam flow is a function of several input variables to the digester; primarily, the volume of chip flow as measured by the speed (in r.p.m.) of feed conveyor screw 13s and liquor flow; as well as blow valve openings; and also, anticipatory of how plugging conditions affect the power requirements of reaction chamber screws and defibrators; the present invention is concerned with the computation of the interaction of these variables to produce control signals for controlling the blow valve opening.

It will be noted in connection with digester shown in FIG. 1 that the basic control of steam flow through the digester from regulated steam source 17 is by way of

blow valve

21 and 22 at the output of the defibrator 15. Control signals to the blow valve control apparatus 32B are effected by the output from computer 30. It has been determined that the proper opening of the blow valves (there may be more or less blow valves controlled in accordance with the invention) in order to optimize steam consumption, can be determined by solution of the following equation:

F=system function x =feed screw conveyor speed (r.p.m.)

x =reaction chamber screw conveyor speed (r.p.m.) x b =steam flow rate x =steam pressure (p.s.i.)

and d d d d d :1 and AP bias, are constants and their values are established for a given digester according to digester conditions such as wear and tear of feed screws, drive motors, etc.

MEASUREMENT OF DIGESTER VARIABLES As diagrammatically illustrated in FIG. 1, screw feed revolutions per minute (r.p.m.) is measured by a tachogenator 12T, on screw feed drive motor 12M, and applied as input variable voltage x to computer 30. Liquor flow rate (in gallons per minute) is measured by a magnetic TYPICAL COMPUTER OUTPUT FOR EXCESS STEAM FLOW CALCULATION [Five minute intervals selected over a five-hour time period] Reaction Reaction Blow Actual Theor. Excess chamber, Liquor chamber Chip valve steam steam steam Screw feed, r.p.m. r.p.m. ratio tempJ flow 1 opening 3 flow I flow 4 flow UNITS: 1 F. i Lb./min. 1 Percent.- Lb./h0ur.

type fiow meter 40 and is applied as input variable voltage x to computer 30. The speed of reactor chamber screws 138 is measured by a tachogenerator 13T on screw drive mechanism 13D and is applied as input variable voltage x;; to computer 30. The rate of steam flow is measured by steam flow rate meter 60 in steam line 16 which produces a signal applied as input variable voltage x to computer 30. Steam pressure is measured by gauge 61 on steam line 19 which produces a signal applied as input variable ar to computer 30.

Reaction chamber screws 138 are commonly driven from an electric motor 13M. In order to obtain the plugging anticipation signal, current, indicative of power supplied to motor 13M, is measured by ammeter 13a and applied as variable voltage signal x to the plugging anticipation section of computer 30, such signal indicating loading on the conveyor screws in anticipation of a plug condition. A corresponding signal may be obtained by measuring power or current to defibrator screw motor 15M by meter 15A.

DESCRIPTION OF COMPUTER For purposes of economy and simplicity, an analog computer was chosen to perform the calculation although it is apparent that a digital computer may be used to perform the computations. Although electrical quantities are manipulated in performing the computations, corresponding pneumatic computers may be used.

The elements of the analog computer 30 shown in FIG. 2 are conventional and, per se, form no part of the present invention. It essentially consists of means for receiving analog voltage inputs proportional to measured values as modified by selected constants which are applied through

amplifiers

31, 32, 33, 34, and 35, to summing amplifiers 36. Thus, the signal voltage e corresponding to steam pressure (x is applied as one input to operational amplifier 31; whereas a voltage from potentiometer P9, proportional to the AP bias voltage (equation (6) above), is applied as another input to amplifier 31 so that the output of amplifier 31 will be a voltage proportional to the steam pressure less a constant (x -AP bias). The output from the tachogenerator 12T measuring the speed of feed screw conveyor 12 is applied through a voltage device 12VC (which proportions the voltage output of tachogenerator 12T to the range used by the computer), to amplifier 32 which applies its output signal (x through potentiometer P (set at constant d to summing amplifier 36.

Liquor flow is measured by a magnetic flow meter 40, and since magnetic flow meter 40 produces a current proportional to liquor flow, this current is passed through resistor 41 so that the voltage drop across the resistor 41 is proportional to liquor flow voltage e (variable x in equation (6)), and this is applied to operational amplifier 33, as one input, and a voltage proportional to the constant d from potentiometer P5 is applied to the other input of operational amplifier 33 so that the output from amplifier 33 will be proportional to the quantity x d This voltage is then applied through potentiometer P2 which selects a proportion corresponding to the constant d in equation (6) so that a voltage proportional to the expression d (x -d is applied as a third input to summing amplifier 36.

The reaction chamber screw speed is measured by tachogenerator MT, and as in the case of screw feed tachogenerator, this voltage is likewise converted to a voltage in the range being utilized in the computer by means of a voltage conversion device 14VC and applied to amplifier 34, voltage 2 corresponding to the reaction chamber revolutions per minute (x in equation (6)). The output voltage from amplifier 34 is applied to a potentiometer P3 having its wiper set to correspond to the constant d so that a voltage proportional to the expression d e is applied as a further input to summing ampli-v fier 36.

The rate of steam flow is measured by a conventional steam flow measuring instrument 60 which produces a current proportional to steam flow. This current is passed through a dropping resistor 60DR to produce a voltage proportional to steam flow and, with this voltage, is applied as one input to operational amplifier 35. The second input to operational amplifier 35 is from a potentiometer P which applies the other input to operational amplifier 35 so as to meet the expression x b (equation (6)). The output from amplifier 35 is a voltage proportional to the expression x b and this voltage is applied to square root circuit 63 which includes an amplifier having a diode D or other nonlinear device in the feedback path. The output from square root circuit 63 is a voltage proportional to the square root of the expression x b This voltage is applied through potentiometer P4 which has been set at a position to correspond to the constant (1., in equation (6). The voltage from proportioning device 64 is, therefore, a voltage proportional to the expression d /x b and this voltage is likewise applied to summing amplifier 36.

A voltage of fixed magnitude is applied through potentiometer P having its wiper set at a position to correspond to the voltage proportional to the constant d and this voltage is applied as the final input to summing amplifier 36. Thus, there has been applied to summing amplifier 36 six voltages proportional, respectively, to each of the terms set out in equation (6). The output from summing amplifier 36 then is a control signal voltage which is applied to amplifier to produce a voltage converted by voltage to pressure converter 32A to a pneumatic pressure suitable for controlling pneumatic controller 32B which controls the

blow valves

21 and 22.

Potentiometer P is a conventional output scaling potentiometer used in analog computers.

Although the control signal from the computer may be applied to an electrical operator for the blow valve, pneumatic operators for the blow valves are preferred since, if electrical operators are used, the required feedback potentiometer may not hold up in the surrounding atmosphere. A suitable pneumatic transducer is 701-TD- 122 Taylor electropneumatic transducer, manufactured by the Taylor Instrument Company, Cleveland, Ohio; such transducer receiving actuation DC voltage of 0-10 VDC, and having an output of 3-15 p.s.i.g., and a regulated 20 p.s.i.g. supply. A suitable pneumatic control drive is type AC 08l6-PBA-model 212-N by the Bailey Meter Company, Southfield, Michigan. A suitable analog computer to solve the equation is Model PC-lZ analog computer by Electronic Associates, Inc. Although not shown, a separate control line to pneumatic controller 32B may be used for remote manual control in the event of failure of the continuous computer control.

PLUGGING ANTICIPATION AND AVOIDANCE A further feature of the invention is in the method and apparatus for preventing plugging of blow valves or lines under computer controlled conditions. Mill experience shows that an increase in power to electrical motor driving reaction chamber screws 138 indicates a plugging tendency. Accordingly, the invention contemplates measuring the current (herein called reaction chamber current) to the electrical motor 13M driving the reaction chamber screws 13S; and continuously comparing reaction chamber current with a selected known safe value or limit; and causing automatic opening of the blow valves when the reaction chamber current exceeds the safe limit; the

blow valves

21 and 22 will be opened automatically until the plug is cleared (by increased steam flow through the digester above the amount necessary for normal operation as computed by the computer) as indicated by a decrease of reaction chamber current to a normal range. This safety feature overrides the control over the blow valves by the computer and can be set to open the blow valve to specific openings. For example, in

the system to be described, when the reaction chamber current (x exceeds the safe limit, the blow valves are automatically opened to 60%. Of course, as will be apparent to one skilled in the art, if desired the blow valves can be opened to values other than 60% to prevent plugging. A further comparison may be made of reaction chamber current and the selected safe limit, and if the reaction chamber current still exceeds the safe limit, the blow valves will be automatically opened to 100%. If after a time delay, the reaction chamber current continues to exceed the selected limit, a warning signal (not shown) may be given to the operator. Of course, this limit may be adjusted or changed, if desired, by the operator.

The plug preventing or anticipating circuit is shown in FIG. 2 and includes a meter 13A which produces a signal current through resistor 71 proportional to current flow to the motor 13M driving the screw conveyors 13S in the reaction chamber tubes 13. Voltage across resistor 71 is proportional to reaction chamber current (and hence, power) and is applied through amplifier 72 having a feedback filter 73. The output from amplifier 72 is a signal voltage proportional to reaction chamber current, and this voltage is applied as one input to comparator circuit 74. The other input to comparator 74 is obtained from potentiometer P which has its wiper set to correspond to a voltage proportional to the selected normal limit for the reaction chamber current. Any time the value of reaction chamber current signal voltage exceeds the voltage from potentiometer P there will be an output from comparator 74 which energizes relay 78.

Relay 78 actuates switch element 78-1 which, when relay 78 is de-energized, connects the output of sum ming amplifier 36 and potentiometer P to output amplifier 90. However, when relay 78 is energized, switch 78-1 opens the circuit from summing amplifier 36 and connects a constant signal voltage to a source 104 to the input of amplifier 90. This voltage is of a mangitude to cause pneumatic controller 32B to open the blow valves to a preselected value (including 100%) above the opening computed to be necessary by the computer. Thus, on a plugging condition being anticipated by an increase in current to the drive motor 13M of reaction chamber screws 13S above a selected value, relay 78 is energized to cause the potential of constant voltage source 104 to be applied as the output of the computer to cause the blow valves to be opened to pass relatively large quantities of plugging tendency. Contacts 78-2 of relay 78 may be used to control indicating devices, lights, etc., to indicate the controlled condition of the blow valves.

COMPUTED POWER FAILURE Relay 100 is connected directly across electrical supply lines (not shown) to the computer circuits and, hence, is energized as long as electrical energy is being supplied to the computer circuits. In the event of failure of the electrical supply to the computer, relay 100 de-energizes. Relay switch 100S-1 connects battery 101 to the computer output to cause voltage to pressure converted 32A to receive a constant safety signal from battery 101. The voltage on battery 101 is of a magnitude to cause pneumatic controller 323 to fully

open blow valves

21 and 22. Contacts 1005-2 may be used to control signal lights (not shown).

Steam consumption of the digester shown in FIG. 1 was about 17,500 lb./hr. prior to application of the invention thereto as compared to steam consumption of about 11,500 lb./hr. with the method and apparatus of the present invention applied. In addition to significantly reducing steam consumption by controlling the blow valves in the manner of this invention, wide variation in steam fiow and pressure in the digester, per se, are reduced so as to obtain a more uniform yield from the digester.

While we have described and illustrated a preferred embodiment of the invention, it is to be understood that the invention is not intended to be restricted solely thereto, but it is intended to include all modifications thereof which would be apparent to one skilled in the art and which come within the spirit and scope of the invention.

What is claimed is:

1. A method of improving steam consumption in a continuous wood chip digester of the type in which wood chips are conveyed into and through a tubular reaction chamber by mechanical conveyer means which includes a screw feed conveyer means in said reaction chamber which is driven by a motor, steam and liquor are admitted at an input end to the reaction chamber with the wood chips, and the efiiuent substances are continuously discharged from the reaction chamber under pressure through adjustable valve means, comprising the steps of continuously providing an indication of the amount of the wood chips moved into said reaction chamber by measuring the speed of rotation of said screw feed conveyor means, continuously controlling the degree of opening of said valve means as a linear function of said measured speed of rotation of said screw feed conveyor means,

continuously measuring the electrical power requirement of said screw conveyer means and producing a signal proportional thereto,

producing a reference signal corresponding to a safe degree of plugging of said conveyer means or of said blow valves,

comparing said signal proportional to the electrical power supplied to said screw conveyor means with said reference signal to produce a secondary control signal whenever said signal proportional to electrical power supplied to said screw conveyer means exceeds said reference signal, and

overriding the control of said valve as a function of said measured speed of rotation by said secondary control signal to open said valve to a predetermined opening size above the opening size determined as a function of said screw feed conveyer speed rotation.

2. In a continuous wood chip digester having a horizontally oriented tubular reaction chamber, first screw conveyer feed means for feeding wood chips to said reaction chamber, means for admitting steam at a selected pressure and temperature to said reaction chamber with said wood chips, means for admitting cooking liquor to said reaction chamber with said steam and said wood chips, second screw conveyer means for moving the mixture through said reaction chamber, and valve means at a terminal end of said reaction chamber for controlling the exit of effluent material from said reaction chamber, the improvement in apparatus for improving steam flow through said digester comprising,

means for producing a first signal proportional to the rate of rotation of said first screw conveyer means, means for producing a second signal proportional to liquor flow to said reaction chamber, means for producing a third signal proportional to the rate of rotation of said second screw conveyer means,

means for producing a fourth signal proportional to the pressure of steam admitted at the input end of said reaction chamber,

valve control means responsive to the signal applied thereto for producing a valve control signal which is linearly related to the applied signal for controlling the opening of said valve means,

summing means for taking the algebraic sum of the signals inputted thereto and for applying the sum signal to said valve control means, and of means for inputting said first, second, third and fourth signals to said summing means whereby said valve control signal which controls said valve means is linearly related to said first, second, third and fourth signals.

3. The invention defined in claim 2 including means for producing a fifth signal proportional to the rate of steam flow into said reaction chamber,

means for extracting the square root of said fifth signal to produce a sixth signal, and

means for applying said sixth signal to said summing means whereby said valve control means is responsive to the algebraic sum of said first, second, third, fourth, fifth and sixth signals.

4. In a continuous wood chip digester having a horizontally oriented tubular reaction chember, first screw conveyer feed means for feeding wood chips to said reaction chamber, means for admitting steam at a selected pressure and temperature to said reaction chamber with said wood chips, means for admitting cooking liquor to said reaction chamber with said steam and said wood chips, second screw conveyer means for moving the mixture through said reaction chamber and valve means at a terminal end of said reaction chamber for controlling the exit of efiluent material from said reaction chamber, improvement in apparatus improving steam flow through said digester comprising means for producing a first signal proportional to rate of rotation of said first screw conveyer means, valve control means responsive to the said first signal produced by said means for producing a valve control signal which is linearly related to said first signal for controlling said valve means as a function of the rate of rotation of said first conveyer means, a source of a negative constant signal, and means for applying the negative constant signal from said source of a negative constant signal to said valve control means whereby on any failure of said signal producing means said negative constant signal is effective to cause said valve control means to fully open said valve. 5. In a continuous wood chip digester having a horizontally oriented tubular reaction chamber, first screw conveyer feed means for feeding wood chips to said reaction chamber, means for admitting steam at a selected pressure and temperature to said reaction chamber with said wood chips, means for admitting cooking liquor to said reaction chamber with said steam and said wood chips, second screw conveyer means for moving the mixture through said reaction chamber and valve means at a terminal end of said reaction chamber for controlling the exit of effluent material from said reaction chamber, improvement in apparatus improving steam flow through said digester comprising 1 means for producing a first signal proportional to rate of rotation of said first screw conveyer means, and

valve control means responsive to the said first signal produced by said means for producing a valve con trol signal which is linearly related to said first signal for controlling said valve means as a function of the rate of rotation of said first conveyer means,

means for measuring power supplied to said second screw conveyer means and producing a voltage proportional thereto,

reference means for producing a reference voltage corresponding to a safe value of plugging of said conveyer means or of said blow valves, comparator means for comparing said voltage proportional to power supplied to said second screw conveyer means with said reference voltage, and

-means controlled by said comparator means for disabling the output of said summing means whenever the magnitude of said voltage proportional to power to said second conveyer is greater than said reference means and applying a plugging anticipation signal to said valve control means to cause said valve control means to open said valve to a preselected opening and permit steam to flow through said digester at a rate higher than required for normal operation thereof.

6. The invention defined in claim 5, including means responsive to continuance of said second conveyor power signal voltage being greater than said reference voltage 10 for increasing the opening of said valve .to permit maxi mum steam flow through said valve.

7. In a continuous wood chip digester having a horizontally oriented tubular reaction chamber, first screw conveyer feed means for feeding wood chips to said reaction chamber, means for admitting steam at a selected pressure and temperature to said reaction chamber with said wood chips, means for admitting cooking liquor to said reaction chamber with said steam and said wood chips, second screw conveyer means for moving the mixture through said reaction chamber, and valve means at a terminal end of said reaction chamber for controlling the exit of eflluent material from said reaction chamber, the improvement in apparatus for improving steam flow through said digester comprising,

means for producing a first signal proportional to the rate of rotation of said first screw conveyer means, means for producing a second signal proportional to liquor flow to said reaction chamber,

valve control means responsive to the signal applied thereto for producing a valve control signal which is linearly related to the applied signal, for controlling the opening of said valve means,

summing means for taking the algebraic sum of the signals inputted thereto and for applying the sum signal to said valve control means, means for inputting said first and second signals to said summing means, whereby the valve control signal is linearly related to said first and second signals,

an electrical battery having a voltage corresponding to a signal requiring maximum opening of said valve from said summing means, and

means responsive to failure of electrical current to apply said battery voltage to said valve control means to cause said valve to open fully and permit maximum steam flow therethrough.

8. Apparatus for optimizing steam consumption in a continuous wood chip digester of the type in which wood chips are continuously fed into a horizontal tubular reaction chamber by a feed screw conveyor means and continuously through said reaction chamber by further screw conveyor means, steam and liquor are admitted at an input end of the reaction chamber with the Wood chips and substances passing through said reaction chamber are continuously discharged therefrom through controllable valve means comprising means for continuously producing a control signal as a function of the equation:

x =feed screw conveyor speed (r.p.m.)

x =reaction chamber screw conveyor speed (r.p.m.)

x =steam pressure (p.s.i.) and d d d d d d and AP bias, are constants having values established for a given digester, and means responsive to said control signal for controlling said controllable valve means.

9. Apparatus as defined in claim 8 including means for measuring the power required to operate said further screw conveyor means and producing a signal proportional thereto,

a reference signal producing means for producing a reference signal corresponding to a safe value of plugging of said conveyor means or said blow valves,

comparator means for comparing said signal proportional to screw conveyor power and producing a valve override signal when said signal proportional to screw conveyor power exceeds said reference signal, and

means for applying said override signal to said means responsive to said control signal to cause said valve 1 1 means to open to a selected value greater then established b said control signal.

10. A control system for preventing plugging in a continuous digester of the type which is equipped with a mechanical conveyor means for moving material therethrough and blow valve means controlling exit of treated material from said digester comprising monitoring means for measuring power supplied to said mechanical conveyor means and producing a first signal proportional thereto, means producing a second signal constituting a reference signal corresponding to a safe value of plugging of said conveyor means or said blow valves,

comparator means for comparing said first signal with said second signal and producing a third signal proportional to the difference, if any, between said first and said second signals, and

means controlling said blow valves when the difference between said third signal exceeds a selected value to 2,673,690 3/1954 Segl 16219 X 2,870,009 l/l959 Richter 162-19 3,322,616 5/1967 Hutchinson et a1. l62238 OTHER REFERENCES Peskovol, B. 1.: Automation of Screw Conveyor Feeders For Continuous Digesters, I.P.C., Biblio. Series #241 Abstract #287.

15 S. LEON BASHORE, Primary Examiner A. DANDREA JR., Assistant Examiner U.S. Cl. X.R.

*zg g UNITED STATES PATENT OFFICE CERTIFICATE OF (IURECTION Patent N0- q g97 17q Dated Julv 2. 1974 Inventor(s) N.c.s. CHARI and w.w. MARTENY It is certified. that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Co1. 2, line 39, "14 should be -l6-. Col. 3 line 14, .1

"date" should be --data; Col. 3 line 40, "J=Sa S should be ---J=Sa s Table, "Chip flow Co1., 11118 12, "518.0" should be 5 13 .O; Table, "Theor. steam flow Col line 4, "17,619" should be -17,419-; Table, "Excess steam f1ow Col. line 31, "5 ,577" should be -6,577-. C01. 4, line 41, "genator" should be generator--. C01. 7, line 47, after "of" add -steam, in a purging fashion, to reduce the plugging or-.

Signedand sealed this 12th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents Attesting Offieer