US3610596A

US3610596A - Method and apparatus for rotary kiln control

Info

Publication number: US3610596A
Application number: US866584A
Authority: US
Inventors: Gordon L Snyder
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-10-15
Filing date: 1969-10-15
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05

Abstract

A rotary kiln control system and method for stabilizing kiln temperature. Measurements of kiln temperature and the air pressure under the cooler grate determine cooler grate speed to vary material depth on the grate to maintain the kiln-lining temperature constant. In addition, the temperature of cooling air from the material is measured and the flow of cooling air through the material is regulated in response to this measurement.

Description

United States Patent [56] References Cited UNITED STATES PATENTS 1/1963 Gieskieng.....................

Primary Examiner-John J. Camby [72] Inventor Gordon L. Snyder Phoenix, Arlz. [21] Appl. No. 866,584

[22] Filed Oct. I5, 1969 [45] Patented Oct. 5, 1971 [73] Assignee General Electric Corporation Attorneys-William S. Wolfe. Frank L. Neuhauaer, Oscar B.

Waddell and Joseph B. Forman ABSTRACT: A rotary kiln control system and method for stabilizing kiln temperature. Measurements of kiln temperature [54] METHOD AND APPARATUS FOR ROTARY and the air pressure under the cooler grate detennine cooler grate speed to vary material depth on the grate to maintain the kiln-lining temperature constant. In addition. the temperature CONTROL 10 Claims, 5 Drawing Figs.

[52] US. of cooling air from the material is measured and the flow of 263/32 R F27b 7/20 cooling air through the material is regulated in response to this 263/32 R measurement.

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PATENTEOUCT SIQTI 3.610.596

SHEEI 3 OF 5 MEASURE UNDERGRATE PRESSURE UGP 0F COOLER GRATE O0 SUCCESSIVE VALUES OF UGP YES DIFFER BY MORE THAN A PREOETERMINEO VALUE? 1 FILTER UGPgmn TO OBTAIN FILTEREO VALUE FUGP DETERMlNE TEMPORARY UNDERGRATE PRESSURE ERROR TEUGP (i.e., VARIATION OF FUGP FROM PREVIOUS UNDERGRATE PRESSURE SETPOINT UGPSP U SET usP T0 PREvwus VALUE C WAS PREVIOUS COOLER YES sPEEn SETPOINT L|AA|TEo. )----l N0 l IS TEUGP wnum LIMITS (AA,

Auo

CLAMP EUGP T0 EUGB 0R EUGP AS APPROPRIATE J ACCEPT A NEW UNDERGRATE PRESSURE ERROR EueP DETERMINE A TEMPORARY COOLER SPEED SETPOINT TCSSP BASED UPON EusP IS rcssn. wnum LIMITS m,

CSSP s TCSSP" 5 cssP w YES CLAMP CSSP TO CSSP 0R cssp AS APPROPRIATE; DETERMINE EUGP in ma EUGP AND INDIC TE CSSP umEb R w El ACCEPT A NEW COOLER SPEED SETPOINT CSSP PATENIED OCT 5 I97| SHEET I OF 5 MEASURE KILN LINING TEMPERATURE LTP Q00 SUCCESSIVE VALUES 0F m DIFFER BY MORE THAN A PREDETERMINED VALUE? SET LTP T0 PREVIOUS VALUE FILTER LTP TO OBTAIN FILTERED VALUE FLTH I DETERMINE TEMPORARY KILN LINING TEMPERATURE ERROR TEITP (in, VARIATION OF FLTP FROM KILN LINING TEMPERATURE SETPOINTLTSP I was PREVIOUS UNDERGRATE PRESSURE YES sETPoIIIT UGPSP LIMITED YES IS TELTP vIITMIM LIMITS IT.8., Em 5 TELTP 1 ELTPMI .P

ACCEPT A NEW ERROR SIGNAL ELTP DETERMINE A TEMPORARY UNOERGRATE PRESSURE SETPOINT TUGPSP T UGPSP 2 C IS TusPsP YES IIIo

CLAMP ACCEPTED ERROR SIGNAL ELTP TO ELTP OR ELTP AS APPROPRIATE YES TO MOVE ow IF CSSP AT A LIMIT, WILL TUGPSP TEND BEYOND LIMIT? SET UGPSP UGPSP SET usPsP, TUGPSP,

YES

UGPSPmin s UGPSP IS UGPSP MITRIM LIMITS TIL,

usPsRmI? FIG. 3A

PATENT EU DCT SIQTI SHEET 5 OF 5 MEASURE SECONDARY AIR TEMPERATURE SAT DO SUCCESSIVE VALUES OF SAT DIFFER BY MORE THAN A PREDETERMINED VALUE? YES I SET 5m TO PREVIOUS vAEuE] FILTER san TO OBTAIN FILTERED VALUE FSAT TREND FSAT TO OBTAIN TRENDED VALUE TSAT DETERMINE A TEMPORARY SECONDARY AIR TEMPERATURE ERROR TESATn (I.e. VARIATION OF FSAT FROM TSAT I ACCEPT A NEW SECONDARY AIR TEMPERATURE ERROR ESATn WAS PREVIOUS COOLER DAMPER POSITION SETPOINT CDSRH LIMITED? YES IS TESAT WITHIN LIMITS (IE, ESATmin s TESAT ESA CLAMP ACCEPTED ERROR SIGNAL ESAT TO OR ESATm x AS APPROPRIATE DETERMINE A COOLER DAMPER POSITION SETPOINT COSP SET cusp T0 cosp 0R cosp AS APPROPRIATE; DETERMINE ESAT AND EsAT INDICATE cus LIMITED ACCEPT NEW COOLER DAMPER POSITION SETPOINT CDSP USE CSSP AND CDSP TO CONTROL FIG. 3B

METHOD AND APPARATUS FOR ROTARY KILN CONTROL BACKGROUND OF THE INVENTION This invention relates to the production of end products in rotary kilns and, in particular, to an improved method and apparatus for controlling and regulating the operation of rotary kilns to provide stable kiln operation.

Typical rotary kilns employed in the production of Portland cement are steel cylinders 8 to 25 feet in diameter and between 100 and 700 feet long. The cylinders are lined with refractory brick and inclined 2 to 3' from the feed end to the discharge end. The steel cylinder is supported at spaced points and rotated through a gear drive by an electrical motor at speeds in the order of 20 to l20 revolutions per hour. Cement raw material, such as finely ground limestone, clay or shale intermixed in the desired proportions and either in the form of a finely ground slurry or a dry pulverized, intermixed material are fed into the upper or feed end of the rotary kiln.

During rotation of the kiln, the raw materials move slowly down the kiln at a rate which is a function of the kiln rotational speed and pass through successive zones known as the drying or chain zone, the preheating zone, the calcining zone and the clinkering or burning zone. If the raw materials enter the feed end of the kiln in the form of a wet slurry, the moisture is evaporated in the chain zone which may extend for 25 percent of the kiln length. Chains are suspended from the kiln to contact the slurry and serve as a heat exchanger to drive off moisture. The drying or chain zone is not necessary in a kiln which is specifically adapted to use only a dry mix. As the materials move down the kiln, they are slowly heated by a stream of hot gases which are produced by a burner positioned at the lower or discharge end of the kiln and which flow counter to the direction of material movement in the kiln. A fan at the feed end of the kiln creates a slightly negative pressure in the kiln and draws the hot combustion gases produced by the burner through the kiln to heat the raw materials moving in the opposite direction, causing the raw materials to undergo successive changes due to the steadily increasing temperature of the materials.

The temperature of the dried raw materials increases until the calcining temperature is reached at which time carbon dioxide is liberated from the raw materials, changing the carbonates to oxides. The calcining zone occupies the major portion of the kiln length. The temperature of the material changes little within the calcining zone since the calcining reaction is endothermic and requires heat. A measurement of the material temperature within this zone gives little indication of the degree of calcination. At a point down the kiln where calcination is complete, a large temperature difference exists between the solid materials and the counterflowing hot gases. Thus, when calcination is complete, the temperature of the solid material begins to increase rapidly to the point where the exothermic clinkering reactions are initiated. The heat generated by these chemical reactions causes the solid material temperature to rapidly increase 700800 F. The clinkering or burning zone is near the discharge end of the kiln and the material remains at or near the high temperature until it leaves the kiln and is thereafter cooled. The degree of completion of the chemical reaction in the clinkering or burning zone depends upon the feed composition, the temperature in this zone and the residence time of an increment of feed within the zone.

The kiln must be controlled in such a manner as to produce a clinker product having a satisfactorily quality and preferably a uniform quality. The variables over which a kiln operator has immediate control and which directly influence the kiln operation are the kiln feed rate, i.e., the rate at which the raw materials are fed into the upper end of the kiln, the kiln rotational speed, the fuel rate, i.e., the rate at which fuel is injected into the kiln and burned, and the exit gas rate, i.e., the rate at which the combustion gases and other gaseous kiln products are drawn through the kiln and exhausted from the feed end into the atmosphere. In addition, the kiln operator also has control over the cooling of the materials after they are discharged from the kiln. Usually, the materials are discharged onto a movable grate through which cooling air is forced and transmitted along the movable grate to a conveyor for further processing. The kiln operator has immediate control over the rate at which cooling air is forced through the material and the operating speed of the movable grate. The kiln operator attempts to select values for each of these control variables which will result in stable kiln operation producing a desirable product at the required product volume.

One primary control approach is that of maintaining conditions in the burning zone or clinkering zone constant. To this end, control systems sense parameters such as burning zone temperature and other kiln temperatures, torque required to rotate the kiln and others to regulate the rate of fuel input to the kiln or the speed of the kiln. In this manner, the effects of process disturbances are minimized and kiln stability is greatly improved. Such stability minimizes kiln operating cost by producing a more uniform product and increasing the efficiency of the equipment. For example increased kiln stability increases the kiln-lining life and thereby reduces the frequency at which the refractory lining has to be replaced in the kiln.

Therefore, it is an object of this invention to provide a control system which improves stability in a rotary kiln.

Another object of this invention is to provide a control system which is compatible with and complements other kiln control systems for improving kiln operating stability.

Yet another object of this invention is to provide a control system for improving rotary kiln stability by stabilizing kilnlining temperature.

SUMMARY In accordance with one aspect of this invention, the kiln-lining temperature and heat transfer characteristics of materials on a cooling grate are analyzed and the measurements implemented to control the depth of materials on the grate to maintain a constant kiln-lining temperature. A secondary control function includes measurement of the air temperature leaving the material to control the air flow through the materials.

The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The above and further objects and advantages of this invention may be better understood by reference to the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram depicting a rotary cement kiln embodying and utilizing the present invention;

FIG. 2 is a block diagram illustrating a control system incorporating the invention and employed to control the operation of the rotary cement kiln of FIG. 1;

FIG, 3, FIG. 3A and FIG. 3B are Ilow diagrams illustrating the operation of the control system of FIG. 2.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT Referring to FIG. 1, a typical rotary cement kiln with its associated equipment is schematically illustrated. Rotary cement kiln I0 has at its upper or feed end a kiln feed hopper l I and a kiln feed pipe 12 for feeding blended raw materials l3 into the upper end of kiln I0. The raw materials normally include Al,0,, Silk, Fe fl MgCO, and CaCo plus small amounts of K,0, Na,0 and sulfur. The blended raw materials or feed may be in the form of a dry powder or a slurry and may be preheated in a he'atexchanger utilizing the kiln exit gases. The specific embodiment shown is particularly adapted for use with slurry mixtures. Chains 16 are suspended along the kiln length from the feed end 14 to remove moisture from the slurry. Such chains could be used to preheat a dry mixture but are not necessary for dry mixtures. Kiln l0, inclined downward at an angle of approximately 3 from feed end 14 to discharge end 15, is rotated by an electric motor 20 shown here driving a pinion gear 21 that engages a ring gear 22 encircling and attached to kiln 10. As kiln 10 is rotated by kiln drive motor 20 through gears 21 and 22, the kiln rotation causes the raw materials or feed to slowly cascade forward, the rate of forward progress of the feed within kiln being approximately proportional to kiln rotational speed. Motor is normally controlled to drive kiln 10 at a predetermined constant rotational speed.

At the discharge end of the kiln, a fuel supply line 25 and a primary air-supply line 26 are connected to a fuel-air mixing chamber 27 which injects a high-energy flame into kiln 10. Natural gas, pulverized coal, oil or combinations thereof may be employed as fuel, the fuel being fed into line 25 from a suitable source. The primary air is forced through line 26 and into chamber 27 by fan 28.

The interior of kiln 10 is lined with a refractory material (not shown in detail) generally designated by numeral 29 which is capable of absorbing heat from flame 30 and transmitting it to the gases and feed travelling through kiln Ill. The combustion gases and other gaseous kiln products are drawn through the kiln by an induced draft fan 31 which exhausts the gases through a dust collector and stack 32. Induced draft fan 31 creates a slightly negative pressure in the kiln drawing secondary air from clinker cooler 35 through the kiln. The gases emerging from feed end 14 of kiln 10 pass through a series of dust collectors 37 which recover the dust and an exit gas damper 38. The dust may be reintroduced to the kiln through a conduit 33 to a dust feeder 34.

As the feed proceeds slowly down the kiln, it is heated by hot gases flowing counter to it and also by the heated refracto ry walls of the kiln. The temperature of the dry feed increases until the calcining temperature is reached. At this point, the calcium carbonate CaCtl, and the magnesium carbonate MgCO, begin to decompose, forming C30 and Mg0. The released carbon dioxide CO, joins the combustion gas and is drawn from kiln ill by fan 31. The zone of kiln 10 where this reaction occurs is called the calcining zone. This reaction continues over a major portion of the kiln length. The temperature of the feed changes very little within this zone since the calcining reaction is endothermic and requires heat. A measurement of feed temperature within this zone will not give a meaningful indication of the degree of calcination of the feed.

At the point in kiln 10 where calcination of the feed is complete, a large temperature difference exists between the feed and the combustion gases and therefore a rapid increase in feed temperature results. The temperature at which the exothermic clinkering reaction occurs is reached quickly and the heat generated by the clinkering reaction causes the temperature of the feed to increase still further to the point where the solids become partially liquefied. The clinkering reaction for the formation of (CaO),-(Si0, (CaO),-(Al,0,,) and (Ca0),-(AB1"L0:,)-(Fe,0,), which are the crystalline compounds that determine the physical properties of the cement, occurs rapidly. The resulting partly fused mass of varying size continues to move down the burning zone of the kiln and remains near its maximum temperature until it nears discharge end 15 of the kiln. While at this temperature, most of the remaining CaO combines with the (Ca0),-(SiO,) to form (CaO);,-(Si0,). The degree of completion of this clinkering reaction depends upon the feed composition, the temperature in the burning zone and the residence time of an increment of feed within the zone.

As the hot clinker material approaches the end of the kiln, it begins to lose some of its heat to the incoming secondary air. At the exit end of the kiln, the clinker drops onto the travelling grate 40 usually reciprocated by a motor 4i. Air is blown through grate 40 by fan 42 to cool the clinker. Part of the resulting heated air becomes secondary air which is drawn through kiln 10 by fan 31, the remainder being exhausted by fan 43 through dust cyclone 44 to the atmosphere. The cooled clinker is transported by conveyor 45 to grinding apparatus (not shown) which pulverizes the clinker to form cement.

A number of sensors are provided to monitor various parameters of kiln operation and to generate electrical signals representing the values of these parameters. These signals are employed by the control system of the invention to affect the improved kiln stabilization. As illustrated in FIG. 1, control system 50 has a signal coupled thereto from undergrate pressure sensor 51 on line 52 indicating the air pressure in the chamber under movable grate 40. A kiln-lining temperaturesensing device 53, for example a thermocouple. is provided near the discharge end 15 of the kiln to provide a signal transmitted to control system 50 on line 54 indicating the kiln-lining temperature at that point. Another temperature sensing device 55 is located to measure the average temperature of the secondary air passing from the material on grate 40 to the kiln. This sensor is coupled to control system 50 by line 56. Control system 50 utilizes the information concerning kiln operation provided on

lines

52, 54 and 56 to produce control signals on

lines

60 and 61. The control signal on line 60 represents a cooler speed set point and is applied to controller 62 to control the speed of motor 41 and therefore the depth of material on grate 40. The control signal on line 6] represents a cooler damper set point signal and is applied to controller 63 to control positioners 47 and, thereby, the position of dampers 46. Varying the position of the individual dampers 46 thereby varies the flow rate of cooling air from fan 42 through grate 40 and the material thereon. Controllers 62 and 63 are standard analog controllers as known in the art and will not be described in detail.

FIG. 2 illustrates the details of control system 50 shown in FIG. 1. Referring to FlG.2, the signal on line 54 representing the temperature of the kiln lining is applied to check logic 70 as measured by the kiln-lining temperature-sensor 53. The signal on line 54 LTP, is constantly being applied to check logic 70 which compares successive values of signal LTP When two successive values differ by more than a given value. it is assumed that a disturbance exists or that the temperature measuring device has failed and the former, or earlier, value of LTP,,,,,. is used. An alarm function may be initiated if such differential values continue to exist. The functions of check logic 70 may be conveniently performed in a digital computer with successive values of LTP,,.,,,, being stored in the computer memory. Check logic 70 thereby serves to mask momentary or short term disturbances and to sense the failure of the kilnlining-temperature-measuring device 53.

Filtering and smoothing of signal LTP to remove noise and other signal variations unrelated to the kiln-lining temperature is performed in filter 71. The output signal of filter 7l is FLTP,,. The filtering action of filter 71 is described in the equation:

FLTP is the new filtered value,

FLTP, is the last filtered value,

LTP is the present scan value and K is the filter constant.

The function of filter 7l may conveniently be performed in a digital computer with signals FLTP... FLTP, and LTP and constant K being stored in the computer memory. This calculation is performed at short intervals to insure that FLTP, represents the current condition of the kiln-lining temperature thereby forming an accurate basis for control action. K is selected to be small enough to eliminate noise and other unrelated disturbances on the signal, but not so small as to damp out the signal.

Output signal FLTP of filter 71 is applied to summing amplifler 72. A kiln-lining-temperature setpoint signal LTSP is also applied to summing amplifier 72. The kiln-lining-temperature setpoint represented by LTSP is controlled by the operator and generated by means of a potentiometer, a value stored in a digital computer or equivalent means. Typical kilnlining-temperature setpoint values for a particular kiln used in the cement process will be based upon past experience.

Summing amplifier 72 is of the type well known in the art and provides a temporary kiln-lining-temperature error TELTP, proportional to the difference between the lining temperature setpoint LTSP and the present filtered value of the kiln-lining temperature represented by signal FLTP,,, as expressed by the equation:

Temporary kiln-lining-temperature error TELTP, is positive if the present filtered value of kiln-lining-temperature FLTP, is less than the lining-temperature setpoint LTSP. The error TELTP, is negative if the present filtered value of kilnlining-temperature FLTP, exceeds the lining-temperature setpoint LTSP. The function of summing amplifier 72 may con veniently be performed in a digital computer.

Temporary kiln-lining-temperature error signal TELTP, is applied to proportional windup logic 73. Proportional windup logic 73 is used in conjunction with setpoint generator 74 and pressure limit logic 75 to substantially eliminate adverse effects caused by proportional windup of the control system which occurs when a two-mode control algorithm is implemented.

Various types of windup occur when control algorithms are implemented in nonlinear systems, as is known in the art. Proportional windup occurs in a control system which implements a control algorithm on an incremental basis. Proportional or reset windup will occur when either of the normally utilized twoor three-mode control algorithms are implemented with the type of windup being dependent upon the particular control algorithm. if either proportional or reset windup are encountered, the process will not respond properly. Therefore, both proportional and reset windup problems must be overcome if a control system is to be effective. Either proportional or reset windup can be eliminated by the proper selection of the control algorithms. In this particular embodiment, twomode control algorithms which eliminate reset windup are utilized; and means for eliminating the resulting proportional windup problems are disclosed. Equivalent means could be utilized to eliminate reset windup if the control algorithm were selected initially to eliminate proportional windup.

Proportional windup logic 73 responds to an indication from pressure limit logic 75 that the previous control signal was limited. Such a limiting action indicates that the control system was operating outside its effective range and that proportional windup could be encountered in this control action. Proportional windup logic 73 responds to such an indication to substantially eliminate adverse effects. Such compensation is necessary because setpoint generator 75 generates an output control signal which is a function of a two-mode control algorithm. The output signal from setpoint generator 74 is examined by pressure limit logic 75 to determine whether the control signal will place the system into a proportional windup range. Pressure limit logic 75 is coupled to proportional windup logic 73 to indicate, therefore, whether a proportional windup problem existed on the previous control action. lf such a condition did exist, proportional windup logic 73 would clamp TELTP, to a limit determined by pressure limit logic 75 during the previous control action. As a result, the output of proportional windup logic 73 represented by signal ELTP, is generated in accordance with the following relationships:

ELTP,,=TELTP,, if ELTP,,,,,, s TELTP, s ELTP ELTP,,=ELTP,,,,,, if TELTP,, ELTP,,,,,

ELTP,=ELTP,,,,,, if TELTP,, ELTP,,,, where ELTP, is the present error function to be used as a control variable, and

ELTP and ELTP,,,,,, are process dependent limits.

The function of proportional windup logic may be conveniently performed in a digital computer with signals ELTP,,, ELTP, and TELTP, and limits ELTP,,,,,, and ELTP being stored in computer memory.

The resulting error signal ELTP, is applied to setpoint generator 74 which determines a temporary undergrate-pressure setpoint signal TUGPSP, in accordance with two-mode control algorithm:

TUGPSP, is the new temporary undergrate-pressure setpoint,

UGPSP is the previous undergrate-pressure setpoint,

ELTP, is the present kiln-lining-temperature error, and

ELTP, is the previous kiln-lining-temperature error.

K and K, are controller constants which are determined in accordance with the following equations:

K,=K,+l00(R)(T)/(PB) and K,=l00/(PB) Where R is the resets per minute for the setpoint generator,

T is the calculation interval in minutes, and

PB is the proportional band figure for the setpoint generator which is a function of the setpoint generator gain.

The function of setpoint generator 74 may be conveniently performed in a digital computer with signals TUGPSP... UGPSP FLTP, and ELTP and constants K, and K, being stored in computer memory.

The output of setpoint generator 74 represented by TUGPSP, is applied to pressure limit logic 75 to be compared with the previous undergrate-pressure setpoint signal UGPSP Pressure limit logic 75 determines whether the cal culated change in the undergrate-pressure setpoint would require the prevention of proportional windup by examining both the previous cooler speed setpoint and undergrate-pressure limits. If no undergrate-pressure setpoint change will result or if the control system senses the proportional windup condition and the proposed change would tend to cause the system to move further into proportional windup, the previous undergrate-pressure setpoint is used (i.e., UGPSP,,==UGPSP Otherwise, UGFSP, is set equal to TUGPSP. and is compared against limits UGPSl and UGPSP,,,.,,. If UGPSP, is limited, limits for kiln-lining-temperature error signal ELTP. are determined; and the fact that the undergrate-pressure setpoint has been limited is recognized. The limits and limit indication are fed back to proportional windup logic 73 for use in the succeeding control action. Therefore, the output of pressure limit logic 75 may be represented by the following relationships:

CSSP, at a maximum or minimum limit and TUGPSP, would decrease or increase the setpoint respectively, otherwise UGPSP,,=TUGPSP,,if UGPSPmin eruorsr, s UGPSP The function of pressure limit logic 75 may be conveniently performed in a digital computer with the signals UGPSP,,, TUGPSP. and UGPSP and the constant limits UGPSP,,,,,, and UGPSP being stored in computer memory.

As specifically shown, the output signal UGPSP,,is coupled through a delay circuit 76, whose output, therefore, represents the previous value for the undergrate-pressure setpoint, UGPSP It is also possible to obtain and utilize the signal UGPsP directly; however, system stability and the embodiment adapted for implementation in a computer is simplified if the delay is incorporated. Such a delay may be conveniently implemented by a digital computer with signals UGPSP, and UGPSP being stored in the computer memory.

The previous undergrate-pressure setpoint signal UGPSP is applied to summing amplifier to determine a temporary undergrate-pressure error represented by TEUGP,,. The other input to summing amplifier 80 is obtained from undergrate-pressure sensor 5|, illustrated in FIG. which generates a signal UGP on line 52. This signal is constantly being applied to check logic M which compares successive values of signal UGP to determine if a disturbance or sensor failure has occurred. An alarm function may be initiated if a failure exists. As check logic 81 produces the earlier value of UGP if the earlier and present values differ by more than a predetermined value, check logic 81 serves to mask momentary disturbances and sense failures of the measurement circuitry. The functions of check logic 81 may, like check logic 70, be conveniently performed in a digital computer.

The output signal of check logic 8! is applied to filter 82. Filter 82 smooths signal UGP and removes noise and other signal variations unrelated to the undergrate pressure. The output signal of filter 82 is FUGP, and is generated in response to FUGP is the new filtered value,

FUGP is the last filtered value,

UGP is the present scan value, and

K is the filter constant.

The function of filter 82, like filter 70, may also conveniently be performed by a digital computer with the signals FUGP,,, FUGP and UGP and the filter constant K being stored in computer memory. This calculation is also performed at short intervals to insure that FUGP represents the current condition of the undergrate pressure thereby forming an accurate basis for control action. K is selected to be small enough to eliminate noise and other unrelated disturbances, but not so small as to damp out the signal.

Output signal FUGP, is a filter value representing the air pressure under the grate. This air pressure is a function of the depth and porosity of materials on the grate and the charac teristics of the fan or source of cooling air. As the characteristics of the cooling air source are substantially constant, air pressure variations under the grate are primarily a function of variations in the depth and porosity of the materials. Further, the temperature of the cooling air leaving grate will vary in accordance with heat transferred to the cooling air as it passes through the materials, the initial heat in the cooling air being substantially constant. Further. the heat transfer from the materials is a function of the material depth and porosity. Therefore, changes in the filtered signal FUGP, represent changes in the heat transfer characteristics of the material on the grate and the heat transferred to the kiln from the cooler can be varied by changing the depth of material on the grate.

Output signal FUGP, is applied to summing amplifier 80 where it is combined with the previous setpoint signal UGPSP,,,, from delay circuit 76 to obtain temporary undergrate-pressure error signal TEUGP. in accordance with the equation:

Temporary undergrate-pressure signal TEUGP, is positive if the present filtered value of undergrate pressure represented by FUGP, is less than the pressure setpoint represented by UOPSP The error signal is negative when the present filtered value FGUP, is greater than the setpoint signal. Summing amplifier 80 may also be implemented in a digital computer.

Temporary undergrate-pressure error signal TEUGP, is applied to proportional windup logic 83. Proportional windup logic circuit 83 responds to previous indications that the system is in a proportional windup range to eliminate adverse effects. The output signal from controller 84 is examined by speed limit logic 85 to determine whether it will place the control system in a proportional windup range. Speed limit logic 85 is coupled back to proportional windup logic 83 to indicate. therefore, whether a proportional windup problem existed on the previous control action. If such a situation did exist, proportional windup check logic 83 clamps EUPG,,, representing the undergrate-pressure error to limits determined by speed limit logic if the error signal TEUGP, exceeds those limits. As a result, the output of proportional windup logic 83 is represen ed by signal EUGP, which is generated in accordance with the following relationships:

EUGP, is the present error function to be used as a control variable, and

EUGP and EUGP are process dependent limits.

The function of proportional windup logic 83 may be conveniently implemented in a digital computer with signals TEUGP, and EUGI and limit signals EUGP and EUGP being stored in computer memory.

The resulting error signal EUGP,is applied to controller 84 which generates a temporary speed setpoint signal TCSSP, in accordance with the two-mode control algorithm TCSSP, is the new temporary cooler speed setpoint signal,

CSSP is the last cooler speed setpoint signal,

EUGP, is the present error function, and

EUGP is the previous error function.

R is the resets per minute for the controller,

T is the calculation interval in minutes, and

PB is the proportional band figure for the controller which is a function of the controller gain.

Controller 84 may be conveniently implemented with a digital computer with signals TCSSP... CSSP EUGP, and EUGP and constants K, and K, being stored in computer memory.

The output signal TCSSP, from controller 84 is applied to speed limit logic 85 1 compares TCSSP. with predetermined maximum and minimum constant values CSSP ,r and CSSP to determine if proportional windup will be encountered. CSSP is therefore representative of the lower end of the effective speed control range while CSSP represents the upper end. If either constant is exceeded, TCSSP, is clamped. Also EUGP and EUGP,,.,,, are determined. They represent the undergrate-pressure errors for present process conditions while will take the control system to proportional windup. Both error limit signals EUGP and EUGP,,,,,, and the indication that proportional windup exists are fed back to proportional windup logic 83. if TCSSP, lies within the acceptable range, then CSSl, is set equal to TCSSP,,. Therefore, the output of speed limit logic 85 may be represented by the following relationships:

CSSP, is the present cooler speed setpoint signal, and

CSSP and CSSP are constant limits indicating the lower and upper proportional windup points.

The function of speed limit logic 85 may be conveniently performed in a digital computer with signals TCSSP, and CSSP and constant limits CSSP,,,,,, and CSSP being stored in computer memory.

Cooler speed setpoint signal CSSP, is then transferred back to the kiln and instrumentation on line 60 to cause controller 62, shown in FIG. I, to adjust the speed of motor 41 ac cordingly. Controller 62 is a conventional analog controller which is known in the art. Adjusting the speed of motor 41 varies the material depth on movable grate 40. if the motor speed increases, the depth of materials decreases. Such a control action occurs when the lining temperature represented by LTP,.,,,, exceeds the setpoint signal LTSP. As the depth of materials decreases, less heat is transferred from this material to the kiln thereby tending to decrease the kiln-lining temperature. Likewise, if the kiln-lining temperature becomes cool, motor 41 moves grate 40 more slowly allowing a buildup of materials on grate 40 with the result that heat transfer at the kiln discharge end increases. This causes an increase in the kiln-lining temperature. Hence the action of this control system tends to stabilize the kiln-lining temperature with the result that the thermal stability of the overall system is increased.

Still further increased thermal stability can be obtained by altering the position of the dampers 46 in response to the average secondary air temperature as measured by sensor 55 shown in FIG. 1. Sensor 55 generates a signal SAT on line 56 which represents the temperature of the secondary air entering kiln 10. The signal SAT is initially transferred to check logic 90 which compares successive values. When two successive values differ by more than a given value, it is assumed that a disturbance exists or that the temperature sensor 55 has failed; and the former, or earlier, value of SAT is used. An alarm function may also be initiated if such differential values continue to exist. The functions of check logic 90 may conveniently be performed in a digital computer, with successive values of SAT being stored in computer memory. Check logic 90 thereby serves to mask momentary short term disturbances and to sense the failure of secondary air-temperature measuring device 55.

Filtering and smoothing of the signal SAT to remove noise and other signal variations unrelated to the kiln-lining temperature is performed in filter 91. The output signal of filter 91 is FSAT,,. The filtering action of filter 91 is described in the equation:

FSAT,=FSAT,.,1+K,,,, (SAT 3] FSAT Where FSAT, is the new filtered value,

FSAT,. is the last filtered value,

SAT is the present scan value, and

K, is the filter constant.

The function of filter 91 may conveniently be performed in a digital computer with signals FSAT, l-SAT and SAT and constant K, being stored in computer memory. This calculation is performed at short intervals to insure that FSAT, represents the current condition of the secondary air-temperature thereby forming an accurate basis for control action. K is selected to be small enough to eliminate noise and other unrelated disturbances, but not so small as to damp out the signal.

OUtput signal FSAT of filter 9] is applied to summing amplifier 92 where it is compared with a trended secondary air temperature signal TSAT, from trend filter 93. Trend filter 93 produces a filtering action as described by equation TSAT is the new trended temperature value.

TSAT, is the last trended temperature value,

FSAT, is the present filtered value, and

K is the filter constant.

The function of trend filter 93 may be conveniently performed in a digital computer with signals TSAT,., TSAT and FSAT, and filter constant K being stored in computer memory.

A trended-air-temperature value is utilized as a base for determining errors rather than a constant setpoint signal. This allows the secondary air-temperature control loop to control the air temperature but not to override the cooler speed control loop. Hence, the setpoint signal represented by trend-airtemperature signal TSAT, factors in variations caused by control actions of the cooler speed control loop and responds to short term disturbances.

Summing amplifier 92 is of the type well known in the art and may be implemented in a digital computer. Summing amplifier 92 provides a temporary error signal TESAT, proportional to the difference between the secondary air-temperature setpoint signal TSAT, and the present filtered value of secondary air temperature represented by signal FSAT as expressed by the equation TESAT,,=TSAT,,FSAT,

The temporary secondary-temperature error signal TESAT, is applied to proportional windup logic circuit 94. Proportional windup logic 94, like proportional windup logic 83, responds to previous indications that the system is in proportional windup to eliminate adverse effects. The output signal from controller 95 is examined by damper check logic 96 to determine whether the control signal will place the system in a proportional windup range. Damper limit logic 96 is coupled back to proportional windup 94 to indicate. therefore, whether a proportional windup problem existed on the previous control action. if such a situation did exist, proportional windup check logic 94 clamps ESAT, to limits determined by damper limit logic 96 if the present error signal TESAT. exceeds those limits. As a result, the output of proportional windup logic 94 represented by signal ESAT. is generated in accordance with the following relationships:

ESAT.,=TESAT, if ESAT,,.,,,s TIESAT, s ESAT ESAT,,=ESAT,,,,, if TESAT,, ESAT,, and

l-ZSAT,,=ESAT,,., if TESAT,, E$AT,,,,

Where ESAT, is the present error function to be used as a control variable. and

ESAT and ESAT are process dependent limits.

The functions of proportional windup logic 94 may conveniently be performed in a digital computer with signals ESAT, and TESAT, and limits ESAT and ESAT being stored in computer memory.

The resulting error signal ESAT. is applied to controller 95 which generates a cooler damper position setpoint signal TCDSP, in accordance with the two-mode control algorithm 'ICDSl, is the new temporary cooler damper position setpoint signal,

CDSP is the last cooler damper position setpoint signal,

ESAT is the present error function, and

ESAT, is the previous error function.

K, and K are controller constants which are determined in accordance with the following equations:

R is the resets per minute for the controller,

T is the calculation interval in minutes, and

PB is the proportional band figure for the controller gain.

The function of controller 95 may conveniently be performed in a digital computer with signals TCDSP... CDSP ESAT, and ESAT. and constants K; and K being stored in computer memory.

The output signal TCDSP, from controller 95 is applied to damper limit logic 96 which compares TCDSP, with predetermined maximum and minimum constant values CDSP,,,,,, and CDSP to determine if proportional windup will be encountered. CDSP therefore represents a value of TDCSP, which will move the dampers to a minimum position; CDSP.,.., indicates the value of TCDSP, will open the dampers to a maximum position. if either constant is exceeded, TCDSP, is clamped. Also ESAT and ESAT are determined. The represent the errors for the present process conditions which will just take the process control system to proportional windup. Both error signal limits and the indication that proportional windup exists are fed back to proportional windup logic circuit 94. If TCDSP lies within the acceptable range. then CDSP, is set equal to TCDSP,,. Therefore. the output of damper logic circuit 96 may be represented by the following relationships:

CDSP,,=TCDSP,, if CDSP s TCDSP,,,,,,.

CDSP,=(IDSP,.,,. if TCDShs CDSP CDSP, =CDSP if TCDSP, zCDSP Where CDSl, is the present cooler damper position setpoint, and

CDSP and CDSP are constant limits indicating the proportional windup range limits.

Damper limit logic 96 may be conveniently implemented in a digital computer with CDSP, and TCDSP,,,. and constant limits CDSP and CDSl being stored in computer memory.

Cooler damper position setpoint signal CDSP, is then transferred back to the kiln and instrumentation on line 61 to cause an analog controller 63 or equivalent shown in PK 1 to adjust to the position of dampers 46 accordingly. As shown the controller 63 produces an output signal which is applied to an intermediate positioner 47 which controls the position of each damper. Both controller 63 and positioner 47 are known in the art.

By adjusting the position of dampers 46, air flow through grate 40 can be altered independently of the variations due to control actions from the cooler grate speed control loop. However, sustained changes in the operating conditions caused by control actions from the cooler speed control loop are factored into the secondary air-temperature control loop by trending the setpoint signal as a function of actual process temperature. Assuming a constant bed depth, the secondary air-temperature control loop opens the dampers as secondary air temperature increases and closes the dampers as the temperature decreases. Changes in secondary air temperature caused primarily by speed variations of motor 41 are reflected as corresponding changes in the trended secondary air-temperature setpoint. As a result, variations, in the sensed secondary air temperature will be nullified so the the secondary airtemperature control loop will take no action.

FIGS. 3, 3A and 3B (hereinafter FIG. 3) illustrate a flow chart of the operation of a control system such as that shown in FIG. 2 which is especially adapted for implementation by a digital computer. Signal UGlL representing the undergrate prenure and, hence, the depth of material on the grate, is checked to determine whether successive values differ by more than a predetermined value. The previous value of UGP is saved and used if the difference exceeds the predetermined value. The signal is filtered periodically to obtain a filtered value FUGP, and compared to the previously calculated undergrate pressure setpoint UGPSP and a temporary undergrate-pressure error signal TEUGP, is generated representing the difference between UGPSP and FUGP,,.

If the previous cooler speed was not limited, a temporary cooler speed setpoint signal TCSSP, is calculated. However, if the previous cooler speed setpoint signal was limited indicating that proportional windup could exist, the error signal TEUGP, is compared with previously determined limits EUGP... and EUGP lf TEUGP, lies between or equals the limits, TEUGP, becomes the error signal EUGP,,', if not, EUGP. is set equal to the appropriate limit value EUGP or EUGP Then EUGP, is used to determine the temporary cooler speed setpoint. The temporary cooler speed setpoint is checked against limits CSSP and CSSP which represent the range of eflective control for the particular apparatus. if within or equal to the limits, the calculated setpoint TCSSP, becomes the efi'ective setpoint CSSP,,; otherwise, the output signal is limited to the maximum or minimum values. When limiting occurs, EUGP,,,,, and EUGP are calculated as those values which will, under the present process conditions, cause an output of CSSP or CSSP respectively. These signals are stored in a digital computer until the next control action. When limiting occurs, an indication of the fact is made.

The next series of steps in the control system operation outlined in FIG. 3 is that of measuring the kiln-lining temperature. Signal LTi representing the kiln-lining temperature is checked to determine whether successive values differ by more than a predetermined value. The previous value of LTP. is saved and used if the difi'erence exceeds the predetermined value. The signal is filtered periodically to obtain a filtered value FLTP. and compared with the desired kiln-lining temperature represented by setpoint signal LTSP. The resulting error signal TELTP, is used to determine a temporary undergrate-pressure setpoint signal TUGPSP. if the previous value of the undergrate-pressure setpoint signal USPSP, was within limits. If it was not, then the error signal is compared with temperature error limits ELTP,,,,,, and ELTP generated during the previous control action and set to those limits as appropriate. Therefore, either the actual kiln-lining temperature or a limiting value will be used as an error signal ELTP, to calculate a temporary undergrate-pressure setpoint signal TUGPSP,,.

The value of the temporary undergrste-pressure setpoint signal TUGPSP, is examined to determine whether it will be used as the new undergrate-pressure setpoint signal UGPSP,,. 1f the temporary and previous setpoint signals are equal (l.e., TUGPSP -UGPSP the temporary setpoint signal becomes the control variable. If there is a difference in the two values, then the magnitude and direction of the resulting control action is examined to determine whether a control action generated from implementing TUGPSP, would place the process into proportional windup. If it would not, then the temporary setpoint signal is used as a control variable UGPSh; if such control action would cause proportional windup, then the control variable is set to UGPSP or UGPSP. which are constant limits. When such limiting occurs. new lining temperature error limits ELTP... and ELTP are calculated; and an indication of limiting is made.

if the secondary air-temperature control loop were not utilized, the control system would utilize the value of cooler speed setpoint signal CSSP, to adjust the system. Where however, such a control loop is utilized, the control system then measures the secondary air temperature and generates a signal SAT Successive values of SAT are checked to determine whether they differ by a predetermined value. If they do, the earlier value is saved and used. The signal is filtered periodically to obtain a filtered value FSAT... This signal is trended to obtain a trended secondary air-temperature signal TSAT... These signals are compared and a temporary error signal TESAT, representing the difl'erence between the trended and filtered secondary air temperatures is obtained. if the previous cooler damper setpoint signal did not drive the dampers to a limiting position, the diflerence between the trended and filtered secondary air temperatures is utilized as a control signal BSAL. However, if the previous control signal did cause proportional windup, the error or difference signal is utilized only if it does not exceed limits calculated on the previous control action. if the newly calculated difference does exceed one limit, that limit is utilized.

Once the secondary air-temperature difi'erence signal ESAT, has been selected, a new cooler damper position setpoint signal TCDSP. is generated. If it is within the limits of damper position, it becomes the cooler damper position setpoint signal CDSP. Otherwise either the limit CDSP, or CDSP, is utilized. When a limit is required, an indication of proportional windup is made and new values for the temperature difference limits ESAT and ISAT,.,,, are calculated. These are the limits which will cause the calculated value of the cooler damper position setpoint signal to reach the minimum or maximum limits respectively. Therefore, either the calculated value or a limit is used as the cooler damper position setpoint signal CDSP... At this point, the control system of FIG. 3 would cause both the values of C881. and CDSP. to be transmitted to the

controllers

60 and 63 respectively shown in FIG. 1. After a predetermined interval, the control system of FIG. 3 would be implemented again to determine new setpoint values in accordance with the present operation of the kiln.

As indicated previously, the control system described herein tends to stabilize the thermal characteristics of a rotary cement kiln by controlling the kiln-lining temperature at the discharge end and the temperature of the air entering the kiln. Primarily, the control system will regulate the depth of discharged materials on a cooler grate to thereby vary the heat transfer from the materials to the kiln lining. Long term variations in the secondary air temperature are not effective to alter the damper positions because the control action is dependent upon a trended temperature value as a setpoint. However, damper position is controlled to compensate for short term changes in the secondary air temperature. It will be recognized by those skilled in the art that many modifications may be made to the various embodiments of the disclosed system without varying the true spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such modifications and variations which are within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A control system for rotary kiln apparatus including a kiln lining desirably maintained at a predetermined temperature and cooling apparatus including movable grate means adapted to receive materials from a discharge end of the kiln and a source of cooling air, said control system comprising means for indicating the kiln-lining temperature at the discharge end and means for indicating the heat transfer characteristics of discharged materials on the movable grate and control means responsive to said kiln-lining temperature measuring means and said characteristic indicating means and adapted for controlling said movable grate means so as to maintain the kiln-lining temperature at the predetermined value.

2. A control system as recited in claim 1 wherein said characteristic indicating means includes means operably coupled to said grate means for sensing the air pressure under said grate means.

3. A control system as recited in claim 2 wherein variable speed drive means are operably connected to said movable grate means, said control means including means responsive to said kiln-lining temperature means and the predetermined temperature value for determining the temperature difference and means coupled to said temperature difference means for generating a signal representing the desired grate means pressure as a function of the temperature difference and means responsive to said desired grate-pressure generating means and said grate-pressure sensing means for determining the pressure difference and means responsive to said pressure-difference generating means for producing a speed control signal, said variable speed drive means being responsive to the speed control signal 4. A control system as recited in claim 3 wherein said source of cooling air includes controllable means for transferring the cooling air through the grate and material on the grate, said control means additionally comprising means for measuring the cooling air temperature after the cooling air passes through the material and means responsive to said measurement of air temperature for controlling said source of cooling air.

5. A control system as recited in claim 4 wherein said source of cooling air comprises a fan and damper means for controlling air flow to the grate. said control means being responsive to said air temperature sensing means for generating a predetermined air temperature and means responsive to said air temperature sensing means and said predetermined temperature signal generating means for controlling said damper means.

6. A method for controlling a rotary kiln apparatus including a kiln lining desirably maintained at a predetermined temperature and cooling apparatus including movable grate means adapted to receive materials discharged from a discharge end of the kiln and a source of cooling air comprising the steps of measuring the kiln-lining temperature at the discharged end, analyzing the heat transfer characteristics of the discharged materials on the movable grate and, in response to said kiln-lining-temperature-measuring and heat transfer characteristic analyzing steps controlling the movable grate means so as to maintain the kiln-lining temperature at the predetermined value by varying the amount of materials on the grate means.

7. A control method as recited in claim 6 wherein said analyzing the heat transfer characteristics includes the step of measuring the air pressure under the grate means caused by discharged materials, thereon,

8. A control method as recited in claim 7 wherein the grate means may be moved at variable speeds to thereby vary the amount of materials thereon, said control method including the additional steps of determining the difl'erence between the measured kiln-lining temperature and the desired kiln-lining temperature and, in response thereto determining an undergrate-pressure setpoint and varying the movable grate speed so as to maintain the actual undergrate pressure equal to the undergrate-pressure setpoint.

9. A control method as recited in claim 8 wherein said cooling air source includes means for controlling cooling air flow through the grate means, said control method additionally comprising the steps of measuring the temperature of the air after it exits from the material and responding to said measurement for controlling the quantity of cooling air passing through the grate.

10. A control system as recited in claim 9 wherein said source of cooling air includes controllable damper means and a fan, the controllable damper means controlling the air flow through the material, said control method additionally comprising the step of determining a desired air temperature from measured air temperatures and, in response to a diflerence therebetween, controlling the damper positions to maintain the measured air temperature at the desired air temperature.

Claims

1. A control system for rotary kiln apparatus including a kiln lining desirably maintained at a predetermined temperature and cooling apparatus including movable grate means adapted to receive materials from a discharge end of the kiln and a source of cooling air, said control system comprising means for indicating the kiln-lining temperature at the discharge end and means for indicating the heat transfer characteristics of discharged materials on the movable grate and control means responsive to said kiln-lining temperature measuring means and said characteristic indicating means and adapted for controlling said movable grate means so as to maintain the kiln-lining temperature at the predetermined value.

3. A control system as recited in claim 2 wherein variable speed drive means are operably connected to said movable grate means, said control means including means responsive to said kiln-lining temperature means and the predetermined temperature value for determining the temperature difference and means coupled to said temperature difference means for generating a signal representing the desired grate means pressure as a function of the temperature difference and means responsive to said desired grate-pressure generating means and said grate-pressure sensing means for determining the pressure difference and means responsive to said pressure-difference generating means for producing a speed control signal, said variable speed drive means being responsive to the speed control signal

4. A control system as recited in claim 3 wherein said source of cooling air includes controllable means for transferring the cooling air through the grate and material on the grate, said control means additionally comprising means for measuring the cooling air temperature after the cooling air passes through the material and means responsive to said measurement of air temperature for controlling said source of cooling air.

5. A control system as recited in claim 4 wherein said source of cooling air cOmprises a fan and damper means for controlling air flow to the grate, said control means being responsive to said air temperature sensing means for generating a predetermined air temperature and means responsive to said air temperature sensing means and said predetermined temperature signal generating means for controlling said damper means.

8. A control method as recited in claim 7 wherein the grate means may be moved at variable speeds to thereby vary the amount of materials thereon, said control method including the additional steps of determining the difference between the measured kiln-lining temperature and the desired kiln-lining temperature and, in response thereto determining an undergrate-pressure setpoint and varying the movable grate speed so as to maintain the actual undergrate pressure equal to the undergrate-pressure setpoint.

10. A control system as recited in claim 9 wherein said source of cooling air includes controllable damper means and a fan, the controllable damper means controlling the air flow through the material, said control method additionally comprising the step of determining a desired air temperature from measured air temperatures and, in response to a difference therebetween, controlling the damper positions to maintain the measured air temperature at the desired air temperature.