US3483363A - Method and apparatus for maximizing the output of a rotary kiln - Google Patents

Description

C. W. ROSS Dec. 9, 1969 METHOD AND APPARATUS FOR MAXIMIZING THE OUTPUT OF A ROTARY KILN 4 Sheets-Sheet 1 Filed April 22, 1966 JOKRPZOQ omwnmm INVENTOR JOWFZOQ Qmmaw CHARLES W. ROSS BY fll lluu AGENT C. W. ROSS Dec. 9, 1969 METHOD AND APPARATUS FOR MAXIMIZING THE OUTPUT OF A ROTARY KILN 4 Sheets-Sheet- 2 Filed April 22, 1966 JOKPZOQ ommam .4 ma 2% 1 26d m fig E Dec. 9, M969 C. W. ROSS METHOD AND APPARATUS FOR MAXIMIZING THE OUTPUT OF A ROTARY KILN Filed April 22, 1966 4 Sheets-Sheet 4 AKS'o I 15 MODEL MGDEL SIMULATOR saMuLAmR 544 AKSoL KT FROI 15 t5 FRQ KETO t5 A E m \A/@ A/D/ \A/D t6 551? #348 522 n 6 mam RATE -546 gmw TEMP AFRO ERROR s5 PQWT COMPUTER CGMPUTER 950 390 ET-H KEOo t6 KEO 36 KET comma. 112 L pm COMPUTER E T} g 57 55O\A/D A/D Al's PULSE 540 558 -356 GENERATOR {if 534\ ID FAN SPEED 00mm. -fl

SY$TEM ID FAN MOTOR jQ/M nited States Patent 3,483,363 METHOD AND APPARATUS FOR MAXIMlZlNG THE OUTPUT 01 A ROTARY KILN Charles W. Ross, Hatboro, Pa., assignor to Leeds &

Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Apr. 22, 1966, Ser. No. 544,597 Int. Cl. G06f /46, 15/20; G06g 7/48 US. Cl. 235-151.1 17 Claims ABSTRACT OF THE DISCLOSURE The rate of product flow in a cement kiln is maximized by controlling the induced draft fan at its maximum speed and modifying the product flow rate so as to maintain the desired oxygen concentration at the kiln exit. This is accomplished in one arrangement by modifying the fan speed in response to changes in the oxygen concentration at the kiln exit and then modifying the product flow in response to changes in the fan speed until the changes in oxygen concentration at the exit resulting from the product flow changes are sufiicient to cause the fan speed to control at the desired maximum. The fan speed control and the kiln firing rate control are modified by feed-forward signals to anticipate the effect on exit conditions and the clinker temperature which will result from a change in product flow rate.

This invention relates to a method and means for effecting the maximization of the output of a rotary kiln and is more particularly directed to a method and means for controlling the rate of product discharge from a rotary kiln while at the same time preventing interaction of the results of such control on the control of other quantities in the kiln process.

A typical rotary kiln is the type used in the production of cement. These kilns are usually in the form of a long cylinder which is tilted from the horizontal at a slight angle and is slowly rotated about its axis. The raw materials to be formed into the cement clinker, which is the product, are introduced into the upper or higher end of the kiln. These materials slowly travel along the length of the kiln as the kiln is rotated and the rate at which the materials travel through the kiln and hence the product output rate is directly related to the speed of rotation of the kiln and/or to the rate of feed of raw material to the kiln.

As the feed material travels through the length of the kiln during a time period which may involve several hours it is heated by flames introduced at the lower or hot end of the kiln as well as by the hot air which travels through the kiln from the lower end to the upper end. As the material progresses along the kiln length it becomes fused to form clinker. To form suitable clinker it is, of course, desirable that the material be heated to a predetermined temperature and also that the temperature profile along the kiln have a desired character. Many variables affect kiln operation including the rate of feed of the raw materials to the kiln, the speed of rotation of the kiln, the rate of feed of air through the kiln and the control of the firing rate as determined by the fuel feed.

It is, of course, desirable to maintain the production rate of a rotary kiln as high as possible. It has generally been found that the production rate is limited by the capacity of the induced draft fan which draws air through the kiln and thus the control of the several variables which are important in kiln operation should be such as to allow for a maintenance of the maximum possible speed for the induced draft fan consistent with the necessity to maintain a range of control to accommodate variations in kiln ice conditions. It has been found that this can best be accomplished by exerting a control on the rate of flow of product through the kiln in such a manner as to maintain the desired conditions for the process with control of the induced draft fan speed.

It is therefore an object of this invention to provide an improved method and means for controlling the rate of flow of product through a rotary kiln so as to maximize the output of the kiln.

Another object of this invention is the provision of a method and means for preventing an upset of other variables which may be subject to control in the kiln as a result of the control of the rate of product flow as by controlling kiln speed and raw material feed rate to obtain maximum output for the kiln.

A still further object of this invention is the provision of a method and means for controlling the product flow rate of a rotary kiln so as to maintain a maximum air flow without exceeding the desired conditions in the kiln exhaust and without upsetting the control of the firing rate as the product flow rate of the kiln is varied by its control system.

In accordance with this invention, there is provided a method and means for the control of the rate of product flow by measuring the oxygen concentration in the kiln exit and determining the speed of the induced draft fan and changing the product flow rate is responsive to either the deviation of the oxygen concentration from its desired value or the deviation of the induced draft fan speed from its particular desired value so as to simultaneously establish the desired oxygen concentration while maintaining also the desired induced draft fan speed as required to maximize the product flow rate of the kiln consistent with controlability.

A more detailed understanding of this invention may be had from the following description in conjunction with the drawings in which like reference characters identify like elements.

FIG. 1 is a block diagram of an analog circuit showing a novel control system for controlling the speed of rotation of a rotary kiln, and/or its raw material feed rate,

FIG. 2 is a block diagram of another analog control system for controlling the speed of rotation of a rotary kiln, and/or its raw material feed rate,

FIG. 3 is a diagram showing how FIGS. 3A and 3B should be placed in juxtaposition to obtain a complete block diagram of a digital control system for controlling the speed of rotation of a rotary kiln,

FIG. 3A is one portion of the digital control system shown in block diagram form,

FIG. 3B is the other portion of the digital control system which is also shown in block diagram form.

In FIG. 1, the rotary kiln 10 is shown with its lower end to the left. At the lower end of the kiln, there is provided the source of heat for the process provided by the combusion of fuel supplied by a pipe which includes a fuel flow control valve 14 subject to being positioned by motor 16 through the mechanical coupling 18. The control of the fuel flow and hence the firing rate to the kiln is determined by a measurement of the temperature of the clinker 20 in the burning Zone.

The temperature of the clinker 20 is measured by radiation pyrometer 24 which has one of its electrical output terminals connected to ground and the other electrical output terminal connected by line 2-6 to the firing rate control system 23 shown as a block. The firing rate control system 28 receives as a feedback, along line 30, an indication of the actual flow of fuel through line 12 as measured by the fuel flow instrument 32 which responds to the differential pressure across the orifice plate 34- in fuel line 12.

The rotary kiln has at its upper or exhaust end the backhouse 35 which may include a tampering fan shown as 37. The backhouse leads to dust collecting chamber 38 which in turn leads to the induced draft fan 40 which continually draws air from the lower end of the kiln up through the kiln, the backhouse, and the dust collecting chamber and forces the air up through a baghouse 44 to chimney 48. The air thus circulated through the kiln serves not only to support the combustion of the fuel supplied by fuel line 12 but also to carry the heat generated by the combustion of the fuel along the length of the kiln to heat the feed material supplied to the kiln by Way of feed line 50 shown as having a funnel-shaped input. As mentioned before, these materials are sufficiently heated so as to form clinker which is discharged at the lower end of the kiln into the clinker cooler area 22.

In FIG. 1 the speed of rotation of the induced draft fan 40 which is driven by motor 54, is determined by the speed control system shown here as block 56 which has a speed setting supplied by knob 58 which is mechanically coupled :by shaft 60 to the speed control system. Knob 58 is preferably so adjusted as to cause the speed control system 56 to keep the motor 54 rotating at the maximum useful speed whereby the induced draft fan tends to draw through the kiln a maximum amount of air.

In the system of FIG. 1, since the air flow through the kiln is essentially fixed by the fixing of the speed of rotation of the induced draft fan, the firing rate control system 28 will serve to maintain the valve 14 adjusted so as to supply the amount of fuel necessary to provide the clinker 20 in the area at the lower end of the kiln at the desired temperature as determined by whatever setpoint is set into the firing rate control system 28.

Not only must the temperature of the clinker be controlled but also the percentage content of oxygen in the exhaust gases flowing from the kiln must be subjected to control. The concentration of oxygen in the exhaust area is indicative of the efliciency of combustion of the fuel and for proper operation of the kiln combustion efiiciency must be maintained at optimum value.

In the system of FIG. 1, the desired oxygen concentration can be maintained by the trimming action obtained by the adjustment of either the speed of rotation of the kiln or the rate of raw material fed through feed line 50. It will be evident that if the rate of kiln rotation is increased and if there is a simultaneous increase in the rate of raw material feed, the rate of flow in the material will be increased and hence the temperature of clinker 20 in the area sighted by the rayotube 24 will tend to decrease for the material fed through the kiln will have the same depth but will have had less chance to become heated by the hot gases due to the resulting increase in the rate of product flow to cooler 22. The result of this reduction in the temperature of the clinker 20 as viewed by the radiation pyrometer 24 will be an increase in the firing rate due to the response of the firing rate control 28. The increased firing rate would then naturally use up more oxygen from the air drawn through the kiln by the induced draft fan 40 and tend to therefore reduce the oxygen concentration in the gases in the exhaust area. Thus, the detection of an excess concentration of oxygen in the exhaust gases is desirably corrected by an increase in the speed of rotation of the kiln 10 and the simultaneous change in rate of flow of raw material. To accomplish this control there is provided in FIG. 1 an oxygen analyzer 60 which obtains a sample of the gases from the exhaust area of the kiln 10 by way of sample line 62. The oxygen analyzer which may be any one of a number of standard oxygen analyzers available on the market produces on its output line 64 a signal KEO representa tive of the actual oxygen concentration measured in the kiln exit gases. This signal KEO is supplied as one output to a controller 66 which includes both proportional and integral control actions as indicated by the PI designation in the block 66. Another input to controller 66 is the signal on line 68 which is obtained from the variable tap 69a of potentiometer 69. The variable tap 69a is adjusted along the slidewire 69 by the adjustable knob 69]).

The potentiometer slidewire 69 is supplied by a potential source E which is connected across the slidewire 69. It is thus evident that the signal supplied on line 68 represents the setpoint KEO and the controller 66 has for its purpose the control of speed of rotation of the rotary kiln so that the signal on line 64 tends to be brought to equality with the signal on lines 68.

The output of controller 66 is provided on line 67 to motor 70 which is a positioning motor which serves through the connecting shaft 72 and clutch 73 when it is engaged to position an element in the speed control, shown as block 74. This control is effective to determine the speed of motor 78, which is connected to the output of the speed control 74, so that the motor 78 will be rotated at a speed such that through its driving pinion 80 it will cause the coupled gear 82 to rotate the kiln 10 at a speed which will tend to bring about the previous mentioned equality between the signals on lines 64 and 68.

Also the motor 70 can be connected through shaft 75 and clutch plates 77 to modify the raw feed rate through pipe 50.

The modification of the raw feed rate through pipe 50 is accomplished by the controller 90, a PI controller providing proportional and integral response to the deviation between the setpoint input signal KFR and the actual raw material feed rate measured by load cells 92 and 93 and transmitted to controller by way of line 94. KFR is derived from potentiometer contact 91a on slidewire 91. Slidewire 91 is supplied from voltage source E and contact 91a is positioned by shaft 75 or manually set by knob 91b. The material supplied from hopper 95 to a constant speed moving belt 96 which carries the raw material to the funnel shaped entrance of pipe 50. The output of hopper 95 is controlled by butterfly valve 97 which is positioned b shaft 98 of motor 99. The positioning of motor 99 is controlled by controller 90.

The product flow rate to cooler 22 can be controlled by either controlling the speed of rotation of the kiln and the rate of feed of the raw material as by engaging clutch 73 and clutch 77. Alternatively the control of product flow rate may be controlled by varying the raw material feed rate only while keeping constant the kiln rotation speed. This later arrangement would result from a disengagement of clutch 73 while clutch 77 is maintained engaged.

With the speed of rotation and the feed rate of the raw materials simultaneously varied the bed depth in the kiln would remain essentially constant but if only the raw material feed rate is varied the bed depth would vary.

With a system such as that shown in FIG. 1, the control of the product output of the rotary kiln may be maximized. However, by virtue of the fact that the effect on kiln conditions such as burning zone temperature, for example, is slow in response to changes in kiln speed and the raw material feed rate, it is difficult to compensate in such a control system for any sudden fluctuations in the air flow through the kiln and the resulting fluctuations in the 0 concentration in the exhaust gases. Such fluctuations may result from ring breaks in the kiln or from the introduction of clean bags in the baghouse 44 or other conditions which will tend to suddenly reduce the load on induced draft fan 40 and thus cause a sudden change in the air flow through the kiln. In order to overcome this disadvantage a variation in the control system of FIG. 1 may be utilized. Such a variation is shown in FIG. 2.

In the system of FIG. 2 the speed of the induced draft fan 40, instead of being maintained at a predetermined value as in FIG. 1, is subject to control by the control system shown as block 100 which operates to maintain the exhaust conditions of kiln 10 at their desired values. The particular exhaust conditions which are subject to control by the control system 100 are the kiln exit ternperature KET and the kiln exit oxygen concentration KEO. The kiln exit temperature is measured by thermocouple 110 while the kiln exit oxygen concentration may be measured by an oxygen analyzer which is part of the control system 100. The oxygen analyzer would obtain its sample of the exhaust gases by way of sample line 62.

The control system 100 is preferably designed to maintain the kiln exit oxygen concentration KEO at a predetermined setpoint KEO as established by the signal on line 112 as obtained from the variable tap 114a of slideware 114 in accordance with the adjustment of knob 11412. As shown in FIG. 2, the slidewire 114 is supplied by a potential source E so that by adjustment of the knob 11417 the signal on line 112 may be varied as desired to represent the kiln exit oxygen concentration desired in the exhaust gases.

The setpoint for the kiln exit temperature may be determined by the setting of knob 22%. However, it is desirable as will be explained further in the subsequent portion of this description to modify the kiln exit temperature setpoint by a signal KET to obtain KEi as supplied to the control system 100 on line 116 as the efiective setpoint. The control system 109 may advantageously be constructed as set forth in copending application Ser. No. 509,348, of which I am a co-inventor, or other circuits capable of similar performance.

In FIG. 2, the control system 100 supplies on its output lines 118 a signal which determines the speed of the induced draft fan motor 54 to which the output lines are connected.

The speed of the motor 54 is measured by tachometer 120 which has one terminal of its output connections connected to ground and the other terminal connected to output line 122 which in turn forms an input to controller 144 providing a signal IDS representing the induced draft fan speed as actually measured.

The controller 144 is shown as a controller having both proportional and integral action as indicated by the PI notation in block 144.

The other input to the controller 144 is the setpoint for the induced draft fan speed, identified as a signal IDS and appearing on line 146 from variable tap 148a of slidewire 148 as adjusted by adjustable knob 14812. As shown in FIG. 2, the slidewire 148 is provided with a potential supply E across its terminals. The signal IDS is desirably of such a magnitude as to represent the maximum induced draft fan speed which can be maintained while still having some leeway for controlling the fan.

The control system 144 may be any of a number of standard control units which are available to provide both proportional and integral action in their output signal. In FIG. 2, the output signal of the controller 144 on line 149 is a signal AKS representing a change in the product output rate setpoint above an initial product output rate setpoint K8 the magnitude of the change being es tablished by controller 144 in accordance with the difference between IDS and IDS The initial setpoint KS is set by knob 15012 which adjusts the variable tap 150a on slidewire 150. The slidewire 150 being supplied by potential source E so that there is provided on line 152 the signal KS representing the initial product output rate desired at the start-up of the process.

The signal on line 152 is added to the signal on line 149 by the apparatus represented by the summing terminal 156, the summing terminal 156 being a schematic showing of a point at which two signals are combined. This combination may of course be supplied in many of a number of ways. One example would be by use of an operational amplifier.

The output of the summing junction 156 on line 158 is then a signal KS representing the new product output rate setpoint. This signal is supplied to controller 169.

The controller 160 is a PI controller of a type similar to that shown as controller 144 except that in the case of controller 160 the output instead of being a signal on a line such as line 149 out of controller 144 is instead a signal on one of the output lines 162 to a positioning motor 164.

The signal to motor 164 will be supplied on that one of the output lines 162 associated with the desired direction of motor rotation. The motor 164 has three input terminals. The center terminal is grounded while the other terminals provide the connecting points for the lines 162.

The shaft 166 of motor 164 is positioned so as to control the speed of the kiln 10 when clutch plates 167 are engaged. The rotation of the shaft 166 of motor 164 then modifies the input to the speed control, shown here as block 168, so as to produce on the output lines 170 of the speed control the necessary signal to cause the kiln drive motor 78 to rotate its driving gear by way of the connecting shaft. Driving gear 80 in turn causes the gear 82, which it engages, to drive or rotate kiln 10 at a speed determined by the speed of rotation of drive motor 78.

Also engaged with driving gear 80 is another gear 180 which is connected by way of shaft 182 to tachometer 184. The tachometer 184 has one of its output terminals coupled to ground while the other output terminal is shown as being coupled to a line 136 which provides another input to controller 160 so as to supply the signal KS which represents not only the actual kiln speed, as measured by the tachometer 184, but also the actual product feed rate since the raw material feed rate is controlled to be directly varied with the kiln speed as will be evident from the following description of the raw material feed rate control.

Motor 164 can also operate through shaft 169 and engaged clutch plates 171 to position contact 91a, which may be present manually by knob 91b on potentiometer slidewire 91 to provide a setpoint signal KFR to a controller of the type shown in FIG. 1. The remaining portion of the raw material feed control is omitted from FIG. 2 since it would be similar to that shown in FIG. 1. Controller 90 also receives as an input the signal KFR on line 94 so as to produce a position for motor 99 which will adjust the connected butterfly valve 97 through shaft 98 and allow a controlled flow of raw material from hopper on to the constant speed belt 96. By this means the weight of the raw material fed to the kiln is controlled along with the control of kiln speed by controller 160. Alternately, of course, the signal KFR could be derived from signal KS but for the purposes of this description it is assumed that the alternative is not preferred.

As pointed out in connection with FIG. 1 the clutch plates 167 and 171 may both be engaged or the clutch 171 may both be engaged or the clutch 171 alone may be engaged to effect the desired maximization of product flow. If the latter arrangement is desired certain obvious modifications would be necessary to the circuit of FIG. 2 to eliminate the control of kiln speed.

It will thus be evident that the controller normally serves to modify the speed of rotation of kiln 10 and the rate of raw material feed until the measured actual kiln speed, which represents by the signal KS, a value for the product output rate, is equal in magnitude to the setpoint represented by the signal KS It will be evident that the control system as so far described uses the induced draft fan speed as a means for controlling rapid fluctuations in the air fiow through the kiln and it varies the kiln speed and raw material feed rate so as to tend to maintain the induced draft fan speed at a predetermined value which is normally a value close to its maximum speed. As previously mentioned, the effects of kiln speed variations and raw material feed rate changes can only aifect process conditions at a slow rate as compared with the variations in speed of the induced draft fan. Therefore, kiln speed and raw material feed rate controls can only return the induced draft fan speed to its desired value over a period of time which is relatively long compared with the rate at which the induced draft fan speed can fluctuate in response to control from the exhaust conditions as established by controller 100.

In establishing the above mentioned method for maintaining the exhaust conditions at their desired values as well as at the same time controlling the kiln speed and raw material feed rate so as to tend to maximize the product output of the rotary kiln, it is also desirable to prevent as much as possible these controlled changes from introducing into the process any unnecessary upsets which might be anticipated. In order to prevent such upsets it is desirable to establish feed-forward signals to the other control systems used in controlling the process to provide a means for anticipating the changes inthe other variables under control. For example, in controlling the exhaust conditions and the clinker temperature the induced draft fan speed and the firing rate respectively can be modified to anticipate changes which might be required to maintain the exhaust conditions and the clinker temperature substantially unchanged under changing kiln speeds and raw feed rates.

The signal AKS on line 192 is introduced by way of line 194 to a first model network 196 which is shown as comprising a circuit for producing a first order lag. In the arrangement shown in FIG. 2, the first order lag circuit 196, which serves as the approximate model network, provides the feed-forward signal for the control system 100. It is comprised of a potentiometer 200 which introduces a factor K into the input for amplifier 202 which is an integrating type of operational amplifier having a feedback circuit including line 204 which incorporates a potentiometer 206 to introduce the factor [3 in the feedback circuit of amplifier 202. The output from amplifier 202 and hence from the model network is on output line i 210 and is a signal AKET representing a lagged value of the change in the kiln exit temperature which is to be expected as a result of the change in kiln product output rate represented by the input signal AKS on line 194. The model network 196 thus includes potentiometer 200, amplifier 202 and potentiometer 206. By adding the signal AKET on line 210 to the signal KET which appears on line 212 at the summing junction 214, there is obtained a new kiln exit temperature setpoint KET on line 116.

The signal on line 212 representing the preset setpoint for the kiln exit temperature KET is supplied by potentiometer slidewire 220 who has its variable tap 220a adjusted by knob 22011 in accordance with the desired kiln exit temperature. The potentiometer slidewire 220 is supplied by a potential source E shown in FIG. 2 as a battery.

By virtue of the feed-forward signal supplied on line 210 the kiln exit temperature setpoint is varied so as to anticipate in the control of the kiln exhaust conditions by the control system 100 the variations in the induced draft fan speed which will be required to compensate for the anticipated changes in the kiln exhaust conditions resulting from the change in product output rate to be effected by controller 160.

It is also desirable to supply a feed-forward signal to the firing rate control system, and for this purpose the signal on line 194 is introduced into another approximate model network 230 which is shown in FIG. 2 as comprising both a first order lag as well as a dead-time circuit. The first order lag portion of the model network includes potentiometer 232 which is in circuit with line 194 and introduces a factor K 5 into the input to amplifier 234. Amplifier 234 is an integrating type of amplifier whose feedback by way of line 236 is through another potentiometer 238 incorporating the factor ,8. The combination of the potentiometers 232 and 238 with amplifier 234 in the configuration shown supplies the first order lag effect previously mentioned.

The output of amplifier 234 on line 240 is introduced into block 242 which represents a dead time circuit which can be any one of a number of several types of dead time circuits providing on its output line 244 a signal AKS which represents a lagged value of the change in kiln speed and which therefore can be used as a feed-forward signal for the firing rate control system. The model network 230 therefore includes potentiometer 232, amplifier 234, potentiometer 238, and circuit 242.

The firing rate control for kiln 10 as shown in FIG. 2 comprises a radiation pyrometer 24 which detects the temperature of the clinker by citing on the clinker 20 in the burning zone. One of the outputs of the radiation pyrometer 24 is grounded While the other output is by way of line 26 into a controller 250 which is a controller providing both proportional and integral action, as noted by the PI designation in block 250. The other input to controller 250 is by way of line 252 and represents the signal RLT which is the clinker temperature setpoint, whereas the signal supplied on line 26 represents the signal RLT which is the actual measured temperature as determined from the radiation pyrometer 24.

The signal for line 252 is provided by adjusting the variable tap 254a of slidewire 254 by turning knob 254b so as to obtain the desired potential on line 252. The slidewire 254 is supplied by a source of potential E.

The output from the controller 250 represents the change in firing rate setpoint from the initial value FR and is designated AFR The signal FR is supplied by potentiometer contact 251a on slidewire 251 as set by knob 251b. Slidewire 251 is supplied by a potential source E. The signal on line 253 is introduced into a summing point 260, at which point the firing rate setpoint AFR is added to FR supplied from contact 2510 by line 253, and the result is compared with the actual fuel feed rate measurement FR which is supplied on line 262 from the flow measuring device 264. Also there is combined with the signals FR AFR and FR the signal AKS from line 244 so as to produce as an output from the summing junction 260 a signal AFR which represents the change in the firing rate which is desired. This signal AFR is supplied as an input to another PI controller 272 Whose outputs on lines 274L and 274R are to a positioning motor 276 and are of such a nature as to cause the motor 276 to position the fuel control valve 14 in one direction or another, depending upon the direction of rotation of the shaft 18 connecting motor 276 with valve 14 in response to the signals from controller 272.

It will be evident that the firing rate control system operates so as to vary the feed of the fuel to tend to maintain the temperature detected by the radiation pyrometer 24 to the predetermined setpoint for that temperature with the firing rate being modified by the anticipating signal supplied from the product output rate control on line 244. It will also be evident that when the signal on line 194 (AKS represents a change in product output rate which is to be in a direction such as to increase the kiln speed and raw material feed rate, then the feedforward signal supplied to the exhaust control system 'on line 210 should be such as to tend to cause the control system 100 to increase the induced draft fan speed and also the feed-forward signal provided on line 244 should be in such a polarity as to cause the firing rate control system to increase the fuel feed through line 12. With such a relationship there is anticipated both the decrease in the temperature of the clinker 20 as well as the decrease in the kiln exit temperature which would normally result from the increased feed of product through the kiln 10.

FIGURES l and 2 show analog systems for effecting the control of kiln speed and/or raw material feed rate so as to maximize the output of the rotary kiln; the same functions can be carried out by digital systems and the circuits of FIGS. 3A and 3B when joined as shown in FIG. 3 provide the digital equipment necessary to effect a control similar to that of the system shown in FIG. 2.

In FIG. 3A the signals IDS and IDS are introduced over lines 146 and 122 respectively which correspond with those same lines in FIG. 2. These signals are introduced into the respective analog-to- digital converters 211 and 213 in response to the timing signal I; so that during a fixed period after the timing signal t the signals are converted to their digital equivalent and are outputted on the respective lines 215 and 216 at the end of that period and before the timing signal t The digitized signals on lines 215 and 216 provide inputs to the error computer 218 which make a comparison between the two input values in accordance with the equation:

and establishes an output signal on line 221 AIDS. This computation and outputting on line 221 is carried out in response to the timing signal t which occurs after the inputs are available on lines 215 and 216. It will be evident that the timing signals such as t and t represent the time of initiation of a certain function in the digital computer and also represent a fixed period after that particular time during which that function is not only carried out but also has the output of the function outputted from the device involved so that that output will be available upon the occurrence of the next timing signal for the subsequent calculation to be made.

The output on line 221, namely the signal AIDS is supplied as an input to the control computer 222 and also as an input to a delay device 224 which is indicated as D indicating that the delay device is etfective to delay the signal supplied to it on its input line 226 for a period of two time periods so that the output of the delay device 224 on line 228 is supplied to the input gates 16 of memory device 231 and is gated into that memory device at the time 12;. As indicated, the memory device 231 also has output OG which are effective upon the occurrence of the next time pulse t to output the signal AIDS inputted into the memory during the previous 2 period.

The output of the memory 231 when gated out by the timing signal t through the output gates 06 to line 241 provides an input into the control computer 222 which may be considered as AIDS (n1) or in other words, the AIDS signal which occurred as a result of the computations made following the previous sampling period. The signal on line 220, AIDS, may be considered as AIDS(n), or in other words, the signal used on line 220 as a result of the present sampling of the inputs on lines 146 and 122. The control computation made by the control computer 222 is initiated by the timing signal t to produce on its output line 243 a signal KS which is computed in the control computer in accordance with the equation The quantity KS appearing as the first quantity in the above equation is obtained from the storage device 255 and is gated into the control computer 222 through the output gates 0G at the time the timing signal i appears to initiate the outputting from those gates on output line 257.

The input signal to the storage device 255 represents KS or in other words, the output from the control computer 222 on line 243 except when the process is first being started and at that time the signal stored in the storage device 255 is K the initial setpoint for the kiln speed, which is obtained from the input line 259 derived as an output from the analog-to-digital converter 256 as it appears on line 261. The input to the analog-to-digital converter 256 is from line 152 an is representative of the signal KS This Signal is obtained as shown in FIG. 2 from the potentiometer 150.

The output from the control computer 222 not only goes into the storage device 255 but also by way of line 263 is introduced into input line 265 for the control computer 267. The other input to the control computer 267 is by way of line 266 from the analog-todigital converter 268 whose input on line 186 is a signal KSa corresponding with the output of the tachometer 184 in FIG. 2. The control computer is initiated in block 267, namely in accordance with the following equation:

The output of the control computer 267 is thus a signal T which appears on line 273 as an input to pulse generator 275. The signal T produces as an output from pulse generator 275 a series of pulses in number corresponding with the magnitude of the signal T The stepping motor 278 is then stepped a number of steps depending upon the value T so as to rotate the shaft 280 and thereby adjust the input to the speed control 168 as well as the input of feed rate control 269 so as to provide on output lines 170 a signal to kiln drive motor 78 so as to drive the kiln at a speed such that the signal IDS will tend ultimately to be returned to equality with the signal IDS The feed control 269 simultaneously providing on output lines 172 a signal to tthe raw material feed control motor 99, which signal is operable to cause the feed rate of the material from hopper to be directly related to kiln speed.

In order to obtain the feed forward signals as called for in the description of FIG. 2, there is provided an error computer 300 which receives as one of its inputs the signal KS from line 302 which is connected with line 263 which in turn receives the output from control computer 222 from line 243. The other input to the error computer 300 is on line 304 which is an output line from the delay device 306 shown as having a single period delay and having the input derived from line 261. As a result, there is provided on line 304 a signal KS which appears at the correct time so that upon the initiation of the timing pulse t the error computer 300 makes a computation in accordance with the following equation and it produces on its output line 310 the signal AKS which is the feed forward signal required for the several feed-forward paths shown in FIG. 3B.

Considering first the feed-forward to the control system which controls the induced draft fan speed; it will be seen from FIGURE 3B that the line 310 is connected to the model simulator 314 as an input thereto so that upon the initiation of the model simulator by the timing pulse t the model simulator may make the necessary computation so as to produce an output AKET on out put line 316. The model simulator 314 is representative of a computer capable of producing an output signal which represents a lagged signal so that the model simulator 314 is in essence a first order lag, for example, comparable to the first order lag 196 of FIG. 2, but effected in a digital fashion rather than by an analog device.

The signal on line 316 (AKET provides an input to the kiln exit temperature setpoint computer 320. The other input to the computer 320 is provided on line 322 from the analog-to-digital converter 324 whose input is a signal KET representing the kiln exit temperature setpoint as derived from a potentiometer 220 of FIG. 2. The analog-to-digital conversion made by the converter 324 is carried out on the initiation by the timing pulse t so that the kiln exit temperatures setpoint computer 320 can be initiated at time 1 During the time period t the indicated computations are carried out in accordance with the following equation:

KETd-i- AKET =KET The output of the computer 320 is supplied on line 332 as one of the inputs to the LD fan speed control system 334. Other inputs to the LD. fan speed control system 334 are supplied on lines 336, 338 and 340. The

input provided on line 336 is derived from the signal KET supplied on line 110a and representing the signal supplied from the thermocouple 110 of FIG. 2. This signal is converted by the analog-to-digital converter 342 at the time t so as to provide the signal KET in digital form on line 336.

Another input to the control system 334 provided on line 338 is derived from the signal KEO which represents the actual measured kiln oxygen concentration and is supplied on line 62a as a signal corresponding with the kiln exit oxygen measured through the sampling tube 62 of FIG. 2. The signal on line 62a is converted to a digital signal at time i The other input to the control system 334 provided on line 340 is derived from the signal KEO which represents the kiln exit oxygen setpoint and corresponds with the signal supplied .on line 112 in FIG. 2. This signal is shown as supplied here on line 112 to the analog-todigital converter 350 which in turn provides the digitalized signal on input line 340 when initiated by timing signal i At time n; the ID. fan speed control system is effective to perform control functions similar to those set forth in the description of FIG. 2 and similar to the digital arrangement shown and described in my co-pending application, Ser. No. 509,348, filed Nov. 23, 1965.

The output of the LD. fan speed control system, 334, is produced on lines 118 to ID. fan motor 54 so as to modify the speed of the induced draft fan motor as required to ultimately maintain the kiln exit oxygen at its desired setpoint and also to maintain the kiln exit temperature at its setpoint with the control being modified by the anticipating feed-forward signal supplied on line 310 in accordance with the characteristics of the model simulator 314.

A similar feed-forward is provided to the firing rate control, as mentioned in the description of FIG. 2. In FIG. 3B, this feed-forward is supplied on line 340 which is connected with line 310. The line 340 introduces the signal AKS into the model simulator 342 which upon initiation by timing signal i effects a digital execution of a first order lag plus a dead time so as to provide on its output line 344 a signal AKS which represents the lagged change in kiln speed setpoint after the introduction of not only the lag but also the dead time which is required to provide the proper timing for the anticipation signal into the firing rate control network.

The signal on line 344 is one of the inputs to the firing rate error computer 346. The other inputs are provided on lines 348, 350 and 351, respectively. The input supplied on line 348 is derived by way of the analog-todigital converter 352, which has as an input the signal PR which is supplied on line 262, comparable to the similar numbered line in FIG. 2. The conversion from analogto-digital form of signal FR is carried out at the time t by the initation from the timing signal t The output signal on line 348 then appears as an input to the firing rate error computer in time to be utilized in the computation initiated at the time t Another input to computer 346 is supplied on line 351 from the analog-to-digital converter 355 which receives an input signal from line 253, namely FR representing the initial setpoint for the firing rate. The conversion is made at t as indicated.

Still another input to the firing rate error computer 346 supplied on line 350 is derived from the output of the firing rate setpoint computer 354 in FIG. 3A. As Shown in FIG. 3A, the input signal RLT representing the setpoint for the temperature of the clinker 20 is supplied on line 252 while the actual measured value of the temperature of the clinker 20 is supplied on line 26 as signal RLT. Both of these signals are introduced into their respective analog-to- digital converters 360 and 362 which at the time t are caused to initiate corresponding digital output signals on the respective lines 364 and 366 which then provide inputs to error computer 368 which computes the signal ARLT in accordance with the following equation:

RLT -RLT ARLT The error computation is carried out by the initiation of the timing signal L; as shown in FIG. 3A and at the end of the period initiated by that timing pulse there is produced an output on line 370 from the error computer 368 which output represents ARLT and is supplied as an input to the firing rate setpoint computer 354.

To provide the other input to computer 354 on line 389, there is provided a memory device 384 which 0btains its input from line 370 through the two period delay device 386 into the input gates IG which are initiated at the time 1 to read the signal ARLT into the memory 384 at the time t As shown in FIG. 3A, the output gates OG of the memory 384 are timed to read out the signal ARLT at the time L; on to the line 380 into computer 354. Thus, the signal on line 380 may be considered to represent ARLT(n1) While the signal supplied to the computer 354 on line 370 may be considered to represent ARLT(n) where n represents the signal supplied as a result of the present sampling, whereas the signal ARLT (nl) represents the signal which was present as a result of the computation carried out by computer 368 following the previous sampling period.

The firing rate setpoing computer 354 makes the computation in accordance with the following equation so as to obtain signal AFR ARLT(n-1)]=AFR The signal AFR produced as an output on line 350 from computer 354 is entered into storage 357 by way of line 359 and is then available at the next sampling period to be read out by output gates 0G activated by timing signal i as an input to computer 354. This input appears on line 361.

Having now explained the manner in which signal AFR is obtained to provide the input on line 350 into the firing rate error computer 346, it will be evident that the firing rate error computer can make the computation as set forth in the following equation so as to obtain on output line 390 a signal AFR which is used in the control computer 392, in accordance with the following equation to obtain a signal T The computation made by computer 346 provides because of the nature of the computation made only a reset type of control action in the firing rate control.

As shown in FIGURE 3B, the control computer calculation is initiated by the timing pulse t so that at the end of the period starting at t the signal T appears on the output line 394 as an input to pulse generator 396. The pulse generator 396 then in turn causes the initiation of either relay 398 which may be considered the raise relay, or the initiation of relay 400, which may be considered the lower relay for the fuel valve positioner motor 276.

A potential source E is provided at terminal 410 as a source for energizing through either a closed raise relay 398a or a closed lower relay 400a, either the raise or lower winding of motor 276. It will thus be seen that if it is necessary that the fuel valve positioner receive a raise pulse to its motor 276 the pulse generator 396 will cause the energization of relay 398 pulling in relay contact 398a to make a complete circuit between the source E at terminal 410 through line 274R to motor 276.

By similar means, the pulse generator 396 upon energization of relay 400 may cause motor 276 to lower the fuel flow rate by operating fuel valve motor 276 through the closed relay contact 400a so as to make a complete 13 circuit between the source E at terminal 410 and the line 274L to motor 276.

While FIGS. 3A and 3B show one arrangement whereby the kiln speed may be controlled with digital equipment in a manner similar to that shown in FIG. 2 with analog equipment, it will be evident to those skilled in the art that other similar digital equipment and circuit arrangements may be utilized for effecting the same type of control action and in fact, a general purpose digital computer may be programmed so as to carry out the described computations by time sharing the elements of the computer in the usual fashion so that they may produce in the output devices of the computer the same control signals as are produced in the system of FIG. 2 as well as the system of FIGS. 3A and 313.

What is claimed is:

1. A control system for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a fiow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising:

an oxygen measuring device for producing a signal indicative of the measured oxygen concentration in the kiln exit,

a fan speed control system operable to maintain the speed of the induced draft fan at the maximum usable speed, and

means responsive to the deviation of said signal from its desired value for changing said product output rate so as to simultaneously establish the desired oxygen concentration while maintaining said maximum usable speed for the induced draft fan speed to thereby maximize the product output rate of said kiln.

2. A control system as set forth in claim 1 in which the fan speed control system includes a means for presetting a fixed value for said fan speed.

3. A control system as set forth in claim 1 in which said fan speed control system is responsive to the exit conditions of said kiln and is operable to vary said fan speed to maintain said exit conditions at predetermined values.

4. A control system for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising:

means responsive to the difference between the desired value for the speed of the induced draft fan of said kiln and its measured value for producing a first signal representative of a value for the product output rate setpoint,

means for producing a second signal representative of the measured value of said product output rate, and

means for controlling said product output rate to tend to bring said second signal into equality with said first signal.

5. A control system as set forth in claim 4 which includes: I

a first model simulator responsive to said first signal and operable to produce a first feed-forward signal representative of the change expected in the exit temperature of said kiln as a result of the change in the product output rate effected by said control means, and 1 means responsive to said first feed-forward signal for modifying the speed of the inducted draft fan of said kiln in direction and extent to c rrect for the anticipated effect on the kiln exit temperature of the change in product output rate called for by said control means.

6. A control system as set forth in claim 5 in which said first model simulator comprises means for producing a first order lag as the response of said model simulator to said first signal, the lagged signal from said simulator being said first feed-forward signal.

7. A control system as set forth in claim 5 in which said means responsive to said first feed-forward signal comprises:

a control system responsive to both the kiln exit temperature and the kiln exit oxygen concentration for controlling the induced draft fan speed to tend to maintain said kiln exit temperature and said kIln exit oxygen concentration at their respective setpoint values, and

means for modifying the setpoint of said kiln exit temperature in response to said feed-forward signal.

8. A control system as set forth in claim 4 which includes:

a second model simulator responsive to said first signal and operable to produce a second feed-forward signal representative of the change required in the kiln firing rate to prevent any substantial change in the clinker temperature in the kiln as a result of the control of said product output rate, and

means responsive to said second feed-forward signal for modifying the kiln firing rate to correct for such anticipated changes.

9. A control system for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising:

means for producing a first signal representative of the measured speed of the inducted draft fan of said kiln,

means for producing a second signal representative of the desired value for the speed of said induced draft fan speed,

means for producing a third signal representing a desired initial value for the product output rate setpoint,

control means responsive to the difference between said first and second signal to produce a fourth signal representative of the desired change in the product output rate setpoint for said initial value,

means combining said third and fourth signals to produce a fifth signal representative of the setpoint for the product output rate,

means for producing a sixth signal representative of the value of said product output rate, and

means for controlling said product output rate so as to tend to bring said sixth signal into equality with said fifth signal.

10. A control system for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising:

means for maintaining the speed of the induced draft fan of said kiln at a preset maximum value,

means for producing a first signal representative of the measured oxygen concentration in the exhaust gases from the kiln,

means for producing a second signal representative of the desired value for said measured oxygen concentration,

and control means responsive to the difference between said first and second signals for changing the product flow rate in direction and extent to bring said first and second signals to equality while the speed of said fan is maintained at its preset maximum value.

11. A control system as set forth in claim in which said control means changes the feed rate of the raw material to the kiln and the kiln speed in related amounts.

12. A control system as set forth in claim 10 in which said control means changes only the feed rate of the raw material to the kiln without making any corresponding change in the kiln speed.

13. A method for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising the steps of measuring the oxygen concentration in the kiln exit,

producing a first signal, the magnitude of which is varied in accordance with the measured oxygen concentration,

producing a second signal, the magnitude of which is indicative of a predetermined desired maximum usable speed for the induced draft fan,

controlling the speed of the induced draft fan in response to said second signal,

producing a third signal, the magnitude of which is indicative of the desired oxygen concentration in the kiln exit, and

modifying the product output rate of the kiln under the control of said first and said third signal so as to tend to simultaneously establish the desired oxygen concentration and the maximum usable speed for the induced draft fan.

14. A method for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising the steps of:

measuring the oxygen concentration in the kiln exit,

producing a first signal, the magnitude of which is varied in accordance with the measured oxygen concentration,

producing a second signal, the magnitude of which is indicative of the desired oxygen concentration in the kiln exit,

controlling the speed of the induced draft fan in response to the difference between said first and said second signals, producing a third signal, the magnitude of which is indicative of the desired initial setpoint for the product output rate upon initiation of control,

producing a fourth signal, the magnitude of which is indicative of the desired setpoint for the speed of the induced draft fan of said kiln,

measuring the speed of said induced draft fan,

producing a fifth signal, the magnitude of which is varied in accordance with the measured speed of the induced draft fan, producing a sixth signal, the magnitude of which is established from the magnitudes of said fourth and fifth signals so as to have a magnitude which varies in accordance with the change in product output rate setpoint above the initial setpoint, producing a seventh signal, the magnitude of which is established from a combination of the magnitudes of said third and sixth signals so as to have a magnitude which varies in accordance with the setpoint of said product output rate at times other than the time of initiation of the control,

measuring the. speed of rotation of said kiln,

producing an eighth signal, the magnitude of which is varied in accordance with the measured kiln rotation speed, and

modifying the product output rate of the kiln in response to the difference between said seventh and eighth signals so as to tend to bring said eighth signal into equality with said seventh signal.

15. A method as set forth in claim 14 which includes as additional steps:

producing as a first feed-forward signal a signal whose magnitude varies in accordance with a lagged sixth signal,

producing a ninth signal, the magnitude of which is in accordance with the desired kiln exit temperature setpoint,

producing a modified kiln exit temperature setpoint signal by combining said first feed-forward signal and said sixth signal,

producing a control signal, the magnitude of which is varied in accordance with said modified setpoint signal and other signals produced in accordance with the measured kiln exit oxygen concentration, the desired kiln exit oxygen concentration and the measured kiln exit temperature, and

modifying the speed of the induced draft fan of said kiln in response to said control signal to anticipate the efi'ect on the kiln exit temperature of the modification of the product output rate.

16. A method as set forth in claim 14 which includes as additional steps:

producing a second feed-forward signal, the magnitude of which is varied in accordance with a lagged and delayed sixth signal,

producing a tenth signal, the magnitude of which is indicative of the desired firing rate for said kiln, measuring the actual firing rate for said kiln, producing an eleventh signal, the magnitude of which is varied in accordance with the measured value of the actual firing rate for said kiln,

producing a control signal in accordance with the sum of said second feed-forward signal and the difference between said tenth and eleventh signals so that said control signal is indicative of the desired change in the firing rate necessary to anticipate the effect on said firing rate of the modification of said product output rate, and

modifying said firing rate in response to said control signal.

17. A method for maximizing the product output rate of a rotary kiln in which an induced draft fan produces a flow of air from the heated end of the kiln to the exit at the cool end while raw material is fed into the cool end and flows toward the heated end as the kiln rotates, comprising the steps of:

producing a first signal, the magnitude of which is indicative of the initial product output rate for the kiln speed existing upon initiation of control and which is representative of a new value for said kiln speed at other times during the control of said kiln,

producing a second signal, the magnitude of which is indicative of the setpoint for the speed of the induced draft fan for said kiln,

measuring the speed of said induced draft fan,

producing a third signal, the magnitude of which is varied in accordance with the measured speed of said induced draft fan,

producing a fourth signal, the magnitude of which is varied in accordance with the combined values of said first, second and third signals so that said fourth signal has a magnitude indicative of the new value for the setpoint of said kiln speed at said other times,

producing a fifth signal, the magnitude of which is varied in accordance with the measured speed of rotation of said kiln so as to be indicative of the product output rate,

producing a control signal, the magnitude of which is varied in accordance with the difference between said fourth and fifth signals,

modifying the product output rate in response to said control signal so as to tend to bring said fourth and fifth signals to equality,

producing a sixth signal, the magnitude of which is varied in accordance with the difference between said second and third signals,

producing as a first feed-forward signal a signal whose magnitude is in accordance with a lagged sixth signal,

producing a seventh signal, the magnitude of which is indicative of the desired kiln exit temperature setpoint,

producing a modified kiln exit temperature setpoint signal by combining said first feed-forward signal and said seventh signal,

producing a control signal from said modified setpoint signal and other signals varying in accordance with the measured kiln exit oxygen concentration, the desired kiln exit concentration and the measured kiln exit temperature,

modifying the speed of the induced draft fan of said kiln in response to said control signal to anticipate the effect on the kiln exit temperature on the modification of the product output rate,

producing as a second feed-forward signal a signal whose magnitude is in accordance with a lagged and delayed sixth signal,

producing an eighth signal, the magnitude of which is indicative of the initial firing rate setpoint,

producing a ninth signal, the magnitude of which is indicative of the desired change in firing rate for said kiln above said initial firing rate setpoint,

measuring the actual firing rate for said kiln,

producing a tenth signal, the magnitude of which is varied in accordance with the measured actual firing rate for said kiln,

producing in accordance With the sum of said second feed-forward signal and the difference between the sum of said eighth and ninth signals and said tenth signal, a control signal whose magnitude is indicative of the desired change in the firing rate for anticipating the effect on said firing rate of the modification of said product output rate, and

modifying said firing rate in response to said last named control signal.

References Cited UNITED STATES PATENTS 3,366,374 1/1968 Bay et a1 26332 3,346,250 10/1967 Strassburger 235151.1 3,300,196 1/1967 Bendy 26332 3,280,312 10/1966 Sandelien 235151.3 3,162,325 12/1964 Hall et a1 235-151.1

OTHER REFERENCES Sid Levine: Mineral Processing, April 1963; pages 38 and 39; Modernization at Giant Cement-Close-Up on Instrumentation.

EUGENE G. BOTZ, Primary Examiner EDWARD J. WISE, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,483,363 December 9, 1969 Charles W. Ross It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 34, after "The" insert load cells 92 and 9E measure the mass flow rate of the raw Column 9, line 38, after "output" insert gates Column 10, line 6, after "initiated" insert by the timing signal t to carry out the computation shown line 23, "tthe" should read the line 63, after "potentiometer" insert such as potentiometer Column 11, line 19, "digitalized" should read digitized Column 14, line 31, after "while" insert raw Signed and sealed this 27th day of October 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. JR-

Attesting Officer I Commissioner of Patents

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