US3556496A

US3556496A - Furnace profile temperature controller with ancillary signal producing means to maintain furnace at optimal temperature control levels and to minimize adverse effect of upsets

Info

Publication number: US3556496A
Application number: US839449A
Authority: US
Inventors: Ernest Hucke
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1969-07-07
Filing date: 1969-07-07
Publication date: 1971-01-19
Anticipated expiration: 1988-01-19
Also published as: DE2032844A1; GB1266099A; CA935900A

Abstract

A temperature-measuring control apparatus having opposing ancillary set point adjusting loop circuits for regulating the heat required to convert a granular material, for example: a cement mix into quality clinker which control apparatus is responsive to changes taking place in the overall temperature profile of the furnace and whose control action is not adversely effected when sudden upsets, such as a sudden surge of feed material being passed into the furnace occurs, which causes lengthening and/or shortening of the calcinating and burning zones.

Description

1 1 United States Patent 1 13,556,496

[72] Inventor Ernest Hucke, [56] References Cited Glenside- UNITED STATES PATENTS El Q J' 33 ;2 3.366374 1/1968 Bay 6! a1. 263/32 1 19 R 263 32 45 Patented Jan. 19, 1971 3381946 5/ 68 [73] Assignce Honeywell Inc., Primary Examiner.lohn J. Camby Minneapolis, Minn.. Attorney-Arthur H. Swanson, Lockwood D. Burton and a corporation of Delaware John Shaw Stevenson [54] FURNACE PROFILE TEMPERATURE CONTROLLER ANCILLARY SIGNAL ABSTRACT: A temperature-measuring control apparatus PRODUCING MEANS TO MAINTAIN FURNACE AT havmg opposing anc1llary set point ad ustmg loop circuits for OPTIMAL TEMPERATURE CONTROL LEVELS 1 AND To MINIMIZE ADVERSE EFFECT OF UPSETS regulating the heat requ1red to convert a granular material, for 10 Cl 4 D example: a cement mix into quality clinker which control aprawmg paratus is responsive to changes taking place in the overall [52] US. Cl i. 263/32; temperature profile of the furnace and whose control action is 236/15 not adversely affected when sudden upsets, such as a sudden [51] Int. Cl F27b 7/00 surge of feed material being passed into the furnace occurs, [50] Field of Search... 263/32, 33; which causes lengthening and/or shortening of the calcinating 236/15B and burning zones.

CALCINING 74 PREHEATING ZONE SHEET 2 [IF 2 PROCESS VARIABLE 1 SET POINT MAN- STATION VOLTAGE ERROR SIGNAL VOLTAGE DUAL POT SETTING IN!!- SET IN 4-20 MA LEFT LOOP IN V. DC RIGHT LOOP SOIL INVENTOR. ERNEST HUCKE W AGENT.

FURNACE PROFILE TEMPERATURE CONTROLLER WlTlHl ANCHLLAIRY SIGNAL PRODUCING MEANS TO MAlNTAlN FURNACE AT OPTIMAL TEMPERATURE CONTROL LEVELS AND TO MINIMIZE ADVERSE EFFECT 01F UPSETS It is an object of the present invention to provide control apparatus to increase or decrease the amount of heat that is being applied to varying qualities and quantities of material such as a cement mix that moves at any one of a selected number of different speeds through a rotating kiln so that a quality end product material such as clinker will continuousfy be produced at the highest possible rate, with minimum fuel consumption and with minimum wear and tear on the kiln parts, such as the kiln liners.

During the final stage of a cement manufacturing process, it is necessary to introduce the cement mix into a preheating zone and a heating zone that is located at the right end of a tilted gravity fed-rotating kiln so that it will be heated to a prescribed temperature by hot gases that are passed through the kiln. The cement mix is then moved by gravity and the rotation of the kiln into a calcining zone where the mix is heated by the hot gases to a temperature which is below the melting temperature of the mix. In this zone, the mix is formed into certain desired chemical compounds. The cement mix is then moved in a similar manner to thatjust described into the burning zone of the kiln, a zone into which the flame of a burner is introduced so that an end product formed of a hard chemical mixture known as clinker is produced.

It is another object of the present invention to provide a unique ancillary error signal-adjusting apparatus for controlling the temperature of a cement mix as it passes through the aforementioned zones of the kiln so that an electric set point control signal can be transmitted to a master temperature controller by a set point adjusting control circuit that will more accurately regulate the rate at which fuel flows to a burner in the burning zone in respect to the profile tempera ture of the kiln than has heretofore been possible and which will enable an improved quality clinker to be produced.

it is another object of the present invention to disclose an apparatus of the aforementioned type that employs a process variable indicator and sensor associated with the preheating heating and calcining zones to indicate the temperature condition of each of these zones and a separate manually adjusted set point indicator electrically connected to each process variable indicator manually set at the same value as the process variable to initially provide a zero electric error signal which can be introduced into the master set point control loop without any change taking place at that time in the value of the electric set point signal being transmitted by the set point controller.

It is another object of the present invention to provide an electrical connection between the circuit associated with each of the aforementioned process variable indicators and their respective manually adjusted set point indicators which connection electrically connects current-producing set point and process variable looped circuits whose currents are arranged to oppose one another when the aforementioned process variable and set point values are equal so that a zero ancillary set point signal will be produced thereby.

It is another object of the present invention to provide manually adjusted electrically connected potentiometers as a means by which each of the aforementioned process variable and set point loops are interconnected so that different potentiometric resistance settings can be introduced into each of the aforementioned combined process variable set point indicator current-opposing loops circuits and so that an operator will be able to select a pair of potentiometric settings for any one of the set point process variable loops which will provide a greater percent of the error signal effect than provided by the other remaining pairs of potentiometers.

More specifically it is another object of the invention to allow the error signal produced by any one of the set point process variable loops to be given greater importance in the roll it plays in producing the overall ancillary error signal that is produced by the sum of these loops.

It is another object of the present invention to provide a dual potentiometer for each of the aforementioned process variable set point loops that will limit the excursion of the set point of the master controller over a preselected range so as to prevent the fuel control valve which the master controller is regulating from being moved to an undesired position that would cause an improperly heated poor quality clinker to be produced.

It is another object of the present invention to provide a control apparatus of the aforementioned type that is constructed to automatically effect a control action which will maintain the heat being added to the cement mix at a desired level which will produce a high quality clinker even during a time when, for example, dust is present in a forced air line passing through the kiln or during a time in which a furnace upset occurs such as a surge in feed which causes a shift in the length of the respective calcinating and burning zones.

A better understanding of the present invention may be had from the following detailed description when read in connection with the accompanying drawings in which:

FIG. 1 is a view showing the set point process variable apparatus associated with the respective zones of a kiln being employed to produce an error adjusting ancillary signal whose values are used to algebraically modify a master set point signal that is being transmitted to a master controller and how a calcining zone temperature transmitting circuit shown in dotted line form can be substituted for the master set point transmitter.

FIG. 2 shows how the resistant values of each of the potentiometers in the process variable-set point loop circuits can be adjusted by a separate mechanical means in place of the single mechanical means employed to simultaneously adjust the resistance of the process variable and set point loops to the same resistance value.

FIG. 3 shows a converter which can be employed in the circuit shown in FIG. 1 to convert a DC input signal into a milliamp DC signal and FIG. 4 shows the values that the potentiometer associated with the process variable and set point loops will be in to obtain a zero ancillary set point signal.

Referring now to the drawings in detail there is shown in FIG. 1 a control apparatus 10 for making optimum use of the heat required to convert a granular cement mix or other similar material 12 into a material such as a clinker 14 having a preselected density and hardness prior to, during and after a furnace upset.

This control apparatus 10 employs a master indicating controller 16 that receives a process variable signal by way of a conductive electrical connection 18 from any primary flowsensing element such as from any differential pressure to current transducer 20 having

conduits

22, 24 connected to opposite sides of an orifice plate 26 that is located in a conduit 28 that supplies fuel from a source, not shown, to a burner 30.

The control apparatus 10 also employs a set point indicating transmitter 32. The master set point transmitter 32 is connected to an AC power supply 34 and has two

conductors

36, 38 connected across a resistance 40 and an electrical

conductive transmission line

42, 44. These

lines

42, 44 are in turn connected by way of a voltage to current transducer 46, an electric-transmitting line 48 to transmit the set point signal, the burning zone temperature transmitter 50, the electrical transmission line 52 to the master controller 16.

A process variable electric loop circuit 54 having current flowing in the direction of the arrow 56 is shown with one end of its

electrical conductors

58, 60, 62 connected to a thermocouple to current transducer 64 which in turn has a pair of electrical leads 66, 68 connected to a protecting well enclosed thermocouple 70 that is employed to sense the temperature of the right end or preheating zone 72 of the furnace 74. The electrical circuit for thermocouple to current transducer 64, its thermocouple 70 may be of the type disclosed in detail in the Edward T. E. Hurd lll L'S. Pat. application. Ser. No. 670,822.1iled Sept. 8, l967.

The other end of the conductors is connected to a process variable vertical scale indicator 76 for indicating the mag nitude of the temperature of the preheating zone 72.

The manually adjusted set point electrical loop circuit 78 having a current flowing in the direction of the arrow 80 is shown with its conductors 82 to 84 connected at one end to a manually adjustable vertical scale set point indicator 86 and at its other ends to the master signal transmission line 44.

The indicator 86 in turn receives alternating

current power conductors

87, 88 from a suitable AC power supply source 89. The wiper 90 and resistance 92 form the potentiometer 94 in the process variable electrical loop 54 and the wiper 96 and resistance 98 form a potentiometer 100 in the manually adjustable set point electrical loop 78.

Since the conductor 102 electrically connects the process variable loop 54 with the set point loop 78 by way of the

wipers

90, 96, the electrical conductor 102 electrically con nects the process variable loop 54 with the set point loop 78, the

wipers

90, 96, can as shown, have a connection 104 for manually moving each of them jointly along their

respective resistances

92, 98, to increase or decrease the resistance in the process variable and set

point loop circuits

54, 78, by rotation of the knob 106.

The switch 108 is initially kept in a closed position during the initial heating up stages of a kiln and until a manual adjustment of the set point adjustment wheel 110 of the master set point transmitter 32 has been established which retains the flame heat being added to the furnace at a value that will produce a good quality clinker 14. This clinker 14 is formed just prior to the time it is passed out the left burning zone end of the rotary furnace or kiln 74, as this kiln is rotatably driven by a conventional electric motor 112 and reduction drive 114.

During this initial period of time, the vertical scale type process variable indicator 76 will indicate the temperature that is present in the preheat zone of the furnace 74. As soon as the furnace 74 starts to produce a good quality clinker the manually adjustable set point adjusting knob 116 is moved to an ancillary set point control-indicating position that matches the value that is shown on the process variable indicator 76. The switch 108 is then opened and since the instantaneous setting of the process variable and set

point indicators

76, 86 at that time are equal the

opposing loop circuits

54, 78 will be of the same magnitude so that no change in the set point signal being transmitted by the master set point transmitter through

leads

42, 44, will occur due to the electrical output of these

loops

54, 78.

After the switch 108 has been opened the thermocouple 70 then senses a cooling off of the cement mix entering the preheating zone due to, for example, an upset such as an abnormal increase in the cement mix that is fed into the furnace 74 by the conveyor 118. The value of current being sent through the process variable loop 54 will thus be changed from the value of the current passing through the electrically opposing set point loop 78 and an ancillary error signal will be introduced that will change the magnitude of the set point signal being transmitted by the master set point transmitter 32 by way of the

conductors

42, 44, 48, transmitter 50 to the master controller 16. This action will thereby cause the set point of the master controller 16 to change the set point 110 and an error signal which is representative of the differences in magnitude of its process variable and set point signals to be sent to the motorized valve 16 to allow, for example, more fuel to be sent to the furnace to heat the increase in cement mix flowing into the right end of the furnace 74.

As this increase; in cement mix passes into the calcinating zone its temperature may be raised by the last heat-adding control action to a value that exceeds a temperature level at which a good quality clinker can be produced.

if this undesired higher temperature condition should occur another pair of set point process variable opposing

electrical loops

120, 122, whose representative current flow is in the direction of the

arrows

124, 126. are employed that are the same type and function in the same manner as the process variable and set point loops 56, 80, previously described. The process loop 120 has. for example, a vertical scale indicator 128 similar to indicator 76 and the thermocouple 130, thermocouple leads 132 and the thermocouple to current transducer 134 are similar to the previously described

parts

70, 68 and 64.

A pair of

leads

136, 138, are shown connecting the transducer 134 with associated slip rings 140, 142 that are in turn fixedly mounted by insulating support members, not shown, on the outer surface of the kiln 74.

The output signal produced by the transducer 134 in response to the temperature sensed by the thermocouple is taken from the slip rings 140, 142, by means of

brushes

144, 146, and

electrical leads

148, 150.

Part way between the opposite ends of the

leads

148, 150, there is shown a voltage to current converter 152 whose circuit is shown in HO. 3 as comprising a source of current 153, a transistor 154 and a resistor 156. This voltage to current converter compares the input voltage signal against the reference bias voltage applied to a complementary connected transistor amplifier. The magnitude of the input voltage will affect the conduction of the transistor circuit and produce a proportional output current.

The set point loop 122 has an indicator 158 that is connected to an AC source 160 by way of

conductors

162, 164, similar to the previously described set point indicator 86, its AC source 89 and

conductors

87, 88.

Resistors

166, 168,

wipers

170, 172, conductor 174, manual connection 176, its associated knob 178 and switch 180 function in the same manner as that previously described under the description of the

opposing circuits

54, 78.

In lieu of operating either the pairs of

wipers

96, 90, or the

wipers

170, 172, jointly as shown in FIG. 1, it is possible to separately actuate each of these wipers by separate mechanical connections and

knobs

182, 184; 186, 188 shown in FIG. 2. In this way individual resistance settings of the

aforementioned wipers

96,90, 170, 172 can be accomplished.

A suitable position of the wiper 90 of the process variable potentiometer 94 and the wiper 96 of the set point poten tiometer 100 shown in FIGS. 1 or 2 that is found to produce a good quality clinker may be, for example, 100 ohms resistance indicated in the first column of the chart shown in FIG. 4 with settings on the manual resistance values from the joint or in dividual settings of the

wipers

170, 172 at the second men tioned control station. In this way greater importance can be given to a particular control station in the determination of the total overall corrective ancillary error signal that is introduced by that particular selective one of several control stations.

As an alternative part of the aforementioned control system an automatic set point master controller 194 shown in dotted line form can take the place of the manually adjusted master set point transmitter 32. When this automatic set point master controller 194 is employed it is connected to a suitable power source, for example AC source 34, and receives a signal whose magnitude varies in accordance with changes in temperature of the calcining zone as sensed by the thermocouple 196. The thermocouple 196 has a pair of leads 198 connected to a thermocouple to current transducer 200 which may be similar to the previously described

parts

130, 132, 134.

A pair of leads, 202, 204 are shown in dotted line form connecting the transducer 200 with associated

slip rings

202, 204 that are in turn fixedly mounted by insulating support members, not shown, on the outer surface of the kiln 74.

The output signal produced by the transducer 200 in response to the temperature sensed by the thermocouple 196 is taken from the slip rings 202, 204 by means of

brushes

206, 208 and

electrical leads

210, 212.

The pair of

conductors

214, 216, in turn, are shown connecting the automatic set point master controller 194 to opposite sides of the resistance 40 in transmission conductor 42.

A radiation pyrometer 218 which may be of the'type disclosed in the T. R. Harrison U.S.Pat. No. 2,357,193 is employed to take a measurement of a temperature representative of the clinker 14 being fed through the burning zone of the kiln 74 and to send a signal proportional to this temperature by way of electrical transmission line 220 to a millivolt to current transducer 222. The circuit employed in the millivolt to current transducer 222 is similar to that disclosed in the Edward T. E. Hurd III U.S. Pat. application, Ser. No. 670,822, filed Sept. 8, I967.

The current flowing from the transducer 222 is transmitted by a suitable conductor means 224 to a single mode transmitter 50 as a process variable (P.V.) input signal. 7

The controlling transmitter 50 receives the set point signal flowing into it by way of conductor 48 compares the aforementioned process variable with this set point signal and produces and transmits an error signal, or signal representative of the difference between the magnitude of process varia ble (P.V.) and set point (S.P.) signal that it receives as a set point input signal by way of the conductor 52 to the master three mode controller l6.

The master controller 16 in turn electrically compares the input process variable signal (P.V.) it receives from the differential pressure to current transducer20 by way of conductor l8 with the previously mentioned error signal in the form of a set point signal (S.P.) and produces and transmits another error signal equal to the difference in the two input signals by way of the conductor 226 to a motorized valve 228. The signal applied to the motorized valve 228 in the aforementioned manner will regulate the flow of the fuel that is allowed to pass through conduit at 28 to the burner 30 to a desired value that will continue to produce a good quality clinker 14.

It should be noted that the circuitry employed for the manually adjusted vertical scale set

point indicators

86, 158; the vertical scale process

variable indicators

76, 124; the manually adjusted set point master transmitter 32; the automatically adjusted set point-transmitting controller 194; the single mode-transmitting controller 50 and the three mode master-indicating controller 16 is covered by the circuitry shown in the Newbold U.S. Pat. No. 3,081,425, filed Aug. 28, 1959 and its associated reissued U.S. Pat. No. 26,516.

It should be understood that the

wipers

90, 96, 170 and/or 172 can be jointly or individually manually adjusted to other resistance values, for example 50 ohms or 150 ohms, to obtain a lower or higher range of ancillary DC voltage value that can be introduced into the

transmission lines

42, 44. This may be desirable to correct for any undesired heating or cooling of the material 12 as it passes through the furnace because it is of a different quality and quantity to the material just described.

Furthermore, it should be understood that resistance values lower than the 50 ohm values or higher than the 150 ohm values can be used where an application requires a different range in ancillary signal than that described supra and shown on the drawings to correct the temperature condition of the material 12 passing through any particular portion of the furnace so that preliminary desirable steps will be taken to obtain a good quality clinker before the material 12 reaches the burning zone of the furnace 74.

It should also be understood that the joint or individual settings of the

wipers

90, 96 of the first mentioned S.P.-P.V. control station shown on the right side of FIG. 1 can be set at different potential and the master set point transmitter 32 that is required to obtain this quality clinker may be found to be the 12 milliamps value shown in the second column chart shown in FIG. 4. The set point-adjusting knob 116 in FIG. 1 is the process variable value on indicator 76 is 4. 8. l2, 16, or 20 milliamps.

Depending on the values of the algebraic sum of the voltages produced by the right process variable and left set

point loops

54, 78, a DC signal will be produced as shown in Column four of FIG. 4 that will either buck, aid or have no effect on the set point signal being transmitted through the

transmission lines

42, 44, by the transmitter 32.

Iclaim:

1. A control apparatus to optimize the heat required to convert a granular material passing through a multizone furnace into a material having a preselected density and hardness prior to, during, and after a furnace upset occurs, comprising the combination of a master controller, a means operable to transmit a process variable signal to the controller whose magnitude is proportional to the rate of fuel being supplied by way of a final control means through the final-end zone of the furnace, a transmitter operably connected to transmit a preselected set point signal to the controller and at least one pair of electrical current opposing set point process variable loops forming an ancillary signal-producing circuit operably connected with the transmitter to algebraically alter the magnitude of the set point signal that is being transmitted by the transmitter to the controller in accordance with temperature changes taking place in any other different zone of the furnace and the final control means being operably connected with the controller to regulate the rate of fuel flowing into the furnace in accordance with the difference in magnitude of the combined set point and process variable signals.

2. The control apparatus defined in claim I wherein a pair of said opposing set point process variable loops are associated with two heating zones of the furnace which precede the final end zone of the furnace, said apparatus being operably connected with the transmitter to alter the magnitude of the set point signal being transmitted by thetransmitter in accordance with the temperature of the-last two mentioned zones and wherein the granular material is a cement mix and the end product produced by the furnace is clinker.

3. The control apparatus defined in claim 1 wherein each of said pair of opposing set point process variable loops are electrically connected to one another by means of a potentiometer, and a mechanical means is operably connected to said potentiometers to provide the same simultaneous joint resistance adjustment of each potentiometer.

4. The control apparatus as defined in claim I wherein each of said pair of opposing set point processvariable loops are electrically connected to one another by means of a poten' tiometer and a mechanical means is operably connected to said potentiometers to provide a separate resistance adjust ment of each potentiometer.

5. The control apparatus as defined in claim 1 wherein each of the process variable loops are connected to an associated process variable indicator and each of the set point loops are connected to an associated manually adjusted set point indicator, separate switches are employed to electrically cut off each of the set point loops with the transmitter, manual adjusting means is employed to line out the value of the set point indicator with the value on the process variable indicator while the switch is in a closed position and means to open the switch and connect each of the set point loops with the transmitter after the value of their set point indicator has been manually lined out with the value of the process variable indicator and a resulting zero ancillary error signal is being produced by each of said process variable set point loops.

6. The control apparatus defined in claim 1 wherein each of said pair of opposing set point process variable loops are electrically connected to one another by means of a potentiometer and a mechanical means is operably connected to said potentiometers to provide a separate resistance adjustment of each potentiometer and wherein an adjustment of the mechanical means associated with one pair of potentiometers associated with one of the pairs of loops that is different from the adjustment of the mechanical means connected with another pair of potentiometers associated with a different pair of process variable set point loops will provide a selective means by which one of said set point process variable loops is given greater importance in the roll it plays in producing the overall ancillary error signal that is produced by the sum of these pairs of loops.

7. The control apparatus defined in claim I wherein a temperature-sensing thermocouple to current-transducing means associated with the preheating zone of the furnace is operably connected to said process variable loop forming a portion of the ancillary signal-producing circuit to supply an electrical error signal proportional to the temperature of the preheating zone thereto.

8. The apparatus defined in claim 1 wherein a second transmitter operably connected with a primary temperaturesensing element in a burning zone of the furnace is employed to form a second process variable signal-producing circuit operably connected to the first mentioned transmitter to alter the magnitude of the set point signal produced thereby in accordance with temperature changes taking place in the burning zone.

9. The control apparatus defined in claim 1 wherein a pair of said opposing set point process variable loops are as sociated with the preheating and calcining zones of the furnace and are operably connected with the transmitter to alter the magnitude of the set point signal being transmitted by the transmitter in accordance with the temperature of the respective preheating and calcining zones andwherein the granular material is cement mix and the end product produced by the furnace is clinker. Y

10. The control apparatus defined in claiml wherein a pair of said opposing set point process variable .loops are associated with the preheating and calcining zones of the fur nace and are operably connected with the transmitter to alter the magnitude of the set point signal'being transmitted by the transmitter in accordance with the temperature of the respec tive preheating and calcining zones and wherein the granular material is cement mix and the end product produced by the furnace is clinker and a second transmitter operably connected with a primary temperature-sensing element in a burning zone of the furnace is employed to form a second process variable signal-producing circuit operably connected to the first mentioned transmitter to alter the magnitude of the set point signal produced thereby in accordance with temperature changes taking place in the burning zone.

Claims

1. A control apparatus to optimize the heat required to convert a granular material passing through a multizone furnace into a material having a preselected density and hardness prior to, during, and after a furnace upset occurs, comprising the combination of a master controller, a means operable to transmit a process variable signal to the controller whose magnitude is proportional to the rate of fuel being supplied by way of a final control means through the final end zone of the furnace, a transmitter operably connected to transmit a preselected set point signal to the controller and at least one pair of electrical current opposing set point process variable loops forming an ancillary signal-producing circuit operably connected with the transmitter to algebraically alter the magnitude of the set point signal that is being transmitted by the transmitter to the controller in accordance with temperature changes taking place in any other different zone of the furnace and the final control means being operably connected with the controller to regulate the rate of fuel flowing into the furnace in accordance with the difference in magnitude of the combined set point and process variable signals.

2. The control apparatus defined in claim 1 wherein a pair of said opposing set poinT process variable loops are associated with two heating zones of the furnace which precede the final end zone of the furnace, said apparatus being operably connected with the transmitter to alter the magnitude of the set point signal being transmitted by the transmitter in accordance with the temperature of the last two mentioned zones and wherein the granular material is a cement mix and the end product produced by the furnace is clinker.

4. The control apparatus as defined in claim 1 wherein each of said pair of opposing set point process variable loops are electrically connected to one another by means of a potentiometer and a mechanical means is operably connected to said potentiometers to provide a separate resistance adjustment of each potentiometer.

7. The control apparatus defined in claim 1 wherein a temperature-sensing thermocouple to current-transducing means associated with the preheating zone of the furnace is operably connected to said process variable loop forming a portion of the ancillary signal-producing circuit to supply an electrical error signal proportional to the temperature of the preheating zone thereto.

8. The apparatus defined in claim 1 wherein a second transmitter operably connected with a primary temperature-sensing element in a burning zone of the furnace is employed to form a second process variable signal-producing circuit operably connected to the first mentioned transmitter to alter the magnitude of the set point signal produced thereby in accordance with temperature changes taking place in the burning zone.

9. The control apparatus defined in claim 1 wherein a pair of said opposing set point process variable loops are associated with the preheating and calcining zones of the furnace and are operably connected with the transmitter to alter the magnitude of the set point signal being transmitted by the transmitter in accordance with the temperature of the respective preheating and calcining zones and wherein tHe granular material is cement mix and the end product produced by the furnace is clinker.

10. The control apparatus defined in claim 1 wherein a pair of said opposing set point process variable loops are associated with the preheating and calcining zones of the furnace and are operably connected with the transmitter to alter the magnitude of the set point signal being transmitted by the transmitter in accordance with the temperature of the respective preheating and calcining zones and wherein the granular material is cement mix and the end product produced by the furnace is clinker and a second transmitter operably connected with a primary temperature-sensing element in a burning zone of the furnace is employed to form a second process variable signal-producing circuit operably connected to the first mentioned transmitter to alter the magnitude of the set point signal produced thereby in accordance with temperature changes taking place in the burning zone.