US2359885A - Furnace temperature control - Google Patents

Description

J. -F. VOLLRATH FURNACE TEMPERATURE CONTROL I oct., 1o, 1944.

File -March 7, 1941 T/MER INVENTOR. JOSEPH P. VOLLRATH WMM;

Patented Oct. 10, 1944 FURNACE TEMPERATURE CONTROL Joseph P. Vollrath, Glenside, Pa., assignor to The V Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application March 7, 1941, Serial No. 382,123.

(Cyl. 236-) 9 Claims.

The present invention relates to furnace control systems and more particularly to a system for controlling the temperature in an openhearth or' other regenerative furnace in such a manner that the metal will be heated at a maxi- Y mum rate with a minimum of damage to the furnace.

In the use of furnaces of this type itis customary to raise the metal to some desired temperature in the shortest possible time and with the least possible fuel. In heating the furnace charge, however, care must be exercised not to raise the furnace temperature too high, since if this is done the furnace walls will be damaged and expensive and time consuming repairs will be necessary. In furnaces of this type it has been customary heretofore to control the furnace in response to the hottest temperature in the regenerator system, usually near the slag pockets, or to depend upon the experience of the operator. The various prior art methods of control are satisfactory to a certain extent but none of these methods takes into consideration the heat potential against the furnace roof. Furthermore, it has been found that the temperature of the furnace wall, particularly the roof, is a good criterion of the maximum temperature to which the furnace may be raised. In operation of furnaces of this type, fuel and `air are supplied through one end of the furnace While exhaust gases are removed from the other end. After the exhaust passages have been heated and the intake passages have been cooled down by the iiow of gases, to some predetermined extent, the furnace is reversed. This means that the fuel and air are supplied through the end that was formerly used for the exhaust thereby preheating the combustion supporting air. During each reversal of the furnace the temperature momentarily drops but rapidly rises to its desired point and any control system that is used in a furnace of this type must take into account this dropping of temperature so that the fuel supply will not be increased at, this time, causing the temperature to over A l shoot its mark when the flame from the new end is started. During the melting and refining periods in the-operation of an open-hearth furnace large volumes of combustible gas are liberated from the charge. The burning of these gases rapidly raises the temperature of the furnace above the danger point and unless the normal supply of fuel is reduced at this time the furnace is liable to be damaged due to the excess heat. My control system also compensates for the heat liberated by the combustible gases during the periods that the gas is liberatedland burning. Y

It is an object of my invention to provide a* furnace control system which will regulate the "supply of fuel to a regenerativeV furnace in atxcordance with the temperature of one of the furnace Walls. It is a further object of my invention to provide in this furnace control system a means for automatically preventing opening of the fuel valve during the time that reversal is taking place and for a short period of time thereafter.

It is a further object of my invention to automatically reduce the supply of fuel to the furnace when the temperature of a wall of that furnace rises some predetermined amount above the normal value thereof,

yThe description of my furnace temperature control system will be in connection with an open-hearth furnace. This is merely used, however, for descriptive purposes and it is not-to be taken as limiting the use of my invention to a furnace of this type. This is true because this control system is adapted to be used with various other types of furnaces.

The various features of novelty which char-'- acterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specication. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawing and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawing:

Fig. 1 is a more or less diagrammatic view of the furnace showing the fuel and air supply connections and the controls therefore; and

Fig. 2 is a. wiring diagram showing the various parts of my control system.

Referring first to Fig. yl, there is shown a furnace l of the open-hearth type in which a charge of metal is to be heated. This furnace is supplied with fuel, eithenliquid or gas, through a pipe 2 that is provided with an adjustable valve 3. This pipe divides into branches 4 and 5 each of which is used to supply fuel to one end of the furnace. Air to support combustion of theV fuel is supplied through a duct 6 andbranches 'l and 8, each of which contain a checker work, to. the respective ends of the furnace. When fuel and air are being supplied to one end of the furnace, say through branches 4 and 'l respectively, the gases of combustion pass out the opposite side of the furnace through branch 8 andan exhaust stack 9. In order to direct the fuel and air to the proper end of the furnace there is Vprovided a valve Il in the fuel line and a valve Il in the airline which valves are actuated in unison by amanually operated lever l2. The arrangement is such that when the furnace is to be fired from its left end the lever I2 is thrown to the left thus directing fuel from pipe 2 tobrsn'ch I The gases of combustion will then be exhausted through ducts 8 and S.

The temperature of this furnace is measured from the roof by means of some suitable temperature responsive device, shown herein as a thermocouple I3. This device can also take the form of a radiation pyrometer, an optical pyrometer or another suitable device depending upon the temperature at which the metal is to be maintained. The selection of the type of the temperature measuring device will also depend upon the temperature range through which it operates. The measurement of the temperature of the roof of the furnace by the device I3 is recorded through some suitable measuring system and instrument such as the potentiometer type of instrument shown at I4. A valve operating motor I5 is then energized for rotation in a direction to open or close the fuel valve 3 de pending upon the value of the temperature as recorded by the potentiometer I4.

The manner in which these operations are carried out is best shown from the wiring diagram'in Fig.` 2. It will be seen in that figure that the instrument I4 contains a conventional potentiometer mechanism in which the voltage from a battery I6 is impressed across a resistance I1. The E. M. F. produced by the measuring element I3, which is shown in this case by way of example only as a thermocouple, is balanced against a portion of the current from the battery I6 which balance is measured by a galvanometer I8. As the temperature of the furnace roof varies the galvanorneter will be unbalanced in one direction or the other to energize a motor I9 for rotation in one direction or the other, which motor serves to rebalance the potentiometer mechanism. This is accomplished by having the motor rotate a threaded shaft upon which a pen carriage, 2| is mounted such carriage carryinga contact 22 that is electrically connected to the thermocouple I3 and in engagement with the resistance I1. Therefore, as the shaft 20 is rotated in one direction or the other the pen carriage 22 will bemoved up or down scale to rebalance the galvanometer I8 by inserting more or less of the resistance I1 in the thermocouple circuit. The pen carriage also carries a pen 23 which serves to make a record of the value of the temperature upon a suitable chart 24. I

Movement of the pen carriage, the position of which is a measurement of the value of the temperature of the'furnace roof, is also used to operate the supply of fuel to the furnace. This is accomplished by having a normally balanced electrical network unbalanced in proportion of the position of the pen carriage, which unbalance serves to energ1ze the valve adjusting motor I5 for rotation in one direction or the other. Rotation of this motor serves to adjust the valve and simultaneously rebalance the network. The control system includes a variable resistance 25 located in the instrument parallel to the potentiometer resistance I1 and another variable resistance 26 connected in circuit therewith which is located at some point suitably near the motor I5. These resistances 25 and 25 are each provided with sliders or contacts 21 and 28 respectively, the former of which is Aattached to and movable with the pen carriage and the latter of which is moved by the valve motor. In opera tion, as the pen carriage moves it will adjust the contact 21 along the resistance 25. This unbalances the network and causes the inductance of one of the coils 30 to exceed that of the other coilthereby raising or lowering the armatureil, depending upon the direction of unbalance, to move a contact 32 carried by the armature into engagement with either of a pair of cooperating contacts 33 or 34 as the case may be. This'is to be described in detail below. Engagement of contact 32 with contact 33 or 34 serves to energize the motor I5 which then moves contact 28 in a direction to rebalance the electrical network while simultaneously adjusting the valve 3.

The chart 24 of the potentiometer is driven by a constant speed motor 34 that also serves to drive an yinterruptor switch 35 which is located in the circuit of valve motor I5. The switch 35 is mounted on a support 36 that is pivotedv at its left end and is provided on its lower surface with a cam edge that is periodically engaged by a roller on the end of an arm 31 which is rotated by the motor 34.

In the control of afurnace of the type disclosed herein it is often necessary to slow down the speed at which the control valve is opened or closed. This is accomplished by inserting the interruptor 35 in the motor circuit. This interruptor switch is continually opened and closed, and by adjusting the proportion of the closed to the opened periods, the length of time the motor is energized for each cycle may be adjusted. To this end the switch support 3G may be shifted relative to its pivot in order to vary the time that the rod 31 is in engagement with the support during each rotation of the former.

'I'he common terminal of .motor I5 is supplied with current from one side of the supply line through a conductor 38, interruptor switch 35, and a conductor 39. The other side of the line is connected through conductor 40, manual-automatic switch 4I and conductor 42 to the contact 32. From the contact 32 the motor I5 is energized through its field I5a, that serves to open the valve 3, through contact 33, conductor 43, timer switch 44, conductor 45, manually operated switch 45 and conductors 41 and 48. The purpose of the timer switch 44 will be described below. From contact 32 the motor is energized through its field I5b, that serves to close the valve 3, through contact 34, conductor 49, manually operated switch 58 and conductor 5I.

In the operation of a furnace of the type shown herein the furnace is periodically reversed or fired from rst one end and then the other. When this occurs there is a momentary drop in the temperature of the furnace and unless -some provision is made to take care of this the control system will call for an increase in fuel, -with a consequent over shooting of the temperature, when the furnace starts to fire normally from the other end. In order to overcome this disadvantage, atimer is inserted in the control circuit which will prevent opening of the fuel valve for a predetermined time -after the reversal, has taken place. To this end a conventional motor driven timer 52 is inserted in the line, the timer serving to prevent energization of the eld I5a, .thatoperates to open the control valve, for some predetermined time after reversal takes place. This timer is set in operation by opening the switch 53. Thereafter the timer will retain contact-44 open for whatever length of time it is set. This length of time will be suihcient for the furnace to normally regain its temperature after which the switch 44 is closed and the valve motor is permitted to be operated in the usual manner. The switch 53 is open to reset the timer and start it operating each time that a reversal takes place. This is accomplished by mounting a cam 54 on the control lever I2 so that as the control lever is moved from one side to the other to reverse the furnace, the switch 53 will be actuated.

'I'he operation of' the control system should be apparent-from the above description but a summary of its operation will now be given.

.As the temperature of the furnace roof varies the potentiometer circuit will be unbalanced causing the galvanometer I8 to deflect in one directionor the other to energize motor I9. This motor will then rotate shaft 20 to shift the pen carriage 2| and make a record of the change, simultaneously rebalarrcing the potentiometer circuit. This same movement of the pen carriage will move contact 21 along resistance 25 to unbalance the control network, thereby causing core 3|, if the temperature was increasing, to be lowered to engage contact 32 with contact 34.v 'I'he motor I5 is then energized intermittently for periods of time depending u'pon the adjustment of the switch 35 and will operate to close the control valve 3 to bring the temperature back to normal and will simultaneously adlust contact 28 and rebalance the control bridge circuit. If the temperaturecontinues to increase the core 3| will again be lowered to bring contacts 32 and 34 into engagement to thereby energize motor eld |5b and cause a further closing movement of the valve 3. On the other hand if the furnace roof temperature is falling,l the contacts 32 an'd 33 will be engaged to energize motor eld |5a to open the valve 3.

At times during the heating of the metal some abnormal condition may occur which will cause the temperature of the roof ,to rise appreciably, thus endangering the furnace walls.- When such an abnormal condition occurs the fuel valve should be closed, and t this end my present control system is provided with the following features. Mounted in the instrument 4 adjacent the shaft20 is a switch 55 that is on a pivoted 'support 56, which support has its lower surface in the form of a cam engaged by a cam roller 51 mounted on the pen carriage. Thus, as the temperature of the furnace risesunduly the pen y carriage will move to the right to accomplish two things. Such a movement will first shift contact 21 along resistance 25 to unbalance the control network in a direction to energize field b of the valve motor to start to close the valve. Movement of the pen carriage at this time will also bring roller 51 into engagement with the cam edge of support 56 to move the same counterclockwise to close switch 55. This latter operation will short out the interrupter switch 35, thereby permitting the motor l5 to be continuously energized` to close the valve 3 as long as the temperature is too high. This switch 55 and its support 56 may be adjustably positionedrelative to the pen carriage so that it will be closed after the temperature has risen any predetermined amount above the normal for which the instrument has been adjusted. To this end the support 55 is pivoted to a projection extending downwardly from a block 56A.' This block is provided with an elongated opening whereby it may be adjustably fastened to the frame of the machine. f

In any control system of the type disclosed herein it is desirable .if not essential that the one side of the supply line through conductors 38, 59, switchv 4|, and conductors 58 and 39. Thereafter, the motor can be run in a direction to open the valve by moving switch 50 to its left hand position. 'I'his serves to connect conductor 48 directly with conductor 4|) and to vthe other side of the supfply line. The motor |5 can also be run in a direction .to close the valve by moving switch 46 to its left hand position. This serves to connect conductors 5| and 40 ofthe other side of the supply line.

In the operation of the system of this type the resistance 25 is generally but not necessarily adjusted so that a narrow throttling range is obtained. By this it is meant that the resistance 25 is so formed 'thatI a small movement of the pen carriage will move the contact 21 from one end to the other of the resistance 25 and thereby run 'the motor I5 to completely open or shut the valve 3. Such operation means that if the temperature is low and rising, just prior to the time that the temperature reaches the control point the valve 3 will begin to be closed. Conversely if the temperature is high and falling just prior to the time the temperature reaches th control point the valve 3 will again begin to be opened. It is'often desirable to have the control valve operated automaticallythrough only a small portionof its full range of movement. This can also be accomplished by properly adjusting the effective length of theresistor 25. In such a case the switches 46 and 50 when operated will run the motor I5 ,an amount to completely open or close the valve 3. From the above description it will be seen that I have provided ,a control system for a furnace which is responsive to the temperature of a wall of that furnace and which system serves to adjust the fuel supply valve in accordance with this temperature. I have also provided a system in which the adjustment of the fuel valve is in steps and in which the adjustment of a fuel valve in an opening direction is positively prevented during reversal of the furnace. The control can he adjusted in a closing direction at any time and is also adjusted in a closing direction continuously in response to an abnormal rise in the furnace Wall temperature.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of this invention now known to me, it will be apparent to 'those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of this invention as set forth in the-appended claims, and that in some cases certain features of this invention may sometimes be used to advantage without a corresponding use of other l features.

Having new described this invention, what r claim as new and desire to secure by Letters Patent is: e

1. In a temperature control system for a regenerative furnace adapted. to be supplied with fuel from any one of a plurality of the combination of fuel regulating means, an eleture a predetermined amount above normal,..

means to shift the supply of fuel to the furnace from one point to another and means to render ineffective the operation of said fuel regulating means in afuel increasing direction for a predetermined time after the shifting of the fuel supply takes place.

2. In combination with a regenerative furnace adapted to be supplied with fuel from any one of a plurality of points, means to increase and decrease the supply of fuel to the furnace, means operative in response to variations in the temperature of a furnace wall to adjust said supply means, means to shift the supply of fuel to said furnace, from one point to another, and means operable upon a shift in the fuel supply of the furnace to render said fuel supply means ineffective to increase the supply of fuel for a predetermined time after the shifting takes place.

3. In an open hearth furnace control system in which the furnace is provided with two ends and is adapted to be supplied with fuel from either end1 means to adjust the supply of fuel thereto, means to reverse the supply of fuel to said furnace from one end to the other, an element responsive to the temperature of the roof of said' furnace to adjust said fuel supply in an increasing or decreasing manner depending upon whether the furnace roof temperature is below or above normal respectively, and means operative in response to furnace reversal to render said fuel supply adjusting means inoperative to increase the fuel supply for a predetermined time. 4. In a regenerative furnace control system y means to render intermittent the operation of in which the furnace is adapted to have fuel sup.. l

plied from one of two points, the combination of means to regulate the supply of fuel to said furnace, means to measure the temperature of y a wall of said furnace, mechanism responsive to the measurement made by said measuring means to adjust said regulating means to increase or decrease the supply of fuel to the furnace, means -to render the regulating means intermittent in operation, means to reverse the supply of fuel from one point to the other, and means to render said regulating means ineffective to increase the supply of fuel for a predetermined time after a reversal takes place.

5. In a regenerative furnacecontrol system, means to supply fuel tothe furnace from a plurality of points, means to control the supply of fuel to said furnace, means to measure the temperature of a wall of said furnace, means to regulate said supply means in response toa measurement made by said Imeasuring means in responsev said valve, means to supply fuel from one to another of the furnace ends, and means responsive to operation of `said supply means to prevent said valve from being operated in an opening direction for a predetermined time afterl said supply means has been operated.

'7. In an open hearth furnace temperature con`- trol system in which the furnace is heated from opposite ends alternately at periodic intervals by the injection of fluid fuel and oxidizing gases into said furnace, the combination of a fuel supply, a valve to regulate the supply of fuel, means to reverse the operation of said furnace, an element responsive to the roof temperature of said furnace, mechanism operative in response to measurements made by said element to adjust said valve in an opening or closing direction as the roof temperature is below or above a normal value, means to render the operation of said valve intermittent, means responsive to an increase of the roof temperature a predetermined amount above normal to produce continuous operation of said valve in a'closing direction, and means operated by said reversing means to render said valve inoperative to be adjusted in an opening direction for a predetermined time after reversal takes place.

8. A furnace temperature control system comprising means to measure the temperature of the furnace, a valve through which fuel is supplied to the furnace, a motor to operate said valve, a proportioning control system for said motor operated by said measuring means to produce an adjustment of the valvein accordance with the temperature of the furnace, means to render the operation of 4said motor periodic during normal operation of the same while said control system is being operated by said measuring means, and means operated by said measuring means to energize continuously said motor for rotation in a direction to close said valve upon the attainment of a vpredetermined temperature.

9. A furnace temperature control system comprising means to measure the temperature of the furnace, means including a motor operated valve to control the supply of fuel to said furnace, a

balanced electrical control system, means operated by said measuring means to unbalance said system as the furnace temperature changes, relays operated upon unbalance of said system to energize the motor for said valve in one direction or theopposite direction depending upon the direction of system unbalance to adjust said valve in a direction to return the furnace temperature towardnormal, means simultaneously operated by said motor to rebalance said control system, means to renderV the operation of said motor periodic when the furnace temperature is below :some predetermined maximum and while said control system is being unbalanced by said measuring means, and means operated by said'measuring means to energize continuously said motor for rotation Iin a direction to close said valve if the furnace temperature exceeds said maximum. JQSEPH P. VOILRATH.

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