US2025542A

US2025542A - Automatic furnace control system

Info

Publication number: US2025542A
Authority: US
Publication date: 1935-12-24
Anticipated expiration: 1952-12-24

Description

7 Dec. 24, 1935. c w U A 2,025,542

AUTOMATIC FURNACE CONTROL SYSTEM Filed June 16, 1934 amnion CHARLES w. LUGAR wM-WM Patented Dec. 24, 1935 AUTOMATIC FURNACE CONTROL SYSTEM Charles W. Lugar, Cleveland Heights, Ohio, as-

signor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application June is, 1934, Serial No. 731,003

20 Claims. (01. 236-45) The present invention relates to the automatic control of furnaces and particularly to the automatic control of such furnaces as heat treating furnaces wherein it is desired to maintain some furnace condition, such as a temperature, at a predetermined value or within predetermined limits, .the furnace normally being subjected to varying loads so as to require varying heat or fuel supply under these varying load conditions.

In the case of a heat treating furnace, for

example, there are times during which there is no load whatsoever upon the furnace whereas during other periods the furnace will be subjected to a quite heavy load. In heat-treating processes, it is of course necessary that the furnace temperature be maintained reasonably constant or that it be varied in a predetermined manner in order to obtain the particular treating desired. It is therefore advantageous, particularly where the furnace temperature should be constant during the treatment, to maintain the furnace at a given temperature not only dur ing such times as articles are actually being heat treated but during the standby period as well, so

25 that the furnace will always be ready to treat further articles.

In the past, it has been proposed to control the supply of heat in such a furnace, as by varying the supply of fuel thereto, in accordance with the deviation of furnace temperature .within some predetermined range. Now if this range be made quite small, the likelihood of hunting with the result of over-shooting and under-shooting is very great whereas, if this range is made rela- 35 tively wide, then as the load upon the furnace is increased, the furnace temperature will have a I drooping characteristic in that while it will remain within the predetermined relatively wide,

range,'the furnace temperature will approach the 40 lower end of such range as the'load thereof is increased.

One of the objects of the present invention is the provision of an automatic control system for a furnace by which-the control action of some 45 device responsive to a furnace condition, such as furnace temperature, upon the mechanism for maintaining the furnace condition constant is varied upon changes in the load upon the furnace.

A further objectof the invention is the pro- 50 vision of an automatic control system for a furnace by which the position which a furnace control member assumes in accordance with fluctuations in a furnace condition, such as the furnace temperature, is automatically changed a 5 predetermined amount when the furnace is sub jected to a load after having been operating under standby conditions.

Other objects of the invention will be found in the drawing, the detailed description and the appended claims. 5 For a more complete understanding of the invention, reference may be had to the following description and the accompanying single drawing which is a diagrammatic showing of the preferred form of the invention. 10

Referring to the single drawing, a heat-treating furnace, generally indicated at H), is provided with a conveyor II by means of which articles to be treated'may be carried through the furnace and thereby subjected to the furnace l5 temperature. The supply of heat to the furnace I0 is herein shown as being controlled by a valve I2 which controls the flow of fuel through a fuel pipe I3 and to a burner (not shown). The valve I2 includes an operating stem M which is provided with teeth I5 that cooperate with a pinion l6 secured to a main operating shaft H. The

main operating shaft II carries a gear 3 which is driven by a motor pinion l9 through suitable reduction gearing, generally indicated at 20. The

motor pinion i9 is carried by a motor shaft 2| to which is secured a pair of motor rotors 22 and 23 which rotate in opposite directions upon energization of their

respective field windings

24 and 25.

Energization of field windings24 and 25 is controlled by a pilot relay, generally indicated at 30,

whichcomprises a plunger 3! that is controlled by 'a pair of relay coils 32 and 33. The plunger 3| controls, a relay switch arm 34 through the me- 35 dium of a connecting member 35,- the relay switch arm 34 being adapted to alternately engage stationary contacts 36 and 31. -The' pilot relay 30 is controlled by a balanced relay, generally indicated at 3 8, which comprises a single'plunger 39 40 that is controlled by a pair of balanced "solenoid coils and 4| The plunger 39 controls a flexible switch arm 42 by means of a connecting member 43, the flexible switch arm 42 being adapted to alternately engage contacts '44 and 45.

Thesolenoid coils '40 and,4l are primarily co'ntrolled 'or'unbalancedby means of a furnace condition responsive controller generally indicated at 41. This furnace conditionj respdnsivecontroller, I in the present instance, comprises a potentiometer which includes a control resistancewinding .48 that is adapted 'tobe traversedby a'moyable control member or contact 49; The movable control member or'contact' 49 is hereins'howri as operated by means of a coiled ther' ostaticflelement 50 which responds-to the temperature within the heat-treating furnace I8. The un'balancing in the energizations of solenoid'coils 48 and 4|, as a result of changes in the furnace temperature or by other means to be hereinafter explained, is adapt- -ed to be rebalanced by a balancing potentiometer comprising a balancing resistance winding 5| and a balancing contact finger 52 adapted to traverse the same and which is operated by the main operating shaft I1.

The action of the controller 41 upon the balancing relay mechanism 38 during standby periods when the furnace is operating under minimum or no load conditions is determined by means of a first auxiliary resistance means comprising a pair of resistances 54 and 55 which are provided with cooperating contact fingers 56 and 51 that are preferably controlled by a single manually operable adjusting device 58, to which they are connected by means of a shaft 59. Similarly, the action of the controller 41 upon the balanced relay 38 is determined during those periods when the heat-treating furnace I8 is subjected to a load by means of a second auxiliary resistance means which comprises a pair of resistances 68 and 6|. A pair of contact arms 62 and 63 respectively cooperate with the resistances 68 and 6| and are preferably simultaneously adjustable by means of a single manual operating device 64 to which they are secured by means of a shaft 65.

.One or the other of these auxiliary resistance means is selectively electrically'connected in circuit with the controller 41 by means of a relay 61 which includes a relay coil 68 that operates apair of switch arms 69 and 18 through an armature 1|. When relay coil 68 is deenergized, switch.

- of conveyor I I and terminates thereabove so as to act as an operator which is adapted to be engaged by articles to be heat-treated which are placed upon the conveyor I I, it being understood that suitable mechanism is provided for causing the conveyor -II to pass. through the furnace I8. The switch 16 is normally maintained in open circuit position by means of a coiled spring 82.

The system of the present invention further includesa manual controller 88 which takes the form of a manually operable potentiometer comprising a resistance 84 and a cooperating contact finger adapted to be manipulated by means of a manual operating device 86. A manually operable reversing switch 81 is capable of placing the automatic controller 41 or the manual controller 83 in control of balanced relay 38.

The system further includes suitable protective resistances, 89 and 98 and also preferably includes'a pair of limit switches 9| and 92 which prevent excessive movement of valve I2. in any of the usual manners well-known in the art.

High voltage power is supplied to-the

field windings

24 and 25 by means of line wires 94 and 95' whereas low voltage power is supplied to the control circuits by means of a step-down transformer 96 having a high'voltage primary 9! and a low voltage secondary 98. I

Line wire 94 is connected to one end of each of

field windings

24 and 25 by means of a wire The otherend of field winding 24 is connected to contact 31 of pilot relay 38, through 'limit switch-92, by means of wires I8I and I02 whereas the other end of field winding 25 is connected to the contact 36 of pilot relay 38, through limit switch 9|, by means of wires- I83 and I84. Line wire 95 is connected to relay switch arm 34 by means of a wire I85.

The solenoid coils 48 and 4|, in series; are connected across the secondary 98 oLtransformer 96 by means of wires I86, I81, I88 and I89. The balancing resistance 5| is connected in parallel with the series connected solenoid windings 48 and 4|, through protective resistances 89 and 98, by means ofwires II8, III, H2, H3, H4, and H5. One end of the controlresistanc'e 48 is connected to one end .of each of the resistances 55 and 6|, through the manually operable reversing switch 81, by means of .wires H8, H1 and H8, whereas v its other end is connected to one end of each of the resistances 54 and 68, through the manually operable reversing switch 81, by means of wires II9, |28'and |2|. Relayswitch arm 18 is con- .the parts in the position shown, the control resistance '48 is connected in parallel with the series connected solenoid coils 48 and 4| through the protective resistances 89 and 98 and through the effective portions of resistances 68 and 6|. On the other hand, when relay coil 68 is energized by means of switch 16, the control resist; ance 48 is connected in parallel with the series protective resistances 89 and 98 and through the effective portions of resistances 54 and 55. The resistance 84 of manually operable pdtentiometer 83 is connected to the manually operable reversing switch 81 by means of wires I 28 and I29 so that upon operation of manually operable reversing switch 81 to the position opposite to that shown in the drawing, the manual potentiometer 83 is substituted for the automatic controller 41. The control member or contact 49 and balancing contact finger 52 and the contact finger 85 are all connected to the junction of solenoid windings 48 and 4| by means of wires -|3I, I32, I33, I34 and I35.

nected towires III and H2 by means of a wire I connected solenoid coils 48 and 4| through the Therelay windings 82 and 33 of pilot relay 38,

in series, are connected across the secondary 98 of transformer 96 by wires I86, I35, I31 and I89. The junction of relay coils 32 and 33 is connected to flexible switch arm 42 through protective resistance 88 by means of wires I38 and I39. A

small number of turns of solenoid coil 48 is connected to contact 44 by a wire I48 and a small number of turns of the solenoid winding 4| is connected to the contact 45 by means of a wire Operation 5 temperature is at the desired point since the con-' control resistance 48. With this control member 49 engaging the mid-portion of control resistance 48, it will be noted that there is a larger amount of resistance between the outer end of solenoid coil 4| and this control member 49 than there is between the outer end of resistance 40 and the control member 49. This is true since the larger portion of the resistance 60 is effective by reason of the position of its contact arm 62 whereas only' a small portion of resistance 6| is effective by reason of the position of its contact arm 63. Therefore, in order for the energizations of solenoid coils 40 and 4| to be equal, the balancing contact arm 52 is engaged with balancing resistance 5| near one end thereof. The valve I2 is therefore only about one quarter open. This amount of opening of valve I2 should maintain the furnace temperature constant under these no-load conditions. If the furnace temperature should rise slightly causing control member 49 to move along control resistance 48 towards the right hand end thereof, the voltage drop across solenoid winding 4| will be increased over that across solenoid winding 40 whereupon plunger 39 will move towards the left and bring flexible switch arm 42 into engagement with contact 45. When this occurs, current from secondary 98 will flow by way of wires I06, I36 and relay winding 33 whereupon the circuit will branch, a Dart going by way of relay winding 32 and wires I31 and I09 to the other side of secondary 98 whereas the other part will go by way of wire I38, protective resistance 88, wire I39, flexible switch arm 42, contact 45, wire I4I, a small number of turns of solenoid winding 4| and wires I08 and I09 to this same side of secondary 98. In this manner, the relay coil 33 is more highly energized than the relay coil 32 and a small additional pull is exerted on plunger 39 by the flow of current through the small number of windings of solenoid coil 4| whereby the flexible switch arm 42 is held more firmly in engagement with contact 45. The larger current flow through relay winding 43 moves the plunger 3| to the left whereupon relay switch arm 34 engages contact 31 to energize field winding 24 as follows: line 95, wire I05, relay switch arm 34, contact 31, wire I02. limit switch 92, wire IOI, field winding 24 and wire I00 to line 94. Motor rotor 22 is thereupon rotated in a clockwise direction as viewed from the left whereupon main operating shaft I1 is rotated in a counter-clockwise direction as viewed froml the left: This counterclockwise rotation of main operating shaft I'I moves balancing contact finger 52 toward the left hand end of balancing resistance 5| to rebalance the voltage drops across solenoid windings 40 and 4| whereupon flexible switch arm 42 is again moved to its mid-position, as shown in the drawing, wherein it is disengaged from both contacts 44 and 45. This action therefore rebalances the current flows through relay windings 32 and 33 and the plunger 3| moves to the position shown in the drawing wherein relay switch arm 34 is between and disengaged from both the contacts 36 and 31 whereby field winding 24 is deenergized. This counter-clockwise rotation of main operating shaft I! in order to rebalance the voltage drops across solenoid windings 40 and 4| also moves valve I2 to a more nearly closed position. Similarly, upon a tem-' perature fall, control member 49 will move along control resistance 4 8 towards the left hand end thereof whereupon the voltage drop across solenoid winding 40 becomes larger than the voltage drop across solenoid winding 4| and the plunger 39 moves to the right. Such movement of plunger 39 to the right brings flexible switch arm 42 into engagement with contact 44. Current then flows from secondary 98 through wire I09, wire I31, and through relay winding 32 whereupon the circuit branches, one part going by way of relay winding 33 and wires I36 and I06 to the other side of secondary 98, whereas the 1 other part goes by way of wire I38, protective resistance 88, wire I39, flexible switch arm 42, contact 44, wire I40, a small number of turns of solenoid winding 40, and wires I01 and I06 to the same side of secondary 98. In this manner, 1 the contact pressure between flexible switch arm 42 and contact 44 is increased and the relay winding 32 becomes more highly energized than relay winding 33.- Plunger 3| therefore moves to the right whereupon relay switch arm 34 en- 2 gages contact 35 to energize field winding 25 as follows: line 95, wire Hi5, relay switch arm 34, contact 36, wire I04, limit switch 9|, wire I03, field winding 25 and wire I00 to line 94. Motor rotor 23 thereupon rotates in a counter-clockwise 2 direction as viewed from the left whereupon main operating shaft I! is rotated in a clockwise direction also when viewed from the left. Balancing contact finger 52 therefore moves along balancing resistance 5| towards the right hand 3 end thereof to rebalance the voltage drops across field windings 40 and 4| so as to bring flexible switch arm 42 and relay switch arm 34 to the positions shown in the drawing. Such clockwise rotation of main operating shaft I'I slightly 3 opens valve I2 so as to increase the supply of fuel to furnace I0 and return the temperature thereof to the desired point. In this manner, for each position of the control member 49 upon the control resistance 48, the valve I2 assumes a corresponding position. The relationship between the positon of the valve I2 for any position of the control member 49 can be changed by manipulating the manually operable device 64 so as to change the position of contact arms 62 and 63 4 upon their cooperating resistances 60 and 6|.

Now if articles to be heat treated are placed upon the conveyor II for movement through the furnace I0, such articles will engage the operator 8| of switch 16 and cause the same to close. Relay coil 68 is then energized as follows: line wire I50, switch I6, wire I5I, relay coil 68 and line wire I52. Switch arms 69 and I0 are therefore moved from engagement with contacts 12 and I3 and are moved into engagement with contacts I4 and I5. The control resistance 48 is thereupon connected in parallel with the series connected solenoid windings 40 and 4| through the effective portions of resistances 54 and 55. It will now be seen that there is a relatively small amount of resistance connected between the right hand end of control resistance 48 and the right hand end of solenoid winding 40 whereas a considerablylarger amount of resistance is connected between the left hand end of control resistance 48 and'the left hand end of solenoid winding 4|.

The voltage drop across solenoid winding 40 is therefore immediately increased considerably in respect to the voltage drop across solenoid winding 4| whereupon plunger 38 moves right and brings flexible switch arm 42 into engagement with contact 44. Plunger 3| of pilot relay 30 therefore moves to the right, as previously explained, and brings relay switch arm.

to the T0 34 into engagement with contact 36. Field winding 25 is therefore energized and the main operating shaft- I1 is rotated in a clockwise diranging the resistances in circuit with the con- 7 trol resistance .48. In this manner, the relationship between the controlmember 49 .and the opening of valve I2 is varied or changed in such mannerthat the valve I2 is in some position,

such .as half open position, when the control. member 49 is in the central position shown in the drawing. -A larger amount of heat is therefore furnished to the furnace III in anticipation of theload about to be placed upon the furnace by the articles on the conveyor 1 I. This increased opening of valve I2 should maintain the furnace I 0 at the proper desired temperature and the position which the .valve I2 will assume when the control member 49 is in the central position shown in the drawing can be adjusted by manipulation of manually operable device 59., .If itwere not for this additional opening of valve I2 when a load isplaced on the furnace I9, it will be obvious that the furnace temperature would drop considerably and, while such drop would be accompanied by an increase in the fuel supply, still the furnace temperature could never return to the exact desired value while the furnace was subjected to this increased load so that the furnace temperature would have a drooping characteristic. Small fluctuations in the load during the passage of articles through the fur- .nace III will of course be followed by adjustments of valve I2 in response to slight changes in the furnace temperature in the same general man- 'ner .as heretofore set forth in connection with furnace temperature changes under no load conditions. As soon as the articles on the conveyor II pass the operator 8| of switch 16, the switch 16 will again move to open circuit position to deenergize relay-coil 68 whereupon the relationship between the valve position and the position of the control member 49 will be returned to that shown in the drawing. Y

In this manner, when the furnace is operating under minimum or no load conditions, the valve I2 is positioned in accordance with the movements of control member 49 but there is aninitial or'basic relationship between the valve position and the position of control member 49 which has been herein set out as the valve being one fourth open when the control member 49 is in.

the central position shown-in the drawing. On the other hand, as 'soon as a load is placed on the furnace III as indicated by closure of switch 16, while the valve I2 continues to be positioned in accordance with the movements of control member 49, the basic relationship is automaticallychanged so that the valve I2 is half open when the control member 49 isin the central position shown in the drawing. In this manner, a controller such as the controller 41 having a sumciently wide range to prevent huntingcan be employed without its allowing a falling off or droop-' ing in the furnace temperature upon an increase in the load upon the furnace.

If it should be desired to position the valve I2 manually irrespective of the furnace temperature, the reversing switch 81 is thrown to the position opposite that shown in the-drawing whereupon the manually operable potentiometer 83 is substituted for the automatic control potentiometer 41 and the position of the valve I2 can then be adjusted as desired by manipulation of manually operable device 86 of potentiometer 8.3.

For a more thorough understanding of the general manner in which the valve I2 is positioned in accordance with the movements of control member 49 reference may be had to the copending application of Lewis L. Cunningham, Serial No. 673,246 which was filed May 24th, 1933. i5 While a specific embodiment of the invention has beenherein described, it is to be understood that many modifications could be made therein and I therefore intend to be limited only by the scope of the appended claims. 20

, I claim:

1. The combination with a heat treating fur-. nace and a conveyor for conveying articles to be treated therethrough, of means responsive to the temperature of the furnace, means in control of 5 the heat supplied to said furnace controlled by said. 'furnace temperature responsive means,

means associated with said furnace temperature responsive means and heat controlling means for varying the control action of the former upon the 30 latter, and a device controlled by the articles on said conveyor for controlling said last-mentioned means. I

2. The combination with a heat treating furnace and a conveyor for conveying articles to be 35 treated therethrough, of means responsive to the temperature of the furnace, a valve in control of the supply of fuel to said furnace controlled by said temperature responsive means, means associated 'with' said temperature responsive 40 means and valve for varying the control action of the former upon the latter, and a device controlled by the presence or absence of a load on said conveyor for controlling said last-named means. H 45 3. The combination with a heating furnace having a conveyor for conveying articles to be heated therethrough, of 'yalve means in control of the flow of fuel to said furnace, a control member movable back and forth over a predetermined 0 path of travel in response to changes in furnace temperature, connections between said control member and valve means by which the latter is positioned in accordance with the movements of the former, means for changing the relationship 55 between said valve means and control member, and a device controlled by the load on said furnace for controlling said relationship changing means.

4. The combination with a heating furnace having a conveyor for conveying articles to be heatedtherethrough, of valve means in control of the flow of fuel to said furnace; a control member movable back and forth over a predetermined path of travel in response to changes'in furnace 65 temperature, connections between said control member and valve means by which the latter is positioned in accordance with the movements said furnace, a control member movable back and forth along a predetermined path of travel in response to changes in furnace temperature, connections between said control member and valve means by which the former positions the latter means being such as to allow a greater flow of fuel to said furnace for any position of said control member, and a device responsive to an increased load upon said furnace for rendering said last-named means operative to change the relationship between-said control member and valve means.

6. The combination with a heating furnace having a conveyor for conveying articles through the furnace, of means for controlling the supply of heat to the furnace, means responsive to the temperature of the furnace in control of said heat supply controlling meansmeans for varying the action of said temperature responsive means upon said heat supply controlling means, and a device in the path of movement of the articles placed on said conveyor for controlling said lastnamed means.

7. The combination with a heating furnace having a conveyor for conveying articles through the furnace, of means for controlling the supply of heat to the furnace, means responsive to the temperature of the furnace in control of said heat supply controlling means, electrically operated means for varying the action of said temperature responsive means upon said heat supply controlling means, and a switch including an operator located in the path of movement of the articles on said conveyor for controlling said electrically operated means.

8. The combination with a heating furnace and a conveyor for conveying articles to be heated thereto, of valve means in control of the supply of heat to the furnace, a furnace temperature responsive control member movable back and forth across a predetermined path of travel in response to variations in furnace temperature, connections between said control member and valve means by which the control member positions said valve means in accordance with the movements of said control member, means associated with said control member and valve means for changing the position which said valve means will assume for any given position of said control member, and a device located in the path of movement of the articles on said conveyor for controlling said means.

9. In combination, electrically operated heat supply controlling means, a temperature responsive variable resistance in control of said electrically operated heat supply controlling means, first and second resistance means adapted to be placed in circuit with said temperature responsive variable resistance means for changing the control action thereof upon said electrically operated heat supply controlling means, and automaticmeans for selectively placing said first or second resistance means in circuit with said temperature responsive variable resistance means.

10. In combination, electrically operated heat supply controlling means, a temperature responsive variable resistance in control of said electri cally operated heat supply controlling means,

first and second resistance means adapted to be placed in circuit with said temperature responsive variable resistance means for changing the control action thereof upon said electrically operated heat supply controlling means and an automatically controlled switching means for selectively placing said first or second resistance means in circuit with said temperature responsive variable resistance means.

12. The combination with a heating furnace for heating articles, of electrically operated means in control of the supply of heat delivered to said furnace, variable resistance means responsive to the temperature of said furnace for controlling the electrically operated means, auxiliary resistance means adapted to be placed in circuit with the temperature responsive resistance means for varying the control action of said temperature responsive resistance means on said electrically operated means, and means responsive to the load on said furnace for placing said auxiliary resistance means in and out of circuit with said temperature resistance means.

13. The combination with a heating furnace for heating artices, of electrically operated means in'control of the supply of heat delivered to said furnace, variable resistance means responsive to the temperature of said furnace for controlling the electrically operated means, auxiliary resistance means adapted to be placed in circuit with the temperature responsive resistance means for temperature responsive resistance means and a switch controlled by the load on the furnace for controlling said relay.

14. The combination with a furnace having a conveyor for conveying articles thereto to be heated thereby, of valve means in control of the supply of fuel to said furnace, furnace temperature responsive variable resistance means for variably positioning said valve means in accordance with furnace temperature, auxiliary resistance means associated with said furnace temperature responsive variable resistance means for varying the control thereof upon said valve means, and a switch located in the path of movement of the articles conveyed to said furnace for controlling the auxiliary resistance means.

15. The combination with a furnace having a conveyor for conveying articles thereto to be heated thereby, of valve means in control of the supply offuel to said furnace, furnace temperature responsive variable resistance means for va-' riably positioning said valve means in accordance .with furnace temperature, first and second auxilcontrol action thereof -upon said valve means, and switching means including an operator located in the path of movement of the articles being conveyed to said furnace for selectively connecting said first or second auxiliary resistance means to said furnace temperature responsive resistance means. 4

16. In combination, a furnace, a member movable to'a plurality of positions for varying the operation of said furnace, a control member movableback and forth along a predetermined path of travel in response to changes in a furnace condition, connections between said movable memher and control member by which the latter positions the former in accordance with changes in said furnace condition, and means responsive to an increase in the load on said furnace for'changing the relative positions of said control member and movable member a predetermined amount.

17. The combination with a furnace adapted to be subjected to varying loads, of a movable memberadapted to control the operation of said furnace, electrical motor means in control of said movable member, switching means in control of said motor means, electromagnetic means in con,- trol of said switching means, a control resistance means associated with said electromagnetic means for variably energizing the same in response to changes'in a furnace condition, an auxiliary resistance means adapted to be placed in circuit with saidicontrol resistance means to vary the effect thereof upon the electromagnetic means,

. perature first and second ,resistancemeans for varying the same.

response to changes in a furnace condition, first and second auxiliary resistance means adoptedv to be placed in circuit with said control resistance means to vary the effect thereof upon said electromagnetic means, and switching mechanism operated in response to changes in the load on said furnace for selectively placing one or the other of said auxiliary resistance means in circuit with said control resistance means. 19. The combination with a furnace having a conveyor for conveying articles to the furnace, of a valve in control of the supply of fuel to the furnace, electric motor means in control of said valve, switching means in control of said electric motor means, normally electrically balanced electromagnetic means in control of said switching means, a control potentiometer connected to said electromagnetic means. for varying the energization' thereof and responsive to the furnace temadapted to be placed in circuit with said control potentiometer to vary the effect thereof 'upon said electromagnetic means; switching means including an operator located in the path of movement of the articles on said conveyor for selectively 2 5 placing one or the other of. said resistance means in circuit with said control potentiometer, and a balancing potentiometer for rebalancing the energizations of said normally electrically balanced 4 electrical means controlled by said motormeans. 30

CHARLES W. IBIUGAR.