US2105686A - Combustion control system - Google Patents

Description

Jan. 18, 1938. CUNNINGHAM 2,105,536

COMBUSTION CONTROL SYSTEM Filed July 15, 1935 2 Sheets-Sheet l v gkam Lewzbl 62am HHIHII lllllllll llllll'lll Jan. 18, 1938.

L. L. CUNNINGHAM COMBUSTION CONTROL SYSTEM Filed July 15, 1935 2 Sheets-Sheet 2 fiat 610W" lien 1191;. Cunningham Mame/g 10 means is controlled in response Another object of Patented Jan. 18, 1938 UNITED STATES 2,105,636 COMBUSTION CONTROL SYSTEM Lewis L. Cunningham,

signor to Minneapolis-Honeywel Company, Minneapolis, Minn,

of Delaware Application July 15,

1 Claim.

This invention relates to combustion control system.

The prime object of this invention is to provide a combustion control system which will 5 maintain desired combustion conditions in a boiler or furnace regardless of the effects of outdoor atmospheric conditions on the draft. H

An object of this invention is to provide a com bustion control system wherein the fuel feeding to the demand on the boiler and the rate of flow of combustion gases through the boiler is controlled in response to the demand on the boiler and the rate of flow of combustion gases through the boiler.

this invention is to provide a combustion control system wherein the fuel feeding means is controlled in response to the demand on the boiler wherein the air supply means is controlled in response to the over-fire 20 pressure and wherein the draft through the boiler is controlled in response to the demand on the boiler and in response to the combustion gases through the boiler.

Another object of this invention is-to provide I 25 a combustion control system for a boiler having a draft regulating damper, a variable speed fuel feeding mechanism, and an air supplying mechanism with a floating motor for controlling the damper. a proportioning motor for controlling the speed of the fuel feeding means, a proportioning motor for controlling the supply of air to the boiler.

Still another object of this invention is to provide a damper for controlling the rate of flow of 5 gases of combustion through the passes of a boiler which damper is operated by a motor under the control of a pressure controller, responsive to boiler steam pressure and a pressure regulator responsive to the pressures existing in the 40 various passes of the boiler. Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claim and drawings, in which drawings:

Fig. l is a diagrammatic illustration of my combustion control system as applied to a boiler, and Fig. 2 is a wiring diagram of a portion of the control system disclosed in Fig. 1. l

Although a furnace could be equally well used, 50 for purposes of illustration I have shown my invention as applied to a boiler designated at l0. Contained within the boiler H! are water tubes l which are connected between a supply header l2 and a steam header l3. Steam generated in 55 the boiler l0 and collected in the header I3 is rate of flow of Minneapolis, Minn, as-

l Regulator a corporation 1935, Serial No. 31,462

taken therefrom through a pipe l4 which may lead to some point of use for any desired purpose. Fuel is shown as supplied to the boiler It by means of a chain-grate stoker l5, the fuel being fed on the chain-grate from a hopper |6. Located beneath the stoker is an air duct for supplying air to the fuel for combustion purposes. Located in the boiler are a plurality of bafiles |9, forming passes in the boiler through which the gases of combustion pass over the tubes II and into a stack 20. Located in the stack is a draft'damper 2| for regulating the rate of flow of the combustion gases through the boiler It.

The draft damper 2|, is controlled by a floating motor 23 through a crank arm 24 operated by the motor 23, a link 25 and a lever 26 connected to the draft damper 2|. 7

The chain-grate stoker is operated by a steam engine 28 through any suitable driving mechanism such as a chain 29. Steam is supplied to the steam engine 28 through a pipe 30 leading from some source, not shown, and the flow of steam to the steam engine 28 is regulated by a throttle valve 3|. The throttle valve 3| is shown to be operated by a proportioning motor 32 through a crank arm 33 operated by the proportioning motor 32, a link 34 and a lever 35 secured to the valve.

Air is supplied through the duct |8 for combustion purposes by means of a fan 31 which is driven by an electric motor 39 through any type of driving means such as a belt 38. The flow of air through the duct I8 is controlled by a damper 40. The damper 40 is positioned by means of a proportioning motor 4| through a crank arm 42 operated by the proportioning motor 4|, 9. link 43 and a lever 44 connected to the damper 40.

A pressure controller 41, having a bellows chamber 48, is connected to the steam supply pipe H by a pipe 49 so that the pressure controller 41 responds to variations in boiler steam pressure or the demand on the boiler. The details of this pressure controller 41 are shown and described in my copending application S. N. 752,481, filed November 10, 1934.

The low pressure side of a pressure regulator 50 is connected by a pipe 5| toone of thepasses in the boiler l0 such as the pass next preceding the stack 20. The high pressure side of the pressure regulator 50 is connected by pipes 52 and 53 to another pass in the boiler In which may be the portion of the boiler immediately above the fuel bed. Therefore, the pressure regulator 50 responds to furnace pressures existing in different passes within the boiler and since these pressures vary in accordance with the rate of flow of combustion gases through the boiler, the pressure regulator 50, in effect, is responsive to the rate of fiow of combustion gases through the boiler.

A static pressure regulator 54, identical with the pressure regulator 50, is connected by pipes 55 an 53 to one of the passes of the boiler which may be the portion of the boiler immediately above the fire bed. Therefore, the pressure regulator 5 responds to a pressure existing within one of the passes of the boiler and, more specifically to the over-fire pressure. The details of construction of the pressure regulators 50 and 54 are shown and described in my copending application 5. N. 31,463 filed July 15, 1935. The pressure regulator 41 responding to boiler steam pressure or the demand on the boiler and the pressure regulator 5!] respond to the rate of flow of combustion gases through the boiler operate through a relay 5'! in control of the floating motor 23 to accurately position the draft damper 2I. The pressure controller 4'I responding to boiler steam pressure or the demand on the boiler also operates through a relay 58 to control the proportioning motor 32 to regulate the speed of the power driven fuel feeding mechanism I5. The pressure regulator 54 responding to over-fire pressure conditions operates through a relay 59 for controlling the proportioning motor 4| for regulating the supply of air for combustion purposes.

Line wires leading from some source of power, not shown, are designated at BI and 62. The relay 5? receives its power from the line wires GI and 52 by being connected thereacross by wires 68 and 64. The relay 58 receives its power from the line wires 6| and 62 and is connected across these line wires by wires 55 and 66. In a like manner, the relay 59 receives its power from line wires 6! and 82 and is connected across the line wires GI and 62 by wires 51 and 68. A wire 69 extending from the line wire 6| is connected to a manually operated switch I which in turn is connected by a wire II to the electric motor 39. Electric motor 39 is also connected to the line wire 62 by a wire I2 so that when the switch I0 is closed, the electric motor 39 is placed in operation to drive fan 37.

Referring now to Fig. 2, I have shown the manner in which the various pressure controllers and regulators operate through their associated relays to control the operation of the above referred to floating and proportioning motors. The particular portion of the control system illustrated in detail in Fig. 2 is that portion shown in the upper right hand part of Fig. l. The floating motor 23 comprises a shaft 15 to which is secured the crank arm 24 for operating the damper 2I. Rigidly secured to the shaft 15 is a gear 16 which is driven through a reduction gear train I! by either of the rotors I8 or 19 upon energization of their respective field' windings 80 and 8|. Also rigidly secured to the shaft 15 is a disc 82 carrying a pin 83. The pin 83 is adapted at extreme positions of the shaft I to open limit switches 84 or 85. Upon energization of field winding 89, the shaft I5 is rotated to move the damper 2I towards an open position and upon energization of field winding 8|, the shaft 15 is rotated to move the damper 2| towards a closed position.

Although I have described in detail the structure of my pressure controller 41 in application S. N. 752,481, filed November 10, 1934/ for purposes of illustration in this application I have shown'it to comprise a. plunger 81 operated by the bellows contained within the bellows casing 48. The plunger 81 is adapted to rotate a lever 88 above its knife-edge pivot 89 in a counterclockwise direction. The lever 88 is urged in a clockwise direction by an adjustable tension spring 90. The lever 88 carries an arm 9I to which is secured in an insulating manner a slider 92 adapted to slide across a resistance coil 93. The arm 9! also has secured thereto in an insulating manner a slider 94 adapted to slide across a resistance coil 95. Upon an increase in pressure within the steam line I4, the bellows contained with the bellows casing 48 is expanded to rotate the lever 88 in a counter-clockwise direction to move the sliders 92 and 94 to the left with respect to their associated resistance coils 93 and 95. Upon a decrease in pressure within the steam line I4, the lever 88 is moved in a clockwise direc tion by the action of tension spring 90 to move the slider 92 and 94 to the right with respect to their respective resistance coils 93 and 95. The range of pressure required to move the sliders 92 and 94 across their coils is controlled by a range adjusting spring assembly 96 which is clearly shown and described in my above referred to copending application.

Although I have disclosed in detail the construction of my pressure regulator 50 in my copending application S. N. 31,463 filed July 15, 1935 I have shown it to comprise a casing 98 containing a liquid therein. Located in the casing 98 and sealed by the liquid is an inverted bell 99 which is urged in an upward direction by the adjustable tension spring I00. The tension spring I00 may be adjusted by the handle IOI in the manner pointed out in my above referred to copending application. The space above the inverted bell 99 is in communication with the over-fire pressure by means of pipe 52 and the space within the inverted bell 99 is in communication with the pressure of the last pass of the boiler by means of the pipe 5I. Therefore, the bell 99 is operated by the differential in pressure existing between these two passes within the boiler I0. Since the pressure acting through the pipe 52 is greater than the pressure acting through the pipe 5I, the bell 99 is urged downwardly against the action of the adjustable spring I00 and the bell 99 assumes a position corresponding to the differential of these two pressures. Therefore, the bell 99 is positioned according to the rate of flow of combustion gases through the boiler I0. One arm of a bell crank lever I03 is connected by a link I02 to the bell 99 and the other end of the bell crank lever I03 carries a slider I04 adapted to slide across a resistance coil I05. Therefore, it follows that upon an increase in the rate of flow of combustion gases through the boiler I 0, the slider I04 is moved to the left with respect to its resistance coil I 05 and upon a decrease in the rate of flow of combustion gases through the boiler, the slider I04 is moved to the right with respect to its resistance coil I05. The range of changes in the rate of flow of combustion gases through the boiler I0 required to move the slider I04 across the resistance coil I05 may be varied by means of a range adjusting spring assembly I06 in the manner pointed out in my copending application.

The wires 63 and 64 connect a primary I08 of a step-down transformer I09, having a secondary IIO across the line wires SI and 62, respectively, the transformer I09 being located in the relay 51. Also located in the relay 51 are oppositely acting coils III and H2. These oppositely acting coils III and H2 operate a core or armature H3 which is connected by a spring H4 to a pivoted switch arm H5. Cooperating with the pivoted switch arm I I5 are spaced contacts H8 and H1 so that when the oppositely acting coils III and H2 are equally energized, the switch arm H5 is maintained midway between the contacts II6 and H1. When the coil III is energized more than the coil H2, the switch arm H5 is moved into engagement with the contact H6 and likewise, when the coil H2 is energized more than the coil III, the switch arm. H5 is moved into engagement with the contact H1. Also located within the relay 51 are oppositely acting coils H8 and H9 which control theoperation of an armature or core I20. The armature or core I20 is connected by a spring I2I to a pivoted switch arm I22.- Cooperating with the pivoted switch arm I22 are spaced contacts I23 and I24 so that when the coils I I8 and I I9 are equally energized, the switch arm I 22 is spaced midway between the contacts I23 and I24. When the coil H9 is energized more than the coil H8, the switch arm I22 is moved into engagement with the contact I24 and when the coil H8 is energized more than the coil II9, the switch arm I22 is moved into engagement with the contact I23.

One end of secondary .IIO of the step-down transformer I09 is connected by wires I30 and I3I to one end of the coil H2. In a like manner, the other end of secondary H0 is connected by wires I32 and I33 to one end of coil III. One end of coil H9 is connected by awire I34 to the junction of wires I30 and I3I and one end of the coil H8 is connected by a wire I35 to the junction of wires I32 and I33. The junction of wires I30, I3I and I34 is connected by a wire I36, a protective resistance I31 and a wire I38 to the right hand end of resistance coil 85. The junction of wires I32, I33 and I35 is connected by a protective resistance I39 and wires I40 and MI to the left hand end of the resistance coil 95. The junction of oppositely acting coils III and H2 is connected by wires I42 and I43 to the slider 94. The switch arm H5 is connected by a wire I44 to the junction of oppositely acting coils H8 and H9. The contact H6 associated with the switch arm nected by a wire I45 to a small number of turns of coil III and in a like manner, the contact I I1 is connected by a re I46 to a small number of turns of coil' I I2. e left hand end of the resistance coil I05 of the pressure regulator is connected by a wire I41 to the junction of wires I40 and I4I and the right hand end of resistance coil I05 is connected by a wire I48 to the junction of protective resistance I41 and the wire I38. The slider I04 associated with the resistance coil I05 is connected by a wire I49 to the junction of wires I42 and I43.

The switch arm I22 is connectedv by a wire I50 to the wire 64. The contact I23 associated with the switch arm I22 is connected by a wire I5I to thelimit switch 85 and the contact I24 is connected by a wire I52 to the limit switch 84. The limit switch 85 is connected by a wire I53 to one end of field winding BI and the limit switch 84 is connected by a wire I54 to one end of the field winding 80. The field windings 80 and BI are connected together and to a wire I55 which in turn is connected to the wire 63.

. coil III and decrease H5 is con- From the above related wiring connections, it is seen that the upper end of the secondary H0, the right hand end of the coil I I2, the right hand end of resistance coil 95, and the right hand end of resistance coil I05 are connected together. Likewise, it is seen that the lower end of secondary H0, the left hand end of coil III, the left hand end of resistance coil 95 and the left hand end of resistance'coil I05 are connected together. It is also seen that the junction of coils III and N2, the slider 94 and the slider I04 are connected together. It is therefore seen that the secondary H0, the coils III and H2, the resistance coil 95 and the resistance coil I05 are connected in parallel.

With the parts in the positions shown in Fig. 2. the slider 94 is midway of its resistance coil 85, the slider I04 is midway of its coil I05, the switch arms H5 and I22 are midway of their contacts and the coils III, H2, H8 and H9 are equally energized whereby the damper 2I is .maintained in a mid position which position allows the correct rate of flow of combustion gases through the boiler I0 for a given-boiler steam pressure. When the boiler steam pressure decreases, the lever 88 'is rotated in a clockwise direction to move the slider 94 to the right with respect to its coil 95. By reason of the parallel relationship set out above, movement of slider 94 to the right causes shunting or short-circuiting of the coil I I2 to increase the energization of the energization of coil H2. This unbalanced relationship of coils III and I I2 causes movement of switch arm H5 into engagement with the contact H5, movement of switch arm H5 into engagement with the contac H6 completes a circuit from the secondary a circuit from line wire 62, through wires 64 and wire I52, limit I50. switch arm I22, contact I24, switch 84. wire I54, field winding 80, and wires I55 and 63 back to the other line wire 6I. Completion of this circuit causes energization of field winding 80 to rotate the shaft 15 to move the damper 2I towards an open position. Movement of damper 2I towards an open position increases the rate of fiow of combustion gases through the inverted bell 89 of the pressure regulator 50 to move the slider I04 thereof to the left with respect to its resistance coil I05. Due to the parallel relationship oi the various coils, as pointed out above. movement of the slider I04 to the left boiler I0 and causes downward movement of the of coil I05 causes shunting or short circuiting of the coil I I I to increase the energization of the coil H2 and decrease the energization of the coil III. When the rate of flow of the gases of combustion through the boiler I0 has been increased in response to the opening of damper 2I to a value which corresponds to the new low stream pressure value, the slider I04 will be moved s ciently far to the left with respect to its coil I05 to rebalance the coils III and H2 to move the switch arm H5 out of engagement with its contact H6 and into mid position as shown in Fig. 2. Movement of switch arm H8 out of engageshort circuit through the coil H9 equally energize the coils H8 and H to move the switch arm i22 out of engagement with the contact i24. This causes deenergization of field winding 80 to stop rotation of the shaft l to maintain the damper 2i in its newly adjusted position whereby the draft through the boiler it is maintained in accordance with the value of the boiler steam pressure.

When the boiler steam pressure increases, the lever 08 of the pressure controller "ii is rotated in a counter-clockwise direction to move the slider 0% to the left with respect to the resistance coil 05. This left hand movement of slider 04 causes shunting or short-circuiting of the coil iii to increase the energization of coil E555 and decreases the energization of coil This causes movement of switch arm 355 into engage- M vii.

. ment with the contact ill to complete a circuit from the secondary ti l, through wires 530 and lti, a small number of turns of coil H2, wire M6, contact ill, switch arm H5, wire [144, coil M8 and wires 535 and i32 back to the secondary H0. This causes short circuiting or shunting of the coil M0 to increase the energization of coil i is and decrease the energization of coil ii? to move the switch arm l22 into engagement with the contact i23 to complete a circuit from the line wire 62, through wires 04 and i501, switch arm 5222, contact 523, wire i56 limit switch 85, wire field winding 8i and wires M5 and 63 back to the other line wire 6i. tion of field winding M to move the shaft 75 in the opposite direction to move the damper 2i towards a closed position. Movement of damper 2! towards a closed position decreases the rate of flow of the combustion gases through the boiler 50 and in response to this decrease in the rate of flow, the inverted bell 99 of the pressure regulator 50 is raised by the adjustable tension spring Hi0. This upward movement of the inverted bell 99 causes right hand movement of slider I04 with respect to the coil W5 and this right hand movement shunts or shorts out the coil H2 to increase the energizatlon of the coil ill and decreased the energization of the coil H2. When the rate of flow of gases of combustion through the boiler I0 have decreased to a value corresponding to the new increased boiler steam pressure value, the coils HI and H2 will become equally energized to move the switch arm H5 out of engagement with the contact Ill and into mid position shown in the drawings, This causes equal energization of the coils H8 and H0 and consequent movement of the switch arm i122 out of engagement with the contact I23 and into the mid position shown in the drawings. This causes equal energization of the coils H8 and H9 and consequent movement of the switch arm I22 out of engagement with the contact I23 and into the mid position shown in the drawings. This causes deenergization of field winding 8| to stop rotation of shaft I5 to maintain the draft damper 2i in its newly adjusted position.

It is noted that the circuits completed by the I movement of switch arm H5 into engagement with either contact H6 or II! include a small number of turns of coils III and H2 which increases the holding effort of the coils to prevent chattering between the switch arm H5 and the contacts H6 and Ill. The limit switches 84 and 85 are opened when the motor is moved to either of its extreme positions to break the circuit to This causes energiza- -by a wire I48 aromas ment with the contact H8 breaks the above the field windings 80 and 8! whereby overtravel of the motor 23 in either direction is prevented.

It is well-known that the draft and the rate of flow of gases of combustion in the boiler are affected by outdoor atmospheric conditions, including outdoor temperatures and wind so that for a given position of a draft damper, the rate of flow of gases of combustion through the boiler will vary in accordance with the variations in the outdoor atmospheric conditions. Therefore, with a draft damper that is merely positioned in accordance with some condition without any compensating effect under the control of the draft or rate of flow of combustion gases through the boiler will not afford an accurate control. By having my pressure regulator 50 perform the balancing function in the manner above described, this compensating efiect is accomplished and for a given condition of steam pressure or demand on the boiler, the rate of flow of the gases of combustion through the boiler i0 is maintained at a given value regardless of outdoor atmospheric conditions. Therefore, it is seen that I have provided means whereby the rate of flow of gases of combustion through the boiler is controlled by a floating motor in response to the boiler steam pressure or demand on the boiler and the rate of how of gases of combustion through the boiler i0.

The proportioning motor 32 which controls the steam valve 3i of the steam engine 28 is also controlled by the pressure controller 41 through the medium of the relay 58. The slider 02 and the associated resistance coil 93 of the pressure regulator 41 as disclosed in Fig. 2 therefore performs the control function of the proportioning motor 32. The relay 58 associated with the proportioning motor 32 is in all respects identical to the relay 5'I disclosed in Fig. 2 and the proportioning motor 32 is likewise identical to the floating motor 23 disclosed in Fig. 2 with the exception that the balancing potentiometer is controlled by the motor instead of by the changes in the condition produced by the motor in a manner disclosed in my copending application S. N. 673,236 filed May 27, 1933. To illustrate this, reference is made to Fig. 2 wherein slider I04 is moved in accordance with the position of the proportioning motor 32. The slider I04 is adapted to slide across the resistance coil I05 and one end of resistance coil I05 is connected by a wire I41 to the junction of wires I40 and MI while the other end of the resistance coil I05 is connected to the junction of the protective resistance I31 and wire I38. The slider I04 is connected by a wire I49 to the junction of wires I42 and I43. When a proportioning motor such as 32 is used, the resistance coil I05, the slider I04, and the wires I47, I48 and I49 are substituted for the resistance coil I05, the slider I04 and the wires I41, I48, and I49, respectively. In Fig. 1, the wiring connections are indicated to correspond to the wiring connections disclosed in Fig. 2.

Upon a decrease in steam pressure or upon an increase in the demand upon boiler I0 theproportioning motor 32 is placed in operation through the relay 58 to move throttle valve 3I toward an open position to increase the speed of operation of the chain-grate stoker to supply more fuel to the boiler I0. When the throttle valve 3| has been moved to a position corresponding to the decreased steam pressure, the balancing potentiometer operated by the proportioning motor 32 stops operation of the proporconstant regardless of the 2 to the tioning motor 32 and maintains the throttle valve 3! in its newly adiusted position whereby fuel is supplied to the boiler it; in proportion to the demand for steam or in inverse proportion to the boiler steam pressure.

The pressure regulator 54 responding to overfire pressure is identical to the pressure regulator 58 disclosed in Fig. 2 but instead of performing a balancing function as is done by the pressure regulator 50, the pressure regulator 54 performs a controlling function. The pressure regulator E lis therefore connected by wires I38, Ml and M3 to the relay 5% which relay controls the operation of the proportioning motor 4!. The propcrtioning motor it operates a balancing potentiometer in the same manner as proportioning motor 32 so that the motor 4| is positioned in accordance with the control of the pressure regulator 54. When the over-fire pressure decreases, the pressure regulator 54' operates through the relay 59 and the proportioning motor 4! to move the air supply damper 40 towards an open position to supply more air i'or purposes of combustion and to restore the over-fire pressure to normal. In a like manner, when the over-fire pressure increases above its normal value, the pressure regulator 54 operates through the relay 59 and the proportioning motor M to move the air supply damper to towards a closed position to decrease the supply of air for combustion and restore the over-fire pressure to its normal value. Therefore, the over-tire pressure is maintained efiects of the outdoor atmospheric conditions andof the condition of the fuel bed.

Summarizing the mode of operation of the entire control system as disclosed in Fig. 1, upon a decrease in steam pressure or upon an increase in the demand for steam, the draft damper 2| is opened to increase the rate of flow of the gases of combustion through the boiler l0 and by reason of the balancing function afforded by the pressure regulator 58 this rate of flow of the gases of combustion is maintained at the desired value regardless of outside atmospheric conditions. This increase in the rate of flow of the combustion gases through the boiler increases the heat supplied to the water in the water tubes ii to generate more steam and bring the steam pressure in the supply pipe It back up to normal. Increasing the rate of the how of combustion gases through the boiler It would tend to dethe over-fire pressure but by reason of the pressure regulator ti l, controlling the air supply damper the overdire pressure is maintained constant regardless of changes in the rate of flow of gases of combustion. increase in the rate of flow of gases of combustion causes faster hurnof the fuei. and requires more fuel to be fed tire bed. is taken care of by means of the pressure controller l'i responding to the pressure or to the demand for steam to pply more to the boiler it where the steam as u decreases to 2 am restore the steam c normal. manner, when .ond potentiometer,

the steam pressure rises above normal, the rate of flow of gases of combination through the boiler I0 is decreased regardless of the outdoor atmospheric conditions. The over-fire pressure is maintained constant even though the decrease in the rate of flow of gases of combustion tend to increase the over-fire pressure and the amount of fuel supplied to the boiler Ill is decreased.

From the above it is seen that I have provided a combustion control system which is extremely accurate and reliable in its operation and which is not effected by varying conditions such as outdoor atmospheric conditions and fuel bed conditions.

Although I have disclosed one form of my invention other forms may become apparent to those skilled in the art and this invention is to be acted only by the scope of the appended claim and prior art.

I claim as my invention:

In a combustion controlv system for a furnace having means for feeding fuel to the fuel bed of the furnace, means for supplying combustion supporting air and a stack for inducing draft through the furnace, the combination of, means responsive to the load on the furnace for controlling the rate of fuel supply to the fuel bed of the furnace,

a damper associatedwith the stack for controlling the draft through the furnace, a reversible electric motor for positioning said damper, a relay including a plurality of coils for controlling the operation of the electric motor, a first adjustable potentiometer, means responsive to variations in the load on' the furnace for adjusting said potentiometer, a second adjustable potentiometer, means responsive to variations in the draft through the furnace for adjusting the seccircuit connections between the potentiometers and the coils of the relay to position the damper in accordance with the load on the furnace and the draft through the furnace for maintaining the draft through the furnace at predetermined values as determined by the load on the furnace regardless of changes in the draft inducing efiect of the stack, a second reversible electric motor for controlling the rate of supply of combustion supporting air by the supplying means, a relay including a plurality of coils for controlling the operation of the second electric motor a third adjustable potentiometer, means responsive to over-fire pressure for adjusting said potentiometer, a fourth adjustable potentiometer, means responsive to a condition indicative of the position of .ne second electric motor for adiusting the fourth potentiometer, and circuit connections between. the third and fourth poten tiometers and the coils of the last mentioned 'iosition the second electric motor and the rate oi supply combustion air ;.tatin UVQi filE pressure y constant reg dless of changes in the Road on the furnace, in the condition re fuel hed or the draft through

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