US2811120A

US2811120A - Control system for powdered coal-fired burners

Info

Publication number: US2811120A
Application number: US452778A
Authority: US
Inventors: George R Anderson
Original assignee: Hagan Chemicals and Controls Inc
Current assignee: Hagan Chemicals and Controls Inc
Priority date: 1954-08-30
Filing date: 1954-08-30
Publication date: 1957-10-29
Anticipated expiration: 1974-10-29

Description

Oct. 29, 1957 e. R. ANDERSON 2,311,120

CONTROL SYSTEM FOR ROWDERED COAL-FIRED BURNERS Filed Aug.' 30, 1954 3 Sheets-Sheet 1 IN V EN TOR.

GEORGE R. ANDERSON G. R. ANDERSON 2,811,120

Oct. 29, 1957 CONTROL SYSTEM FOR POWDERED COAL-FIRED BURNERS Filed Aug. 30, 1954 3 Sheets-Sheet 2 INVENTOR. GEORGE A. Awe/e50 BY Z /s Oct. 29, 1957 e. R. ANDERSON CONTROL" SYSTEM-FOR POWDERED COAL-FIRED BURNERS 3 Sheets-Sheet 3 &

Filed Aug. 30, 1954 INVENTOR. GEORGE R. ANDERSON I es 3 CONTROL SYSTEM For POWDERED COAL-FIRED BURNERS Application August 30, 1954, Serial No. 452,778 9 Claims. ((31. 110 103 This invention relates to automatic control systems for the regulation of combustion of fuels such as powdered coal supplied by .a pulverizer, in accordance with the demand for combustion as reflected by temperature, steam pressure, or other condition which indicates the demand for combustion. While not necessarily limited to the control of combustion in boiler furnaces, the system is particularly adapted to the control of the rate of supply of pulverized coal 'by a pulverizer mill to the burners of the furnace in accordance with the demand for steam.

An object of this invention is to provide a control system that will automatically regulate the rate of supply of coal to a pulverizer mill in accordance with the demand for combustion as reflected by a condition such as temperature or steam pressure, and the rate of delivery of pulverized coal to the burners of the combustion de- VICC.

Another object of this invention is to provide a con; trol system that will automatically regulate the rate of supply of coal to a pulverizer mill in accordance witha predetermined relationship between boiler steam pressure and the rate of delivery of pulverized coal to the burners of the boiler furnace.

Another object of the invention is to provide a system as above set forth having means for maintaining a substantially constant pressure drop across an orifice in aduct leading from the mill classifier to an enhauster that supplies the pulverized coal to the burners.

A still further object of the invention is to provide a system as above set forth having means for controlling the rate of supply of secondary .air to the furnace or combustion device in accordance with a predetermined relationship between the condition to be regulated, for example, temperature or steam pressure, or other condition reflecting the demand for combustion, and the rate of supply of pulverized coal to the burners of the furnace or combustion device.

And a still further object of the invention is to provide a system as above set forth having means for controlling' the rate of supply of secondary air to the furn'ace in accordance with a predetermined relationship between the steam pressure generated by .the' boiler and the rate of supply of pulverized coal to the burners of the boiler furnace.

The above and other objects of the invention will be apparent to those of ordinary skill in the art to which the invention pertains from the following description taken in conjunction with the accompanying drawings.

In the drawings:

Figure 1 is a more or less diagrammatic View of a steam boiler furnace supplied with powdered coal as fuel;

Fig. 2 is a more or less diagrammatic view of a pulverizer mill and a control system for supplying pulverized coal to the burners of the furnace shown in Fig. 1 at such a rate that the heat input to the furnace is in ac cordance with the rate" at which steam is generated by the furnace as measured by the steam pressure;

atent 2,311,120 Patented ea. 2s, rear Fig. 3 is a more or less diagrammatic view of a regulator included in Fig. 2 that responds to the pressure ofthe steam generated by the boiler and to the rate of delivery of pulverized fuel to the burner for controlling the rate of supply of raw or unpulverized coal to the pulverizer;

Fig. 4 is a more or less diagrammatic view of a regulater that measures the rate of delivery of pulverized coal to the furnace and develops a control force for regulating the rate of supply of secondary air to the burners of the furnace;

Fig. 5 is a view of a regulating device for maintaining a substantially pressure drop across an orifice in a duct leading from the classifier of the pulverizer to an eirhauster which receives the coal from the classifier and delivers it to the furnace burners;

Fig. 6 is a more or less diagrammatic view of a regulator for operating a damper by which the pressure drop across the orifice in the duct leading from the mill classifier to the mill exhauster is maintained substantially constant; and p Fig. 7 is a view of a power cylinder embodied in Figs. land 2 p The control system to be described infra, and-as illustrated in the drawings, is applicable to the control of combustion in steam boiler furnaces, and it is also applicable to the regulation of combustion in furnaces or other com' bustion devices where certain temperature conditions are to be maintained. In the one case theprimary or moti v'a'ting force of the control system is the steam pressure in the boiler, whereas, as in cases where temperature is to he the motivating factor, the system responds to tem perature rather than steam pressure. Whether the system is initiated by changes in steam pressure or by changes in temperature, the control system operates to control the supply of raw coal to a pulverizer mill in accordance with the demand for combustion and to so regulate the secondary air that the coal will be burned with efficiency While meeting the demands for heat;

In the following description the control system 'be described with particular reference to its application to" a steam boiler furnace.

'In Fig. 1 of the drawings is shown a boiler furnace I having a steam drum 2, a superheater 3 and a steam header 4 by means of which steam is delivered to' steamconsumirig apparatus such as turbines, etc. The furnace is provided with burners 5 to which pulverized coal is delivered by an exhauster 6 through a duct 7 to the burner 5. The exhauster 6 exhausts pulverized coal from a pulverizer 8 (see Fig. 2) and delivers it through the duct 7 to the burners 5.

Secondary air is supplied to the furnace burners by means of a forced draft fan 9 and a duct 10. The air delivered by the fan 9 is controlled by a damper 11in duct 10, the damper being actuated by a power cylinder 12} The" power cylinder 12 is automatically controlled by the system shown in Fig. 2. Power cylinder 12 may be of the type shown in Donaldson Patent No. 2,044,936,

granted June 23, 1936, and as illustrated in Fig. 7 hereof.

The constr'u ction of power cylinder 12 is such that the stroke of its piston for a given input signal pressure may be so controlled that the secondary air will be propor tional to the weight of coal delivered through duct 7 to the burners 5;

As schematically illustrated in Fig. l, a regulator 13 responds to the pressure of the steam generated by the' boiler. That pressure may be sensed at the boiler drum or at the steam header 4, as shown. The steam pressure in the header 4 is the initiating force of the control system and the control system is so arranged that combustion will be regulated to maintain the steam pressure substantially constant at the value desired.

Regulator 13 is provided with a pressure defiectable member 14 such as a bellows disposed in a housing 15. The bellows divides the housing 15 into pressure chambers 16 and 17. The pressure in header 4 is communicated to chamber 16 while chamber 17 is connected to the atmosphere. The bellows 14 actuates a beam 18 that extends through a pressure flexible seal such as a bellows 19. Beam 18 is connected to bellows 14 by a push rod 20. Beam 18 actuates an escapement valve 21 that is supplied with a pressure medium such as compressed air at a substantially constant pressure by a supply pipe 22. The valve delivers a variable pressure to a sending pipe 23 in accordance with pressure changes on bellows 14. The pressure in pipe 23 varies with the position of the beam 18 and that pressure is supplied to a balancing device 24 which includes a diaphragm 25. The pressure acts on diaphragm 25 which in turn, acts through a push rod 26 on the beam 18. Therefore, if the pressure acting on bellows 14 is increasing, indicating that the steam pressure is increasing, beam 18 tends to turn counterclockwise, thereby actuating the valve in a direction to increase the pressure in pipe 23. That pressure acting on diaphragm 25 exerts a balancing force on bellows 14. Therefore, for every value of steam pressure there will be definite value of pressure established in the sending line 23. The function of the pressure in line 23 as utilized in the system shown in Fig. 2, is to adjust the control point of a steam pressure flow regulator, as will be explained infra.

Fig. 2 illustrates schematically the pulverizer mill 8 and the control system associated therewith. Mill 8 may be what is known as a bowl mill, to which coal is supplied by a feeder 26. The coal is fed to the mill through a duct 27, the coal entering the mill through the bottom thereof. The feeder 26 is driven by a motor 28 preferably an adjustable speed, direct current motor. The speed of the motor is controlled by a field rheostat 29 which is adjusted or positioned by means of a power cylinder 30. The position of the power cylinder 30 is controlled by a steam pressure-air flow regulator 31. The regulator 31 responds to the sending pressure in line 23 of regulator 13. Increasing pressures in the steam header 4 above the regulated value to be maintained results in increasing pressures being delivered to the power cylinder 30 with a resulting decrease in speed of the feeder motor 28 and a reduction in the supply of coal to the pulverizer mill 8. More precisely stated, the pressures in sending line 23 of regulator 13 act on regulator 31 to establish the particular set or regulating point thereof that is de-' termined by the steam pressure. In other words, if the steam pressure is lower than it should be, the set point of the regulator 31 is increased so that more coal will be delivered by the feeder 26 to mill 8 until the amount of coal delivered corresponds to the particular control point established by regulator 13 on regulator 31. Conversely, if the steam pressure is higher than the desired value, regulator 13 lowers the control or set point of regulator 31 whereby the coal feeder motor 28 is decreased in speed until the feed of coal is reduced to the value required. Regulator 31 also responds to the pressure difference as measured between the outlet of the exhauster 6 and the pressure in the combustion chamber of the furnace as measured at a point at the same elevation as the burners. The pressure point or connection to the exhauster 6 may be designated the P1 connection and the pressure connection to the furnace at the burner level may be designated the P2 connection. Since the regulator 31 responds to the pressure in pipe 23 which is a function of the rate of steam flow, and to the difference between the pressures at connection points P1 and P2, the control pressure delivered to power cylinder 30 through line 33 will be a function of the steam pressure and the rate at which fuel is delivered to the burners of the furnace.

The coal as pulverized in mill 8, is carried upwardly by the air flow induced by the exhauster fan 6 into a classifier 34. The coal is conveyed from the classifier 34 through a duct 35 to the intake of the exhauster 6. The exhauster 6 and the pulverizer mill may be driven by a motor 37. In the duct 35 is an orifice 38. The pressure drop across that orifice is measured by a regulator 39 which, in turn, controls a damper operator 40. Damper operator 40 as shown, is connected to a damper 41 in the pipe 36 down stream of orifice 38. Regulator 39 operates to so position damper positioner 40 and damper 41 that the pressure drop across orifice 38 will be maintained substantially constant at a preset value.

As stated supra, the secondary air is supplied by fan 9 through duct 10. The secondary air is regulated by and in accordance with the rate at which pulverized coal is delivered through duct 7 to the furnace burners. In order to so regulate the rate of supply of secondary air a regulator 42 is provided. The regulator 42 responds to the pressure drop across the pressure connections P1 and P2 and sends out a control impulse that causes the power operator 12 to so position the damper 11 that the secondary air will be in the correct relationship to the supply of pulverized coal to the burner. The pressure drop P1P2 is a linear function of the weight of coal carried in suspension in the primary air flowing in duct 7. Regulator 42 is adjusted to zero output pressure when the primary air contains no coal. When so adjusted, the control impulse from regulator 42 will vary from a minimum gauge value, for example, Zero, to a maximum value corresponding to the value of the supply pressure when the primary air contains more or less coal as required.

In the operation of a pulverizing mill such as the one indicated in Fig. 2, hot or heated air may be supplied to the suction side of the mill for preheating the coal. As shown, the heated air enters at the bottom of the mill through a duct 44. A damper 45 which may be manually operated or adjusted for regulating the amount of heated air supplied, is provided. Damper 45 as shown, is operated manually by a bell crank 46 connected by a cable 47 operating over pulleys 48. The bell crank may be manually positioned by means of a screw 49 and handwheel 50. On the screw is a traveling nut 51 connected to the crank .46.

In order to further regulate the air supply to the mill and to temper the heated air, a counter-balanced damper 52 is provided. Damper 52 is mounted on a shaft 53 to which a lever arm 54 is connected and counter-weighted by a weight 55. The weight 55 is so adjusted that the damper 52 will take a position depending upon the suction on the mill. If the suction in the mill increases, damper 52 automatically swings open to allow more air to flow into the mill. If the suction decreases, the damper swings towards a more closed position.

The details of construction of regulator 31 are shown more particularly in Fig. 3. As there illustrated, the regulator includes a parted housing 57 at the part line of which the marginal edge of a diaphragm 58 is clamped. The diaphragm 58 divides the housing into pressure chambers 59 and 60. The diaphragm is provided with a push rod 61 having a knife edge 62 that bears against the upperside of a lever 63. The lever 63 actuates an escapement valve 64 having a valve body 65 in which is a valve 66 that is connected by a stem 67 to the lever' 63. The valve 66 has comically tapered ends that control the inlet port 68 and the exhaust port 69 in the valve body. Air pressure at a substantially constant pressure is supplied to the inlet 70 of the valve body. The valve body is provided with an outlet port 71 that is connected to sending line 33 leading to the power cylinder 30.

The regulator 31 is provided with a dash-pot 72 that stabilizes the regulator. The dash-pot includes a cylinder 73 which is open at its upper end and connected at its lower end to the interior of a bellows 74 mounted in a housing 75. The bellows works against a compression spring 76. The dash-pot includes a piston 77 that is connected by a spring 78 to the diaphragm 58. A sealing diaphragm 79 seals the opening through which the connection between the diaphragm and spring 78 extends. Similarly, there is a sealing diaphragm 80 that seals the opening through which the push rod 61 extends.

The cylinder 73 and bellows 74 contain fluid such as oil having a substantially constant viscosity. The oil level rises to a point above the piston 77. A bypass 78 connects the space above and below piston 77, and a needle valve 79 is provided in the by-pass to regulate the rate of interchange of oil between the space above the piston and that below it. The pressure delivered to the sending line 33 is communicated to the housing 75 which compresses the bellows in proportion to the pressure, the spring. 76 causing the bellows to take a definite deflection for each value of pressure. The regulator 31 regulates to a set point that is governed by the sending pressure developed in line 23 by regulator 13. The sending pressure is delivered to a chamber 82 of regulator 31, having therein a diaphragm 83. The diaphragm works against a compression spring 84. Motion of the diaphragm is communicated to lever 63 by a push rod 85, but that motion is resisted by a spring 85' that urges the lever to a position at which the output pressure in line 33' would be at a minimum value.

Chamber S9 of regulator 31 is connected to the pressure connection P1 in the exhauster 6 and chamber 60 is connected to the pressure connection P2 in the furnace chamber at a point located preferably at the same level as the burners. Therefore, the difference in pressure at the connections P1 and P2 will urge the diaphragm 58- downwardly. The pressure in chamber 82 acts to tilt lever 63 clockwise and in the same direction as it would be turned by an increase in pressure difierence acting on diaphragm 58. Therefore, for every loading pressure applied to chamber 82, the regulator 64 will establish such a pressure in sending line 33 that the rheostat 29 would be adjusted to a position in which the raw coal is delivered to the mill at the rate required by the steam pressure air flow relationship.

The stabilizing action of dash-pot 72 on regulator 31 is as follows: If an increase in pressure on diaphragm or bellows 74 results from deflection of diaphragm 58, that pressure acting on the piston through spring 78 will impose a loading, on diaphragm 58 that resists sudden deflections of the diaphragm in response to changes in the pressure acting in chambers 59 and 60. The resisting force is dissipated as oil flows through the needle valve 79. On decreasing pressures the reverse action takes place.

From the foregoing, it will be seen that regulator 31 so controls through regulator 30 and rheostat 29, the speed of operation of motor 28, that feeder 26 will deliver raw coal to the mill 8 at the rate required by the combined effects of steam pressure and the rate of delivery of pulverized coal to the furnace burners.

Regulator 42 is illustrated more in detail in Fig. 4. As there shown, the regulator comprises a parted housing 90 which clamps the marginal edge of a diaphragm 91 at the part line of the housing to provide opposed chambers 91a and 91b. The diaphragm operates a beam 92 through a push rod 93 and a knife edge 94 that acts downwardly on the beam. The beam is provided with a stationary fulcrum 95. The point where the push rod 93 passes out of the housing is sealed by a flexible dia phragm 96. The lever 92 actuates an escapement valve 64' which is similar in construction and operation to valve 64 shown and described in connection with Fig. 3. The inlet of valve 64' is supplied with compressed air at substantially constant pressure and it delivers a variable pressure to a sending line 92. The sending line 92' is connected to the pilot valve operator 94 of power cylinder 12.. Pressure chamber 91a is connected to the pressure connection P1 of the exhauster 6 and chamber 915 to the furnace pressure connection P2. Thus the diaphragm 91 responds to the difference between the t t 6 pressures at the connections P1 and P2; That pressure difierence is a direct measure of the amountofcoal carried through the duct 7 to the burners.

Power cylinder 12 and the pilot valve operator 94'may actuated by a diaphragm 97d to which the signal pressure is supplied by pipe 92 of regulator 42. The pressure works against a spring 97e carried in a socket 977. Socket 971 is carriedby one arm of a bell crank 97g, the other arm of which has a follower roller'97h' that rides on a cam or compensating bar 97i. Bar 97i is' secured 'to'a' frame 97 attached to the piston rod 97k of the power cylinder 12. As the piston rod 97k moves either upwardly or downwardly, the bar 971 so adjusts the tension in spring 97e that the pilot valve is returned to neutral when the piston has moved a certain distance in either direction in response to a particular pressure in pipe 92'. 'By properly shaping the bar 971', the throw of'crank 97g can beincreased or decreased at different portions of the stroke of power cylinder piston so that the damper 111will be'posi tioned to cause the secondary air to be proportional to the pressured difference P1P2. as measured by regulator 42. Consequently, the rate of flow of secondary air-will be proportional to the weight of coal carried by the primary air in duct 7 to the burners. i In order that the regulator 42 will have a definite send-'- ing pressure in line 92' for every value of pressure diflerence on diaphragm 91, the regulator is provided with a diaphragm housing 98 having therein a diaphragm 99 that acts through a push rod 100 on the beam 92, and exerts a force opposing the force of diaphragm 91 on the beam. As shown, thediaphragm 99 acts against a' compression spring 101 so that the diaphragm'will take aj definite deflection for every value of sending pressure in line 92'. Since regulator 42 develops a sending pressure in line 92' that is proportional to the rate of flow of coalto the burners, the secondary air supplied by forced draft fan 9 through duct 10 to the furnace will be proportionah to the rate at which coal is delivered to theburners.

Regulator 42 may be provided with an adjustable ten 7 sion spring 104 whereby the zero'of the regulator can be suppressed more or less as required. That is to say, that'- the spring can be so adjusted that the output pressure of valve 64 will be zero gauge when there is nocoal'in theprimary air flowing through duct 7 to the burners. When so adjusted, the output pressure of valve 64' will increase from zero gauge to a maximum value as the amount of 7 coal carried in suspension in duct 7 increases from zero to full capacity. As the pressure difference P1-P2 is lin-- early proportional to the coal weight suspended in the primary air, the regulator 42 will transmit a pressure out put that varies from zero gauge to the maximum of its range on the basis of the presence of coal in the primary air and the density of the coal in that air.

The regulator 39 of the control system is illustrated schematically in Fig. 5. The regulator comprises atwopart housing 106 having therein a diaphragm 107 whose marginal edge is clamped between the two. parts of the.

housing at the part line thereof. The diaphragm 107 thus provides

pressure chambers

108 and 109. Pressurechamber 108 is connected by a pipe 110 to the up stream. side of orifice 38 in duct 35, while chamber 109 is con.-. nected by a pipe 111 to the down stream side of that The diaphragm is connected by a push rod 112 and a.

knife edge 113 to a beam 114 that is mounted at one end sure through a line 116 to the power operator 40 that adjusts the position of damper 41 so as to maintain a substantially constant pressure drop across the orifice 38.

Beam 114 is connected to an adjustable tension spring 118 so that the dead weight of the diaphragm, the strut or push rod 112, and the knife edge 113 may be balanced, thereby causing the pressure acting on the diaphragm 107 to effect a motion of beam 114 that is proportional to the pressure.

Since regulator 39 responds to changes in the pressure difference across orifice 38 and causes the power operator 40 to adjust damper 41 in accordance with those changes, the pressure drop across the orifice 38 will be maintained at a substantially constant value. The tension spring 118 may also be used to establish the set point of the regulator 39, the set point being the pressure difference to be maintained across the orifice 38. V

In case necessity requires, the control system may be operated manually by means of a hand sender 120 by means of which the pressure in line 33 leading to the regulator 30 may be manually adjusted and a hand sender 121 by means of which the pressure delivered to the power operator 40 may be adjusted manually. Where the control is to be effected manually, valves 122 and 123 are provided for disconnecting the sending line 33 leading from regulator 31 to the valve 122 and shutting off the pressure leading from the escapement valve of regulator 39 to the operator 40. t

The power operator 40 of Fig. 2 is illustrated more or less schematically in Fig. 6, and comprises a two-part housing 125 having a diaphragm 126 therein, the marginal edges of which are clamped between the housing parts at the part line thereof. The diaphragm is urged downwardly by an adjustable tension spring 127. Pressure delivered by regulator 30 to the operator 40 is delivered into chamber 128. That pressure acts upwardly on the diaphragm 126. Diaphragm 126 is connected by a. push rod 129 to a lever 130 which is pivoted at one end as at 131. The opposite end of the lever is connected by a link 132 to a lever arm 133 connected to the shaft 134 of damper 41. For every value of pressure delivered to chamber 128, lever 130 will take a definite position and therefore actuate damper 41 to a definite position. That position will depend upon the deviation of the pressure drop across orifice 38 from the preselected value.

From the foregoing description it will be seen that a regulating system is provided whereby raw coal is fed to a pulverizer mill at such a rate that the steam pressure in the boiler is maintained substantially constant. It will also be seen that the system includes a regulator for automatically adjusting the secondary air in accordance with the total amount of coal delivered to the burners.

Since pulverized coal carried in a stream of air will cause a pressure drop between the pressure connections P1 and P2 that is directly proportional to the weight of coal flowing through the duct 7 per unit of time, the provision for maintaining the constant pressure drop across the orifice 38 will cause the pressure drop between the pressure connections P1 and P2 to be a true and accurate measure of the total weight of coal delivered to the burners per unit of time.

The foregoing description has dealt with the system as applicable to a steam boiler. If the system is to be used for regulating combustion to maintain a given temperature in a given combustion space or the like, a thermostat can be substituted for regulator 13. Such a thermostat would be equipped with an escapement valve for delivering a signal to regulator 31 to adjust the set point of that regulator in accordance with the departure of the regulated temperature from the predetermined value. If the temperature is above the regulated value the set point of regulator 31 will be modified by the pressure loading rom the thermostat to decrease the combustion rate if the temperature is below the regulated value, the regulator 31 will be adjusted by the thermostat impulse to a set point that will require a higher rate of combus. tion until the temperature condition is satisfiedby the combustion rate. A thermostat suitable for substitution of the regulator 13 may be of the type shown ingFig. 5 of A. A. Markson Patent No. 2,497,783, granted February 14, 1950. The thermostat shown in Fig.5 of that patent has an escapement valve with an outlet port 1'7. through which pressures of varying magnitude are transmitted, depending upon the temperature to which the thermostat responds. That pressure would bev communicated to the diaphragm 83 of regulator 31. j

Having thus described the invention, it will he appar-' ent to those of ordinary skill in the art to which the invention pertains, that modifications and changes may be made in the illustrated embodiments without departing from either the spirit or the scope of the invention. Therefore, what is claimed as new and desired to be secured by Letters Patent is:

l. The combination with a steam boiler furnace having powdered coal burners provided with means for sup plying secondary air thereto, a pulverizer mill, a coal feeder for the mill, a motor for driving the coal feeder, the motor having means for adjusting the speed thereof, said powdered coal mill having a classifier, an exhauster and a duct from the classifier to the exhauster, and a discharge duct for conveying air and coal suspended in said air from the exhauster to the burners, the duct from the classifier to the exhauster being provided with an orifice, of a control system therefor comprising a first regulator responsive to the pressure drop across the orifice for developing a control force, the magnitude of which varies by and in accordance with said pressure drop, damper means in the duct from the classifier to the exhauster, an operator controlled by the control force of the first regulator for so adjusting the damper means as to maintain the pressure drop across the said orifice substantially constant, a second regulator responsive to the boiler steam pressure and to the pressure drop from the exhauster to the burners for developing a control force that varies by and in accordance with the combined effect of said steam pressure and pressure drop, an operator responsive to the control force of the second regulator for actuating the speed adjusting means for the coal feed motor, whereby raw coal is delivered to the mill in accordance with the response of the second regulator to the steam pressure and the coal laden air flow pressure drop, and a third regulator responsive to the rate of flow of coal laden air through the duct leading from the exhauster to the burners for controlling the secondary air supply means whereby the supply of coal to the burners and the supply of secondary air are caused to bear a predetermined relationship to the boiler steam pressure.

2. The combination with a boiler furnace provided with powdered coal burners and secondary air supply means therefor, a coal pulverizer, a coal feeder for the mill, an adjustable speed motor for driving the same, said motor being provided with means for adjusting the speed thereof, an exhauster for conveying powdered coal from the mill to the furnace burners and means between the mill and the exhauster for developing a pressure drop that varies in accordance with the fiow of air and coal to the exhauster and regulator means for adjusting the secondary air supply of a control system therefor comprising a first regulator responsive to the boiler steam pressure for developing a control impulse whose magnitude varies by and in accordance with variations in the steam pressure, a second regulator responsive to the rate at which coal by weight as suspended in the carrier air from said exhauster, is delivered to the furnace, said second regulator having means responsive to the control impulse of the first regulator and to the rate of coal delivery to the burners, whereby said second regulator develops a control force that is proportional to the boiler steam pressure control force and to the coal weight feed rate for controlling the speed of the coal feeder motor,

a third regulator responsive to the flow of air and the weight of coal suspended therein as delivered to the furnace burners for developing a control force for so adjusting the secondary air supply regulating means that the rate of flow of secondary air to the burners is by and in accordance with said air and the coal flow to the burners, means for developing a pressure drop between the coal pulverizer and the exhauster that varies by and in accordance with the air flow from the mill to the exhauster, and means responsive to said pressuredrop for so regulating said flow to the exhauster that the pressure drop is maintained at a substantially constant predetermined value.

3. A control system in accordance with claim 1 in which the third regulator is provided with means whereby the control impulse developed thereby may be adjusted relative to the control impulse developed by said second regulator.

4. A control system in accordance with claim 2 characterized by the fact that means are provided in the third regulator for adjusting the control impulse thereof relative to the control impulse developed by the second regulator that controls the speed of the coal feeder motor.

5. The combination with powdered coal burners provided with a source of supply of secondary air and means for regulating the supply of secondary air, a pulverizer mill for supplying coal to the burners, the mill being provided with a classifier, an exhauster, a first duct leading from the classifier to the exhauster, and having an orifice in the same, and a second duct leading from the exhauster to the burners, of a control system for the same comprising a regulator responsive to the pressure drop across said orifice for developing a control force the magnitude of which varies by and in accordance with said pressure drop, means in the first duct for adjusting the flow of coal laden air therethrough, and means operated by the control force of said regulator for so actuating said flow adjusting means that the pressure drop across said orifice is maintained substantially constant, and means responsive to the pressure drop through the second duct for developing a control force whose magnitude varies by and in accordance with the weight of coal carried by air therethrough, the control force of said pressure drop responsive means actuating the secondary air supply regulating means.

6. A control system according to claim 5 in which the means responsive to the weight of coal flowing to the burners responds to the pressure drop as measured from the outlet of the exhauster to the burners.

7. The combination with a powdered coal combustion device for burning such coal in accordance with a condition to be maintained, the combustion device being provided with a pulverizer mill, means for delivering raw coal to the mill at adjustable rates, said mill having a classifier, an exhauster and a duct leading from the classifier to the exhauster, an orifice in said classifier duct, a second duct from the exhauster to the burners, and means for supplying secondary air to the burners of a control system therefor comprising means responsive to the condition to be controlled for developing a control force whose magnitude varies by and in accordance with changes in said condition, means responsive to the flow of air and coal from the exhauster to the burners and to said condition responsive control force for adjusting the coal feed means to increase coal feed as the condition responsive control force decreases below a predetermined value and to decrease the same as the condition responsive control force increases above said value, means responsive to the pressure drop across said classifier duct orifice for maintaining said drop substantially constant, and means responsive to the rate of flow of air containing coal in suspension, to the burners for adjusting the secondary air supplying means by and in accordance with the weight of coal delivered in the exhauster air to the burners.

8. The combination with powdered coal burners provided with a pulverizer mill, means for supplying raw coal to the mill at regulatable rates, an exhauster, a classifier duct connecting the mill classifier to the intake of the eXhauster through which powdered coal is withdrawn from the mill with air constituting primary air for the burners, a duct leading from the exhauster to the burners, an orifice in the classifier duct and means for supplying regulatable amounts of secondary air to the burners, of a control system therefor comprising means responsive to the pressure drop across the orifice in the classifier duct for developing a control force whose magnitude varies by and in accordance with said drop, damper means in said duct and an operator therefor having means responsive to said control force for causing the same to so actuate the damper means that the flow through the classifier duct is controlled to maintain a substantially constant pressure drop across said orifice, and means responsive to a condition to be controlled and to the pressure drop from the exhauster outlet to said burners for developing a control force whose magnitude varies by and in accordance with the same, and means responsive to said control force for s0 regulating the coal feeding means as to maintain said condition at a preselected value, whereby changes in the feed of coal to the mill are effected substantially instantaneously with changes in weight of coal carried to the burners.

9. A system as in claim 8 having a regulating means substantially instantaneously responsive to changes in the pressure drop from the burners to the exhauster as affected by changes in the rate at which coal is carried to the burners by the primary air, for developing a control force whose magnitude varies by and in accordance with the same regulating the secondary air supply means in accordance with the rate at which coal is delivered to the burners.

References Cited in the file of this patent UNITED STATES PATENTS