US2000270A - Combustion control - Google Patents

Description

May 7, 1935. v. ANDREWS ET AL 1 3 GOMBUSTION CONTROL Filed Nov. o, s 2 Sheets-Sheet 1 1/5 e ai" s o //4 r n Enm ANDREWS CHARLES M. TERRY ATTRNEY May 7, 1935. v. ANDREWS ET AL COMBUS TI ON CONTROL Filed Nov. 10, 1931 2 Sheets-Sheet 2 ATTORNEY A i l ,I /i

VENTORS y w w .m/ W W 0/ a m m 7 Patented May 7, 1935 UNITED STATES" COMBUSTION CONTROL L. V. Andrews, Worcester, Mass., and Charles M. Terry, Decatur, Ill., assignors to A. W. Cash Company, Decatur, Ill., a corporation of Dela- Ware Application 'November 10, 1931, Serial No. 574,156

16 Claims.

This invention relates to combustion control, and more particularly to a method and apparatus for controlling the combustion of pulverized fuel.

In burning pulverized fuel by the direct fired system, it is the practice to reduce the fuel to a desired degree of fineness in a pulverizing ap'- paratus and deliver the finely divided uel by means of a current of so-called primary air directly from the pulverizer to a furnace. The additional or secondary air required to complete the combustion is usually supplied at the furnace. The rate of combustion is controlled by varying both the supply of secondary air delivered to the furnace and the supply of coarse fuel delivered to the pulverizer. It has already been proposed to control both of these variables automatically in accordance with the demand on the furnace.

These automatic control systems as heretofore arranged have not, however, proven entirely Satisfactory. Certain dificulties have been encountered in operation, particularly when the demand for heat is unsteady and subject to .rapid fluctuations. It is found that the efliciency of combustion under these conditions is considerably less than that theoretically obtainable, and the ratio of the fuel and air supplied to thefurnace is at times too high and at other times too low. This 'results' in a waste of fuel and the occasional production of smoke.

It is accordingly the main object of the present invention to overcome these dimculties and to provide a method and apparatus whereby a pulverized fuel burning system may be controlled to provide eflicient combustion at all times, re-

gardless of frequent and rapid changes in thede- 35 mand for heat.

It is a further object of the invention to prcvide a method and apparatus which will ensure the delivery of the correct proportions of fuel and air to the furnace at all combustion rates and particularly during periods of change in demand.

It is a further object of the invention tq provide a combustion control apparatus which may be easily and simply adjusted to compensate for variations in Operating conditions encountered at different furnace installations.

With these and other objects in View, as will be apparent to those skilled in the art, the invention resides in the combination of parts and the steps of the process set forth in the specification and covered by the claims appended hereto.

Referring to the drawings illustrating one embodiment of the invention, and in which like ref-l erence numerals indicate like parts,

65 Fig. 1 is an elevation of an automatically controlled direct fired pulverized fuel burning installation;

Fig. 2 is an enlarged view, partly in section, showing the Construction of the master regulator;

Fig. 3 is a View similar to Fig. 2, showing the construction of the secondary regulator;

Fig. 4 is an enlarged cross section of the fourway pilot valve which controls the movements of the master regulator pisten; and

Fig. 5 is an enlarged cross section of' one-of the special valves provided on the secondary regulator.

In the embodiment of the invention illustrated in the drawings, we have shown a steam generating furnace arranged to burn pulverized fuel, comprising a combustion chamber o and a boiler ll, the boiler being connected by a pipe |2 to a main steam header l 4, which may lead to a steam turbine or other apparatus arranged to use the steam. In order to supply pulverized fuel for combustion in the furnace, weprovide a unit type pulverizer !5 connected by a pipe |6 with a suitable burner !8 mounted on the furnace wall. The pulverizer may be, of any Construction which will pulverize the fuel to a sufiicient degree of fineness and deliver it in a current of air to the furnace.

secondary air is supplied to the burner IB through a branch duct [9 leading from a main duct 20, which is shown as located in the basement below the boiler room floor. A forced draft fan 22 is arranged to discharge air into the main duct. Obviously, the main duct may be extended if desired to supply air to a plurality of furnaces,

but for Simplicity of illustration one furnace only 3 trolled rate, and the finely pulverized fuel passes to the furnace ID, where it meets the secondary air supplied by thefan 22. combustion of the fuel takes place in the furnace, and the gaseous products of combustion pass through the boiler I l,

thereby generating steam, and are discharged which steam is being generated, the steam pressure will increase. It is ordinarily desirable to maintain the steam pressure substantially uniform, and this may be accomplished by controlling the supply of fuel and air in accordance with the demand for heat. In order to obtain this result, it is preferable to utilize automatically actuated regulating apparatus, thereby climinating the necessity for constant attention on the part of the operators in charge and at the same time increasing the accuracy of the regulation.

As an indication of the demand for heat, we preferably use the steam pressure, 'and permit a slight Variation in the pressure for purposes 'of regulation. For example, we may provide for a pressure of 400 pounds per square inch at the lowest demand or rating, and allow this pressure to drop to 390 pounds at the highest rating, thereby establishing an Operating range of 10 pounds. In order to make use of this pressure change for regulation, we provide a suitable master regulator 30, which is connected by a small pipe 3! to the steam header !4 and which is preferably of the compensating type having a hydraulic piston arranged to move in accordance with the steam pressure variations. In the example mentioned, the regulator would be adjusted to make a full stroke throughout the lo-pound Operating range, taking one extreme position at 400 pounds, the other extreme position at 390 pounds, and an intermediate position corresponding to each intermediate pressure. With such a Construction the regulator floats with the pressure. A slight increase in the demand is accompanied by a cor- 'respondingly slight increase in the rate of combustion, and sudden extreme fluctuations in fuel feed or air flow with the resulting inefiiciency are avoided.

A number of devices suitable for use as the master regulator 36 are available commercially, one particularly satisfactory Construction being disclosed in the patent to Paul C. Temple, No. 1,890,4"72, granted December 13, 1932. such an apparatus is illustrated in Fig. 2, the steam pressure is transmitted through the pipe 3! to the lower surface of the flexible pressure responsive diaphragm 32 of the regulator. A knife edge 34 mounted on the upper surface of the diaphragm supports a horizontal lever arm or beam 35 which is pivoted-on a second knife edge 36 mounted in the regulator rame. The pressure of the steam on the diaphragm is balanced by weights 38 and 39 carried by the beam 35, and this beam will assume a definite position for each steam pressure within the Operating range. This, it will be understood, is due to' the well known fact that the effective area of a flexible diaphragm decreases as it moves in response to' a pressure increase, the result of the decreased area being to arrest the movement of the beam.

The movements of the beam 35 in response to steam pressure changes are utilized to control the movements of a piston 40 slidably mounted in a stationary vertical cylinder 42. A piston rod 43 joined to the piston extends upwardly through a stufing box 44 in the upper end of the cylinder, and is connected to the regulatable feeding device 21 of the pulverizer l5 by means of a link 45, a bell crank 46 and a rod 41. These parts are so proportioned and arranged that downward movement of the piston will decrease the feed of fuel, and a complete stroke of the piston will move the fuel feeder throughout the complete range of fuel burning rates desired for the particular installa- The piston 40 is actuated by fluid pressure under the control of a pilot valve 48 connected to the beam 35. This valve is preferably of the well known balanced piston type and is mounted in a valve casing 50. The stem 5l of the valve extends upwardly through a stufing box 52 in the casing and is connected to a horizontal lever 54 which is joined to the beam 35 by a vertical link 55. The lever 54 is supported intermediate its length'on a fulcrum 56 formed on the upper end of a vertical link 51. These parts are so arranged that whenever the beam 22 is raised by an increase in steam pressure, the pilot valve will move downwardly, and vice versa.

The valve casing 50 is divided internally by means of partitions into four separate chambers. The central chamber 59 is supplied with fluid under pressure through a pipe 60. This Operating fluid' may be air, water, oil or any other suitable gas or liquid as desired. The exhaust chamber Bl extends to both ends of the casing and is connected to an exhaust pipe 63, which is intended to carry away the spent Operating fluid, the disposal of which depends upon its character. If compressed air is being used, the pipe 63 may discharge to atmosphere. If Water is being used, the discharge pipe will lead to a drain. If oil is being used, it is returned to a reservoir. Above the central inlet chamber 59, and between the inlet chamber and the exhaust chamber, we provide a chamber 64 which is connected by a pipe 65 to the lower end of the cylinder 42. The upper end of the cylinder is connected by a pipe 61 to a chamber 68 directly beneath the inlet chamber 59. For convenience in manufacture we preferably provide a hollow cylindrical sleeve 69 in which the valve 48 is slidable and which extends vertically through the chambers 64, 59 and 68. Ports 10 in the wall of the sleeve provide passages leading between each of these chambers and the interior of the sleeve, and the ends of the sleeve open into the exhaust chamber Bl.

The pilot valve 48 and its associated parts are so arranged that whenever the valve is in the neutral position shown in Fig. 4, both ends of cylinder 42 are cut off from communication with the inlet chamber 59 and the exhaust chamber 6 I, and as a, result the piston 40 is held stationary. If the valve is moved downwa-dly from its neutral position, chamber 68 and the upper end of the cylinder are connected to the inlet chamber 59, while at the same time chamber 64 and the lower end of the cylinder are connected to the exhaust chamber 61. This will admit fluid above the piston 40 and cause it to move downwardly. In a similar manner, if the valve 4| is moved upwardly from its neutral position, fluid will be admitted beneath the piston 40, the upper end of the cylinder will be connected to the exhaust, and the piston will move upwardly.

With the Construction as so far described the piston will travel all the way to the end of its stroke after it has once been started. Ths of course is not desirable, and We therefore provide a compensating mechanisn which serves to return the pilot valve to its neutral position and thus stop the piston after a' predetermined movement, the extent of which bears a definite relationship to the change of pressure in the diaphragm chamber.

The compensating mechanism illustrated comprises a pair of relatively slidable members '12 and 13. drical rod having its upper end pivoted to the piston rod 43 by means of a pin 14. This rod extends downwardly at a considerable angle with respect to the piston rod. The member 13 is The member 12 is shown as a cylinv through the full length of its stroke.

pivoted to the frame by means of a pin 15, and is 'shaped to slidably. receive the rod 12. The member 13 has formed integral therewith a horizontally projecting slotted extension or arm 16, which is connected by means of a link 11 to one end of a lever 18. The efiective length of the extension 16 may be varied by adjusting the lower end of the link 11 along the slot. The lever 18- is pivoted to the frame by means of a pin 19 intermediate of its length, and the end opposite the link 11 is pivoted to the lower end of link 51. In this way the compensating mechanism is ar- -ranged to coact with the connection between the weighted beam and the pilot valve.

It will now be clear that if the pressure in the control line 3! drops slightly, because of increased demand for steam, then the beam 35 will'be lowered a distance proportional to the pressure drop, and will come to rest in its new position because of the change in effective area of the diaphragm. As beam 35 moves downwardly, it will act through link 55 to rock lever 54 about pin 56 as a fulcrum, thus raising valve 48. This will admit fluid to the lower end of the cylinder 42 through pipe 65, causing piston 40 to move upwardly and increasing the fuel feed to take care of the increased demand. As the piston rod moves upwardly it will carry with it the slide rod 12, which will slide through the member 13 while at the same time turning it about its pivot 15 in a clockwise direction because of the change in the angular position of the slide rod. This movement of the member 12 will be transmitted through link 11, lever 18, and link 51, thus rocking lever 54 about the upper end of link 55 as a fulcrum, lowering pilot Valve 48 to its neutral position and stopping piston 40 in a new position.

By adjusting the lower end of link 'IT along the slot in arm 16 it is possible to alter the operating range of the regulator, that is, the amount of pressure change required to move the piston 40 An increase in the efiective length of arm 16 will increase the Operating range, and Vice versa. Where the demand for steam fluctuates greatly, an increased Operating range is desirable as tending toward smoother operation.

It is necessary to control not only the rate of fuel feed to the pulverizer !5, but also the rate of secondary air flow to the burner s. For this purpose we utilize an adjustable damper 8| located in the branch duet 19 and provided with an external Operating arm 82. In order that each position of the damper Bl may result in a predetermined air flow in the duct !9, we preferably provide means for automatically controlling the pressure in the main duct 20 anterior to the damper and the pressure in the furnace chamber !0 posterior to the damper. In the embodiment illustrated, the pressure in the main duct 20 is transmitted through a pipe 83 to a regulator 0 which is connected by a cable 85 to a damper 88 located at the discharge of the fan 22. The regulator 8& controls the position of the damper 86 and maintains the pressure in the main duct 20 constant at a value sufiicient to allow operation at the highest rating desired. The pressure in the urnace chamber !0 is transmitted through a pipe 88 to a regulator 89, which is connected by a cable to a damper Bi located in the furnace uptake 28. The regulator 89 controls the discharge of the gaseous products of combustion and maintains a predetermined pressure in the furnace, preferably slightly below atmospheric pressure. These regulators 84 and 89 may be of various conobtainable.

structions. one suitable apparatus being disclosed in the prior patent to Terry No. 1,596,031.-

We have discovered that if the damper 8! is arranged to be operated directly by a master regulator 30, the efliciency of combustion under avarying load is not as high as that theoretically From inspection of the combustion flame, there appear to be periods during which the air flow is excessive, and other periods during which the air flow is deficient. We have furthermore discovered that this phenomenon is caused by a lag in the response of the pulverizer to a change in demand for fuel at the burner. For example, if the fuel feeder 21 and the damper 8! are both moved at the same time to a new position corresponding to a higher steam demand, the air flow in duct !9 will increase instantly to the new value, but the fuel flow at the burner will increase much more slowly, and in some cases as much as 30 seconds will elapse before the fuel flow into the furnace will reach the value corresponding to the new feeder position. A similar lag in the change of fuel flow will occur when the feeder is adjusted to decrease the rate of feed, but in some cases the time lag under these circumstances has been found to be less than when the feed is increased. We believe that this slowness in response is caused by the fact that during operation an appreciable quantity of fuel is present in the pulverizer in the process of being pulverized, and that this quantity increases as the rate of pulverization increases. Hence, when the feed of coarse fuel is increased, a portion of the increased incoming coarse fuel must be used to build up the quantity of fuel in the pulverizer, and the entire increased flow is not at once available at the burner. v similarly, a decrease in the fuel feed does not produce an immediate corresponding decrease in flow at the burner, since a portion of the fuel in the pulverizer must first be discharged. Apparently certain types of pulverizers are able to clear themselves more quickly *than they can load up, and hence will follow a decreasing demand more closely than an increasing demand. A similar time lag is encountered in the so-called storage system of burning pulverized fuel, though usually to a considerably less extent;

Pulverized fuel is burned in suspension in the furnace chamber, and each particle of fuel is` present within the chamber for a very' few seconds only. It is therefore essential that the correct amount of air 'to burn the fuel should enter the urnace at exactly the same time as the fuel enters. It is not enough that the average rato of fuel and air flows should be maintained correct, but it is necessary that the ratio should be correct at every instant during operation.

We have found that we can obtain the desired result by causing all changes in the rate of air flow to be made more slowly than the corresponding changes in the adjustment of the fuel feeder. This is preferably accomplished by initiating a change in the position of the damper BI substantially simultaneously with the start of each change in the adjustment of the feeder 21-, and moving the damper more slowly than the feeder, so that the damper will reach its final position after each change in demand at substantially the same instant as the rate of fuel flow at the burner i& completes its change in value. In this way the desired fuel and air ratio can be maintained at all times.

The preferred form of apparatus for carrying out this method comprises a secondary regulator p end of a horizontal lever !25.

93 controlled by the master regulator 30 and connected to the air damper 8 This secondary regulator comprises a vertical cylinder 94 within which a piston is slidable. A piston rod 9'! extends upwardly from the piston and through a stufiing box 98 in the upper portion of the cylinder. The piston rod is connected by a link 99, bell crank !00, and rod !0! to the arm 82 of the burner air damper 8!, these parts being so proportioned and arranged that downward movement of the piston will close the damper, and a complete stroke of the piston will move the damper suia ciently to cover the entire air flow range desired.

The movements of the piston 95 are controlled by a pilot valve mounted in a valve casing !02, these parts, preferably being identical with the corresponding parts of the master regulator 30. An inlet pipe !03 and an exhaust pipe !04 are connected to the valve casing !02. Pipes !05 and !06 connect the valve casing with the lower and upper ends respectively of the cylinder 94. The valve stem !01 is actuated by a horizontal lever !08 which is connected by a vertical link !09 to an arm !0 pivoted to the regulator frame and formlng a control member. The lever !08 is pivoted intermediate its length to the upper end of a vertical link !!2.

The arm !!0 isacontrolled by the master regulator 30, and for this purpose we have illustrated a. vertical rod !!3, a lever !!4, and a link !!5. The master regulator is shown as provided for convenience with a piston rod extending downwardly from piston 40 and through a stuing box !8 at the lower end of cylinder 42, the lower end of the piston rod being connected to the link !!5. The master regulator thus serves to move the arm 0 in a manner corresponding to the movements of the piston 40.

The secondary regulator 93 is shown as provided with a compensating mechanism indentical with that of the master regulator. This comprises a rod !20 pivoted at its upper end to the piston rod 9'! and slidable through a member !2! pivoted to the regulator frame. The member 12! has a horizontally extending slotted arm !22 which is connected by a vertical link !24 to one The lever !25 is pivoted' intermediate its length to the regulator frame, and is connected at one end to the link !21 It will-now be seen that movements of the 'master regulator will move arm !0 and open the pilot valve, thus adm'tting fluid to the cylinder 94 and causing movement of the piston 95 and the air damper 8! actuated thereby. The compensating mechanism will return the pilot Valve to its neutral position and arrest the piston in a definite predetermined position dependent upon the extent of the master regulator movement.

i In order thatthe movements of the air damper 8! may be slower than the movements of the fuel feeder 21, whereby we way overcome the difficulties caused by the lag in pulverize' response as heretofore explained, we preferably provide means to throttle or restrict the rate of flow of fluid to or from the cylinder 94. This serves to 'retard the piston 95 without afiecting its final position after a change in demand. Since the lag may be different in opposite directions, we preferably provide separate adjustable throttling means to control the fluid flow to each end of the cylinder. Obviously, it is immaterial whether the speed of the piston is retarded by restricting the flow into the cylinder or the flow out of the cylinder, and either arrangement will, it is believed, provide the dcsired operation.

'! 28 comprises a hollow casing The preferred construction illustrated comprises a valve !28 connecting the pipe !05 with the pilot valve casing !02, and a second valve !29 connecting the pipe !06 with the pilot valve casing. The valve !28 is shown in detail in Fig. 5, and the valve 29 may be and preferably is an exact duplicate thereof. Referring now to Fig. 5, the valve I3!` forming a passage for the fioW of fluid thereth'ough. A partition wall !32 extends across the interior of the casing, and an opening !33 is provided in this partition, the flow through the opening being controlled by an adjustable needle valve member !34 screw threaded in the casing. A stuing box !35 prevents external leakage past the valve member.

It will be clear that when one of these valves is conducting fluid to one end of the cylinder, the other valve is conducting fluid away from the other end of the cylinder. In order that the adjustment of each valve may affect the piston speed in one direction only, we provide a check-valve controlled by-pass around each valve. such a by-pass is shown incorporated in the valve casing itself, and comprises a second opening !31 in the partition' !32, controlled by a ball check valve !38 held in place by a light compression spring !39. A pipe nipple !40 serves to hold these parts in the proper relationship, as well as to connect the Valve casing !3! to the pilot valve casing !02. The ball check !38 allows fluid to flow freely in a direction away from the cylinder 94, but fluid flowing toward the cylinder must pass through the opening !33 controlled by the needle valve. Hence each needle valve may be adjusted to give an independent control over the speed of the piston movement in opposite directions. Valve !28 controls the upward piston speed, and valve !29 Controls the downward piston speed.

The operation of the invention will now be apparent from the above disclosure. Whenever a change occurs in the demand for heat, the piston of the master regulator 30 will assume a new position corresponding to the new demand, and will at the same time adjust both the fuel feeder 2'! and the secondary regulator control arm !!0. The secondary regulator piston 95 will immediately begin to move and adjust the air damper 8! to change the rate of air flow. The needle valves !28 and !29 are so adjusted, preferably after installation, that they retard the movements of the piston 95, and allow the damper 8! to reach its final position of adjustment at the same instant that the change in fuel feed has become fully effective at the burner !8. If there is a difference between the rate at which the pulverizer !5 will clear itself and the rate at which it will pick up a load, the valves !28 and !29 can be readily adjusted to take care of these conditions. It is merely necessary to throttle one valve more than the other. The adjustment of the piston speed in one direction has no effect on the speed in the other direction, since the check valves !38 allow free dischargepf the fluid from that end of the cylinder 94 toward which the regulator piston 95 is moving.

Our invention makes it possible tu obtain a desired predetermined ratio between the rates of flow of the fuel and aii` into the urnace at all ratings and particularly during periods of change in the demand for heat. variations in the performance characteristics of difierent pulverizers are overcome and if a change in the character of the fuel alters the 'time lag in the pulverizer, it is a simple matter to Difficulties cau'sed by F conditions. The apparatus disclosed is of simple and inexpensive Construction, and will result in a marked increase in combustion eiflciency and a consequent saving in fuel.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. The method of controlling a pulverized fuel burning system comprising the steps of varying the feed of fuel in accordance with the demand for heat, initiating a change in air flow substan- 'tially simultaneously with the start of each change in fuel feed, and making all air flow changes more slowly than the corresponding changes in fuel feed.

2. The method of controlling a pulverized fuel burning system having a fuel feeder and an air damper, comprising the steps of adjusting the fuel feeder in accordance with the demand for heat, initiating an adjustment of the air damper substantially simultaneously with the start of each fuel feeder adjustment, and making all air damper adjustments more slowly than the corresponding fuel feeder adjustments.

3. The method of controlling a direct fired pulverized fuel burning system comprising the steps of controlling the feed of coarse fuel to the pulverizer in accordance with th`e demand for heat, initiating an adjustment of the air flow to the furnace substantially simultaneously with the start of' each change in fuel feed, and making all air flow changes more slowly than the corresponding changes in fuel feed.

'4. The method of controlling a direct fired pulverized fuel burning steam generating system comprising the steps of controlling the feed of coarse fuel to the pulverizer in accordance with the demand for steam, initiating an adjustment of the air flow to the furnace substantially simultaneously with the start of each change in fuel feed, and making all air flow changes more slowly than the corresponding changes in fuel feed.

5. The method of controlling a direct fired pulverized fuel burning systemcomprising the pulverizer in accordance with the demand for heat, initiating an adjustment of the air flow to the furnace substantially simultaneously with the start of each change in fuel feed, and making all air flow changes so slowly in comparison with the corresponding fuel feed changes that the air flow reaches its final changed Value as substantially the same instant as the change in fuel feed becomes fully effective at the furnace.

6. The method of controlling a direct fired pulverized fuel burning system having a forced draft damper comprising the steps of controlling the feed of coarse fuel to the pulverizer in accordance with the demand for heat, intiating an adjustment of the damper substantially simultaneously with the start of each change in fuel feed, making all damper adjustments more slowly than the corresponding changes in fuel feed, maintanng a constant air pressure anterior to the damper, and controlling the discharge ofthe gaseous products of combustion from the furnace in accordance with the pressure in the furnace.

'7. A pulverized fuel burning system comprising a furnace, means to deliver a stream of primary air to the furnace, controllable means to feed pulverized fuel at a definite rate to the primary air stream, a duct for supplying secondary air to the furnace to effect complete combustion of the fuel, meansto adjust the fuel feed in accordance with the demand for heat, means to ondary air to the furnace to effect complete comv bustion of the fuel, means to adjust the fuel feed in accordance with the demand for heat, means to initiate an adjustment of the air supply substantially simultaneously with each change in 'fuel feed, and means to cause all air supply adjust- 'ments to be made at a slower rate than the corresponding fuel feed adjustments.

9. A pulverized fuel burning system comprising a furnace, a pulverizer arranged to deliver pulverized fuel and primary air directly thereto, controllable means to feed coarse fuel to the pulverizer at a definite rate, a duct to supply secondary air to the furnace to effect complete combustion of the fuel, a master regulator arranged to control the fuel feed in accordance with the demand for heat, and a reversible secondary regulator arranged to control the secondary air supply, the secondary regulator being controlled by the master regulator and arranged to start substantially simultaneously therewith following a change in demand, the secondary regulator being arranged to operate at a slower speed than the master regulator in each direction.

10. A pulverized fuel burning system comprising a furnace, means to deliver a current of primary air to the furnace, controllable means to feed pulverized fuel at a definite rate to the primary air current, a duct for supplying secondary air to the furnace to effect complete combustion of the fuel, a master regulator arranged to control the fuel feed in accordance with the demand for heat, and a secondary regulator arranged to control the secondary air supply, the secondary regulator being controlled by the master regu-' lator but arranged to operate at a slower speed than the master regulator.

11. A pulverized fuel burningsystem comprising a furnace, a pulverizer arranged to deliver pulverized fuel directly thereto, controllable means to feed coarse fuel to the pulverizer at a definite rate, a duct to supply air to the furnace, means to control both the feed of coarse fuel to the pulverizer and the air flow to the furnace in accordance with the demand for heat, means to cause each change in air flow to be made more slowly than the corresponding change in fuel feed, and means to adjust the speed of response of the air flow to an increased demand independently of the speed of response of the air flow to a decreased demand.

12. A pulverized fuel burningsystem comprising a furnace, a pulverizer arranged to deliver pulverized fuel directly thereto, controllable means to feed coarse fuel to the pulverizer at a definite rate, a duct to supply air to the furnace, a piston operated by fluid under pressure and connected to control the feed of the coarse fuel, a second piston operated by fluid under pressure and arranged to control the rate of air flow in the duct, means to move both of saidpistons in accordance with the demand for heat, means to cause the second piston to make`all air flow changes more slowly than the first piston makes the corresponding changes in fuel feed, and

definite rate, a duct to supply air to the furnace a master regulator arranged to control the fuel feed in accordance with the demand for heat, a reversible secondary regulator arranged to con- .trol the air fiow in the duet, the secondary regulator' being controlled by the master regulator and arranged to operate at a slower speed than the master regulator in each direction, and means to adjust the Operating speed of the secondary regulator independently in each direction.

14. Combustion control apparatus f or a direct fired pulverized fuel burning system having a pulverizer and a feeding device to supply coarse uel to the pulverizer comprising a master regulator of the compensating type arranged to control the feeding device in accordance with the demand for heat, and a reversible secondary regulator actuated by the master regulator and arranged to control the air supply to the furnace, the secondary regulator being arranged to operate at a slower speed than the master regulator in each direction.

15. Combustion control apparatus for a direct fired pulverized fuel burning system having a pulverizer and a feeding device to supply coarse fuel to the pulverizer comprising a master regulator of the compensating type arranged to control the feeding device in accordance with the demand for heat, a cylinder, a double acting hydraulic pisten slidable therein and connected to control the air supply to the furnace, a pilot valve, means connecting the master regulator with the :pilot valve, compensating mechanism connecting the piston with the pilot valve, means providing .fired pulverized fuel burning system having a.

pulverizer and a eeding device to supply coarse fuel to the pulverizer comprising a master regulator of the co'mpensating type arranged to control the feeding device in accordance with the demandior heat, a secondary regulator of the ompensating type actuated by the master regu-' lator and arranged to control the air supply to the furnace, the secondary `regulator being arranged to make all changes in air supply more slowly than the corresponding changes in fuel supply, and means to adjust the Operating speed of the secondary regulator independently in each direction.

L. V. ANDREWS. CHARLES M. TERRY. I

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