US2831637A

US2831637A - Control for pulverizing mill

Info

Publication number: US2831637A
Application number: US395412A
Authority: US
Inventors: Harvey C Mittendorf; Konrad S Svendsen
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1953-12-01
Filing date: 1953-12-01
Publication date: 1958-04-22
Anticipated expiration: 1975-04-22

Description

April 22, 1958 H. c. MITTENDORF ET AL 2,831,637

CONTROL FOR PULVERIZING MILL Filed Dec. 1, 1953 2 Sheets-Sheet 1 INVENTORS Harvey C. Mittendorf Konrad S. Svendsen ATTORNEY -H. c. MITTENDORF ET AL 2,831,637

April 22, 1958 CONTROL FOR .PULVERIZING MILL 2 Sh eets-Sheet 2 Filed Dec. 1, 1953 4 INVENTORS Harvey C. Mittendorf Konrad S. Svendsen J T'I'OR NE Y CONTRQL FOR PULVERIZlNG NULL Harvey C. Mittendorf, East Grange, N. J., and Konrad S. Svendsen, Eellerose, N. V assignors to Combustion V Engineering, lino, New Yorlr, N. Y;, a corporation of Delaware This invention relates to control systems which regulate the amounts of fuel and air delivered for combustion to the furnaces of vapor generating units; and is specifically concerned with controlling the amount and temperature of primary air conveying the fuel from a pulverizing mill to the burners into the furnace of a vapor generator.

The invention can be applied with great benefit to a steam generating unit having a furnace subjected to variations in static pressure with fluctuations in steaming load.

If a furnace of this type is fired with pulverized fuel for varying steam generating loads, the amount of primary air, i. e. air conveying the pulverized fuel entrained therein from the mill to the burners into the furnace, must bear a preferred quantitative relationship with the amount of coal delivered to the furnace. This relationship directly ar'fects the grinding capacity of the mill, the conveying capacity of the fuel pipes from mill to burners and the performance of the burners.

Accordingly the proper relationship between amount of coal and amount of primary air depends primarily on a coal-air mixture velocity maintained in the pulveriz ing mill, coal piping and burner that permits these units to deliver optimum performance at all loads. For a given flow area and coal loading this velocity is determined by the volume of primary air, which in turn depends on the temperature of the air-coal mixture leaving the mill.

l-leretofore the amount of primary air for each pound of coal burned had been controlled in part by measuring the static pressure at the mill outlet. However such a control system does not take into proper account fluctuations in pressure at the mill inlet as may occur in mills operating under pressure, and/or fluctuations in pressure in the furnace chamber as may occur in pressurized furnaces.

It is accordingly 2. primary object of the invention to provide an improved control system for regulating the amount and temperature of the air conveying the pulverized fuel from the mill to the burners into the furnace under widely varying operating and load conditions.

It is a further object of the invention to provide improved control means for regulating the air flow from mill to burners which control means are responsive in part to variations in steam generating load and responsive in part to variations in pressure differential between the static pressure in the mill and that in the furnace.

It is an additional object of the invention to provide improved control means for regulating the air fiow from mill to burners which control means are also responsive to the quality of the fuel being pulverized as reflected in the heating value and moisture content thereof.

Other objects and advantages will become apparent from the following description of illustrative embodiments of the invention when read in conjunction with the accompanying drawings wherein:

Fig. 1 is a diagrammatic representation of apparatus embodying the invention applied to a mill supplying fuel to the furnace of a steam generator.

Fig. 2 is a portion of Fig. 1 modified to show another form of the invention.

In carrying out the invention in one form thereof, there is provided control means responsive to the steam pressure prevailing in the steam outlet header which pressure is indicative of fluctuations in steam generating load. These pressure responsive control means serve a dual purpose: first, to regulate the flow of fuel to the pulverizing mill, and second, to maintain a predetermined ratio of primary air to fuel in the mill and in the fuel pipe by controlling the amount of primary air entering the mill. The invention further provides corrective control measures imposed upon the aforesaid primary air control means. These measures comprise apparatus which is responsive to the pressure differential existing between the static pressure in the mill and that in the furnace,

said pressure diiferential being an indication of the volumetric quantity of the air-fuel mixture flowing from the mill through the fuel pipe and burner into the furnace for combustion. The invention further provides for control means responsive. to the temperature of said air-fuel mixture as it leaves the mill for the purpose of regulating the quantity of hot air required for fuel drying, such quantity being a portion of the primary air entering the mill in controlled amounts as above outlined.

Referring now to Fig. 1 the apparatus includes a steam generator 1 and its associated furnace 2 both shownin diagrammatic fragments, amill 3, a mill fan 4, an 'air preheater 5 and a main air fan 6. Fanj6 delivers cold air via conduit 7 into the air heater 5, the heated air leaving via conduit 8 to be delivered into the furnace 2 adjacent the fuel nozzle 9 of the burner 10. A duct 11 connects into the cold air conduit 7 and a duct 12 connects into the heated air conduit 8, the two ducts joining at right angles into a common duct 13 which is the suction line of millfan-4. p

A damper 14 is mounted -at the junction of ducts 11 and 12 in such a manner that its movement will vary the opening to either duct to proportion the relative amounts of cold and heated air flowing into the fan duct 13 and thereby regulate the temperature of the mixture. A damper 15 in fan duct 13 varies the resistance to airflow through the duct and thereby the amount of primary air. Fan 4 delivers the mixture into the mill '3 via duct 16. Obviously the amount of primary air flowing through duct 16 can also be controlled by other means such as regulating the speed of fan 4 by employing a variable speed motor (not shown). After the heated air mixture has passed through the mill'3 wherein it serves as-a drying and a transporting medium for the fuel being pulverized, the fuel-air mixture leaves the mill 3 via duct 17 and is delivered into the furnace 2 through burner fuel nozzle 9. j I

The mill3 is'provided with a fuel feeder 18 which d elivers the raw fuel in measured quantities into the mill 3. The feeder 18 may by way of example be operated in a well known fashion by a motor driven oscillating pawl which engages one or more teeth of a ratchet wheel (not crease or decrease the rate of fuel fed to the mill 3.

The apparatus here illustratively shown for controlling the fuel supply to the mill 3 includes a Bourdon tube 21 connected via pipe 22 to the steam header 23 of steam generator 1, which tube 21 is responsive to thesteam pressure in said header. The moving end 24 of the Bourdon tube'21 raises or lowers the stem 25 of a pilot valve 26. The pilot valve 26 has an air supply inlet 27 receiving air under pressure, an air outlet 28 and an air connection 29. The valves 30, by moving up or down, regulate the relative sizes of openings leading. into the outlet 28 and into the connection 29, opening one while closing the other thereby controlling the air pressure to the connection 29. As the steam pressure in the header 23 increases or decreases from the normal (depending on the load demand) the air pressure in connection 29 respectively decreases or increases.

Pipe 31 conveys the air pressure from the connection 29 of the pilot valve 26 into a chamber A of a relay 32. The relay 32 has four chambers A, B, C, D. Chamber A is separated from chamber B by a flexible diaphragm 33 and chamber .C from chamber D by a flexible diaphragm 34. The diaphragms are fastened to and move a rod 35 which rocks a lever 36 which is fulcrumed at 37, and acts to open the spring closed

pilot valves

38 and 39 by means of pins 68 and 69 respectively.

Pilot valve 38 when lowered or raised respectively increases or decreases the flow of supply air under pressure into chamber D and pilot valve 39 when lowered or raised respectively increases or decreases the flow of air out of chamber D to the atmosphere. The pilot valves 38 and 39are arranged to be both closed by spring action for the mid position of the lever 36. The chamber B is open to the atmosphere through port 4%. The chamber C is connected to chamber D via a needle valved connection 41 through which the air under pressure in chamber D may leak into chamber C to equalize the pressures within the chambers after a period of time depending upon the extent of the valve leak. A spring 42 opposes the downward movement of the

diaphragms

33, 34 or rod 35. a

In operation, the relay 32 receives a pressure such as 15 pounds in chamber A when the steam pressure in the header 23 is normal and the spring 42 balances the 15 pounds pressure in chamber A. Chamber Bis at atmospheric pressure, and

pilot valves

38 and 39 are then both in closed position. The pressure prevailing in chambers D and C is accordingly sustained. When the pressure in steam header 23 rises or falls from the normal due to decrease or increase in the steam generators load, the pressure conveyed via pilot valve 26 and pipe 31 to chamher A respectively decreases or increases from the 15 pound pressure and the pilot valve 38 respectively remains closed or opens while the pilot valve 39. respectively opens or remains closed. Consequently with an increase in pressure in chamber A from 15 pounds, air under pressure fiows into chamber D through valve 38 (to increase the pressure therein) and with a decrease in pressure chamber A, air flows out of chamber D through valve 39 (to decrease the pressure therein).

Chamber D of relay 32 is connected via pipe 43 to apparatus such as bellows 44, which actuates a lever 20 attached to mechanism 18 for feeding fuel to the mill 3. An increase or decrease in pressure in chamber D causes bellows- 44 to move lever 20 to increase or decrease the rate of fuel fedto the mill 3. Should for example, the fuel feed be inadequate to maintain the load on the steam generator, the steam pressure will fall, the Bourdon tube 21 will cause pilot valve 26 to raise the pressure in chamber A of relay 32 above 15 pounds which will open valve 38 and in turn will raise the pressure in chamber D and in bellows 44 to cause an increase in fuel feed. Upon return of the steam pressure to normal the Bourdon tube 21 will adjust the air pressure in chamber A back to 15 pounds thereby causing the pilot valves 38 act directly on bellows 44 causing the rate of fuel feed to increase or decrease with steam demand over a small load range.

The air pressure at the mill outlet as measured at connection 45 varies with the load on the steam generator. In conducting the fuel and air mixture from mill 3 to the furnace 2 the mill air pressure must overcome the resistance to flow through the duct 17 and through burner 9 plus the pressure in the furnace. For general purposes the duct losses and the burner losses can be assumed to vary substantially as the square of the air velocity through the duct. The difference in pressure between the mill and furnace which is the equivalent of the duct losses and burner losses is measured by a manometer 46 for the various loads of the steam generator.

The manometer 46 is constructed to convert the square functional relationship between the velocity and flow resistance of the air-fuel mixture from mill 3 through duct 17 into a linear measurement throughout all loads. One side 47 of the manometer 46 is connected to the mill at 45 via a pipe 48 to measure the mill outlet pressure and the other side 49 is connected to the furnace 2via pipe 50 to measure the furnace pressure. The resulting movement'of the lever 51 of the manometer 46 actuates a pilot valve 52 which is constructed and operates in the same manner as pilot valve 26 described above and has the same reference numbers applied to its individual parts. Pilot valve 52 provides a rise and fall of air pressure in the outlet thereof and in pipe 53 connected thereto which is consistent with a rise and fall in the volume of the air-fuel mixture flowing from the mill 3 through duct 17 into furnace 2.

Air pressure pipe 53 communicates with relay 54 which is constructed in the same manner as relay 32 described above and has similar reference characters. It has its chamber B connected to pipe 53 and thereby receives the rise and fall of the pressure from pilot valve 52 as determined by manometer 46 .and represents variations in the volume of the air-fuel mixture flowing from the mill 3 into the furnace 2. The chamber A of relay 54 is connected via pipe 55 into pipe 43 which in turn communicates with the outlet 41 of relay 32 thereby receiving the rise and fall impulses of the air pressure from relay 32 which determine the rate of fuel feed. The pressures in chambers A and B of relay 54 are arranged to be counter balanced when the proper ratio exists between the fuel fed to the mill and the air in the fuel-air mixture flowing from the mill to the furnace as is reflected in the air pressure in pipe 53.

When such proper ratio exists the lever 36 in relay 54 is in midposition at any load. Both pilot valves 38' and 39 are then in closed position and the pressure in chambers D and C of this relay 54 is sustained. This pressure is conveyed through pipe 56 from chamber D into a bellows 57 which acts to set the damper 15 in the duct 13 to a certain position. This position is such as to cause a proper amount of air-fuel mixture to flow from the mill to the furnace, the manometer 46 delivers an air pressure to chamber B of relay 54 which is equal tothe pressure in chamber A.

Should for example, the air flow from the mill 3 be inadequate for the existing rate of fuel being fed into the mill, the manometer 46 Will then cause the pilot valve 52 to deliver a too low relative pressure into the chamber B of relay 54. The higher pressure in chamber A of relay 54 will then tilt lever 36' counterclockwise, open pilot valve 38 allowing air under pressure to enter which will raise the pressure in chambers D and C of this relay 54 and in bellows 57 thereby turning damper 15 to a more open position. The resulting increased air flow through the mill 3 will determine a new setting of manometer 46, thereby relatively raising the pressure in chamber B of relay 54; and if the proper setting of damper 15 is effected in this manner the pressure in chamber B' of relay 54 will equal the pressure in chamber A and pilot valves 38.- and .39 will be in'closed position-to sustain the pres sure in chambers D and C and bellows 57 to thereby maintain the damper 15 in its proper setting.

As stated earlier herein above other means,,not shown,

may be employed to control the air flow through duct 13, fan 4 and duct 16 with equally beneficial results. Thus a variable-speed motor (not shown) may be used to drive fan 4, the speed of the motor being determined by a resistor (not shown) actuated by means responsive to air pressure fluctuation in pipe 56.

A thermometric element 58 projects into the mill 3 adjacent its outlet and measures the temperature of the air leaving the mill. it actuates the piston of a pilot valve 59 which is constructed and operates in a similar manner as pilot valve 26 described above and has similar reference characters. The valves 30' of this pilot valve 59 move in such a manner that an increase or decrease of air temperature in the mill outlet causes a corresponding decrease or increase in air pressure in pipe 60 leading from the outlet 29 of pilot valve 59 into the chamber A"of a'relay 61.

Relay 61 is identical with relay 32 described above, it has similar reference characters, and its operation is the same. The pressure in chamber A is maintained at a given value, such as 15 pounds, when the temperature in the mill is normal. The tension of spring 42" is such, with chamber B" open to the atmosphere, that the diaphragm 33" is held in midposition, and pilot valves 38" and 39" in closed position, thereby sustaining a certain pressure in chambers D" and C". Said chamber D" is connected by pipe 62 to a bellows 63 which, depending upon the pressure, holds the damper 14 in a certain position.

The damper 14 controls the relative amounts of hot and cold air of which the air mixture delivered into the mill 3 is composed, and when this mixture, after drying the fuel within the mill, attains the proper temperature as measured by the thermometric element 58, the damper 14 isin the desired position. Should the air mixture contain insufficient heat to adequately dry the fuel the temperature leaving the mill will be low and the element 58 will cause the pilot valve 59 to increase the pressure in chamber A" of relay 61. This in turn will increase the pressure in chambers D" and C" and in bellows 63 and the damper 14 will thereby be moved to relatively increase the hot air flow. if the proper damper setting is effected, the temperature of the air leaving the mill will become normal and the pressure in chamber A" of relay 61 will return to 15 pounds.

Fig. 2 is a portion of Fig. 1 showing a modification of the invention. As described hereinabove and as illustrated in Fig. 1, the thermometric element 58 in mill 3 effects an air flow under pressure from pilot valve 59, through pipe 60 into chamber A" of relay 61. In turn relay 61 effects an air flow under pressure from its chamber' 'D through pipe 62 to bellows 63 to move damper 14 for adjusting both the hot and cold air flow. In Fig. 2 damper 14 is replaced by damper 14a to only regulate the amount ofhot air flowing through conduit 12 and damper 65 for the purpose of separately controlling the cold air which in the Fig. 2 organization is taken in from the room through conduit 64 joining both conduits 12 and '13 at the junction thereof. The check damper 65, pivoted at 66 and counter weighted at 67' permits cold air to flow through conduit 64 into conduit 13 to form a mixture with the hot air flowing in from conduit 12.

If the proper setting of damper 14a is effected, the temperature of the air leaving the mill 3 will be normal and the pressure in the chamber A" of relay 61 will be of predetermined value such as 15 pounds for the reasons given earlier. Shoud the air mixture contain insuflicient heat to adequately dry the fuel in the mill 3, the temperature leaving the mill will be low and the element 58 will cause the damper 14a to move to relatively increase the amount of hot air in the mixture for the reasonsalsot;

given earlier herein.

As stated earlier .in the description of Fig. ratus, damper 15 in conduit 7 amount of hot and cold air delivered by fan dinto the mill 3.

The arrangement of Fig. 2 permits the mill 3 to operate under a pressure required to only overcomethe mill and piping resistance because the pressure at the entrance of conduit 13 is near atmospheric, while in the arrangement 1 appaof Fig. l the mill 3 operates under an additional pres-.-

amount and temperature of the air flowing through the mill in a novel manner to satisfy the primary air require-- ments regardless of the actual furnace pressure against which the air is delivered and regardless of the temperature of the air necessary to dry the fuel.

While we have here shown certain preferred embodiments of our invention as applied to a system including one mill and one burner, our new control facilities o bviously may be modified to apply to a multiplicity of mills and/ or burners serving the furnace of a steam generator or the like; and wherefor it ,will be understood that changes in apparatus and in combination and arrange ment of parts may be made without departing from the spirit and scope of the invention as claimed.

What we claim is:

l. A system comprising a pulverizing mill, a boilerfurnace subjected to variations in static pressure with av fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burner for conveying pulverized fuel to the furnace by air flotation,

a feeder for introducing fuel into said mill at a controllable rate, means forforcing air under pressure into and through the mill causing a flow of air whereby to convey the pulverized fuel into the furnace by way of said conduit and said burner, variable flow resistance means to control said flow of air, and control means for 7 said flow resistance means whichare in part responsive to the steaming load ofsaid boiler furnace and in part responsive to the difference between the pressure in the mill and the pressure in the furnace said control means causing said flow resistance means to allow air to flow through said mill to said furnace at a rate that is adequate to permit operation of the mill to sustain said steaming load.

2. The combination of a pulverizing mill, a boiler fure.

nace subjected to variations in static pressure with a fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burner for conpulverized fuel to the furnace by air flotation, a I

veying feeder for introducing lable rate,

fuel into said mill at a controlthe pressure in the mill and the said control means causing said air forcing means to supply air at a rate that is adequate to permit operation of the mill to sustain said steaming load.

3. In a system comprising a pulverizing mill, a boiler furnace subjected to variations in static pressure with a fluctuating steam load, a burner for said furnace,- a confrom the mill to said burner for 13 controls the combinedin response to steam means for forcing air under pressure into and through the mill whereby to convey the pulverized fuel into' the furnace by way of said conduit and said burner, and control means for said air forcing means which are in part responsive to the steaming load of said boiler furnace and in part responsive ,to the difference between pressure inthe furnace,

spares? means which are in part responsive to the steaming load of said boiler furnace-and in part responsive to the diiference between the pressure in the mill and the pressure in the furnace, said feeder control means causing said feeder to supply fuel, and said air forcing control means causing said air forcing means to supply air, both fuel and air, respectively, at a rate'that is adequate to permit operation of the mill to-sustain said steaming load.

4. The combination of a pulverizing and fuel dryingmill, a boiler furnace subjected to variations in static pressure with a fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burnerfor conveying pulverized and dried fuel to the furnace by air flotation, a feeder for introducing moisture carrying fuel into said mill at a controllable rate, means for forcing heated air under pressure into and through the mill whereby to dry said fuel while being pulverized and to convey the dried and pulverized fuel into the furnace by way of said conduit and said burner, and control means for said air forcing means which are in part responsive to the steaming load of said boiler furnace and in part responsive to the difference between the pressure in the mill and the pressure in the furnace and in part responsive to the temperature of said conveying air as it leaves the mill.

5. In a system comprising a pulverizing and fuel drying mill, a boiler furnace subjected to a variations in static pressure with a fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burner for conveying pulverized and dried fuel to the furnace by air flotation, a feeder for introducing moisture carrying fuel into said mill at a controllable rate, and means for forcing heated air under pressure into and through the mill whereby to dry said fuel while being pulverized and to convey the dried and pulverized fuel into the furnace by way of said conduit and said burner, the combination of control means for said feeder which are responsive to the steaming load of said boiler furnace and control means for said air forcing means which are in part responsive to the steaming load of said boiler furnace, in part responsive to the difference between the pressure in the mill and the pressure in the furnace and in part responsive to the temperature of said conveying air as it leaves the mill.

6. The combination of a pulverizing and fuel drying mill, a boiler and furnace subjected to variations in static pressure with a fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burner for conveying pulverized and dried fuel to the furnace by air flotation, a feeder for introducing moisture carrying fuel into said mill at a controllable rate, means for forcing heated air under pressure into and through the mill whereby to dry said fuel while being pulverized and to convey the dried and pulverized fuel into the furnace by way of said conduit and said burner, instrument means for receiving a first control impulse responsive to the temperature of said conveying air as it leaves the mill and for transmitting said impulse to apparatus for regulating the temperature of said heated air, instrument means pulverized and to convey the dried and pulverized fuel into the furnace by way of said'conduit and said burner,

receiving and transmitting a second control impulse responsive to the steaming load on said boiler, instrument means for receiving and transmitting a third control impulse responsive to the difference between the static pressure in the mill and the pressure in the furnace, and instrument means receiving and correlating said second and third control impulses for regulating said air forcing means, whereby said air forcing means supply heated air at a controlled temperature and rate that is adequate to permit operation of the mill, including suflicient drying of said fuel to sustain said steaming load.

7. In a system comprising a pulverizing and fuel drying mill, a boiler furnace subjected to variations in static pressure with a fluctuating steam load, a burner for said furnace, a conduit leading directly from the mill to said burner'for conveying pulverized and dried fuel to the furnace by air flotation, a feeder for introducing moisture I carrying fuel into said mill at a controllable rate, and means for forcing heated air under'pressure into and through the mill whereby to dry said fuel while being the combination of control means for said feeder which are responsive to the steaming load of said boiler furnace, instrument means for receiving a first control impulse responsive to the temperature of said conveying air as it leaves the mill and for transmitting said impulse to apparatus for regulating the temperature of said heated air, instrument means receiving and transmitting a second control impulse responsive to the steaming load on said boiler furnace, instrument means for receiving and transmitting a third control impulse responsive to the difference between the static pressure in the mill and the static pressure in the furnace, and instrument means receiving and correlating said second and third control impulses for regulating said air forcing means, whereby said feeder is caused to supply fuel and said air forcing means is caused to supply heated air at a controlled temperature, both fuel and air, at a rate and temperature that is adequate to permit operation of the mill, including sufficient drying of said fuel to sustain said steaming load.

8. In a fuel pulverizing and burning system for generation of steam in a boiler, the combination of a pulverizing mill including means for feeding thereto fuel to be pulverized; means for supplying air to the mill to provide a carrier for the pulverized fuel; a boiler furnace including a burner for burning said fuel for the generation of steam, said furnace being subjected to variations in static pressure with fluctuations in steam load; a discharge conduit for the pulverizer and through which the air and pulverized fuel in suspension is passed to said burner; control means responsive to the steam load for varying the rate of supply of fuel to the pulverizer; and automatic means for proportionally varying the rate of air supply in simultaneous response to fuel supply and the static pressure in said furnace.

References Cited in the file of this patent Dickey Nov. 14, 1950