US2403976A - System of pulverizing, feeding, and transporting of material - Google Patents

Description

July 1, 3946. J. L A Y 2,403,976

SYSTEM OF PULVERIZING, FEEDING, AND TRANSPORTING OF MATERIAL Filed May 22,.1943

INVENTOR.

JamesL Harvey ATTORNEY Patented July 16, 1946 UNITED STATES PATENT OFFICE SYSTEM OF PULVERIZING, FEEDING, AND TRANSPORTING OF MATERIAL Application May 22, 1943, Serial No. 488,013

12 Claims. 1

The invention disclosed herein pertains to a system providing for the pulverization of a solid material capable of being transported in a finely divided state by means of a current of air or other gaseous medium. The material may suitably be a fuel such as coal, although other fuels, or other materials of a non-combustible character, are equally within the scope of the invention. The invention further contemplates transportation of the pulverized product to one or more points of use.

A specific application of the invention is to an air-swept pulverizing and distributing system wherein a current of air or other gaseous medium is maintained throughout the system for removing pulverized fuel from a pulverizer and for transporting the pulverized fuel direct to one or more burners in the manner commonly referred to as direct-firing. Nevertheless certain features of the invention may be found useful in other known systems employing such additional apparatus as cyclones, storage bins, and feeders incident to the transportation of the pulverized material from the pulverizer to the ultimate point of use. r

In a direct-fired system employing a plurality of burners in one or more furnaces, it may often be found desirable to utilize a single pulverizer for supplying the entire fuel requirements of several burners operating simultaneously. Where a number of small furnaces are involved, it may also be found both convenient and economical to distribute fuel to the burners of several furnaces from a single continuous main conduit system having both ends connected to the pulverizer to provide a closed circulating path or circuit for the fluent. fuel mixture; individual burners being supplied through individual branch lines connected to the conduit at intervals along its length, and any surplus quantity of the mixture above that required by the burner or burners in operation being returned in whole orin part to the pulverizer or other advance point in the system for recycling,

If one or more burners should be operated at varying capacities, or if the cycles of operation of individual burners should be different, the resulting changes in overall fuel consumption make it necessary to provide suitable adjustment of the rate at which material is fed to the pulverizer. Such conditions may be found in metallurgical plants, for example, where a different number of furnaces for the same or diiierent processes may be in operation over different periods, thus causing abrupt and Wide variations in fuel consumption for which there must be adequate control of pulverizer operation to suit each new condition.

In the usual direct-fired system where the entire output of the pulverizer is utilized and none of the mixture is returned for recycling, it has been found that the quantity of pulverized coal discharged from the pulverizer is substantially proportional to the rate of flow of primary air therethrough so long as a proportional amount of coal is maintained therein. This characteristic of air-swept pulverizer operation is utilized in U. S. Patent 1,965,643 to R. M. Hardgrove whereby the feed of material to the pulverizer is controlled to maintain a certain level or reserve quantity of material in the pulverizer for the existing rate of flow of primary air.

According to the present invention, an airswept pulverizer and transport system is proposed wherein any desired proportion of the pulverizer output may be withdrawn, and any remaining proportion circulated by being returned to the pulverizer, or in part to the pulverizer and in part to some other point in the system in advance of a point of withdrawal, the proportions of the mixture so withdrawn and returned being varied as desired for a given value of pulverizer output, or for different values.

As an accompaniment to such an objective, it is proposed to maintain under all conditions a proper relation between fuel and air in the mixture available for withdrawal.

An additional object concerns the supply of raw material to the system and the maintenance of a proper reserve of material in the process of pulverization within the pulverizer, each in relation to varying demands on the System.

Another object contemplates controlling the rate at which raw material is fed to the pulverizer in accordance with fluid flow conditions within the system, including for example, the rate at which a portion of the pulverizer output is returned for recycling.

A further object is to correlate indications of fluid pressure conditions existing at different points in the systemfor the purpose of controlling pulverizer operation.

These and other related objects of the invention may be achieved in the manner about to be described, an arrangement of apparatus suitable for the purposes of the invention being diagrammatically indicated in the accompanying drawmg.

In the drawing, an air-swept pulverizer I0 is shown with its associated feeder I2 by which raw unpulverized material such as coal, is fed to the pulverizer from a suitable bin 14 or other source of supply, the feeder being driven by a multispeed electric motor l6, for example, as a means of providing different rates of feed.

Air or other gaseous medium to provide a carrier for the pulverized material is admitted to the pulverizer through an inlet duct M3 by which the carrier medium at an elevated temperature is supplied from a source not shown, a branch inlet 20 conveniently open to the atmosphere providing a means for admitting tempering air to the duct I8; regulating dampers 22 and 2:1 being suitably provided in the respective inlet connections for maintaining the temperature of the carrier medium entering the pulverizer at a desired predetermined value.

A fan 26 connected to the pulverizer outlet by the discharge duct 28 induces a draft through the pulverizer which causes the carrier air to pass into the pulverizer from the duct 18 and to pass through the pulverizer where it picks up the pinverized material to provide a fluent mixture, which for the purpose of this example may consist of pulverized fuel suspended in a stream of primary combustion air, the term air being used generally whether or not a portion of the carrier medium is obtained, as is sometimes the case, from a source of hot flue gases. The mixture is discharged from the pulverizer through the duct 28 as a fluent stream, passing through the fan 26 and into the fuel main 39 from which it is delivered to one or more points of use.

The illustrated embodiment provides for delivery of the pulverulent fuel mixture to a plurality of burners 32, 3'4, 36 and 38 associated with a plurality of furnaces 4%, t2 and i l, each burner line 43 preferably being fitted with a plug valve dfl adjacent its connection to the fuel main to, and with a second valve 58 of the butterfly type intermediate the plug valve and the burner. The plug valves are for the purpose of cutting off and completely isolating the supply of fuel from individual burners so that any number of burners maybe operated, as desired, while the butterfly valves serve as a means'for regulating the flow of fuel to such burners as are in operation. In practice, the fuel main 333 may be so arranged that the .greater part of its length is horizontally disposed and at a level higher than the level at which the burners are located, thus enabling the burner lines 56 to be conveniently connected to the under side of the fuel main 3!] and to extend verticall downward therefrom for connection to individual burners. A secondary supply of air may be admitted to the burners or to the furnaces for combustion purposes, but the details of such secondary supply do not enter into the subject of the present invention and are therefore omitted from the drawing.

The fuel main 3.9 is continued beyond the last burner to be served, here indicated as burner 38 for furnace 34, to provide a conduit length 52 for returning to the pulverizer any surplus quantity of the fuel-air mixture which is not delivered to the-burners, the return length 52, as shown, being connected to the pulverizer ill at a location opposite to th location at which the primary air is admitted through duct l8, and at a somewhat higher level. There is thus provided a single continuous main conduit system consisting of the .disohargeduct 28, fan and fuel main 38 including its return length 52, having both ends connected to the pulverizer iii and thereby forming a closed path or circuit over which at least a portion of the mixture initially discharged from the pulverizer maybe circulated. The fuel main may be formed of different sizes of conduit with reductions in cross section at intervals as at 56, 56 and 58 to maintain a sumciently high velocity of fluid flow throughout to prevent fuel from settling out of the flowing mixture.

The pulverizer shown is of a known type utilizing a circular series of steel balls 63 operating between a stationary lower grinding ring '52 and a revolving upper grinding ring 64, the latter 4 being operatively connected to the rotating spindle -56 to which an extension casting {i8 is fitted to provide an annular passage or throat 18 for directing the carrier air into the grinding zone. A duct 12, formed in the base of the pulverizer and open at its upper side to the throat 10, provides a mixing chamber for fresh incoming primary air entering at one end from inlet duct i8 and for the surplus fuel-air mixture entering at a location adjacent its opposite end from the return line 52. The introduction of the surplus mixture at the level indicated takes advantage of the conical formation of the pulverizer casing as at M which serves to deflect the returning,

mixture downwardly into the duct 12 and obliquely against its bottom wall, thus producing a degree of agitation which tends to promote a redistribution of the fuel particles within the chamber and a more thorough mingling of the two streams to provide a relatively homogeneous total mixture of fuel and primary air for admission through the annular throat It.

It is evident that in the operation of a directfired circulating system such as has been described, there can be wide variations in fuel consumption due to the ability to operate a different number of burners at different times, and, if necessary, to operate individual burners at varying capacities. The resulting variations in fuel consumption impose varying demands on the pulverizer unit so that it becomes necessary to increase or decrease the rate at which raw coal is fed to the pulverizer, such changes in load requiring a change in the rate of fuel input which while satisfying the demand for a dilferent output of pulverized fuel will also maintain the proper level or reserve of material in the pulverizer for the particular load involved; the level of material being preferably increased as the load is increased and decreased as the load is decreased.

Since the pulverized fuel is maintained throughout the system in a fluent state, that is, in suspension in a current of primary air or other gaseous carrier medium, an indication may be obtained of the rate of flow of the fluent fuel mixture through a given portion of the system by measuring the pressure differential across that particular portion. A similar measure may be obtained of the rate at which air is supplied to the system. These are factors which enter into the control of pulverizer operation in the present system.

Referring again to the drawing, a measure of air flow to the pulverizer is conveniently obtained by employing an orifice 16 in the inlet duct l8, the resulting differential pressure across the orifice being transmitted by tubes 18 and 8D to the pressure sensitive diaphragm 82 suitably mounted for movement within a stationary casing 84. The pressure differential resulting from the flow of air and coal in suspension through a selected portion of the pulverizer, including particularly the grinding zone thereof, is transmitted by tubes 86 and 88 to the pressure sensitive diaphragm 9!] within the casing 92, the tube 86 being connected to a pressure point in advance of the grinding zone, for convenience to the downstream side of the orifice 16 as in the case of tube 86 for diaphragm 82, and tube 88 being connected to a pressure point following the grinding zone, for convenience to a point adjacent the pulverizer outlet. The rate at which surplus fuel and air are returned to the pulverizer under varying conditions is measured in terms of the pressure differential across a selected ortion of the return conduit 52, tubes 94 and 06 being connected to the conduit at relatively widely separated locations and the existing pressure differential transmitted thereby to the pressure sensitive diaphragm 98 within the casing I 00.

The relative pressures acting upon diaphragm 82, 90 and 98 are coordinately utilized to effect movement of the lever I02 which is pivoted about a point I04 fixed in relation to the casing 84 at one side. The lever I02 carries a contactor element I05 whereby upon movement of the lever I02 abouts it pivot point I04 certain electrical circuits to be later described are selectively energized to control the operation of the feeder motor I ii and thereby the rate at which coal is fed to the pulverizer.

The force exerted by diaphragm 82 is transmitted to the lever I02 by means of a pin I03 movable with the diaphragm 82 and extending through opposite walls of the casing 84, one end of the pin engaging the lever I02 at a predetermined distance from. the fulcrum point I04, The opposite end of pin I00 engages a second pivoted lever IIO also at a predetermined distance from a fixed fulcrum point II2, the lever arm distances in both instances being suitably equal as shown.

The force exerted by diaphragm 99 is applied to the lever I02 by means of a pin II4 extending through a side wall of casing 92, and similarly, the force exerted by diaphragm 98 is applied to the lever IIO by means of a pin 'IIB extending through a side wall of easing 60. The

casings 92 and I00 preferably have adjustable mountings as at H8 and I20, whereby their position may be varied relative to the respective fulcrum points I04 and H2, to thus increase or decrease the lever arm distances at which the forces exerted by diaphragms 90 and '98 are applied to the respective levers I02 and H0.

Springs I22 and I24 are provided to apply an initial loading to the levers I02 and H0 respectively, each spring having an adjustment I26 by which a desired degree of tension or compression may be obtained.

It may be assumed that the burners shown are of such capacities that when all are operating simultaneously at full ratings the total delivery offuel and air tothe burners is practically equal to the total output of fuel and air from the pulverizer, in which case there is a minimum of surplus fuel and air being returned to the pulverizer and the differential pressure transmitted through tubes 04 and 90 approaches zero. Under such conditions, virtually clean air alone is admitted through the pulverizer throat and, in accordance with known characteristics of airswept pulverizer operation, the differential pressure across the clean air orifice l6, as transmitted .through tubes I8, 80 to diaphragm 82, bears a definite relation to the difierential pressure across the pulverizer I0, as transmitted through tubes 86, 88 to diaphragm 00; this relation being determined by the pulverizer design and the grindability, sizing, and moisture content of the fuel being handled. As long as the fuel characteristics remain the same, and air flow through the pulverizer is kept constant, the pressure differentia1 across the pulverizer cannot be increased or decreased without an accompanying increase or decrease of the pulverizer output; and conversely, the pulverizer output cannot be increased or decreased without an accompanying increase or decrease of the pressure differential across the pulverizer.

Therefore, with return flow through conduit 52 practically zero, the entire control of pulverizer operation is subject to the relative forces exerted by diaphragms 82 and on lever I02, whereby motor IE will be operated at intervals at the proper speeds to provide a fuel-air mixture of a predetermined fuel-air ratio at the required pul verizer output; the term fuel-air ratio being defined as the cubic feet of air at pulverizer outlet temperature per pound of fuel.

When the opposing forces exerted on lever I02 by the diaphragms 82 and 90 are balanced, the contactor I06 is held clear of all upper and lower contacts I48, I42, I44 and I46, and feeder I2 is operated tov feed material to the pulverizer at a predetermined low rate. If the level or reserve quantity of material within the pulverizer should decrease, ,thus decreasing differential pressure across the pulverizer, the force exerted by diaphragm 02 overcomes the force exerted by diae phragm 90, and lever I02 is moved counterclockwise to cause contactor I06 to successively con tact the upper contacts I40 and I42, and feeder I2 is operated to feed material to the pulverizer at a predetermined high. rate to restore th normal operating level for the required rate of pulverizer output. If the level or reserve quantity of material within the pulverizer should increase beyond the normal operating level, the differential pressure across the pulverizer is correspondingly increased, and the force exerted by diaphragm 90 overcomes the force exerted by diaphragm B2, and lever W2 is moved clockwise to cause contactor W5 to successively contact the lower contacts I44 and MB, and operation of feeder i2 ceases.

When operation of the system is such that an appreciable fiow of the pulverized material and air mixture occurs in the returnline 52, the density of the fluid admitted through the pulverizer throat I0 is increased and the same relation of pulverizer differential to fresh air differential no longer holds. Furthermore the new relation may be subject to variations due to the varying rates at which the mixture is returned, and to the relation Which the proportion of the mixture returned bears to the total pulverizer output for a given load.

Assuming one of the burners, burner 32 for example, to be shut off, the proportion of the total pulverizer output formerly delivered to that burner becomes a surplus proportion of the total mixture which is returned through conduit 52 to the pulverizer I0 where it is mingled with the fresh air entering through inlet I8 for combined flow through the pulverizer throat 10, the resultant modified mixture constituting in effect a modified carrier medium of increased density. Tests have indicated that when an appreciable proportion of the mixture is returned to the pulverizer, as through the conduit 52, the relation between the differential pressure across the pulverizer and the differential pressure across the clean air orifice I6 is modified, as compared with the relation for zero return flow, and that the relative response of the two diaphragms 82 and 90 is also modified.

In order to compensate for the effect which return flow has on the functioning of diaphragms 82 and 90, the third diaphragm 98 is made responsive to fluid flow conditions in the return line 52, as indicated by differential pressures transmitted through tubes 94, 96, the diaphragm 98 acting to oppose the operation of diaphragm 90 and to assist the operation of diaphragm 82; the force exerted by diaphragm 98 being applied to lever H and transmitted through pin I08 to lever I02, so that the operation of diaphragm 98 is superimposed on the operation of diaphragms B2 and 90 which normally act to control the supply of raw material in relation to fresh air to maintain a fuel-air ratio of constant value, or of a predetermined range of values, throughout the entire range of pulverizer output capacities. Based on the known pulverizer characteristics as heretofore mentioned, such predetermined values of fuel-air ratios may be Obtained by suitable adjustment of the position of diaphragm 60 relative to the fulcrum I04, and by suitable adjustment of the spring I22; the diaphragm adjustment being made for maintaining either a constant or varying fuel-air ratio throughout a given range of pulverizer capacities, and the sprin adjustment transposing the value of fuel-air ratio by the same amount for all capacities within thegiven range. The combination of these two simple adjustments provides a means for securing the desired value or values of fuel-air ratio for the particular arrangement of burners. The return of a proportion of the outgoing mixture to the pu1- verizer adds to the amount of pulverized material and air flowing through the pulverizer, and accordingly tends to increase the differential pressure across the pulverizer, thereby modifying the normally fixed relation between the differential pressure across the mill I0 and the difierential pressure across the air inlet'orifice 15. The diaphragm 98 acting in opposition to the pulverizer differential diaphragm 96 serves to cancel out that component of the pressure drop through the pulverizer which is due to the returned pulverized material and air, and enables the operation of feeder I2 to be kept in step with the incoming fresh air as in the case where none of the mixture is circulated and, in consequence, fluid fiow through the return line 52 is zero. The differential pressure transmitted through tubes 94 and 98 to the diaphragm 58 need not be of any specific value since a compensating adjustment may be made as at I to suitably position the diaphragm relative to the fulcrum I I 2 for lever I It.

Any number of burners may be cut in or out as desired, and in accordance with operations reported to date, varying loads may be carried with thecontrol apparatus functioning satisfactcrily throughout a range as high as 8:1 in. pulverizer capacities.

The manner in which movement of the lever I 02 is correlated to operation of the feeder motor I5 will be understood from the wiring diagram which by way of example is based on a source of three-phase alternating current supply for operation of an electric motor having separate windingsfor different speeds. A transformer I28 suitably connected to the main supply lines AC provides reduced voltage at its secondary terminals I38 and I32 for the several control circuits, the secondary terminal I being connected by lead I34 to the arm I36 of a selector switch I38,

the switch arm I being shown in the off position.

Four adjustably fixed contacts I40, I42, I44 and I43 are shown arranged in pairs at opposite sides of the movable contactor element I06, the contacts I48 and I44 being adjusted to points nearer to the contactor I06 than the contacts I42 and I46, and the contactor I 05 being made flexible so that movement of the lever I62 in one direction causes contact to be made first with contact I40, and then additionally with contact I42. Movement of lever I82 inv the opposite direction causes contact to be made first with contact I44 and then additionally with contact I46. Lead I48 connects the contactor I08 with a contact I58 at the automatic position of switch I38.

, Upper contact I42 is connected to one terminal of the relay coil I52 by lead I54, while upper contact I40 is connected to the same terminal through the normally-open relay switch I56. The other terminal of relay coil I52 is connected to terminal I32 of the transformer secondary by means of lead I58 which serves as the common transformer return connection for the various relay coils shown.

Lower contact I46 is connected to one terminal of the relay coil I60 by lead I62, while lower contact I 44 is connected to the same terminal through the normally-openrelay switch I64 Theother terminal of relay coil Ififl'is connected to terminal I32 of the transformer secondary through the commonconnection I58.

A second contact I66 at the automatic position of the selector switch I33 is connected by lead I68 to one side of the normally-closed relay switch I10, the other side of which is connected through the normally-open relay switch Hi2 to one terminal of relay coil I14, and through the normally-closed relay switch I16 to one terminal of relay coil I18. The other terminals of relay coils I14 and I18 are connected to terminal I32 of the transformer secondary through the common return I58.

Switches I88 and I82 are three-pole switches, operated respectively by the relay coils I14 and H8, for establishing connections between the main power supply lines AC and the high and low speed windings of the feeder motor I6; a three-wire connection I84 leading from switch I to the high speed winding, and a three-wire connection I86 leading from switch I82 to the low speed winding.

When the selector switch I38 is set for autov matic operation the motor I6 will operate at low speed as long as contactor I06 is held clear of all contacts I40, I42, I44 and I46. Under such conditions, the relay coil I18 is alone energized, whereby the associated switch I82 is held closed, and ,power is supplied only through connection I86 to the low speed winding of the motor; the control circuit extending from the transformer terminal I30, through lead I34, switch arm I36, switch contact I66, lead I68, normally-closed relay switches I10, I16, relay coil I18, and return connection I58 to the transformer terminal I32.

Counterclockwise movement of the lever I02 causing contactor I05 to contact upper contact I 40 results in the control circuit voltage being applied to one side of the normally-open relay switch I56. When contactor I85 is brought into contact with upper contact I42, there is a circuit established through the relay coil I52 whereby the associated relay switches I5 I12 and I16 are actuated, and the motor I6 caused to run at high speed. The control circuit in this case is from the transformer terminal I30, through lead I34, switch arm I36, switch contact I36, lead I68, normally-closed relay switch I10, through normally-open relay switch I12 now closed, relay coil I14, and return connection I58 to the transformer terminal I32. The opening of switch I16 opens the circuit through relay coil II'8'to interrupt the power supply to the low speed winding, while the closing of switch I12 completes the circuit ,9 through relay coil I14 to close switch I80 and thereby transfer the power sup-ply to the high speed winding. The motor will continue to operate at high speed. as long as contactor I06 remains in contact with upper contact I40, whether or not contact is broken with upper contact I42.

Clockwise movement of lever I02 results in similar successive contacting of lower contact points I44 and I46 by contactor I06. When contact i made with point I46 the power supply to the motor I5 is cut off and the motor stops, the contact with point I46 causing relay coil I60 to be energized and the associated relay switches I64 and I I0 to be actuated; the opening of normally-closed switch I interrupting the control circuit through transformer I28 so that neither relay coil I14 nor I18 is energized and both power switches I80 and I82 remain open.

Additional contacts I88 and I90 may be provided on the selector switch I38 if it is desired to include manual control of motor operation. For this purpose, a connection may be made from contact I88 direct to point I92 of the circuit, and another from contact I90 direct to point I94, whereby lowspeed or high speed operation may be obtained by moving the switch arm I36 either to contact I88 or to contact I90.

Certain features disclosed herein are also disclosed and claimed in my copending application, Serial No. 531,776, filed April 19, 1944.

The invention as herein disclosed in accordance with the provisions of the statutes will be understood by persons skilled in the art to be applicable in arrangements other than those specifically described, and to include features which may be used to advantage without a corresponding use of other features, within the scope of the appended claims.

I claim:

1. In combination with a pulverizer having an annular grinding zone in a lower portion thereof wherein a reserve of material in the process of pulverization is maintained during normal operation, said pulverizer having an outlet at the upper side of said grinding zone, means for feeding material to be pulverized into said pulverizer above said grinding zone, means forming an an nular throat passage marginally of said grinding zone, means for maintaining an upward current of air through said annular throat passage for discharging through said outlet a fluent mixture comprising pulverized material suspended in said air, a main conduit having one end connected to said outlet and having its opposite end open to the interior of said pulverizer at the lower side of said grinding zone, mean for withdrawing from said conduit varying proportions of said pulverizer output mixture, said conduit constituting a conductor for returning a remaining proportion of said mixture to said pulverizer at a density substantially equal to the densityof the total mixture initially discharged, means separate from said conduit for admitting said air to said pulverizer, means for causing said returned proportion of said mixture to become mingled with said air at a location axially below and adjacent said annular throat passage for combined upward flow through said pulverizer, and means for regulating said material feeding means comprising pressure sensitive means responsive simultaneously to pressure differentials indicative respectively of said air admission to said pulverizer and of said combined fluid flow through a predetermined portion of said pulverizer inclusive of said annular throat passage.

2. In a system of theclass'described, an airswept pulverizer having an annular grinding zone within the lower portion thereof in which material in the process of pulverization is maintained during normal operation, said pulverizer having an outlet from its upper portion fordischarging a fluent mixture comprising pulverized material suspended in air, means for withdrawing from said system varying amounts of" the total mixture discharged from said pulverizer outlet, means for supplying air to saidpulverizer at a level below said grinding zone, means"sep arate from said air supplying means for returning to said pulverizer a remaining portion of said mixture at a density substantially equal to the density of the. total mixture initially discharged. means for mingling the returned portion of said mixture with said supplied air, means for directing said returned portion mingled with said supplied air upwardly through said pulverizer inclusive of said grinding zone, means for feeding material to be pulverized into said pulverize'r above said grinding zone, means for regulating said feeding means comprising pressure sensitive means responsive to a pressure differential indicative of said mingled fluid flow through said pulverizer inclusive of said grinding zone, and means for rendering said regulating means operative in accordance with a measure of the .return flow of said remaining mixture portion.

3. In combination with an air-swept pulverizer arranged to discharge a fluent mixture of pulverized material and air, means for feeding ma terial to be pulverized to said pulverizer, means for returning a portion of said mixture to said pulverizer,.means for regulating the rate at which said material to be pulverized is fed to said pul verizer, and means responsive to the flow of said portion being returned for controllingtheloper;

ation of said last named means to maintain a predetermined ratio of air to material in said mixture.

4. In combination with an air-swept pulverizer arranged to discharge a fluent mixture of'pulverized material and air, means for feeding material to be pulverized to said pulverizer at varying rates to provide varying output capacities,

mean for returning'a portion of said mixture to said pulverizer, means for regulating the rate at which said material to be pulverized is fed to said pulverizer, and means responsive to the flow of said portion being returnedffor' controlling the operation. of said last named means to main tain the ratio of air to material in said mixture substantially constant at said varying output capacities of said pulverizer.

5. In combination with an airswept pulver izer arranged to discharge a fluent mixture of pulverized material and air, means for feeding material to be pulverized to said pulverizer, means for delivering a portion of said mixture to a point of use, mean for returning the remain der of said mixture to said pulverizer, means for measuring the rate at which said remainder is returned to said pulverizer, and means controlled by said last named means for regulating the operation. of said feeding means. I

6. In a continuous conduit system including a pulverizer, means for supplying material to be pulverized ,to said pulverizen'mean for supplying carrier air to said pulverizer for transporting pulverized material therefrom and through said system, means for withdrawing a portion of said pulverized material and air from a part of said system other than said pulverizer, means for maintaining another portion of said pulverized material and air in circulation throughout said system, means for regulating the rate at which said material to be pulverized is supplied to said pulverizer, and means responsive to the rate of circulation of said other portion of pulverized material and air for controlling the operation of said regulating means.

7. In combination with an air-swept pulverizer having separate inlets for fuel and air and having an outlet for pulverized fuel mingled with said air, a continuous conduit having one end connected to said pulverizer outlet and its opposite end connected to said inlet for air, means for supplying fuel to said pulverizer, means for withdrawing a portion of said fuel-air mixture from said conduit, means for measuring the rate at which the remaining portion of said mixture is returned to said pulverizer air inlet through said opposite conduit end, and means controlled by said last named means for regulating the rate at which fuel is supplied to said pulverizer.

8.. In an air-swept conduit system including a pulverizer, wherein a portion of the mixture of pulverized material and carrier air discharged from the pulverizer is withdrawn from said system and a remaining portion of said mixture is returned to said system for recycling through said pulverizer, the method of controlling the operation of said pulverizer at varying capacities which comprises feeding material to be pulverized to said pulverizer, supplying carrier air to said pulverizer, each from a source outside said system, measuring in terms of differential pressure the rate at which carrier air is supplied to said pulverizer, measuring in terms of differential pressure the rate of fluid flow through said .pulverizer, measuring in terms of differential pressure the rate at which pulverized material and carrier air are returned to the pulverizer, and regulating the rate at which material to be pulverized is fed to the pulverizer in accordance with the resulting three measures of differential pressure.

9. In a system of the class described, an airswept pulverizer arranged to discharge a fluent mixture of pulverized fuel suspended in air, means for supplying fuel to said pulverizer, means for returning a portion of said mixture to said pulverizer and for mingling said portion with air supplied to said pulverizer, means for measuring the rate of air flow to the pulverizer, means for measuring the differential pressure across a predetermined portion of said pulverizer, means for measuring the rate at Which said portion of the mixture is returned to the pulverizer, and means conjointly responsive to said three named measuring meansfor regulation the rate at which fuel is supplied to saidpulverizer.

10. In combination with an air-swept pulverizer arranged to discharge a fluent mixture of pulverized material suspended in a stream of carrier air, means for supplying material to be pulverized to said pulverizer, means for returning a mixture of pulverized material and air to said pulverizer at a rate less than the rate at which said initial mixture of pulverizable material and carrier air is discharged from the pulverizer, means for measuring the rate of flow of carrier air to said pulverizer, means for measuring the difierential pressure across a predetermined portion of said pulverizer, means for measuring the rate at which said pulverized material and air are returned to the pulverizer, and means under thejoint control of said three named measuring means for regulating the rate at which said material to be pulverized is supplied to said pulvermen 11. In combination with a pulverizer having a grinding zone in a lower portion thereof comprise ing an annular grinding surface together with a circular row of reliable grinding elements cooperating with said surface, means for feeding material to be pulverized to said grinding zone from above, mean forming an annular throat passage marginally of said grinding surface for admitting air into proximity with said grinding zone from below, means for maintaining an upward current of air through said throat passage for discharging from said pulverizer a fluent mixture comprising pulverized material suspended in said air, a main conduit having its opposite ends open to the interior of said pulverizer above and below said annular throat passage, said conduit forming with said pulverizer a closed circulating path for a portion of the total mixture initially discharged from said pulverizer, means for regulating the rate at which said material is fed to said grinding zone comprising means responsive to therate of air delivery to said annular throat passage relative to the rate of fluid flow through a predetermined portion of said pulverizer inclusive of said annular throat passage, and means for rendering said regulating means operative in accordance with the rate at which said portion of said output mixture is re turned to said pulverizer.

12. In combination with a pulverizer having a grinding zone in a lower portion thereof wherein a reserve of material in the process of pulverization is maintained during normal operation, said grinding zone being defined by upper and lower grinding rings and a circular series of metal balls operating therebetween, means for feeding material to be pulverized to said pulverizer above said grinding zone, means forming an annular throat passage interiorly of said lower ring for admitting air to said grinding zone from below. means for maintaining an upward current of air through said throat passage and grinding zone for discharging from said pulverizer a fluent mixture comprising pulverized material suspended in said air, a main conduit having its opposite ends open to the interior of said pulverizer at the upper and lower sides of aid grinding zone, said conduit forming with said pulverizer a closed circulating path for at least a portion of the total mixture initially discharged from said pulverizer, means for measuring in term of differential pressure the rate at which air is delivered to said throat passage, means for measuring in terms of difierential pressure the rate of fluid flow through a predetermined portion of said pulverizer including said throat passage and said grinding zone, means for measuring in term of differential pressure the rate of flow of the portion of said mixture being returned to said pulverizer, and means rendered operative in response to said three named measuring means acting jointly for controlling the rate at which material to be pulverized is fed to said pulverizer.

JAMES L. HARVEY.

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