US2335853A - Pulverizing system - Google Patents

Description

M. FRISCH Dec. 7, 1943. I

PULVERIZING SYSTEM Filed Aug. 5, 1940 1 NVENTOR fiver/1v FE/SCH ATTOR Patented Dec. 7, 1943 PULVERIZING SYSTEM Martin Frisch, New York, N. Y., minor to Foster Wheeler Corporation, New York, N. Y.,' a oorporation of New York Application August 3, 1940, Serial No. 350,347

v 16 Claims. (Cl. 839) This invention relates to pulverizing systems and more particularly pertains to systemsfor pulverizing fuel such as coal, and feeding the fuel to a zone of combustion such as the furnace oi a'steam generator, or the like.

In systems of this character, the. fuel. after ordinarily carried by, a. single current of air Like characters of reference refer to the same parts in the several views.

" Referring to the drawing, reference character having been pulverized to the desired fineness is moving through a conduit directly to one or 4 more fuel burners. This air is preheated tothe desired temperature and isilrst conducted under pressure to the pulveri'zing means, such as a single ball mill, by an air duct, and the flow of air through this duct is controlled ordinarily by one or more dampers, the arrangementbeing such that when for any reason the burners or the mill are shut down, the flow of hot air to the 'mill will be,stopped, as otherwise, a serious explosion might occur. vHowever, where a pluralityof fuel-air streams flow from the mill to the combustion zone,'it is desirable frequently to operate with less than all these fuel-air streams. In these circumstances, it is necessary to maintain the flow of preheated air to the mill so long as combustion is-maintained, but to discontinue .the supply of preheated air to the mill as soon as combustion entirely ceases to avoid the possibility of explosion in the system.

The present invention provides improved means for controlling the flow ofpreheatedair to .thepulverizer in pulverizing systems wherein -a plurality of fuel-air streams flow from the pulverizer to the combustion zone and for controlling these fuel-air streams in response to Y varying operating conditions and fluctuations in ,pulverizer output requirements.

The particular features and advantages of the invention will be understood from the following -dmcription when considered in connection with the accompanying drawing forming a part thereof, and in which:

Fig. lris a more or less diagrammatic view, partly in elevation and partly in section, of a pulverlzing and fuelfeeding system embodying .the invention; f Fig. 2 is a diagrammatic view of the automatic control means: g

Fig.- 3 is an enlarged elevational view of part of the'apparatus shown in Fig; 1;.

Fig. 4 is a'partial elevational and longitudinal sectional view of a further part of the apparatus shown in Fig. 1, and

Fig. 5 is an elevational view of damper control means for controlling in part the output-of the pulverizins means.

the conveyors l6 and I! to the interior of the to a separate burner or to separate burner' ll designates generally 'a ball mill pulverizer comprising a cylindrical shell II which contains the material to be pulverized and grinding balls of various sizes. The shell is supported by hollow-trunnions i 2 and' I3 and is adaptedto be rotated about its horizontal axis. In the form of the invention chosen for illustration, the

unground coal or other material, is introduced to the mill at both ends through feed conduits I4 and I5, which communicate with the hollow v Helical conveyors l6 and I1 within 7 trunnions. the trunnions, and connected to the shell for rotation therewith, convey, the unground mate-' 1 rial-into the shell. .Air in sufllcient quantity andat the proper temperature is conducted to the mill through a main air duct l8 and branch ducts l9 and which communicate with the interior of the shell at .both ends thereof by means of conduits 2| and 22 extending through the trunnions l2 and i3 respectively. As the material is pulverized, it is picked upby' the air currents flowing into the shell-and carried by the'air from the shell in separate streams through each of the trunnions .to classifiers 23 and 24 wherein undesirablycoarse or. oversize particles of the' material are separated from the air streams,

dropout of the classifiers, and are returned by shell. The'material which has been pulverized to the desired extent is borne by the air streams through conduits 25 and 28 to a pair of similar exhausters individually designated A and B, which are driven byelectric motors 21 and 28 respectively. It will be understood that each exhau'ster discharges through a suitable conduit groups, which may be employed in the firing of a single furnace or a plurality of separate.

furnaces. k Connected to exhauster A at points 29 and 30,

Q are pressure .tubes ti and 32 respectively, while connected to .exhauster B at points 33 and 34 are similar pressure tubes 35 and 38. The con- '-duits .25 and 28 are respectively connected to the exhausters A and B through exhauster inlet elbows 31. and 38. In the elbow 31 is a damper 39 for controlling the flow of fuel to the exhauster, and the mill output is controlled in part by regulation of this damper. As shown in the drawing, the damper 39 is in fully opened position. The position of .the damper 39 may be adjusted to points between fully opened and fully closed positions, and maintained in any adjusted position in. response to variations in combustion requirements by suitable means actuated either manually or automatically. One such means will be described hereinafter. When the exhauster is shut down, or ceases to operate normally due to emergency conditions, the dam automatically closes, thereby, shutting off completely the supply of fuel to the exhauster.

. The damper is held in the desired open position by-the adJusting means referred to, and lever ll which releases the damper from the adjusting means, is retained in the position to eflect the release of the damper by a latch 02 which engages the keeper l3 thereon and is actuated by a solenoid ll. In normal operation, the solenoid "II is deenergised and the latch is positioned to hold the lever CI in the position shown in Fla. 3 for an open position of the damper. Upon failure of the exhauster for any reason, or upon normal shut down of the furnace ormill, the solenoid II will be energised-and the latch I! willrrbe tripped to release the damper so that it will-move to closed'position by gravity. An air inlet" is'provided in the elbow 31 which is controlled by a damper ll, and means are provided -to cause this damper ll to open when the damper it closes so that fresh air may be drawn intothe elbow and the associated elements of the system to prevent explosions. Inlet elbow ll of exhauster B -is similarly constructed and is provided with similar dampers and control means. therefor. An emergency shut off damper l! is disposed in the hot air duct ll "leading to the mill. nected to this damper is a lever II for manually Conmoving the damper to open position. Within the duct- II is a vertically inclined seat It for the damper when it is in closed position. The damper is maintained in open position in normal operationby a latch ll controlled by a' solenoid ll and which engages the keeper on the lever ll.

The arrangement is such that when the solenoid is deenergized,.the latch will hold the damper in open position., but when the-solenoidls energisedthe latch will trip and the damper will move to closed position by gravity, it being understood that the' .damper is of greaterweight than the lever 43. -The hot air duct it is also provided with throttling dampers 41 for maintaining the v desired suction'on the mill. These damper may be nianuallyoperated by alever ll, or they may be automatically controlled by a mill suction regulator, if desired. Connected to the hot air duct between theemergency shut off damper l2 and the' throttlingdampers I! is a tempering air inlet duct 4!. Flow of air at atmospheric temperature through the duct 49 into the duct II is controlled by a damper IO.

' In the present. system, where one mill supplies fuel to two exhausters operatingv in parallel, it

1 may be frequently desirable, or at times neceshauster is operating and until both exhausters are shut down, and when both exhausters are no longer functioningthe damper automatically closes to cut off the flow of hot air to the mill. Referring to Fig. 2, an exhauster controller II is connected across theexhauster A by the pressure tubes SI and I2, and a similar exhauster controller 5| is connected across exhauster B by thepressure tubes 38 and It. The exhauster contrgller 5| includes a mercury sealed bell 52 and mercoid switches Hand 54 which are closed by downward movement of the bell and opened by upward movement thereof, and exhauster controller 5| includes a similar mercury sealed bell 52' and similarly operated mercoid switches 53' and M. Shut down or failure of either exhauster will causepressure changes on the inlet and discharge-sides of each .exhauster which will be transmitted to the controller connected to the exhauster. The mercoid switches 53, 54 and 53', II which are all shown in open position, are connected to a three-phase electrical circuit a, b, c which is connected to a source of power. As-

suming that both exhausters' are operating, if ex-- close-open cycle upon downward movement of the bell 52,.with the result that the damper 39 is caused to close by the momentary energization of the solenoid 0 through lead 55 and contactor I. The switch," is arranged to move back to open position to prevent burning out of the coil so of solenoid ID. The switch II remains in closed position until the bell I2 moves upwardly. When exhauster B is shut down, the pressure changes resulting will be transmitted to the controller 5|,

the bell 52' will move downwardly and the switches 53' and 64' will be moved to closed po- I sition. .The switch-IV is mounted and actuated so that it passes from open to closed to open position upon downward movement of the bell 52',

with the result that the solenoid M is momentarily energized through lead It and contactor II and the damper I! is moved to closed position. The switch ll remains closed, andsince this switch is in series with the closedswitch 54,

the hot air shut-off damper cir'cuitis closed so through lead 51' and contactor 88, which enersizes the solenoid l6 and thus permits the'hot air shut-oilv damper 42 to close. As the damper l2 approaches closed position, a limit switch 59 in lead ill is automatically tripped open by any suitanism. This opens the circuit to'solenoid 4i an?1 prevents bumin g out of the coil of the soleno y In starting the system, the several dampers are opened manually and latched in open position. After normal pressure is attained in the controllers by operation of the exhausters, the bells l2 and 52 in the controllers will move up wardly and will open the switches 54 and 54'. The limit switch 59 is then closed and the system will then operate normally with both exhausters functioning.

From the foregoing it will be perceived that with this arrangement, preheated air will continue'to be supplied to the pulverizing means so long [as any fuel-air mixture is flowingto the burners or to the combustion zone or zones supplied by' the pulverizing means, but that as soon as the flow of fuel-air mixture is cut oil entirely for any reason, the supply of preheated air to the pulverizing means ,is also cut 01!; thereby avoiding any chance of an explosion in the mill or in the system. This arrangement includes 7| control of each fuel-air stream from the mill to able means, not shown, connected to and operated by the damper 42 or its associated mechthe extent that upon failure or shut down of any exhauster, that exhauster and the burners served thereby will be isolated from the mill by a cut- 7 of! damper such as damper 39 in elbow 31 which is connected to the exhauster A, thus avoiding explosion due to flame propagation to the mill.

The use of a plurality of exhausters with a single mill provides greater flexibility oi control of the firing means and greater efllciency of opv eration of the pulverizing system.

Manually operated means are shown ior regulating the position of the damper 39. Thedamper is fixed to a shaft 95 (Fig. which extends beyond the elbow in which the damper is mounted, and on the outerend portion of the shaft 33, a control arm 33 is mounted for freemovement with the shaft. Secured to the hub of arm 99 by bolts 31 is a clutch member 33 hav ing a notch or recess 39 which is shaped to re- Iceive a roller dog ll secured to the outer end of -a clutch member II which is pivoted to the end of arm as is placed in the recess of the lifting cam 93 in order that the upward movement ofthe solenoid lever will cause damper ll to close the auxiliary air opening 64. The cam 99 on solenoid lever 3| has a contour such that when the lever is in the latched position shown in Fig. 3; the clutch comprising the members 98 and "II. is engaged to connect the damper adjusting damper shaft 93. A link. I! is pivoted at one end v to an intermediate point of member H and pivoted at its other end to one .face of a circumferentially slotted collar I3 which is freely slidable on shaft 95. oppositely disposed pins 14 on the inner forked legs 15 of a bell crank lever are received .in the slot in the collar 13. The bell crank lever is pivotally mounted at 19 between 1 the forked ends of'a support 11 which is fixed to the elbow or to some. other stationary member.

The other arm 19 of the bell crank lever carries a roller 19 which is continually urged into engagement with a cam 99 or other part of the solenoid lever 9|, uponwhich the cam is fixed,

by a spring 9| which is disposed between theouter endofarm I5 and a stationary part. of the elbow orother part of the apparatus. The solenoid lever 6| is mounted on and is freely movable which is pivotally connected to a stationary member 86. Rotation of the wheel 92 in one diarm 33 to the damper, and when the solenoid lever is'in a substantially vertical unl-atched position the clutch will be disengaged so that the damper will be free of adjusting arm 68 and will move underits own weight to closed position.

Accordingly, movement of the solenoid lever 3| 'i'romunlatched to latched position will cause the bell crank lever comprising arms I! and 19 to move in a counter-clockwise direction about pivot 13 which will move collar I3 to the right, as seen in Fig. 5, and will cause clutch member II to move into engagement with member 68. With the clutch engaged, damper 39 is moved to the desired initially adjusted position by movement 0 the hand wheel". When the damper is to be closed for any reason, solenoid 43 is energized, latch 92 is tripped to free it from engagement with keeper 63, solenoid lever Si is released and moves in a counter-clockwise direction to a substantially vertical position, during which movement of the lever cam 80 will cause clockwise movement of the bell crank lever about pivot I3,- thereby'sliding collar 13 on shaft "to the left as seen in Fig. 5, and disengaging clutch member II from member 99, so that the damper is freed from'the adjusting means and moves to closed position underits own weight. During normal operation when the damper is connected to the adjusting means, the damper may be moved to any'desired adjusting position between open and closed positions by proper movement of the hand wheel 92, thereby controlling the quantity oi fuel-air mixture flowing from the mill through the exhauster to the one or more burners to control the rate of firing of the burners and the furnace temperature in consequence.

Each damper 39 is adjusted independently so that. the rate of tiring of the burners served by rection causes the rods to move into sleeve 93,

and rotation in the opposite direction causes the rods to move ouwardly of the sleeve. thus causing the damper to move toward open and closed position respectively at such times as roller dog -Ill engages the clutch member 98. A weighted arm 91 is fixed to the damper shaft OI to counter balance the weight of the damper. The damper II which controls the air inlet 9, has a link 99' fixed to the damper shaft and pivotally connected to one end of an arm 99, the other end of which rests in a recess in a lifting 1 cam 99 on solenoidlever 9|.

In operation ofthe damper control mechanism; assuming that the damper 39 is in closed position and the solenoid lever 9| is in a substantially vertical position, having been released by the energization of solenoid l9, to open the.

damper and to set the damper in the position initially desired, the solenoid lever 9i is'moved in a clockwise direction until it is in the position shown in Fig. 3 with the solenoid latch 92 eneach exhauster is independently controlled.

1 Where the exhausters serve burners in different furnaces, the firing of the furnaces may be individually controlled. 7 A

In lieu'of the manually operated means disclosed for regulating the position of each damper 39, automatic means may be employed. One

gaging the keeper 33 to retain the lever in the position shown. It will be understood that the solenoid 49 is deenergized, and that the lower such means suitable for the purpose is disclosed in Heat Engineering, January, 1940, volume XV, number 1, pages 2 to 8, published by Foster Wheeler Corporation, Broadway, New .York city. It will be understood that separate regulating means are employed for each damper 39.

It is to be understood that changes may be made in the'particular form, location and relative arrangement in the sev ral parts of the apparatus disclosed without departing from the principles of the invention, which is not to be limited .excepting'by the scope of the appended claims.

What is claimed is:

l. Pulverizing apparatus comprising a mill,

means for supplying to the mill material to be pulverized, means for supplyin to the mill a medium for carrying a pulverized .material, means for withdrawing from themill a plurality of streams of said medium with entrained pulverized material, flow controlling means in each of said streams, and means for controlling the supply of said .medium to the mill in response to the flow. of said streams from the mill and operative to continue the flow of said medium to the mill so long .as said flow from the mill continuesin any stream. 2. Pulverizing, apparatus comprising a mill, means for supplying to the mill material to be pulverized, means for supplying to the mill a medium for carrying a pulverized material, separate conduits connected to the mill through inlet and discharge of each of said exhausters for controlling the operation of the first-mentioned damperitdmaintain said damper in operative pos'itioni toradmit the. medium tothe mill and each of-which a stream of said medium and suspended pulverized material is withdrawn from the mill, means in each of, said conduits for varying the quantity of said medium and pul- -verized material withdrawn in each ,stream, an

Y =duits tocut, oil the, flow oi-jthe, medium to the mill only when now in all of said conduits is discontinued.

3. Pulverizing apparatus comprising a mill, means for supplying 'to the mill material to be pulverized, means for supplying to the mill a medium ioracarrying a pulverized material, sep- '"-arate -conduits connected to the mill through each; 'which a stream of said medium and suspended pulverized material is withdrawn from *the mill, means in each of said conduits for vary- ,ing the quantity or said medium and pulverized ,material withdrawn in each stream, an exhauster connected with each conduit, means for controlling the ,supply of said medium to the mill, and-.tmeans operative in response to the differential-Tin the pressure between th inlet and discharge ,of each of said exhausters to actuate said controlling means so; a to maintain th controlling means lnoperative position to admit the medium-to-themill and responsive to -a reduction in saidpressure difierence betweenthe in-. let and discharge of all of said exhausters to cut ofl the flow of the medium to the mill only when 1 flow in all of said conduits is discontinued.

4. Pulverizing apparatus comprising a mill, means for supplyingto the mill material to be pulverized, a conduit for supplying to the mill a .medium for carrying a pulverized material, a i-iamper for controlling the conduit, a plurality of separate conduits connected to the mill through each of which a streamof said medium and suspended pulverized material is withdrawn from the mill, an exhauster connected with each of tion in said pressure diflerence between the inlet and discharge of each of said exhausters. to cut on the flow of the stream in said conduits, and electrically actuated means operative in responseto the difference in pressure between the responsive to ,a reduction in said pressure difference between the inlet and discharge of all of said exhausters to cut of! the flow 01. the medium to the mill when the ilow in all of said conduit is discontinued.

5. Pulverizing apparatus comprising .a mill,

means for supplying to the mill material to be pulverized, means for supplying to the mill a medium forcarrying a pulverized material, control meansior controlling the supply of said medium to the mill, separate conduits connected to the mill through each of which a stream 01' said medium and suspended pulverized. material is withdrawn from th mill, an exhauster connected with each conduit, a damper for controlline each conduit, means for regulating-the position of the damper in each conduit to vary the quantity of medium and suspended pulverized material flowing through the conduit, and means responsive to the flow of said streams through the separate conduits adapted to operate said first-mentioned control means to cut off th flow of the medium to the mill when flow from the mill in all of said separate conduits is discontinued.

' 6..Pulverizing apparatus comprising a mill, means for supplying to the mill material to be pulverized, means for supplying to the mill a medium for carrying a pulverized material, con+ trol means for controlling the supply of said medium to the mill, separate conduits connected to the mill through each of which a stream of said medium and suspended pulverized material is withdrawn from-the-mill, an exhauster con- 'nected with each conduit, a damper for controlling each conduit, means for regulating the position of the damper in each conduit to vary the quantity of medium and suspended pulverize'd material flowing through the conduit, means for disconnectingthe regulating means from the damperto permit the damper to move to closed position, and means responsive to the flow oi said streams through the separate conduits adapted to operate said first-mentioned control means to cut oil the flow oithe medium to'th mil; when'all of said dampers are moved to closed position and flow irom the mill' in-all the separate conduits is thereby discontinued.

7. Pulverizing apparatus comprising a mill, means for supplying to the mill material to. be pulverized, a conduit for supplying to the mill 9. medium for carrying a. pulverized material, a damper for controlling the conduit, a plurality oi separate conduits connected to the mill through each or which a stream of said medium'and suspended pulverized material is withdrawn from the mill, an exhauster connected with each of said plurality of conduits, a damper for each of said plurality of conduits, means for regulating the position of the damper in eachof said plurality of conduits to vary the quantity or medium and suspended pulverized material flowing through the conduits, means for disconnecting the regulating means fromthe damper in' each of said plurality of conduits to permit the damper to move to closed position, and means for controlling the damper in the conduit through which medium is supplied to the mill operative to maintain said damper in an open position so long as at least one of said other dampers is openand to close said damperw-hen all oi said other dampers. are closed, 7

8; Pulverizing apparatus comprising a mill. meansior supplying to the mill material to be pulverized, means for supplying to the mill a medium for carrying, ;a pulverized material, a. plurality of. means. forjcanslng said-med m to flovathrough nd out-oitl'iev mill in a plurality oi. streams :canyingmulverized material in. sus- .pensiommeans in cache! said treams iprvary ing the quantity or said'medillm and suspended pulverized material withdrawn in. each stream,

and means for controlling the supplyof medium to'the millin response to the flow of the streams irom said mill operativeto continue the flow of the position of the damperin each of said plurality of conduits to vary the quantity of medium and'suspended pulverized material flowing through the conduits, andmeans {or controlling the damper in the conduit through which medium is supplied-to the mill operativ to maintain said'damperin an open position so long as at least one .oi-saidother dampers is open and to close said damper when all of said other dampers are closed.

10. Pulverizing apparatus comprising a mill, means for supplying to the mill material to be pulverized, means for supplying to the mill a medium for carrying a pulverized material, a plurality of means forcausing said medium to flow through and out of the mill in a' plurality of streams carrying pulverized materialin suspension, -flow controlling ,means .in each oi. said streams-operative to 'cut oil the flow of .any

*streamlrom the mill when the flow of said streamdecreases, and means tor controlling the s ply oi said medium to the mill in response to the flow of said streams-from the mill and operative to cut oil the flow oi. said medium to.

the mill when all flow from the mill stops.

11. Pulverizing apparatus comprising a mill, means for supplyin to the mill material to be pulverized, means for supplying to themill a medium for carrying a pulverized material, control means for controlling the supply of said medium to the mill, means for'withdrawing from the-mill a plurality of "streams or saidimedium with entrained pulverizedunateriaLflow controlling means in each oi said streams operative to cut oil the flow '01 any stream from the millwhen the flow of said stream decreases, "and means responsive-to the flow of said streams means for supplying to the mill material to be 1 pulverized, a conduit for supplying to-the mill eachjoi which a stream' or, said medium'iand suspended pulverized material is withdrawn from the mill, an exhausterconnected with each of said plurality of conduits, a damper for each of said plurality of conduits,,.ilow controlling means in each or said separatejconduits operative to cut off the flow or mediumand pulverized 3 material through any one of said s'ep'a'r'ate conduits when the f flow thereinYdecreasesQmeans for regulating the position'oithedampei" in each of said plurality or conduits to" vary. the quantity of medium and SuspenGedZpulverizedmaterial flowing through the conduits, and me'ansffor controlling the damper in the conduit through which medium. is supplied to the mill operative to maintainsaid damper in an open position so long as at least one of said other dampers is open and to close said damper when all of said other dampers are closed.

' 13. Pulverizlng apparatus comprising a mill,

a medium for carrying a pulverizedmaterial, a damper for controlling the conduit. a plurality of separate conduits connected to the mill through each of which a stream ofsaid medium and suspended pulverized material is withdrawn from the mill, an exhauster connected with each of said plurality of conduits, a damper for each duits at a point intermediate the mill and the exhauster, and electrically actuated means operative in response to the diiierentialin the pressure between the inlet and discharge 0! each of said exhausters for controlling the operation of the damper in each of the conduits through which said medium is withdrawn from the mill to cut off the flow of the stream in any one conduit when' the flow of said stream in that conduit decreases, and electrically actuated means operativein response to the difierential in pres sure at the inlet and discharge of each of said exhausters for controlling the operation of the first-mentioned damper to cut oil the flow oi the medium to the mill when flow in all or said plurality of conduits is discontinued.

. .14. Pulveri'zing apparatus comprising a mill,

means for supplying to the mill material to be pulverized, means for supplyin to the mill a medium for carrying a-pulverized material, control means for controlling the supply or said mediumio the mill, means for withdrawing. from the mill aplurality oi streams. of. said medium with entrained'pulverized material, flow controls line means in each of said streams, and electrically actuated means responsive to the flow of said streams from the mill adapted to operate the first-mentioned control means to cut oil the flow of the medium to the mill when flow from the mill in all of said streams is discontinued.

15..Pulverizing apparatus comprising a mill,

. plying t 'the mill material to be irom the mill adapted to operate said flrst-men- I tioned control means to cut oilthe flow oi. the

medium to the mill when flow irom the mill in all of said streams stops.

l2. Pulverlzing apparatus comprising a mill, means for supplying '00 the mill material to be pulverized, a conduit for supplying to the mill a medium for carrying a pulverized material, a damper for controlling the conduit, a plurality of separate conduits connected to the mill through 75 trol means to cut oil the flow. oi the medium to pulverized, means for supplying to the mill a medium for carrying a pulverized material, control means for controlling the supply of said medium to the mill, means ior withdrawing from the mill a plurality of streams of said medium with entrained pulverized material, electrically actuated means operative in response to the flow of said streams from the mill for controlling the operation of the flow controlling means in each of said streams, and electrically actuated means responsive to the flow of said stream from the mill adapted to operate the first-mentioned con- 01' said plurality of conduits for closing said constreams is discontinued.

16. Pulverizing apparatus comprising a mill, means for supplying to the mill material to be pulverized, means for supplying to the mill a medium for carrying pulverized material, separate conduits connected to the mill through each of which a stream oi. said medium and suspended pulverized material is withdrawn from the mill, an exhauster connected with each conduit, flow controlling means in each or said conduits responsive to the diflerence in pressure produced by the exhauster connected to the conduit and operative to maintain the flow controlling means in each 01' said conduits in operative position to permit the flow 01 medium through said conduits and responsive to a reduction in said pressure difierence between the inlet and discharge of each of said exhausters to cut oil. the flow of the stream in said conduits, means for controlling the supply of said medium to the mill, and means operative in response to a difference in pressure produced by said exhausters to actuate said means for controlling thesupply of said medium so as to maintain said means for controlling the Supp y of medium in operative position to admit the medium to the mill and responsive to .a reduction in said pressure diflerences produced by the exhausters in all of said conduits to cut of! the flow oi the medium to the mill only when flow in all of said conduits is discontinued.

Y MARTIN FRISCH.

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