US2354997A - Control system - Google Patents

Description

Aug. 1, 1944.

FIG. 2

A. E. KROGH CONTROL SYSTEM Filed June 25, 1943 3 Sheets-Sheet l 6 y l? 4i.;

o Q alii" INVENTOR.,

ANKER E. KROGH ATTO NEY;

A118- 1,'1944 A A. E. KROGH A 2,354,997

CONTROL SYSTEM Filed June 25, 194s 5 sheets-sheet 2 FIG. 4

y INVENTOR. ANKER E. KROGH 1% MM? ATTO EY.

A. E. KROGH Aug. 1, 1944.

CONTROL SYSTEM Filed June 25,V 194sl :s sheets-sheet 5 FIG. 6'

INVENTOR. ANKER E. KRoGH |29 i 2i l 1 y ATTOEY.

Patented Aug.,1,` 1944 CONTROL SYSTEM Anker E. Krogh, Glenside, Pa., asslgnor to The Brown Instrument Company, Philadelphia, Pa., a corporation'ofrennsylvania Application .lune 25, 1943. Serial No. 492,243 s claims. (ci. zasig) The present invention relates to` automatic temperature controlling systems, and is particularly designed to regulate within very narrow limits the temperature of the hot blast supplied to a blast furnace.

In the operation of a blast furnace the air for u combustion is heated in one of a plurality of stoves that are connected in parallel between the air supply, or cold blast, and the air to the furnace, or hot blast.' Air for the furnace is heated by forcing it .through one of the stoves at a time. While the stove through which air is being forced is cooled down by the air the rest of the stoves are being heated. In order to keep the air at a constant temperatureas the stove temperature varies a by-pass pipe between the hot blast main and the cold blast main is used, and a valve in this by-pass is adjusted so that more or less of the air is passed through the stove.

Some systems, such as the one disclosed herein, have two valves, one in the cold blast main and one in the by-pass, which are alternately or simultaneously adjusted to vary the amount of air Vpassing through the stove. In this manner .as the stove cools down a larger percentage of moved from one extreme position to the other; It is advisable therefore to provide some means to reverse rapidly the position of the valves when a stove change takes place. l

It is an object of the present invention to provide a. system for hot blast control which will be capable of adjustingj the air valves in such a manner that a substantially constant tempera.-

' ture of the air is maintained at the blast furnace.

It is a further object of theinvention to provide a hot blast control system in which means is provided to operate individually each ofthe valves in sequence or in vsuch a" manner that. one may begin to operate when the other is in any' predetermined portion of'its range of movement.

It is a further object of the invention to proi 'I the valves quickly whenever the blast temperature devlates' more than ya predetermined amount from the said control point.

The variousk features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification.` For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying vdrawings and descriptive'matter in which is illustrated and described a preferred embodiment4 View of a blast Figure 5 is a view of a. by-pass assembly to be used with the instrument of Figure 4,

Figure is a View, partly in section, of an air pressure regulating` instrument, and p y Figure 'I is 'a view on an enlarged scale of certain levers-of Figure 6. l

'I'here is shown in Figure 1 a diagram of a blast furnace system indicating the manner in which combustion air is supplied vto this furnace. There is shown at I a blowing engine which forces air into the blast furnace `2. The' air is forced through a pipe 1 known as a cold blast main and then through one o f the stoves 3, 4 and 5 that are'connected in parallel between the cold blast main and the hot blast main 6 whichv supplies air to the furnace., The cold blast main has branches leading from it to the other jstoves and each of thesel blanches is lsupplied with a valve 8, 9 or I0. In alike mannen-the hot blast main .is supplied with air through branches leading to that main from the stoves and thesey branches have in them valves Il, l2 or lirespectively. There is also provided in ythe-cold blast main a valve I4 which controls the air iiowing through this main to the stoves, vand there is provided a by-pass i! extending between the cold blast and the hot blast mains which bypass has avalve I8 in it to control the amount of air flowing through the same. It is notedV that in their closed positions these valves do not entirely shut oil' the flow of air through the pipes in which they are located.

In the normal operation of a blast furnace system a. blower i is used to force a given volume of air to the furnace through the by-pass i5 and through one of the stoves, while the other two stoves are being heated up so that in their turn they can be used to heat the air supplied to the furnace. If, for example, the air is being heated by stove 3, the valves 3 and would be open while valves 3, I0, I2 and I3 would be In order to regulate the air temperature, the

valves I4 and I8 are adjusted by apparatusy set up in a control layout as shown in Figure 2. The details of the various instruments are disclosed in the remaining figures of the case. lIn Figure 2 the thermocouple I1 which is located in the hot blast main is shown as being connected by means of a thermocouple lead wire Il to a potentiometer type of air controller I3. This con- .,troller is supplied with air under a regulated pressure through a'pipe 2| which contains in it a yfilter 22 and a pressure regulator 23. I'he air passes throughf a so called by-pass panel 24 to.

the instrument I3 which serves to'vary the pressure` o! this air and apply it through pipes'25v and 23 tov an air control instrument 21. This ,latter instrument is also supplied with air from the pipe 2| and is provided with apparatus which serves to adjust this air and apply a regulated air pressure through pipes 23 and 29 which pressures are varied in accordance with the pressure supplied by the instrument I9. Pipes 28 and 29 lead respectively to piston positioning mechanisms 3| and 32 that serve to adjust the valves I4 and Il. These mechanisms may take the form of the piston positioner shown in Moore. Patent Y 2,237,038, granted April 1, 1941, and which are l* supplied with a suitably regulated air pressure,

that is generally considerably higher. than that in pipe 2|, through a pipe 33. The piston positioners 3| and 32 are connected by means of suitable links -34 and 3l with the valves I4 and I3 respectively.

Located between pipes 23 and 23 is a so-called manual-automatic station 33 which includes a pressure regulator and a three-way valve 31.

This manual-.automatic station is generally located at some point adjacent the lblast furnace itself while the instruments I3 and 21 mayy be located in a control housey containing the rest of the apparatus and control instruments that are needed for the' proper operation of the furnace. In the normaloperaticn of the instrument the three-way 'valve 311s adjusted to the position shown in which pipes 25 and 26 are connected so that changes in the temperature of the thermocouple as measured by the instrument I3 will be converted into air pressure of a suitablevalue and applied to the control point changing mechanism of the instrument 21. At times, however, it may be advisable to adjust the instrument 21 manually, in which case valve 31'is moved 90 in a counter-clockwise direction in orderto connect the pipe 23 with a pipe 33 that forms a branch of.; the air supply pipe 2|. When this is done, the pressure regulator in the station 433 may be adjusted to apply any desired pressure direct from the pipe 2|.through pipe 23 to the instrument 21. Also connected to the pipe and 4| which are connected in circuit with an alarm device 42. One of the switches 39 or 4| is designed .to close the circuit for the alarm when a predetermined minimum of pressure occurs in the pipe 26, while the other of the switches is designed to close the circuit when a predetermined maximum pressure occurs in the pipe 2l. Thus the attendant will be notified that one of the limits of the instrument system is being approached and he can take the proper steps to correct this condition. This usually occurs when the stove which has been heating the air supplied to the blast furnace has cooled to such a degree that it is necessary to place a new stove in the line. A pushing button station 43 is also provided so that the alarm may be operated manually if for some reason it is desired to call the operator's attention to a condition of the furnace.

The instrument I9 may be anytype of air control potentiometer but is shown herein as being of the continuous balance type disclosed in the application of W. P. Wills, Serial No. 421,173, which was illed December 1, 1941. In that instrument there is provided, as shown in Figure 3, a slide wire 45 across which a potential drop is obtained by means of a battery 46. One end of the slide wire is connected with one terminal of thel thermocouple I1 whose other terminal is connected to the central contact of a vibrator 41, the cooperating contacts of which are connected to the ends of the primary winding of transformer 48. The secondary winding of the transformer is connected toan amplifier 43. A center tapvlil of the primary winding of the transformer 48 is connected to a slide wire contact 52 which is shifted along the slide wire by means of a motor 53 to vwhich it is mechanically connected. When the temperature in the hot blast main 6 is constant, no current will flow in the potentiometer circuit. When, however, a change in temperature occurs, a current flow in one direction or the other will be set up in the. potentiometer circuit, and this current flow will be amplified through the transformer 48 .and the amplifier 49 to energize the motor 53 for rotation in one direction or the other. The motor which is connected to the contact 52 serves to move this contact along the slide wire until such time as the flow of current is balanced by the potential drop across a portion of the slide wire, at which time the potentiometer circuit will be balanced and the motor will again be de-ener. gized.

As shown in Figure 4, the motor 53 is provided with a pinion 54 that meshes with and rotates a large gear 55 as the motor rotates. This gear is mechanically connected in a suitable fashion with the slide wire contact 52 and also serves to move a segment member 58 around a pivot which is formed by a shaft 51 to which this segment is attached. Also fastened to the shaft 51 is a pen arm 58 that is moved by the segmentv across a i suitable calibrated chart to make a record of 23 are a pair of pressure operated switches 39 75 "to the gear l5 so that this indicator is moved in the temperature of the hot4 blast. VAn indicator 30 which is shown in Figure A2 is also connected synchronism with the pen arm.

.ll-second arm 39 is attached to the shaft 51 and serves by means of a link 3| 'to raise or lower the left end of a diiferential floating lever 32 whose other end is supported by means of a segment 63. This segment is pivoted at 64 and has an index member 65 movable with it. In the operation oi' the instrument, the segment 63 is moved around its pivot to raise or lower the right end of the differentialv lever 82 until the index 85 is y' at a. position across the chart corresponding .to

- of any suitable type but which is shown herein as taking the form of a control instrument which may be of the type that is fully disclosed in Moore Patent 2,125,081, that was'granted on July 26, 1938.

To this end, the lever 62 is connected by means of a link 86 with one `arm of a three-armed lever 81 that isv pivoted at 88. This lever has a pin 89 on it which serves to move a apper 1| against a suitable biasing force away from or permit the fiapper to move toward a nozzle 12 to throttle the flow of air through this nozzle. The nozzle, along with a chamber 13 that is l formed in a pilot valve 14, is supplied with air from a pipe 15 that is connected to the pipe 2|. A restriction 18 is formed in thesupply pipe to the nozzle and chamber so that air is supplied thereto at a slow rate.

As the pressure in the chamber 13 varies clue to the throttling of the ow through nozzle 12 the size of this chamber is varied due to the expansion and contraction of a bellows 11 that forms one wall of this chamber. 'Ihe bellows 11 and a secon'd and smaller bellows 18 that is attached to it each serve to raise or lower an exhaust nozzle 19 which is connected with the. atmosphere through the inter-bellows space. This nozzle operates in connectionwith a valve, 8|, that is biased in a counter-clockwise direction, to either open or close this nozzle and to simultaneously close or open a supply nozzle 82 which is connected directly with the pipe 15 and which terminates in a second chamber 83 of the pilot valve 14. The chamber 83 is connected by means of a pipe 84 with a chamber 85 formed in the air control unit. This'pressure acts through a pair of -liquid filled bellows 86 and 81 to move these bellows and a rod 88 which is fastened thereto. The rod extends across the unit and into engagement with a second bellows 81a that forms one wall of a second liquid lled chamber whose other wall is formedby a bellows 88a. The liquid filled bellows chambers are connected by means of a pipe 95 having a variable restriction 96 in it.

As the pressure in chamber 85 follow-up movement is given to the flapper 1|.

This is accomplished by meansof the action of the rod 88 and a projection 89 thereon on a first lever 90 that is pivoted at 9|, a pin 92, and a second lever 93 that is pivoted at 93a, which second lever supports the three-armed lever 81.

The pin 92 is attached to a supporting arm 94 and may be moved upwardly and downwardly between levers 90 and 93 to change the throttling range of the instrument. As the le'ver 93 is moved, the ilapper is shifted in a direction opposite to its original movement in order to bring the instrument back to equilibrium. This second or follow-up movement is followed by a third compensating or reset movement as the pressure of the liquid inthe chambers between bellows 8S and, 81. and bellows V86a and 81a is equalized through the connection 95. The speed at which this reset movement can take place is dependent upon the adjustment of the restriction 98.

The total amount of follow-up movement that a-can be obtained may be limited by the amount of movement that can be imparted to the lever is varied, va`

93. To this end the lever 93 is formed with a pair of ngers 91 which extend on either side of a shaft 98. Each of these fingers has adjustably attached to it a member 99 that can be used to, in effect, change the spacing between the ngers 91 and the shaft v98.

In the operation of the instrument, if for example, the temperature of the hot blast has been increased, thepen 58 will be moved in a counterclockwise direction outwardly across its chart. This same movement will cause the left end of the differential lever 62 to be lowered so that link 83 will move lever 61 in a clockwise direction. Such a movement will move the pin 69 to the left so that apper 1| will be moved away` from nozzle 12 permitting more air to escape from this nozzle with a consequent decrease in pressure in the chamber 13. The bellows 11 and 18 will accordingly be elongated so that nozzle 19 will move upwardly away from the valve 8| permitting air to escape from chamber 83 to the atmosphere. The escaping of this air will also cause a reduction of pressure in the chamber 85 therebyV permitting the rod 88 lwith its projecto the right. The levers 98 and g tion 89 to move 93 and the pin 92 will therefore cause a move- .l ment of the pivot point 68 and lever 61 to the right so that the ilapper 1| will be given a followup movement toward the nozzle 12. If the temon-oi instrument, since no further follow-up can be obtained.

In alike manner, a decrease in the temperature of the hot blast in main 6 will cause a reverse operation to that above described so that an .ine

- crease in pressure is obtained throughout the system. with the lever 493 moving in a counter-` clockwise direction to an extent limited by engagement between the upper member 99 and the shaft 98. It will therefore be seen that an airpressure corresponding to the temperature of the hot blast will be set up in the chamber 83 of the pilot valve 14, and this pressure will be varied as the hot blastchanges in temperature within reasonable limits on either side of its control point. When the temperature changes beyond these limits, however, further follow-up movement cannot be obtained due to engage-y ment of one ofthe members 99 with the shaft' 98. Thereafter, the pressure in the chamber 83 will go immediately to one of its limits. The amount of deviation from the control point which is permitted the temperature of the hot blast prior to the time that the members 99 engage the shaft 98 can be varied by adjusting these members on their supporting flngers 91. Air is also supplied from chamber 83 through a pipe |0| which is connected to pipe 25 and to the instrument 21.

The above description of the air control connections was made with the assumption that no by-pass panel 24 was used. If such a panel is used as is shown in Figure 2 the operation of the instrument will be the same, but the pipe connections will be slightly different. lIn this case, as shown in Figure 5, the supply pipe 2|"to the panel 24 has in it a pressure regulator .192y and has connected to it a pressure gauge |83. pipe 2| terminates in one connection of a fourway valve |04 to which the'pipes 15 and ||l| .and-

a fourth pipe are also connected. 'I'he pipe |08 has two branches, the first one 84 connects with the follow-up provisions of the air control unit while the second branch is the pipe 25 leading toward the instrument 21.

The valve |04 is shown in Figure 5 in the position it will assumewhen the control is automatic. Air is supplied through pipe 2| the valve |04 and pipe 15 to the air control unit. Air is supplied from the pilot valve through pipe |0| through the valve |04 to the air control unit and to the instrument 21. If it is desired to control the process manually from a point near the instrument i9 the valve |04 is rotated 90 in either direction. This will connect pipes 15 and |0| so thatthe pilot valve will have no function in the apparatus. At the same time, pipes 2| and |05 are connected so that the pressure applied through pipe 25 may be manually adjusted by manipulation of the pressure regulator |02. At this time the operator will use the pressure gauge |03 to *determine the adjustment of the pressure in pipe 25. It is noted that because of the arrangement ofthe pipes I4 and 25, air will be applied to the followup and reset provisions of the air control unit both when the process is on manual as well as i when it is on automatic control. The reason for thisffis to prevent reset from building up in the instrument so that it will control properly when Y the `system is returned to automatic control. It may be advisable at times to control the process 'anually for various reasons, but it is particularl important that a by-pass panel of this type used with a potentiometer, since when the instrument is being standardized, the control will be upset unless the air pressure from the instrument is disconnected from the controlled valve. Pressure is applied through pipes 25 and 26 `to a receiving unit |06 in the instrument 21. This receiving unit consists of an expansible chamber |01, one wall of which is formed by a bellows |03 that is biased toward its elongated position by means of a spring |09. Received within a socket on the end of the bellows |08V is a bellows rod whose upper end is connected with a lever ||2 that is pivoted at ||3 and ythat is biased in a clockwise direction by means of a spring ||4. rThe other arm of the lever ||2 is connected by means of a link 5 to the lower end of a curved arm ||6 that issattached to a pivoted shaft ||1. Also attached to the shaft are a pointer ||3 which moves across a suitable scale ||0 on this instrument and a pair of arms ||9 4and |23. The latter arms are connected respectivelyV by links |2| and |22 with the left end of differential levers |23 and |24. The former of the differential levers or lever |23 has its right end attached to a segment |25 that is pivoted at |26. This segment has fastened to it an index |21 so that as the segment is rotated by means of gearing |28, the index 21 will be moved across the scale ||0. The right end of the'differential lever |24 is attached to the end of an arm |29 whose left end has also a segment formed on it. The arm |29 is formed with a pointer |30, which pointer is moved with the lever as the Agearing |3| is rotated. The differential lever |23 is connected by a link |32 to an air control unit 33. The differential lever |24 is connected by means of a link |34 toan air control unit |35. These air control units are similar and a description of one will suffice for the purposes of this disclosure.

The lower end of the link |34y is connected to one arm oi' a lever |38 which is in turn pivoted at point |39. The lever |36 is provided with a pin 4| that serves to move a fiapper |42 against its normal bias and away from a nozzle |43 or which serves to permit the iiapper to move toward the nozzle. This nozzle is connected by means of a pipe |44 to a pilot valve |45 which is identical in form with the pilot valve 14 that was described above in connection with instrument I9. As air pressure in the pilot valve chamber corresponding to chamber 13 is varied due to a change in the ilapper position, a variable pressure is applied through a pipe |46 to a chamber |41 in the control unit |35. A wall of this chamber is formed of a bellows |48 that has attached to its end wall a rod |49, that is axially moved as the pressure in the chamber |41 is varied. Movement of the rod serves to give a follow-up movement to the flapper. This is accomplished by having a pin |5 on the right end of the rod |49 bear against one side of a cam |52 that is pivoted at |53. The right side of the cam acts on a pin |54 projecting from the rear surface of a lever |55 that is pivoted at |56. Movement of the lever |55 is imparted to the lever |38 through a pin |51 projecting from a supporting arm which pin extends between the edges of levers |55 and |38. The support |58 for the pin |51 is mounted on a rack member |59 that may be raised and lowered by means of a pinion |60 to shift the position of the pin relative to the levers 55 and 38. It is noted that the cam |52 may be raised and lowered to adjust the distance between pins |5| and |54 by rotating around its pivot a member |6| upon which the cam is supported. The member |6| is held in place by a clamp screw |62 that extends l through an arcuate slot in the support |6I.

The pipe 28 extends from the pilot valve |45 to the powerv positioning mechanism 3| for the valve I4. A second pilot valve |64 is located in the instrument 21 and cooperates with the unit |33 in exactly the same manner that the pilot valve |45 cooperates with the unit |35. The pipe 29 extends from the pilot valve |64 to apply variable pressure air to the piston positioning unit 32 for the valve I6.

v It will be vnoted that a change in the pressure supplied to the unit |06 will actuate the lever system in such a fashion that the units |33 and |35 will be operated in opposite directions. vIi

for example the pressure is increased in chamber |01, the shaft ||1 will Ibe movedin a counterclockwise direction so that the left ends of levers |23 and |24 will be moved downwardly. This movement of the .lever |23 will shift link |34 Jdownwardly to move flapper 42 of the-unit |35 away from the nozzle |43, therefore decreasing the pressure supplied-by that unit. In a like manner, a downward movement of the link |32 by the lever |24 will operate the ilapper of the unit |33 in a direction to permit this dapper to movev toward the nozzle of that unit, and thereby .increase the pressure that is produced thereby.

pass pipe |5. When a new stove has been'put on y the line its temperature is very high so that only a small proportion `of the air will flow through the stove then being used, whereas a large 'proportion of the air will flow through the by-pass. As the stove cools down a larger proportion of the air will have to be forced through the stove lin because of the fact that the units |33 and |35 are 4 purely throttling instruments which serve to set up a pressure which is proportional to the position of the actuating lever and the adjustment of the throttling range of the unit. The throttlins range of each of the units |33 and |35 may be adjusted by moving the pin |51 upwardly or downwardly with respect to levers |55 and |33. The higher the pin the smaller the throttling range. In other words the higher the pin |51 is between the two levers |55 and |38 the smaller the change in pressure in thechamber |41 that is necessary to move the lever |38 through its full range. When the unit |35 is adjusted so that its throttling range is 100%, this means that a maximum pressure change in chamber |01 will produce a complete pressure change by the unit |35 for full scale movement of the instrument. If the throttling range of the unit |35 is at 50%, this means that a maximum change in pressure in the chamber |01 will cause the unit |35 to produce `its maximum pressure change through one half of the scale of the instrument.

Suppose it is desired to have the valve lI4 begin to open only after the valve I3 has -completely closed. In such a case the index for the unit |33 would be 25% of the scale while the index for the unit |35 would be set to '15% of the scale and the throttling range of each unit would be set for 50%. Therefore, starting from a minimum and changing to a maximum pressure in the chamber |01, the unit I 33 would operate` to move the valve I3 from its completely open to'its completely closed position, and thereafter the unit |35 would operate to move the valve I4 from its completely closed to its completely open position. Generally speaking, however, the instrument is so arranged that there will be overlapping of the operation of valves I4 and I3. This means that the control indices |21 and |30 would .be moved respectively to slightly above 25% and slightly below 75% of the scale ||0,

vwhilev the throttling range of the two units |33 and |35 would be set to slightly more than 50%. Therefore, during the operation of the instri'- ment, the unit |35 would begin to increase the pressure in pipe 28' vbefore the unit |33 had reached its maximum pressure so that the valve I4 would begin to open before the valve I6 had completely closed.

The above operation will continue until such time as the temperature of the stove which is then on thelne has been reduced to a point at which it is unable to maintain the temperature of the hot blast. The attendant of the blast furnace system will then close off .that stove and .put one of the vhot'stoves on the line. When this operation takes place the valves I6 and I4 whichy have each been gradually closed and opened respectively must be rapidly reversed in their positions in order to properly control the temperature of the hot blast. When the hot stove is placed on the line there will be a rapid rise in the temperature of the hot blast above the control point for which the instrument I 3 has been set. When the temperature ofthe blast reaches a vcertain number of degrees above the control point, one of the members 90 will engage the rod 93 on that instrument to give that instrument, in effect, an on-of! operation so that the air pressure 'will go completely to one of its limits substantially instantaneously. The pressure in the chamber |01 will then be rapidly changed from one of its limits to the other so that the valves I4 and I5 will be rapidly operated Y to their opposite positions in order that they may control the flow of air through the new stove inl a manner above described. Thereafter the operation will be such that the valve I5 gradually closes as the valve I4 is gradually opened.

From the above description it will be seen that I have provided instruments for and a system maintaining constant the temperature of the hot blast for a blast furnace in such a fashion that the valves will be either sequentially or simultaneously operated to maintain the temperature of the hot blast at some desired value. It will also be seen that I have provided a temperature responsive instrument which will act to moverapidly the valves from one extreme position to the other when a change in stoves takes place. The ability of the instruments of my .invention to rapidly reverse the pition of the valves I4 and I5 when a new stove is placed on the line is of the utmost importance, since it will quickly bring the temperature of .the hot blast back to the desired point with a minimum upset when a new stove is placed on the line. v While in accordance with the provisions ofthe statutes, I have illustrated and describedthe best form of my invention now known to' me, it will be apparent to those skilled in the art that changes may be made inthe form of the apparatus disclosed without departing from the spirit of my invention as set forth in the kappended claims. and that in some cases certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure byletters Patent is:

l. In a hot blast control system having means including a plurality of paths to supply air to a throughout a given range, means responsive to each one of said last mentioned air pressures and operative to adjust one of said control devices, and means responsive to the air pressure set up by said air control mechanism to adjust said air control instrument.

2. In a blast furnace control system having means including a plurality of paths through which air may be supplied to a blast furnace, one of said paths having in it means to heat the air passing therethrough, the combination of a control device in each path operative to vary the amount of air passing through that path, means to measure the temperature of the air after it has passed through said paths, a control mechan'ismV responsivey to said measuring means and operative to set up an air pressure proportional to the value of the temperature of the' air, an air control instrument operative to set up a plurality of air pressures each of which may vary throughout a given range of saidinstrument, means to .vary individually the point in the range of said instrument where each oi' the pressures produced thereby will be at a maximum and a minimum, means responsive to each of the air pressures produced by said air control instrument to adjust said control devices individually, and means to Y apply the air pressure produced by said air control' mechanism to operate said air control instrument.

' 3. In awhot blast control system having means including` a plurality of paths through which air is supplied to a blast furnace, one of said paths having means in it toheat the air passing thereof its limits, an Aair control instrument operative' to set up a plurality of air pressures. means to operate each of said control devices from one of said last mentioned air pressures, and means to transmit the air pressure set up -by said air control mechanism to said air control instrument to operate the latter.

4. In a hot blast control system having means including a plurality of paths through whichtair is supplied to a blast furnace. one of said paths having means in it to heat the air passing therethrough, the combination of a control device in each path operative tq vary the amount of air passing through that path, means to measure the temperature of the air after it has passed through,

said paths, a control mechanism responsive to said measuring m ns and operative to set up an air pressure val ing throughout a given range as said temperature varies, means in said mechanism to run said air pressure to the end of its range immediately upon the deviation of said temperature beyond a given amount from a predetermined control point, an air control instrument operative to set up a plurality of air pressures each of which may vary through a given range, regulating means j operated individually by the air pressures set up by said control instrument to adjust said control devices, means in said instrument to vary theI point in the range thereof at which the air pressures set up thereby may be a maximum and, a

' minimum whereby the points at which each of said control devices is operated may be varied, and means to operate said control instrument by the air pressure produced bysaid control mechanism.

5. In a hot blast control system having means to supply a given amount of air -to a blast furnace including a plurality o! paths through which the air passes. one of said paths having in it a means to heat the air passing therethrough, the combination of control devices in said paths operable to vary the proportion oi' the total air passing through each path, mechanism to measure the temperature of the air and set up an air pressure proportional thereto, pressure responsive means to operate said control devices. an air control in-4 strument provided with a plurality oi pressure regulating units each operative to supply an air 'pressure through av given range to said pressure responsive means, means to apply the air pressure produced by said mechanism to said air control instrument to operate the same, 'and means to adjust individually each oi said units so that they will vary the air pressure produced thereby for given values oi air pressure produced by said mechanism.

. ANKER E. KROGH.

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