US2877281A

US2877281A - Electric furnace regulators

Info

Publication number: US2877281A
Application number: US651393A
Authority: US
Inventors: Eaton Milton
Original assignee: Shawinigan Chemicals Ltd
Current assignee: Shawinigan Chemicals Ltd
Priority date: 1957-04-08
Filing date: 1957-04-08
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

March 10, 1959 M. EATON ELECTRIC FURNACE REGULATORS 2 Sheets-Sheet 1 Filed April 8, 1957 VOLTAGE ADJUSTER VOLTAGE STABILIZER Inventor M EATON AGENT M. EATON ELECTRIC FURNACE REGULATORS March 10, 1959 2 Sheets-Sheet FIG. 3

Filed April 8, 1957 Inventor M. EATON AGENT United States Patent ELECTRIC FURNACE REGULATORS Milton Eaton, Shawinigan Falls, Quebec, Canada, as-

signor to Shawinigan Chemicals Limited, Montreal, Quebec, Canada, a corporation of the Dominion of Canada Application April 8, 1957, Serial No. 651,393

8 Claims. (CI. 13-43) This invention relates to improvements in automatic electric furnace regulators of the current-voltage balance type.

Conventional regulators of the class referred to operate on the principle of balancing the electrode current with the voltage between the electrode and hearth in such a way that, if the control effect'indicated by the ratio of current to voltage becomes greater or less than unity, the electrode is moved in the direction to return this ratio to unity. Regulators of this class are sometimes referred to as balanced beam regulators or im pedance regulators.

An advantage of this type of regulator is that the electrode currents are held substantially uniform within limits and at the same time the electrode tips tend to be maintained at a uniform level. For example, if the length of the arc in one phase of a 3-phase are furnace becomes greater than that of the'other two phases, then the corresponding voltage between the lower end of each electrode and the hearth becomes proportionally greater for the one phase and lesser for the other two phases. The regulator then functions to lower the high electrode and to raise the other two, thus equalizing the arc lengths and voltages between the electrodes and the hearth without changing appreciably the mean load current. If the regulator were made to respond to variations in the load current only, it would be possible to have the phase currents fairly well balanced but at the same time to have a considerable difference in the voltages between the electrodes and hearth. The resulting unequal distribution of power between phases would prevent normal furnace operation.

In an arc furnace it is also important that the arc lengths be held within limits such that an electrode is not allowed to operate in contact with the furnace charge; otherwise an undesirable transfer of carbon to the charge may result.

In a resistance furnace, an important consideration is that while maintaining substantially equal power distribution between phases there must be suflicient tolerance in the regulation to compensate for such differences as (a) nonuniformity in the resistance of 'the furnace charge, (b) unequal lengths of the electrodes, and (c) variations in the hearth level.

In this specification the hearth is understood to be the furnace bottom at or just below the tap hole level.

The hearth level is the upper surface level of the molten charge in the furnace.

Although the application of the conventional currentvoltage type of regulation is excellent in theory, the performance is not satisfactory for the following reasons in particular:

(l) The voltages between the electrode tips and hearth 1 2,877,281 Patented Mar. 10, 1959 other points, remote from the tips, for measurement of voltage, and the voltages between the hearth and these other points are fluctuating and seldom directly propor-' tional to or representative of the voltages between the corresponding electrode tips and hearth. Because the voltages between the hearth and the remote points are not truly indicative of the voltages between the hearth and electrode tips, the controller action of conventional current-voltage balance type regulators is seldom the specific controller action that is required.

(2) These voltages, especially of the open arcs in mult-i-electrode arc furnaces, are inherently unstable.

The principal objects of this invention are (l) to improve the performance of electric furnace regulators of the current-voltage balance type and (2) to provide control means to increase the power consumption in an electric furnace without increasing the maximum demand (i. e. to improve the load factor). A further object of the invention is to provide control apparatus which is simple in construction and operation. Other objects will become apparent from the description to follow.

The invention broadly consists, in automatic electric furnace regulators of the current-voltage balance type, in the improvement comprising, (1) means for providing a control reference voltage which is independent of the voltages between the furnace electrodes and the furnace hearth, said reference voltage being applied to the voltage control elements of the regulator connected in series, (2) means for adjusting the value of the said reference voltage, (3) variable impedances, each connected in parallel with a different one of the said voltage control elements, (4) means responsive to the vertical movement of the furnace electrodes for adjusting the impedance value of the said variable impedances where'- by the impedance value of each of the said variable impedances increases as its associated furnace electrode moves vertically in one direction and decreases as its associated furnace electrode moves vertically in the opposite direction, and (5) means for moving each of said furnace electrodes vertically in response to controller action initiated by the imbalance of the said current-voltage balance regulator.

More specifically, one embodiment of the invention comprises an apparatus for improving the performance of automatic electric furnaceregulators of the currentvoltage balance type, comprising, (1) a current-voltage balance relay for each furnace electrode, each relay having its current coil energized with electric current proportional in value to the current taken by its associated furnace electrode and its voltage coil connected in series with the voltage coils of the current-voltage balance relays pertaining to the other furnace electrodes, (2) .a control reference voltage independent of the voltages between the furnace electrodes and furnace hearth, said reference voltage being applied to the seriesconnected voltage coils of the said balance relays, (3) means for adjusting the value of the said control reference voltage, (4) variable resistors, each connected in parallel with a different one of the voltage coils of the said balance relays, (5) means responsive to the movement of the furnace electrodes for adjusting the resistance of the variable resistors whereby the resistance of each of the said variable resistors increases as its associated furnace electrode moves upwards and decreases as its associated furnace electrode moves downwards, and (6) means for moving each of said furnace electrodes vertically in response to controller action initiated by the imbalance of said current-voltage balance relays.

Also more specifically, another embodiment of the in vention comprises an apparatus for improving the performance of automatic electric furnace regulators of the current-voltage balance type comprising (1) a direct current electric generator associated with each electrode of a furnace to be regulated, each generator having two opposing field coils one of which is energized with direct current proportional in valve to the current taken by its associated furnace electrode and the other of which is connected in series with corresponding lield coils of the direct current generators associated with the other furnace electrodes, (2) a direct current control reference voltage independent of the voltages between the furnace electrodes and furnace hearth, said reference voltage being applied to the series-connected generator field coils, (3) means for adjusting the value of the said control reference volt age, (4) variable resistors each connected in parallel with a different one of said series-connected generator field coils, (5) means responsive to vertical movement of the furnace electrodes for adjusting the resistance of the variable resistors whereby the resistance of each of the said variable resistors increases as its associated furnace electrode moves upwards and decreases as its associated furnace electrode moves downwards, and (6) a direct current reversible motor associated with each direct current generator and providing motive power for vertical movement of the associated electrode, said motor receiving uniform field coil current from an independent source and receiving armature current from said associated direct current generator, whereby said motor rotates to move said electrode vertically in response to imbalance in the field coil currents of said direct current generator.

The invention will be understood from the following detailed description of preferred embodiments of it, reference being made to the accompanying drawings in which:

Figure 1 shows schematically a sectional view of a resistance furnace with control apparatus of the invention.

Figure 2 shows schematically electrical connections of the control apparatus.

Figure 3 shows schematically how the invention is applied to a regulator using hoisting gear of the Ward- Leonard type and Figure 4 shows schematically the adaptation of the invention to an Amplidyne control.

Figure 5 is a schematic wiring diagram showing connections of a double-pole, double-throw transfer switch mounted to provide alternative voltage supplies to the field winding of a motor.

The objects of. the invention are achieved by using a stabilized reference voltage, which is independent of the furnace voltages, to energize the voltage control elements of the current-voltage balance control apparatus, and varying the proportion of the reference voltage applied to each of the voltage control elements by impedance means which make the applied voltage proportions directly proportional to the distance between the respective electrode tips and the hearth.

The following characteristics are in principle common to the regulation of electric furnaces by regulators of the current-voltage balance type when the voltages applied to the regulator control elements are either proportional to the furnace voltages between the electrodes and hearth or are the reference voltages as obtained with the apparatus of this invention:

(1) As long as the distances between the lower ends of the electrodes and hearth (arc lengths in an arc furnace) remain equal, the voltages applied to the voltage control elements of the regulator remain equal and normal.

(2) If the distance betweenthe lower end of one electrode and the hearth of a 3-phase 3-electrode furnace becomes greater as less than that of the other two electrodes, the corresponding voltages applied to the control elements of the regulator become proportionally greater or less thannormal thus causing corrective controller ac 4 tion to lower the high electrode and to raise the low electrodes.

A sectional view of a typical closed resistance furnace, such as a calcium carbide furnace, is shown schematically in Fig. 1.

Electrodes

1, 2 and 3 penetrate the charge in the furnace 3%. Radial lines at the bottom of the electrodes indicate the passage of electric current through the charge to the hearth. Electrode 3 is indicated as being supported by contact shoes 4 and elements 7 and S which are connected with the electrode hoisting gear. Current is carried to the electrode by connections 5 and 6, the former being made flexible to permit electrode travel.

Current-voltage balance relay 14 has its current element 141 connected with the current transformer 22 and load-adjusting rheostat 23. The

current elements

151, and 161 of

relays

15 and 16 respectively are similarly connected with current transformers and. load-adjusting rheostats pertaining to phases (electrodes) 2 and 1 respectively. The current transformers, used to measure the electrode currents, can be located on either the primary or secondary side of the furnace transformers. The

voltage elements

142, 252, and 162

ofrelays

14, 15, and 16 respectively are connected in series through a voltage adjuster 21 and a voltage stabilizer 20 with an alternating current source R. V. A. C. (reference voltage A. C.). The voltage adjuster 21 is preferably of the autotransformer type such as a Variac or a Powerstat. An adjustable resistor 17-3 and a variable resistor 11-3 in series are connected in parallel with the voltage element 142 of relay 14. Similar adjustable resistors 17-2 and 17-1 pertaining to phases 2 and l are connected by

leads

24 and 25 in series with the variable resistors 11-2 andll-l (not shown) pertaining to electrodes 2 and 1 and in parallel with relay elements 152 and 162 respectively. The use of the adjustable resistors, in series with the variable resistors, is a feature which is optional and can be omitted if sensitivity adjusment is not required.

The movable contact Tall-3 of variable resistor 11-3 is connected with its associated electrode 3 by suitable means indicated by element 9 so that the resistance of resistor 11-3 is increased or decreased as the electrode moves up or down respectively. It is preferable that the range of resistance of resistor 11-3 be made to accommodate-the range of electrode travel during normal furnace operation. The resistance of the variable rcsistor 11-3 should therefore remain unchanged as its associated electrode is raised above its normal operating range of travel. This operation can be accomplished, as shown schematically, by the use of

extended contacts

12 and 13. LS1 and LS2 indicate limits of electrode travel as determined by limit switches. The position of moveable contact 10-3, relative to the distance between the lower end of the associated electrode and furnace hearth, is made adjustable. This is shown schematically by means 101 for adjusting the position of moveable contact 19-3 relative to element 9. This adjustment may be used to compensate for variations in the extension of the electrodes below the contact shoes 4. Similar apparatus is provided forelectrodes 2 and 1.

Initial settings of the control apparatus, prior to commencement of operation of the apparatus, are made as follows:

(1) Load adjusting rheostat 23, and the rhcostats per taining to phases 2 and 1 (not shown) are adjusted so that normal current passes through the current elements of

relays

14, 15, and 16 when the electrode current is at its desiredvalue.

(2) The voltage adjusting device 21 is adjusted so that the voltage. applied to the voltage elements of

re lays

14, 15, and 16, connected in series, is three times the normal voltage of the voltage element pertaining to each relay.

(3') The separation of contacts R and L of the

relays

14,15, and 16 are made to correspond with the desired differential gap, i. e., the range .of load current in which no corrective action is required.

(4) Resistors 17-3, 17-2, and 17-1 are'adjusted to obtain the desired sensitivity of the control relays in response to changes in the resistance of the variable resistors resulting from movement of the electrodes.

The control apparatus operates as follows:

Assuming the electrodes to be positioned in the furnace so that the distances between their lower ends and the hearth are equal, normal voltage will be applied to the voltage elements of

relays

14, 15, and 16. If the current taken by electrode 3 rises above normal, .the contact arm of control relay 14 becomes unbalanced and contacts R close to initiate upward movement of the electrode. If the current falls below normal, the relay is unbalanced to close contacts L, thus initiating downward movement of the electrode. Assuming that, in the operation of the furnace, electrode 3 reaches a level higher than electrodes 1 and 2, the resistance of variable resistor 11-3 will then be greater than that of the variable resistors associated with electrodes 1 and 2. For this reason the voltage applied to voltage element 142 of relay 14 will be greater than one third, and the voltage applied to each of the voltage elements of relays 1S and 16 will be less than one third, of the reference voltage supplied by voltage adjusting device 21. The control relays will therefore become unbalanced and function to lower electrode 3 and to raise electrodes 1 and 2, thus tending to bring them to the same level. Regulation of electrodes 1 and 2 is the same as that of electrode 3. Y Figure 2 shows schematically the electrical connections of control apparatus for one of the electrodes. Leads 31 and and 32 are connected with a suitable source of control power supply. If contacts L of control relay 14 close to initiate control action to lower electrode 3, relay R1, with which limit switch LS1 is in series, becomes energized. Contacts R1-1 then close to complete a holding circuit in series with normally closed contacts R2-2 of relay R2 and normally closed contacts T-l of time relay T. Contacts R1-2 of relay R1 open to prevent relay R2 being energized at the same time as relay R1. Contacts R1-3 close to energize time relay T, and contacts R1-4 of relay R1 close to energize magnetic switch S1, which in turn controls the motive power to lower the electrode. The electrode continues to lower for a period at least as long as the timing period of time relay T, at the termination of which contacts T-1 open to break the holding circuit of relay R1. If contacts L of control relay 14 are then open, the control apparatus is deenergized and the electrode comes to rest. A similar operation to raise the electrode occurs if contacts R of control relay 14 close; in this case however, relay R2 in series with limit switch LS2 becomes energized. Contacts R2-1 close to complete a holding circuit in series with normally closed contacts R1-2 of relay R1 and normally closed contacts T-1 of time relay T. Contacts R2-2 of relay R2 open to prevent relay R1 being energized at the same time as relay R2. The time relay T is energized through contacts R2-3, and contacts R2-4 of relay R2 close to energize magnetic switch S2 controlling the motive power to raise the electrode.

I Identicalcontrol apparatus is used to control the movement of each of the other electrodes.

, The time-relay holding circuits prevent repeated operations (chattering) of relays and magnetic switches which would otherwise occur when the contacts R or L of the control relays are making imperfect contact. These circuits are an improvement of the invention of earlier U. S. Patent No. 2,026,617. An advantage in using timerelay holding circuits for this purpose is that the timing period of the time relays may be adjusted to correspond with the rate at which the controller condition is returned to the control point when the electrode hoisting gear is in operation;

According to ASME terminology, the regulator described above provides single-speed floating controller action with a neutral zone. This type of controller action is satisfactory for resistance type furnaces, such as calcium carbide furnaces, in which the electrode current changes slowly. For certain arc furnaces, which are subject to rapid changes in load current, it is desirable to provide proportional-speed floating controller action. Figures 3 and 4 show the application of the invention to regulators of this class. 1

Figure 3 shows schematically the application of the invention to a regulator including electrode hoisting gear of the Ward-Leonard type. Motor M-13, connected with an alternating current power supply A. C., through switch 19, drives a direct current generator G-3 at a constant speed. The generator has two field windings 14-3 and 15-3 connected in opposition respectively with current transformer 22 and a source of direct current reference voltage R. V. D. C. Current from current transformer 22, adjusted by load adjusting rheostat 23, is rectified by a suitable rectifier 40. The reference voltage is adjusted to a suitable value by means of a potentiometer 18. E and F indicate connections to field windings 15-2 and 15-1 (not shown) of the generators for the hoisting gear of electrodes 2 and 1 respectively, which field windings are connected in series with field winding 15-3 of generator G-3. Winding 15-3 is in parallel with series connected resistances 17-3 and 11-3, and similarly winding 15-2 is in parallel. with resistances 17-2 and 11-2 and winding 15-1 is in parallel with resistances 17-1 and 11-1. The proportion of the direct current reference voltage applied to each of the generator field windings connected in series is determined by the operation of variable resistors 11-3, 11-2, and 11-1 which are automatically adjusted by movement of moveable contacts 10-3, 10-2, and 10-1 in accordance with electrode movement as described with reference to Fig. l for moveable contact 10-3. Adjustable resistances 17-3, 17-2, and 17-1 are in series respectively with variable resistors 11-3, 11-2, and 11-1 as in Fig. 1..

Direct current motor M-3 receives uniform D. C. current for its field coils 61 from an independent source and receives its armature current from generator G-3. Motor M-3 controls the movement of electrode 3 by hoisting gear shown schematically as a cable 35 passing over pulleys 36 and 37, and connected with the electrode and a winch 34 driven by motor M-3. Electrode 3 receives its current through connections 5 and 6.

When the field strengths of field coils 14-3 and 15-3 are equal, generator G-3 develops no voltage and motor M-3 remains at rest. When the field strength of field coil 14-3 exceeds that of field coil 15-3, generator G-3 generates a voltage of polarity such that motor M-3 rotates in a direction to raise electrode 3. When the field strength of field coil 15-3 exceeds that of 14-3, a similar controller action occurs to lower the electrode. During either operation, the generator voltage and motor speed are proportional to the difference between the magnetomo'tive forces of the opposing field windings, thus proportional-speed floating controller action is obtained. Similar apparatus isused for each of electrodes 2 and 1 in furnace-30.

Figure 4 shows schematically the application of the invention using an Amplidyne generator as the power generator 6-3 for the hoisting motor M-3. Forconvenience, reference numbers identical with those in Figure 3 indicate equivalent parts. Forjsimplicity the motor driving the Amplidyne generator 6-3 is not shown.

Adjustment of the position of moveable contact 10-3 relative. to. element-9 (Figure 1) may be made to compensate .for unequal electrode lengths. This adjustment, however, is not practicable for making allowance for uneven surface levels in the charge of an arc furnace which occur at the beginning of a heat. For this reason it is usually preferable to use the voltage between the.

electrode tips and the hearth for the voltage control elements of the regulator during the melting period; when the charge is melted down sufficiently to form a bath of molten metal, then the voltage control elements are transferred to the reference voltage. Provision for this transfer is indicated in Fig. 4. Rectifier 50 provides a direct current voltage proportional to the voltage between the lower end of electrode 3 and the hearth. A double-pole, double-throw, transfer switch is provided for transfer of the terminal connections of field winding l-3 from terminals A1, B1 of the reference voltage supply to terminals A2, B2 of the direct voltage supply proportional to the voltage between each electrode tip and the hearth of the furnace. It is preferable that the three double-throw switches (one for each phase) should be gang-operated. To avoid confusion in the drawing these switches are omitted, but the connections for one such switch are shown in Figure 5. D. C. voltage proportional to the voltage between electrode 3 and the hearth is supplied to terminals A2, B2 by rectifier 50. D. C. reference voltage is applied to terminals A1, B1. Doublepole, double-throw transfer switch TS connects the terminals of field winding -3 either to terminals A2, B2 in one closed position, or terminals A1, B1 in the other closed position. The switch is shown in the open position.

Although the invention has been described as applied to regulators for 3-phase electric furnaces having one electrode per phase, it is applicable to any number of electrodes or any number of phases in any multi-electrode furnace. Obviously there must be at least two electrodes to be kept in balance.

It should be understood that the illustrations are schematic and that the invention is not confined to the use of such apparatus as is indicated on the drawings. For example, various means may be employed for automatically changing the resistance of variable resistor 1L3, Fig. l, in response to electrode movement. The con nection between the electrode hoisting gear and moveable contact 10%, indicated by element 9, may be of a design suitable for remote operation or it may be a direct connection with part of the hoisting gear.

The function of the variable resistors 11-3, 11-2, and 11-4. is to change the resistance or the impedance of the circuit between the terminals of the voltage control elements with which they are connected. If the control reference voltage is alternating, other means, for example variable reactors, can be used for this purpose.

What I claim is:

1. In an automatic electric furnace regulator of the current-voltage balance type, the improvement comprising, (1) means for providing a control reference voltage which is independent of the voltages between the furnace electrodes and the furnace hearth, said reference voltage being applied to voltage control elements of the regulator connected in series, (2) means for adjusting the value of the said reference voltage, (3) variable impedances, each connected in parallel with a different one of the said voltage control elcmcnts, (4) means responsive to the movemerit of the furnace electrodes for adjusting the impedance value of the said variable impedances whereby the impedance value of each of the said variable impedances increases as its associated furnace electrode moves vertically in one direction and decreases as its associated furnace electrode moves vertically in the opposite direction, and (5) means for moving each of said furnace electrodes vertically in response to controller action initiated by the imbalance of the said current-voltage balance regulator.

2. An apparatus for improving the performance of automatic electric furnace regulators of the current-voltage balance type, comprising, (1) a current-voltage balance relay for each furnace electrode, each relay having a current coil energized with electric current proportional in value to the current taken by its associated furnace assign electrode and a voltage coil connected in series with voltage coils of the current-voltage balance relays pertaining to the other furnace electrodes, (2) a control ref erence voltage independent of the voltages between the furnace electrodes and furnace hearth, said reference voltage being applied to the series-connected voltage coils of the said balance relays, (3) means for adjusting the value of the said control reference voltage, (4) variable resistors, each connected in parallel with a different one of the voltage coils of the said balance relays, (5) means responsive to the movement of the furnace electrodes for adjusting the resistance of the variable resistors whereby the resistance of each of the said variable resistors increases as its assoicated furnace electrode moves upwards and decreases as its associated furnace electrode moves downwards, and (6) means for moving each of said furnace electrodes vertically in response to controller action initiated by the imbalance of said current-voltage balance relays.

3. An apparatus according to claim 2 including adjustable resisors each connected in series with a different one of the variable resistors, for sensitivity adjustment.

4. An apparatus according to claim 3, including means for stabilizing the said control reference voltage.

5. An apparatus for improving the performance of automatic electric furnace regulators of the current-voltage balance type comprising (1) a direct current electric generator associated with each electrode of a furnace to be regulated, each generator having two opposing field coils one of which is energized with direct current proportional in value to the current taken by its associated furnace electrode and the other of which is connected in series with corresponding field coils of the direct current generators associated with the other furnace electrodes, (2) a direct current control reference voltage independ ent of the voltages between the furnace electrodes and furnace hearth, said reference voltage being applied to the series-connected generator field coils, (3) means for adjusting the value of the said control reference voltage, (4) variable resistors each connected in parallel with a different one of said series-connected generator field coils, (5) means responsive to vertical movement of the furnace electrodes for adjusting the resistance of the variable resistors whereby the resistance of each of the said variable resistors increases as its assoicated furnace electrode moves upwards and decreases as its associated furnace electrode moves downwards, and (6) a direct current reversible motor associated with each direct current generator and providing motive power for vertical movement of the associated electrode, said motor receiving uniform field coil current from an independent source and receiving armature current from said associated direct current generator, whereby said motor rotates to move said electrode vertically in response to imbalance in the field coil currents of said direct current generator.

6. An apparatus for improving the performance of automatic regulators of the current-voltage balance type for electric furnaces having a plurality of electrodes, comprising: l) a current-voltage balance regulator for each electrode, each regulator having a current coil energized with current proportional to the current through its associated electrode and a voltage coil connected in series with voltage coils of the current-voltage balance regulators of the other electrodes, (2) means for providing a reference voltage independent of the voltages between the furnace hearth and the electrodes, said reference voltage being applied to the series-connected voltage coils of the said balance regulators, (3) means for adjusting the said reference voltage, (4) variable impedances each connected in parallel with a difi erent one of the voltage coils of the said balance regulators, (5) means responsive to the movement of the electrodes for adjusting the impedance values of the said variable impedances whereby the impedance value of each of the said variable impedances increases as its associated furnace electrode moves upwards and decreases as its associated electrode moves downwards, and (6) means for moving each of said electrodes vertically in response to controller action initiated by imbalance of the said current-voltage balance regulators.

7. An apparatus according to claim 6, wherein the current-voltage balance regulators are current-voltage balance relays and the variable impedanccs are variable resistors.

8. An apparatus according to claim 6 wherein the reference voltage is a direct current voltage, the variable impedances are variable resistors, the means for moving each electrode is a direct current reversible motor associated with a direct current generator having two opposing field coils, said motor receiving uniform field (oil current from an independent source and receiving armature current from said associated direct current generator, and the current coil and the voltage coil of said balance regulator are the two opposing field coils of said direct current generator, whereby said motor rotates to move the associated electrode vertically in response to imbalance in the field coil currents of said direct current generator.

References Cited in the file of this patent UNITED STATES PATENTS 1,241,499 Dixon Oct, 2, 1917 1,573,095 Saklatwalla ct a1 Feb. 16, 1926 1,901,426 Young Mar. 14, 1933 2,026,617 Eaton Jan. 7, 1936 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,877,281 March 10, 1959 Milton Eaton It is hereby certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 5 for valve" read value line '72, for greater as" read greater or column 4, line '74, for "separation" read separations column 5,- line 36, for "control" read controller column 8, line 45,- for "assoic'ated" read associated Signed and sealed this. 25th day of August 1959:.

( SEAL) Attest:

KARL H. AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION N,

Patent should read as corrected below.

Column 3, line 5, for "valve" read value line '72-, for "greater as" read greater or column 4, line 74, for "separation" read -separations column 5, line 36, for "control" read controller column 8, line 45, for "assoicated" read associated Signed and sealed this 25th day of August 1959,.

( SEAL) Attest:

KARL H. AXLINE ROBERT c. WATSON Attesting Ofiicer Commissioner of Patents