US2458947A

US2458947A - Method and means for improving blast furnace operations

Info

Publication number: US2458947A
Application number: US491822A
Authority: US
Inventors: Clarence D King; Peer D Nielsen
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1943-06-22
Filing date: 1943-06-22
Publication date: 1949-01-11
Anticipated expiration: 1966-01-11

Description

FIG- 1- Jan. 11, 1949. c. D. KING ETAL 2,458,947

' IETHOD AND MEANS FOR IMPROVING BLAST FURNACE OPERATIONS Filed June 22, 1943 2 Sheets-Sheet 1 if v i;

' I r E I Jan. 11, 1949. c. D. KING ETAL 2,458,947

METHOD AND MEANS FOR IMPROVING BLAST FURNACE OPERATIONS Filed June 22, 1943 2 Sheets-Sheet 2 i l l I i WY I I J u% C) k w" w w 1 1 Q I Q l i [L 0 A I x 1 C) E INVENTORS: b; QfiPf/Vff A K/A fi 40/ a P559 Q Mflfif/V Val Patented Jan. 11, 1949 Manson AND MEANS roa mrhovmo ams'r FURNACE orammons Clarence D. King, Pgtsburgh, 2a.; and Peer D.

1 Nielsen, Lorain,

lilo,

assignora to United States Steel Corporation of Delaware, a corporation of Delaware Application June 22, 194:, Serial No. 491,822

, 11 Claims. 1

The present invention relates to an improved method and means for improving blast furnace operations. More specifically, it contemplates a novel combination of mutually interdependent instrumentalities effective to determine the humidity of the-air in the region of the cold air supply and to automatically vary the temperature of the hot-blast of air supplied to blast furnaces, in a manner to compensate for fluctuations in the determinedhumidity content of the cold air supply, so as to automatically reduce or nullify the normally detrimental effect on furnace performance due to variations in the moisture content of the air. Thus theinvention provides for maintenance of optimum blast furnace operating conditions automatically at all periods, regardless of fluctuations in humidity due to changing weather conditions at all seasons of the year.

It is well known to those-skilled in the art that in normal blast furnace practice, in smelting either ferrous or nonferrous metals, the air required for'combustion of the carbon used is preheated inlarge heat exchangers called stoves, containing checker brickwork or tubular fines of various types. The volume of air used in a modern blast furnace for smelting iron ore normally constitutes, as measured by weight, a greater total than all other materialsentering the furnace. For example, the weight of air in a conventional iron blast furnace may range from 3 to 5 tons of air per ton of iron. Variations in the moisture content orhumidity of the air are very quickly reflected in, the change of furnace temperature conditions and furnace practice. The decomposition of moisture in the blast furnace hearth absorbs heat. and since this moisture content may be as high as 50 tons or more per day, a very material amount of heat energy is required to compensate for the introduction of water entrained in the air blast supplied to the furnace. The introduction ofmoisture with the air blast deleteriously affects the hearth, which is the most vital and sensitive zone of the furnace. It is highly desirable to maintain a substantially stable temperature in the hearth zone, so as to achieve smooth furnace operation and thus improve the control and quality of the product, increase production, and maintain minimum fuel consumption. To offset the deleterious effects of moisture introduced into a blast furnace, attempts have heretofore been made to maintain a dry blast so as toreduce the moisture content of the air and thereby obtain fuel economies and improved furnace operation.

It has also been proposed, in lieu of maintaining a dry air blast, to adjust furnace practice in two different ways to conform with changes in the humidity of the atmosphere. One of these proposed prior practices is commonly known as "cutting the burden. If the humidity increases in the atmosphere, the air used for combustion in the furnace will contain increased amounts of moisture. Since the decomposition of such moisture absorbs additional heat in the furnace hearth, one expedient resorted to has been to "cu" the burden, which means to reduce the ratio of the metallic bearing materials to carbon. In practice this has been accomplished by maintaining the coke input substantially constant, and reducing the input of metallic burden materials. However, such a cut in burden does not become effective until the materials, charged at the top, descend the entire height of the furnace stack and reach the hearth, in which region the carbon and solid fuel are burned by the oxygen of the blast-introduced at the tuyres. Since a typical blast furnace is about feet high, the interval of elapsed time between the introduction of the burden at the top of the furnace and its effectiveness upon reaching the hearth near the bottom, is about eight hours under ideal conditions. In normal practice the time lag frequently ranges from 10 to 12 hours. Consequently, any heat deficiency or excess would continue to exist in the furnace for an interval of eight hours or more if restoration of temperature equilibrium were entirely dependent upon changes in the components of burden charged to the furnace. Furthermore, we have determined that compensation for small changes in humidity by burden variations are ineffective, and small units of burden reduction do not produce corresponding changes in hearth conditions. If signiflcant or material humidity changes transpire within a short period, we have determined that it is impossible for the blast furnace operator to accurately and fully compensate for these changes by adjustments in the burden. Accordingly, such prior practice is successful only to the degree that the blast furnace operator can anticipate short interval variations in humidity. Hence both the human element and changing weather conditions enter into the operation. These variables, we have found, make it extremely difllcult and practically impossible to maintain optimum conditions at all periods of furnace operation.

The second method heretofore attempted for compensating the humidity changes in the air blast is to raise or lower the temperature of the air. thereby increasing or decreasing the sensible heat input directly into the hearth of the furnace. Since blast furnaces are normally provided with stoves yielding a hot air blast at a temperature somewhat in excess of that required for routine purposes, reserve heat is available by reducing the amount of cold air admitted to the hot blast line by manipulation of a conventional mixing valve. Accordingly, under prior practice, if humidity of the atmosphere increases, the blast temperature has been increased, and if the humidity decreases the blast temperature has been decreased. But We have determined from careful observation and analysis of blast furnace operations that it is impractical to constantly make the required changes in blast furnace temperature under manual control, in order to compensate for rapid changes in humidity. Such changes as have been made in practice heretofore were made on a somewhat hit-or-miss basis, and were made at relatively long spaced time intervals; thus during the periods between changes, material variations are apt to and'frequently do take place in the humidity of the atmosphere. It is apparent that since variations in humidity of the atmosphere may be very sudden and of considerable magnitude, the variable operating conditions cannot be reliably compensated for by manually adjusting controls to change the blast temperature.

Since the prior attempts at controllin conditions by means of burden adjustment are not adequate to compensate for rapid changes in humidity of the atmosphere, and since manually operated adjustments of blast temperature have been made too infrequently or haphazardly to be completely effective, it is apparent that the prior practices known to those skilled in the art have fallen short of solving the problem. In other words, heretofore, so far as we are aware, there has not been any known method or means for reliably and continuously maintaining optimum operating furnace conditions in a manner to compensate for variations of the moisture content or humidity of the atmosphere.

Accordingly the chief object of our invention is to provide an improved method and apparatus for automatically and continuously maintaining optimum operating conditions in a blast furnace to compensate for the moisture content or humidity of air introduced into the furnace.

An ancillary object of our invention is to provide a practical method and means for automatically ascertaining the moisture content of the atmosphere and adjusting the temperature of the air introduced into the furnace automatically in accordance with such moisture content or humidity of the atmosphere.

The novel characteristics of the method and combination of apparatus will be fully apparent from consideration of the following disclosure when read in connection with the accompanying drawings, and will be defined with particularity, in accordance with the requirements of the patent statutes, in the appended claims.

In the drawings:

Figure 1 is a diagrammatic view illustrating a blast furnace and related equipment suitable for making available the invention herein claimed.

Figure 2 is a view illustratin an alternative arrangement of equipment for practice of the invention.

In its broadest aspect, the present invention contemplates the provision of a novel met od, a d

means for correcting the hot blast temperature automatically, in accordance with the continuous moisture determination by means of suitable automatically operating equipment. As will hereinafter more fully appear, in both examples of the invention illustrated, the temperatures of the hot air blast supplied to the furnace at any desired level are automatically maintained by the use of means which continuously measure the humidity of the air and automatically actuate a. hot blast temperature control means, according to the moisture content of the air.

By calibration, the adjustment in temperature of the air can be made on any desired basis. For example, if it is determined that an increase of 50 F. is required for one grain of atmospheric moisture, this temperature increase can be accomplished automatically. Or a setting can be made for a 25 F. increase per grain of moisture or a 75 F. increase or any other ratio which the specific blast furnace practice may dictate is desirable. ,V

Referring now to the drawings, in both of the figures, turbo-blowers or blowing engines I deliver a supply of cold air through cold blast mains 2 to stoves 3 which function as heat ex changers. These stoves are of conventional construction known to those skilled in the art, and they contain masses of checkerbrick which are heated in the usual manner. The heated checker-brick form fiues through which the cold air supply is forced so as to be heated to a temperature slightly in excess of I the desired hot blast temperature. Cold air is added automatically in a manner later set forth so as to accurately control the temperature of the hot blast.

introduced into the furnace. Leading from the heat exchanger or stoves 3 is a hot blast main 4 which delivers the air to support combustion to the bustle pipe b of the blast furnace F. The; combustion air is supplied to the furnace from the bustle pipe through a plurality of tuyres t in the hearth region of the furnace in a manner controlling the temperature of the hot blast, is.

all conventional, and constitutes a component part of every modern blast furnace plant. These conventional items of equipment are not materially changed in the practice of our invention, except that means to be presently described in greater detail are provided for automatically actuating the mixer valve 6 in such a way that the amount or magnitude of flow of cold air bypassed from the cold blast main 2 to the hot blast main 4 is automatically and continuously controlled with great precision and in such a way that fluctuations in the humidity or moisture content of the air in the region of the inlet of the blowing engine are automatically and continuously compensated for to thus maintain optimum operating conditions in the hearth zone of the furnace by continually and accurately varying the temperature of the hot blast delivered to the furnace by and in accordance with fluctuations of humidity or moisture s pp content of the cold air Referring first to the embodiment of themvention shown in Figure 1, an air sample is continuously drawn oil through -a branch pipe or sampling line I, the volume of air withdrawn being regulated by manipulation of a suitable throttle valve 8. The air sample withdrawn is led to an insulated chamber 9 where it is maintained at a constant temperature by means of a steam heating coil it! which in turn is controlled In the lower portion of the differential so as to make available a wet and dry bulb effect which is a convenient manner or determining the humidity or moisture content of the air. The voltage thus developed by the thermopile is substantially or approximately. in direct proportion to the deviation from saturated air at the controlled temperature, and this-voltage is utilized for control purposes in a manner more fully set forth hereinafter.

An important feature of the invention resides in the installation of a thermocouple [3 or its equivalent located in the hot blast main 4, a potentiometer controller I! electrically connected with the thermocouple l6, and an operating motor l8 for actuating the mixer valve 3. The parts are so combined and coordinated with the thermopile N that they mutually cooperate to accurately control or vary the temperature oi the hot blast air introduced to the furnace, so .as to make adequate compensation for variations in the humidity or moisture content of the cold 'air supply.

As shown, a galvanometer relay It includes an element 20 movable with the galvanometer needle 2i capable of completing respective circuits through contacts 22 or 23 so as to cause I rotation of a reversible rotor 24 in either one direction or the other; thus the galvanometer controls the rotor 24 and motor l8 which does the actual required work.

In the operation of the combination of equipment shown in Figure 1, the voltage developed'by the thermopile it appears across the

terminals

25 and 26 of a sensitivity adjusting potentiometer 21, and a. portion of this voltage is picked up by the sliding contact 28 and added to the voltage developed by the thermocouple i8 located in the hot blast main 4.

For example, assume that the moisture content 0! the air can vary from approximately 20 grains per cubic foot (which corresponds to saturation at 100 F.) to 2 grains per cubic foot, and that a 25 F. variation in hot blast temperature is required to compensate for a one-grain change in moisture content of the air blast. Let us further assume that a hot blast temperature of 1400" F. is used with air having a moisture content of 20 grains. Now, assume that the potentiometer controller I! is set for 1400 F., and the constant temperature of the moisture sampling device is set at 100 F. Under such assumptions, with the 20 grains of moisture per cubic foot of air present in the sample withdrawn, said sample will be the thermopile H. The potentiometer controller I? will be affected under the assumed conditions only by the voltage set up by the thermocouple II, and will so control the motor l8 and the extent of movement of the mixer valve 6 that the hot blast temperature will be 1400 F. Now, assume that the humidity or moisture content drops to the minimum of 2 grains. The evaporative effect on the wet side of the thermopile M will drop the wet bulb temperature to 64 F., the dry bulb temperature remaining at 100 F., or in other words. therewill exist a temperature difference of 36 F. If it is desired to reduce the hot blast temperature by 25 times 20'grains less 2 grains (i. e., 450 F.), it is apparent that the hot blast temperature should be 1400 F. minus 450 F. or 950 F. Thus the sensitivity adjusting potentiometer 21 must be set so that the portion of the thermopile voltage equals the voltage developed by 450 F. on the thermocouple. Under such conditions, with the mixer valve 6 so adjusted that the blast temperature is 950 F., the additional voltage developed by the thermopile i4, and added to the voltage developed by thermocouple 16, is equivalent to 450 F., and the potentiometer controller I! functions to hold the 950 F. temperature, although set for 1400 F. The adjustment obtained is not absolutely or theoretically correct, as shown by the following tabu lation:

Moisture Actual Center}: ghermigiitlfe gi gig' Temp. oi tggg fi cu. m P s e p 1 Equivalent Hot Blast 0r F. F. F. F. 5 27% 344 1,056 1,025 10 17 213 1,18 1,150 15 7% 94 1,306 1,275

or a departure from theoretical of 1 to 1 /2 grains of moisture correction.

It will be obvious that complete control over the operation of the device is assured from the following:

The maximum temperature of the hot blast cannot exceed the temperature for which the potentiometercontroller I1 is set.

The greatest moisture content for which it is desired to compensate is established by control- As shown in Figure 1, the thermopile i4 is connected across the sensitivity adjusting potentiomsaturated, and no voltage will bedeveloped by eter 21 and the portion of the potentiometer between the fixed terminal 26 and the adjustable contact 28 is connected in series with the thermocouple i6, as shown.

The potentiometer controller indicated as a whole at I'I in Figure 1 includes the usual elements of battery 29, battery current adjustment rheostat 30, fixed resistor 3|, and slide wire 32. The fixed contact 33 which engages the slide wire 32 is connected electrically through the galvanometer i9 to the hot blast thermocouple i6 and thermopile it.

The slide wire 32 is in the nature of a resistance aasaou mounted .on the periphery cl 8. disc 34 secured to the shaft 35 of the rotor 24. The rotor shaft 35 also carries a control cam 36 having a rise portion 31 and a-depre'sscd portion 38. This cam coacts with a. follower 39 carried on the extremity of a control arm 43'pivotally secured to a stub shaft 4| carried by a gear sector 42 mounted loosely on the shaft 34. The angular position of I this sector can be varied by a manipuating and motor i8 by circuit wires 48 and 43. The contact arm 45 is connected by wire 50 with one side 5i of the power supply line. Similarly the motor I8 is connected with the other side 52 of the power supply line by a wire 53. As thus arranged, when the cam follower on the control arm 40 coacts with the depression 38 of the cam 36, the motor I8 will turn in one direction, and when the rise 31 of the cam coacts with said follower, the motor l8 will turn in the opposite direction. In normal operation, the rheostat is adjusted so that a known standard magnitude of current flows in circuit through the slide wire 32 and fixed resistor 3|. Then the voltage produced by the hot blast thermocouple l6 plus the voltage across the portion of the sensitivity adjusting potentiometer 21 are balanced against the voltage drop across that portion of the slide wire 32 between contact 33 and fixed resistor 3! plus the voltage drop across the fixed resistance. It said two voltages are unequal, the galvanometer needle deflects and completes a circuit through either contact 22 or 23, thus energizing rotor field coils 22H or 23L, causing the rotor 24 and slide wire 32 to turn, thus varying the eflective resistance of the slide wire so as to balance or equalize the two voltages. Consequently the galvanometer returns to neutral position (out of contact with 22 and 23), deenergizing the field coils of rotor 2a.

With the galvanometer needle in neutral position, the angular rotary movement of the slide wire bears a definite relation to the voltage developed by the thermocouple 16. Thus the galvanometer I9 can be calibrated to read directly in degrees by choosing slide wires and fixed resmtors of appropriate values. With the parts arranged and coacting as above described, it is apparent that when the temperature is higher than desired in the hot blast main 4, the voltage from the hot blast thermocouple I6 is above standard, whereupon galvanometer contact 22 is made, energizing rotorfield coil 22H, causing the rotor shaft 35 to turn. Thereupon the slide wire turns until the galvanometer relay returns to neutral or zero position. Meanwhile, control cam 36 actuates control arm 40, causing contact 46 to be made. This energizes the motor 18, causing the mixer valve 6 to open wider, allowing more cold air to bypass the stoves 3 and thereby reducing the temperature of the hot blast flowing through main 4, thus compensating for a decrease in the humidity of the cold air supply.

0n. the other hand, when the hot blast is below normal the reverse action takes place, contacts 22 and 41 being made to cause movement of the motor IS in a direction to partially close the mixer valve 6, thus by-passing more cold air and consequently raising the temperature of the hot increase in the humidity of the cold air supply.

-blast in the main 4, which compensates for an In the modification illustrated in Figure 2, much of the apparatus is identical with that shown in Figure 1, and repetition of detailed description is therefore'unnecessary. In the modification of Figure 2, there are two potentiometer controllers. The first potentiometer controller 54 operates in response to the thermocouple i4, and the second potentiometer controller ,il operates in response to the hot blast thermocouple it. The controller 54 operates to vary the regulated temperature of the hot blast controller, which action can be achieved by various methods. In the embodiment of Figure 2, a gear 55 is fastened to rotor shaft 53 of a rotor 51. This gear 55 meshes with a gear 58 loosely mounted between collars 53 on the rotor shaft 35 ofthe potentiometer controller i1. This potentiometer controller M is substantially identical with that shown at ii in Figure 1, and corresponding reference characters are applied to the corresponding parts. In this modification, however, it is noted that the gear 53 is substituted for the sector 42 shown in Figure 1. Thus in the arrangement of Figure 2, instead of the angular position of the control arm 4!! being varied manually by manipulation of the adjusting knob 43, as in Figure 1, its position is varied automatically by the coaction 0t gears 55 and 58. In other words, instead oi setting the control point by positioning control arm 40 manually as in Figure l, the control point in the arrangement 01' Figure 2 is determined automatically b the potentiometer controller 84 acting through the meshing gears 55 and 58. The amount of rotary movement of gear 58 desired for any glven'change in voltage of thermopile M, can be predetermined by calculation and selection of the proper ratio of pitch di- 1 ameters of gears 55 and 58.

In both embodiments of the invention illustrated in Figures 1 and 2, substantially equivalent method and means for'determining the humidity or moisture content of the air are used. These means are capable of many modifications. In the arrangement illustrated in both figures, the air is sampled at the inlet side of the blowing engine. However, it is to be understood that the sampled air could be taken either at the inlet or outlet side of the blowing engine, or from the free atmospheric air inside or outside of the blowing engine room.

In both embodiments of the invention illustrated, the thermopile i4 comprises a group of thermocouples connected in series and so arranged that the cold junctions are kept wet and the hot junctions maintained dry. The inclusion of both junctions of the series of thermocouples constituting the thermopile I4 within the humidity measuring device 9, overcomes the problem of correcting for one junction temperature, and the presence of both junctions in the sampled air reduces the efiect of any change in the dry bulb temperature. Thus one aspect of our invention involves the provision of the method and means for continuously determining the moisture content of air by passing a sample of air maintained at a predetermined temperature in contact with the series of thermocouples (the thermopile I4), the alternate junctions of which are kept dry and wet with water, respectively, by measuring the voltage produced by such series of thermocouples.

Many variations are possible, and various types of conventional equipment can be used. For ex-- ample, the sample of air could be taken from the cold blast main by. means provided for cooling the same after compression,.to a point just above through the sampling line I and the volume regulated by the throttle valve 8. Or, instead of heating the insulated chamber 9 by steam, electric heating means can be used, and other known expedients can be employed for obtaining the wet and dry bulb temperature differential. The equipment shown and described has been combined and coordinated in a novel manner to perform the novel function of automatically and continuously controlling or varying the temperature of the hot blast supplied to the blast furnace in accordance with fluctuations of the humidity or moisture content of the cold air supply. Thus optimum conditions in the furnace are automatically maintained continuously.

From the foregoing, it is apparent that in its broadest aspects the invention covers the method comprising automatically regulating the temperature of the hot air blast in accordance with variations in the humidity of the cold air supply. It is also apparent that a more detailed aspect of the invention comprises forcing a supply of cold air through the heat exchanger 3 to raise its temperature, withdrawing a sample of air from the region of the cold air supply source or main 2, and automatically and continuously determining the humidity of the withdrawn sample, shunting a controlled amout of air via pipe 5 around the heat exchanger and mixin it with the hot blast air in main 5, the said shunted amount of air being automatically varied by and in accordance with fluctuations in the humidity of the air of the withdrawn sample, to thereby automatically vary the temperature of the hot air blast supplied to the blast furnace F in such manner as to maintain optimum furnace performance.

While the invention has been described with reference to the use of specific control equipment illustrated, believed to be very efficient and reliable in use, it is not to be construed that we are limited thereto, since modifications and substitutions of equivalents may be made by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1 In the operation of a blast furnace, the method which comprises automatically and continuously varying the temperature of the hot air blast supplied to the furnace in accordance with variations in the humidity of the cold air supply.

2. In the operation of a blast furnace, the method which comprises determining the humidity of the cold air supply and automatically and i0 said cold air mixed with the-heatedair in accordancewith variations in the humidity of the cold air supply, and passing the thus-temperature-controlled hot air blast to the furnace.

5. In the operation of a blast furnace, the method which comprises automatically and continuously determining the humidity of the cold air supply, forcing air from the cold air supply through a heat exchanger to raise its temperature, by-passing cold air from the cold air supply around the heat exchanger to the hot blast 7 air, and automatically varying the amount of continuously varying the temperature of the hot air blast supplied to the furnace so as to compensate for the fluctuations of the humidity of said cold air supply.

3. In the operation of a blast furnace, the method which comprises continuously determining the humidity of the atmosphere in the region of the cold air supply and automatically and continuously varying the temperature of the hot air blast supplied to the furnace by and in accordance with fluctuations in the thus determined humidity of the cold air supply.

4. In the operation of a blast furnace, the method which comprises forcing a supply of cold air through a heat exchanger to raise its temperature, automatically determining the humidity of the cold air supply, mixing air drawn from the cold air supply with the air treated by the heat exchanger, automatically varying the amount of said by-passed cold air in accordance with the determined humidity of the cold air supply.

6. In the operation of a blast furnace, the method which comprises forcing a supply of cold air through a heat exchanger to raise its temperature, withdrawing a sample of air from the region of the cold air supply source and automatically and continuously determining the humidity of the withdrawn sample, shunting a controlled amount of air around the heat exchanger and mixing it with the hot blast air, and varying said shunted amount of air automatically by and in accordance with fluctuations in the humidity of the air of the withdrawn sample to thereby automatically vary the temperature of the hot air blast supplied to the furnace in such manner as to maintain optimum furnace performance.

'7. In combination with a blast furnace, the stoves for heating the air blasts supplied thereto,

ture of hot blast air supplied to the furnace iseffective to maintain optimum operating conditions therein. I

8. The combination with a blast furnace, of a heat exchanger connected therewith by a hot blast main, 9, cold blast main connected to the heat exchanger, ablower for forcing a cold supply of air to the heat exchanger, means for automatically and continuously determining the humidity of the cold air supply, and means responsive to variations of the determined humidity of the said cold air supply for controlling the temperature of the hot air blast supplied to the furnace so as to reduce or nullify the detrimental effect on furnace performance due to variations of the humidity of the cold air supply.

9. In combination with a blast furnace, a stove for heating a blast of cold air supplied thereto,

a hot blast main for conveying heated air from the stove to the furnace, a blower and a cold blast main supplying cold blast air to the stove, or a by-pass connection around the stove for conveying cold air from the cold blast main to the hot blast main, means for automatically and continuously determining the humidity of the atmosphere in the region of the cold air supply, a valve in said by-pass effective to controlthe magnitude of flow of cold air through said bypass and means responsive to variations in humidity of the cold air supply for actuating said valve, whereby the temperature of the hot blast supplied to the furnace is automatically varied thereto, a hot blast main'ior conveying heated air from the stove to the furnace, a blower and a cold blast main supplying cold blast air to the stove, of a by-pass connection around the stove ior conveying cold air from the cold blast main to the hot blast main, means for withdrawing and isolating a sample of cold blast air, means for automatically and continuously determining the humidity of said "withdrawn air sample, a valve in said by-pass connection eflective to control the magnitude of flow 01' cold air therethrough, and. means responsive to variations in the humidity of the said isolated sample withdrawn for controlling the operation of said valve, whereby the temperature of air supplied to the furnace is automatically varied in a manner to reduce or nullify the detrimental efiect on furnace performance due to variations of the humidity of the atmosphere in the region of the cold air intake of the blower.

11. The method for continuously determining the moisture content of air, comprising passing a sample 01' air maintained at a predetermined temperature in contact with a series of thermocouples the alternate Junctions of which are kept dry and wet with water, respectively, maintaining the dry junctions at a substantially constant temperature to preclude the necessity of compensating for changes in the temperature thereof and measuring the voltage produced by such series oi thermocouples.

CLARENCE D, KING.

PEER D. NIELSEN.

REFERENCES ClllEll The following references are of record in the OTHER REFERENCES Clements, "Blast ace Practice," vol. 3, page 46, published by Earnest Benn, Ltd., London, England, 1939.

Certificate of Correction Patent No. 2,458,947. January 11, 1949. CLARENCE 1). KING ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 8, line 7, for the word thermocouple read thermopile; column 9, line 74, claim 4, for treated read heated;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 31st day of May, A. D. 1949.

THOMAS F. MURPHY Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,458,947. January 11, 1949. CLARENCE D. KING ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 8, line 7, for the word thermocouple read thermogn'le; column 9, line 74, claim 4, for treated read heated;

Signed and sealed this 31st day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant Gammz'ssz'oner of Patents.