US2491828A - Continuous strip furnace - Google Patents

Description

Dec. 20, 1949 A. N. oTls CONTINUOUS STRIP FURNACE 2 Sheets-Sheet 1 Filed March 2, 1949 n www 59696962 x16? PIA.

verwtor Albert NA Otis, by. Wasn H s Attorney Dec. 20, 1949 A. N. ons

CONTINUOUS STRIP FURNACE 2 Sheets-Sheet 2 Filed March 2, 1949 Inventor: Albert N. Otis,

by @JK/(afm H i s Attorney Patented Dec. 20, 1949 CONTINUOUS STRIP FURNACE Albert N. Otis, Schenectady, N. Y., assigner to General Electric Company, a corporation o! New York Application March 2, 1949, Serial No. 79,252

1 claim. 1

My invention relates to furnaces which provide for the continuous movement of the material to be heated through the furnace, and more particularly to furnaces adaptable for the heating of material in form of a relatively thin continuous strip as it passes through the furnace.

One object of my invention is the provision of a vertically disposed furnace for the continuous heating of strip material which occupies a relatively small volume, and especially one which requires a minimum of head room for a given length of heating path.

Another object of my invention is the provision of an improved type of construction for continuous strip furnaces of the type adapted to contain an artificial atmosphere.

A further object of the invention is to provide a furnace in which a relatively large percentage of the strip material in the furnace at one time is exposed directly to the source of heat.

A further object of my invention is the provision of a furnace which will satisfactorily heat strip material which travels through the furnace at a relatively high rate of speed.

A further object of the invention is the provision of a continuous strip furnace in which the temperature to which the strip material is raised can be accurately and uniformly controlled.

A still further object of the invention is the provision of a continuous strip furnace having means for maintaining the temperature in the strip, transversely thereof, substantially uniform so that the edges of the strip are heated uniformly with the center of the strip irrespective of the width of the strip being put through the furnace, Whether it be relatively narrow or relatively wide.

In carrying out my invention in one form, I provide a furnace having four vertical heating chambers arranged in a continuous row. Adjacent chambers are interconnected alternately at the top and at the bottom and each chamber has a relatively small transverse roller at each extremity thereof in the interconnecting passageway which communicates with the adjacent heating chamber. The strip material being heated is supported by these rollers as it passes longitudinally through consecutive heating chambers. Resistance type electrical heating units connected in groups of varying capacities are located on the side Walls of the heating chambers, with the larger groups of heating units opposite the center of the strip and the groups of smaller capacity opposite the edges of the strip. Control means is provided which automatically adjusts the relative currents in the various groups of heating units so that the temperature of the strip, transversely, is substantially uniform, regardless of the width of the strip being heated. As the strip material passes through the furnace, the temperature of the edge portions of the strip increases uniformly with the temperature of the center portion while the strip is heated rapidly to a predetermined temperature.

For a clearer and more complete understanding of my invention, reference should be had to the accompanying drawings. In the drawings:

Fig. 1 is a side elevational view, partly in section, of a furnace incorporating my invention;

Fig. 2 is a sectional view along the line 2-2 of Fig. 1;

Fig. 3 illustrates the difference in heating chamber height resulting from the substitution of a single large drum for two of the small rollers of my invention;

Fig. 4 is a fragmentary sectional view showing the detailed construction of these rollers;

Fig. 5 is a schematic electrical circuit diagram of my invention;

Fig. 6 shows the heat input level and disposition of heat required for heating various widths of strip material; and

Fig. 7 is a simplified schematic diagram of a portion of the electrical circuit.

Referring to Fig. 1 of the drawing, a typical furnace embodying my invention is illustrated generally by the numeral I. The furnace I is of the vertical chamber type and is provided with lateral support by vertical structural members 2 and horizontal structural members 3. The ends of the furnace are supported by additional horizontal structural members 4. The furnace proper has an outer metal casing 5 inside of which is located a layer 6 of heat insulating material. Inside of heat insulating layer 6 is another layer 1 of refractory brickwork.

The furnace has a removable top comprising a group of longitudinal structural members 8 which support a layer of heat insulating material 9 and an inner layer of refractory material I0. Longitudinal structural members 8 are secured to transverse structural members I I. At each extremity of members II is a lifting lug I2 which is secured to a projection I3 on the side of the furnace by a chain I4 during operation of the furnace. A downward projection I5 extending along each side and across each end of the cover fits into a corresponding rectangular recess in the top surface of the furnace structure. A corresponding upward projection I6 extends entirely 5e around the top of the furnace structure along the inner edge of the recess. Together projections I and I6 provide a seal around the removable cover for the artificial atmosphere of the furnace.

The furnace preferably has an atmosphere of non-oxidizing gas at a pressure sufficiently above atmospheric pressure to prevent the entry of air into the furnace. Such an artificial atmosphere which may be of hydrogen, or a mixture of hy-- drogen and nitrogen, is maintained by injecting the gas continuously during operation of the furnace through ports, which are provided at various points on the furnace for this purpose.

The furnace i is separated into four vertical heating chambers 2i by three vertical partitions Il of refractory brickwork. One of these can be clearly seen in the sectional portion of Fig. 1, while the other two partitions are located at equally spaced intervals in the remaining portion of the furnace. The three partitions il are identical, except that the center partition does not have a lower foundation section i8 resting on the bottom casing of the furnace as do the two outer partitions. The center partition il is supported by arch members i9, which are shown in greater detail in Fig. 2. The arch members i9 are of refractory material and are, in turn, supported by metal structural members 2li of triangular cross section which runs longitudinally of the furnace and are welded to side casings 5. Also, the center partition i'? extends entirely to the top of the furnace where it contacts refractory layer lil instead of terminating below the rollers 26 like the two outer partitions il. Each heating chamber 2i has an access door 22 in the rear wall of the furnace, These doors provide entrance to the heating chambers for cleaning and maintenance purposes when the furnace is not in operation. During operation, the doors are sealed with insulating material and refractory brick, as illustrated in Fig. 2.

At the lower end of the first heating chamber 2i on the entrance end of the furnace is an entrance chamber 23. A transverse roll or roller 24 is located in chamber 2t and there is an opening 8i in the front wall thereof equipped with sealing means (not shown), to admit the strip material which is to be heated. At the top of the rst heating chamber, interconnecting this chamber with the adjacent heating chamber, is an interconnecting or roller chamber 25 in which are located two transverse rollers 2S. A similar interconnecting chamber 25 and two transverse rolls or rollers 26 are located at the bottom of the two center heating chambers, with another charnber 25 and two rollers 26 at the top of the two chambers on the exit end of the furnace. An. exit chamber 2l and a transverse roller 28 are therein located beneath the last of the four heating chambers in the same relative position as entrance chamber 23 and roller 24 occupy with respect to the rst heating chamber 2i. The two upper roller chambers 25 are separated from the main heating chambers 2l by refractory arches 29 which are provided with slots for passage of the strip to be annealed. The bottom roller chamber 25, entrance chamber 23, and exit chamber 21 are separated from the main heating chambers by refractory arches I9.

The transverse rollers 24, 26 and 28 support a continuous strip of steel 30 which is heated as it is moved through the furnace by external means (not shown). The steel strip 3D moves vertically through consecutive heating chambers 2| along the path indicated by the arrows and is Cil supported in the correct location in each heating chamber by the transverse rollers situated in the various roller chambers at the ends of the heating chambers. The strip 3D enters the furnace l through opening 8|, passes beneath roller 24, and after passing over and under consecutive pairs of rollers 28, passes beneath roller 28 and leaves the furnace through an opening in the wall of exit chamber 2l at location 82. After leaving the furnace, the strip may go through a cooler 80, in which the rate of cooling of the strip can be controlled, to complete the annealing process and provide the annealed strip with the desired characteristics. One suitable cooler for this purpose is disclosed in copending application Serial No. (D. 80,684) of Carl L. Ipsen and Norman B. Jones, which is assigned to the assignee of the present invention and which is filed concurrently herewith.

The manner in which rollers 24, 26 and 28 are supported in the furnace is shown more clearly in Fig. 4, the fragmentary view shown in this figure being typical of all rollers. Each roller is composed of a hollow cylindrical metal shell 3i which is supported co-axially on a hollow shaft 32 by a flange member 33. At the end illustrated in Fig. 4, shaft 32 has a reduced diameter portion 34 connected to the larger portion of the shaft by a tapered portion 35, the portion 34 being arranged to fit into a lifting device for putting in or removing the rollers from the furnace structure. At the opposite end of shaft 32, reduced diameter portion 34 may be omitted. At this end of the shaft, at the outer extremity of portion 34, 'there is a second tapered portion Et to which is connected a coaxial rod-like portion 3l, the outer surface of which forms the journal or bearing surface. A collar 3B, having the same outside diameter as portion 34 of the shaft, is mounted on the inner extremity of journal portion 3l' 'to form, in effect, an extension of surface Bil in order to provide a longer gripping area for the above mentioned lifting device. Journal 3l is supported by an outer annular member 39 which forms the housing for a suitable bearing, preferably of the ball or roller type. Member 39 is secured to a flange 452 which is bolted by means of bolts 83 to a correspondingr flange 4l. The holes in iiange 4U for bolts 83 are elongated, or over size, to provide for the radial adjustment of bearing housing 39 for the alignment of the roller. Flange 4i is, in turn, attached to a flanged cylindrical member 42 which is fastened by bolts 84 to the casing 5 of the furnace. A group of annular fins 43 is positioned on the outer cylindrical surface of member 42 to conduct heat away from the extremity of the roller shaft, thus helping to keep the bearing cool. Additional assistance in preventing heat from the furnace from reaching the bearing is provided by heat insulation B8 which fills the end of hollow shaft 32, being held in position by a retainer l0.

Prior to my invention, continuous strip annealing furnaces of the vertical chamber type utilized a single large drum instead of a pair of small rollers for supporting the strip in adjacent heating chambers. Referring to Fig. 3 on the accompanying drawing, there is shown a fragmentary view of a single large drum 44 which could be substituted for two of the rollers 25. That is, if partition l1 and refractory arch 29 were constructed at the height shown in Fig. 3, a single large drum 44 could be used in place of two adjacent rollers 26 to maintain the strip in the correct relative location in two adjacent heating chambers 2|. It can readily be seen by comparing Fig. 3 With Fig. 1 that the effective heating length of the heating chambers would be substantially reduced if a single drum were used instead of two rollers. The use of large drums at the bottom of the furnace in place of the smaller rolls 26 would further decrease the heating length of each chamber. The net result would be that the heating length of furnace I would be decreased by approximately double the difference in height indicated by Figs. 1 and 3 'or each heating chamber.

In one typical furnace incorporating the dual rollers of my invention, the rollers 26 are 18 inches in diameter. Partition l1 must be of substantial thickness in a furnace of this type for structural reasons, and the width of chambers 2l must be sufficient for workmen to install or repair the heating units. A single drum would have to be more than double the diameter given, or not less than 36 inches. in order to replace two adjacent rollers 26 and maintain the strip 30 in the same spaced relationship in two adjacent heating chambers 2|. As a matter of fact. similar strip annealing furnaces have been built with drums 40 inches in diameter. The same calculations would apply in order to substitute a single drum for the two rollers 26 at the bottom of the furnace, Therefore, if single drums were used instead of the dual rollers, there would be a decrease of not less than 2 times 18 inches, or 36 inches in the height of the heating chambers attributable to the use of large drums instead of small rollers. Assuming that the same vertical clearances would be required above and below a single large drum. as for the smaller and dual rollers. the heating length of each heating chamber would be decreased more than 3 feet.

In addition, the small rollers are much lighter in weight than large drums and they can be removed frorn the furnace, when necessary, much more readily. Also, small rollers have less inertia, so that less tension is required on a strip in the furnace to start from rest after a shutdown. Furthermore` the bearings are much smaller and the heat lost through the bearings and through the cooling fins 43 is much less than would be the case for large drums. A still furthere advantage of using the smaller rollers of my invention is that the openings required for removing the rollers from the furnace are smaller and can be more readily sealed and kept tight against leaks of the artificial atmosphere out of the furnace, or air into the furnace.

The walls of heating chambers 2| are provided along the entire height with projections 45 of refractory material which support ribbon type electrical resistance heating units 46. The resistor units 46 are arranged in rows separated by horizontal projections 4l of refractory material to provide mechanical protection for the heating units. In order to provide uniform heating of the steel strip material 30. the heating units 46 for each heating chamber are arranged vertically in three groups. These three groups are connected to receive electrical energy from the three phases of a three-phase alternating cur rent system.

In Fig. 5 on the accompanying drawing, the connections of the three groups of heating units are shown schematically for one heating chamber. A group 48 is disposed vertically along the center portion of one wall of the heating chamber and is connected to one phase of the three-phase electrical supply circuit. A second group 49 is disposed vertically along the center of the opposite wall of the heating chamber and is connected to a second phase of the supply circuit. The third group consists of four small sections .5D-53 inclusive, along the edges of the strip on opposite walls of the heating chamber connected in series across the the third phase of the source of electrical energy.

The heating chamber cross-sectional dimensions, indicated in Fig. 5, are those which were used in one typical four chamber continuous strip furnace. which was constructed primarily to heat steel Strip of 30 inch width to approximately 1350 degrees Fahrenheit in order to anneal it. This furnace was required to be suitable for heating strip wider than 30 inches at a constant output of 20.000 pounds of steel per hour for all widths 30 inches and greater. The furnace was also required to heat strip narrower than 30 inches at reduced tonnage output. Under these conditions, for strip of a given thickness, it is necessary in order to secure the desired constant output, to operate the strip at a correspondingly slower speed in the furnace if the strip is greater than 30 inches in with. For strip less than 30 inches in width the strip speed remains the same in this particular case; therefore, the furnace output is reduced proportionally with the amount the strip width is less than 30 inches.

If the heating units of Fig. 5 were to deliver energy at a uniform rate across the width of the furnace, and narrow steel strip, for example, strip less than 30 inches in width were being put through the furnace, it is apparent that excess temperatures would exist beyond the edges of the strip, as there is no strip in those areas to absorb the heat being delivered. This would tend to overheat the edges of such strip. For this condition, therefore, it is desirable to reduce the heat input along the edges, and increase it in the center portion, where it would do more work.

For wider strip, 48 inches for example, which travels at a slower speed. more heat energy is required at the edges of the chamber, and less is required at the center than for narrow strip. This is due to less pounds of steel per riit of time passing through said center portion, because of the slower travel speed.

It is, therefore, desirable when heating wider strip to reduce the heat input opposite the center of the strip and to distribute the heat input over a wider surface. This is illustrated graphically in Fig. 6, in which the desired heat input level and distribution are shown graphically for 30 inch, 36 inch and 48 inch wide strip for a given output of 20,000 pounds per hour. Pounds is considered equivalent to required heat input for the purpose of this illustration, inasmuch as the desired heat input per hour for any given longitudinal section through the furnace one foot wide is substantially proportional to the weight of steel strip which passes through that one foot section of the furnace in one hour. The latter. in turn, is smaller for wider strip in order to keep the total hourly output of the furnace constant.

In order automatically to adiust the heat level and distribution in the heating chamber in accordance with my invention, I provide a temperature responsive device 54 positioned adjacent the center portion of the strip and a second temperature responsive device 55 positioned adjacent one edge of the strip. Devices 54 and 55 are preferably thermocouples at the temperatures used in annealing steel; however, other types of temperature responsive devices can also be used, and in Fig. I have shown bulb type devices for the sake of simplicity. )Devices 54 and 55 actuate control circuits which govern, by means of saturable reactors in the alternating current electrical supply lines, the relative currents in the three groups of heating units and thereby automatically control the distribution of heat and the heat level to provide the desired temperatures across each heating chamber.

The furnace is supplied with three phase alternating current electrical venergy at consumers voltage and frequency from three mains, 59, 60 and 6I. One central group of heating units 48 is connected in series with reactor 51 across alternating current mains 60 and 6i. The opposite central group of heating units 49 and reactor 56 are connected in series across alternating current mains 59 and 60. The two groups of heating units 48 and 49 jointly are of suiiicient capacity to supply the heat required for the full output of 20,000 pounds per hour of 30 inch wide strip. The third group of heating units composed of sections 50-53 inclusive, is connected in series with reactor 58 across alternating current mains 59 and 6i. Taken together, the three groups of heating units are connected to the alternating current mains in delta with one reactor in each leg of the delta connection, as shown schematically in Fig. 7,

Reactors 56, 51 and 58 are of the saturablecore type and act as valves to regulate the amount of alternating current energy which iiows to the heating units. Each reactor has an alternating current winding and, in addition, a second winding for unidirectional current. When there is no current iiowing in the latter, the impedance of the alternating current winding is high and most of the alternating current voltage appears across the reactor; hence, the energy input to the furnace is minimum. When maximum current is iiowing in the unidirectional current winding, the impedance of the alternating current winding of the reactor is low and energy input to the furnace is maximum. -Intermediate values of energy are obtained by varying the current in the unidirectional current winding.

The unidirectional current windings of reactors 56 and 51 are connected in parallel in this instance, and the current in these windings is varied automatically through the action of temperature responsive device 54. Device 54 is i'llled with a liquid or gas whose volume changes in response to changes in temperature and actuates a bellows 62 through a tube 63. The bellows 62 operates a contact arm 64 over a potentiometer resistor 65 to vary the amount of resistance in a grid control circuit 66 which operates on` the potential diierence between one end of resistor 65 and movable arm 64. One grid control circuit suitable for this purpose is disclosed in Patent No. 2,266,569, issued on December 16, 1941, to Elbert D. Schneider and August R. Ryan, although other control circuits having similar characteristics may also be used.

The unidirectional current windings of reactors 56 and 51 are supplied with unidirectional current from the mains 59 and 60 through the action of an electron discharge device 61 acting as a half-wave rectifier. Device 61 is preferably of the three element gas-filled type characterized by large power output controlled by a small amount of grid energy. The control electrode or grid 68 of device 61 is energized by a signal voltage from grid control circuit 66 which is responsive to the temperature at point 54 in the heatw ing chamber. If additional heat is required to raise the temperature at this point to a predetermined value, control electrode 68 acts to cause an increase in the average unidirectional current through device 81 which, in turn, increases the current in the unidirectional windings of reactors 56 and 51. This increases the amount of alternating current energy which flows through reactors 56 and 51 and, as a result, increases the input to heating units 48 and 49. If a decrease in temperature is indicated at location 54 in the heating chamber, the control circuit functions in the opposite manner to cause a decrease in the alternating current which flows through heating units 48 and 49.

A half wave rectifying device 1i is connected in parallel with the unidirectional current windings oi. reactors 56 and 51. Rectifying device 1I passes current during the half cycles that electron discharge device 61 is not operating so as to provide full wave rectication and maintain a substantially uniform current through the unidirectional current windings during the entire alternating current cycle. The current through device 'Il is caused by the inductive voltage across the unidirectional current windings of reactors 56 and 51 when device 61 ceases to conduct current.

The third group of heating units composed of sections -53 inclusive, along the edges of the heating chamber, is controlled by reactor 58 in a manner similar to that described above for the control of the other groups of heating units by reactors 56 and 51. The liquid or gas in bulb 55 acts on bellows 'l2 through tube 13 to actuate arm 14. The movement of arm 14 on a potentiometer resistor 15 causes a control voltage proportional to the difference in potential between one end of resistor 15 and the movable arm 14 to be supplied to a grid control circuit 16. The latter supplies a signal voltage responsive to the temperature at location in the furnace chamber to control electrode 11 of electron discharge device 18 which is similar in type and characteristics to device 61. Device 18 functions as a half wave rectifier to supply unidirectional current to the unidirectional winding of reactor 58 from alternating current mains 59 and 6|. An additional electron discharge device 19 functions in a manner similar to device 1l to provide unidirectional current for reactor 58 when device 1B is not conducting.

It is apparent from Fig. 5 that device 55 is responsive to the temperature adjacent the edge of the strip material 30. If a comparatively narrow strip of material, 30 inches or less in width, for example, is being heated in the furnace, device 55 acts through its associated control circuit to reduce the unidirectional current in reactor 58 and, thus, reduce the alternating current which ilows through reactor 58. This reduces the current in heating units 50-53 inclusive, which reduces the temperature along the edges of the heating chamber 2| and prevents the overheating of the edges of strip 30 which would result from the build-up of heat along the edges of the chamber with nothing to absorb and remove it. Concurrently, heat being absorbed by the strip tends to lower the temperature at point 54, which causes an increase of input to units 48 and 49 sufficient to maintain a substantially constantI provide uniform heating transversely across the strip, while less heat is required near the center because the strip moves at a slower speed. In this case, device 55 acts through its control system to cause reactor 58 to allow increased alter` nating current to flow and provide more heat along the edges of the strip, and device' 55 acts to reduce the input to the center portion. Through the action of device 54 on reactors 56 and 51 and the action of device 55 on reactor 58, the heat input is automatically proportioned from the center to the edges so that the strip 30 is uniformly heated to the desired temperature, irrespective of the width of the strip material.

While I have illustrated and described one preferred embodiment of my invention, many modifications will occur to those skilled in the art and, therefore, it should be understood that I intend to cover by the appended claim any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

In a furnace of the artificial atmosphere type for the continuous heating of metal strip material, the combination of a plurality of vertically disposed heating chambers positioned in a continuous row, a quantity of interconnecting chambers equal to one less than the number of said heating chambers interconnecting adjacent heating chambers alternately at the top and bottom extremities thereof, a' quantity of transverse rollers equal to double the quantity of said interconnecting chambers positioned two in each interconnecting chamber at the respective extremities of the heating chambers interconnected thereby. means including said transverserollers for moving said strip material through consecutive heating and interconnecting chambers, a plurality of electrical resistance heating umts positioned in said heating chambers for the heating of said strip material, said heating units being connected in a plurality of vertically disposed groups adjacent the center and edge portions of said strip material, alternating current supply means for energizing said heating units, means for deriving a unidirectional current responsive to the temperature of the center portion of said strip material, a saturable reactor having an 10 alternating current winding connected in circuit with the alternating current supply to the heating units adjacent the center portion of said strip material and a unidirectional current saturating winding connected to said unidirectional current deriving means, means including said saturable reactor for varying the current in said center heating units to heat said center strip portion to a predetermined temperature, means for deriving a second unidirectional current responsive to the temperature of an edge portion of said strip material, a second saturable reactor having an alternating current winding connected in circuit with the alternating current supply to the heating units adjacent the edge portions of said strip material and a unidirectional current saturating Winding connected to said second unidirectional current deriving means, and means including said second saturable reactor for varying the current in said edge heating units to heat said edge portions to said predetermined temperature, Where by the temperature transversely across said strip material is substantially uniform irrespective of the width of said strip.

. ALBERT N. OTIS.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,515,511 Milner Nov. 11, 1924 1,664,965 Bonds Apr. 3, 1928 1,765,955 Von Maltitz et al. June 24, 1930 1,775,682 Martin Sept. 16, 1930 1,776,823 Summey Sept. 30, 1930 1,811,522 Shover et a1 June 23, 1931 1,842,967 Hegel Jan. 26, 1932 1,844,658 Holcomb Feb. 9, 1932 1,968,442 Clark et al July 31, 1934 2,060,634 Otis Nov. 10, 1936 2,215,080 Hess Sept. 17, 1940 2,266,569 Schneider et al. Dec. 16, 1941 2,269,236 Wellmar Jan. 6, 1942 2,320,172 Brooke et al May 25, 1943 2,345,181 Cooper et al Mar. 28. 1944 2,465,137 Van Nordstrand Mar. 22, 1949

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