US3454267A - High performance blast furnace stoves - Google Patents

Description

Filed D c. 13, .1967 fIQJ 9 5:5. 2

y 8, 1969 w. E. SLAGLEY ET AL 3;. 454,267

HIGH PERFORMANCE BLAST FURNACE STOVES Sheet of 2 INVENTOES fW/fa/n A. Egg/e away ATTY5.

United States Patent M HIGH PERFORMANCE BLAST FURNACE STOVES William E. Slagley, Crown Point, and Lawrence G. Maloney, Munster, Ind., assignors to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed Dec. 13, 1967, Ser. No. 690,315 Int. Cl. F231 9/00 U.S. Cl. 263-19 9 Claims ABSTRACT OF THE DISCLOSURE An improved blast furnace stove having a metallic heat exchanger installed between the lower brick checkers and the checker supports can be operated at higher average checker temperatures without increasing the dome temperature or the stack temperature.

Our invention relates generally to metal-making apparatus, and a process of operating such apparatus. More particularly, our invention pertains to a new and improved blast furnace stove as well as an improvment to existing blast furnace stoves.

In the operation of current day blast furnaces, tremendous quantities of air must be heated from near ambient temperatures up to between 1000 F. and 2000 F. or even higher. The quantity of air required may range from 50,000 cubic feet per minute (measured at standard conditions) to as high as 150,000 cubic feet per minute. The heating of these quantities of air (hereinafter referred to "as clod blast) is accomplished in a blast furnace stove. Blast furnace stoves are generally utilized in groups of three or four for each blast furnace. Blast furnaces are normally operated by utilizing one stove at a time to heat the air for the furnace on blast while the other two or three stoves are being reheated on heat using the blast furnace gas as the fuel.

Blast furnace stoves generally range in size of from 24 to 32 feet in diameter and from 90 to 140 feet high. The shell usually has an 18-inch firebrick lining. The interior portion of the stove is divided by a bridge or breast wall which separates the combustion chamber, having a lens shape, from the checkerwork chamber. The checkerwork chamber generally comprises approximately 70 percent of the volume and a combustion chamber comprises the remaining 30 percent of the volume. The checkerwork chamber is filled with checkers which are masses of brick placed in the stove for the purpose of storing heat. These checkers absorb heat from the prodnets of combustion of the blast furnace gas (which may be enriched with other combustible fuel gases) and air during the heating cycle and give up this heat to the air during the blast cycle. The total weight of checkers in a stove may vary from 1.5 to 4 million pounds, and their volume may exceed 40,000 cubic feet.

As the technology of iron and steel making progresses, increased qualities of hot blasting air at higher temperatures are required. Various expedients have been tried to help satisfy this need. Additional stoves have been built, existing stoves have been increased in height, external combustion chambers have been built and checkerwork has been placed in the initial combustion chamber. All of these solutions are expensive, and while they may be thermodynamically eflicient, they are not economically attractive.

Blast furnace stoves have long been used to preheat air used in blast furnaces before the air is admitted to the blast furnace through tuyeres. The purpose of preheating the air blast is to intensify and speed up the burning of the coke in the furnace with the reduction 3,454,267 Patented July 8, 1969 of the coke required for smelting the ore. The reduction in coke consumption turns out to be more than that which would correspond to merely the additional principal heat carried in by the heated air blast.

Essentially a blast furnace stove consists of two parts, the first being a combustion chamber which is a vertical passageway extending from a point near the bottom of the stove to the top of the stove and through which the hot products of combustion cast upward to the top of the stove. It is in this combustion chamber that the cleaned blast furnace gas is burned in order to heat the checkerwork described above.

The second main part of the blast furnace stove is the checkerwork, which is actually a mass of brick containing a multiplicity of small passageways through which the products of combustion from the blast furnace gas pass downwardly from the top of the blast furnace stove to a point near the bottom of the stove.

The air to be used for the blast furnace referred to as blast passes through the heated stove in a direction countercurrent through the passage of the gas during the heating cycle (with respect to the stove). In a two-pass stove during the heating cycle, the gas passes upward through the combustion chamber and then downwardly through the checkers and up the stack, while the blast (air to be heated) passes upward through the checkers and downward through the combustion chambers, and out into the hot blast main. When a blast furnace is equipped with three stoves, one stove is generally on blast at a time, while the remaining two stoves are on the heating cycle referred to as on gas. The result of this is that each stove is on its heating cycle nearly twice as long as it is on its on blast cycle. The changeover from the gas cycle to the blast cycle is usually accomplished by valves.

In the operation of blast furnace stoves, the heating cycle is carefully controlled in order to avoid structural damage to the stove. Typically, the on heat cycle is started by burning a fuel gas (such as enriched blast furnace gas) with air until the dome temperature reaches a predetermined maximum. At that point, the temperature of the burning gases is reduced through the introduction of excess air, in order to maintain the dome temperature close to, but below the maximum temperature. The stack gas temperature is then followed and used to control the heating cycle. When the stack gas reaches a certain predetermined temperature, the heat ing is discontinued, and the on heat cycle is complete. Both the maximum dome temperature and the maximum stack gas temperature are set at levels which will avoid any structural failures to the stove.

I This invention is based on the discovery that the installation of a metallic heat exchanger between the lower checkers and the checker supports permits the blast furnace stove to be operated at a higher average temperature, and thus to give higher performance. The heat exchanger is installed under the checkerwork in a manner which enables it to cool the combustion products which are used to heat the checkerwork during the heating cycle before these combustion products are fed to the stack. Thus, the high performance stove of this inventino will operate at conventional stack gas temperatures. In order to provide a heat sink for cooling these combustion products, the heat exchanger of this invention is so located that it will heat the air which is used to burn the blast furnace gas (or enriched blast furnace gas) and thus provide a more efiicient combustion for heating the checkerwork. A blast furnace stove using a heat exchanger of this invention may be operated at conventional dome temperatures. Thus, the uppermost brick checkerwork will achieve only conventional temperatures when fully heated. The lowermost brick checkerwork (those bricks adjacent to the heat exchanger) will oper ate at much higher than conventional temperatures, since the combustion products used to heat the checkerwork are cooled by the heat exchanger in order to achieve conventional stack temperatures. The net result is that when the stove is fully heated the average temperature of the checkwork is higher than conventional temperatures.

The result of the checkerwork having a higher average temperature is that during the blast cycle, the stove can heat more air or heat the air to a higher temperature or heat the air over a longer duration. In any case, the effect is to increase the effectiveness of the blast furnace stove.

Present construction methods employ alloy cast iron or alloy steel columns to support steel bridegwork which in turn supports the checkerwork within the blast furnace stove. It has been found that by using a heat exchanger of proper dimensions and construction in place of some of these supporting members, the volume of brick checkerwork within the blast furnace stove is substantially unchanged, but that because of the use of the heat exchanger, the average operating temperature of the blast furnace stove can be substantially increased thereby increasing the efliciency and/or capacity of the stove.

Additionally, the use of the heat exchanger in accord ance with this invention permits the preheated air, necessary to burn the fuel gas when the blast furnace stove is on the heating cycle, to be sent directly through the breast work which separates the checkerwork chamber from the combustion chamber. This gives rise to two advantages which are: (1) that the gases can be more efl rciently mixed since the air will be approaching the gas from the opposite direction, providing for more efficient combustion, and (2) gas and air valves of conventional design can be used, since both will be cold lines, and can be controlled by valves outside the stove.

A better understanding of our invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a vertical cross-section, showing a side elevation of a blast furnace stove, indicating the position of the checkerwork chamber and the combustion chamber as well as the heat exchanger used as contemplated by the present invention.

FIGURE 2 is a top plan view of a blast furnace stove taken in section along line 22 of FIGURE 1.

FIGURE 3 is a perspective view of a cut-away fragment of the heat exchanger where it meets the checkerwork, showing the vertical openings in the checkerwork being aligned with the vertical openings of the heat exchanger.

FIGURE 4 is a vertical section showing a side elevation fo a second embodiment of the heat exchanger.

FIGURE 5 is a plan view of a brick checker.

FIGURE 6 is a cross-section of a fragment of a brick checker.

Referring now in detail to the drawings:

FIGURE 1 shows generally the blast furnace stove having an outer wall 10 and an inner dividing wall or breastwork 12 which separates the stove generally into a checkerwork chamber 14 and a combustion chamber 16. Also shown are the cold blast inlet 18 and the hot blast outlet 20.

The metallic heat exchanger of this invention is shown generally at 22. It can be seen that the heat exchanger is in fluid communication with a cold air inlet 24 and a preheated air opening 26, which is in fluid communication with the opening between the vertical tubes of the heat exchanger and the combustion chamber 16. It can be seen that while the stove is on the heating cycle, cold air can be fed in through cold air inlet 24 through heat exchanger 22 where it makes several passes through the heat exchanger and is heated. The heated air passes out of the heat exchanger through opening 26 into combustion chamber 16 where it is used to burn blast furnace gas (which may be enriched) which is fed to combustion chamber through fuel gas opening 28.

The exact configuration of the burners which comprise gas opening 28 and preheat air opening 26 are not shown in any great detail. While variations in the burner construction may be made in order to get proper mixing and complete combustion of the said gases, the general configuration whereby the combustible gas is fed from one direction and the air is fed from the other gives rise to effective mixing and combustion. More particularly, very effective combustion can be had by feeding the gas streams into each other at approximately right angles. This may conveniently be accomplished by directing the gases upwardly from the horizontal. Most advantageously, the gases should impinge each other at right angles in such a way that the resulting flame is substantially vertical. This results in the flame going straight up the combustion chamber, and not impinging on the refractory; hence the life of the refractory lining the combustion chamber is extended.

During the heating cycle, the hot products of combustion rise vertically upward through the combustion chamber 16, and then downwardly through the vertical openings in the checkers 14. These gases heat up the brick checkers as the gases are cooled, As the gases pass through the vertical tubes of the heat exchanger 22, they are further rapidly cooled and then are exhausted via opening 18 to a conventional stack (not shown).

FIGURE 2 shows a cross section of slightly modified blast furnace stove taken above the level of the heat exchanger. The figure illustrates that the cold air may be admitted through a plurality of inlets 24 from a common bustle pipe 25. Likewise, the fuel gas may be admitted to the combustion chamber through a plurality of openings 28 through a common bustle pipe 29. Further, the preheated air may be admitted through the breastwork to the combustion chamber through a plurality of openings 26.

FIGURE 3 shows an expanded view of one embodiment of the heat exchanger of this invention. The heat exchanger comprises a plurality of tubes 32 which are horizontally spaced and held by at least two horizontal spacers and 41.

FIGURE 3 illustrates the brick checkers 44 with vertical opening 46 therein being in substantial alignment with openings 34 of the heat exchanger 22.

FIGURE 3 also illustrates the FIGURE 1 embodiment of the heat exchanger, wherein further horizontal dividers such as shown at 36 can be used to allow the air to pass back and forth through the heat exchanger during its travel from the cold air inlet 24 through the breast wall opening 26. This invention is not limited to a heat exchanger having any particular number of passes, but generally contemplates the use of from about 3 passes to about 7 passes as being most efficient.

FIGURE 4 is an expanded view of the heat exchanger, shown generally at 22, operating on the heating cycle. The fuel gas enters the combustion chamber 16 through pipe 28 which goes through the outer wall 10. The combustion chamber is preferably lined with a refractory material 11, as shown. The cold air enters the heat exchanger 22 through pipe 24. The air passes through the heat exchanger 5 times and is heated and it then passes through opening 26 where it is used to burn the fuel gas. The combustion products go vertically up the combustion chamber and down through the checkerwork 14, down through the vertical tubes of the heat exchanger 22, and then out pipe 18 to the stack.

FIGURE 4 illustrates the fuel gas being directed at an upward angle into the combustion chamber opposite to the preheated air, which is also directed upwardly (with respect to the horizontal). As described above, best results may be achieved by adjusting the angle of impingement to about 90 and at such an attitude that the resultant flame is substantially vertical.

FIGURE 5 shows a brick checker which is of the type preferred for use in connection with this invention. Generally, this checker comprises the brick 44 which contains relatively small uniformly spaced holes 46. Generally, this invention contemplates the use of bricks providing at least 30 holes per square foot.

FIGURE 6 shows fragmentary views of such bricks, and illustrates the vertical alignment of the checkers, one on top of the other. Further, FIGURE '6 illustrates the taper of the holes or openings 46. In the case of checkers having relatively small openings, it is highly desirable that openings be tapered in the manner illustrated by FIGURE 6. FIGURE '6 shows the solid portion of the brick 44, and the opening 46, as well as illustrating the uppermost part of the brick 48 being somewhat rounded -(when viewed in section) in effect accentuating the taper of the hole near the top of the brick.

Although bricks having holes in them generally and checker bricks in particular are made with the holes having a slight taper, such a taper is minimal, and the only reason for the taper is to facilitate manufacturing the brick. In is standard practice in the industry to taper the holes in a six-inch brick checker between A; and inch on each side (from to inch difference in diameter). Such a taper is so slight it cannot readily be determined visually, and bricks having holes so tapered have no top or bottom in the sense none is designated. The present invention, on the other hand, contemplates the use of checker bricks in which the hole has a much greater taper. The taper of the hole should be suflicient to be observed visually and should be at least 5 inch on each side or $4; inch difference in diameter for a brick checker .6 inches high. Advantageously this taper is about A: inch on each side or A inch dilference in diameters for a 6-inch brick. Shorter or taller bricks should have holes with proportionate tapers.

In installing the brick checkers having tapered openings, as contemplated by this invention, the brick is placed so that larger diameter opening faces up, and the smaller diameter opening is down, as illustrated by FIGURE 6. This construction obviates the problem of ledges in the checkerwork which can accumulate soot and dirt of various types, which in turn may tend to block the openings of the checkerwork. This tends to decrease efliciency of the checkerwork by reducing its ability to exchange heat and by increasing its resistance to 'gas flow.

As described above, the preferred embodiment of this invention includes the use of checker bricks having a large number of relatively small openings and thinner walls, and in particular having a tapered opening, e.g. the opening on one end of the brick being larger than the opening on the other end. The theory on which the smaller openings are preferred is that such a configuration will cause a greater percentage of the checker weight to reside at a closer distance to the flue heating surface. It has been found that in actual practice, the effective performance of the checker shape is proportionate to its heating surface per cubic foot of checker volume, so long as the total weight of the checkers is maintained constant. From this it can be seen that it is desirable to have numerous very small flues. The only disadvantage with small flues is that they are more diflicult to install, in that they are relatively diflicult to align, and when not aligned properly, these smaller flues are plugged more readily by a relatively small amount of dirt.

The use of the tapered flue tends to overcome the disadvantage of plugging. Thus the upper opening of the checker will be larger than the adjacent lower opening of the next higher checker. In the event of a slight misalignment, no ledge will exist, where dirt can collect and build up during the gas firing cycle.

This configuration of brick checkers gives rise to a further advantage in that gas being fed through the tapered flues will undergo an expansion and a contraction cycle for each brick through which is passes. This typically would yield between and 300 such pulsations over the full height of a single flue. Such pulsations will help to improve heat transfer from gas to brick or brick to air, and as much as a 25% increase in the heat exchange capacity has been demonstrated.

The use of checkers having a relatively high number of small holes is particularly advantageous for use in connection with the heat exchanger of this invention. Since it is necessary that the holes in the checkerwork be aligned with the openings of the vertical tubes of the heat exchanger, the number of tubes in the heat exchanger will be the same as the number of holes in the checkerwork. Thus it can be seen that the available heat exchange surface in the heat exchanger will vary directly with the number of openings in the checkerwork, assuming the same heat exchanger height. Naturally, the minimum feasible height of heat exchanger is desired, since the heat exchanger displaces checkerwork, or at least occupies a volume of the checkerwork chamber which could otherwise be occupied by brick checkers.

Additionally, the use of checkers having small closely spaced holes, at least within certain limits, results in stronger heat exchangers, since they will have more vertical tubes. While the drawings illustrate checkerwork openings which have both square and round cross-sections, this invention is not so limited. Any convenient cross-section may be used for the brick checker opening.

Through the use of the heat exchanger of this invention, it is possible to operate the blast furnace stove approximately 500 F. hotter than the same blast furnace stove could operate without the heat exchanger, and at the same time maintain an equally low stack temperature. While it is possible, of course, to operate blast furnace stoves at this increased temperature without the use of a heat exchanger, more fuel is required to achieve such operating temperatures, and the stack gas temperature is increased to the point where the stove would fail.

This invention also contemplates heat exchangers which comprise vertically separable sections. Such sections are more easily handled and installed than a heat exchanger which occupies the whole horizontal area of the checkerwork chamber. It is contemplated that the most eflicient way to use the heat exchanger would be to weld it into a one-piece unit, after installation in the stove.

While the invention herein has been described in terms of a two-pass blast furnace stove, i.e. the gas passing upward through the combustion chamber and thence downwardly through the checkerwork and out vented through the stack, the invention contemplates a three-pass blast furnace stove as well as a four-pass design. Likewise, this invention contemplates the use of the heat exchanger in stoves in which the combustion chamber is external to the stove. It is essential only that the heat exchanger be within the stove and that it be located as close as possible to the stack end of the stove.

Although we have described our invention with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention, and the scope of the claims appended hereto.

We claim:

1. A blast furnace stove comprising a checkerwork chamber, a combustion chamber, and a breast wall separating said chambers, said checkerwork chamber being essentially filled with brick checkers having a plurality of horizontally spaced checkerwork openings in vertical alignment, said checkerwork openings being in fluid communication with said combustion chamber, said stove containing a metallic heat exchanger comprising a plurality of horizontally spaced vertically positioned tubes, the uppermost openings of said tubes being in substantial vertical alignment with the lowermost portion of said checkerwork openings, the lowermost openings of said tubes being in fluid communication with a stack, said stove having a cold air inlet in its outer wall adjacent to said heat exchanger, said breast wall having a preheated air opening adjacent to said heat exchanger, whereby the space between the tubes of said heat exchanger is in fluid communication with both said cold air inlet, said preheated air opening and said combustion chamber.

2. A blast furnace stove as described in claim 1, wherein said metallic heat exchanger is located beneath said checkerwork chamber.

3. A blast furnace stove as described in claim 2, wherein said heat exchanger is a multiple-pass exchanger with respect to the air being preheated.

4. A blast furnace stove as described in claim 2, wherein said heat exchanger occupies an area, measured hoizontally substantially equal to the area, measured horizontally, occupied by said checkerwork.

wherein said checkerwork openings are tapered.

8. A blast furnace stove as described in claim 7, wherein the checkerwork opening taper is equivalent to at least a A inch taper on each side, for a 6-inch brick.

9. A blast furnace stove as described in claim 7, wherein the checkerwork opening taper is equivalent to at least a inch taper on each side, for a 6-inch brick.

References Cited UNITED STATES PATENTS 2,185,559 1/1940 Mohr et al. 26351 2,188,289 1/1940 Schwarze 26320 FREDERICK L. MATTESON, JR., Primary Examiner.

EDWARD G. FAVORS, Assistant Examiner.

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