US2994288A

US2994288A - Expansion and contraction control of refractory furnace roofs

Info

Publication number: US2994288A
Application number: US811304A
Authority: US
Inventors: John D Keller
Original assignee: Harbison Walker Refractories Co
Current assignee: Harbison Walker Refractories Co
Priority date: 1959-05-06
Filing date: 1959-05-06
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01

Description

Aug. 1, 1961 J D. KELLER 2,994,288

EXPANSION AND CONTRACTION CONTROL OF REFRACTORY FURNACE ROOFS Filed May 6, 1959 2 Sheets-Sheet 1 INVENTORH r/ol/N 0. KslLER BY M442,

JLM 5 M 2,994,288 EXPANSIQN AND CONTRACTION CONTROL OF REFRACTORY FURNACE ROOFS John D. Keller, Pittsburgh, Pa., assignor to Harbison- Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 6, 1959, Ser. No. 811,304 13 Claims. (Cl. 110-99) This invention relates to industrial furnaces, and more particularly to the problems resulting from thermal expansion and contraction of arched refractory roofs, especially those made of basic materials such as magnesitechrome mixtures.

As the arched refractory roof of a furnace becomes hotter when the furnace is heated up, the arch attempts to expand laterally. Since the sides of the roof normally are held in fixed position by the skewbacks, the only direction the roof can expand is upward. It will be seen that as such expansion occurs, the pressure against the lower ends of basic bricks in the central part of the arch is greatly increased as the joints between the bricks open at the top of the roof. The concentration of pressure at the lower ends of the bricks may crush them or cause them to pinch spall, and then when the roof cools down and the pressure is relieved, pieces of the brick wall fall away from the roof. Moreover, this higher than desirable compressive stress on the bricks occurs at temperatures where the strength, even of basic brick, is comparatively low.

In roofs made of silica brick, as has been usual practice for open-hearth furnaces, this trouble has not been so serious, because silica refractories not only have less total expansion than basic refractories, but their relation of expansion to temperature is such as to mitigate to a considerable extent the trouble from pinch spalling. The silica material attains almost its full expansion by the time it is heated up to only about 800 F., and does not expand much more on up to the maximum working temperature of about 3000 F. The roof temperature decreases almost in direct proportion to the distance from the lower surface of the roof where it is about 3000 F to the upper surface, where it is usually between 600 and 700 F., but silica bricks will expand laterally nearly uniformly from bottom to top because the temperatures above about 800 F. will not affect their expansion materially.

When the form on which a roof arch has been laid during construction is removed, the slight elastic yielding of the steel buckstays and other parts of the furnace binding, together with the bedding-in of the high spots of the contacting surfaces of adjacent bricks on each other, allows the crown of the arch to drop slightly and small gaps to open up between the lower ends of the bricks. Then, when the furnace is heated up, silica bricks will expand and raise the arch to its original position where the bricks will make contact over their entire depth, except near the top where there may be slight gaps between them. The thrust or load is thus distributed over so great an area of each brick that the unit pressure is low. When temperature changes occur in the roof as it burns or wears thinner, the shape of the contacting brick surfaces changes hardly at all, due to the nature of the silica expansion curve and any danger of pinch spalling is largely avoided.

Conditions are entirely different with basic bricks. They have an expansion curve in which the rate of expansion actually increases in the high-temperature range and the total expansion up to 3000 F. is considerably greater than that of silica. The basic bricks therefore expand laterally progressively more as their Patented Aug. 1, 1961 lower or hot ends are approached, thereby becoming.

wedge shaped. The compensation due to the elastic yielding of the furnace binding may be insulficient to make up for the greater expansion, and even if it were suflicient, the bricks at operating temperature would still engage one another only at their lower ends. The entire load thus being concentrated on a very small area where the temperature is highest and the brick weakest, pinch spalling is sure to result.

Hitherto, the only attempt which has been made to mitigate this trouble has consisted in the insertion of steel plates about A inch thick between the sides of the roof bricks. About 1 inch to 1 /2 inches of the lower ends of the plates oxidize, soften and impregnate the bricks, leaving gaps between the lower ends of the bricks, while the part of the plates just above the melted or absorbed portion becomes comparatively soft under the high temperature and acts as a sort of plastic cushion. However, in any case this is only a half way measure. To remove the pinch-spelling trouble entirely, the skewbacks of the arch should be moved as the roof expands on heating and as it burns thinner, and the movement should be such as to insure uniform distribution of the load over the contacting brick faces under all conditions.

One specific way that has been proposed for moving the skewbacks is to press laterally movable skewbacks against the sides of a roof so that the skewbacks can move outward as the roof expands. That solution is not satisfactory, because the thrust of the roof against the skewbacks decreases as the roof burns thinner, and the spring loaded skewbacks then cause rising of the arch and therefore still greater concentration of stress in the lower ends of the bricks, with danger of buckling and collapse of the roof.

It is among the objects of this invention toprovide apparatus which will assure the load or thrust on the arched refractory roof being more uniformly distributed over the vertical length of the bricks as the roof expands, which will do this automatically regardless of the thickness of the roof, and which is controlled by the temperature of the roof to compensate for its thermal expansion and contraction.

In accordance with this invention, an arched refractory furnace roof spans the space between the tops of its side walls. One side of the roof is held on top of one of the walls, in any suitable manner, while the opposite side engages a skewback that is slidably mounted on top of the other wall. The skewback is held tightly against the roof by movable means. Means responsive to the temperature of the roof actuate the movable means in the directions and for the distances necessary to allow sliding of the skewback outward on the underlying side wall when the roof expands and to maintain the skewback tightly against the roof when it contracts. For best results, the movable means just mentioned include cams that are shaped to exert pressure against the skewback at progressively higher elevations as the roof continues to expand after a predetermined amount of expansion. The cams may be controlled by screws operated by a motor controlled by the temperature of the roof.

The preferred embodiment of the invention is illustrated in the accompanying drawing, in which FIG. 1 is a fragmentary transverse section through a furnace showing the roof hot and expanded;

FIG. 2 is a fragmentary side view of the furnace with parts broken away;

FIG. 3 is a fragmentary view, similar to FIG. 1, but showing the roof partly worn away and expanded more than in FIG. 1;

FIG. 4 is an enlarged fragmentary vertical section of the side of the roof opposite to that shown in FIG. 3;

FIG. 5 is a fragmentary vertical section through one side of a roof showing a modification;

FIG. 6 is a fragmentary view of a small modification of FIG. 5; and

FIG. 7 is a fragmentary side view of an expanded roof arch, showing how the upper ends of basic brick spread apart when my invention is not used.

Referring to FIGS. 1 and 2 of the drawing, skewbacks l and 2 are mounted on the tops of furnace side walls 3, which are reinforced and prevented from spreading apart by means of the usual buckstays 4. The opposite sides of an arched refractory roof 5 engage the skewbacks, which hold the arch in place. In accordance with this invention, both of the skewbaoks may be slidable transversely of the walls supporting them, but that is not necessary, so only one of them has been shown slidable in order to simplify this description. The other skewback 2 is mounted in fixed position as is customary, except that it is allowed to tilt through a limited angle with its backing up channel 6, as will be described later. The slidable skewback 1 is backed up by a rigid supporting member, such as a channel 7, engaging its outer surface and slidable back and forth across the wall with it. Overlapping the top of the channel is an angle bar 8 secured to the inner surfaces of the buckstays. This bar is spaced a short distance from the channel in order to permit the channel and skewback to tilt if the expanding roof tends to cause such tilting. Another angle bar 9, supported by the buckstays underneath the channel, supports the weight of the left-hand part of the arch 5 and provides a metal surface, on which skewback support 7 can slide outward and inward.

The channel 7 is held against the skewback, which in turn is held against the side of the roof, by movable means supported by the buckstays. Preferably, such means include a number of parallel substantially horizontal screws 19 extending in a direction transverse to the channel. Each screw may be journaled in a bearing housing 11 secured to one side of a buckstay, and the central portion of the screw has a worm gear 12 keyed onto it. The line of gears can be turned simultaneously by worms 13 keyed on a rigid line-shaft 14 that extends lengthwise of the furnace through the buckstays. At a suitable location on the shaft there is a sprocket 15 that can be turned in either direction by a chain 16 extending around it and also around a sprocket 17 on the shaft of a reversible electric gear-motor 18 supported by one of the buckstays. It will be seen that when the motor is in operation, all of the screws 10 will be turned simultaneously and will move either toward or away from the center of the furnace. Although the inner ends of the screws may bear directly against channel 7, for best results they engage vertical cams 19 interposed between them and the channel. Each cam has a bifurcated upper end pivotally suspended from a link 20 that in turn is pivoted to a bracket 21 projecting inward from a bearing housing 11. The vertical faces of the cams that engage the channel are curved in a manner that will be described presently.

The operation of the motor is under the control of apparatus responsive to the temperature of the roof. Thus, a number of thermocouples 22 are inserted in the top of the roof, generally for a distance of about /3 of its thickness. These thermocouples are wired to a controller 23 in such a manner that the latter responds to the average temperature of the roof. Assuming that the roof is cold with the slidable skewback I pressed tightly against it by the screws and cams, when the furnace is put in operation and the roof becomes hot the roof tends to expand laterally. The thermocouples transmit to the motor controller the fact that the roof is at a high enough temperature to expand, so the controller operates the motor to back otf the screws 10 at the same rate as the roof expands. Of course, the screws always maintain the adjoining skewback tightly against the roof to preserve the arch. Since lateral expansion of the roof is not permitted by a stationary skewback, but is permitted by the sliding skewback, the line of thrust remains the same and the joints at the top of the arch do not open up and concentrate stress at the lower ends of the bricks. Controller 23 is so set that for each increment of brick temperature increase, the screws permit the sliding skewback to move outward a definite calculated amount.

The shape of cams 19 is such that they engage channel 7 at points about half way between the upper and lower edges of the adjacent side of the roof. Those contact points therefore are at the center of the thrust exerted by the expanding roof against the sliding skewback, which is the correct location for preventing a change in the position of the line of thrust as expansion occurs. However, as the roof wears away from beneath, as shown in FIG. 3, the center of pressure will rise on the skewback so that the cams should engage channel 7 at higher points than before. Fortunately, this can be done because, since the upper part of a thin roof is hotter than the upper part of a thick roof, the entire roof will be at a higher temperature than a thick roof and therefore will expand more. The increased expansion means that the screws will allow the cams to swing farther outward, and the shape of the cams is such that as this occurs they will progressively engage the channels at higher levels.

When the furnace is cooled down for repairs or other reasons, it is obvious that the roof will contract and might fall into the furnace if the sliding skewback is not moved inward during the contraction and at the samerate. Such movement of the skewback is provided for because the thermocouples in the cooling roof will signal controller 23 to operate the motor in a reverse direction in order to push the cams inward to maintain the adjoining channel and skewback tightly against the side of the roof as it contracts. Temperature controller 23 preferably is a type in which the motion of the movable element is not in constant proportion to the roof temperature throughout the entire temperature range, but decreases in proportion to the roof temperature in the higher part of the range. For example, when the thermocouples are inserted in a basic roof to a depth of about 30% of the initial roof thickness, it is desirable for a roof span of 20 feet to have screws 10 move outward about 0.58 inch for each F. increase of measured temperature when that temperature is below about 1350 F., but when the roof tempera ture is between 1350" and 2400 F. the screws should move outward only about 0.24 inch per 100 F. increase in temperature. Such variable scale temperature controllers are on the market and do not themselves form part of this invention. Also, the specific way in which the temperature controller causes the motor to rotate the screws the proper number of turns is not part of this invention, because apparatus capable of doing that is well known.

The right-hand skewback 2 shown in FIG. 1 may be fixed, in the sense that it does not slide horizontally across the supporting wall, but, like the other skewback 1, it should be allowed to tilt as may be required in order to conform to the different positions assumed by the roof arch as the latter expands and contracts. The position of skewback 2, while the roof is hot and has its original thickness, is shown in solid lines. The broken lines show its position when the roof is cold and contracted and therefore has a shorter radius. Channel 6- is spaced from the adjacent buckstays by suitable strong springs. One type of spring that is satisfactory is a heavy U-shaped member 24. When the roof is cold, the skewback channel can rock or tilt on the adjoining springs to the position shown in broken lines. When the roof becomes hot, the arch will tend to straighten out and that will cause channel 6 to be rocked into upright position as shown in full lines. In both positions the springs will be compressed because of the thrust of the roof against them. When the roof wears thin, it will expand even more as has been explained above and will therefore tend to straighten transversely still more. This will cause it to tilt skewback 2 in channel 6 in the opposite direction, as shown in FIG. 4. At the same time, the lateral thrust of the roof decreases, because the thrust is approximately proportional to the roof thickness. This allows the springs to expand, and in doing so to engage the channel at a higher level to keep the point of reaction near or at the line of horizontal thrust of the roof. For this purpose the channel-engaging side of each spring is specially shaped like a cam so that the point of reaction will move up and down the channel as it is tilted back and forth. The toe 25 at the bottom of the spring prevents the bottom of the channel from moving outward too far. Consequently, the degree of tilting of the channel and skewback is definitely limited between the position corresponding to the cold position for any thickness of roof and that corresponding to the hot position of the roof when worn thin, but within those limits the skewback is free to tilt in order to conform to the arch position under any conditions.

Where only one skewback is movable back and forth to take care of expansion and contraction of a wide roof made of basic refractories, which have great expansion, it is preferred that the weight of the arch not be carried by the skewback channel sliding against its support. Instead, it is more suitable to support this weight from swinging members such as shown in FIG. 5. Thus, an arm 26 beside each buckstay is pivotally suspended from a bearing 27 mounted on the buckstay, and has its lower end pivotally connected by a pin 28 to the lower part of an upwardly and outwardly inclined lever 29 secured to a channel 30 supporting a movable skewback 31 of a refractory roof arch 32. The upper end of the lever is pivotally connected by a pin 33 to one end of a link 34. The opposite end of the link is pivotally connected with the upper end of the arm, so that the angle which the channel makes with the underlying side wall 35 of the furnace will be controlled by the lever as the arm is swung inward or outward. Adjusting screws 36 are provided as before and are operated in accordance with the temperature changes of the roofs, but instead of engaging pivoted cams, they engage bearing plates 37 rigidly mounted on the outside of the channel.

In FIG. 5 the skewback is shown in the position it occupies when the roof is hot and still has its original thickness. When it cools, the screws move inward to keep the skewback tight against the arch. As the skewback and its supporting channel 30 move back and forth, they are supported and guided by the lower ends of the arms and levers. The skewback will move inward and outward in a substantially horizontal line, but it should also tilt a controlled amount if the pressure is to be uniformly distributed throughout the thickness of the roof arch. Tilting is controlled by the levers 29. Therefore, the location of pivot pin 33 is carefully chosen with reference to the temperature changes in, and the corresponding expansion changes of, the roof. The pin must not only be free to move upward as the skewback moves outward, but it also must be guided along a slanting path. This can be accomplished either by link 34 or, as shown in FIG. 6, mounting a pin 40' in the upper end of a modified lever 41 and slidingly mounting the pin in a slotted guide 42 attached to the buckstay 43.

According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I drwire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. In an industrial furnace, side walls, an arched refractory roof between the tops of said walls, means limiting outward movement of one side of the roof on top of one of the walls, a skewback supported at the top of the other wall in engagement with the adjoining side of the roof, said skewback being movable across the underlying wall, movable means holding said skewback tightly against the roof, and means other than the roof itself but responsive to the temperature within the upper part of the roof for actuating said movable means in the directions and for the distances necessary to allow movement of the skewback outward across the underlying wall when the roof expands and to maintain the skewback tightly against the roof when it contracts.

2. In an industrial furnace as recited in claim 1, said movable means including vertical cams shaped to exert pressure against said skewback at progressively higher elevations as the roof continues to expand after a predetermined amount of expansion.

3. In an industrial furnace, side walls, an arched refractory roof between the tops of said walls, means engaging one side of the roof and tiltably mounted on top of one of the walls, buckstays beside said one wall and spaced from said roof engaging means, springs compressed between said means and buckstays, a skewback supported at the top of the other wall in engagement with the adjoining side of the roof, said skewback being movable across the underlying wall, movable means holding said skewback tightly against the roof, and means other than the roof itself responsive to the temperature within the upper part of the roof for actuating said movable means in the directions and for the distances necesesary to allow movement of the skew-back outward across the underlying wall when the roof expands and to maintain the skewback tightly against the roof when it contracts.

4. In an industrial furnace as recited in claim 3, each of said springs having a vertically convex inner face engaging said roof-engaging means, said face being shaped to engage said means at a higher level as the spring expands.

5. In an industrial furnace, side walls, an arched refractory roof between the tops of said walls, means limiting outward movement of one side of the roof on top of one of the walls, a skewback slidably mounted on top of the other wall in engagement with the adjoining side of the roof, movable means holding said skewback tightly against the roof, and means other than the roof itself but responsive to the temperature within the roof for actuating said movable means in the directions and for the distances necessary to allow sliding of the skewback outward on the underlying side wall when the roof expands and to maintain the skewback tightly against the roof when it contracts.

6. In an industrial furnace, side walls, an arched refractory roof between the tops of said walls, means limiting outward movement of one side of the roof on top of one of the walls, a skewback slidably mounted on top of the other wall in engagement with the adjoining side of the roof, supporting means engaging the outer surface of the skewback, a plurality of horizontally spaced screws extending transversely of the furnace with their inner ends pressing said supporting means against said skewback to hold it tightly against the roof, a motor for turning the screws in opposite directions, and motor-control means responsive to the temperature within the roof for driving the motor to move the screws axially in the directions and for the distances necessary to compensate for the thermal expansion and contraction of the roof, so that said skewback will be allowed to slide outward on the underlying side wall as the roof expands and will be forced inward as the roof contracts.

7. In an industrial furnace, side walls, an arched refractory roof between the tops of said Walls, means limiting outward movement of one side of the roof on top of one of the walls, a skewback slidably mounted on top of the other wall in engagement with the adjoining side of the roof, supporting means engaging the outer surface of the skewback, a plurality of horizontally spaced screws extending transversely of the furnace with their inner ends pressing said supporting means against said skewback to hold it tightly against the roof, a shaft extending transversely of said screws, gears operatively connecting the shaft of the screws, a motor for turning the shaft in opposite directions, and motor-control means responsive to the temperature of the roof for driving the motor to move the screws axially in the directions and for the distances necessary to compensate for the thermal expansion and contraction of the roof, so that said skewback will be allowed to slide outward on the underlying side wall as the roof expands and will be forced inward as the roof contracts.

8. In an industrial furnace, side walls, an arched refracto-ry roof between the tops of said walls, means limiting outward movement of one side of the roof on top of one of the walls, a skewback slidably mounted on top of the other wall in engagement with the adjoining side of the roof, supporting means engaging the outer surface of the skewback, a plurality of horizontally spaced screws extending transversely of the furnace beside said supporting means, vertical cam spacing the inner ends of the screws from said supporting means and holding said skewback tightly against the roof, means spaced vertically from the screws for pivotally supporting the cams for movement in a direction lengthwise of the screws, a motor for turning the screws in opposite directions, and motor-control means responsive to the temperature of the roof for driving the motor to move the screws axially in the directions and for the distances necessary to compensate for the thermal expansion and contraction of the roof, so that said skewback will be allowed to slide outward on the underlying side wall as the roof expands and will be forced inward as the roof contracts, said cams being shaped to engage said supporting means at progressively higher elevations as the roof expands beyond a predetermined amount.

9. In an industrial furnace, side walls, buckstays beside the walls, an arched refractory roof between the tops of said walls, means for limiting outward movement of one side of the roof on top of one of the walls, a skewback at the top of the other wall in engagement with the adjoining side of the roof, a skewback support, vertically extending arms above said support, means pivotally connecting the upper ends of the arms with the adjoining buckstays, the lower ends of the arms pivotally supporting said skewback support so that it can move across the underlying wall, movable means pressing said support and skewback toward the roof, means other than the roof itself but responsive to the temperature of the roof for actuating said movable means in the directions and for the distances necessary to allow movement of the skewback and support outward across the underlying wall when the roof expands and to maintain the skewback tightly against the roof when it contracts, levers having one end rigidly connected to said skewback support and extending outward and upward therefrom, and means guiding the upper ends of the levers as the lower ends of said arms are swung inward and outward, whereby to control the tilting of said support.

10. In a furnace as recited in claim 9, said guiding means being links pivotally connected to the upper ends of the levers and to the adjoining buckstays.

11. In a furnace as recited in claim 9, said guiding means comprising pins mounted in the upper ends of the levers and slidably mounted in slotted members attached to the buckstays.

12. In an industrial furnace, side walls, an arched refractory roof between the tops of said walls, tiltable skewbacks supported at the tops of the walls in engagement with the adjoining sides of the roof, means limiting outward movement of the skewbacks across said walls, means other than the roof itself but responsive to the temperature within the upper part of the roof for actuating one of said limiting means in the directions and for the distances necessary to allow movement of the adjoining skewback outward across the underlying Wall when the roof expands and to maintain that skewback tightly against the roof when it contracts, and means for controlling the tilting of the skewbacks as the roof expands and contracts.

13. In an industrial furnace, side walls, an arched refractory roof between the tops of said Walls, tiltable skewbacks supported at the tops of the walls in engagement with the adjoining sides of the roof, means limiting outward movement of the skewbacks across said walls, means other than the roof itself but responsive to the temperature within the upper part of the roof for actuating one of said limiting means in the directions and for the distances necessary to allow movement of the adjoining skewback outward across the underlying wall when the roof expands and to maintain that skewback tightly against the roof when it contracts, said limiting means applying pressure to the skewbacks at progressively higher elevations as the roof expands beyond a predetermined amount.

References Cited in the file of this patent UNITED STATES PATENTS 2,641,207 Pollen June 9, 1953 2,698,588 Davies Jan. 4, 1955 2,834,306 R-oxburgh May 13, 1958 FOREIGN PATENTS 618,216 Germany Sept. 4, 1935