GB2104635A - Heating apparatus - Google Patents

Abstract

Apparatus for heating hollow objects, in particular for preheating metal-pouring ladies, comprises a low thermal mass plug body (90) to be inserted into the object to be heated and means for directing a hot gas flow, conveniently produced by a burner (78) in the bottom or inner region of the plug, through a relatively narrow gas path that is either formed by a gap between the body and the interior wall of the object or is in a conduit formed between the body and a thin-walled surrounding shroud (92) on the body. The hot gas flow is thereby used more efficiently than if it were simply allowed to fill the interior of the body to be heated. Efficient heat transfer is obtained from the flow of gas in contact with or close to the interior wall of the object, and in the latter case the flow is enclosed by a shroud arranged to reach the hot gas temperature quickly and radiate heat to the inner wall of the object. <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus. It is particularly, but not necessarily exclusively, concerned with such apparatus for preheating ladles for pouring molten metal, in particular for molten steel.

The ladles used for pouring molten steel from steel-making furnaces are refractory-lined and have to be preheated before the molten metal is poured into them, initially to remove moisture present that could expand and damage refractory material, and subsequently to prevent chilling of the metal and reduce damage to the refractory material through thermal shock.

The preheating methods used are extremely inefficient. Combustible material may be heaped into a ladle and fired, or a burner flame may be heaped into a ladle and fired, or a burner flame may be played upon the ladle interior walls. It is known to fit a lid onto the ladle carrying a gas burner so as to direct its flame into the ladle, but this offers only a limited improvement because the heat transfer effect is inefficient and most of the heat is carried away by the waste gases. External heat recuperation techniques are known in which a separate heat exchanger recovers waste heat to pre-heat combustion air but such recuperators are bulky and costly due to the inherently inefficient heat transfer process within the ladle itself.

An apparatus according to the present invention for heating a hollow object, in particular for preheating a metal-pouring ladle, comprises a low thermal mass plug body adapted to be inserted into the object and to provide a relatively narrow gas path in a space between the plug body and the inner wall of the object, and provided with means for directing a hot gas flow into said path to heat said inner wall.

By "low thermal mass" is meant a plug which has a total heat capacity in use (i.e. the product of the specific heat of the plug material, its mass and its mean temperature) that is substantially less than the total heat capacity of the walls of the hollow object, in the particular case of a metalpouring ladle that of the ladle refractory material, when the adjacent walls of the plug and the object are at the same temperature under steady state conditions. Generally the total heat capacity of the plug should not be substantially greater than 20% of the refractory heat capacity, and preferably it is not substantially more than 10% of the refractory heat capacity.

Because of its low thermal mass the plug aids heat transfer to the inner wall, e.g. the refractory lining of the ladle, by heating up quickly and then re-radiating to the inner wall. In use, if the gas path occupies all the space between the plug body and said inner wall the ladle or other object is preheated both by increased convection due to the restricted path for the gases past the inner wall and by re-radiation of heat from the plug. Even if, as may be required for practical reasons, the gas flow is kept out of contact with said inner wall by a thin-walled shrouding member to that only the re-radiation effect is experienced, the efficiency of heating is greatly enhanced as compared with conventional methods.

Apparatus according to the invention for preheating metal-pouring ladles can be arranged to make use of existing ladles, provided with a refractory lining. The apparatus can also use existing burner appliances, as in the example of a lid-mounted gas burner in which case the means with which the plug is provided for directing a hot gas flow may be simply a duct leading to its bottom or inner end region for the combustion gases from the burner, but in a preferred arrangement a burner is disposed adjacent the bottom of the plug whereby to reduce the heat transmission to the radially inner region of the plug.

Particularly in the case of a burner mounted in the lower region of the plug, it is also possible to provide a combustion air flow through a passage or passages in or adjacent a circumferential wall of the plug in order to preheat the air by heat conducted through said wall during the preheating operation. In this way the heating efficiency is increased and the maximum temperature of the inner region of the plug can be limited.

To increase the heat transfer from the hot gas flow, the plug may have a profiled outer wall facing onto the inner wall of the object. As another measure to increase the transfer of heat from the hot gas, the plug outer wall may be adapted to fit the inner wall closely and have a recessed profile that forms an extended gas path leading the hot gas around its circumference.

For economy of production, the plug may be made up of a number of sections that can be assembled in different combinations to suit different sizes and shapes of object. If moulded blocks of ceramic fibre material are used for their high-temperature capabilities, these sections may comprise a series of annular or plate-like members that can be superimposed on each other, or a series of segmental members which are preferably fitted with mutually interlocking elements to locate them relative to each other.

If permitted by the maximum operating temperatures the outer face of the plug may be made of metal, but even if ceramic fibre materials are required to face the gas path, it is possible for these to be provided in the form of an outer circumferential jacket or lining on a supporting wall structure.

Embodiments of the invention will be described by way of example with reference to the accompanying drawings, in which: Figs. 1 to 3 are axial sectional views of three alternative forms of apparatus according to the invention mounted in pouring ladles, Figs. 4 and 5 illustrate two forms of profile outer faces for the plug of apparatus according to the invention, Fig. 6 is a plan illustration showing the formation of a core for apparatus according to the invention from a series of segmental members, Figs. 7 and 8 are, respectively, an axial sectional view and a plan view with the lid removed of a further form of apparatus according to the invention, Figs. 9 and 10 are a pair of similar views of another form of apparatus according to the invention, Fig. 11 illustrates a ladle preheating installation in which apparatus according to the invention is used, and Fig. 12 illustrates another installation using ladle preheating apparatus according to the invention.

In Fig. 1, the pouring ladle L with its refractory lining R is shown chain-dotted lines, as is also a lid 2 placed over the ladle for a preheating operation.

The lid 2 is of known form, with a central gas burner 4 to direct a hot gas flow downwards into the ladle interior, and with vents 6 for the exhaust gas flow.

For improving the efficiency of heating operation a plug 10 is inserted in the ladle. This is a one-piece vacuum-formed block of ceramic fibre material having a generally annular shape that rests centrally in the bottom of the ladle to leave a central passage 12 for the combustion gases from the burner and a narrow annular path 14 for the combustion gases between the plug outer face and the inner wall of the ladle ceramic lining, as the gas flows to the exhaust vents 6. Connection between the inner passage and outer annular path is provided by integrally formed recesses 1 6 in the bottom face of the plug. The top face of the plug fits the underside of the lid sufficiently closely to prevent a significant leakage flow from the central passage past this face to the exhaust vents.The top face is also formed with an inner lip 1 8 that is engageable with a portion (not shown) of the lid so that the plug is lifted and lowered with the lid.

In use, the plug has the effect of increasing the velocity of the gas flow in contact with the ladle refractory lining, thereby improving heat transfer and reducing the amount of fuel needed to maintain a given heating rate. The low thermal mass of the ceramic fibre plug also heats up rapidly and then assists the heat transfer process by radiation to the refractory lining of the ladle.

During this operation the whole of the plug insert will reach the hot gas temperature because it has a very much smaller heat capacity than the refractory lining of the ladle.

If the plant is equipped with a number of different sizes of ladle, it is possible to limit the number of moulds required to produce the ceramic fibre plugs by building up each plug from a range of simpler mouided members. As illustrated in Fig. 2, the plug 20 comprises a number of annular members 20a of different outer diameters selected to form between them and the interior contours of the ladle a relatively narrow tubular hot gas path 22. The members are pinned together by high-temperature metal staples 24.

Lifting eyes 26 are secured to a mounting ring 28 on the underside of one of the members 20 for lifting and lowering the assembled plug independently of the ladle lid 2 if required.

Another composite plug construction is shown in Fig. 3. The base member 30a of the plug 30 with the gas flow recesses 1 6 and an inner sleeve 30b are clamped together by tie bolts 32 that extend between upper and lower clamping rings 34, 36. The remaining ceramic fibre members 30c of the plug insert simply rest upon this inner structure and have radial engagement faces to locate them concentrically. The upper ring 34 projects radially from the inner sleeve to provide a lifting lip for the complete plug.

By profiling the outer wall of the plug and so increasing the surface area of the plug exposed to the hot gas flow, the plug temperature can be raised more quickly to radiate heat to the ladle ceramic lining. Fig. 4 shows a tubular plug body 40 with a fluted outer surface 42 for this purpose.

It is also possible to increase the heat transfer from the hot gases to the ladle ceramic lining by increasing the gas dwell time in the ladle. A configuration particularly suitable for large ladles is shown in Fig. 5 in which the tubular plug 50 is intended to fit more closely to the ladle inner walls than the previously described embodiments and a spiral groove 52 around the plug outer surface provides the main gas path, which accordingly has a length many times greater than the ladle height.

In all the examples described, to permit simpler moulds to be used and/or to give a greater variety of plug shapes and sizes from a given number of moulds, the plugs can be composed of a number of segmental members, these extending the whole or a part of the height of the plug. Fig. 6 is a composite view illustrating a number of alternative configurations of segmental members for building up a tubular plug body, including members 60a with planar radial faces, members 60b with inerlocking portions that locate the members radially of each other, and members 60c with interlocking portions that locate the members radially and circumferentially of each other. Where supplementary location means are required between the members as with the radial-faced members 60a, high-temperature adhesive and/or metal pins or staples 62 may be employed.

Whereas Figs. 1 to 3 illustrate the use of a known form of ladle lid in which a gas burner is mounted, it may be preferred to position such a burner in the bottom region of the plug in an apparatus according to the invention, thereby reducing the temperature stresses on the inner core of the plug. Figs. 7 and 8 illustrate one construction in which this is done, the plug 70 in this instance comprising an inner metal shell 72 with radial braces 74 supporting concentric gas and air tubes 76a, 76b that extend into a combustion chamber 78 formed in a bottom ceramic fibre member 70a of the plug. In the combustion chamber there is an ignitor 80 and a flame sensor 82. The ceramic fibre members of the plug are all supported by the inner metal structure and comprise an outer fluted jacket or liner 84 and an upper ring 86. The ladle lid 88 rests on the upper ring 86 to be lifted with the plug using the lugs 74a attached to the inner metal braces 74.

Figs. 9 and 1 0 show a modified form of the construction of Figs. 7 and 8, in which the plug 90 has a double-walled metal shell 92. The combustion air inlet 94 is directed into the space between the shell walls, where there is an internal divider 96 forming a spiral path for the flow to a series of bottom radial pipes 98 leading into the combustion chamber 78 which receives a gas supply through central pipe 100. The incoming air is thereby preheated by conduction through the plug outer lining 84 before reaching the combustion chamber, so improving the fuel economy and also limiting the temperature to which the metal shell 92 is subjected. The arrangement permits a thinner ceramic fibre outer lining 84 to be used for a given maximum gas temperature.

It may be possible, particularly for use with larger sizes of ladle, to have extremely thin outer ceramic fibre linings.

Fig. 11 illustrates a typical installation using apparatus according to the invention. Ladles L are moved by a shop gantry crane G to and from a station at which the preheating operation is performed. At the station there is a frame structure 110 comprising a vertical column 112 on which a beam 114 can be raised and lowered and the beam can also be pivoted to one side or the other on a pivot 11 6 so as to service a number of ladles standing side by side at the station at any one time. The preheating apparatus 11 8, comprising in this instance the pre-assembled plug bodies, ladle lids and gas burners, are suspended from a trolley 1 20 that runs along the beam.When a ladle has been placed at the station, an apparatus 118 can thus be swung over and lowered into it, and the burner and ignited.

The operation being completed, the apparatus is removed and the ladle is taken by the shop crane G to the steelmaking furnace to be filled.

Fig. 12 illustrates another installation using apparatus according to the invention, in this instance with the ladle positioned horizontally. In this figure a form of plug is shown which is adapted to cater for variations in the inside diameter of the ladle, such as may occur over a period of use due to wear of the refractory lining or a build-up of slag around the ladle rim.

The plug comprises an inner metal shell 122 with radial braces 124 supporting concentric gas and air tubes 126a, 126b leading to a combustion chamber 128 in a bottom ceramic fibre member 130. An ignitor and a flame sensor are provided in the chamber 1 28. The hot combustion gases leave the chamber through ports 1 36 but the flow is contained by an outer high-temperature alloy shroud 138 which comprises a bottom plate 140 secured to the pipe 1 26b and a frusto-conical wall 142 secured to the inner metal shell 122.

The gas flow from the combustion chamber thus goes through the ports 1 36 into the annular space 144 between the bottom member 130 of the plug and the bottom plate 140, then to the tubular space 146 between the main ceramic fibre body 148 of the plug and the wall 142 to reach a collector space 150 in the upper region of the plug where the top of the shroud wall 142 seals against an outer ceramic member 1 52. Finally a flue 1 54 discharges the gas from the collector space.

In use, the outer metal shroud heats up very rapidly because it has a minimal heat capacity and it will radiate heat with nearly 100% efficiency. At the same time, however, it determines the size of flow passage available for the combustion gases within the ladle and so ensures a constant performance independently of any wear of the ladle walls or build up of deposits which would otherwise create variations in the spacing between the ladle inner wall and the opposed faces of the ceramic fibre body of the plug.

Another feature of the construction is that variations in the dimensions of the ladle inner walls and top lip will not give rise to leakage of combustion gas. However, in some circumstances it may be preferred to perforate the shoud wall so as to allow a small proportion of hot gas to contact the refractory walls of the ladle, and the plug will then be provided with means to ensure that the top of the ladle is closed off and all the combustion gas is directed into the flow passage.

The plug is shown mounted on a trolley 1 62 movable along rails 164 to run the plug in and out of a stationary ladle held with its central axis horizontal. The trolley is moved using a handwheel drive 1 66. The trolley also carries a blower 168 for the combustion air supply to the plug.

It should be clear that the form of plug illustrated in Fig. 12 can equally well be used in an installation in which the ladle axis is upright, and that the plugs of the earlier figures can be used in a horizontally oriented preheating station such as, is shown in Fig. 12. More generally, it is of course possible to combine individual features from the embodiments in other ways than has been shown in the illustrated embodiments, which have been included by way of example.

Also, although the examples described above show the use of gas burners for providing the heating gas flow, it will be understood that it is possible to utilise any hot gas of suitable temperature from any source, such as waste furnace gases.

While the examples described above are concerned with the preheating of pouring ladles, the invention can be applied to other heating operations, such as the preheating of refractorylined tun dishes, e.g. for continuous casting, and the heat-treatment of metal tubes, and also including operations in which heat is used for drying purposes and other relatively low temperature applications. Thus, a plug body can be provided according to the invention for assisting the drying of hollow concrete products, including spin-cast pipes. In the heating of tubular or pipe-like objects, one end of the object may be blocked and the body, analogously to the illustrated examples, inserted into the other end to leave a relatively narrow gas path for the hot gas flow between it and the inner wall of the object.

Claims (14)

1. Apparatus for heating a hollow object comprising a low thermal mass plug body adapted to be inserted into the object and to provide a relatively narrow gas path in a space between the plug body and the inner wall of the object, and provided with means for directing a hot gas flow into said path to heat said inner wall.

2. Apparatus according to claim 1 wherein the plug has a total heat capacity not substantially more than 20% of the total heat capacity of the object to be heated.

3. Apparatus according to claim 2 wherein the plug has a total heat capacity not substantially more than 1 0% of the total heat capacity of the object to be heated.

4. Apparatus according to any one of claims 1 to 3 wherein a gas entry duct extends into the plug body to an inner or bottom region thereof.

5. Apparatus according to any one of claims 1 to 3 wherein a fuel burner is provided in an inner or bottom region of the plug body.

6. Apparatus according to any one of the preceding claims wherein at least one airflow inlet passage is provided in the plug at or adjacent a circumferential wall thereof.

7. Apparatus according to any one of the preceding claims wherein the plug body comprises conduit means at its outer periphery for the hot gas flow, the outer wall of the conduit means comprising a thin-walled metal shroud.

8. Apparatus according to claim 7 wherein said shroud has apertures for a proportion of the hot gas flow to pass between the shroud and the interior wall of the object to be heated.

9. Apparatus according to any one of the preceding claims wherein the plug has a profiled outer wall arranged to face onto the inner wall of the object to be heated.

10. Apparatus according to claim 9 wherein said outer wall has a recessed periphery arranged to form an extended gas path leading hot gas around its circumference when the plug is inserted in the associated object to be heated.

11. Apparatus according to any one of the preceding claims wherein the plug body comprises a ceramic fibre material.

12. Apparatus according to claim 11 wherein the plug body comprises a plurality of preformed blocks of ceramic fibre material.

1 3. Apparatus according to claim 11 or claim 12 wherein the ceramic fibre material extends circumferentially around a supporting wall structure of the plug body.

14. Apparatus according to any one of the preceding claims wherein the plug body is adapted to rest on a bottom wall of the interior of the hollow object and is provided with recesses in its bottom face for the passage of hot gases past said bottom wall to said narrow gas path at the outer periphery of the plug body.

1 5. Apparatus for heating a hollow object constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.

1 6. An installation for heating a hollow object comprising apparatus according to any one of the preceding claims and means for inserting and removing the plug body as a unit into and out of the hollow interior of the object to be heated.