Technical Field

The present invention relates to furnace cooling elements, and also relates to a method of forming cooling elements in general to improve- their thermal efficiency. Background Art

Furnace cooling elements are utilised for. cooling the refractory lining and shell of a high temperature ^' furnace to prevent excessive heating and cracking of the external shell of the furnace and so extend the operating campaign life of the furnace.

. he cooling elements take the form of cast metal plates or blocks with elongated metal cooling tubes cast therein and through which coolant is passed. More particularly, traditionally cast iron plates and steel tubes are used in blast furnaces and electric furnaces and are called staves.

In producing the cooling elements, the metal tubes may be protectively coated to prevent harmful metallurgical bonding. Such protective coatings are conventionally provided by thermosprayed alumina, which is a costly requirement.

It is an object of a first aspect of the invention to provide a furnace, cooling element construction in which the necessity for such costly protective coatings is dispensed with, whilst at the same time the thermal efficiency of the element is improved.

Furthermore, during the formation of conventional furnace cooling elements, a small gap is formed, as a result of casting, between the outside of the metal cooling tube and the surrounding material of the cast metal plate or block.

We have also discovered that the size of the gap has a direct and significant effect on the heat exchange between the components due to the insulating effect of the gap.

It is therefore an object of a second aspect of the present invention to overcome this problem by providing a controlled method of reforming the metal tubes of the cooling elements in which the amount of gap is significantly reduced and the thermal efficiency of the cooling element is consequently improved. Disclosure of the Invention

According to the first aspect of the invention, there is envisaged.a method of forming a furnace cooling element, including the steps of positioning an external shroud member around an elongate metal cooling tube over a substantial length thereof, and thereafter casting a metal body around the combination of the inner metal cooling tube and its surrounding shroud member. According to the second aspect of the invention, there is envisaged a method of forming at least part of a furnace cooling element, including the step of sharply increasing the pressure of a fluid within an inner component thereof to expand said component closer

to a surrounding outer component.

Preferably the pressure of the fluid in said inner component is sharply increased by detonating an explosive charge in the fluid in said component, or in communication with the fluid in said component, although other means of sharply increasing fluid pressure, such as the application of percussive forces to elements acting on said fluid, to sharply increase the pressure within said fluid, may be utilised. In one form of this second aspect of the invention, as applied to a basic furnace cooling element, the metal body is metal plate or block which is cast around one or more elongated metal tubes to form a cooling element and thereafter the metal tube is explosively expanded closer to the surrounding material of the metal plate. It has been found that the reduction of the gap between the, or each, tube and metal- plate significantly improves the thermal efficiency of the cooling element. The, or each, metal tuba may have a protective coating applied thereto.

In another form of this second aspect of the invention, the protective coating on the metal tube is replaced by an external shroud, according to the first aspect of the invention, and the inner metal cooling tube is explosively expanded closer to the surrounding shroud prior to casting to minimise the gap between the outside wall of the metal cooling tube and the . inside wall of the protective shroud. Alternatively, or in addition to, the metal cooling tube may again be explosively expanded closer, to the protective shroud after casting.

Expansion may be achieved by exploding a single relatively large charge of explosive, or alternatively, a series of repeated explosions using smaller explosive

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charges may be utilised to reduce the possibility of cracking or other damage to the cooling tube or surrounding metal body of the cooling element.

The invention also envisages a furnace cooling element produced according to any one of the methods hereinabove defined. Brief Description of the Drawings -

Preferred embodiments of both aspects of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a longitudinal cross-sectional view of a stave assembly (furnace cooling element) showing a steel tube cast within a cast iron plate preparatory to expansion by an explosive charge. Figure 2 is a cross-sectional view of an arrangement incorporating a continuous shroud pipe, to protect the inner steel cooling tube against harmful carburisation, and shows the arrangement prior to expansion of the inner tube by an explosive charge. a Figure 3 is a cross-sectional, view of a s .ave incorporating a continuous shroud pipe prior to explosive treatment to minimise the contraction gap resulting after casting.

Figure 4 is a schematic representation of a secti taken transversely across one of the steel cooling tubes in a stave that has been protected by a continuous shroud pipe, and prior to expansion by an explosive charge. Best Modes for Carrying out the Invention

Referring to Figure 1 of the drawings, in accord- ance with one preferred embodiment of the method of one aspect of the invention, and as an example of the effect¬ iveness thereof, a stave, comprising a cast iron plate or block containing four steel cooling tubes 11 (only one of which is shown) and protected by a coating of thermo-

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sprayed alumina, was, after casting of the plate or block 10, explosively expanded to improve the thermal efficiency of the stave.

An explosive charge weight of one strand of 20gm of instantaneous detonating fuse (generally indicated as 12) was attached at one end to a pull cord 13 with an electric detonator 14 attached to the other end. The pull cord was used to locate the explosive charge in the section of steel tube 11 parallel to the stave hot face 15. The steel tube was filled with water to the level indicated at 16. Each steel tube 11 within the

stave was similarly prepared. The- explosive charges were. initiated at separate time intervals to avoid undue stress on the stave iron.

The explosively treated stave was then placed in a test furnace together with a stave that had been manufactured by the same method, except for the explosive expansion treatment, to compare the heat extraction rate of each stave. The results of the furnace trial indicated that a significant improvement in the heat extracted from the explosively treated stave had been achieved as shown in the following Table 1. It will be observed from Table 1 that the exposed hot face temperature of the explosively treated stave had been lowered by between 80 C and 100 C compared to the hot face temperature of the standard trial stave under the same furnace conditions. TABLE 1

Furnace Temperature of Stave Flue Gas Hot Face C Temperature C Standard Exploded Difference

850 380 290 - 90 °

900 435 335 -100

950 490 390 -100

1000 540 640 -100

1050 580 490 - 90

1100 640 560 - 80

'Following the furnace trial it was observed that the explosive expansion treatment of the trial stave had resulted in extremely fine intermittent surface cracking of the stave iron. The cracking only occurred along the flat stave sides at one end where the thickness of cast iron surrounding the steel tubes was a minimum. Sectioning of the stave revealed that the depth of cracking was in the order of 6mm deep and that cracking

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had _, ceased when the thickness of stave iron 10 covering the .steel tube 11 exceeded 61mm depth. Examination of additional explosively expanded staves ^' revealed that the strength and thickness of cast iron 10 surrounding the steel tube 11 will determine if surface cracking would occur. Table 2 shows this effect when an explosive charge weight of 20gm is utilised.

TABLE 2 Explosive Charge Weight of 20gm -1

Cast Iron Thickness of Cast Type of Surface Strength Iron Covering Cracking

MPa Steel Tube

192 ,, Extremely fine and intermittent 186 60 mm Fine and Intermittent

212 60 mm NIL

It will be observed from Table 2 that the critical thickness of cast iron of 200 MPa tensile strength would be approximately 60 mm. To avoid surface cracking a design thickness of 70 mm of cast iron of 200 MPa strength covering the steel tubes would be required for a 20gm explosive charge. Using known transverse rupture stress formula other design thickness limits may be set. For example a stave iron of 150 MPa strength has a calculated critical thickness of 69 mm, the addition of a 10 mm safety factor would give a design thickness of 79 mm for a stave requiring explosive expansion by a 20gm explosive charge.

A preferred form of another aspect of the invention, which has been developed to completely eliminate the high cost -of alumina spraying the steel tubes, cost of •quality control etc and the resistance of the protective alumina coating to heat transfer, will now be described with reference to Figures 2 to 4 of the drawings.

In place of the alumina coating an external continuous steel pipe 21 (shroud pipe) of 5 mm minimum wall thickness is placed over a steel cooling tube 20 and a close fit between the outside of the cooling tube 20 and the inside of the protective continuous shroud pipe 21 is provided. The continuous shroud pipe 21 serves two purposes, the first is. to absorb all carbon diffusion from the cast iron during casting, this normally occurs within the outside 3 mm of the protective pipe wall thickness. The second purpose is to promote metallurgical bonding of the cast iron to the continuous shroud pipe 21 to give good heat transfer and to retain sections of stave iron that would normally be lost due to thermal fatigue cracking towards the end of the stave life.

Figure 2 of the drawings also shows, a method of reducing the "fit-up" gap between the steel cooling tube 20 and continuous shroud pipe 21 by explosive expansion, in accordance with a preferred form of the other aspect of the invention. Prior to bending tb Steel tubes the outside surface of the steel cooling tube 20 is cleaned and placed inside the continuous shroud pipe 21. The gap 22 between the concentric tubes may then be reduced by detonating a small linear explosive charge of lOgm of instantaneous detonating fuse 23. The explosive charge with detonator 24 attached at one end . is attached at the other end to a rubber plug 25 by a tie cord 26 and the inside of the steel cooling tube 20 is then filled with water to the level indicated at 27. Depending on the quality and strength of the internal steel cooling tube 20 an explosive charge of greater weight may be required to give a tighter fit. Also the outside surface of the continuous shroud pipe 21 may require support to prevent expansion of this pipe or possible splitting of the steel cooling tube 20.

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Following bending of the combination of tubes 20 and 21 to shape, to suit the stave mould, the tubes are cast within the stave body 28. Following casting, a small shrinkage gap results between the inside steel cooling tube 20 and the external shroud pipe 21 due to differential contraction caused by the bonding of the shroud pipe to the stave body, thereby limiting the contraction to the shroud pipe to the lesser contraction of.the cast iron stave body. This gap can be eliminated by a further explosive expansion treatment, resulting in excellent heat transfer between the stave hot face and the cooling medium within the steel cooling tubes.

Referring to Figure 3 the arrangement of one of the continuous shroud pipes 21, protecting the cooling tube 20, can be seen within the cast iron stave body 28, ^' prior to explosive expansion. A cross-sectional view of one of the protected tubes 20 is depicted in Figure 4 showing the metallurgical bonding of the continuous shroud pipe 21 to the cast iron stave 28, the contraction gap between the two tubes 20 and 21 and the approximate location of the explosive charge within the water filled steel cooling tube 20 prior to explosive expansion. In Figures 3 and 4 the detonating fuse is designated-by 23', the detonator by 24', the pull cord by 26', the water level by 27', and the remaining gap which is to be further explosively reduced is designated by 22'. The amount of explosive charge required for successful explosive treatment following casting of the stave depends on the following factors. A. The average width of the gap between the steel cooling tube 20 and the continuous shroud pipe 21. B. The diameter and wall thickness of the steel cooling tube 20.

C. The strength of the stave cast iron 28.

D. .- The minimum thickness of cast, iron 28 surrounding the continuous shroud pipe 21.

For example an average gap between tubes 20 and 21 of 0.5 mm with a steel cooling tube 20 of 65 mm diameter and 6 mm wall thickness would require an explosive charge weight of 20gm of instantaneous detonating fuse (V.O.D. 7000ms ) . This could safely be -carried out for a stave of 200 MPa tensile strength having a minimum, coverage of 70.mm of cast iron 28 surrounding the continuous shroud pipe 21. For staves of lower strength and/or reduced cast iron coverage of the continuous shroud pipe 21, multiple applications of an explosive charge of lower charge weight may be used to achieve the desired expansion of the steel cooling tube 20.

The .lower hot face operating temperatures achieved with the improved heat transfer efficiency of the furnace cooling elements of the present invention o has the effect of prolonging the service life of the

_furnace cooling elements due to a reduction in the incidence of erosion and thermal fatigue cracking.