GB2174985A

GB2174985A - Furnace sill cooling

Info

Publication number: GB2174985A
Application number: GB08512566A
Authority: GB
Inventors: Joseph Declaye; Marcel Bodart
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1985-05-17
Filing date: 1985-05-17
Publication date: 1986-11-19
Also published as: GB2174985B; GB8512566D0; BE904790A; JPS61266315A; JPH0587449B2

Abstract

A glass melting tank furnace incorporates at least one sill provided with internal cooling means, for controlling the flow of currents in the molten glass. The sill 3 is of hollow construction defining at least one internal passageway 11 which runs lengthwise of the sill and which contains a conduit 8 spaced from the roof 13 of that passageway for the conveyance of liquid coolant along and through the sill. Means 23,24 is provided for maintaining a flow of liquid coolant through the conduit 8 and for maintaining a flow of gaseous coolant through the space 12 in the passageway 11 above the conduit 8. Spacing the conduit 8 from the roof 13 of the passageway 11, and also from its side walls, reduces thermal gradients in the refractory blocks 7,10 forming the sill, so promoting long life. The conduit 8 may carry means 25,26,27 via which compressive forces can be exerted on the sill blocks. <IMAGE>

Description

SPECIFICATION Glass melting tank furnaces The invention relates to a glass melting tank furnace incorporating at least one sill which is provided with internal cooling means. This invention also relates to a method of manufacturing glass in which glass-forming batch material is melted in a glass melting tank furnace and the melt is caused to flow over a sill on the sole of the tank while the sill is cooled internally.

In a conventional glass making process, batch material is fed to one end of a tank where it is melted in a melting zone. The melt passes downstream along the tank to a refining zone, and molten glass flows from the refining zone of the tank for working. Somewhere in the tank, near the centre of its heated zone, the melt is at its hottest and accordingly has its lowest density. Because of the relatively low density of the melt near this hot spot, a rising current is formed in the melt and this feeds surface currents in the melt which radiate out towards the side and end walls of the tank. The rising current is fed by bottom currents of cooler melt which travel towards the hot spot along the sole of the tank.These bottom currents are in turn fed by melt which has been cooled by contact with the side and end walls of the tank furnace, in particular by a return current of melt which returns to the hot spot from the working end of the tank. This continuous cycle of reheating a mass of molten glass in the tank leads to very high fuel consumption.

Various attempts have been made to impede the flow of the bottom return current towards the hot spot by locating a sill in the tank so that the quantity of melt which is cyclically reheated is reduced, thus in turn reducing fuel consumption. Glaverbel's British Patent Specification No 1,168,974 goes further and proposes cooling such a sill in order to slow down the return current as it traverses the sill, and this is found to promote greater overall heat economy despite the fact that heat is necessarily extracted from the tank furnace by the sill cooling means.That Specification proposes the use of a hollow sill having a longitudinal interior passage which is either provided with a lining box serving as a conduit for liquid coolant, or which accomodates a spray conduit from which liquid coolant is sprayed directly against the interior surface of the refractory material from which the sill is formed. In the latter case, cooling may be augmented by blowing air along the sill's interior passage.

There are a numer of problems which are encountered in practice which are not adverted to in that specification. For the greatest benefit in controlling the bottom return current, the sill must be located not too far downstream of the hot spot and it must have an appreciable height in relation to the depth of the melting tank. Usually, such a sill has a height in excess of half that depth. This im plies that the top of the sill will be exposed to intense radiation from burners used for supplying heat to the furnace.It is an unfortunate fact that when refractory material is exposed to a highly corrosive environment such as molten glass, it will be eroded more rapidly as its temperature rises, and furthermore that as the temperature gradient across a refractory block becomes steeper, the useful working life of that block is also reduced because of the thermal stresses set up within it.

Because of this, the liquid cooling processes described in Glaverbel's British Patent Specification No 1,168,974 are detrimental to the working life of the sill.

As for air cooling, unless the interior of the sill is open at its base, the cooling air must be blown from one side to the other, and at the exit side it will be rather hot so that side of the sill will hardly be cooled. Also, the heat capacity of air is rather low, and it is not possible to have a very high mass flow rate of air through the sill. If it is desired to use a sill whose interior is open at its base, very considerable difficulties in construction will be encountered. It will be appreciated that a typical glass melting furnace for the commercial manufacture of glass has a width of the order of 5 to 12 metres. It is simply not feasible to construct a sill of that length in one piece.Accordingly when the sill is first built, a number of blocks must be used, and these must be laid down spaced apart by an amount sufficient to allow for the thermal expansion they will undergo as the furnace is brought up to its working temperature. Because of this, and because the sill must be able to resist the forces which will be exerted upon it by the currents in the bath of molten glass, a rather complicated sill construction becomes necessary.

It is an object of the present invention to provide, in a glass melting tank furnace incorporating at least one sill which is provided with internal cooling means, a cooling system which is of high efficiency without either being highly detrimental to the useful working life of the sill or requiring a complicated sill construction.

According to the present invention, there is provided a glass melting tank furnace incorporating at least one sill which is provided with internal cooling means, characterised in that said sill is of hollow construction defining at least one internal passageway which runs lengthwise of the sill and which contains a conduit spaced from the roof of said passageway for the conveyance of liquid coolant along and through the sill, and in that means is provided for maintaining a flow of liquid coolant through said conduit and for maintaining a flow of gaseous coolant through said passageway above said conduit.

The present invention provides a glass melting tank furnace with a sill having cooling means which can be run at high efficiency without either substantial shortening of the useful working life of the sill or requiring a complicated sill construction.

The most suitable coolants are water and air. It has in particular been found that as compared with a prior art air cooling system in which air is blown through the sill at the same rate, and provided that the volume flow rate of water is sufficient that water leaves the sill conduit as liquid rather than steam, the adoption of the present invention gives a better protection to the sill blocks. A better protection is also afforded than when using a prior art water cooling system. The present invention enables the sill blocks to be cooled efficiently so that they are less subject to erosion by the melt but without subjecting them to severe thermal shock.

By adopting the present invention, the upper part of the sill, which in use will be the hottest part of it and so gives rise to the greatest problems, is cooled by the gas acting as a heat transfer medium between the refractory material and the liquid coolant flowing through the conduit, so that the upper part of the sill is cooled at a rate which is beneficial for its longevity. The upper part of the sill will also cool by radiation towards the coolant conduit.In practice it is found that this radiant cooling may usually remove only a minor proportion of the heat from the sill, but the heat extracted by this radiation becomes extremely important where the joints between blocks of refractory material from which the sill is built up are not tight: under such circumstances, any molten glass penetrating a joint will be cooled so that it freezes within the joint thus sealing it and preventing the egress of any of the melt.

Advantageously, the space between the roof of said passageway and said conduit is within, or intrudes into, the upper half of the sill. This promotes cooling of the hottest part of the sill.

Preferably, said conduit is spaced from the side walls of said passageway. Spacing the conduit from the side walls of the sill prevents direct conductive heat loss from those side walls to the coolant conduit, so limiting the thermal gradient in those side walls to allow them a longer useful working life.

In the most preferred embodiments of the invention, the space between the roof of said passageway and said conduit is greater than the space between the side walls of said passageway and said conduit. This gives a favourable distribution of heat flux density over the interior surfaces of the walls and roof of the sill for promoting good control of the currents of glass flowing over it, and also for promoting a long useful working life.

Advantageously, the or at least one said conduit carries means for exerting longitudinal compressive forces on said sill. This helps to maintain the refractory blocks from which the sill is built up in position against the forces which will be exerted on them by the currents in the melt.

Preferably, the or at least one said conduit defines two liquid flow paths separated by a conductive partition, and means is provided for maintaining a flow of liquid coolant in opposite directions along said flow paths. The adoption of this feature promotes uniformity of cooling along the length of the sill. It has for example been found advantageous to construct a conduit defining two liquid flow paths by welding cheek pieces across the flanges of a steel I- or H- section girder.

In preferred embodiments of the invention, the sill is built up of upper and lower prefabricated refractory blocks, the upper blocks having downwardly opening channels which provide said passageway for the flow of gaseous coolant. This is a very simple and convenient way of building up a sill of the required cross section.

Advantageously, said upper and lower blocks are relatively located by key members which intrude into said channels. The adoption of this feature militates against relative displacement of the blocks due to forces exerted by currents in the melt.

Preferably, there is a said conduit which rests on said key members within said channels. This is a very simple way of supporting the conduit in a suitable location. Furthermore, especially if the conduit is massive, its own weight and the weight of coolant within it help to stabilize the position of the key members and thus further promote resistance to displacement of the sill blocks.

In some preferred embodiments of the invention, a second conduit is located within the sill for the conveyance of liquid coolant through the sill at a different level from that of the first conduit, and means is provided for maintaining a flow of liquid coolant through said second conduit. This permits the extraction of more heat from the sill. It will be appreciated that when such a second conduit is provided, it is preferable for means to be provided for maintaining said flow of gaseous coolant above the upper of said conduits, in order to achieve the full benefit of this invention.

Advantageously, said key members (when provided) rest on such a second conduit, since this simplifies construction of the sill.

This invention includes a method of manufacturing glass in which glass-forming batch material is melted in a glass melting tank furnace and the melt is caused to flow over a sill on the sole of the tank while the sill is cooled internally. Such method is characterized in that said sill is of hollow construction defining at least one internal passageway which runs lengthwise of the sill and which contains a conduit spaced from the roof of said passageway for the conveyance of liquid coolant along and through the sill, and in that a flow of liquid coolant is maintained through said conduit and a flow of gaseous coolant is maintained through said passageway above said conduit.

By adopting the present invention, the upper part of the sill, which in use will be the hottest part of it and so gives rise to the greatest problems, is cooled by the gas acting as a heat transfer medium between the refractory material and the liquid coolant flowing through the conduit, so that that upper part of the sill is cooled at a rate which is beneficial for its longevity. The upper part of the sill will also cool by radiation towards the coolant conduit. The present invention thus provides a method of manufacturing in which the sill can be cooled means in a highly efficient manner without either substantially shortening the useful working life of the sill or requiring a complicated sill construction.

Preferably, the space between the roof of said passageway and said conduit is within, or intrudes into, the upper half of the sill, so as to promote cooling of the hottest part of the sill.

Advantageously, longitudinal compressive forces are exerted on said sill. This helps to maintain the blocks from which the sill is made up in their correct position against the forces which will be exerted on them by currents of molten glass flowing over the sill.

Preferably, the or at least one said conduit defines two liquid flow paths separated by a conductive partition, and a flow of liquid coolant is maintained in opposite directions along said flow paths. This promotes uniformity of cooling along the length of the sill.

In preferred embodiments of the invention, the sill is built up of upper and lower prefabricated refractory blocks, the upper blocks having downwardly opening channels which provide said passageway for the flow of gaseous coolant. This is a very simple and convenient way of building up a sill of which the hottest parts form a wall of the gaseous coolant flow passageway so that they are cooled in the most favourable manner.

Advantageously, the gaseous coolant flow rate is maintained at such a level that the temperature of the gaseous coolant as it leaves the sill passageway is not more than 150 . This implies an effective cooling of the sill passageway and avoids that the exiting air shall be dangerously hot.

Preferably, the gaseous coolant flow rate is maintained at such a level that a greater quantity of heat is extracted by the gaseous coolant than by the liquid coolant. It is found that the adoption of this feature is especially beneficial for the extraction of heat without subjecting the sill blocks to undue thermal stress.

It is especially preferred to maintain the gaseous coolant flow rate at a rate of at least 30 normal cubic metres per second per square metre of the cross sectional area of the gaseous coolant passageway.

In the most preferred embodiment of the invention, the gaseous coolant is air, and preferably, the liquid coolant is water. These are the least expensive and most convenient gaseous and liquid coolants available.

A preferred embodiment of the present invention will now be described in greater detail by way of example only with reference to the accompanying diagrammatic drawings in which: Figure 1 is a detail side view, partly in cross section, of part of a glass melting tank furnace including a sill; Figure 2 is a cross section of the tank furnace viewed in the direction of the arrow II of Figure 1 showing the sill in elevation, and with the tank superstructure omitted; and Figure 3 is a plan view of an end of a conduit for the conveyance of liquid coolant along and through the sill, and shows means for exerting longitudinal compressive forces on the sill.

In Figure 1, the sole 1 of a glass melting tank furnace is supported by a plurality of I-section girders such as 2. Except beneath a sill 3, the sole 1 of the tank furnace comprises a first layer of blocks 4 of an insulating material. The insulating blocks 4 and beneath the sill 3, girders 2 support a continuous layer of paving blocks 5 of refractory material, and these in turn support a layer of refractory lining blocks 6 and the sill 3 which interrupts that lining layer.

In a typical regenerator furnace for the control of bottom return currents, the sill 3 would usually be located between the last (counting from the charging end) and the next to last pair of burner ports (not shown), though the sill could be located elsewhere and for other purposes.

The sill 3 is built up using two spaced lines of lower refractory blocks 7 which rest on refractory paving blocks 5 between refractory lining blocks 6.

An optional lower conduit 8, shown in end elevation in Figure 1, rests on the paving blocks 5 between the lines of lower sill blocks 7, and supports a line of key blocks 9 which stand proud of the upper surfaces of those lower sill blocks 7. An upper conduit also allotted reference numeral 8 and shown in cross section rests on the line of key blocks 9, and the sill 3 is completed by a line of upper refractory blocks 10 each of which is provided with a downwardly opening channel defining an internal passageway 11 in the sill 3 accommodating the proud portion of the key blocks 9 and the upper conduit 8, and leaving a space 12 extending along the length of the sill 3 between that conduit 8 and the roof 13 of the passageway. The upper and lower conduits 8 are suitably substantially identical.The upper conduit is shown spaced from the sidewalls of the passageway 11, and the lower conduit is likewise spaced from the lower sill blocks 7.

As shown in Figure 2, the upper and lower sill blocks 7, 10 at each end of the sill project beyond side blocks 14 of the furnace tank at each side thereof for feeding liquid coolant such as water through the conduits 8. Also shown in Figure 2 is a blower 15 and gas supply line 16 for feeding gaseous coolant, normally air, through the passageway 11 around the upper conduit 8.

An end of a conduit 8 is shown in plan view in Figure 3. The conduit 8 is made by welding cheek pieces 17 between the flanges of an I- or H- section girder 18 to give two liquid flow paths 19, 20 separated by a conductive partition 21 which is formed by the web of the girder 18. Each end of the conduit is closed by a end plate 22, also welded on, through which lead a liquid coolant feed line 23 for one flow path (19) and a liquid coolant extraction line 24 for the other flow path (20). This arrangement is made in order to maintain a flow of liquid coolant in opposite directions along those flow paths so as to promote uniformity of cooling along the length of the sill 3.

Figure 3 also illustrates means for exerting longitudinal compressive forces on the sill. Wing plates 25 are welded to each end of the conduit via brackets 26, 27 and support bolts 28 provided with buffer pads 29 for engaging the end faces of a line of upper or lower sill blocks 10, 7 as the case may be. A desired load may be brought to bear against the end of the line of sill blocks by turning nuts 30 against cradles 31 each containing a stack of Belleville washers 32. An optional deflector baffle such as that shown at 33 in Figure 2, or other means, may be provided for leading away heated air coolant to safety. Alternatively, such heated air may be made use of by conducting it to a burner port as preheated comburent, or for preheating, by exchange, fuel and/or air fed to one or more burners or indeed for preheating the batch to be fed to the furnace.

In a specific practical embodiment, a glass melting tank furnace has a width of 11.1 metres and a design depth (melt depth) of 1.28 metres. The sole of the furnace has a layer of insulating blocks 4, which supports a thicker layer of refractory paving blocks 5. These paving blocks are formed of a silico-aluminous refractory. The lining blocks 6 are of a higher grade refractory. The sill 3 is built to a height of 880mm (above the top of the layer of lining blocks 6) to allow a depth of 400mm of melt above its top surface. The lower sill blocks 7 are rectangular having a height of 400mm, a width (the dimension shown in Figure 1) of 300mm, and a length (the dimension shown in Figure 2) of 325mm. These lower sill blocks 7 are also of the higher grade refractory, and their lines are laid a nominal 200mm apart. The upper sill blocks 10 are of an even higher grade refractory.These upper sill blocks 10 have a height of 600mm, a width (the dimension shown in Figure 1) of 800mm, and a length (the dimension shown in Figure 2) of 325mm. A 200mm wide channel 11 is formed in those upper sill blocks to leave a semi-circular roof 13 with a thickness of 300mm of refractory material above it. The key blocks 9 are 300mm high and 200mm wide. They do not need to form a continuous line along the length of the sill, and they are preferably arranged to overlap the joints between successive blocks in the lines of upper and lower sill blocks. The key blocks 9 do not need to be made of the highest grade refractory and may be formed of a silico-aluminous refractory. The crosssectional area of the free space 12 in the passageway 11 above and to each side of the conduit 8 is approximately 165cm2.

In use air may be blown through the space 12 at a rate of 2800Nm/hour3 (which implies a velocity of between 30 and 50m/s, and a flow rate of about 47 normal cubic metres per second per square metre of the cross sectional area of the gaseous coolant passageway) while water is caused to flow through the upper conduit 8 alone at a rate such that it exits at a temperature of 40 C to extract heat from the sill at a total rate of 120,000kcal/hour (approximately 500MJ/hour), the temperature of the melt near the sill being 1400 C. Of the total heat extracted, some 40,000kcal/hour is extracted by the water, and 80,000 kcal/hour by the air.The air exits from the passageway at a temperature of 125 C. It is calculated that the mean temperature of the inner surfaces of the upper sill blocks 10 is then about 320 C, and this miltates against erosion of those blocks. When the furnace is operated in that way to produce soda-lime glass at a rate of 550 tonnes per day, it is found that the use of the cooled sill gives rise to a net fuel saving of about 6 to 10%, depending on other operating conditions.

If, on the other hand, no air is blown along the space but water is still caused to flow through the upper conduit 8 alone at a rate such that it exits at a temperature of 403C, somewhere between 85,000 and 90,000kcal/hour is extracted so that it is calculated that the mean temperature of the inner surfaces of the upper sill blocks 10 would then be about 600"C. If the upper conduit 8 is removed and the sill is cooled solely by blowing air through the passage 11, then rather less than 80,000kcal/hour can be extracted which implies a mean internal sill surface temperature in excess of 600=C. it is to be noted that at 800 C, molten glass is sufficiently free-flowing to penetrate even narrow joint spaces where it can cause quite severe erosion leading in turn to further penetration of molten glass.

Claims

1. A glass melting tank furnace incorporating at least one sill which is provided with internal cooling means, characterised in that said sill is of hollow construction defining at least one internal passageway which runs lengthwise of the sill and which contains a conduit spaced from the roof of said passageway for the conveyance of liquid coolant along and through the sill, and in that means is provided for maintaining a flow of liquid coolant through said conduit and for maintaining a flow of gaseous coolant through said passageway above said conduit.

2. A glass melting tank furnace according to claim 1, wherein the space between the roof of said passageway and said conduit is within, or in trudes into, the upper half of the sill.

3. A glass melting tank furnace according to claim 1 or 2, wherein said conduit is spaced from the side walls of said passageway.

4. A glass melting tank furnace according to claim 3, wherein the space between the roof of said passageway and said conduit is greater than the space between the side walls of said passageway and said conduit.

5. A glass melting tank furnace according to any preceding claim, wherein the or at least one said conduit carries means for exerting longitudinal compressive forces on said sill.

6. A glass melting tank furnace according to any preceding claim, wherein the or at least one said conduit defines two liquid flow paths separated by a conductive partition, and means is provided for maintaining a flow of liquid coolant in opposite directions along said flow paths.

7. A glass melting tank furnace according to any preceding claim, wherein the sill is built up of upper and lower prefabricated refractory blocks, the upper blocks having downwardly opening channels which provide said passageway for the flow of gaseous coolant.

8. A glass melting tank furnace according to claim 7, wherein said upper and lower blocks are relatively located by key members which intrude into said channels.

9. A glass melting tank furnace according to claim 8, wherein there is a said conduit which rests on said key members within said channels.

10. A glass melting tank furnace according to any preceding claim, wherein a second conduit is located within the sill for the conveyance of liquid coolant through the sill at a different level from that of the first conduit, and means is provided for maintaining a flow of liquid coolant through said second conduit.

11. A glass melting tank furnace according to claim 10, wherein means is provided for maintain ing said flow of gaseous coolant through the upper of said passageways.

12. A glass melting tank furnace according to claim 8 or 9 and claim 10 or 11, wherein said key members rest on said second conduit.

13. A method of manufacturing glass in which glass-forming batch material is melted in a glass melting tank furnace and the melt is caused to flow over a sill on the sole of the tank while the sill is cooled internally, characterised in that said sill is of hollow construction defining at least one internal passageway which runs lengthwise of the sill and which contains a conduit spaced from the roof of said passageway for the conveyance of liquid cool ant along and through the sill, and in that a flow of liquid coolant is maintained through said conduit and a flow of gaseous coolant is maintained through said passageway above said conduit.

14. A method according to claim 13, wherein the space between the roof of said passageway and said conduit is within, or intrudes into, the up per half of the sill.

15. A method according to claim 13 or 14, wherein said conduit is spaced from the side walls of said passageway.

16. A method according to claim 15, wherein the space between the roof of said passageway and said conduit is greater than the space between the side walls of said passageway and said conduit.

17. A method according to any of claims 13 to 16, wherein longitudinal compressive forces are exerted on said sill.

18. A method according to any of claims 13 to 17, wherein the or at least one said conduit defines two liquid flow paths separated by a conductive partition, and a flow of liquid coolant is maintained in opposite directions along said flow paths.

19. A method according to any of claims 13 to 18, wherein the sill is built up of upper and lower prefabricated refractory blocks, the upper blocks having downwardly opening channels which provide said passageway for the flow of gaseous coolant.

20. A method according to any of claims 13 to 19, wherein a second conduit is located within the sill for the conveyance of liquid coolant through the sill at a different level from that of the first conduit, and a flow of liquid coolant is maintained through said second conduit.

21. A method according -to claim 20, wherein said flow of gaseous coolant is maintained through the upper of said passageways.

22. A method according to any of claims 13 to 21, wherein the gaseous coolant flow rate is maintained at such a level that the temperature of the gaseous coolant as it leaves the sill passageway is not more than 150 C.

23. A method according to any of claims 13 to 22, wherein the gaseous coolant flow rate is maintained at such a level that a greater quantity of heat is extracted by the gaseous coolant than by the liquid coolant.

24. A method according to any of claims 13 to 23, wherein the gaseous coolant flow rate is maintained at a rate of at least 30 normal cubic metres per second per square metre of the cross sectional area of the gaseous coolant passageway.

25. A method according to any of claims 13 to 24, wherein the gaseous coolant is air.

26. A method according to any of claims 13 to 25, wherein the liquid coolant is water.