GB2157674A - Electrically heated forehearths - Google Patents

Abstract

A forehearth having a delivery channel 2 for molten liquid (e.g. glass) 1 is provided with electrical heating elements 6 adapted to produce a preferential heating of the channel sides relative to the channel centre and so reduce the non-uniformity in temperature of the molten liquid in the channel. There are also described heating elements 6, in the form of rods or tubes of electrically conductive material having regions 7 of high electrical resistance about a central region 8 of lower electrical resistance, suitable for use in the forehearth to achieve the preferential heating of the channel sides. <IMAGE>

Description

SPECIFICATION Forehearths This invention relates to forehearths, in particular to forehearths provided with electrical heating means.

Forehearths are used, particularly in glass production, as heated conduits through which molten fluid, e.g. glass or metal, may flow to forming apparatus.

In general, a forehearth comprises a channel resistant to the molten fluid and provided with heating means, such as gas burners arranged above and along the sides of the channel, to maintain the fluid in its molten state and to bring it to the correct temperature for delivery to the forming apparatus.

Hitherto however it has not been possible using a forehearth to supply glass to a forming apparatus at a uniform temperature. This has resulted from various factors such as the inadequate homogenising of glass temperature during the passage along the forehearth of a stream of molten glass supplied thereto at a non-uniform temperature, the heat losses from the sides and the bottom of the channel, and the non-uniformity of the radiative heating of the glass within the channel which has tended with conventional gas-heated forehearths to provide a greater heating effect at the channel centre than at the channel sides even though the gas heating jets are themselves arranged along the channel sides.

It is thus an objective of the present invention to provide a forehearth having heating means which achieve an improved uniformity of the temperature ofthe molten fluid delivered from the forehearth.

We have found that the desired improvement in temperature uniformity can be achieved if electrical heating elements which result in a preferential heating of the sides of the forehearth channel are used.

According to one aspect of the present invention we thus provide a forehearth having a molten glass deliery channel, e.g. a molten liquid delivery channel such as a molten glass delivery channel, with arranged thereover electrical heating elements adapted to provide a greater heating effect towards the sides of said channel than towards the centre thereof.

Particularly suitably, the electrical heating elements in the forehearth of the invention may be in the form of silicon carbide tubes. To create a region of enhanced heating effect in such tubes, an elongate aperture, for example a helical cut, may be formed along the tube surface to reduce the crosssectional area of the path through which electrical current may flow along the tube and thus increase electrical resistance and the heating effect in the apertured section of the tube. A heating element with "hot" regions, i.e. regions of enhanced heating effect, on either side of a central "cold" region may thus take the form of a tubular element having helical apertures in the tube surface on either side of a central tube portion in which either no helical aperture appears or in which the helix frequency (i.e.

the number of turns per unit length) is reduced.

Where a sharp distinction between "hot" and "cold" regions is not desired, the helix frequency can be varied gradually to be greater in those regions requiring greater heating. Alternatively, regions of enhanced heating effect can be achieved by reducing the cross-sectional area of the silicon carbide tube or rod in the areas where enhanced heating is required. Again the change-over to areas of reduced cross-sectional area (the "hot" regions) may be gradual or sharp.

As a further alternative "hot" and "cold" regions of the heating element may be achieved by varying the chemical composition along the element to achieve a variation in resistivity.

According to a further aspect of the present invention we provide an electrical heating element for a forehearth comprising an elongate tube or rod of electrically conductive material, such as silicon carbide, said tube or rod having along its length a region of relatively lower electrical resistance between regions of relatively higher elecrical resistance whereby the heating effect of said tube or rod is greater in said regions of higher resistance than in said region of lower resistance.

In a particularly preferred embodiment, the heating element of the invention comprises an elongate tube of electircally conductive material, such as silicon carbide, having in the surface thereof an elongate helical aperture defining a path for electrical current along said tube, said path having regions of relatively lower cross-sectional area about a region of relatively higher cross-sectional area thereby enabling the tube to exert a non-uniform heating effect along its length.

Electrical heating elements having different relative lengths of the "hot" and "cold" regions can thus readily be formed thereby allowing a great variation in the preferential heating effects to be achieved.

Thus for example in certain forehearths the extra heating required at the channel sides is asymmetric.

In such cases heating elements having suitably asymmetric regions of enhanced heating effect can be employed.

It is particularly preferred however that the heating elements used in the forehearths of the invention shall have the same overall resistance as and so be readily interchangeable with standard heating elements of uniform heating effect.

The forehearths of the invention may comprise several electrical heating elements disposed above and extending across at least a major part of the molten fluid delivery channel. The successive heating elements underneath which the molten fluid passes as it moves along the channel may have the same or different relative hot and cold regions according to the extent of preferential channel side heating required and indeed not all of the electrical heating elements need be of the type capable of providing the preferential heating. A preferred arrangement of electrical heating elements however is one in which the relative size of the central cold region increases along the direction of flow of the fluid to enable the cooling at the channel sides to be tackled more vigorously as the fluid approaches the forehearth outlet or spout.

The regions of preferential heating in the forehearths of the invention may be well defined and do not vary with the current passing through the heating elements. This contrasts with the heating effect of gas burners in conventional forehearths where with a reduction in gas supply the flame lengths shorten and the most radiant regions of the refractory ceiling to the channel move towards the channel sides.

Afurther advantage of the use according to the invention of electrical heating elements rather than gas heating in forehearths is that it permits the refractory ceiling to the channel to be positioned at a lower height above the channel thus allowing the "hot" regions ofthe ceiling to have a more localised heating effect on the fluid in the channel below. A further advantage is that in the absence of the high velocity turbulent gases which result from gas flame heating, the electrical heating of the forehearth of the invention permits the use of light-weight ceramic fibre material as the refractory material for the channel ceiling and the forehearths of the invention advantageously are provided with channel ceilings of such ceramic fibre material.

The ceramic fibre material is suitably such as to be capable of withstanding temperatures of up to 1300 C and preferably up to at least 15000C. Such lightweight ceramic fibre materials allow a channel ceiling to be constructed with a thermal mass of as low as about 10% of that of the conventional gas heated forehearths. This reduction in thermal mass permits much faster response speeds and allows a greater precision in temperature control. In turn this allows glass of increased temperature uniformity to be delivered to the forming apparatus.

As the weight of glass is closely related to its temperature on delivery to the forming apparatus, improved temperature control results in improved product weight control and a lower rate of rejection of products for being overweight or underweight.

Thus in a further aspect, the present invention provides a forehearth having a molten liquid, e.g.

glass or metal, delivery channel with arranged thereover electrical heating elements and having arranged over said channel and said heating elements a channel ceiling member of ceramic fibre refractory material.

Preferred embodients of the present invention will now be described by way of example and with reference to the accompanying drawings, in which Fig. 1 is a cross-sectional view of a conventional gas heated forehearth; Figs. 2A and b are schematic cross-sections, showing isotherms, through molten glass streams in forehearth channels; Fig. 3 is a cross-sectional view of a forehearth according to the present invention; Fig. 4 is a cross-sectional view of an alternative construction of a forehearth according to the present invention; Fig. 5 is a schematic plan view of a forehearth according to the present invention with the channel ceiling removed; and Fig. 6is a schematic plan view of a further forehearth according to the present invention also with the channel ceiling removed and showing an alternative arrangement of the heating elements.

Referring to figure 1, molten glass 1, generally at a temperature of 1050 to 1300"C, flowing along channel 2 (in the out of plane direction) is heated by gas burners 3 which cause the refractory material of channel ceiling 4 to radiate heat down onto the glass. A vent 5 is provided in channel ceiling 4 to permit cooling of the glass where required.

Figures 2Aand2B are provided to show the degree of temperature inhomogeneity across the glass streams in the channels of conventional gas-heated forehearths at positions close to the outlets of the channels into the forming apparatus.

In figure 3, electrical heating element 6 is shown extending across channel 2. Element 6 has "hot" regions 7 about a central "cold" region 8. The element 6 is in the form of a silicon carbide tube with helical apertures 9 cut into its surface to provide the "hot" regions. Electrical current to the element 6 is supplied through flexible braids 10 electrically connected to sections of the element which protrude out through the walls of the forehearth.

The channel is suitably formed of MULLITE, a refractory glass contact material which is commonly used in forehearths and glass furnances and is resistant to glass attack.

The channel ceiling 11 is formed of a ceramic fibre board, which suitably has a density of about 0.25 g/cm3 (as compared with the 2.5 g/cm3 density of MULLITE). This material is preferably made from pure alumina fibres which, together with a binder, are vacuum formed into boards.

The channel 2 and the channel ceiling 11 are provided with insulating backing layers 12 in the form of ceramic fibre boards, such as those discussed above, or blankets of ceramic fibre material. The blankets are also of about 0.25 g/cm3 density and may conveniently be made from a mixture of silica and alumina fibres.

The backing layers 12 are themselves provided with further insulating layers 13 of a microcellular silica material such as that obtainable under the trade name MICORTHERM.

In the alternative embodiment shown in figure 4, channel 2 is shown as having conventional forehearth insulation 14 and channel ceiling 11 is shown provided with a vent 5.

Figures 5 and 6 show embodiments of the forehearth of the invention with channel ceiling 11 removed. Molten glass flows along channel 2 towards spout 15 below the electrical heating elements 6. In the embodiment shown in figure 5, the central "cold" regions of elements 6 becomes successively wider towards spout 15 to enable cooling at the channel sides to be tackled more vigorously. In the alternative embodiment shown in figure 6, heating elements having a central "cold" region alternative with heating elements of uniform heating effect; in this embodiment, the uniform and non-uniform heating elements can be controlled separately and, by varying the relative power supply to the two sets of heating elements, the preferential channel side heating effect can be varied as required.

While the present invention has been described in terms of forehearths for molten fluids, it should be realised that delivery channels constructed in accordance with the present invention may be suitable in processes where constant heating across the delivery channel is required e.g. in the reheating of billets or partially formed metal sections. Such delivery channels are deemed also to fall within the scope of the present invention.

Claims (11)

1. Aforehearth having a molten liquid delivery channel with disposed thereover electrical heating elements arranged to provide a greater heating effect towards the sides of said channel than towards the centre thereof.

2. Aforehearth as claimed in claim 1 comprising a plurality of electrical heating elements one or more whereof are adapted to provide a greater heating effect towards the sides of said channel than towards the centre thereof.

3. Aforehearth as claimed in either of claims 1 and 2 comprising tubular or rod-like heating elements wherein the electrical resistance per unit path length of the electrical path of a said element is higher on either side of a central portion of said element thereby to create regions of enhanced heating effect on said element.

4. Aforehearth as claimed in claim 3 comprising at least one electrical heating element in the form of an elongate tube of electrically conductive material having in the surface thereof an elongate helical aperture defining a path for electrical current along said tube, said path having regions of relatively lower cross-sectional area about a region of relatively higher cross-sectional area.

5. Aforehearth as claimed in any one of claims 1 to 4 comprising a plurality of electrical heating elements each having substantially the same overall electrical resistance.

6. A forehearth as claimed in any one of claims 1 to 5 wherein said heating elements are arranged to provide a preferential heating of the channel sides relative to the channel centre which increases in the direction of liquid flow along the channel.

7. Aforehearth as claimed in any one of the preceding claims comprising a channel ceiling member of ceramic fibre refractory material.

8. A forehearth substantially as hereinbefore disclosed with electrical heating elements arranged to produce a preferential heating of the delivery channel sides.

9. An electrical heating element comprising an elongate tube or rod of electrically conductive material, said tube or rod having along its length a region of relatively lower electrical resistance per unit length of electrical path between regions of relatively higher electrical resistance per unit length of electrical path whereby the heating effect of said tube or rod is greater in said regions of higher resistance than in said region of lower resistance.

10. An element as claimed in claim 9 comprising an elongate tube having in the surface thereof an elongate helical aperture defining a path for electrical current along said tube, said path having regions of relatively lower cross-sectional area about a region of relatively higher cross-sectional area thereby enabling the tube to exert a non-uniform heating effect along its length.

11. Electrical heating elements substantially as hereinbefore disclosed adapted to exert a nonuniform heating effect along their lengths.