IL40020A - Apparatus for use in manufacturing drawn sheet glass including protective means against gas currents in the drawing chamber - Google Patents

Description

40020/2 APPARATUS FOR USE IN MANUFACTURING DRAWN SHEET GLASS INCLUDING PROTECTIVE MEANS AGAINST GAS CURRENTS IN THE DRAWING CHAMBER C nn it Ίλ2 Π3ΛΠ *»»SDR nm naioo maiat mm*? *ns^a ©ieW7 ipiin This invention relates to apparatus for use in manufacturing sheet glass, comprising a kiln along which molten glass can be fed to a drawing zone, a drawing chamber and a contiguous annealing shaft , means for drawing a continuous ribbon of glass from said drawing zone and along a path through said drawing chamber and annealing shaft , and at least one cooler through which cooling fluid can be circulated and which is located within said drawing chamber and serves to assist cooling of the said ribbon of glass.

The invention also relates to a process of manufacturing sheet glass by means of such apparatus.

Apparatus of the kind referred to is well known.

During the drawing of the glass ribbon it is exposed to the influence of thermally heterogeneous environmental gas currents which are due to various causes. One of the principal causes is the chimney effect of the annealing shaft. Under the strong natural draught forces, hot gas currents flow upwardly along the ribbon from the intensely hot drawing zone, through the drawing chamber and into the annealing shaft, and cooler currents of gases flow back into the drawing chamber from the annealing shaft along the walls of the apparatus. The said convection currents tend to be propagated over the surface of the molten glass in regions of the molten glass supply kiln adjacent the drawing chamber so that there tends to be a non-uniform heat distribution not only in the gaseous environment in contact with the ribbon but also in the gaseous environment \ in contact with the molten glass flowing into the drawing zone from which the ribbon is drawn.

Another cause of thermal heterogeneities in the environmental conditions to which the glass is exposed 5 is the cooler or coolers provided in the drawing chamber.

The provision of such a cooler or coolers is required for promoting the setting of the glass ribbon but the maintenance of the cooler(s) at a sufficiently low temperature to promote cooling of the glass ribbon inevitably influences 10 the pattern of currents which prevails in the drawing chamber and the nature of this influence is such that the heat distribution in the atmosphere in contact with the glass is further adversely affected.

In the operation of the aforesaid apparatus there are 15 various other causes of environmental conditions which militate against uniform cooling of the ribbon over its entire width but for the purposes of introducing the present invention they need not be detailed.

The non-uniform temperature profile across the path 20 of the drawn ribbon, due to the aforesaid main convection currents induced by natural draught forces, and the existence of thermally heterogeneous gas currents induced by the action of the cooler(s) are important causes of defects in the geometry of the drawn sheet glass. Any irregularity in the 25 cooling action on the glass is an impediment to the drawing of sheet glass with faces which are truly. flat and parallel at all positions. Due to the lack of true flatness and parallelism of the sheet faces, the sheet glass causes Ύ angular deflections of light waves travelling through the glass so that objects viewed through the glass under certain conditions appear distorted.

The flow along the central and marginal portions of the glass ribbon of convection currents which have different speeds and different temperatures tends, if the temperature disparity is excessive, to cause "necking-in" of the ribbon at its central region. Moreover the difference in tempera-ture between the aaid currents contributes to non-uniform cooling of the molten glass flowing along the supply kiln into different parts of the width of the ribbon. The consequential glass viscosity variations across the drawing zone affects in an unfavourable way what is known as the thickness profile of the glass ribbon. The thickness profile can be represented by a line plotting points on a graph representing the thickness of the ribbon at intervals of, say, 10 cms across its width. The quality of the sheet glass insofar as its geometry is concerned depends in part on the shape of this profile and in part on the magnitude of the difference between the minimum and maximum thickness values, which magnitude is known as "the overall thickness variation".

Thermally heterogeneous gas currents caused by the action of a cooler or coolers in the drawing chamber, can affect the geometry of the sheet glass in different ways depending inter alia on the position or positions of such - - cooler or coolers in relation to the ribbon path and to the boundaries of the drawing chamber. In most cases, one or more coolers is or are provided in the lower part of the drawing chamber, fairly close to the bottom of the glass ribbon, in order to accelerate the dimensional setting of the ribbon. Unless special precautions are taken, gases cooled by such coolers will fall onto the molten glass adjacent the meniscus and as there is inevitably some variation in temperature of such falling gas currents from one position to another they cause unequal cooling of the glass which gives rise to surface defects in the form of waves running more or less parallel to the line of draw of the glass.

These wave defects are quite apparent when viewing objects at a shallow angle through the sheet glass in a plane normal to the line of draw, particularly during change in the viewing angle. In some cases, one or more additional coolers (called "secondary coolers") is or are provided at a higher level in the drawing chamber, such secondary cooler(s)1 being normally at a greater spacing from the ribbon path than the main cooler or coolers in the lower portion of the drawing chamber. The secondary cooler or coolers tends or tend to influence the heat distribution in the drawing chamber in a way which gives rise to surface defects known as "martelage or "hammering" in the drawn sheet glass. This kind of defect takes the form of an haphazard distribution of shallow surface depressions usually measuring from 1 to cm across. It should be recognised that while the different defects in the sheet glass which have been described are attributable to the different causes which have been explained, there is inevitably an interaction between superimposed systems of gas currents. Consequently the precise characteristics or the severity of a given type of defect nay in any given case depend upon main and secondary causes. Thus there is inevitably an interaction between cool gas currents falling onto the surface of the molten glass flowing to the drawing zone and the central and outer main convection currents propagated along that surface and there is likewise in any given process, some interaction betx^een the main convection currents generated along the ribbon path under the natural draught forces on the one hand and the flow of gases due to the cooling action of the secondary cooler or coolers (if any) on the other hand.

Many proposals have been made to modify the influence of a cooler or coolers on the pattern of gas currents in the drawing chamber with a view to reducing the occurrence of surface defects in the sheet glass, notably waves*. Thus it has been proposed to prevent the currents of cooled gas from falling from the main coolers onto the surface of the molten glass bath by providing heating means which generates hot gas currents upwardly over the inner .surfaces of the coolers. This measure is in itself useful but it often does not sufficiently improve the heat distribution to permit the drawing of sheet glass of the very high optical quality demanded for certain specialised purposes. The limitatione in the improvements attainable in this way are thought to be due to a lack of stable dynamic equilibrium in the gas currents . Δ further proposal which has been made is to fit around the lower portion of the coolers, metal elements which retain heat radiated from the molten glass and define gas spaces immediately adjacent at least one side of the cooler. In effect, when adopting this proposal, there are no descending currents of cooled gas alongside the cooler. On the contrary, gas in the said gas spaces adjacent the cooler is heated by the said metal elements sufficiently to cause it to rise in a continuous upward current. The physical form of the metal elements required for functioning in this way would in practice give rise to turbulence in the upwardly moving stream of gas, which would be a fresh cause of uncontrolled variations in the heat distribution.

The present invention aims to influence the flow of gas induced by the cooling action of a cooler located in the drawing chamber, in such a way that the action of the cooler is less liable to give rise to defects in the geometry of the drawn sheet glass.

According to the present invention, apparatus for use in manufacturing sheet glass, comprising a kiln along which molten glass can be fed to a drawing zone, a drawing chamber and a contiguous annealing shaft, means for drawing a continuous ribbon of glass from said drawing zone and along a path through said drawing chamber and annealing shaft, and at least one cooler through which cooling fluid can be circulated and which is located within said drawing chamber and serves to assist cooling of the said ribbon of glass, is characterised in that at the side of the or at least one said cooler facing the ribbon path there are two or more baffles extending inwardly fron said cooler at vertically spaced levels thereof, ni n i hn f f T r n nrt ond i ng the pnrtlinin n f th r r rm l i | l i i i .i il nlmvr that nff o, The provision of baffles in accordance with the invention, while still allowing continuous downward flow of cooled gas along the side of the cooler facing the ribbon, has the inportant consequence that the movement of the gas which has been cooled by contact with the parts of the cooler located above the baffles is controlled in such a way that it does not adversely affect the heat distribution in the atmosphere in contact with the glass. This control is attributable in part to the fact that a zone immediately to the inside of the cooler is divided by such baffles into two or more superimposed sub-zones in each of which the volume rate of flow of cooled gas is substantially less than the volume rate of flow of cooled gas along the cooler considered as a whole. Moreover each of the different quanta of cooled gas descending in the said superimposed sub-sones is deflected inwardly towards the ribbon by the baffle at the bottom of that sub-zone so that such deflected gas enters the main convection currents of gas along the ribbon. The said subdivision of the zone immediately to the inside of the cooler and the said deflection of gas from the sub-zones towards the ribbon, results in more stable thermal conditions across the surface of the glass.

In the most preferred embodiments of apparatus according to the invention there is at least one main cooler, i.e., at least one cooler located in the lower part of the drawing chamber close to the drawing zone, which is provided with said baffles. The provision of baffles as aforesaid has been found to be particularly beneficial in the case of a main cooler. In particular the provision of baffles on such a cooler has been found to reduce or eliminate the occurrence of waves in the corresponding face of the drawn sheet glass. It has also been found that the provision of baffles on a main cooler in many cases leads to an improvement in the thickness profile of the sheet glass. These results are undoubtedly due to the control of the movement of cooled gas adjacent the inside face of the cooler so that currents of cooled gas do not fall onto the surface of the molten glass flowing to the drawing zone. There are preferably two main coolers, located at the opposed sides of the ribbon path and each having baffles in accordance with the invention as hereinbefore defined.

The invention includes apparatus in which there are said baffles which are provided on at least one cooler \ " V" located in the upper part of the drawing chamber. It has been found that the provision of baffles as aforesaid on a cooler or coolers located in the upper part of the drawing chamber reduces or avoids the occurrence of 5 hammering in the surface of the sheet glass due to the action of such cooler or coolers.

In the event that the apparatus includes one or more main coolers and one or more secondary coolers, baffles may be provided in accordance with the invention, on the or at 10 least one main cooler and/or on the or at least one secondary cooler.

Preferably, there is at least one said cooler which is provided with baffles as aforesaid which extend over substantially the full projected width of the ribbon path 15 past such cooler. In that case the baffles cause cooled gas descending onto the baffles to be deflected inwardly towards the ribbon substantially over its full width.

However, it is within the scope of the invention for the said baffles on at least one cooler to extend over only a 20 part of the projected width of the ribbon path. In that case the baffles will preferably extend over at least a central region of the projected width of the ribbon since it is in that region that the effects of the baffles on the gas flow is most beneficial. 5 is at least one said baffle which is of open construction. For example, each baffle may be in the form of a trellis or it may be a perforated ""v or slotted plate. A trellis construction is preferred.

The advantage of an open construction is that the baffle can be maintained at a higher temperature by radiant heat from the glass as compared with a baffle in the form of an unapertured plate. In consequence of the higher temperature of the baffle, the gas flowing along the baffle is heated to a greater extent and this is beneficial because gas moving away from the baffle and towards the ribbon path can flow-more easily into the main upward convection currents along the glass ribbon without giving rise to turbulence. Moreover, said open construction promotes the dispersion into the main upward convection currents of cooled gas currents generated along the walls of the coolers.

In certain apparatus according to the invention, the baffles associated with the or each cooler provided with such baffles are vertically spaced by at least .

Preferably, the said vertical spacing is at least 2". The vertical spacing of neighbouring baffles is a factor of relevance to the attainment of the required results. If the spacing is too snail, the required continuous downward flow of cooled gas within the sub-zones between the baffles, and inwardly along the baffles towards the ribbon path, does not take place. Instead, quantities of gases tend to eddy or circulate in the sub-zones between the baffles.

Tests have shown that it is generally beneficial to adopt a vertical spacing between baffles of at least but of course the optimim spacing in any given case will depend on various factors, including the width of the baffles □easured from the cooler to their free outer edges. The optimum spacing can in any given case be determined by tests .

In apparatus incorporating at least one main cooler having baffles in accordance with the invention, the baffles associated with that main cooler or coolers preferably project over at least one third of the distance between the associated cooler and the ribbon path. This minimum baffle width is recommended in order to promote a well controlled inward movement of the cooled gases towards the ribbon path.

Advantageouslyj, each baffle is raovably mounted so that its orientation in the vertical plane can be varied. For arresting downward movement of cooled gas the angle between the upper surface of each baffle and the contiguous portion of the inner face of the cooler should not be more than 0° but each baffle may be hingewise mounted on the cooler to permit this angle to be varied over a range below 90°. By varying the angle of the baffle it is possible to influence the direction in which the cooled gas is directed by the baffle towards the ribbon path. The smaller the said angle the greater is the upward component of movement given to such gas by the baffle. An upward component is beneficial in avoiding turbulence or eddy currents where the cooled gas leaving the baffles encounters the vertical current of gas moving upwardly along at least a part of the width of the ribbon path under the natural draught forces.

Preferably the apparatus comprises at least one cooler which is provided with baffles in accordance with the invention and which comprises a series of superimposed tubes or tube portions running transversely of the projected width of the ribbon path. Such a cooler is easy to manufacture and instal while ensureing that the conditions determining the convection currents are the same in the different superimposed sub-zones adjacent the inside face of the cooler. There may be one baffle to each tube or tube portion. Alternatively, there may be a lesser number of baffles than tubes or tube portions. For example one baffle may be provided at each level coinciding with a line of division between neighbouring tubes or tube portions. The use of coolers composed or superimposed tubes or tube portions as above specified, although preferred, is not essential. Other types of cooler, e.g., box-coolers can be used.

The provision of baffles in accordance with the invention may be supplemented by other means which assists in preventing cooled gas from falling in a random way from the cooler or coolers.

In certain embodiments of the invention, at least one cooler is provided with a plurality of baffles in accordance with the invention, which baffles are distributed over an upper part of the height of the cooler, and heating means is provided at or near the bottom of the cooler to maintain an upward current of heated gas adjacent the inside face of the cooler, beneath the lower or the lowest baffle. The said heating means can for example comprise one or more gas burners through which combustible gas can be fed and ignited to maintain a flame or flames at or near the bottom of the cooler. The upward current of heated gas not only prevents cooled gas currents from descending from the bottom of the cooler but assists in causing the gases discharging inwardly off the free inner edges of the baffles to flow upwardly into the main upward convection current along the ribbon. This however occurs without creating the turbulent conditions which are liable to occur if the heater is used without the baffles.

The invention includes apparatus wherein there is at least one cooler iirhich is provided with baffles in accordance with the invention as hereinbefore defined and also with baffles which project from the outer face of the cooler, i.e., from the face which is remote from the ribbon path. Experience has shown that the provision of baffles also at the outer face of a cooler sometimes results in further improvement in the environmental conditions in the drawing chamber, such that the geometry of the drawn sheet glass is improved. This particularly is so in certain embodiments accordin to the invention wherein there is at least one main cooler having baffles as aforesaid on its inner and outer faces.

In the case of a cooler composed of superimposed tubes or tube portions, the baffles adjacent the inner and outer faces of the cooler can be formed by different portions of a single set of elements, e.g., strips of metal or pieces of trellis which extend through the cooler between neighbouring tubes or tube portions.

The improvement in the environmental conditions attributable to the presence of the baffles in accordance with the invention can be further enhanced if in operation of the apparatus a displacement of gas constituting the environment within the drawing chamber is brought about in a direction or directions across the ribbon path. Accordingly, in certain special embodiments of the invention means is provided for exerting gas displacing forces within the drawing chamber in a direction or directions across the ribbon path.

The exertion of such gas-displacing forces, in combination with the provision of baffles on at least one cooler as hereinbefore specified has been found to result in a further improvement in the surface quality of the sheet glass.

The gas-displacing forces may for example be exerted at a position behind the or a said cooler provided with baffles or at any other position or positions, whether in the lower or in the upper part of the drawing chamber.

Preferably said gas-displacing forces are of such magnitude and are exerted at such a position or positions that said forces cause significant displacement of gas along at least a portion of the sub-zones between baffles associated with at least one said cooler. In that case the further improvement in the surface quality of the sheet glass, attributable to the said gas displacement is 5 particularly marked.

Preferably the apparatus comprises at least one main cooler provided with baffles adjacent at least its inner face, in accordance with the invention, and means for exerting said gas-displacing forces at a postion or positions in the lower portion of the drawing chamber such as to cause displacement of gas along at least a portion of the sub-zones between baffles at one or each side (preferably both) of the or a said main cooler.

In many glass drawing machines, the supply of molten 1 5 glass to the drawing zone takes place from a kiln into which the molten glass is fed at one end from a glass-7.72. meltirg furnace. The side of the ribbon facing the end of the kiln into which the molten glass is fed is called the "front" side, whereas the other side is called the "cul-20 de-sac" side. In preferred embodiments of the invention, gas-displacing means as aforesaid is provided in the drawing chamber at the cul-de-sac side of the ribbon path. It is found that the exertion of gas-displacing forces at that side of the ribbon is particularly beneficial.

I 5 Of course, means may also be provided for exerting gas-' displacing forces in the drawing chamber at the front side the ribbon if so desired.

In stating that means is provided for exerting gas-displacing forces in a direction or directions across the ribbon path, what is meant is that the said means operates to exerted forces having at least a component of direction normal to the direction of movement of the ribbon. The forces may but need not be exerted in directions normal to such ribbon movement.

The improvements which result from exerting gas-displacing forces as aforesaid are in most cases particularly marked when gas-displacements occur first in one direction across the ribbon path and then in the opposite direction across the ribbon path, on the same side thereof, and accordingly the invention includes apparatus having means for exerting gas-displacing forces alternately in opposite directions across the ribbon path to cause to and fro displacements of gases across such path on the same side thereof.

As an alternative to providing means for exerting gas-displacing forces first in one direction and then in an opposite direction across the ribbon path, the apparatus ? in certain embodiments of the invention, is provided with means for exerting gas-displacement forces in one direction across the ribbon path, periodically, thereby to cause pulsations of gas in the said direction. Tests have shown that by periodically exerting gas-displacing forces in a given direction across the ribbon path, the environmental conditions are enhanced more than when exerting a gas- displacing force continuously in one direction.

Preferably the means for exerting gas-displacing forces in the drawing chamber comprise means for blowing gas into the drawing chamber. By blowing gas into the drawing chamber it is possible to exert displacement forces in a well defined direction on the gases constituting the environment within the drawing chamber. Preferably at least one ejector device is provided within the drawing chamber and means is provided for blowing gas into the drawing chamber through the injection tube of such ejector. By using an ejector device, it is possible to bring about a relatively high volume rate of displacement of gases within the drawing chamber for a given volume rate of supply of gas through the injection tube. As the volume of gas fed through the injection tube can be quite small and this gas becomes mixed with gases drawn through the diffuser of the ejector, the gas supplied through the injection tubes does not have to be heated to a very high temperature in order to avoid an inadmissible reduction in environmental temperature within the drawing chamber.

As an alternative to providing means for blowing gas into the drawing chamber, mechanical means, e.g., one or more reciprocating plates or one or more propellers, may be; provided within the drawing chamber for exerting the required gas-displacing forces.

The invention includes sheet gXass drawing apparatus of surf ce-drawing type, i.e., of a type in which the ribbon of glass is drawn from the surface of a supply of molten' glass as in a classical Pittsburgh type apparatus or a classical Libbey-Owens type apparatus incorporating a horizontal annealing shaft. The invention also includes sheet glass drawing apparatus of extrusion type, i.e., of a type in which the molten glass which enters the drawn ribbon is extruded from beneath the surface of the supply of molten glass, as in the classical Fourcault process.

Preference is given to apparatus according to the invention which is of a surface-drawing type because in that type of apparatus the advantage of providing baffles, with or without means for causing gas-displacements across the ribbon path, is particularly marked.

Apparatus according to the invention may be of any one of a variety of different basic designs. For example, the invention includes apparatus comprising a molten glass feed channel for holding a bath of molten glass of such depth that nolten glass flows into the ribbon from the full depth of such bath; apparatus comprising a molten glass feed channel for holding a bath of molten glass of such depth that the nolten glass which flows into the ribbon is derived from the upper levels of such bath; and apparatus comprising a molten glass feed channel along which molten glass feeds to the drawing zone while floating on a mass of material of higher specific gravity.

Various embodiments of the invention, selected -by way of example, will now be described with reference to the accompanying diagrammatic drawings in which; Fig. 1 is a cross-sectional elevation of part of a Pittsburgh-type .· glass drawing machine, equipped with means for carrying out the invention; Fig. 2 is a cross-sectional plan view of part of that machine, on line II-II in Fig. 1; Fig, 3 is a detail view showing part of a cooler and of an associated baffle 5 Fig. h is a cross-sectional elevation of a Colburn-type drawing machine, constructed to embody the invention; Fig. 5 is a cross-sectional elevation of another machine for drawing flat glass and constructed so that it embodies the invention; Fig. 6 is a cross-sectional elevation of part of another glass drawing machine embodying the invention; Fig. s a cross-sectional plan view of part of that machine, on the line VII-VII in Fig. 6 and Fig. 8 is a cross-sectional elevation of part of a Fourcault-type glass drawing machine embodying the invention.

In the machine represented in Figs. 1 and 2 a glass ribbon 1 is drawn from a bath 2 of molten glass contained in a supply channel 3« The molten glass is fed into this channel at what in the aspect of Fig. 1 X ' is the right-hand end of the channel and the molten glass flows away from that feed end towards the terminal end wall k of the channel. The ribbon 1 is drawn upwardly through a drawing chamber 5 and through a contiguous 5 annealing shaft 6 surmounting the drawing chamber. The ribbon passes from the drawing chamber into the annealing shaft via a slot defined between catch pans 7 » 8. The ribbon is drawn upwardly by pairs of drawirg rollers 9 mounted in the annealing shaft. The drawing chamber is 10 in accordance with conventional practice formed in part of rear and front L-blocks 10 , 1 1 . In the lower part of the drawing chamber between these L-blocks there are main coolers 1 2 , 1 3 . In the upper part of the drawing chamber there are secondary coolers 1 , 1 5 · The upper part of the 1 5 chamber is bounded at the rear and front by upper wall portions 1 6 and 1 7 which extend from the tops of the L- blocks 10 , 1 1 to the bottom of the annealing shaft.

The region 18 above the molten glass in the supply channel, which region is in communication with the glass 20 melting furnace (not shown) from which the molten glass is fed into the channel, is isolated from the region 19 adjacent the drawing chamber by a shut-off 20 which extends across the full width of the supply channel and dips into the surface layers of the molten glass. The 25 position of the bottom of the drawn ribbon is stabilised by a draw bar 21 which is immersed in the molten glass at the drawing zone .

X The main coolers 1 2 and 1 3 are formed by tubes through which cooling fluid, e.g., water, is circulated.

The cooling fluid is supplied into the upper tube portions of the coolers via conduits 22 and 23 which extend into 5 the drawing chamber through the side wall 2k i After circulating through the series of superimposed tube portions constituting the coolers, the cooling fluid passes from the coolers via conduits 25 and 26 which are in effect prolongations of the bottom tube portions of the 10 cooleire and which pass out of the drawing chamber via the side wall 7 * In the normal way streams of cold air tend to descend along the coolers and unless corrective measures are taken they fall upon the surface of the molten glass 1 5 in the vicinity of the meniscus at the bottom of the ribbon, causing defects in the geometry of the drawn sheet glass as hereinbefore referred to. In the illustrated embodiment gas burner tubes are provided along the coolers adjacent the bottom thereof and 20 combustible gas is supplied to these burner tubes and discharges through series of discharge orifices there- along and the discharging gas is burned to form a series of flames extending along each side of each of the main coolers, near the bottom of the cooler. Consequently 25 there is an upward movement of gas adjacent the coolers.

However the mere production of these heated gas currents would not enable the sheet glass to have the desired high surface quality, since there is a tendency for turbulent thermally heterogeneous gas currents to be produced which affect the gas flow adjacent the ribbon.

In accordance with the invention each of the main coolers is provided with vertically spaced baffles which extend inwardly from the cooler towards the ribbon path. Thus the cooler 12 is provided with a series of vertically spaced baffles 28 and cooler 1 3 has a similar series of vertically spaced baffles 29. These baffles are made of stainless steel and are of openwork trellis construction and they serve to deflect and to disperse the streams of cooled air descending onto them from the portions of the coolers immediately above the baffles. These cold air streams are deflected and dispersed into the hotter atmosphere enveloping the glass ribbon and are entrained upwardly by the main upward convection current of gas ascending immediately adjacent the ribbon of glass, as indicated by the arrows in Fig. 1. In this way the formation of thermally heterogeneous turbulent gas currents in the neighbourhood of the glass ribbon is avoided with the result that the surface quality of the drawn sheet glass is improved, there being a notable reduction in the occurrence of defects.

Part of a cooler with baffles of the kind represented in Figs. 1 and 2 is shown on a larger scale in Fig. 3.

This Figure shows in cross-section, two of the superimposed tube portions of the cooler, namely tube portions 30 and 31 , Between these tube portions there is a tube or sleeve 32' which is welded to the tube portions 30 and 31 · The tube or sleeve 3 serves as a bearing for a rod 33 which is rotatable within such tube or sleeve and carries a baffle 3 in the form of a stainless steel trellis or woven wire screen. The rod 33 extends along the length of the cooler and extends through at least one of the side walls of the drawing chamber in order to permit the rod to be turned to bring the baffle 3^ to a required angle with respect to the general vertical plane of the cooler. It is thus possible at any given time to adjust each of the baffles so that the deflection and dispersion of the cooled gas is appropriate to the prevailing temperature conditions for achieving the required improvement in the surface quality of the drawn sheet glass.

Reference is now made to Fig, k. In the machine represented in this Figure, a glass ribbon 35 is drawn from the free surface of a bath 36 of molten glass held in a supply channel 37 which is fed continuously with molten glass from a glass melting furnace. The drawn glass ribbon initially passes vertically upwardly into the drawing chamber 39 » from the bath of molten glass and then bends around a bending roll O from which the ribbon is entrained along a substantially horizontal annealing lehr , by means of conveying rollers 2.

The drawing chamber 39 is formed in the conventional manner. It is defined in part by the end wall 3 and by - 2k - a roof wall 44 and at the bottom of the drawing chamber ' there are lip tiles 45 » 46 immediately above the surface of the molten glass. Edge rolls 7 grip the margins of the drawn ribbon and maintain its width substantially constant .

In the lower part of the drawing chamber, in front of the glass ribbon, i.e., between the glass ribbon and the lip tile 46, there is a coaler 48 comprising superimposed tube portions through which a fluid cooling medium, e.g., water, is circulated. In accordance with the invention cooler 48 is furnished with baffles in the form of grilles 49 which extend inwardly towards the ribbon path from the lines of division between superimposed tube portions of the cooler. By this means, normal turbulent gas movement adjacent the ribbon due to inter-action of the cooled gas currents with the main upward convection current along the front face of the ribbon is avoided. Moreover it has been found that the currents of relatively cold air coming from the annealing lehr and descending towards the glass ribbon are better dispersed in the neighbourhood of the ribbon.

In the machine represented in Fig. 5 » a glass ribbon 50 is drawn from the surface of a bath 51 of molten glass held in a supply channel 2 of which the wall 53 is the terminal end wall remote from the feed end of the channel. The region 54 above the molten glass in the supply channel is isolated from the region 55 adjacent the drawing chamber by a shut-off 56. The glass ribbon passes upwardly into v the drawing chamber 57 ? bends around a bending roll 58 , and is conducted through a horizontal annealing lehr 59 by conveying rollers 60. The drawing chamber is formed in part by L-blocks 6 l , 62 an upper end wall portion 63 and a roof wall 6k . Edge rolls 65 are provided for gripping the margins of the drawn glass ribbon at the drawing zone and maintaining the width of the ribbon substantially constant .

In the lover part of the drawing chamber, on opposite sides of the ribbon path, there are main coolers 66 , 67. In accordance with the invention, baffles are provided which extend inwardly from the inner face of the coolers towards the ribbon path, such baffles being formed for example of stainless steel and being e.g., of trellis form.

In this particular embodiment of the invention the coolers are also provided with baffles which extend outwardly from the faces of the coolers which are remote from the ribbon path. In fact, the baffles on the inside and outside of each of the coolers are formed by the same series of elements which extend from one side to the other of the cooler, between the superimposed tube portions through which cooling fluid circulates. In Fig. 5 » the arrows indicate the manner in which the baffles serve to deflect the cooled gas currents which descend onto the baffles from the cooled tube portions immediately above them. This deflection results in a substantial improvement in the surface quality of the sheet glass and a notable reduction in the occurrence of wave defects.

Figs. 6 and 7 show part of another Pittsburgh-type glass drawing machine. As the basic components of this machine are identical with those of the machine represented in Figs. 1 and 2, those basic components are designated by the same reference numerals as in those earlier Figures.

The machine represented in Figs. 6 and 7 differs from that which is shown by Figs. 1 and 2, only in the way in which the invention is embodied. In the machine represented by Figs. 6 and 7» only the main cooler 12 is provided with baffles in accordance with the invention. In general the prevailing temperature conditions in the drawing chamber at the cul-de-sac side of the ribbon path, i.e., at the side which is nearer the terminal end wall h of the supply channel, are lower than at the front side of the ribbon and a marked improvement in the heat distribution in the environment of the ribbon can be achieved by providing baffles only in association with a cooler at the cul-de-sac side of the ribbon. The baffles designated 28, can be of grille or trellis form and be made of stainless or non-oxidisable steel as already described.

In the machine represented by Figs. 6 and 7 means is provided for exerting gas displacing forces in the drawing chamber to cause displacement of gas across the ribbon path. This means comprises a pair of ejectors 68, 6 which are mounted in the cul-de-sac side of the drawing chamber, on the rear L-block 10. These ejectors are respectively located adjacent the side walls 2k and 25 of the drawing chamber and point in opposite directions across the drawing chamber as is clearly apparent from Fig. 7. The injection tubes of these ejectors are connected to conduits 70 and 71 respectively which lead to a distributor 72 which is in turn connected to a reservoir (not shown) of pre-heated gas. The distributor 72 is automatically controlled so as to connect the reservoir first with the conduit 70 for operating the ejector 68 and then with the conduit 71 for operating the ejector 6 , and so on alternately. In consequence, in a first period of the operating cycle, gas is displaced across the chamber in the direction indicated by the full-line arrow in Fig. 7 whereas in the second period of the cycle, in which ejector 68 is inoperative, gas is displaced in the reverse direction across the chamber as indicated by the broken line arrow in Fig. 7. The gas-displacing forces exerted on gas constituting the atmosphere in the drawing chamber, by the discharge of gas through the ejectors, is of course not confined to a localised zone along the rear L-block. In various tests it was found that the combination of the gas-displacing forces, causing to-and-fro displacement of gas within the drawing chamber, with the action of the baffles 28, resulted in an important beneficial effect on the surface quality of the drawn sheet glass.

In a modification (not shown) the cooler 13 was also provided with baffles, as in the case of the machine shown in Figs. 1 and 2, and another pair of ejectors was provided for exerting gas-displacing forces at the corresponding side of the ribbon path, such further ejectors being mounted on the front L-block 11, in order to obtain a symmetrical action at the two sides of the ribbon path.

In a further modification (not shown), the machine shown in Figs. 6 and 7 was provided with a second pair of ejectors which were also located adjacent the side walls 2k and 27 but at positions substantially opposite the ends of the main cooler 12. The four ejectors were operated according to the following cycles in a first period of the cycle the ejector 68 and the ejector located adjacent the wall 27 and substantially in line with the cooler 12 were operated simultaneously, causing a circulation of gas to take place in the atmosphere within the chamber, the gas moving across the chamber adjacent the rear L-block and towards the wall 27» and then back across the chamber in the opposite direction in the vicinity of the cooler 12, part of the gas circuit thus lying between the sub-zones between the baffles 28. In the second period of the cycle those two ejectors were inoperative and the ejector 69 was operated simultaneously with the ejector located in line with the cooler 12 and adjacent the side wall 2k, thereby to cause circulation of gas in the reverse direction. In this case a particularly marked reduction in the occurrence of wave defects was achieved and there was a substantial improvement in the surface quality of the drawn sheet glass.

Referring now to the embodiment of the invention in a Fourcault-type glass drawing process as represented in Fig. 8 , the machine serves to draw a glass ribbon 73 upwardly from a debit euse 7^ through which molten glass is extruded from within a bath 75 of molten glass held in a supply channel 6 into which molten glass is continuously supplied so as to flow along the channel towards its terminal end wall 77 « The region 78 above the molten glass in the supply channel is separated from the atmosphere through which the glass ribbon is drawn by a bridge wall 80 which dips into the surface of the molten glass. The ribbon of glass 73 is drawn upwardly through a drawing chamber 79 and through a vertical annealing shaft 8 1 , by pairs of drawing rollers 82. I/ithin the drawing chamber 79 there are main coolers 83 » S in the lower part of the drawing chamber, and secondary coolers 85 and 86 in the upper part of that chamber. The vertical annealing shaft 8 1 has a chimney effect in that it creates strong natural draught forces causing gas currents to pass upwardly from the drawing chamber at relatively high speed, along the faces of the ribbon and through the slot by which the drawing chamber communicates with the annealing shaft.

Streams of cold air are generated against the walls of the secondary coolers 85 and 86 , which streams normally tend to mix with the ascending convection currents and to form thermally heterogeneous gas currents which act on the faces of the ribbon, causing "hammering".

This adverse effect is avoided or reduced in accordance with the invention by providing on the inner faces of the secondary coolers 85 and 86, baffles 88, 89 which for example can be of trellis form and made from non-oxidisable steel. The baffles on each of the secondary coolers extend over about one third of the distance separating the cooler from the ribbon path. It was found that as a result of providing these baffles the surface quality of the drawn sheet glass was substantially improved, there being a relatively marked decrease in the occurrence of hammering.

In a modification (not shown), the secondary coolers were provided with baffles projecting from their outer faces as well as from their inner faces. The presence of the outwardly extending baffles resulted in a further improvement in the heat distribution within the drawing chamber.

In a further modification very advantageous results were realised by combining the provision of baffles in accordance with the invention with the exertion of gas-displacing forces on gas in the atmosphere within the drawing chamber by means of propellers which caused to-and-fro displacement of gas across the drawing chamber.

The propellers were disposed opposite the ends of the secondary coolers, adjacent the side walls of the upper part of the drawing chamber, and the propellers adjacent the opposed side walls were operated alternately to cause the said to-and-fro displacement of gas. By virtue of the disposition of the propellers within the drawing chamber there was a substantial displacement of gas along the sub-zones between the baffles associated with the secondary coolers.

While in describing the various embodiments illustrated in the drawings, no actual dimensions have been given, tests established that particularly satisfactory results could be attained when the baffles associated with any given cooler were vertically spaced by at least and preferably by more than 2" .

Claims (2)

1. 40020/3 WHAT IS CLAIMED IS: 1. Apparatus for use In manu acturing sheet glass* comprising a kiln along which molten glass can be fed to a drawing zone, a drawing chamber and a contiguous annealing shaft, means for drawing a continuous ribbon of glass from said drawing zone and along a path through said drawing chamber and annealing shaft* and at least one cooler through which cooling fluid can be circulated and which is located within said drawing chamber and serves to assist cooling of the said ribbon of glass, at the side of the or at least one said cooler facing the ribbon path there being two or more baffles extending inwardly from said cooler at vertically spaced levels thereof, characterised thereby that there is at least one said baffle which Is of open, e.g. trellis , construction.! 2. Apparatus according to claim 1, characterised in that at least one said cooler thus provided with baffles is present in the lower portion of the drawing chamber·< 3. Apparatus according to claim 1 or 2, characterised in that at least one said cooler thus provided with baffles is present in the upper part of the drawing chamber.i 4. Apparatus according to any preceding claim, characterised in that there is at least one said cooler which is thus provided with baffles which extend over substantially the full projected width of the ribbon path past such cooler, 5. Apparatus charactepfsed in is of open, t Apparatus according to any preceding claim, characterised in that there is at least one said cooler thus provided with baffles which are vertically spaced by at least l " . Apparatus according to any preceding claim, characterised in that at least one said cooler thus provided with baffles is present in the lower portion of the drawing chamber, and the said baffles associated with this cooler project over at least one third of the distance between the cooler and the ribbon path. -6% Apparatus according to any preceding claim, characterised in that there is at least .one said cooler thus provided with baffles which are movably mounted so that their orientation in the vertical plane can be varied. Apparatus according to any preceding claim, characterised in that there is at least one said cooler which is thus provided with baffles and which is composed of a series of superimposed tubes or tube portions. Τχ· Apparatus according to any preceding claim, characterised in that there is at least one said cooler - 3 - which is thus provided with baffles which are distributed over an upper part of the height of the cooler and in that heating means is provided at or near the bottom of such cooler for maintaining an upward current of heated gas 5 adjacent the inner face of the cooler, beneath the lower or the lowest baffle. 10. JJ-. Apparatus according to any preceding claim characterised in that there is at least one said cooler which is thus provided with baffles and which is also 10 provided with baffles which project from the outer face of the cooler. Π. J.

2. Apparatus according to claim 1 1 , characterised in that there is at least one said cooler having baffles projecting from its inner and outer faces, located in the 1 5 lower part of the drawing chamber. 12. J-3.. Apparatus according to claim 1 1 or 1 2 , characterised in that there is at least one said cooler which is composed of superimposed tubes or tube portions and which has baffles which project from its inner and outer faces 20 and are formed by different portions of a single set of elements which extend through the cooler between neighbouring tubes or tube portions. 13. -1-¾·. Apparatus according to any preceding claim, characterised in that means is provided for exerting gas- 25 displacing forces within the drawing chamber in a direction ' Apparatus according to any of claims 1 to 1 » characterised in that said means for exerting gas- displacing forces comprises means for blowing gas into the drawing chamber. 20. 2J-. Apparatus according to claim 20, characterised in that said means for exerting gas-displacing forces comprises at least one ejector device located in the drawing chamber, and means for blowing gas into the drawing chamber through the injection tube of such ejector device. 21 . , " 22. Apparatus according to any of claims 14 to 21, characterised in that mechanical means, e.g., at least one reciprocating plate or at least one propeller, is provided in the drawing chamber for exerting said gas- displacing forces. 22, 23„ Apparatus according to any preceding claim, which apparatus is of a surface-drawing type. 23. 24. Apparatus for use in manu acturing sheet glass, substantially as herein described with reference to the accompanying drawings. 24. 2.5.. A process of manufacturing sheet glass wherein apparatus according to any preceding claim is used, 25. 2.6. Sheet glass when manufactured by a process according to claim 25. COHEN ZEDE & SPISBACH - 37 - P.0. Box 33116 , Tel-Aviv Attorneys for Applicant