CA1051198A - Glass melting - Google Patents

Abstract

ABSTRACT
A glass melting tank has a melting zone for melting glass forming material, the melting zone being followed by a refining zone and con-ditioning zone. A liquid cooled barrier extends horizontally across the tank adjacent a waist and controls the forward flow of molten glass to the conditioning zone. A plurality of stirrers are mounted side-by-side across the tank adjacent the barrier.

Description

1~5~g~
The present invention relates to glass production and more par-+ arly to a glass melting furnace and the operation thereof.
In the manufacture of glass in tank furnaces, unmelted batch is fed onto an established bath of molten glass at one end o the furnace where it is melted, the molten glass, which forms from the batch, passes down the furnace frcm the melting area, through refining and conditioning zones, and is drawn off from the furnace to be processed in any known manner.
It is difficult in practice to obtain completely homogeneous glass in a glass melting tank. If the glass con-tains discontinuities of properties ! either chemical or physical, it is consi~ered il~honlo-geneous. Such discontinuities may arise from ~ldissolved solids and gases, differences in composition due to differential glass processing or alternatively variations of physical conditions such as temperature.
The difficulty in avoiding inhomogeneity increases at high loads when time and temperature in a particular zone are limited by furnace design and refractory constraints. The glass produced is generally ~heterogeneous in composition, to a lesser or greatèr degree, depending upon the efficiency with which melting and subsequent operations are accomplished. Glass varying in composition9 forms layers in the fur-nace these layers being subject to convective and other flows imposed ~
by the ~urnace operation~ design and other physical operations carried ;
out on the glass. In the final product these layers are generally parallel to the glass surfaces but there may be deviation from this parallel state in areas which have been subject to other modifying ~onditions. ~Yhere the layers of inhomogeneity cease to be parallel or continuous to the faces of the glass 9 optical faults occur~
We have found that the flow conditions can be modified and there-by reduce the severity of composition inhomogeneity by at-tenuating, or ;o horizontally stretching, so as to reduce the thiclllless of t~le layers of ream. It is believed that stirring action results in homogenisa-tion~ because it increases the length of the flow path of the glass melt through the tank, and hence tends -to a-ttenuate the layers of glass of different composition. This itself reduces the influence of , - 2 - ~

- .--g8 composition changes on optical quality and fur-thermore, because the 1q ~ are thim2er 9 allows ~reater freedom of interchan~e of ~le glass in adjacent layers by diffusion and so reduces -the level o~ composi-tion differences between the glass layers.
The present invention provides a glass melting tank in which glass forming materials are converted in a continuous process to a glass melt in a melting zone and -the melt is subsequently refined in a refining zone prior to reaching a workin~ end of t~le ta1lk and disc11arge to a forming process, said tan~ havin~ a liquid cooled barrier extend-~o ing horizontally across at least part of the width of the tank in thepath of forward flow towards the worlcing end~ the barrier bein~ posi-- tioned at a height above the bottom of the tan]c so as to be located in the upper region of the melt and con-trol the forward flow of molten material to the worlcing end, and two or more stirrers mounted side-by-side across the direction of forward flow within a region adjacent the barrier for rotation abnut vertical axes, saidstirrers bein~ connec-ted to drive means and arranged to stir the forward flow of ~lass melt.
The barrier may be located at or adjacent a ~ais-t in the tanlc.
The stirrers may be arranged so that at, at least, one position in each revolution of th~ stirrers -there is no angular difference -between the rotational settings of the stirrersO
The stirrers may include blades or paddles. They may be arran~ed to rotate in the same direction so that the blades or paddles of dif-ferent stirrers remain parallel to each other during rotation and in this case the s-tirrers are maintained in phasc. `If the blades or paddles are ro-tated in opposite directions they are arran~ed so that all the blades or paddles become parallel to each other at one pre-determined position during each revolution so that there is no dif-ference in rotational setting at that position. Alternatively, the stirrers may comprise cylindrical members, such as cylindrical stalks, which are symmetrical about the axis of rotatioll. In thiS c~se tl stirrers do not exhibit differenc~s of rot~tiol~al setti1l~ re~ar(1less of their rotational positions~ In GaseS w~lere the stirreIs have blades or paddles the drive means may be arran~ed to rotate the r ~ i/`rers out~of-phase with one anotherO By "out-of-p}lase", we mean ~ he pairs of stirrers are always arranged -that a -transverse major axis of a paddle or a blade of one stirrer, is at an angle to the : transverse major axis of a paddle or a blade of tlle next stirrer.
The side-by-side arrangement across the forward flow can be at ~ 90 to the forward flow, or at an angle other than 90.
;~ In all cases the stirrers are designed so -that they do not impart , . .
to the glass a substantial vertical component of glass flow.
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In practice each stirrer will be mounted on a rotary shaft. ~'ad-dles or blades may be mounted eccentrically on the shart so as to provide a greater degree of lateral movemen-t than with symmetrically mounted paddles or blades.
In one embodiment the blade is formed from a loop of metal tube.
The space enclosed by the loop may be filled with a plate made of material resistant to attack by the molten glassD The plate may be .
made of molybdenum. ~
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In an alternative, the tube is formed from stainless steel. T~le , tube and shaft upon which the tube is carried may alternatively be '! formed from mild steel and the area out of glass contact may be coated with a sprayed-on refractory or surrounded Wit}l a refractory tube to protect it from attack.
When using a plurality of stirrers, the spacing between stirrers and groups of stirrers may be adjusted in accordance with the desi~n of stirrer e.g. number of blades, effective diameter and syeed of rotation which in -turn will be dependent upon the load operating con-ditions, and tank design.
In order to eliminate alternative paths for the glass flow other than through the stirring zone, it is preferred to use a plurality of stirrers side-by-side across the direction of glass flow. In this way, the stirrers may extend across -the full width of glass flow.
; Furthermore, it is preferred to arrange the stirrers symmetrically with regard to the centre line of glass flow.
The speed of stirring is limited in that the stirrers must not cause bubble in the glass at the glass stirrer interface, or ' :

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` ~S~8 substantial erosion of -the refractories forming -~he furnace.
~ ~referably some or all of -t~le stirrers are liquid cooled. Con-veniently the liquid used is water.
The barrier may project above the surface of the molten glass or in some cases it may be desirable for the upper surface of the bar-rier to lie in the same plane as the surface of the molten glass. The ` barrier may be in the form of a water cooled pipe extending across at - least part of the tank.
The barrier is preferably adjustable in position and preferably is located adjacent the entrance to the waist. It may however in ` some cases be located within or downstream of the waist. The barrîer may extend perpendicular to the direction of flow of the mel-t througl the waist or may be inclined at some other angle to the direction of flow.
Preferably the stirrers are located in the waist at a position downstream from tl~e barrier.
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The invention also provides a method of producing mol-ten glass comprising feeding glass forming material to one end of a glass melt-ing tank, melting the material in a melting .~olle of the ta~ alld ref~l-ing the molten glass in a refining region before the glass passes to aconditioning zone at a working end of the tank from which molten glass ~j~ is discharged from the tank said method including controllin~ the forward flow of molten material from the relining zone to the condi~
tioning zone by provision of a substantially horizontal barrier through which cooling liquid is passed in the upper region of thc melt, and stirring the melt in the waist region by rotation of two or more stir- `~
rers about substantially vertical axes, the stirrers being mounted side-by-side across the direction of glass flow.
The stirrers may be rotated so that at at least, one position in each revolution of the stirrers there is no angular difference between the rotational set-tings of the stirrersO Alternatively the two stirrers may be rotated out-of-phase with each other.
The stirring may be effected by rotating at least three pairs of stirrers arranged side-by-side across the direction of flow, the t~o ,.

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~5~g8 stirrers of each pair being ro-tated out-of-phase with each ot~ler.
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~~ ~referably cooling liquid is passed through tll~ stirrers.
Preferably the stirrers are operated do~nstream o~ tlle barrie and the stirring is effected in the forward flow of glass melt.
The water cooled barrier in co-acting with the water cooled stir-rers to improve the quality-of the glass acts as a physical barrier restricting the fo~ard flow which in turn influences the hea-t trans-fer between the melting area and the conditioning zone in the worlcin~
; end of the tank~
The heat input to the furnace is limited by -the maximum tempera-tures which the superstructure and regenerator refractories are able to withstand so that the times available for each of the relate~ pro-cesses of melting, refining and conditioning, which are dependent on the temperatures in these zones, must be closely controlled if ma~i-mum output is to be achieved. Too short a melting time results in partially melted raw materials in the final product; too shor-t a refining time results in an increase in bubble in the glass; and excessive cooling is necessary if the conditioning zone is too short and results in adverse flows within the molten glass with consequen-t', 2 0 tial deterioration in optical quality of the glass. In general the furnace is operated so as to achieve compromise conditions, by suitably adjusting the thermal gradients along the furnace so as to permit an overall optimum in glass quality.
The glass forming materials are normally fed into a filling poc-ket at the melting end of the tank and the downstream limit of the melting zone is determined by the increasing temperature found on moving from the filling pocket through -the melting zone. The increase in temperature is controlled by regulation of lleating in this pnrt of the tank. On passiIlg beyond the melting zonc, a fall in tempcrat~lre gradient is present as the glass is being refined. The -times for melting and refining may be adjusted to suit a particular load by modifyinG these tcmperature gradien-ts.
The temperature gradients set up in the tanlc cause convec-tive flows within the glass. In the refining and conditioni~ zones the - G -~ s~
~low is generally forwards and ou-twards towards t}le side walls in the ~ ~r glass layers and gellerally bac3n~ards and to~ards tlle t~ll]C Cell~
in the lower layers. The depth of both forward and return flow is dependent on the load and temperature conditions.
~ y use of the liquid cooled barrier it is possible to adj~l~t tlle amount of heat transferred by the glass melt to the conditionin~ zone.
Such adjustment is usually made so as to increase the amowlt of heat available to the glass for melting and refining and thus res-trict the . .
flow of heat into the conditioning zone. Such a cl~ange ~eans a con-sequent reduction in the cooling required to brin~ the glass in the -~ conditioning zone to the temperature at which it shollld leave the fur-:~ .
nace~ Such adjustments as are necessary are usual~y made in response to a change in load on the tank~ i.e. an increase or decrease in the -quantity of glass to be produced over a specified period from the tank.
The adjustment of the heat transfer betweerl the refining zolle and the conditioning zone can be simply done by varying the depth a-t which t~le barrier is immersed in the glass at or about the boundary bet~leen the t~ro zones. The barrier is placed so as to prevent glass flow across the top of it and the position of its lowermost edge in the glass flow is chosen so as to control the flow of glass b~nea-th and thereby con-trol the quantity of heat transfer between the two zones. To achieve satisfactory operation with different fwrnace loads, it i5 desirable , . .
; for the barrier to be adjw table in ver-tical position so as to alter its depth of immersion in the glass.
; It has been found that by introducin~ some form of baffle or res-triction, e.g. a system of water pipes, in the forward flow of the upper layers of glass -these layers are retarded and that secondary systems of return flow are set up upstream and downstxeam of the bar- ;
. .
; rier so that in the longer time available -to the glass in the surface ~-;o layers upstream of the barriers, more hcat is lIItroduccd ill tO the glass and more of this heat is retained in the refining zone by -trans-fer in the increased return flow from the barrier area. At the same time, less heat is transferred from -the refining to conditioning zone~
By adjusting the depth and design of the barrier it is possible to ,.,' ~ `.

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5~198 regulate the flow to satisfy the various limitations imposed by load an ~~~mperature.
Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which:
Figure 1 is a plan view of the furnace showing the . preferred location of the barrier and i .
stirring system, Figure 2 is a section on the line X-X of Figure 1, Figure 3 shows diagrammatically one arrangement i.:10 for the direction of rotation of the . ~, . .
`.` stirrers, Figure 4 is a sectional view through one s-tirrer, Figure 5 shows another embodiment of stirrer, Figure 6 shows one arrangement of water cooled ^ barrier, Figure 7 shows an alternative arrangement for the direction of rotation of the stirrers, Figures 8a to 8d show plan views of different pairs of stirrers, which may be used in accordarlce ~.?0 with the invention, .~ Figure g shows the results o~ model tests correspond-: ing to the stirring arrangement used in .. -~.
Figure 7, ; Figure 10 shows equivalent model test results with a different stirring arrangement, ,i .
: Figure 11 is a graph showing the comparative results .: of the tests used in Figures 9 and 10~
rlgure i2, appearing with rigs. 4,~, & 6, illustrates a pattern of inhomogeneity in a sheet of glass formed without stirring, -0 and Figure 13, appearing with Figs. 4,5, & 6, shows a similar pattern :- . .
within a sheet of glass after stirring in accordance -;~ with the present invention.

With reference to the drawings, and more particularly to Figures 1 and 2, there is shown a po~tion of a furnace 10 in accordance with ., the invention. The furllace comprises an elon~ate~ talll; 11 for Coll-`ing molten ~lass 12. The tanlc has a crown 13, side walls 14end walls 15 and a bottom lG all formed of a suitable refractory material. The batch, from which the glass is to be formed, is fed ~ -by a device (not shown) into a melting end throueh a filling pocket 18 and is melted in the zone 19. The melt is then refined in a refining ~one 20 and passes through a waist 24 into a conditioning ` zone 21 at the working end of the tank in a continuous process. Tlle - glass is then discharged through an outlet canal 22 to a forming process. ` The batch material, fed into the tank 11, floats on the !; molten glass 12 and is carried thereby tl1rough the melting zone 19.
Heat ~or converting the batch to molten glass within the meltin~ zone , 19 is provided by burners mounted in or close to por-ts 2~ opening in-to the meltin~ and refining zones 19 and 20 above the level of the ~ molten glass 12 on opposite sides of the furnace.
; As is shown in Figure 1, a water cooled barricr ~7 i5 provide~a at the entrance to the waist 24. This barrier is convellielltly in the form of a pair of hairpin water cooled pipes which may for example be as shown in Figure 6. The pipes are positioned at a hei~ht above the bottom of the tanlc so as to be located in the upper re~ion of for-- ward flow of the melt into the waist region. In this way, -the bar-rier controls the forward surface flow of molten material into the waist~ The upper surface of the pipes may in some cases project above the surface of the ~lass or alternatively the upper surface may lie ln the same plane as the glass surface. In ordcr to vary the ; affect of the barrier on flot~ into the waist re~iol1 tl~ barrier is adjustable in vertical height so as to vary the depth the barrier is submerged in the molten glass~ The pipes may be ~ixed on adjustable supports 30 at either side of the tank as shown in Fi~ure G. In the ~o arrangement shown in Figure 6 the barrier consists oL two separate `~
hairpin arrangements 27a and 27b. These project in from opposite sides of the tank~ Although in Fi~ure 6, the upper and lower runs of pipe are shown parallel to each other and to the glass surface, they may be designed so that the lower run is sloped upwards or _ 9 _ downwards towards the centre of the waist.
! Downstream of the barrier six stirrers 28 are located in the for-ward flow path through -the waist region. The stirrers are arranged side-by-side so as to extend across the waist and as shown in Figure

2, the stirrers are rotatable about vertical a~es by a drive motor 31.
In the particular example shown three pairs of stirrers 28 are located in the mid-part of the waist region and are symmetrically disposed relative to the central axis of flow through the waist. Each stirrer is mounted on a rotary shaft projecting through the roof 13 of the melting furnace. The upper ends of the sha~ts are connec-ted via ;. ~.
a horizontal drive shaft to the drive motor 31 which is arranged to rotate the stirrers at the same speed. Each stirrer has a blade or paddle at its lower end the paddle being located in the forward flow of glass and barely extends into the return flow at the bottom half of the tank. In the arrangement sho~m in Figure 3, the blades on each stirrer are arranged parallel to each other and the drive means is arranged so that all the stirrers are rotated in the same direction and at the same speed so that they are maintained in phase with each other. It is possible to use stirrers which ha~e no blades or pad-dles. In this case the stirrers may be of cylindrical form. Alter-;.:;
native shapes for blades or paddles which may be used on stirrers areshown in Figures 8a to 8d whlch schematically illustrate alternative arrangements o~ multiple blade members each forming one pair of stir-rers.
The stirrers, including the blade types shown in Figures ~a to 8d, are water cooled and two alternative constructions are shown in Figures 4 and 5. In the arrangement shown in Figure 4 the stirrer consists of a hollow loop formed by a tube 36 connecting an inlet ~7 ~ith an outlet 38. The tube may be formed of stainless steel.
Figure 5 shows the same construction with the space enclosed by tlle hollow loop filled with a central plate 35~ formed of material resis- ~ i tant to attack by molten glass, such as molybdenum. In both cases cooling water is passed continuously around the hollow tube while the stirrer is rotated.
:

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: - ~05~8 As already described, the stirrers may be rota-ted in an "in ~ " arrangement as shown in Figure 3. In such a case all the stirrers may be rotated in the same direction as shown in Figure 3, or alternatively they may be rotated in opposite directions provided -there is no angular difference between the rotational set-tings of the stirrers at, at leas-t, one position in each revolution of the - stirrers.
It may alternatively be arranged that the stirrers are rota-ted in an '70ut-of-phase" arrangement and examples of this are shown in Figures 7 and 8a to 8d. In Figure 7 the stirrers are spaced side-,i by-side across the width of the tank substantially perpendicular to ; the direction of flow and the spacing between adjacent pairs of stir-rers is twice the spacing between the two stirrers of each palr. In this case, the spacing between the axes of the two stirrers in each pair is thirteen inches whereas the spacing between the axes of adja-cent stirrers of different pairs i5 twentysix inches. As shown in Figure 7, each stirrer is arranged to ro-tate in -the opposite direction to the adjacent stirrer regardless of whether the adjacent stirrer is in the same pair of stirrers. Each stirrer in Fi~ures 8a to 8d has blades or paddles which are non-uniform around the axis of rotation, and successive stirrers can be arranged in-phase or alternatively out-of~phase. In accordance with the arrangement showll in Figures ~a to 8d they are arranged out-of phase and in this particular example -the twin bladed stirrers of Figure 8a are 90 out-of-phase, the three bladed stirrers of Figure 8c are 60 out-of-phase and the four bladed stirrers of Figure 8d are 45 out-of-phase. In Figure ~b the stir-rers have essentially only a single blade each moulded eccentrically on the shaft and arranged 90 out-of-phase wi-th each other. In the single blade arrangement, the blade may be entirely offset from the axis of rotation by a horizon-tal arm joining the blade to the stirrer shaft.
As can be seen from Figure 2, the mol-ten glass circulates within the tank before passing through the waist region 24. The upper region of the glass flows towards the working end 21 whereas -the ~s~
~ower region of the glass has return flow towards t~le meltin~ elld.
` is a neutral lirle marlced 33. As it is important that the stirrers 2~ cause attenuation of the glass in the horizontal plane, it is necessary to limit the extent to which the stirrers are sub-; merged in the glass and in this embodiment tlley are shown ju.t cros-; sing the neutral line 33. In this way, they do not appreciably affect the glass which flows along the return line towards the melt-ing end. The stirrers are so shaped that rota-tion of the s-tirrers causes forward and lateral movement of -tlle glass ~u~ does not Ca-lSe any substantial vertical component of movement to be induced in the glass.
As is shown in Figure 1, the barrier extends horizontally across the full width of the waist region of the tank and the two halves of the barrier are inclined to the transverse direction across the tankO
In this particular case, the two halves of the barrier are inclilled such that the central region of the barricr is locatcd closcr to t~lC
feeding end of the tank. The barrier may however be arranged a-t other inclinations and may in some cases extend perpendicular to the direction of flow.
It has been found that the pattern of heterogeneous layers of glass and the difference in intensity or composition between layers is changed to improve the optical quality of -the final product by passin~
the glass in the refining zone 20 into the waisted section 24 of the tank under the water cooled barrier 27 com~ined Witil s~l~se(luelltly stirrin~ the forward flowing glass by the water cooled stirrers ~.
In order to assess the affects of operatin~ -the stirrers in-phase or out-of-phase with the various embodiments of the invention, a model of -the glass melting tank was set up~ The model was a 1/15th scale model of the tank as shown in Figure 1 and the fluid used in the ta~c was castor oil.
The efficiency of stirring was measured as tlle ratio of the total len~th of an attenuated dye trace after passing tllrough tlle stirrers to the lengtll of the original trace which was injected upstream of the stirrers in tlle line of fluicl flow. For e~ample, referrin~ to I _ 12 -'~ ' , ;''38 Figure 9, if rl is -the number of peaks on one sidc of -the a-ttenua-t~d after s-tirring and y their mean wid-tll and x the ori~inal lel~gtl of trace before st:irring, then the efficiency O:r s-tirrin~ is taken as Original trace length before stirring As can be seen from Figures 9 and 10, representin~ out-of-pilase and in-phase stirrin~ respectively, the dye traces form linear paths 30 leading up to thc stirrers bu-t on passin~ thro~ the stirrcrs ; part of the -trace forms a significant zig-zag pat-tern representing ` 10 subs-tantial attenuation and re-orientation of the original trace, the greater the efficiency of s-tirrin~ -the grea-ter -the de~ree of attentla-tion and the less the remaining linear trace passes straight throu~h the stirrers unchanged in orientation; i-t is irnmediately obvious from these figures that less attenuation occurs when the s-tirrers are operating in-phase, Figure 10, than when the stirrers are 90 out-of:-phase, Figure 9.
The summary of various model tests using different speeds of rotation for stirrers shown in Figure 8a are plotted in Figure 11.
One curve marked ~ shows the attenuation results achieved ~Ihen the stirrers of each pair were 90 out-of-phase and the other curve B
shows the attenuation when the stirrers of each pair were in-pllase.
As is shown, improved attenuation resul-ts from the use of stirrers out-of-phaseO
The type of adiustment tllat can ~e achicvc~l b~ nltcril-g ~lle ~c~tl and design of barrier can be illustrated by comparinC the results achieved with a series of barriers in a glass talllc operatin~ at a tanlc ; outpu-t of 2000 tonnes per week. The barriers used were:
(a) A pair of pipes arran~ed in a hairpin, -the pipes havin~ an outer diameterof 3-~ inches and a 3 inch bore, witll a 1 inch gap between tlle legs of the hairpin. The barrier depth was 8 inclles. 'l`llis size of barrier had virtually no effect on the operation of a -tanlc as regards retention of heat in the refining zone.
(b) A pair of pipes formed from 5 inch by 2 inch box sections, again with a 1 inch gap thereby providing a barrier depth of 11 inches.

' .: . - - - . .: :

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Vsing tllis confi~uration it was possible to dcmonstratc all effcc~ on tl Jrkillg end hea-t balance~ Tlle total heat cx-tracted by tlle pair of water pipes was 23 therms/hour (roughly equivalent to the heat removed by 1000 cubic metres per hour of cooling air). Tlle redllc-tion in cooling air requirements downstream of the barrier as a result of using the water pipes was about 3000 cubic metres per hour in -excess of that simpl~r due to the coolin~r cLfcc-t of tlle barrier, indi-catin~ a retention of heat upstream of thc barrier.
(c~ An arrangement similar to (b~ but usin~ 7 inch by 2 inch box sections to give a barrier depth of 15 inches. This arrangement did not increase the total heat extracted by tlle barrier9 but did result in a ~urther significant reduction ir. cooling air required downstream of the barrier. It was also possible to reduce fuel consumption.
Thus by simply altering the dimensions of the pipes used to carry the water one can achieve the variation in dep-th necessary -to meet the various limita-tions imposed b~J load and temperature. In altcrirl~ tlle depth of the barrier, it is important to ensure that tlle for~ard flow-::;
ing glass passing under the barrier does not also pass under the s-tir-rers, and the position and depth of the stirrers should be adjusted to ensure that all the ~lass which eventuall~J is dischar~ed to a for-ming process is stirred.
;-~ The fact that the retention of heat in the refinill~ arca is dependent in any particular tank on the configuratioll of barrier i5 -:
clearly shown by these results.
The influence of the use of the barrier and stirrin~ is most readily seen in the reproduction of the pattern secn tllrou~ll a trans-verse section of a ribbon of glass taken before and after the stirring operation. These patterns are shown in Fi~ures 12 and 13 respective-ly. As can be seen, -the pattern shown after stirring in Figrure 13 has a much more laminar pattern of ream. ~n ordcr to maintai~l t}~e optimwn pattern at differen-t tarllc loads, it is necessary to vary tlle depth of the ~ater cooled barrier 27 follo~Jin,r, a cl~anGc in loadO
The invcntiorl is not rcstricted to tlle del;ails of tlle forc~oing example. For instance, instead of using the si~ stirrers sho~n in ;

~05~
Figures 3 and 7, it is possil~le -to use four, five, si,~ stirrers or ., ` arran~ed side-by-side across the waist re~ion, and more than one bank of stirrers.
The arran~emcnt o~ stirrers may ~e varied arld t~le spaci~ e tWeeIl them may be varied. It is however desirable to maintain a symmetri-cal arran~ement relative to the central flow lil~e throu~h tile ~aist region.
Paddles may be mo~ted centrally on the axis o~ ro-tation of the stirrer. Alternatively, t~le paddle or blade may be moullted eccen-trically on the rotar~ shaft supportin~ the blade.

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Claims (18)

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The embodiments of the invention in which an exclusive property or ??ilege is claimed are defined as follows:-

1. A glass melting tank in which glass forming materials are converted in a continuous process to a Glass melt in a melting zone and the melt is subsequently refined in a refining zone prior to reach-ing a working end of the tank and discharge to a forming process, said tank ha?g a liquid cooled barrier extending horizontally across at least part of the width of the tank in the path of forward flow towards the working end, the barrier being positioned at a height above the bottom of the tanks so as to be located in the upper region of the melt and control the forward flow of molten material to the working end, and two or more stirrers mounted side-by-side across tile direction of forward flow within a region adjacent the barrier for rotation about vertical axes, said stirrers being connected to drive means and arran-ged to stir the forward flow of glass melt.

2. A glass melting tank according to Claim 1 wherein the bar-rier is located at or adjacent a waist in the tank through which the melt flows to the working end.

3. A glass melting tank according to Claim 2, in which the stirrers have blade or paddle members for immersion in the molten galss.

4. A glass melting tank according to Claim 3, wherein the stir-rers and the drive means are arranged so that at, at least, one posi-tion in each revolution of the stirrers there is no angular difference between the rotational settings of the stirrers.

5. A glass melting tank according to Claim 4, wherein the stir-rers are arranged to be rotated in phase with each other.

6. A glass melting tank according to Claim 3, wherein the stir-rers and drive means are arranged so that the stirrers are ratated out of phase with each other.

7. A glass melting tank according to Claim 1 wherein the stir-re?re cooled by flow of cooling liquid through at least part of each stirrer.

8. A glass melting tank according to Claim 1 wherein the bar-rier is adjustable in depth of immersion in the molten glass.

9. A glass melting tank according to Claim 1 wherein the bar-rier comprises at least one water cooled pipe.

10. A glass melting tank according to Claim 9, wherein the barrier comprises at least one water cooled U-shaped pipe providing two horizontal arms on extending above the other.

11. A glass melting tank according to Claim 12 wherein the bar-rier is located adjacent the entrance to the waist and the stirrers are located downstream of the barrier.

12. A method of producing molten glass comprising feed glass forming material to one end of a glass melting tank, melting the material in a melting zone of the tank and refining the molten glass in a refining region before the glass passes to a conditioning zone at a working end of the tank from which molten glass is discharged from the tank, said method including controlling the forward flow of mol-ten material from the refining zone to the conditioning zone by pro-vision of a substantially horizontal barrier through which cooling liquid is passed in the upper region of the melt, and stirring the melt in a region adjacent the barrier by rotation of two or more stir-rers about substantially vertical axes, the stirrers being mounted side-by-side across the direction of glass flow.

13. A method according to Claim 12, wherein the stirrers are rotated so that at, at least, one position in each revolution of the stirrers there is no angular difference between the rotational set-tings of the stirrers.

14. A method according to Claim 13, wherein the stirrers are ? in the same direction, at the same speed and in phase with each other.

15. A method according to Claim 12, wherein the stirrers are rotated out-of-phase with each other.

16. A method according to Claim 15, wherein adjacent stirrers are rotated at the same speed in opposite directions.

17. A method according to Claim 12 wherein the stirrers are water cooled by circulation of cooling water through at least part of each stirrer.

18. A method according to Claim 12 wherein the barrier is arranged to control flow of molten glass into a waist region of -the tank.