This invention relates to the production of float glass.

A conventional float glass production line comprises a glass melting furnace,

otherwise known as a tank, in which batch material is melted and the resultant molten glass is refined and conditioned, a float glass forming chamber, otherwise known as a bath, in which molten glass received from the furnace is formed into a float glass ribbon, and an

annealing lehr in which the ribbon cools during its travel from the bath to a wareroom in which the glass is cut into plates and stacked. The capacity, i.e. the maximum throughput, of such a line is determined by the maximum throughput of the lowest capacity part of the line, and the line may be required to operate at different throughputs and to make different

products, e.g. different glass thicknesses (substances), at different times. This can limit the efficient use of the furnace which may frequently have to operate below capacity and of the float forming chamber which may have to be equipped to produce a variety of products.

There have been prior proposals for feeding a plurality of flat glass forming

chambers from a single glass melting furnace - see for example US Patent 3,932,165. However, such production arrangements run the risk of impaired glass quality, both with regard to discrete faults such as stones or bubble and with regard to the overall optical quality, which with float glass generally has to be of a standard considerably higher than

that for other forms of glass including other forms of flat glass.

According to the present invention there is provided a float glass production facility including a melting furnace having a melting zone in which batch material is melted to form

molten glass, a refining zone in which the molten glass is refined to a standard suitable for float glass manufacture and a working end from which the refined glass is fed for ribbon

float glass manufacture and a working end from which the refined glass is fed for ribbon forming, there being a first canal connecting a first exit from the working end to a first float glass forming chamber and a second canal connecting a second exit from the working end

to a second float glass forming chamber, in which the working end is operable so that flow

of glass through one of the exits is independent of the flow of glass through the other exit, and the second canal has a depth adjacent the second exit such that the flow of glass along the canal is in a direction away from the exit and return flow through the exit back into the

working end is precluded and has a length greater than its width sufficient to permit thermal conditioning and homogenisation of glass flowing along the second canal, there being associated with the second canal means for thermally conditioning and means for homogenising glass flowing along it, and a shut-off device to shut off flow of glass along the second canal from the second exit in the working end. It will be appreciated that such a facility can, in effect, provide two float glass production lines fed from a single melting furnace which can permit more efficient use of the furnace and the float forming chambers

and can enable different products to be produced on the respective different lines at the

same time.

Preferably the second canal constrains glass flow to unidirectional flow throughout its length so that there is no return flow in the second canal. This can be achieved by an appropriate depth for the second canal but it may have at least one gradual change of cross-

section, i.e. of depth and/or width. The second canal may have a bend which can conveniently permit the second line to run parallel to the firsL

The homogenising means associated with the second canal preferably comprise stirrers which may, in particular, be provided in the second canal near its downstream end.

The first and second exits in the working end are preferably sufficiently spaced to

achieve independence of respective glass streams flowing through them. However, stirrers may be provided in the working end close to the first exit to homogenise molten glass flowing towards the first exit thereby assisting to ensure such independence. The first exit

may be in an end wall and the second exit in a side wall of the working end, the second exit preferably being located at least two thirds of the length of the side wall from the end wall. Preferably the second canal jobs the second exit substantially at right angles to the working end wall containing the second exit

All the previously mentioned stirrers are preferably paddle stirrers.

The thermal condition g means associated with the second canal may comprise overhead heaters or may comprise electrodes to effect Joule heating or may comprise a comb ation of these.

The workbg end may have one or more further exits additional to the first and second exits for feedbg molten glass to one or more further float glass forrmng chambers, there bebg a respective further canal havbg similar features to the second canal between each such further exit and the respective float glass formbg chamber.

The bvention also provides a method of producbg float glass including melting batch material to form molten glass a melting zone of a sbgle melting furnace, refinbg the molten glass in the sbgle melting furnace to a standard suitable for float glass

manufacture, feedbg a first stream of refined molten glass from a working end of the single

melting furnace and formbg that glass bto a first float glass ribbon, feeding a second

stream of refined molten glass bdependently of the first stream from the workbg end of the sbgle melting furnace and thermally conditioning and homogenisbg the molten glass b the second stream and then forming that glass into a second float glass ribbon.

The method may bclude stirring molten glass in the workbg end of the sbgle

melting furnace as it passes into the first stream and may bclude stirring molten glass b the second stream just prior to its delivery to a chamber for formation bto the second float glass ribbon.

Surprisingly it has been found that, with a facility or method in accordance with the bvention, a number of ribbons each of good quality float glass can be formed from molten glass fed b respective streams from the working end of a sbgle meltbg furnace, even with

different throughputs for the respective streams and with asymmetrical arrangements.

ϊn order that the invention may be better understood embodiments of it will now be described, by way of example, with reference to the accompanybg drawbgs, b which:-

Figure 1 is a schematic plan view of part of a float glass production facility,

Figure 2 is a schematic representation of a bend in a canal,

Figure 3 is a schematic cross-section showing heating means in a canal,

Figure 4 is a schematic plan view showbg stirrers b a canal,

Figure 5 is a schematic longitudinal section showbg stirrers b a canal,

Figure 6 is a schematic plan view of part of another float glass production facility.

Figure 1 shows a glass meltbg furnace 1 which comprises a meltbg zone 2, a refinbg zone 3 and a workbg end 4. Batch material is fed into the melting zone 2 in well

known manner and is melted there to form molten glass which is then refined, i.e. bubbles are removed, in the refining zone 3. The molten glass then passes to the working end 4 in which conditionbg of the glass takes place. The furnace shown b Figure 1 is of a form

havbg a waist 5 between the refining zone 3 and the workbg end 4. Stirrers and water cooled pipes may be used in or adjacent the waist 5 to homogenise the glass passbg through it, for example b a manner as described in British Patent Specification 1503145.

As will be well understood by those skilled b the art, there may not be precise fixed

boundaries between the meltbg, refining and conditioning zones of the glass meltbg furnace. It will further be understood that the particular form of furnace shown b Figure 1 is given by way of illustration and example only and that any other form of glass melting

furnace capable of producbg molten glass of a quality and refined to a standard suitable for

the manufacture of float glass could be employed.

The workbg end 4, which is shown as rectangular b form, has a first exit 6 b its

end wall 7 through which a first stream of molten glass flows bto a first canal 8, shown as

tapered, by which it is delivered to a first float glass foπr ng chamber 9. A tweel (not shown) is provided to control and, if necessary, halt the flow of glass bebg delivered from the canal 8. The glass so delivered is formed into a first float glass ribbon which is then drawn from the exit of the forming chamber through a lehr (not shown) to a cutting room

b well known manner.

The bstallation as so far described with reference to Figure 1 is a conventional float glass production line.

b accordance with the present invention the working end 4 of the melting furnace has a second exit 10 through which a second stream of molten glass can flow. This second exit 10 is located in a side wall 11 of the workbg end at a distance at least two thirds of the length of the side wall 11 from the end wall 7. In other words the second exit 10 is located

b the first third of the length of the side wall 11 from the waist end of the workbg end but

preferably not immediately adjacent the comer. The second exit 10 is therefore well spaced from the first exit 6 and more particularly the first and second exits are sufficiently spaced so that bteraction between the first and second glass streams flowing respectively through

them can be avoided and bdependence of the streams can be achieved. If desired the avoidance of such bteraction can be assisted and such independence further assured by usbg stirrers 12 b the workbg end 4 close to the first exit 6 b a manner as described b

British Patent Application No. 95/22123.0 (whose disclosure is incorporated by reference) which may permit a closer spacing of the first and second exits 6 and 10. any event, the workbg end can be operated so that flow of glass through one exit does not affect and is bdependent of the flow of glass through the other.

A second canal 13 connects the second exit 10 to a second float glass forming

chamber 14. This second canal 13 is a long canal relatively to the first canal 8, i.e. it has a length greater than its width sufficient to permit thermal conditioning and homogenisation of glass flowing along it as described later. The second canal 13 jobs the second exit 10 substantially at right angles to the side wall 11 and has a depth adjacent the exit such that

the flow of glass along the canal is b a direction away from the exit and return flow through the exit back into the working end is precluded. This ensures that the molten glass

b the working end is not contambated or adversely affected by back flow from the second

canal 13.

Preferably the second canal 13 is effectively uniflow, i-e. it constrabs glass flow to unidirectional downstream flow, throughout its length. If desired, however, it may have

changes b depth but gradual changes b preference to step changes so as to ensure smooth flow and avoid creating pockets of slow-movbg glass which can bcrease the risk of glass faults. Likewise the second canal may if desired have changes b width but agab these are preferably gradual rather than step changes for the same reasons. Thus changes b cross-

section of the second canal are best achieved by tapered transition sections. Figure 1 shows such a tapered section 15, which reduces in both width and depth, near the downstream end of the second canal 13 where it delivers the molten glass to the second float glass formbg chamber 14. A tweel (not shown) is provided to control, and if necessary halt, the flow of

glass bto the formbg chamber. Since the second canal 13 is a long canal it can suffer from significant 'drawdown', Le. the level of the free glass surface drops along the canal due to

frictional pressure losses. This can affect the head of glass effective behbd the control

tweel and the depth of the canal needs to be sufficient to ensure that tweel control can be mabtabed. The delivered glass is formed in the chamber 14 bto a second float glass ribbon which is drawn from the chamber's exit through a lehr (not shown) to a cutting section b well known manner.

The second canal 13 has a right angle bend 16 which enables the second float glass formbg chamber 14 to lie parallel to the first float glass forming chamber 9. The length of the first leg of the canal 13 between the working end 4 and the bend 16 is sufficient to leave

enough room between the forming chambers 9 and 14 for their satisfactory operation, bcluding for example start-up operations, insertion and withdrawal of devices such as top

rolls, coolers, coating equipment, and the like. The length of the second leg of the canal 13 between the bend 16 and the formbg chamber 14 is sufficient for a convenient formbg chamber entrance location, for example approximately level with or slightly downstream of

the entrance to the first formbg chamber 9. This provides a convenient factory lay-out

with the first and second lines (made up of respective formbg chamber, lehr and cutting section) parallel, b practice, with appropriate conveyor arrangements, there may be some sharing of cutting and stackbg facilities.

In order to mbimise adverse effects arising from flow of glass round the bend 16 it is preferably of swan-neck form as used elsewhere in the glass industry. This form bvolves protrusions 17 and 18 providbg a taper to about half width bto the actual bend followed by a smooth expansion back to full width coming out of the bend as shown in Figure 2.

This achieves a more symmetrical glass flow about the centre-lbe of the canal as is well known in the art

Thermal conditioning means 19 are associated with the second canal 13 to condition the molten glass flowing along it Figure 1 bdicates one such thermal conditioning means b each leg of the canal but it will be understood that they may be located anywhere as required along the canal length. Figure 3 shows particular forms of thermal conditioning means comprising overhead radiant heaters 20, which may be gas or oil burners or

electrical, and electrodes 21 immersed b the flowbg glass to effect Joule heating,

practice the thermal conditionbg means may comprise one or other or a combbation of such types of heaters.

Homogenisbg means 22 are also associated with the second canal 13 to

homogenise the molten glass flowing along it Figure 1 shows such homogenisbg means 22 near the downstream end just before the tapered section 15 in the form of stirrers also shown b Figures 4 and 5. The stirrers 23 and 24 constitute a pair of paddle stirrers arranged to rotate b opposite directions 90° out of phase. The spacbgs between the

respective stirrers and between the stirrers and the canal side walls are such as to achieve

effective homogenisation without hindering the throughput The clearance between the stirrers and the canal floor should be small enough to avoid significant leakage but large enough to avoid mechanical contact or severely accelerated corrosion of the canal bottom.

The paddles or blades of the stirrers should be wholly immersed in the glass so that only the shafts break the free glass surface, the blade tops bebg sufficiently below the glass surface to avoid bubble entrabment or excessive wave generation but sufficiently close to it to

preclude leakage of unstirred glass above the blades. The stirrers are preferably not water-

cooled for fear of heavy chilling effects leadbg to glass devitrification, but may be made of resistant refractory ceramics, refractory shapes coated with noble metal, or refractory metal alloys.

Although Figure 1 bdicates stirrers only near the downstream end of the second

canal 13, they may be located at other positions along the canal as required.

A shut-off device 25, analagous to a tweel, is provided as indicated in Figure 1 at the entrance to the second canal 13 to shut off flow of glass along the second canal from

the second exit 10 in the working end 4. This device 25 is preferably located as close as

feasible to the side wall 11 contabbg the exit 10 so as to minimise the amount of glass between the exit 10 and the device 25, and the device 25 is preferably water cooled to freeze the adjacent glass when it is b the shut-off position. The device 25 can be used to stop the flow of glass to the second canal 13 when, for whatever reason, the second float

formbg chamber is not operative.

The manner of operation of the facility will be largely apparent from the above. Glass batch is melted to form molten glass b the meltbg zone 2 of the sbgle melting furnace 1 and the molten glass is refined b the refinbg zone 3. When both lbes are b

operation a first stream of refined molten glass is fed from the first exit 6 of the workbg end of the sbgle meltbg furnace and formed bto a first float glass ribbon b the first forming chamber 9 while a second bdependent stream of refined molten glass is fed from

the second exit 10 of the workbg end of the single melting furnace. The molten glass in the second stream is thermally conditioned and homogenised as it travels along the second canal 13 and then formed bto a second float glass ribbon b the second formbg chamber

14. If desired the molten glass passing bto the first stream can be stirred by the stirrers 12 b the workbg end just prior to its delivery to the first forming chamber 9. The molten glass in the second stream can be stirred at the downstream end of the long canal 13 just prior to its delivery to the second formbg chamber 14.

The forming chambers 9 and 14 can be operated to make different products at the

same time. For example one might have coating equipment to make a coated product while the other makes uncoated glass. They could produce different respective ribbon

thicknesses (substance) and/or widths. Additives to the base glass could be bjected bto the second canal. Further, by suitable product mix adjustment the sbgle melting furnace may operate at nearly constant throughput (load) permitting greater efficiency and cost

advantages, optimised furnace design and enhanced basic glass quality. If it is required to

shut-off the first line, this can be done by the tweel at the delivery to the first formbg chamber 9. If it is required to shut-off the second tine, this can be done by operation of the shut-off device 25 (thereby isolating virtually all the glass b the second canal 13 from the workbg end 4 which is preferable to shutting off by means of the tweel at the delivery to

the second forming chamber 14).

It will be appreciated that a facility in accordance with the bvention may be built as a new plant or may be formed by addbg to an existing plant Thus the embodiment shown b Figure 1 could be arrived at by addbg the second canal 13 and the second float formbg chamber 14 to an existbg plant comprising the meltbg furnace 1, the first canal 8 and the first float formbg chamber 9. b this case stirrers 12 b the workbg end close to the first

exit 6 may be highly desirable and if necessary the capacity of the existbg melting furnace 1

may be bcreased by the application of boost heating so as to raise its output to satisfy both lbes.

It will further be understood that the dimensions and operating parameters of the second canal will be chosen to meet the particular requirements of the plant Such choice is

within the capabilities of those skilled in the art, possibly with some trial and experiment, and the following information, based largely on model work, is given by way of illustration and example for guidance only.

The second canal should generally have a length greater than about four metres and would typically be much longer, for example about forty metres or even more, b the

Figure 1 embodiment for example, the second canal 13 may have a first leg of about 22 metres to the bend 16 and a second leg also of about 22 metres from the bend 16 to its downstream end, giving a total length of about 44 metres. The glass conveybg width of

the second -canal 13 may be about two metres narrowing to about one metre at its downstream end where it delivers the glass to the second forming chamber 14, the length of the tapering section 15 bebg about one metre. At the swan-neck bend 16 the glass conveybg width also reduces to about one metre. The radii of curvature of the inner and

outer protrusions 17 and 18 may be about 300 mm and 1300 mm respectively about the same centre C, the other dimensions of the bner protrusion 17 shown as A and B b Figure 2 bebg about one metre and 600 mm respectively.

The depth of glass b the second canal 13 just upstream of the tapering section 15

may be about 500 mm reducbg to about 300 mm downstream of the tapering section 15. As previously mentioned, the drawdown phenomenon means the glass depth will be greater at the upstream end of the canal 13 but preferably the head loss should not exceed 10% of the glass depth at the control tweel so that with the present example, the glass depth at the

upstream end of the canal 13 is about 530 mm. As also explabed previously, this depth is such that the flow is unidirectional and return flow into the working end 4 through the second exit 10 is prevented, such uniflow also improvbg process stability.

The paddle stirrers 23 and 24 near the downstream end of the second canal 13 are preferably rotated relatively slowly, e.g. at between 6 rpm and 20 rpm, to avoid

btroduction of bubble and to avoid 'blocking', i.e. when the net resistance to flow becomes excessive and the glass tends to flow over or under the stirrers rather than between them, but fast enough to ensure full homogenisation. The blade width for each stirrer of a pair as

shown b Figure 4 is preferably between about 20% and 25% of the canal width, e.g. about

450 mm, the stirrers bebg mounted symmetrically about the canal centre Ibe with a sparing between stirrer shaft centres of between about 35% and 40% of the canal width, e.g. about 760 mm. The nearest the blade edges come to the canal side walls b operation is preferably between about 16% and 23% of the canal width, e.g. about 395 mm. The

clearance between the bottom of the stirrer blades and the canal floor may be between about 5% and 20% of the glass depth, e.g. about 60 mm. The canal floor may be constructed from corrosion-resistant refractories or have noble metal protection to permit a

small clearance without introducbg refractory corrosion faults which would severely impair good quality. The stirrer shafts may be located about 1200 mm upstream of the upstream end of the tapered section 15, this being in practical terms just before delivery of the molten

glass to the forming chamber 14.

The second canal 13 is designed and operated to avoid the btroduction bto the second molten glass stream of discrete faults which might otherwise arise for example from devitrification, stagnant or semi-stagnant glass, contamination with products of refractory corrosion, or events occurring in the joints between refractory glass contact blocks.

Nevertheless, the depth of glass in the canal is such as to achieve uniflow operation and to prevent flow of glass back bto the working end of the meltbg furnace from which such faults might be transmitted bto the first stream.

The thermal conditioning means 19 serve to ensure that the required temperature conditions are maintabed b the glass flowbg along the second canal 13, bcludbg a

desired surface temperature and acceptable side-to-side or side-to-centre and top-to- bottom temperature differences. Temperature control may also be assisted by judicious use of bsulation, particularly around the below glass level structure of the canal bottom. Glass

entering the second canal 13 through the second exit 10 may, for a conventional float glass composition example, be at a temperature between about 1160°C and 1190°C It may be mabtained b the first part of the canal at a surface temperature of about 1180°C which may reduce to about 1170°C at the bend 16. Between the bend 16 and the downstream end of

the canal the glass may gradually cool in a controlled manner to a surface glass temperature of about 1100°C at the delivery control tweel to the formbg chamber 14. The side-to-side temperature differences which may be about 13°C at entry to the canal 13 may reduce to a side-to-centre difference of about 8°C or less and the top-to-bottom temperature difference

may be about 15°C.

The throughput (load) of the glass meltbg furnace 1 may for example be about

7000 tonnes per week (tpw) with the first float Ibe incorporating the first forming chamber 9 operating at about 4000 tpw and the second float Ibe incorporating the second formbg

chamber 14 operating at about 3000 tpw.

It will yet further be appreciated that more than two lines could be taken off a single glass melting furnace, the workbg end of that furnace being provided with the required

number of exits and being operable so that flow of glass through any one exit does not affect the flow of glass through any of the other exits whereby independent glass streams

can be achieved, and there being respective canals havbg similar features to the described second canal connecting the additional exits to respective float glass formbg chambers.

Figure 6 shows a facility havbg three lbes. The arrangement of the first and second lines is essentially the same as in Figure 1 and the same reference numerals are used to bdicate the same parts. The third Ibe is basically similar to the second Ibe and is bdicated

by the same reference numerals as the second Ibe but with the suffix A. Thus the third Ibe is fed from a third exit 10A b the side wall 11 A of the workbg end 4 opposite the side wall 11. A long third canal 13 A havbg a swan-neck bend 16A and havbg associated thermal conditionbg means 19A (shown after the bend only) and homogenisbg means 22A near its

downstream end which has a tapered section 15A, connects the third exit 10A to a third float glass forming chamber 14A. A shut-off device 25A b the third canal 13A adjacent the third exit 10A can be operated to shut-off flow of glass to that canal. When that device is

open the refined molten glass flows as a third stream along the uniflow canal 13A to the

formbg chamber 14A in which it is formed bto a third float glass ribbon. The separation of the respective exits 6, 10 and 10A in the working end and the use of stirrers 12 adjacent the first exit 6 avoids bteraction between the glass streams flowbg through the respective exits and ensures their independence.

Figure 6 also shows modification or addition cells 26 and 26A located b the upstream legs of the long second and third canals 13 and 13A respectively. These cells can be operated to modify the molten glass flowbg through the canal, for example by injecting additive material to alter its colour or composition. With such cells the float glasses

produced on the second and third lines can have different properties from that produced on

the first Ibe, which uses the unmodified standard base glass from the working end 4, and from each other. The facility can therefore simultaneously manufacture three float ribbons not only of different thicknesses and/or widths but also of different compositions or

properties. If desired one or more of the float formbg chambers may be provided with coatbg facilities to coat the float ribbon passing through. The glass meltbg furnace 1 may, for example, operate at about 12000 tpw with the first float Ibe operating at about 8000 tpw, the second float Ibe at about 1000 tpw, and the third float Ibe at about 3000 tpw. b the embodiments of Figures 1 and 6 the workbg end 4 of the glass meltbg furnace is of conventional rectangular form with exits provided in the end wall and side wall(s) the side wall length bebg, for example, of the order of 15 metres. It will be understood that if more than three lbes are to be taken off the sbgle melting furnace, then

the workbg end may need to be of a different geometry, for example of polygonal or semicircular form, so as to permit the respective canals to join up to it and maintain bdependence of the glass streams flowbg bto them.

The embodiments of Figures 1 and 6 have parallel lbes which is normally

convenient for a factory lay-out It will be understood, however, that if particular circumstances require or can tolerate lines in other angular relationships, e.g. at right angles

to each other, this can be achieved by omitting the bend in the relevant canal or havbg the

bend at a different angle.

Further variations from the embodiments specifically described which can be made without departing from the principles of the invention will be readily apparent to those skilled b the art

17

For example, whereas a sbgle pair of stirrers 23 and 24 is shown across the second canal 13, there could be two or more stirring pairs, or an uneven number of stirrers, across

the canal width, the guideline dimensions given above bebg fractioned appropriately. Stirring may take place at a plurality of locations along the second canal 13, the locations bebg separated sufficiently to avoid adverse interaction. While a simple pair of stirrers at

each location is preferred, there could be more as explained above. It will be understood

that effective homogenisation b the long canal is necessary, at least near the downstream end, to remove elliptical ream features which would prejudice the optical quality of the product and to remove side-to-side temperature variations which would prejudice

uniformity of product thickness and it is surprising that this can be achieved by stirring in the canal to give good quality float glass b the second ribbon.

Similarly, more than two stirrers could be provided b the workbg end 4 near the first exit 6 although a single pair is preferred. It will be understood that while under some

favourable design, glass composition and meltbg conditions these stirrers may be unnecessary, they can be helpful in protecting the first Ibe from quality impairment which could otherwise arise due to changed conditioner flow patterns resulting from the secondary line(s), i.e. in renderbg the working end operable to achieve independent glass

streams so that flow of glass through one of the exits does not affect the flow of glass through the other exit(s).

It will be appreciated that a float glass production facility with plural lbes runnbg off a single melting furnace each capable of making good quality float glass has

considerable benefits in enabling efficient operation of the meltbg furnace while producbg

a desired float glass product mix. One (mab) Ibe may be used to manufacture standard clear float glass products at popular widths and thicknesses, the melting furnace producing such clear float glass, while the other Ibe or lbes are used to produce less widely used products. For example one may make coloured float glass through the addition of colouring material b its respective canal thereby also enabling colour adjustments to be rapidly made. A certab Ibe, particularly the float formbg chamber, may be specifically

designed for a particular product output such as thin glass or coated glass, and there may

be provision for selectively modifybg the products made on a particular line at different times to address short-run specialist markets.