WO2016178964A2

WO2016178964A2 - Apparatus and method for conditioning molten glass

Info

Publication number: WO2016178964A2
Application number: PCT/US2016/030001
Authority: WO
Inventors: Zagorka Dacic Gaeta; William Weston Johnson
Original assignee: Corning Incorporated
Priority date: 2015-05-06
Filing date: 2016-04-29
Publication date: 2016-11-10
Also published as: WO2016178964A3; JP2018520969A; TWI681937B; JP6639522B2; KR20180004231A; TW201641450A; TW202023967A; CN107660197A; JP2020063189A

Abstract

Disclosed is an apparatus for processing molten glass, and more specifically to stirring molten glass. The apparatus comprises a vessel including a main stirrer comprising an uppermost stirring blade, and an inlet stirrer, wherein for a horizontal plane extending through the vessel, the uppermost stirring blade being below the horizontal plane, the inlet stirrer is entirely above the horizontal plane. The vessel may include a plurality of inlet stirrers, and may be rotated in a variety of directions relative tom a direction of rotation of the mains stirrer. A method of processing molten glass is also disclosed.

Description

APPARATUS AND METHOD FOR CONDITIONING MOLTEN GLASS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No. 62/157,576, filed on May 6, 2015, the content of which is relied upon and incorporated herein by reference in its entirety.

BACKGROUND

Field

[0002] The present disclosure is directed to an apparatus for processing molten glass, and more particularly, an apparatus for mixing the molten glass.

Technical Background

[0003] The manufacture of glass on a commercial scale is typically carried out within a refractory ceramic melting furnace wherein raw materials (batch) are added to the melting furnace and heated to a temperature at which the batch undergoes chemical reactions to produce the molten glass. Several methods of heating the batch can be used, including gas- fired burners, an electric current, or both. In a so-called hybrid process, a gas flame from one or more gas-fired combustion burners initially heats the batch. As the temperature of the batch increases and the molten glass is formed, the electrical resistance of the material decreases such that an electric current can be introduced into the molten glass through electrodes mounted in the side walls and/or floor of the melting furnace. The electric current heats the molten glass from within, and the gas burners heat the molten glass from above. In some embodiments submerged combustion can be employed.

[0004] Downstream processing of the molten glass, for example fining and homogenizing, can be carried out in certain portions of the furnace structure or in other vessels located downstream from the melting furnace and connected to the melting furnace by conduits. To maintain an appropriate temperature of the molten glass as the molten glass is being conveyed, the molten glass may be heated. In some processes, such as the fining process, the molten glass can be heated in a fining vessel to a temperature greater than the furnace temperature to facilitate a more complete removal of bubbles from the molten glass. In other portions of the manufacturing apparatus downstream of the melting furnace the molten glass may be cooled while flowing through one or more conduits to bring the molten glass to an appropriate viscosity for forming. However, the cooling may be limited by the controlled addition of heat energy to prevent too rapid a cooling rate.

[0005] The melting process can produce a stream of molten glass into downstream processes that may, without further processing, be inhomogeneous for at least one or more of the following reasons: 1) incomplete melting of granular raw materials; 2) dissolution of refractories comprising the melting vessel; 3) volatilization from free surfaces of the molten glass; 4) blending in density-segregated glass layers; and 5) transitioning between glasses of different composition. As a result, chemical and thermal inhomogeneities may exist in the molten glass that can result in visible refractive index variations when the molten glass is formed into a finished article and cooled. If the finished article is a drawn article, such as a ribbon or sheet of glass, the regions of refractive index difference can be stretched to appear as long, thin streaks, or as more commonly referred to, cord. In some instances, cord may further manifest as small variations in thickness of the glass article. Hence, the glass article may include surface corrugation, with a variable spatial period in a range from about 1 to 10 millimeters (mm) and a depth typically on the order of 10 nanometers (nm) or more.

[0006] For the manufacture of optical quality glass, for example glass suitable for the manufacture of display panels (e.g. liquid crystal display panels) used in such devices as televisions, computer monitors, tablets, smart phones and the like, the removal of these inhomogeneous regions is paramount. A large amount of cord is undesirable, as it affects the quality of displays both visually and functionally. Visually, cord can appear as multiple dark lines in the drawn direction as a result of the lensing effect produced by the curvature of the corrugated surface. Functionally, high cord can produce variation in the cell gap of a liquid crystal display (LCD) device, which may affect the liquid crystal operation. Accordingly, efforts are made to mix the molten glass while the molten glass is still of a sufficiently low viscosity that the mixing is effective to remove cord. Mixing can be passive, wherein a flow direction of the molten glass is repeatedly varied by static vanes positioned within the flow of molten glass, or the molten glass may be actively mixed by flowing the molten glass through a mixing vessel, such as a stirring vessel comprising a moving member, e.g. a rotating stirrer. [0007] Over time, the need to improve the mixing capability of mixing vessels, and in particular stirring vessels, has increased in the face of tightening commercial cord specifications, increased cord loading due to increased glass flow rate, or the introduction of new, difficult-to-melt glass compositions.

[0008] In a given stirring vessel configuration, mixing intensity can be increased by increasing the revolutions per minute (RPM) of the stirrer positioned within the stirring vessel. However, there is a hard limit on the RPM that decreases with increasing stirrer size. That is, as the diameter of the stirring bades increases, the rate at which they can be effectively rotated wihthin the stirring vessel decreases. In addition, high RPM can also produce harmful side effects, including increased metal (e.g. platinum or platinum alloy) inclusions produced by erosion of the mixing vessel and reduced stirrer life.

[0009] The RPM setting of an operating stirrer is a trade-off between mixing cord to desired levels while minimizing platinum or platinum alloy inclusions without compromising stirrer life. It is desirable to find a way to achieve improvement in stirring effectiveness that circumvents this tradeoff.

SUMMARY

[0010] In certain glass manufacturing processes raw materials, the mixture of which is typically referred to as batch, are exposed to a high temperature heat source (or sources), whereupon raw materials undergo chemical reactions to produce a molten material, hereinafter referred to as molten glass, that is formed by a forming apparatus into an article which, upon appropriate cooling, becomes a glass article. To those not particularly skilled in the art, the high temperature processing to form the molten glass is typically referred to as a melting process, and for the purposes of the following discussion, such reference is satisfactory if not strictly accurate. In any event, during the melting process, the resultant molten glass is far from homogeneous. To begin, the raw materials are typically granular in nature, and so very small regions of the molten glass may include a chemical composition different from an adjacent small region. There can be variations in temperature as well, wherein one region of the molten glass may be of a different temperature than another region. Such chemical and thermal inhomogenities in the molten glass, if not mitigated, can lead to property variations in the finished glass article, such as, but not limited to, differences in refractive index that can be readily observed by one having only average visual acuity. In processes that employ some manner of drawing (stretching) of the molten during a downstream forming process, small regions of inhomogeneity can be stretched out into long filaments (i.e. cord) generally parallel with the draw direction. Such filaments can be readily observed in the finished glass article. In the instance where the glass article is a glass sheet, for example a glass sheet used in the manufacture of a display device (e.g. a liquid crystal display device), these filaments produce a distraction to the viewer. Additionally, these filaments can produce thickness variations in the glass sheets, thereby resulting in gap differences between the parallel glass sheets comprising the LCD display panel that can interfere with the display panel function. Even in the instance where the glass sheets may be further processed, such as by grinding and/or polishing, the refractive index differences remain.

Accordingly, glass manufacturers employ various means to homogenize the molten glass.

[0011] In some processes, the heat source or sources, such as electrodes for establishing an electric current in the molten glass, are arranged to produce desired convection currents in the molten glass to maximize mixing and produce complete melting. Some processes may utilize bubblers configured to introduce a gas into the molten glass. The bubbles rise through the molten glass to a free surface thereof, and in so doing, introduce a mechanical motion that aids in mixing of the molten glass.

[0012] In some processes mixing in the melting furnace can be augmented by additional downstream mixing. For example, such additional mixing may be performed by flowing the molten glass through a vessel particularly configured to stir the molten glass. In an example stirring operation, a stirrer can be rotatably mounted in a vessel downstream of the melting furnace, wherein as the molten glass flows through the vessel, the stirrer rotates within the vessel and produces mechanical mixing of the molten glass. Typically, the stirrer comprises a shaft and one or more stirring blades arranged on the shaft and extending outward therefrom such that rotation of the shaft "cuts" the filaments and disperses them throughout the mass of the molten glass. In some embodiments, the stirrer may be of the screw-type comprising a shaft with one or more blades wound about the shaft helically or otherwise. In other embodiments, the stirrer may be of the paddle type comprising one or more paddles arranged along the shaft in a

predetermined pattern. Other configurations are also possible and include paddles with openings in the faces of the paddles, and skeletonized members comprising an arrangement of connected bar members, for example reminiscent of mixing blades for a culinary mixer. [0013] One aspect these foregoing mixing vessels can have in common is the need to submerge the one or more mixing blades below the surface of the molten glass. Should the one or more mixing blade become exposed at a free surface of the molten glass, lapping may occur that entrains gas (e.g. air) in the molten glass. The entrapped air, even minute bubbles, can form additional observable defects in the finished article and are therefore undesirable. Thus, the one or more mixing blades are typically positioned well below the free surface of the molten glass. Flow analysis has shown, however, that the positioning of the one or more mixing blades below the free surface of the molten glass can result in the molten glass above the mixing blades to exhibit regions of quiescence. That is, stagnant regions of molten glass that, under steady state condition, undergo little if any rigorous mixing and wherein the molten glass in these regions may, in certain stirrer designs, leak from the quiescent region and bypass the mixing blades, for example along a path between a wall of the vessel and the stirring blade.

[0014] Accordingly, an apparatus for conditioning molten glass is described herein comprising a vessel, a main stirrer positioned within the vessel and rotatable about a first axis of rotation, the main stirrer comprising a main stirrer shaft and an uppermost main stirring blade extending outward therefrom a distance dm relative to the first axis of rotation and a first inlet stirrer positioned within the vessel and rotatable about a second axis of rotation offset from the first axis of rotation a distance da less than dm.

[0015] For a horizontal plane extending through the vessel, with the uppermost main stirring blade positioned below the horizontal plane, the first inlet stirrer does not extend below the horizontal plane.

[0016] The apparatus may further comprise a delivery conduit opening into the vessel and configured to deliver the molten glass to the vessel, the delivery conduit comprising a central longitudinal axis. Subsequently, for a first vertical plane extending through the vessel perpendicular to the central longitudinal axis of the delivery conduit, with the first axis of rotation lying entirely within the first vertical plane, the first inlet stirrer is positioned on a same side of the first vertical plane as the delivery conduit.

[0017] In addition, the apparatus may further comprise a second inlet stirrer positioned within the vessel and rotatable about a third axis of rotation offset from the first axis of rotation the distance da and positioned on the same side of the first vertical plane as the delivery conduit. Accordingly, for a second vertical plane perpendicular to the first vertical plane, with the central longitudinal axis of the delivery conduit lying entirely within the second vertical plane, the second axis of rotation and the third axis of rotation are equidistant from the second vertical plane.

[0018] The apparatus may further comprise a second inlet stirrer positioned on an opposite side of the first vertical plane from the delivery conduit.

[0019] The apparatus may comprise, for example, a plurality of inlet stirrers.

[0020] In certain embodiment the vessel is cylindrical and the first axis of rotation is coincident with a central longitudinal axis of the vessel.

[0021] In another aspect, an apparatus for conditioning molten glass is disclosed comprising a vessel, a main stirrer positioned within the vessel and rotatable about a first axis of rotation, the main stirrer comprising a main stirrer shaft and an uppermost main stirring blade extending outward therefrom a distance dm relative to the first axis of rotation, and a first inlet stirrer positioned within the vessel and rotatable about a second axis of rotation offset from the first axis of rotation. Accordingly, for a horizontal plane extending through the vessel, with the uppermost main stirring blade positioned below the horizontal plane, the first inlet stirrer does not extend below the horizontal plane.

[0022] The first inlet stirrer may, for example, be positioned within an upstream volume of the vessel and the uppermost stirring blade is positioned within a downstream volume of the vessel.

[0023] The upstream volume and the downstream volumes may be cylindrical volumes and a central longitudinal axis of the upstream volume can be coincident with a central longitudinal axis of the downstream volume.

[0024] The apparatus may comprise, for example, a plurality of inlet stirrers. Each inlet stirrer of the plurality of inlet stirrers can be positioned within the vessel and rotatable about an axis of rotation offset from the first axis of rotation a distance da less than dm.

[0025] The apparatus may further comprise a delivery conduit opening into the vessel and configured to deliver the molten glass to the vessel, the delivery conduit comprising a central longitudinal axis, and wherein for a first vertical plane extending through the vessel

perpendicular to the central longitudinal axis of the delivery conduit, with the first axis of rotation lying entirely within the first vertical plane, the first inlet stirrer may be positioned on a same side of the first vertical plane as the delivery conduit. [0026] The apparatus may still further comprise a second inlet stirrer positioned within the vessel and rotatable about a third axis of rotation offset from the first axis of rotation and positioned on the same side of the first vertical plane as the delivery conduit, and wherein for a second vertical plane perpendicular to the first vertical plane, with the central longitudinal axis of the delivery conduit lying entirely within the second vertical plane, the third axis of rotation may be positioned on an opposite side of the second vertical plane than the second axis of rotation. In some embodiments the second axis of rotation and the third axis of rotation are equidistant from the second vertical plane.

[0027] In still another aspect, a method for conditioning molten glass is described comprising flowing molten glass into a vessel from a delivery conduit, the vessel including an upstream volume and a downstream volume relative to the flow of molten glass, stirring the molten glass in the upstream volume with a first inlet stirrer rotatable about an axis of rotation, and stirring the molten glass in the downstream volume with a main stirrer rotatable about an axis of rotation parallel with the axis of rotation of the inlet stirrer, the main stirrer comprising an uppermost stirring blade, wherein for a horizontal plane extending through the vessel, with the uppermost stirring blade positioned below the horizontal plane, the first inlet stirrer is positioned entirely above the horizontal plane.

[0028] The uppermost stirring blade may, for example, comprise a length dm that is the maximum extension of the uppermost stirring blade from the axis of rotation of the main stirrer, and wherein an offset between the axis of rotation of the first inlet stirrer and the axis of rotation of the main stirrer is less than dm.

[0029] The method may further comprise stirring the molten glass with a second inlet stirrer comprising an axis of rotation, and with the second inlet stirrer positioned entirely above the horizontal plane, an offset between the axis of rotation of the second inlet stirrer and the axis of rotation of the main stirrer can be less than dm.

[0030] In some embodiments, for a vertical plane perpendicular to a central longitudinal axis of the delivery conduit, and with a central longitudinal axis of the vessel lying entirely within vertical plane, the first inlet stirrer and the second inlet stirrers are positioned between the vertical plane and the delivery conduit.

[0031] In some embodiments a direction of rotation of the first inlet stirrer may be the same as a direction of rotation of the second inlet stirrer. [0032] In some embodiments a direction of rotation of the main stirrer is the same as the direction of rotation of the first inlet stirrer.

[0033] In some embodiments an angular speed of the first inlet stirrer is equal to an angular speed of the second inlet stirrer. For example, the apparatus may include a plurality of inlet stirrers, for example more than two inlet stirrers, wherein the angular speed of all of the inlet stirrers is equal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic view of an example glass manufacturing apparatus;

[0035] FIG. 2 is a schematic view of a prior art mixing vessel showing regions of the mixing vessel wherein the molten glass therein is not well-mixed;

[0036] FIG. 3 is a plan view of the mixing vessel of FIG. 2 showing glass flow within the mixing vessel;

[0037] FIG. 4 is a plan view of a mixing vessel according to embodiments described herein comprising one inlet stirrer to augment the function of a main stirrer;

[0038] FIG. 5 is a plan view of a mixing vessel according to embodiments described herein comprising two inlet stirrers to augment the function of a main stirrer

[0039] FIG. 6 is a elevational cross sectional view of the mixing vessel of FIG. 4;

[0040] FIG. 7 is a plan view of the mixing vessel of FIG. 5 showing modeled molten glass flow according to one possible direction of rotation for the inlet stirrers and the main stirrer;

[0041] FIG. 8 is a plan view of the mixing vessel of FIG. 5 showing modeled molten glass flow according to another possible direction of rotation for the inlet stirrers and the main stirrer;

[0042] FIG. 9 is a plan view of the mixing vessel of FIG. 5 showing modeled molten glass flow according to yet another possible direction of rotation for the inlet stirrers and the main stirrer;

[0043] FIG. 10 is a plan view of the mixing vessel of FIG. 5 showing modeled molten glass flow according to still another possible direction of rotation for the inlet stirrers and the main stirrer;

[0044] FIG. 11 is an elevational schematic view of two side-by-side inlet stirrers according to embodiments disclosed herein, wherein the inlet stirrers are arranged in an opposing

configuration and separated by a distance such that the blades stirring blades do not contact during rotation of the inlet stirrers; [0045] FIG. 12 is an elevational schematic view of two side-by-side inlet stirrers according to embodiments disclosed herein, wherein the inlet stirrers are arranged in an interlaced

configuration;

[0046] FIG. 13 is an elevational schematic view of two side-by-side inlet stirrers according to embodiments disclosed herein, wherein the inlet stirrers are arranged in an opposing

configuration, wherein the inlet stirrers are spaced apart an insufficient distance to prevent contact of the stirring blades, but wherein rotation of the respective inlet stirrers is timed such that contact of the stirring blades of the inlet stirrers does not occur;

[0047] FIG. 14 is a plan view of an embodiment according to the present disclosure comprising three inlet stirrers, two paired inlet stirrers between the main stirrer and the delivery conduit, and an the third stirrer positioned on an opposite side of the main stirrer from the paired stirrers; and

[0048] FIG. 15 is a plan view of the mixing vessel of FIG. 14 comprising additional inlet stirrers.

DETAILED DESCRIPTION

[0049] Apparatus and methods will now be described more fully hereinafter with reference to the accompanying drawings in which example embodiments of the disclosure are shown.

Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or like parts. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0050] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0051] Directional terms as used herein - for example up, down, right, left, front, back, top, bottom - are made only with reference to the figures as drawn and are not intended to imply absolute orientation.

[0052] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, it is in no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification.

[0053] As used herein, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a" component includes aspects having two or more such components, unless the context clearly indicates otherwise.

[0054] Aspects of the disclosure include apparatus for processing batch into a molten glass, and more particularly to apparatus for processing the molten glass. Furnaces of the disclosure may be provided for a wide range of applications to heat gases, liquids and/or solids. In one example, apparatus of the present disclosure are described with reference to a glass melting system configured to melt batch into molten glass and convey the molten glass to downstream processing equipment.

[0055] Methods of the disclosure may process the molten glass in a wide variety of ways. For instance, the molten glass may be processed by heating the molten glass to a temperature greater than an initial temperature. In further examples, the molten glass may be processed by maintaining a temperature of the molten glass or by reducing the rate of heat loss that might otherwise occur by inputting heat energy into the molten glass and thereby controlling the cooling rate of the molten glass.

[0056] Methods of the disclosure may process the molten glass with a fining vessel or with a mixing vessel, for example a stirring vessel. Optionally, the apparatus may include one or more further components such as thermal management devices, electronic devices, electromechanical devices, support structures or other components to facilitate operation of the glass manufacturing apparatus including conveying vessels (conduits) that transport the molten glass from one location to another location.

[0057] Shown in FIG. 1 is an example glass manufacturing apparatus 10. In some examples, the glass manufacturing apparatus 10 can comprise a glass melting furnace 12 that can include a melting vessel 14. In addition to melting vessel 14, glass melting furnace 12 can optionally include one or more further components such as heating elements (e.g., combustion burners or electrodes) configured to heat batch and convert the batch into molten glass. In further examples, glass melting furnace 12 may include thermal management devices (e.g., insulation components) configured to reduce heat lost from a vicinity of the melting vessel. In still further examples, glass melting furnace 12 may include electronic devices and/or electromechanical devices configured to facilitate melting of the batch material into a glass melt. Still further, glass melting furnace 12 may include support structures (e.g., support chassis, support member, etc.) or other components.

[0058] Glass melting vessel 14 is typically comprised of refractory material, such as a refractory ceramic material. In some examples, glass melting vessel 14 may be constructed from refractory ceramic bricks, for example refractory ceramic bricks comprising alumina or zirconia.

[0059] In some examples, the glass melting furnace may be incorporated as a component of a glass manufacturing apparatus configured to fabricate a glass ribbon. In some examples, the glass melting furnace of the disclosure may be incorporated as a component of a glass manufacturing apparatus comprising a slot draw apparatus, a float bath apparatus, a down-draw apparatus (e.g., a fusion apparatus), an up-draw apparatus, a press-rolling apparatus, a tube drawing apparatus or other glass ribbon manufacturing apparatus. By way of example, FIG. 1 schematically illustrates glass melting furnace 12 as a component of a fusion down-draw glass manufacturing apparatus 10 for fusion drawing a glass ribbon for subsequent processing into glass sheets.

[0060] The glass manufacturing apparatus (e.g., the fusion down-draw apparatus 10) can optionally include an upstream glass manufacturing apparatus 16 positioned upstream relative to glass melting vessel 14. In some examples, a portion of, or the entire upstream glass manufacturing apparatus 16, may be incorporated as part of the glass melting furnace 12.

[0061] As shown in the illustrated example, the upstream glass manufacturing apparatus 16 can include a storage bin 18, a batch delivery device 20 and a motor 22 connected to the batch delivery device. Storage bin 18 may be configured to store a quantity of batch 24 that can be fed into melting vessel 14 of glass melting furnace 12, as indicated by arrow 26. In some examples, batch delivery device 20 can be powered by motor 22 such that batch delivery device 20 can deliver a predetermined amount of batch 24 from the storage bin 18 to melting vessel 14. In further examples, motor 22 can power batch delivery device 20 such that batch delivery device 20 can introduce batch 24 at a controlled rate based on a sensed level of molten glass downstream from melting vessel 14. Batch 24 within melting vessel 14 can thereafter be heated to form molten glass 28.

[0062] Glass manufacturing apparatus 10 can also optionally include a downstream glass manufacturing apparatus 30 positioned downstream relative to glass melting furnace 12. In some examples, a portion of downstream glass manufacturing apparatus 30 may be incorporated as part of glass melting furnace 12. For instance, first connecting conduit 32 discussed below, or other portions of downstream glass manufacturing apparatus 30, may be incorporated as part of glass melting furnace 12. Elements of the downstream glass manufacturing apparatus, including first connecting conduit 32, may be formed from a precious metal. Suitable precious metals include platinum group metals selected from the group of metals consisting of platinum, iridium, rhodium, osmium, ruthenium and palladium, or alloys thereof. For example, downstream components of the glass manufacturing apparatus may be formed from a platinum-rhodium alloy including 70 to 90% by weight platinum and 10 to 30% by weight rhodium.

[0063] The downstream glass manufacturing apparatus 30 can include a first conditioning (i.e., processing) vessel such as fining vessel 34, located downstream from melting vessel 14 and coupled to melting vessel 14 by way of the above-referenced first connecting conduit 32. In some examples, molten glass 28 may be gravity fed from melting vessel 14 to fining vessel 34 by way of first connecting conduit 32. For instance, gravity may drive molten glass 28 through an interior pathway of first connecting conduit 32 from melting vessel 14 to fining vessel 34.

[0064] Within fining vessel 34, bubbles may be removed from molten glass 28 by various techniques. For example, batch 24 may include multivalent compounds (i.e., fining agents) such as tin oxide that, when heated, undergo a chemical reduction reaction and release oxygen. Other suitable fining agents include without limitation oxides of arsenic, antimony, iron and cerium. Fining vessel 34 is heated to a temperature greater than the melting vessel temperature, thereby heating the fining agent. Oxygen bubbles produced by the temperature-induced chemical reduction of the fining agent(s) rise through the molten glass within the fining vessel, wherein gases in the melt produced in the melting furnace can coalesce into the oxygen bubbles produced by the fining agent. The enlarged gas bubbles can then rise to a free surface of the molten glass in the fining vessel and thereafter be vented out.

[0065] The downstream glass manufacturing apparatus 30 can further include a second conditioning vessel, such as a mixing vessel 36 for mixing the molten glass, that may be located downstream from fining vessel 34. Glass melt mixing vessel 36 can be used to provide a homogenous glass melt composition, thereby reducing or eliminating inhomogeneity that may otherwise exist within the fined molten glass exiting the fining vessel. As shown, fining vessel 34 may be coupled to molten glass mixing vessel 36 by way of a second connecting conduit 38. In some examples, molten glass 28 may be gravity fed from fining vessel 34 to mixing vessel 36 by way of second connecting conduit 38. For instance, gravity may drive molten glass 28 through an interior pathway of second connecting conduit 38 from fining vessel 34 to mixing vessel 36. It should be noted that while mixing vessel 36 is shown downstream of fining vessel 34, mixing vessel 36 may be positioned upstream from fining vessel 34. In some embodiments, downstream glass manufacturing apparatus 30 may include multiple mixing vessels, for example a mixing vessel upstream from fining vessel 34 and a mixing vessel downstream from fining vessel 34. These multiple mixing vessels may be of the same design, or they may be of a different design from one another.

[0066] Downstream glass manufacturing apparatus 30 can further include another conditioning vessel such as delivery vessel 40 that may be located downstream from mixing vessel 36.

Delivery vessel 40 may condition molten glass 28 to be fed into a downstream forming device. For instance, delivery vessel 40 can act as an accumulator and/or flow controller to adjust and provide a consistent flow of molten glass 28 to forming body 42 by way of exit conduit 44. As shown, mixing vessel 36 may be coupled to delivery vessel 40 by way of third connecting conduit 46. In some examples, molten glass 28 may be gravity fed from mixing vessel 36 to delivery vessel 40 by way of third connecting conduit 46. For instance, gravity may drive molten glass 28 through an interior pathway of third connecting conduit 46 from mixing vessel 36 to delivery vessel 40.

[0067] Downstream glass manufacturing apparatus 30 can further include forming apparatus 48 comprising the above-referenced forming body 42 including inlet conduit 50. Exit conduit 44 can be positioned to deliver molten glass 28 from delivery vessel 40 to inlet conduit 50 of forming apparatus 48. In a fusion forming process, forming body 42 can comprise a trough 52 positioned in an upper surface of the forming body and converging forming surfaces 54 that converge along a bottom edge (root) 56 of the forming body. Molten glass delivered to the forming body trough via delivery vessel 40, exit conduit 44 and inlet conduit 50 overflows the walls of the trough and descends along the converging forming surfaces 54 as separate flows of molten glass. The separate flows of molten glass join below and along root 56 to produce a single ribbon of glass 58 that is drawn from root 56 by applying tension to the glass ribbon, such as by gravity and pulling rolls (not shown), to control the dimensions of the glass ribbon as the glass cools and viscosity increases such that glass ribbon 58 goes through a visco-elastic transition and has mechanical properties that give the glass ribbon 58 stable dimensional characteristics. The glass ribbon may subsequently be separated into individual glass sheets by a glass separation apparatus (not shown)

[0068] In certain glass manufacturing systems, molten glass is routed to the mixing vessel via a conduit that opens into the mixing vessel near the top of the mixing vessel, wherein gravity pulls the molten glass downward through the mixing vessel. Typically, for a stirring-type mixing vessel the stirrer comprises stirring blades positioned below the entrance to the stirring vessel. Molten glass near the exit of the supply conduit is close to a uni- directional Poiseuille flow with a parabolic flow profile. Cord is aligned with the streamlines (flow vectors). The molten glass enters the stirring vessel above the stirrer blades and follows the rotational motion around one or more stagnant regions created by the stirring blades below. As a result, incomplete mixing may occur, even as the molten glass descends and directly interacts with the top stirring blade.

Consequently, not all cord entering the mixing vessel may be thoroughly mixed upon exiting the mixing vessel.

[0069] FIG. 2 shows modeled flow data for an example mixing vessel 36 using a conventional stirrer. Mixing vessel 36 comprises a vessel wall 100 and a stirrer 102 rotatably mounted within the mixing vessel and configured to rotate about an axis of rotation 104. Stirrer 102 includes a central shaft 106 and a plurality of stirring blades 108 extending outward from shaft 106, stirring blades 108 being arranged along a length of shaft 106. The flow diagram of FIG. 2 depicts regions 110 of generally stagnant molten glass located above the uppermost blade 108 of stirrer 102, wherein insufficient mixing of the molten glass occurs within the stagnant regions even with stirrer 102 rotating. In some instances, unmixed molten glass may escape from one or more of these stagnant regions and travel through the remainder of the mixing vessel without sufficient mixing. For example, the unmixed molten glass may travel downward along shaft 106, or the unmixed molten glass may travel downward along the inside surface of vessel wall 100. The unmixed molten glass may include inhomogeneous molten glass that is entrained within the flow of molten glass being conveyed to the forming body. [0070] FIG. 3 is a partial plan view of a conventional mixing vessel 36 comprising only a main stirrer 102 as described above in respect of FIG. 2 and showing a modeled flow of molten glass therethrough, as represented by arrows 120. The stirring blades of the main stirrer 102 are not shown so as not to obstruct visibility of flow direction arrows 120. Main stirrer 102 is rotating in a clockwise direction, as indicated by arrow 122, and flow direction arrows 120 depict the gross flow pattern for a flow of molten glass 28 into the mixing vessel 36 from conduit 38 (hereafter delivery conduit 38), and the subsequent flow of molten glass 28 within the mixing vessel. As indicated, in a conventional mixing vessel comprising only a single central main stirrer, the molten glass entering the mixing vessel is urged in a single rotational direction (clockwise or counterclockwise rotation) by the motion of main stirrer 102. Accordingly, little mixing may occur within a volume of the mixing vessel above the uppermost stirring blade of the main stirrer. Mixing in this instance must rely entirely on the work of the main stirrer 102.

[0071] Referring now to FIG. 4, a plan view of another exemplary mixing vessel 36 is shown comprising a vessel wall 200, a main stirrer 202 and at least one inlet stirrer 204. More specifically, FIG. 4 illustrates a mixing vessel including only a single inlet stirrer 204. FIG. 5 illustrates another exemplary mixing vessel 36 including two inlet stirrers 204. FIG. 6 depicts a side cross sectional view of the mixing vessel of FIG. 4 showing a vertical arrangement of main stirrer 202 and the at least one inlet stirrer 204 relative to free surface 206 of molten glass 28 within the mixing vessel. While mixing vessels 36 of FIGS. 4 and 5 are shown with one and two inlet stirrers 204, respectively, mixing vessel 36 may include more than two inlet stirrers 204.

[0072] Main stirrer 202 comprises a main stirrer shaft 208 and one or more main stirring blades 210 extending therefrom. The one or more main stirring blades 210 are configured to be submerged below free surface 206 of molten glass 28 within mixing vessel 36 during operation of the mixing vessel. Main stirrer 202 may include, for example, a plurality of main stirring blades 210 arranged in a vertically extending array and also extending outward from main stirrer shaft 208. In example embodiments, mixing vessel 36 is a cylindrical vessel wherein the vessel walls define a cylindrical internal volume. Accordingly, in the embodiments of FIG. 4 and 5, main stirrer shaft 208 may be centrally positioned, and is rotatably mounted within stirring vessel 36. That is, main stirrer shaft 208 may be concentric with vessel wall 200, for example an inside surface of mixing vessel wall 200, such that a central longitudinal axis 212 of mixing vessel 36 is co-located with (coincident with) a central longitudinal axis 214 of main stirrer shaft 208, which is an axis of rotation of main stirrer shaft 208. In the instance wherein mixing vessel 36 is a cylindrical mixing vessel, stirring blades 210 are configured to sweep close to the walls of the mixing vessel to prevent an incoming flow of molten glass from bypassing the stirring blades as the molten glass moves between delivery conduit 38 and outlet conduit 46 (see FIG. 6). For example, in some embodiments, the outermost extent of a stirring blade 210 may be within 3 cm of the inside surface of the vessel wall, for example in a range from about 1.6 cm to about 3 cm or in a range from about 1.6 cm to about 25 cm, or in a range from about 1.6 cm to about 2 cm, and all ranges and subranges therebetween. The one or more stirring blades 210 extend outward from the axis of rotation 214 a distance dm. Main stirrer shaft 208 may also be coupled to a drive assembly (not shown), including, for example, an electric motor that may be coupled to the main stirrer shaft by a belt, chain or other drive coupling means.

[0073] As noted above, mixing vessel 36 may also include one or more inlet stirrers 204. Each inlet stirrer 204 includes an inlet stirrer shaft 216 and may further include a plurality of stirring blades 218 extending therefrom. Each inlet stirrer shaft 216 is rotatable about an axis of rotation 222 offset from main stirrer shaft axis of rotation 214 by a distance da. The distance da may be the same for each inlet stirrer 204, or the distance da may vary from one inlet stirrer to another inlet stirrer. In some embodiments the distance da may be the same for some inlet stirrers 204, and different for other inlet stirrers. Stirring blades 218 may be arranged, for example in a vertically extending array such that stirring blades 218 are arranged along a length of the inlet stirrer shaft 216. In other embodiments, each inlet stirrer 204 may include only a single stirring blade, for example a helically wound stirring blade (e.g., screw). In the embodiment of FIGS. 4 - 6, axis of rotation 222 of inlet stirrer shaft 216 is spaced apart from axis of rotation 214 of main stirrer shaft 208 by a distance da that is less than the distance dm. In addition, the entirety of inlet stirrer shaft 216 and stirring blades 218 (e.g. inlet stirrer 204) may lie within a projection of the sweep of blades 210 of main stirrer 202. That is, as main stirrer 202 rotates, the one or more blades 210 sweep a circular arc with a radius dm. A projection of that circular arc is a cylindrical volume, and inlet stirrer 204 (inlet stirrer shaft 216 and stirring blades 218) may, in some embodiments, lie entirely within that projected cylindrical volume swept by the stirring blades of main stirrer 202. It should be apparent that the cylindrical volume swept by stirring blades 210 is less than the total volume of mixing vessel 36, and inlet stirrer 204 (inlet stirrer shaft 216 and stirring blades 218) lies entirely within the overall cylindrical volume of mixing vessel 36. [0074] As is apparent from FIG. 6, each inlet stirrer 204 is positioned such that the bottom-most extent of inlet stirrer 204 is positioned above the uppermost stirring blade 210 of main stirrer 202. That is, each inlet stirrer 204 occupies a volume within vessel 36 that is between the uppermost stirring blade 210 of the main stirrer 202 and mixing vessel cover 225. Because the axis of rotation 222 of each inlet stirrer 204 extends parallel with axis of rotation 214 of main stirrer shaft 208 and is offset from axis of rotation 214 a distance da less than dm, if inlet stirrer 204 extended below the uppermost stirring blade 210 of main stirrer 202, inlet stirrer 204 would interfere with the rotation of main stirrer 202. This can be viewed in another manner by imagining a horizontal plane 224 extending through mixing vessel 36 (see FIG. 6). Horizontal plane 224 may, for example, be perpendicular to central longitudinal axis 212.

[0075] Consider wherein an uppermost stirring blade of main stirrer 202 lies below horizontal plane 224. Further, consider wherein none of the one or more inlet stirrers 204 extends below the horizontal plane 224. Thus, horizontal plane 224 separates the volume of the mixing vessel into an upstream volume 226 and a downstream volume 228, and the one or more inlet stirrers 204 are positioned within the upstream volume 226 of the mixing vessel, and do not extend into the downstream volume 228 of the mixing vessel.

[0076] Consider also a first vertical plane 230 (see FIG. 4) extending through mixing vessel 36, wherein axis of rotation 214 lies entirely within first vertical plane 230. Consider further that first vertical plane 230 can be perpendicular to a central longitudinal axis 232 of delivery conduit 38. In some embodiments, such as the embodiments of FIG. 4 and 5, the at least one inlet stirrer 204 is positioned such that the at least one inlet stirrer 204 is located between main stirrer shaft 208 and delivery conduit 38. Put another way, the at least one inlet stirrer 204 lies on the same side of first vertical plane 230 as does delivery conduit 38.

[0077] Next, consider a second inlet stirrer 204 positioned within the mixing vessel and rotatable about an axis of rotation offset from the axis of rotation of main stirrer 202 the distance da and positioned on the same side of first vertical plane 230 as delivery conduit 38 (see FIG. 5), and wherein for a second vertical plane 233 perpendicular to the first vertical plane 230, with central longitudinal axis 232 of delivery conduit 38 lying entirely within second vertical plane 231, the axis of rotation of the first inlet stirrer and the axis of rotation of the second inlet stirrer are on opposite sides of second vertical plane 233. For example, in some embodiments the first inlet stirrer 204 and the second inlet stirrer 204 may be equidistant from second vertical plane 233 (see FIG. 5).

[0078] FIG. 7 represents modeled molten glass flow within an example mixing vessel 36 embodied by FIG. 5, including two inlet stirrers 204. FIG. 7 illustrates that the flow of molten glass entering mixing vessel 36 is disrupted by rotation of the inlet stirrers. That is, the flow of molten glass above the stirring blades of main stirrer 202 becomes more chaotic, thereby enhancing mixing. Stirring blades of both main stirrer 202 and the inlet stirrers 204 have been omitted for greater clarity. Only the shafts are shown. In the embodiment of FIG. 7, main stirrer 202 rotates in a clockwise direction as indicated by arrow 234. Additionally, each inlet stirrer is also rotating in a clockwise direction, although it should be apparent that each of main stirrer 202 and each inlet stirrer 204 could be rotating in a counterclockwise direction. As shown, molten glass entering upstream volume 226 undergoes flow disruption owing to the inlet stirrers 204.

[0079] In the embodiment of FIG. 8, the inlet stirrers 204 are counter-rotating, meaning one inlet stirrer 204 is rotating in a clockwise direction while a second inlet stirrer 204 is rotating in a counterclockwise direction. Main stirrer 202 is rotating in the clockwise direction as indicated by arrow 234, although it should be apparent that main stirrer 202 could be rotating in a counterclockwise direction. Again, flow disruption is shown to occur within the mixing vessel, and in particular within upstream volume 226.

[0080] In still another embodiment, illustrated in FIG. 9, both inlet stirrers 204 are counter- rotating as in the embodiment of FIG. 8, however in the present embodiment the rotation direction has switched so that instead of a counterclockwise rotation of the "top" inlet stirrer in FIG. 8, in FIG. 9 the top inlet stirrer is rotating clockwise and the bottom inlet stirrer is rotating in the counterclockwise direction. Main stirrer 202 is rotating in a clockwise direction

[0081] Finally, in FIG. 10, main stirrer 202 is rotating in a clockwise direction as represented by arrow 234, and both the inlet stirrers 204 are rotating in an opposite, counterclockwise direction. As in the case of FIGS. 7, 8 and 9, flow disruption occurs in the region of the inlet stirrers 204 (upstream volume 226) that reduces the work required to be performed by main stirrer 202 to mix molten glass in the downstream volume 228.

[0082] FIGS. 11, 12 and 13 illustrate various arrangements of paired inlet stirrers 204. For example, in the embodiment of FIG. 11 , two inlet stirrers 204 are shown and arranged such that the distance ds separating the respective axes of rotation 222 of the inlet stirrers is greater than twice the length dn of the individual stirring blades 218 (wherein the stirring blade length dn is measured from the axis of rotation 222 of the respective shaft to the stirring blade 218 tip). Thus, while the individual stirring blades are positioned in opposition, sufficient clearance is provided that the opposing blades do not contact each other. For example, opposing stirring blades may be separated by a distance in a range from about 1.2 cm to about 2.5 cm. The inlet stirrers may be identical, or the inlet stirrers may have different designs.

[0083] In the embodiment of FIG. 12, the inlet stirrers 204 are arranged in a vertically offset configuration, wherein the individual stirring blades 218 of the inlet stirrers are vertically interlaced. Accordingly, the distance ds between the respective inlet stirrer axis of rotation 222 is less than two times the length dn of an individual stirring blade, but greater than the length dn of a single stirring blade, i.e., dn < ds < 2-dn.

[0084] In the embodiment of FIG. 12, the inlet stirrers are arranged in opposition similar to those depicted in FIG. 1 1 (vertically aligned), but wherein the distance between the inlet stirrer axes of rotation 222 are such that the distance ds between the inlet stirrer axis of rotation 222 is less than two times the length dn of an individual stirring blade 218, but greater than the length of a single stirring blade as shown for FIG. 12, i.e. , dn < ds < 2-dn. For purposes of the present disclosure, the stirring blades of the two inlet stirrers are referred to as "intertwined". In the present embodiment, rotation of the intertwined inlet stirrers is timed such that no contact occurs between the stirring blades of one inlet stirrer and the stirring blades of the other, adjacent inlet stirrer. For example, the relative rotational phase between the two stirrers can be 45 degrees, for example in a range from about 40 degrees to about 50 degrees.

[0085] In still another embodiment, more than two inlet stirrers 204 may be included within mixing vessel 36. For example, FIG. 14 depicts an embodiment comprising three inlet stirrers 204. The additional inlet stirrers 204 may be substantially the same or identical to previously described inlet stirrers 204, such as those described above, differing only in placement. For example, the additional inlet stirrers 204 may be positioned on the opposite side of plane 230 than is delivery conduit 38 (see FIG. 4). In the embodiment of FIG. 14, a third inlet stirrer 204 is positioned on a side of first vertical plane 230 opposite from the first and second inlet stirrers 204 positioned between first vertical plane 230 and delivery conduit 38. In addition, this third inlet stirrer 204 may be positioned such that an axis of rotation 222 of this third inlet stirrer is intersected by central longitudinal axis 232 of delivery conduit 38, but need not be so located in other embodiments. FIG. 15 illustrates the use of several additional inlet stirrers arranged about main stirrer 202. In the embodiment of FIG. 15, an axis of rotation 222 of each inlet stirrer 204 of the plurality of inlet stirrers is offset from the axis of rotation 214 of main stirrer 202 a uniform distance da that is less than dm. However, such uniform offset is not required, and so in other embodiments da may vary from inlet stirrer to inlet stirrer.

EXAMPLE

[0086] The Table below provides modeled data indicating certain performance characteristics of an exemplary mixing vessel comprising inlet stirrers rotating in various rotational directions (Rot. Dir.) relative to each other (either co-rotation, Co, or opposite rotation, X) the main stirrer under conditions of varying rotational speed in revolutions per minute (RPM) and relative rotational phase between the two inlet stirrers. In each example the main stirrer was rotating in a clockwise direction at a rotational speed of 11 RMP. Stirring effectiveness index (SEI) was derived by optical analysis of a scaled mock mixing vessel constructed from an optically transparent acrylic using similarly constructed stirrers (main and inlet) and wherein a paraffin selected to mimic the viscosity of molten glass was flowed through the mixing vessel to simulate the flow of molten glass. A dye was then injected into the flow of paraffin and the flow imaged with a charge coupled device (CCD) camera and analyzed with suitable imaging software. The output of each pixel was amplified, digitized and scaled to 8-bit grayscale (spanning a range from 0 for black to 255 for white). The resulting data was encoded into a file suitable for characterization as a numerical array with the width and height dimensions equal to the number of pixels in each direction on the CCD. Quantitative information was obtained about specific features in an image as defined, for example, by edges, coloration, intensity, feature size, fractal dimension and position. By assembling a sequence of images of the same area taken at different times, additional time-derivative information can be measured about features, including without limitation velocity, acceleration and growth and/or decay rates. Using these techniques, a numerical metric, stirring effectiveness, can be used to evaluate, relatively, various mixing vessel designs.

[0087] The stirring effectiveness index is the product of the full-width at half maximum

(FWHM) of the residence time distribution (a dispersal term) and the ratio of high frequency band pass image intensity to unfiltered image intensity (a homogeneity term). The SEI is in units of seconds and is, in effect, the time dispersal of the mixing vessel, corrected by the inhomogeneity introduced by the mixing vessel. For example, the dispersal term can be obtained from a series of line scan images of the mock mixing vessel. Because flow rate is maintained constant during the measurement, a measure of time can be used as a surrogate for volume.

Using suitable imaging software, an average intensity profile of a line scan image can be derived. For example, the average intensity of each line of pixels can be calculated and then plotted as a function of its step in time. The resulting profiles depict a distribution of absorption as a function of time that is approximately a log-normal distribution. The dispersal term is then the FWHM of this distribution.

[0088] The homogeneity term is a measure of the contrast in the image. The object is to subtract out the dispersal of the dye and account for the dye that was dispersed. The FWHM region imaged in respect of the dispersal term is then analyzed. The leading edge of the region is expanded to 20% of full height and the integrated intensity of the region is measured. A low pass filter is then applied. The homogeneity term is then equal to the fraction of the total intensity represented by the integral of the intensity of the low pass region.

[0089] The table data below show an advantaged stirring effectiveness for in phase co-rotating inlet stirrers that also co-rotate with the main stirrer, although other configurations shown are possible with a concurrent decrease in stirring effectiveness. "Phase" refers to the relative rotational speed of the inlet stirrers, and "in phase" means the inlet stirrers were both rotated at the same rotational speed in a manner wherein the stirring blades were arranged as in FIG. 11 (by comparison, a 45 degree phase difference would look similar to the stirring blades of FIG. 13).

Table

7 CW 45 X 21137

2.75 cw 0 Co 20972

7 ccw 45 X 19460

No inlet stirrers 19404

2.75 cw 45 X 19286

7 CCW 0 X 19049

2.75 CCW 0 X 18644

2.75 CCW 45 X 18378

2.75 CW 0 X 16849

7 CW 0 X 13553

[0090] It will be apparent to those skilled in the art that various modifications and variations can be made to embodiments of the present disclosure without departing from the spirit and scope of the invention. Thus, it is intended that the present disclosure cover the modifications and variations of such embodiments provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. An apparatus for conditioning molten glass comprising:

a vessel;

a main stirrer positioned within the vessel and rotatable about a first axis of rotation, the main stirrer comprising a main stirrer shaft and an uppermost main stirring blade extending outward therefrom a distance dm relative to the first axis of rotation; and

a first inlet stirrer positioned within the vessel and rotatable about a second axis of rotation offset from the first axis of rotation a distance da less than dm.

2. The apparatus according to claim 1, wherein, for a horizontal plane extending through the vessel, the uppermost main stirring blade positioned below the horizontal plane, the first inlet stirrer does not extend below the horizontal plane.

3. The apparatus according to claim 2, further comprising a delivery conduit opening into the vessel and configured to deliver the molten glass to the vessel, the delivery conduit comprising a central longitudinal axis, and wherein for a first vertical plane extending through the vessel perpendicular to the central longitudinal axis of the delivery conduit, the first axis of rotation lying entirely within the first vertical plane, the first inlet stirrer is positioned on a same side of the first vertical plane as the delivery conduit.

4. The apparatus according to claim 3, further comprising a second inlet stirrer positioned within the vessel and rotatable about a third axis of rotation offset from the first axis of rotation the distance da and positioned on the same side of the first vertical plane as the delivery conduit, and wherein for a second vertical plane perpendicular to the first vertical plane, the central longitudinal axis of the delivery conduit lying entirely within the second vertical plane, the second axis of rotation and the third axis of rotation are on opposite sides of the second vertical plane.

5. The apparatus according to claim 3, further comprising a second inlet stirrer positioned on an opposite side of the first vertical plane from the delivery conduit.

6. The apparatus according to claim 1, further comprising a plurality of inlet stirrers.

7. The apparatus according to claim 1, wherein the vessel is cylindrical and the first axis of rotation is coincident with a central longitudinal axis of the vessel.

8. An apparatus for conditioning molten glass comprising:

a vessel;

a main stirrer positioned within the vessel and rotatable about a first axis of rotation, the main stirrer comprising a main stirrer shaft and an uppermost main stirring blade extending outward therefrom a distance dm relative to the first axis of rotation;

a first inlet stirrer positioned within the vessel and rotatable about a second axis of rotation offset from the first axis of rotation; and

wherein, for a horizontal plane extending through the vessel, the uppermost main stirring blade positioned below the horizontal plane, the first inlet stirrer does not extend below the horizontal plane.

9. The apparatus according to claim 8, wherein the first inlet stirrer is positioned within an upstream volume of the vessel and the uppermost stirring blade is positioned within a downstream volume of the vessel.

10. The apparatus according to claim 9, wherein the upstream volume and the downstream volumes are cylindrical volumes and a central longitudinal axis of the upstream volume is coincident with a central longitudinal axis of the downstream volume.

11. The apparatus according to claim 8, further comprising a plurality of inlet stirrers.

12. The apparatus according to claim 11, wherein each inlet stirrer of the plurality of inlet stirrers is positioned within the vessel and rotatable about an axis of rotation offset from the first axis of rotation a distance da less than dm.

13. The apparatus according to claim 8, further comprising a delivery conduit opening into the vessel and configured to deliver the molten glass to the vessel, the delivery conduit comprising a central longitudinal axis, and wherein for a first vertical plane extending through the vessel perpendicular to the central longitudinal axis of the delivery conduit, the first axis of rotation lying entirely within the first vertical plane, the first inlet stirrer is positioned on a same side of the first vertical plane as the delivery conduit.

14. The apparatus according to claim 13, further comprising a second inlet stirrer positioned within the vessel and rotatable about a third axis of rotation offset from the first axis of rotation and positioned on the same side of the first vertical plane as the delivery conduit, and wherein for a second vertical plane perpendicular to the first vertical plane, the central longitudinal axis of the delivery conduit lying entirely within the second vertical plane, the third axis of rotation is on an opposite side of the second vertical plane than the second axis of rotation.

15. The apparatus according to claim 14, wherein the second axis of rotation and the third axis of rotation are equidistant from the second vertical plane.

16. A method for conditioning molten glass comprising:

flowing molten glass into a vessel from a delivery conduit, the vessel including an upstream volume and a downstream volume relative to the flow of molten glass;

stirring the molten glass in the upstream volume with a first inlet stirrer rotatable about an axis of rotation; and

stirring the molten glass in the downstream volume with a main stirrer rotatable about an axis of rotation parallel with the axis of rotation of the inlet stirrer, the main stirrer comprising an uppermost stirring blade, wherein for a horizontal plane extending through the vessel, the uppermost stirring blade positioned below the horizontal plane, the first inlet stirrer is positioned entirely above the horizontal plane.

17. The method according to claim 16, wherein the uppermost stirring blade comprises a length dm that is a maximum extension of the uppermost stirring blade from the axis of rotation of the main stirrer, and wherein an offset between the axis of rotation of the first inlet stirrer and the axis of rotation of the main stirrer is less than dm.

18. The method according to claim 17, further comprising stirring the molten glass with a second inlet stirrer comprising an axis of rotation, the second inlet stirrer positioned entirely above the horizontal plane, and wherein an offset between the axis of rotation of the second inlet stirrer and the axis of rotation of the main stirrer is less than dm.

19. The method according to claim 18, wherein for a vertical plane perpendicular to a central longitudinal axis of the delivery conduit, a central longitudinal axis of the vessel lying entirely within vertical plane, the first inlet stirrer and the second inlet stirrers are positioned between the vertical plane and the delivery conduit.

20. The method according to claim 19, wherein a direction of rotation of the first inlet stirrer is the same as a direction of rotation of the second inlet stirrer.

21. The method according to claim 20, wherein a direction of rotation of the main stirrer is the same as the direction of rotation of the first inlet stirrer.

22. The method according to claim 19, wherein an angular speed of the first inlet stirrer is equal to an angular speed of the second inlet stirrer.