Patent Description:
Use of glass tubing to produce glass articles, such as vials, cartridges, and syringes, requires a high level of dimensional stability in the glass tubing wall. For example, vials, cartridges and syringes have tight dimensional requirements that require minimum concentricity and wall thickness variation. Industry standards require that wall thickness variations be less than <NUM>% of the product's overall wall thickness. However, dimensional variations in the glass tubing from which the glass articles are formed may result in glass articles with wall thicknesses that are outside of acceptable tolerances. Such dimensional variations may be a result of, for example, processes instabilities or variations in the glass tubing manufacturing process. Apparatuses for manufacturing glass tubes are for example known from <CIT> and <CIT>.

<CIT> relates to glass tube manufacturing method and apparatus and discloses that in operation, the conditions of temperature, relative humidity and air flow will vary depending on the actual tube drawing operation, glass temperature and ambient temperature and durability. The atmosphere within enclosure is exhausted at a somewhat lower rate to maintain a positive pressure within the enclosure and allow for normal leakage. <CIT> further discloses that glass tube is enclosed in an environmentally controlled area from near orifice to a predetermined point on the conveyor. At that point, the glass will have cooled sufficiently to be unaffected by the environment and to no longer be susceptible to frictive damage.

There are many factors that can affect tube outer diameter. Such factors can occur in the tube forming stage and result in significant tube outer diameter and thickness variations.

Accordingly, a need exists for alternative glass tubing forming apparatuses that reduce dimensional variations in the glass tubing formed therefrom.

The embodiments described herein relate to glass tube forming apparatuses with enhanced thermal dimensional stability that provide reduced tube taper during the production of glass tubing. The apparatuses utilize lower extended muffle structures that manage convective air flow as the glass tubing flows from a vertical to a non-vertical or horizontal orientation during the glass forming process. Convection and ambient air flow can be more controlled in these different air flow regimes as the glass tubing is formed to the desired dimensions.

The invention relates to a glass tube manufacturing apparatus for manufacturing glass tubing includes a glass delivery tank with molten glass. The glass delivery tank has a bottom opening. A bell has an upper portion with an outer diameter located at the bottom opening. A heating apparatus is at least partially disposed around the bell. The heating apparatus includes a heating portion and a muffle portion located below the heating portion. A lower extended muffle structure extends downwardly from the muffle portion, the lower extended muffle structure extending along and around the glass tubing as the glass tubing transitions from a vertical orientation to a non-vertical orientation to manage convective airflow therethrough.

The invention further relates to a method for manufacturing glass tubing is provided. The method includes melting a glass composition in a glass delivery tank and producing molten glass. The glass delivery tank having a bottom opening with an inner diameter. The molten glass is drawn around a bell thereby forming a glass tube. The bell has an upper portion with an outer diameter. The glass tube forms in a space between the inner diameter of the bottom opening and the outer diameter of the bell. A heating apparatus is at least partially disposed around the bell. The heating apparatus includes a heating portion and a muffle portion located below the heating portion. The glass tube is directed through a lower extended muffle structure that extends downwardly from the muffle portion. The lower extended muffle structure extends along and around the glass tubing as the glass tubing transitions from a vertical orientation to a non-vertical orientation to manage convective airflow therethrough.

Additional features and advantages of the glass tubing forming apparatuses with enhanced thermal dimensional stability described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

Reference will now be made in detail to various apparatuses and methods for forming glass tubing described herein, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

Embodiments described herein relate to methods and apparatuses for controlling glass tube taper during glass forming processes. The apparatuses utilize a bell that is positioned below a tank of molten glass in order to form molten glass into glass tubing by directing the molten glass over an exterior of the bell and delivering a pressurized gas, such as air, through the bell to an interior of the molten glass to form an inner diameter. The bell is also positioned at least partially within a heating apparatus that delivers heat to the molten glass as the molten glass passes vertically therethrough. The heating apparatus includes a muffle portion that at least partially isolates the glass moving from the bell from the environment during glass tube formation. A lower extended muffle structure may be provided adjacent the muffle portion that at least partially surrounds the glass tube as it moves from the muffle portion of the heating apparatus. The lower extended muffle structure may continue to at least partially isolate the glass tube therein from the environment and manage convective airflow therethrough. The lower extended muffle structure extends along and around the glass tubing as the glass tubing transitions from a vertical orientation to a non-vertical or horizontal orientation.

Directional terms as used herein - for example up, down, right, left, front, back, top, bottom, vertical, horizontal - are made only with reference to the figures as drawn and are not intended to imply absolute orientation unless otherwise expressly stated.

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, nor that with any apparatus specific orientations be required. Accordingly, where a method claim does not actually recite an order to be followed by its steps, or that any apparatus claim does not actually recite an order or orientation to individual components, or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, or that a specific order or orientation to components of an apparatus is not recited, it is in no way intended that an order or orientation 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, operational flow, order of components, or orientation of components; plain meaning derived from grammatical organization or punctuation, and; the number or type of embodiments described in the specification.

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.

Without wishing to be bound by theory, there are many factors that can affect the outer diameter of glass tubing during the formation process. One process for manufacturing glass tubing is the Vello process. The Vello process forms a glass tube by flowing molten glass, using gravity, around a die (also referred to as a "bell head" or "bell") of a known diameter. Some factors that can affect the outer diameter of the glass tubing include inhomogeneity in the tank, air pressure variation in the bell, natural convection and ambient temperature fluctuation. Natural convection, for example, can induce glass flow instability around the tubing during the forming stage. This induced flow instability can result in glass temperature fluctuation, which can cause enhanced tube outer diameter and thickness variations along the length of the tubing. Such air flow instability can be particularly acute when the glass tubing flows in a non-axisymmetric fashion from a vertical orientation to a more horizontal orientation during the glass forming process.

As the air temperature varies around the glass tubing in the forming state inside the muffle portion, heat transfer from the glass tubing to the air varies, which can cause glass temperature fluctuations. Generally, the glass temperature can fluctuate more rapidly downstream of the bell and can peak in the middle of the muffle portion where there is active glass tube formation occurring. The effects of viscosity variation due to temperature fluctuations on glass tubing dimension (outer diameter) can be estimated. For extension tubing flow, the pulling force can be given as <MAT> where F is the pulling force along the glass tubing, µ(T) is the viscosity, dU/dZ is the attenuation rate and A is the tube cross-sectional area. The pulling force F is kept constant during the tube forming process. When the glass viscosity fluctuates, the tube attenuation rate dU/dZ varies if the cross-sectional area A does not change at the location of the disturbance. As a result, the diameter and thickness of the glass tubing varies in the downstream. Differences in tube outer diameter along a length of the glass tube may be referred to as taper.

The industry definition for calculating taper is the maximum of maximum cross sectional outer diameter minus the minimum of the maximum cross sectional outer diameter along a length of the glass tube (e.g., <NUM>), such that out of round tube portions are not considered. In other words, for a round tube, taper is the maximum outside diameter minus the minimum outside diameter over a predetermined length. The lower extended muffle structure provides a barrier between the glass tube and the environment and is sized and configured to manage convective airflow therewithin, which can reduce taper along lengths of the glass tube as the glass tube is being formed.

Referring to <FIG>, an exemplary glass tube manufacturing apparatus <NUM> for forming glass tubing in a continuous fashion is schematically depicted. The glass tube manufacturing system <NUM> generally includes molten glass delivery system <NUM>, a delivery vessel <NUM> for receiving molten glass, and a bell <NUM>. The molten glass delivery system <NUM> generally includes a melting vessel <NUM>, a fining vessel <NUM>, and a mixing vessel <NUM> coupled to the delivery vessel <NUM> of the glass tube manufacturing apparatus <NUM>. The delivery vessel <NUM> may include heating elements (not shown) for heating and/or maintaining the glass in a molten state. The delivery vessel <NUM> may also contain mixing components for further homogenizing the molten glass in the delivery vessel <NUM>. In some embodiments, the delivery vessel <NUM> may cool and condition the molten glass in order to increase the viscosity of the glass prior to providing the glass to the bell <NUM>.

The delivery vessel <NUM> may include an opening <NUM> in the bottom thereof. In various embodiments, the opening <NUM> is circular, but may be oval, elliptical or polygonal, and is sized to permit molten glass <NUM> to flow through the opening <NUM> in the delivery vessel <NUM>. The molten glass <NUM> may flow over the bell <NUM> directly from the opening <NUM> in the delivery vessel <NUM> to form glass tubing <NUM>.

A bell support <NUM> is connected to the bell <NUM> that is part of the glass tube manufacturing apparatus. The bell support <NUM> can have a fluid supply channel <NUM>, such as a pipe, conduit, or similar fluid delivery device, which is fluidly coupled to an internal chamber <NUM> of the bell <NUM>. The fluid supply channel <NUM> may be operable to deliver a supply of pressurized fluid to the internal chamber <NUM>. In some embodiments, the pressurized fluid may be a pressurized gas, specifically air or an inert pressurized gas, including, without limitation, nitrogen, helium, argon, krypton, xenon, and the like. The gas supports the interior of the glass tubing <NUM> after it flows past the bell <NUM> and no longer contacts a side wall <NUM> of the bell <NUM> as schematically depicted in <FIG>. The glass tube manufacturing apparatus <NUM> includes the glass delivery vessel <NUM> for supplying molten glass to the bell <NUM>.

The bell <NUM> can have a top portion <NUM> with a top surface <NUM> and the side wall <NUM>. The side wall <NUM> and a bottom edge <NUM> define the internal chamber <NUM> of the bell <NUM>. The top surface <NUM> of the top portion <NUM> has an outer diameter <NUM> (<FIG>). The bell <NUM> may have a variety of shapes including, without limitation, a substantially conical shape or, alternatively, a substantially parabolic shape. Accordingly, it should be understood that the bell <NUM> may be of any shape and/or configuration suitable for expanding and thinning a tube of heated glass (i.e., molten glass) drawn over the surface of the bell. The material from which the bell <NUM> is formed is stable at elevated temperatures such that the bell does not contaminate heated glass drawn over the bell <NUM>. Examples of suitable bell materials include but are not limited to refractory metals and alloys thereof, platinum-group metals, stainless steels, nickel, nickel-based alloys and ceramics such as, for example, zircon (ZrSiO<NUM>) and alumina (Al<NUM>O<NUM>).

Referring briefly to <FIG>, the glass delivery vessel <NUM> has the bottom opening <NUM> with a bottom portion <NUM> that has an inner diameter <NUM>. The bottom portion <NUM> has a bottom edge. The outer diameter <NUM> of the top portion <NUM> can be less than the inner diameter <NUM> of the bottom opening <NUM>. A clearance CR between the top portion <NUM> and the bottom opening <NUM> governs, at least in part, the wall thickness of the glass tube drawn over the bell <NUM>. In addition, and as the bell <NUM> is bell-shaped or parabolically-shaped, the outer diameter of the bell <NUM> increases along the length of the bell <NUM> in a downward direction. The position of the top portion <NUM> of the bell <NUM> relative to the bottom opening <NUM> may be adjusted to provide uniform flow of the molten glass from the glass delivery vessel <NUM> through the bottom opening <NUM> and over the bell <NUM>.

Referring again to <FIG>, a heating apparatus <NUM> (e.g., a furnace) with heating elements may be disposed around the bell <NUM>. In one embodiment, the heating apparatus <NUM> can include an infrared heating apparatus. However, it should be understood that other types of heating units may be used including, without limitation, focused infrared, resistive, induction and/or combinations thereof. Further, it should be understood that, while <FIG> depicts the heating apparatus as being disposed around the bell <NUM>, the heating apparatus <NUM> may be integrated with the bell <NUM>, such as when the heating apparatus <NUM> is a resistive heating apparatus.

A lower extended muffle structure <NUM> extends downward from the heating apparatus <NUM>. In the illustrated embodiment, the lower extended muffle structure <NUM> is a muffle portion that is part of the heating apparatus <NUM>. The lower extended muffle structure <NUM> may extend about the glass tubing <NUM> on all sides, <NUM> degrees around a periphery of the glass tubing <NUM> and downwardly from a heating portion <NUM> of the heating apparatus <NUM> that contains the heating elements. The lower extended muffle structure <NUM> may extend below the heating portion <NUM> a predetermined distance D (e.g., about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as about <NUM> or more, such as between about <NUM> and about <NUM>, such as between about <NUM> and about <NUM>). In some embodiments, the lower extended muffle structure may be connected to the muffle portion, as will be described in greater detail below.

A model was built to test use of a lower extended muffle structure <NUM> on glass tubing taper. The model was based on the commercially available computation fluid dynamics (CFD) software ANSYS Fluent from ANSYS, Inc. The model is a 2D axisymmetric model that considers the glass tube flow in the presence of natural convection, radiative heat transfer and gravity. The model domain is illustrated by <FIG>. The draw height was assumed to be <NUM>. The geometry and velocity fields of the glass domain were fixed and obtained from a 2D COMSOL model from COMSOL Inc. The model did not consider glass forming and focused on capturing the thermal interactions and the resulting flow patterns inside the lower extended muffle structure.

The glass inlet temperature in the lower extend muffle portion <NUM> was specified to be <NUM> degrees C and the outlet from the lower extended muffle portion <NUM> was assumed to be adiabatic. The inner surface of the glass tubing was also assumed to be adiabatic. The outer surface heat transfer of the glass tubing varies in response to thermal interactions and was calculated by the model. Isothermal (constant temperature) boundary conditions were prescribed on the walls of the lower extended muffle portion <NUM> to reflect actual temperature measurements.

<FIG> illustrate the affect that the length of the lower extended muffle structure <NUM> can have on convective air flow. For cases (A) and (B) having relatively short muffle portions 154a and 154b, significant instabilities in their air flow patterns are seen in the muffle portions 154a and 154b. These unstable air flow patterns are characterized by the random and chaotic formations to toroidal vortices, which result in spurious temperature variations on the surface of the glass tubing and can adversely affect tube quality. This convective air flow pattern is similar to Rayleigh-Benard convection where there is a critical layer dimension for which the convection pattern becomes unconditionally unstable. One key difference here, however, is the effect of radiation which is typically not considered in Rayleigh-Benard convection and can have a significant effect on the stability of the air flow regime. Case (C) having a relatively long muffle portion 154c has a more stable air flow pattern compared to cases (A) and (B) that provides lower temperature fluctuations on the surface of the glass tubing as it cools and travels through the muffle portion 154c.

Referring to <FIG>, a plot <NUM> of temperature variation versus draw height location X is illustrated for each of the cases (A), (B) and (C) of <FIG>. X location is the distance from the bell, where the glass temperature is kept constant and does not change with time. Such glass temperature variation at locations X is measured over time. As can be seen, the longer muffle portion 154c has a more stable glass temperature profile over distances X from the bell (less than <NUM>) than that of the shorter muffle portions 154a and 154b. Indeed, the longer muffle portion 154c has a temperature variation of less than about <NUM> degree C, such as less than <NUM> degree C, while shorter muffle portion 154a has a temperature variation of more than <NUM> degree C.

<FIG> illustrate the affect that the width or diameter of the lower extended muffle structure can have on convective air flow. For <FIG> having relatively wide lower extended muffle structures 354a and 354b (e.g., <NUM> inches or <NUM> and <NUM> inches or <NUM> in diameter, respectively), there tends to be some toroidal flow. For example, <FIG> shows toroidal flow below the bell, near a beginning of the glass tubing formation, while <FIG> shows toroidal flow within the lower extended muffle structure 354b. In either case, the increased turbulence can cause temperature variations along lengths of the glass tubing. Compare <FIG>, which illustrate relatively narrow flow control structures 354c and354d (e.g., <NUM> inches or <NUM> and <NUM> inches or <NUM>, respectively). The lower extended muffle structures 354c and 354d tend to produce less toroidal flow compared to the lower extended muffle structures 354a and 345b due to their narrowed, elongated dimensions controlling convective air flow within the flow control structures 354c and 354d, which can reduce temperature fluctuation along lengths of the glass tubing.

Referring to <FIG>, an exemplary lower extended muffle structure <NUM> is illustrated that encloses a portion of a length of glass tubing flowing from a heating apparatus. The lower extended muffle structure <NUM> is generally a five-sided structure having a top wall <NUM> and four side walls <NUM>, <NUM>, <NUM> and <NUM> that surround the glass tubing on all sides as the glass tubing exits the heating apparatus. In some embodiments, the lower extended muffle structure <NUM> may be connected at the top wall <NUM> to a muffle portion of the heating apparatus and located therebelow to extend the enclosure provided by the muffle portion alone. One side wall <NUM> may be slanted to accommodate providing a catenary for the glass tubing to change the orientation of the glass tubing from vertical to horizontal and also to isolate upstream portion of the glass tubing from perturbations generated during the drawing process or other downstream processes, such as glass tube cutting. The lower extended muffle structure <NUM> may be formed of any suitable high-temperature material or combination of materials, such as metals, metal alloys, ceramics, glass, etc..

Referring to <FIG>, another embodiment of a glass tube manufacturing apparatus <NUM> includes many of the components described above with reference to <FIG> including molten glass delivery system <NUM>, delivery vessel <NUM>, bell <NUM> and heating apparatus <NUM> including muffle portion <NUM>. In this embodiment, a lower extended muffle structure <NUM> is in the form of a flexible enclosure (e.g., a cylinder or tube) that is connected to the muffle portion <NUM> of the heating apparatus <NUM>. The lower extended muffle structure <NUM> may be a thermally engineered device (TED) that is formed of any suitable material, such as metals (e.g., steel, aluminum alloys), fabrics, such as high temperature weaves of Kevlar, alumina, silica and ceramic fibers.

As above, the lower extended muffle structure <NUM> extends downward from the heating apparatus <NUM>. The lower extended muffle structure <NUM> may be sealingly connected to the muffle portion <NUM> of the heating apparatus <NUM>. A seal <NUM> may be formed between the lower extended muffle structure <NUM> and the heating apparatus <NUM> to inhibit entry of air therebetween. The lower extended muffle structure <NUM> may extend about the glass tubing <NUM> on all sides, <NUM> degrees around the glass tubing <NUM> and downwardly from the muffle portion <NUM>. The lower extended muffle structure <NUM> may extend below the heating portion <NUM> and along a length of the glass tubing <NUM> a predetermined length L. With the flexibility of the lower extended muffle structure <NUM>, the shape of the lower extended muffle structure <NUM> can be adjusted to follow and enclose the desired flow path of the glass tubing <NUM>. As can be seen by <FIG>, the glass tubing <NUM> hangs in a catenary <NUM> and is drawn over one or more pull rollers <NUM> as the glass tubing <NUM> shifts from a substantially vertical orientation to a substantially horizontal orientation as the glass tubing <NUM> travels along the flow path.

The lower extended muffle structure <NUM> may be any suitable cross-sectional shape, such as circular or other suitable rounded shape, such as oval or other suitable shapes such as triangular or rectangular that enclose the glass tubing <NUM> on all sides. The length (or distance D) of the lower extended muffle structure <NUM> may be selected based on various factors, such as length of the glass tubing when sectioned or other suitable factor. As an example, the length of the lower extended muffle structure may be between about <NUM> to about <NUM>, such as between about <NUM> to about <NUM> in length.

Referring to <FIG>, a cross-section view of the lower extended muffle structure <NUM> with the glass tubing <NUM> is illustrated. As can be seen, the glass tubing <NUM> travels through the hollow opening <NUM> through the lower extended muffle structure <NUM>. The lower extended muffle structure <NUM> may use any number of support structures <NUM> that can be used to support the lower extended muffle structure <NUM> in a desired shape and location. For example, the support structure <NUM> may include an arch-shaped or otherwise rounded support <NUM> that can support the lower extended muffle structure <NUM> in a rounded cross-sectional shape spaced-apart from the glass tubing <NUM>. Rollers <NUM> may also be located in the lower extended muffle structure <NUM> and be used to draw the glass tubing <NUM> through the lower extended muffle structure <NUM> and along the flow path.

The lower extended muffle structure <NUM> has a width W and a height H, which, in the case of a substantially circular cross-section, are a diameter. In some embodiments, the width W and/or height H may be substantially equal to the width and/or height of the muffle portion <NUM>. A maximum distance Dr from the glass tubing <NUM> to the lower extended muffle structure <NUM> is less than the width W and/or height H of the lower extended muffle structure <NUM>. As examples, the width W and/or height H of the lower extended muffle structure <NUM> may be about the same as an exit diameter Dm of the muffle portion <NUM> (<FIG>). For example, the exit diameter Dm and the the width W and/or height H may be about <NUM> inches and the maximum distance Dr may be about <NUM> inches. In another embodiment, the the width W and/or height H may be between about <NUM> inches and exit diameter Dm about <NUM> inches and the distance Dr may be about <NUM> inches. In another embodiment, the the width W and/or height H may be between about <NUM> inches and about ten inches and Dr may be about <NUM> inches (<NUM>, <NUM>, <NUM>, <NUM> and <NUM> inches correspond respectively to <NUM>, <NUM>, <NUM>, <NUM> and <NUM>).

The lower extended muffle structure <NUM> may include a number of features that can facilitate glass tube removal from the lower extended muffle structure <NUM>, if needed, and glass tubing formation. For example, referring back to <FIG>, the lower extended muffle structure <NUM> may include an openable portion <NUM>, such as a reclosable opening that is formed within an outer wall <NUM> and can be closed using any suitable reclosable fastener or plug. In some embodiments, the lower extended muffle structure <NUM> may include a releasable seal that is connected to the muffle portion <NUM>, such as a spring loaded and/or magnetic seal. As another example, the outer wall <NUM> may include a burn through section or otherwise separable portion that can be readily removed (e.g., chemically, mechanically, thermally, etc.) under certain conditions. Providing internal access from the outside of the lower extended muffle structure <NUM> can facilitate removal of glass tubing in case of a line break or other event where removal of glass tubing or general access to the interior of the lower extended muffle structure <NUM> is desired.

Stabilizing convective air currents in the lower extended muffle structure <NUM> may increase air temperature therein compared to not stabilizing convective air currents using the lower extended muffle structure <NUM>, which can impact glass tubing attributes. <FIG> also shows an embodiment of the glass tube manufacturing apparatus <NUM> that includes a cooling system <NUM> that can be used to cool the lower extended muffle structure <NUM> and the hollow opening <NUM> thereby removing heat from the glass tubing <NUM> in a predetermined fashion. For example, the cooling system <NUM> may be a multiphase cooling system that takes advantage of a phase change (from liquid to vapor) thus harnessing latent heat of vaporization. The cooling system <NUM> may include an evaporator component <NUM> where liquid is converted to vapor by absorbing heat from the lower extended muffle structure <NUM>, a vapor conduit <NUM> that directs the vapor to a condenser component <NUM> that condenses the vapor back to a liquid and a liquid conduit <NUM> that carries the liquid back to the evaporator component <NUM> to repeat the cycle. Other cooling systems may be used, such as recirculating water and forced air cooling. Further, the hollow opening <NUM> may be filled with gasses other than air, such as nitrogen, helium and argon. Ports may be provided at the interface between the lower extended muffle structure <NUM> and the muffle portion <NUM>, at an end exit <NUM> of the lower extended muffle structure <NUM> or both. The outer wall <NUM> of the lower extended muffle structure <NUM> may be a single-piece or multiple-section design (e.g., a clamshell).

Referring to <FIG>, while length of the lower extended muffle structure can affect convection air flow stability, a muffle bottom cover <NUM> with reduced diameter opening can also suppress natural convection induced flow instability. <FIG> illustrates a heating apparatus 242a with a muffle portion 244a without a muffle bottom cover. As an example, an exit opening 246a of the muffle portion 244a may be about <NUM> inches (<NUM>). As can be seen, various unstable air flow patterns are present, which result in spurious temperature variations on the surface of the glass tubing and can adversely affect tube quality. (B) and (C) illustrate some improvement in convection air flow stability through use of muffle bottom covers 240b and 240c having openings 246b and 246c. For example, opening 246b may be about <NUM> in and opening 246c may be about <NUM> in thereby reducing distances between the glass tubing <NUM> and a perimeter 250b and 250c of the muffle bottom covers 240b and 240c. (D) shows the greatest convection air flow stability and has the smallest opening 246d. For example, the opening 246d of the muffle bottom cover 240d may be about <NUM> in.

<FIG> shows a plot <NUM> of average glass tube taper versus gap between the glass tubing and perimeter of the opening of the muffle bottom cover. As can be seen, the plot <NUM> illustrates a non-monotonic effect of opening hole size of the muffle bottom cover on tube diameter taper by increasing in tube taper in section <NUM>, decreasing in section <NUM> and then again increasing in section <NUM> until point <NUM> with no muffle bottom cover.

Referring to <FIG>, an embodiment of a muffle bottom cover <NUM> is illustrated connected to a bottom <NUM> of a muffle portion <NUM> of a heating apparatus <NUM>. The muffle bottom cover <NUM> may be provided with multiple sections <NUM> and <NUM>, which may or may not be symmetric, each including a portion of an opening <NUM>. Providing multiple sections <NUM> and <NUM> for the muffle bottom cover <NUM> can facilitate opening and closing of the muffle bottom cover <NUM> during the tube forming process, if needed. The muffle bottom cover <NUM> may be plate-like or planar in shape and extend to a periphery <NUM> of the muffle portion <NUM> thereby covering an entire exit opening <NUM> of the muffle portion <NUM>. In other embodiments, the muffle bottom cover may be a single plate-like section. While the muffle bottom cover <NUM> is round or circular to correspond to the shape of the muffle portion <NUM>, other shapes may be used for the muffle bottom cover, such as rectangular, as shown by muffle bottom cover <NUM> of <FIG>. Other shapes can also be used for the openings <NUM>, <NUM>, such as other round shapes (e.g., elliptic), rectangular shapes or irregular shapes. The muffle bottom covers can be formed of any suitable material and can perform as radiation shields, for example, using opaque quartz to reduce loss of heat to the environment.

Referring to <FIG>, in another embodiment, the heating apparatus <NUM> includes the muffle portion <NUM> including the muffle bottom cover <NUM> connected thereto. In this embodiment, a lower extended muffle structure <NUM> in the form of a tube having an outer diameter that is sized to extend through the opening <NUM>, thereby extending the muffle portion <NUM>. The lower extended muffle structure <NUM> may extend outwardly into the muffle portion <NUM> and also outwardly from an opposite side of the muffle portion <NUM> to an end <NUM>. The lower extended muffle structure <NUM> may be curved to match or accommodate the catenary path of the glass tubing as the glass tubing exits the heating apparatus <NUM>. The lower extended muffle structure <NUM> may be circular, elliptic or any other suitable cross-sectional shape.

<FIG> illustrate the heating apparatus <NUM> and the muffle bottom cover <NUM> connected to the muffle portion <NUM> of the heating apparatus <NUM>. Again, referring to <FIG>, the muffle bottom cover <NUM> includes the opening <NUM> that, in the illustrated embodiment, provides communication between an interior <NUM> of the muffle portion <NUM> and an interior <NUM> of a lower extended muffle structure <NUM>. In contrast to the lower extended muffle structure <NUM>, the opening <NUM> is located within the interior <NUM> of the lower extended muffle structure <NUM>. The lower extended muffle structure <NUM> may be curved to match or accommodate the catenary path of the glass tubing as the glass tubing exits the heating apparatus <NUM>. A lower extended muffle structure cover plate <NUM> may be used to cover a lower end <NUM> of the lower extended muffle structure <NUM>. In other embodiments, a lower extended muffle structure cover plate may not be used. Additionally, the lower extended muffle structure <NUM> may be flexible and include any number of curved and straight sections. Further, temperature control devices (heating and/or cooling) may or may not be provided within the interior <NUM> of the lower extended muffle structure <NUM>. These temperature control devices can actively control the energy balance of the glass tubing to adjust the catenary length as process conditions change.

<FIG> illustrate another variation of the heating apparatus <NUM> and the muffle bottom cover <NUM> connected to the muffle portion <NUM> of the heating apparatus <NUM> that includes the lower extended muffle structure <NUM> connected thereto as in <FIG>. In this example, a tapering lower extended muffle structure <NUM> is connected to the lower end <NUM> of the lower extended muffle structure <NUM>. The tapering lower extended muffle structure <NUM> may taper in dimension (e.g., inner diameter) from a top end <NUM> that is connected to the lower end <NUM> of the lower extended muffle structure <NUM> to a lower end <NUM>, forming a somewhat truncated cone shape. Another lower extended muffle structure <NUM> is illustrated connected to the lower end <NUM> of the tapering lower extended muffle structure <NUM>. Again, the lower extended muffle structures <NUM> and <NUM> may be curved to match or accommodate the catenary path of the glass tubing as the glass tubing exits the heating apparatus <NUM>. A lower extended muffle structure cover plate <NUM> may be used to cover a lower end <NUM> of the lower extended muffle structure <NUM>. The lower extended muffle structure cover plate <NUM> may also include an opening <NUM> having a central axis <NUM> therethrough that is at an angle α (e.g., between about <NUM> degrees and about <NUM> degrees) to a central axis <NUM> of the opening <NUM> of the muffle bottom cover <NUM>. As above, the lower extended muffle structure cover plate <NUM> may manage air entrainment into the lower extended muffle structure <NUM> through the lower end <NUM>.

Claim 1:
A glass tube manufacturing apparatus (<NUM>, <NUM>) for manufacturing glass tubing (<NUM>) using a non-axisymmetric flow path, comprising:
a glass delivery tank (<NUM>) with molten glass, the glass delivery tank (<NUM>) having a bottom opening (<NUM>);
a bell (<NUM>) having an upper portion with an outer diameter located at the bottom opening (<NUM>);
a heating apparatus (<NUM>, <NUM>, <NUM>) at least partially disposed around the bell (<NUM>), the heating apparatus (<NUM>, <NUM>, <NUM>) including a heating portion (<NUM>) and a muffle portion (<NUM>, <NUM>) located below the heating portion (<NUM>); and
a lower extended muffle structure (<NUM>, <NUM>, <NUM>, <NUM>) that is connected to the muffle portion (<NUM>, <NUM>) and extends downwardly from the muffle portion (<NUM>, <NUM>), the lower extended muffle structure (<NUM>, <NUM>, <NUM>, <NUM>) extending along and around the glass tubing (<NUM>) as the glass tubing (<NUM>) transitions from a vertical orientation to a non-vertical orientation to manage convective airflow therethrough.