Abstract:
A glass tempering furnace having a chamber, rollers extending laterally within a chamber to form a transport surface for the plate glass, radiant coils positioned along the bottom of the chamber underneath the rollers, a plurality of spaced nozzle assemblies arranged in lateral side-by-side fashion within the chamber above the rollers, and fans coupled to the nozzle assemblies to draw heated air from the chamber and force the heated air onto the top surface of the plate glass. Heating elements, preferably electrically heated rods, extend between each of the nozzle assemblies and are positioned within the return path of the heated air after it is flowed onto the plate glass surface. The air then rebounds from the glass plate prior to the air again being drawn up into the fan and blown back down onto the glass. Air is then again drawn from the furnace chamber and forced through ducting to a nozzle assembly.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit from U.S. Provisional Patent Application No. 60/425,886 filed Nov. 12, 2000 whose contents are incorporated herein for all purposes. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to furnaces for heating glass sheets and more particularly to a system and method for preventing the arching of sheet glass, and particularly glass sheets having a low emissivity coating, in a horizontal tempering conductive heat furnace.  
           [0004]    2. Description of the Prior Art  
           [0005]    It is known that patents exist whereby plate glass, as it passes along rollers within a furnace, can be heating using radiative (e.g. heated coils operating on the same principal as a toaster), conductive (e.g. contact with a heated surface such as rollers), or convective (e.g. hot air blown on the glass). Examples of furnaces using these various heating mechanisms are shown in U.S. Pat. No. 4,505,671 (McMaster). Other known patents are U.S. Pat. No. 3,326,654 to Plumat, U.S. Pat. No. 3,488,178 to Welker and U.S. Pat. No. 3,402,038 to Hordis. These patents appear to only disclose convective heating whereby air heated within the confines of the furnace is circulated over the glass via compressed air nozzles.  
           [0006]    Annealed glass sheets are processed to tempering grades in furnaces utilizing radiation heat transfer as the primary energy source. Typically heating coils comprised of serpentine or helical nichrome wire are arranged in a spatial relationship with the glass surfaces such that the sheet is uniformly heated to high temperatures approaching 615° C. (1139° F.) and then air quenched in subsequent processing.  
           [0007]    Manufacturers of glass tempering furnaces have favored radiative designs. These designs are field proven with a minimum of internal components and complexity, and process glass to uniform material and optical qualities. Standard float glass exhibits emissivity values of 0.85 and higher, leading to production times and material qualities in radiative furnaces acceptable to processing plants.  
           [0008]    Architectural styles and building codes have changed, however, thus introducing increasing surface areas suitable for tempered glass such as doors, windows, and exterior glazing in both residential and commercial structures. Glass manufacturers are sensitive to the increasing energy requirements of the building industry. One priority is to reduce the solar load transmitted through this glass exterior. To minimize the solar influx, the exterior exposed face of the glass has been modified by application of sputtered reflecting films, etchings, or surface treatments such that the solar infrared radiation incident on the surface is highly reflected and the visible light spectrum transmitted. The interior glass face, not requiring specialty treatments, retains the emissivity and heating characteristics of typical annealed glass. Glass treated in this manner is marketed as low-E, or low-emissivity glass. Emissivities are generally stated as 0.15 and lower, with special treatments capable of producing emissivity values as low as 0.04. Though usage of this energy efficient glass is popular for the discussed energy reasons, conventional tempering using radiation heating means is quite difficult.  
           [0009]    In a standard radiant furnace, when the high-performance Low-E glass in conveyed into the furnace, the bottom skin of the glass, which does not have the coating, receives its heat at the normal rate from the conduction of the ceramic rolls. The top skin however reflects most of the radiant energy being produced by the heating elements and does not absorb much heat. This causes the bottom skin to expand much more than the top skin and causes the glass to bow or dish up, inside the furnace. This phenomenon occurs on normal uncoated glass also but it is a very short-lived condition. In other words the top will absorb heat at a rate that will allow the skin temperatures to equalize. When this bowing occurs, there are several problems that are caused. One of the problems is related to high heat transfer from the ceramic rolls to the bottom surface of the glass due to the weight of the glass being concentrated in a smaller contact area. One of the most severe problems is that while in this bowed state, the glass is no longer contacting the ceramic rolls except for the reduced area in the middle and is no longer receiving heating from conduction. This will lead to very non-uniform heating of the glass and will result in breakage, warpage, or exceedingly long heating times.  
           [0010]    Accordingly, a need remains for an improved glass sheet heating furnace that overcomes the drawbacks in the prior art.  
         SUMMARY OF THE INVENTION  
         [0011]    To address this drawback with purely radiative tempering systems, the present invention combines conventional radiation heat transfer with an alternate heating method suitable for low-E products using specialty surface treatment for the exterior glass. That is, forced convection air heating principles and related apparatus are applied in combination with radiation heat transfer in a novel fashion, unobvious to those trained in the art.  
           [0012]    The general design of a glass tempering furnace constructed according to the present invention includes a chamber, rollers extending laterally within a chamber to form a transport surface for the plate glass, radiant coils positioned along the bottom of the chamber underneath the rollers, a plurality of spaced nozzle assemblies arranged in lateral side-by-side fashion within the chamber above the rollers, and fans coupled to the nozzle assemblies to draw heated air from the chamber and force the heated air onto the top surface of the plate glass. Heating elements, preferably electrically heated rods, extend between each of the nozzle assemblies and are positioned within the return path of the heated air after it is flowed onto the plate glass surface. The air then rebounds from the glass plate and flows over these heating elements PRIOR to the air again being drawn up into the fan and blown back down onto the glass. Air is then again drawn from the furnace chamber and forced through ducting to the nozzle assemblies.  
           [0013]    The nozzle assembly contemplated for use with the invention includes a chamber into which the heated air is forced and a plurality of holes formed on a bottom plate thereof. The air is forced out these holes onto the plate glass and the return air rebounds from the glass and flows between the plurality of nozzle assemblies. The heated rods are positioned between these nozzle assemblies so that the air must flow past, and is thereby heated by, these rods. The heated rods additionally create radiative energy that impacts upon the top surface of the plate glass and heats it thereby. More specific configurations of the heating rods include stacked rods, where the bottom-most rod (the one closes to the plate glass) includes multizone heating control for greater control of the radiative heat that impacts the glass sheet surface.  
           [0014]    The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention that proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a side elevation sectioned view showing the interior of the tempering furnace constructed according to a preferred embodiment of the invention.  
         [0016]    [0016]FIG. 2A is an elevation front view taken along line  2 A- 2 A in FIG. 1.  
         [0017]    [0017]FIG. 2B is a top plan view of the furnace of FIG. 1.  
         [0018]    [0018]FIG. 2C is a side elevation view of a portion of the furnace of FIG. 1 showing an upper heating zone of the invention.  
         [0019]    FIGS.  3 A- 3 C show orthogonal views of the air delivery ductwork of the furnace of FIG. 1.  
         [0020]    FIGS.  4 A- 4 C show orthogonal views of the nozzle plate and orifice pattern of the ductwork of FIGS.  3 A- 3 C.  
         [0021]    [0021]FIG. 5A illustrates a tubular sheath embodiment of the heating element used in the furnace of FIG. 1.  
         [0022]    [0022]FIG. 5B illustrates an alternate heating element design from that shown in FIG. 5A.  
         [0023]    [0023]FIG. 6 is an elevation front view of an alternate embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    The forgoing disclosure assumes a general assembly as in FIG. 1 where the lower shell  22  is of known means to those trained in the art and industry. The lower shell  22  is constructed to incorporate all known radiation heating means, conveying system, structure, insulation, and exterior features of accepted design to those in the industry. The top shell  23 , however, is a unique invention clearly differentiated by construction, support, operation, and process function from the lower shell  22 . The top furnace shell  23  and bottom furnace shell  22  are to be considered as separate equipment entities. As such, this embodiment of the invention is directed primarily toward the novel features of the upper furnace  23 .  
         [0025]    The furnace constructed according to a preferred embodiment of the invention is shown generally in FIG. 1 at  10 . Furnace  10  is constructed in top-bottom shell construction; the shells split along a horizontal datum generally defined as where the planar glass  20  surface might convey through the equipment.  
         [0026]    The furnace is designed as a functional whole; however, it should be noted that the top and bottom shells might function in independent fashion. The bottom furnace shell  22  is supported and remains in a fixed height position from the floor datum  29  by structure of known means. The top furnace shell  23  is supported from a lift system constructed of integrated jackscrew assemblies  26  or similar apparatus rated for the top load requirements.  
         [0027]    Conveying rolls  21  of known means support the glass during the process. In this embodiment, the conveying rolls  21  are cylinders of ceramic materials resistant to ambient high temperature and suitable for the process environment.  
         [0028]    Fans  31  of high temperature construction are required for air recirculation within the furnace. The fans  31  are considered high flow, forward curved designs with integrated air-cooling. The fans  31  might be top-mounted as shown or of a sideboard layout. The convection layout and mechanicals required for internal furnace air recirculation (fans, ductwork, vanes, nozzles, impingement plates, etc.) are to be associated only with the top furnace shell  23 .  
         [0029]    It is noted that temperatures within the furnace are considered near the material limits of conventional materials and require care in selection. Ambient internal temperatures range from 670° C. to 700 ° C. (˜1300° F.). Insulation  32  of thru-wall dimensions of 6″and greater are used to reduce energy consumption and other undesired environmental factors such as infiltration of plant air and leakage of furnace air to the plant. In this invention, the top furnace shell  23  utilizes a thermal break of insulation between offset structural members. This double-wall construction within the walls and roof minimizes the direct thermal conduction from inside to outside.  
         [0030]    It is also be noted that the top shell  23  of this invention has internal surfaces  34  clad in heat resistant metal such as stainless steel (316SS or 309SS) to prevent insulation fibers from entraining in the recirculated atmosphere and contaminating the processing glass.  
         [0031]    The invention assumes that the processing of low-E glass is such that the upper conveyed surface of the glass  20  is a surface of low-emissivity properties more suitable for convection heating. The bottom conveyed glass surface is typical of high-emissivity annealed glass  20  suitable for conventional radiation furnace designs.  
       Nozzle Design  
       [0032]    The furnace design requires forced convection principles highly dependent on the end-of-the-run air delivery ductwork and the size, velocity, and uniformity of the impingement jets to the glass surface. The air delivery ductwork  40  repeats along the furnace length and remains supported above the processed glass sheet; connecting the air supply plenums  46 R,  46 L on right and left (FIG. 2A).  
         [0033]    This invention utilizes ductwork  40  designed as a sheet metal enclosure (FIGS.  3 A- 3 C); tapering from the ductwork openings toward the furnace center. The taper T is constructed to maintain a generally constant air velocity along the ductwork length as air flow exits the nozzle plate to the glass  20 . The angular taper T from each end is equal; with a recommended taper of 20°(+/−5°) from the horizontal. The x-sectional opening dimensions of the ductwork  40  are calculated to allow for at least 3× the exit orifice area. The aspect ratio of the opening height H versus the opening width W is generally 5:1. The constant width dimension of the ductwork is generally equal to the spacing between ductwork assemblies.  
         [0034]    Air entering the ductwork  40  from both the right and left supply plenums  46  exits along the ductwork length. Where the air supplied from each end might collide, a zone of high pressure exists. This results in higher velocity jets and faster heating, creating non-uniform results at this location. To avoid this scenario, the ductwork center is not coincident with the furnace center. Further, the furnace construction is such that the ductwork  40  is fixed in location at one plenum wall (e.g.  46 R) and allowed to thermally expand through the plenum wall (e.g.  46 L) on the opposite side. This fixed end of the ductwork  40  alternates along the length of the furnace resulting in the off-center location to be exaggerated toward the expanding ductwork  40  end.  
         [0035]    As shown in FIG. 2C, the nozzle plate  42  is parallel and at a predetermined dimension from the glass surface. The nozzle plate  42  is a machined construction and of thermally stable and rigid materials such as 309SS, ¼″ thick plate. As shown best in FIGS.  4 A- 12   4 C, the described nozzle plate 42 seals the air ductwork and incorporates an array of orifice jets  43 . The jet openings have been calculated in size, number, and distribution to have the optimum heating effect on the glass surface. In a preferred embodiment of the invention, the jets are drilled holes with a machined countersink inlet and {fraction (5/16)}″ diameters selected to geometrically overlap when viewed along the furnace length. In general, the additive area of all the jets is no greater than approximately 7.5% of the plan area above the glass  20 . This ratio is critical to allow the impingement jets to diverge between orifice diameters, blanket the entire glass surface, and exit via a generous exhaust area after impingement.  
       Heater Design  
       [0036]    Specialized heaters  50 , 52  (FIG. 5A) maintain the recirculated air at extreme high temperatures. The heaters  50 , 52  also provide a radiant source where the processing of annealed glass is desired. To provide these functions, a 1-zone heater  50  and 3-zone heater  52  are configured as a grouping of the two heaters, located between each of the ductwork assemblies  40 . The multiple zone heater  52  is retained in close proximity to the glass  20 .  
         [0037]    The heaters are generally designed as tubular elements, with the geometry of a small diameter relative to their length. The heaters are externally sheathed in materials suitable for high temperature applications such as Inconel 600 or a similar material. The internal components include heating wire, such as nichrome, wound and configured to produce one or multiple zones of heating along the length. In another embodiment of the heater design (FIG. 5B), a ceramic core is wrapped externally with heating wire to produce equivalent heating zones.  
         [0038]    It is anticipated that a 1-zone heater  50  would be used strictly for general ambient heating. The multiple zones of the 3-zone heater  52  are each instrumented with thermocouples  54  such that a process control system  27  might orchestrate which of these 3-zone heaters  52  and/or individual zones along the length might be controlled for optimal glass processing.  
       Open Layout over Glass  
       [0039]    A unique requirement of the invention is that the upper furnace shell  23  must be constructed with an open and non-obstructed working area over the glass. The convective transfer from the air delivery ductwork  40  and radiant transfer from the specialized heaters  50 , 52  requires that there not be any metalwork, supports, insulation or similar materials between these components and the glass  20 . Introducing any structure between the glass  20  and ductwork  40  or heaters  50 , 52  will scatter or reflect the radiation transfer and impede the convective jets.  
         [0040]    To minimize potential hot and cold spots, constant oscillation of the sheet glass in a forward-back motion within the furnace improves the uniformity of the finished product. Travel distances in each direction of 18″ and greater are common in the industry. Motion of the glass from the side-to-side is precluded by the conveying system of rolls  21  where rotation is only along the length direction of the furnace. Though glass motion reduces inconsistencies within the sheet, localized imperfections and streaks in the length direction continue to result from blocking the energy source from the sheet. The glass may warp, or incur optical irregularities and uneven breakage properties relative to those areas open to the full energy transfer.  
       Open Layout-Mechanical Considerations  
       [0041]    The reader should appreciate that the furnace sides and ends are structurally built to resist the weight load of both the exterior walls and roof, and all exterior mounted components of the upper shell  23 . Similarly, the components on or within the working space of the upper shell  23  are constructed of heavy gauge sheet metals requiring substantial support and connection to the same external structure. Attention is called to the load bearing shelves  60  that seal the air supply plenums  46  (FIG. 6). These shelves extend along both length sides of the upper furnace  23  and extend into the furnace interior. The weight of the air ductwork  40  is transferred to the air supply plenum  46  walls and shelves  60  and allow the ductwork center and nozzle plate  42  to be essentially cantilevered over the glass sheet  20 .  
         [0042]    It should also be noted that the distance from orifice  43  to glass is critical. The dimension cited in forced convection applications is in the range of 4-12× the orifice diameter. In this invention, 3 ″ is conservatively selected to prevent the glass from potential warping and interfering with the overhanging structure. The dimension represents roughly  10  × the {fraction (5/16)}″ jet orifice diameters. To distribute the weight load of the ductwork  40  and maintain the predetermined distance from orifice to glass over the entire glass sheet; a unique rod-spring support system  70  has been constructed. FIG. 6 details a representation of the rod-spring system.  
         [0043]    The shelf rods  72  connect to a pivot arrangement  74  at the interior shelf edges, and extend and terminate on the shell roof  25 . The ductwork rods  73  connect to support piping  41  in-line with a slot in the center of the air ductwork  40 , and similarly extend and terminate on the shell roof  25 . Shelf rods  72  and ductwork rods  73  are spaced at even increments along the furnace and calculated to carry the load.  
         [0044]    Note that any fixed length will thermally expand within the furnace. For example, the expansion length can be calculated as follows:  
                                                       Coeff. Of Expansion for   8.8 × 10**−6 in/(in-° F.)           Rod Materials of 316SS           Furnace Ambient   ˜1250° F.           Room Ambient    ˜70° F.           Temp Rise     1180° F.           Expansion per Ft.   ˜1/8”/Ft.                      
 
         [0045]    If the rods are fixed at each end, the thermal expansion of a typical support rod member 4 ft. in length is ˜½″(4 ′×⅛″/ft.). Expansion will result in undue stress on the structure, rod stretch and/or cracking at the terminating rod at the ends More importantly ,it is possible that the air ductwork  40  will appreciably sag the discussed additional expansion length. (As calculated ˜½″).  
         [0046]    To minimize the sag and reduce the potential of structural problems, a novel solution is forwarded. The rods ultimately terminate at topside attachment points along the exterior of the furnace roof  25 . The system proposes that the expansion be absorbed by springs  76  at these locations, preloaded to support the primary weight of the internal components such as the air ductwork  40 .  
         [0047]    The rods  72 ,  73  penetrate the insulated furnace roof  25  and extend through a sealing base plate  77 . The base is located above internal structure within the roof. The rods are assumed to be threaded at the far end. The rod end is attached to a top nut-plate  78 . Turning the nut plate  78  causes the springs  76  to compress and pre-load, supporting the internal components. Further rotation and compression is equivalent to additional spring load carrying capacity.  
         [0048]    At start-up, the upper furnace shell  23  is displaced at a known and fixed vertical dimension from the lower shell  22 . Primary load support and vertical position of the external walls, roof, and mounted components of the upper shell  23  results from adjustment of the lift system  26 . The internal components are supported from the roof  25  by adjustment of the nut-plate  78  until the shelves  60  are in nearly zero load carrying contact with standoffs  79  attached to the lower shell  22 . Note that only a very minor load attributed to the shelf (and by connection, the air ductwork  40 ) is carried by said standoffs  79 . The discussed rods and adjusted pre-load of springs carry the primary load of internal components.  
         [0049]    As the furnace is heated to operating temperatures, the rods  72 , 73  thermally expand. Since the shelves  60  are constrained in the downward direction by mechanical interference with the standoffs  79 , the rods must expand in an upward direction. This expansion is absorbed by the take-up in the springs  76 .  
         [0050]    The load carrying forces imparted on internal ductwork  40  and shelves  60  are obviously a function of the original pre-load. It is known that allowing the springs to expand will lose some of this pre-load. However, as calculated, the vertical expansion is generally in the range of ˜½″; the original spring length ˜4″. The minor loss of pre-load is now taken by the standoff  79  supports.  
         [0051]    Other furnaces and capital equipment structure require internal cross bracing, beams, and load-carrying columns. These structural allowances will appreciably interfere with the open layout desired for processing large glass sheet approaching dimensions of 100″×168″. The techniques and concepts explored above are considered to be quite novel and unobvious to those experienced in the art.  
       Operation of Furnace  
       [0052]    Convection  
         [0053]    Fans  31  are arranged in groups of two; each fan supplying air within right or left zones ( 91 R,L . . .  96 R,L) spaced equally along the longitudinal length of the furnace. The convection principles are best described by referring to the FIGS. 2A, 2B and  2 C. The grouping is such that the air mass flow may be independently controlled on the right and left furnace sides. In other embodiments, a single fan might be used with mechanical means of dividing the airflow into right and left plenums. This division of air is advisable to provide a uniform air balancing system across the furnace width and required where the furnace may be loaded unequally from side-to-side with glass, or with glass of varying thickness, shape, or tempering qualities.  
         [0054]    Fans suitable for extreme high temperature service are controlled from 0% to 100% of airflow by suggested electronic means  82  such as variable frequency drives. The supply air  84  is ducted from the fan scroll housings  31 A to a transition plenum box  31 B designed to spread the pulse of air along the zone length of the sidewall. These plenum boxes are so ducted as to transfer the air  84  with minimal pressure loss along a gradual internal radius from horizontal to vertical direction. The air is directed into the finger ductwork  40 , entering through the finger openings  40 A and pressurizing the finger cavities  40 B.  
         [0055]    The finger ductwork  40  and orifice plates  42  are so designed to uniformly transform the higher pressure low velocity air mass into individual jets of high velocity that impinge the glass. Velocities are suggested to be in the 1500 to 5000 FPM range for manufacturing economy of the air system and optimized heat transfer on the glass sheet.  
         [0056]    The airjets  85  scrub the insulating boundary layer of air at the glass surface; introducing high temperature convection heat transfer. The spent gas is continually displaced with a continuum of air jets following behind in the circulation system.  
         [0057]    The exhaust path of spent air is designed to flow in a specific manner between each of the finger ducts  40 . The exhaust path is generally split, with ½ of the supply flow exiting to each respective side of the finger duct  40 ; and evenly along the finger length. The spent air follows the upward path of the exhaust stream  86  toward the fan inlets, recirculating in the described pattern.  
         [0058]    Radiation  
         [0059]    The heaters  50 , 52  shown in FIG. 2C are arranged in a specific functional orientation to the finger ductwork  40 . The arrangement allows the spent exhaust air to travel over and around the heaters for efficient reheating of the air stream. Heaters function as a group of two, each group located between the finger ducts and each heater grouping repeating along the longitudinal furnace length. The effective hot length of the heater elements traverse the width of the furnace equal to the minimum width of glass that can be accommodated. The heaters are vertically arranged (See FIGS. 2C, 5A) in a manner such that the upper heaters  50  are typically of one zone and sized to generally re-heat the exhaust stream. The lower heaters  52  closest to the glass are split into multiple zones of unequal heating length and power. The heating zones, generally of right end-long center-left end construction may be turned off or on as manually desired or programmed by the control system  27 .  
         [0060]    The intent is that the lower heater  52  acts as a radiative system with the upper glass surface. It should be noted that the heaters are placed strategically in a specific manner between the finger ductwork  40  to improve the radiative transfer. The infrared emission is bounded such that sides of the finger ductwork  40  reflect the sideways heater infrared, providing a large spatial relationship with the glass.  
         [0061]    In those applications processing high-emissivity glass, the radiation transfer from the lower heaters  52  plays an important role in general heating and tempering, such that the convection energy transfer might be reduced or unused. In those applications of low-E glass processing, the lower heaters act to balance or adjust the furnace processing from side to side and within the glass sheet center. By intermittently turning on or off all or any of the zones of the lower heaters  52 , the glass might better achieve uniformity results in waviness, optical properties, and other parameters requiring fine-tuning of the processing cycles.  
         [0062]    Production vs. Uniformity  
         [0063]    Heating times are highly dependent on the physical properties of surface emissivity and sheet thickness. Good emitters are poor reflectors. Annealed glass, a good emitter, has an emissivity range of 0.85-0.95. Low-E glass, a very poor emitter, can exhibit values in the very low ranges from 0.15 to 0.04. In all cases, the thermal conductivity of glass is nearly equal. As such, convection can dominate radiation where low-E hardcoats and coatings reflects nearly all IR heating. The effects are more pronounced for thinner glass not moderated by conduction effects. Radiation is effective in applications where emissivities are high; including annealed glass and the untreated reverse side of low-E glass. Greater uniformity is also generally shown in radiation systems as the source heaters can be easily configured in a spatial relationship to the glass.  
         [0064]    The industry state of the art incorporating convection is not highly developed. However, literature and early empirical results might be summarized:  
                                                                     Process Times:       (Heat-Up Time/Glass thickness):                Convection   30 Sec./mm           Radiation   40 Sec./mm            Uniformity:                Convection   *** Dependent on furnace design           Radiation   Generally greater than convection designs                      
 
         [0065]    It should be apparent to the reader that the upper furnace shell incorporates infinite design flexibility in heating concepts. Convection is independently adjustable from 0 to 100% in both the right and left zones and along the furnace length. Radiation transfer is adjustable on or off from right side-center-left side at each heater grouping and along the furnace length.  
         [0066]    The invention promotes a dominant convection purpose, radiation purpose, or radiation and convection furnace combinations to balance the glass processing requirements of uniform product qualities and production throughput.  
         [0067]    Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.