Abstract:
A method of heating low emissivity glass sheet for subsequent processing. The glass sheet is loaded onto a longitudinally extending conveyor and oriented such that the lengthwise edge of the sheet is parallel to the length of the conveyor and the direction of conveyance of the glass sheet. The sheet is conveyed into a heating chamber. The glass sheet is then convectively heated in a specified sequence along the entire length of the glass sheet at selected areas measured along the width by creating a downward flow of heated air proximate the selected areas.

Description:
This is a divisional of application Ser. No. 08/911,954 filed on Aug. 15, 1997, now U.S. Pat. No. 5,951,734. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a semi-convective forced air system and method for heating glass sheets for subsequent processing. More particularly, the system and method of the present invention are used for heating low emissivity coated glass before tempering. 
     BACKGROUND OF THE INVENTION 
     Forced air furnaces for heating glass sheets in preparation for subsequent processing, such as tempering, are known in the art. For example, McMaster, U.S. Pat. Nos. 4,529,380 and 4,505,671, discloses a glass sheet processing system which includes a heating furnace and a processing station for processing heated glass sheets to provide bending, tempering, bending and tempering, filming, etc. The furnace of U.S. Pat. No. 4,592,380 and 4,505,671 comprises an array of gas jets spaced above a conveyor within a heating chamber. The gas jets supply a primary gas flow directed toward the conveyor to provide forced convection heating of the glass sheets as the sheets are conveyed through the heating chamber. 
     The gas jets of McMaster are arranged in linear series perpendicular to the length of the conveyor and the direction of travel of the glass sheets. Each series of jets is connected to a common linear supply manifold or conduit. Each supply conduit also extends widthwise in the heating furnace, perpendicular to the length of the conveyor. McMaster teaches that the array of gas jet pumps are spaced from each other transversely to the direction of conveyance so as to uniformly heat each conveyed glass sheet over its entire width. 
     Heating systems such as described by McMaster appear to provide acceptable results for heating clear glass prior to tempering. Other known systems provide acceptable results for heating coated glass having an emissivity rating greater than about 0.2 prior to tempering. However, manufacturers have now begun to produce coated glass products having emissivity ratings in the range of 0.15-0.04. Prior art heating systems, including the system disclosed in U.S. Pat. Nos. 4,592,380 and 4,505,671, do not provide acceptable results for tempering glass having such low emissivity ratings. Therefore, it would be desirable to provide a system and method of tempering low “e” glass sheeting having an emissivity rating below 0.2. 
     When glass sheets are conveyed into a heating furnace, the bottom surface heats at a faster rate than the top surface due to contact with the rolls of the conveyor. This causes the bottom surface to expand at a faster rate than the top surface which results in the glass bowing upward into the shape of a bowl. All of the glass sheet&#39;s weight is supported in the center of the glass which causes the center of the glass to be overheated. This results in excessive distortion in the center of the glass which can be described as an elongated bubble. Non-uniform glass temperatures also cause the glass to oil can (or become bi-stable). Oil canning (or by-stability) and bubbling are undesirable conditions produced in the glass when the glass sheeting is not heated uniformly. Therefore, it would also be desirable to provide a method and apparatus for heating low “e” coated glass sheets which minimizes oil-canning and bubbling. 
     When low “e” glass is tempered in prior art systems, it is typically run lengthwise through the furnace due to the size of the furnace. It is also run lengthwise to mitigate the appearance of inherent distortions because the glass sheets are typically installed lengthwise down a room or hallway. However, low emissivity glass is more sensitive to heating in the longitudinal direction than it is in the widthwise direction. When glass is tempered in prior art systems, heat is only applied uniformly over the width of the glass sheet. This does not allow for separate control from edge to edge across the width of the glass. Without this control, the glass will not be heated as uniformly and the undesirable conditions of center bubble and oil canning will ensue. Therefore, it is also desirable to provide a system and method of tempering which uniformly applies heat over the entire length of the sheet in a longitudinal direction to improve aesthetic quality. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a semi-convective forced air system for heating glass sheets during a heating cycle for subsequent processing such as tempering. The system and method are particularly useful for tempering low emissivity coated glass sheeting having an emissivity rating below 0.2. In accordance with the system and method of the present invention, heat is uniformly applied over the entire length of selected widthwise portions of the glass sheet to reduce or eliminate oil canning and bubbling. 
     The system of the present invention comprises a heating chamber, a longitudinal conveyor extending through the heating chamber, a compressed air source, a plurality of longitudinally-extending air manifolds in fluid connection with the air source, and a controller for restricting the flow of air to selected manifolds at predetermined times during the heating cycle. Each of the air manifolds is oriented parallel to the length of the longitudinal conveyor and constructed and arranged to create a downward flow of heated air toward the conveyor to convectively heat a sheet of glass on the conveyor. 
     The air manifolds preferably comprise elongate tubes having a longitudinal series of radially extending apertures. The apertures are oriented downwardly toward the conveyor at an angle of about plus or minus 30 degrees from vertical. The air exiting the apertures forms an angle of incidence with the conveyor of about plus or minus 60 degrees. Each of the apertures are oriented oppositely than an adjacent aperture. 
     The manifolds are preferably located about 4-6 inches above the conveyor. The manifolds are constructed and arranged in longitudinally-extending rows. One of the rows is preferably located above the widthwise center of the conveyor, and one of the rows is preferably located above each of the two widthwise quarter points of the conveyor. The rows, preferably the outer rows, optionally may be adjustable to different widthwise locations above the conveyor. 
     The conveyor preferably has horizontally-extending rolls constructed and arranged for conveying glass sheets horizontally through the heating chamber. 
     The air manifolds preferably comprise ½ inch pipe having about 0.04 inch diameter apertures longitudinal spaced about 8½ inches apart. The air manifolds are constructed and arranged to simultaneously convectively heat the entire length of a selected widthwise portion of a glass sheet. A distribution manifold is arranged in fluid connection with the air source and each of the air manifolds. 
     A solenoid valve and flow meter are arranged in fluid connection between each air manifold and the distribution manifold. An air regulator and filter/dryer are arranged in fluid connection between the air source and the distribution manifold. A programmable computer is used to open and close the solenoids at predetermined times during the heating cycle. 
     The present invention also provides a method of heating glass sheet for subsequent processing such as tempering. The method comprises the initial steps of loading the glass sheet onto a longitudinally extending conveyor, and orienting the glass sheet such that the lengthwise edge of the sheet is parallel to the length of the conveyor, and conveying the glass sheet into a heating chamber. The glass sheets are then convectively heated in a specified sequence along the entire length of selected widthwise portions of the glass sheet by creating a downward flow of heated air proximate the selected widthwise portion of the glass sheet. 
     In a preferred embodiment, the lengthwise extending edge portions of the sheet are heated before the lengthwise central portion of the sheet. Preferably, the glass sheet is heated proximate its quarter points before the lengthwise central portion. 
     In a preferred embodiment, the heating step comprises constantly convectively heating only the quarter points of the sheet for the first 30-40% of the heating cycle; intermittently convectively heating only the quarter points of the sheet for the next 10-20% of the heating cycle; intermittently convectively heating only the lengthwise central portion of the sheet for the next 10-20% of the heating cycle; and, constantly convectively heating only the lengthwise central portion of the sheet for the final 20-50% of the heating cycle. 
     The method may include the step of transferring the glass sheet from the heating chamber to a second heating chamber of a two zone furnace. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic illustration of the semi-convective forced air system in accordance with an embodiment of the invention; 
     FIG. 2 is a fragmentary, enlarged side elevational view of an air manifold of the system of FIG. 2; 
     FIG. 3 is a bottom plan view of the air manifold of FIG. 2; 
     FIG. 4 is an enlarged, cross-sectional view of an air manifold of FIG. 1 shown relative to a glass sheet on a conveyor; 
     FIGS. 5 a  and  5   b  are schematic illustrations of a bank of air manifolds shown relative to a glass sheet on a conveyor in accordance with an embodiment of the invention; 
     FIGS. 6 a  and  6   b  are schematic illustrations of arrangements of air manifolds shown relative to a glass sheet on a conveyor in accordance with another embodiment of the invention; 
     FIG. 7 is a schematic illustration of the control system in accordance with an embodiment of the invention; 
     FIGS. 8 a-c  are schematic illustrations of arrangements of air manifolds shown relative to glass sheets in a two-zone oven in accordance with a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described with reference to FIGS. 1-7 wherein like reference numerals are used to designate the same components throughout. 
     The semi-convective forced air system of the present invention is schematically illustrated in FIG.  1  and designated generally by reference numeral  10 . The system  10  comprises a furnace with an internal heating chamber  14  in which glass sheets S are heated during a heating cycle in preparation for subsequent processing such as tempering, bending, filming, etc. The furnace housing  12  has a construction known in the art as taught by, for example, U.S. Pat. No. 4,390,359 owned by Tamglass Engineering, Cinnaminson, N.J., incorporated herein by reference. The furnace housing  12  is preferably made of a heat resistant ceramic material. The furnace includes electric resistance heating elements  16  on the top and bottom which provide radiant heat to a work piece located therein. 
     A longitudinal conveyor  18  extends lengthwise through the heating chamber  14 . The conveyor  18  preferably includes a series of rotatably fixed horizontally extending rolls  19  which are driven in unison to convey a work piece, such as glass sheeting S, through the chamber. A conveyor  18  of this type is well known in the art as taught, for example, by U.S. Pat. No. 4,390,359, owned by Tamglass Engineering, Cinnaminson, N.J., incorporated herein by reference. The system  10  has a plurality of longitudinally-extending air manifolds  20  which are arranged in fluid connection with a compressed air source  22  preferably located external to the heating chamber  14 . The air manifolds  20  are arranged parallel to the length of the longitudinal conveyor  18  and create a downward flow of heated air toward the conveyor  18  to convectively heat a sheet S of glass supported on the conveyor  18 . The convection heat provided by the air manifolds  20  supplements the radiant heat provided by the electric resistance elements  16 . 
     The compressed air source preferably includes a compressor  23  which is capable of supplying about 17 CFM at about 50 psi, which is the equivalent of about a 10 H.P. compressor for the largest system. The air source also preferably includes a 120 gallon stationary air tank  25 . The stationary tank has an automatic bottom drain  27  which relieves oil and water build-up from the tank  25 . 
     The system  10  includes a controller  24  which controls the flow of air through each of the plurality of air manifolds  20 . The controller  24  selectively restricts or allows a flow of air to each of the air manifolds  20 , or rows of manifolds, at predetermined times during the heating cycle to control the heating process and minimize oil canning and bubbling in the glass sheets. 
     The air manifolds are shown in greater detail in FIGS. 2-4. In a preferred embodiment, each manifold comprises a pair of elongate tubes  26  connected at one end by a hollow “T” connector  28 . The other ends of the elongate tubes  26  are sealed with a cap, plug or other means. 
     Each elongate tube  26  has a longitudinal series of radially extending apertures  30 . Preferably, the apertures  30  have a diameter of about 0.04 inches and are spaced about 8½″ from each other along the entire length of the elongate tubes  26 . The elongate tubes preferably comprise ½″ schedule  40 , type  304  stainless steel hollow pipe. 
     Referring to FIG. 4, the apertures traverse the walls of the tubes  26  and are oriented downwardly towards the conveyor  18  at an angle theta θ from vertical. Preferably, the angle theta θ is plus or minus 30 degrees from vertical. The angle theta θ is selected to create a compromise wash effect and turbulent effect on the glass sheet. Air is discharged from the apertures (depicted by arrows) and impinges the sheet S at angle of incidence alpha α of about plus or minus 60 degrees. 
     As best seen in FIG. 3, the apertures  30  are alternatively oriented in opposite directions. For example, the first, third, fifth, . . . aperture on the air manifolds are oriented at an angle theta θ of plus 30 degrees and direct air towards one side of the glass sheet; the second, fourth, sixth, . . . apertures are oriented at an angle theta θ of minus 30 degrees and direct air toward the opposite side of the glass sheet. 
     Each air manifold  20  includes a supply tube  32  which is connected at one end to the third port of the “T” connector  28  and at the other end to a distribution manifold  34 . The distribution manifold  34  is arranged in fluid connection with the compressed air source  22  and distributes compressed air to each of the air manifolds  20 . 
     A solenoid valve  36  and a flow meter  38  are arranged in fluid connection between the distribution manifold  34  and each of the air manifolds  20 . Each solenoid valve is connected to a controller  24  which selectively opens and closes each solenoid valve at different times during a heating cycle. Each flow meter  38  controls the volume of air entering the respective air manifolds  20 . Preferably, each flow meter  38  comprises a Dwyer Rate Master Flowmeter, model No. RMC-104-BV having ½ NPT connections and is set at a flow rate of 200 standard cubic feet per hour. Preferably, the solenoid valves comprise Asco two way solenoid valves, model No. 8210C94 having ½ NPT connections and ⅝″ orifice with a maximum operating pressure differential of 100 psi. The controller is preferably a programmable logic computer which is well known in the art. 
     A filter/dryer  40 , air regulator  42  and solenoid valve  44  are arranged in fluid connection intermediate the compressed air source  22  and the distribution manifold  34 . Preferably the filter/dryer  40  comprises a 40 micron filter manufactured by ARO, part number F25242-111 and a coalescing filter manufactured by ARO, part number F25242-311; the air regulator is preferably manufactured by ARO, part number R27241-100 and the pressure gauge is manufactured by ARO, part number 100067; and, the solenoid valve  44  is manufactured by Burkert, part number 453058. 
     The air manifolds  20  are arranged in banks. The air manifolds are capable of providing forced air convective heating over the full length of a glass sheet S during the entire time which the glass sheet S is being heated. However, as described below, typically convective heating is sequentially provided over the entire length of selected widthwise portions of the glass sheet. 
     The system may be used in a batch type furnace or in a continuous furnace during the heating period only. In a continuous furnace, the air manifolds  20  would not extend over the full length of the continuous system. In a preferred embodiment, ambient compressed air is supplied to the air manifolds  20 ; however, heated compressed air may also be supplied to the air manifolds  20  in the system of the present invention. 
     A preferred location of each manifold is illustrated for two different furnaces in FIGS. 5 and 6. FIGS. 5 a  and  5   b  shows the location of the air manifolds in a 48″-60″ furnace. In this embodiment, the air manifolds have a widthwise spacing W of about 15″ and are located at a height H of about 4″-6″ above the surface of the glass sheet S. Referring to FIG. 5 b,  in a preferred embodiment, the bank of air manifolds  20  comprises three longitudinally extending manifolds. The approximate location of each air manifold above the glass sheet S is illustrated in FIG. 5 b  and shows that the entire glass sheet S or the entire length of a selected widthwise portion of a glass sheet S can be heated by the air manifolds. 
     Preferably, one manifold is located proximate the widthwise center of the sheets, and one manifold is located proximate each widthwise quarter point i.e., the location ¼ the width of the sheet from each edge. For example, if the sheet is 36″ wide, a manifold is  210  preferably located 9 inches from each lengthwise edge of the sheet. 
     The location of the air manifolds  20  for an a 72″ or 86″ or 96″ furnace is shown in FIGS. 6 a  and  6   b.  Preferably, this arrangement comprises two separate banks of air manifolds  20  for simultaneously heating a first glass sheet S 1  and a second glass sheet S 2 . The air manifolds  20  have a widthwise spacing W and a heightwise spacing H similar to the spacing of the 60″ furnace described above. In this embodiment, each bank comprises three longitudinally extending manifolds. The furnace may also have a seventh air manifold  20   c  located proximate the center of the furnace for use in heating a single, large glass sheet. 
     During the heating process, the controller  24  restricts or allows the flow of air to selected manifolds at predetermined times during the heating cycle. In the method of the present invention, the entire length of selected widthwise portions of the glass sheet is convectively heated in a specific sequence by controlling the flow of air to selected air manifolds  20 . 
     For example, as described above, when glass sheets are conveyed into a heating furnace, the bottom surface heats up and expands faster than the top surface due to contact with the rolls of the conveyor. As a result, the glass sheet curls up on the lengthwise outer edges. Therefore, in a preferred embodiment, the manifolds proximate the lengthwise edges of the glass sheet are initially turned on to create top side convective heating of the glass sheet edges to prevent the sheet from curling up. Later in the heating cycle, the manifolds proximate the lengthwise outer edge of the glass sheet are turned off and the manifold proximate the center of the glass sheet is turned on to provide convective heating of the center portion of the sheet. By using this general technique, the glass sheet can be heated more uniformly to mitigate oil canning and bubbling. 
     The method of the present invention can be used on small (for example, 20″×20″) or large (for example, 34″×76″) sheets of glass. However, the improved results of the present invention are most noticeable in large pieces of glass since small pieces of glass do not generally tend to exhibit oil canning and bubbling. 
     The actual amount of time during which convective heating is applied to a glass sheet by each of the manifolds will vary depending on the coating emissivity of the glass sheet. Convective heating is preferably only intermittently applied during the transition phase of the heating cycle where convective heat is moved from one part of the glass to another. Preferably, intermittent heating is done by time proportioning up or down the supply of air to a selected row of manifolds. During time proportioning, air is supplied to the selected manifolds for an ascending or descending amount of time per time interval. For example, during time proportioning up, air is supplied to the manifolds for 6 seconds out of every 10 second interval, then for 7 seconds out of every 10 second interval, . . . then for 10 seconds out of every 10 second interval. 
     EXAMPLE 
     A glass sheet measuring 30″×75″ may be heated in a single zone furnace having the manifold arrangement shown in FIGS. 5 a  and  5   b  using the following sequence steps: 
     1) Constantly heat the longitudinal edges of the sheet by supplying full air flow to the rows of manifolds above the quarter points for the first 30-40% of the heating cycle. During this time, air flow to the center row of manifolds is restricted; 
     2) Intermittently heat the edges of the sheet by time proportioning down air flow to rows above the quarter points for the next 10-20% of the heating cycle. Then, restrict all air flow to the rows above the quarter points; 
     3) Intermittently heat the center portion of the sheet by time proportioning up air flow to the center row manifolds for the next 10-20% of the heating cycle; and, 
     4) Constantly heat the center of the sheet by supplying full air flow the center row manifolds. 
     The system and method of the present invention may be used in a single zone or two zone ( 21 ,  22 ) furnace. In a two zone furnace, each zone would preferably have a bank of manifolds, although the arrangement of the banks would not necessarily be the same. For example, the location of the air manifolds in a 48″-60″, two-zone ( 21 , 22 ) oven is shown in FIG. 8 a.  The location of the air manifolds in a 72″ or 86″ or 96″, two-zone oven arranged for simultaneously heating two glass sheets is shown in FIG. 8 b.  The location of the air manifolds in a 72″ or 86″ or 96″, two-zone oven arranged for heating a single large glass sheet is shown in FIG. 8 c.