Patent Publication Number: US-2009217704-A1

Title: Apparatus for manufacturing sheet glass

Description:
This application is a continuation of U.S. application Ser. No. 11/030,294, filed Jan. 7, 2005 which is a continuation of PCT/JP03/08531, filed Jul. 4, 2003, which claims the benefit of Japanese Patent Application No. 2002-198946 filed on Jul. 8, 2002 including specification, claims, drawings and summary is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an apparatus for manufacturing sheet glass, in particular an apparatus for manufacturing glass into a thin sheet, the glass having a high devitrification temperature. 
     2. Background Art 
     As a process for manufacturing thin sheet glass having high quality, there has been known a process called an overflow fusion process. 
     The overflow fusion process is a process wherein a glass ribbon is formed by converging streams of molten glass just under a lower converged edge portion of a forming body having a wedge-shaped form converging downwardly in section while flowing down the streams of molten glass along both lateral sides of the forming body, and wherein the glass ribbon is formed into thin sheet glass by downwardly pulling the glass ribbon so as to remove distortion remaining in the glass ribbon while gradually cooling the glass ribbon. 
     Such a forming process needs to complete forming of the glass ribbon just after the streams of glass has converged just under the lower edge portion of the forming body having a wedge-shaped form in section and to prevent the glass ribbon from being deformed after completion of forming of the glass ribbon. For this reason, it has been difficult to obtain sheet glass having high precision if the glass has too high or low a viscosity in the vicinity of the lower edge portion of the forming body having a wedge-shaped form in section. In the case of forming by the conventional overflow fusion process, thin sheet glass has been manufactured such that the glass has a viscosity in a range from 10 30,000 to 1,000,000 poises at the lower edge portion of the forming body. 
     Since the glass viscosity of a glass ribbon has limits in the vicinity of the lower edge portion of the forming body having a wedge-shaped form in section when the glass ribbon is formed by the overflow fusion process as explained, the viscosity at the devitrification temperature of glass capable of being formed (i.e., the viscosity at a temperature that glass is devitrified) has been restricted. Specifically, glass, which starts being devitrified at a low viscosity (which has a high devitrification temperature), such as glass having a viscosity of less than 30,000 poises at the devitrification temperature, has not been able to be formed without being inherently devitrified. Supposing that glass having a viscosity of less than 30,000 poises, in particular from 10,000 to 20,000 poises at the devitrification temperature, is formed by the overflow fusion process, devitrification might be avoided. However, the glass ribbon is abruptly extended by gravity applied to the glass ribbon per se or a force downwardly pulling the glass ribbon since the glass ribbon has a low viscosity in the vicinity of the lower edge portion of the forming body. As a result, it is impossible to form the glass ribbon so as to have a uniform thickness since the glass ribbon is broken at an intermediate portion thereof or has too thin a thickness even if the glass ribbon is not broken. From this viewpoint, manufacturing apparatuses employing the conventional overflow fusion process have a drawback that it is impossible to form special glass having a high devitrification temperature, such as a substrate for a flat panel display or information recording medium. 
     Additionally, the conventional overflow fusion process has needed to downwardly pull a glass ribbon in a vertical direction until the glass ribbon has been completely solidified and be cut, which has required a tall building and an operation space extending long in a vertical direction. Further, the conventional overflow fusion process has a drawback that the distortion remaining in glass is removed in an insufficient way since it is impossible to have a sufficient annealing time because of a limitation to the height of the building. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been proposed in consideration of such circumstances. It is ail object of the present invention to provide an apparatus for manufacturing thin sheet glass which is capable of drastically easing known limitations to the viscosity of glass that glass, which is likely to be devitrified, cannot be formed by the overflow fusion process, and which is capable of easily forming even special glass having a high devitrification temperature, such as glass to be used as—a substrate for a flat panel display or information recording medium. 
     It is another object of the present invention to provide an apparatus for manufacturing thin sheet glass, which is capable of reliably removing distortion remaining in glass since it is possible to ensure a sufficient annealing time without need of a tall building and an operation space extending long in a vertical direction. 
     In order to attain the objects, the present invention provides an apparatus for manufacturing thin sheet glass, comprising a forming body including a main body having a cross-sectional shape converging downwardly, the main body being configured to converging streams of molten glass into a single glass ribbon at a lower converged edge portion thereof, the streams of the molten glass flowing down along both surface of the main body; and edge members, the edge members being configured to restrict a width of the molten glass, wherein the glass ribbon formed by the forming body is downwardly pulled to form thin sheet glass; characterized in that the apparatus further comprises a non-contact support member disposed in the vicinity of the lower converged edge portion of the main body, the non-contact support member being configured to form a thin gas layer on a supporting surface thereof, wherein the glass ribbon is supported over an entire width thereof in a non-contact way by the non-contact support member in a course wherein the glass ribbon is downwardly pulled. 
     In accordance with the present invention, it is possible to reduce a force applied to a low viscous portion of the glass ribbon in the vicinity of the lower edge portion of the forming body since the glass ribbon, which has been formed by the forming body according to the overflow fusion process, is pulled down via the noncontact support member, which supports the glass ribbon in a non-contact state through the thin gas layer. By this arrangement, even if glass, which has a viscosity of less than 30,000 poises at the devitrification temperature, is formed according to the overflow fusion process, the glass ribbon can be prevented from being abruptly extended by the own weight or a pulling force. Additionally, it is possible to prevent the glass ribbon from being abruptly cooled or being damaged since the glass ribbon is supported through the thin gas layer having a small thermal conduction. 
     In one mode of the present invention, the molten glass has a viscosity of less than 30,000 poises at the lower edge portion of the main body. Thus, the present invention can form even special glass having a high devitrification temperature (having a viscosity of less than 30,000 poises at the devitrification temperature), such as glass to be used as a substrate for a flat panel display or information recording medium, since it is possible to apply the overflow fusion process without limitations to the viscosity of the glass ribbon. 
     The non-contact support member in one mode of the present invention comprises a rotary roller, the rotary roller including a roller base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, wherein a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state into the roller base member, wherein the vapor-film forming agent is evaporated by high heat from the glass ribbon, and wherein the glass ribbon is supported through a thin gas layer, which is formed from an evaporated portion of the vapor-film forming agent. By employing the rotary roller as the non-contact support member, the entire width of the pulled-down glass ribbon is supported by an upper portion of the peripheral surface of the rotary roller, and then the glass ribbon slides down along the peripheral surface of the rotary S roller and is pulled in a downward direction again or is pulled in a horizontal direction. By this arrangement, the glass ribbon can smoothly move without staying on the non-contact support member. In this case, the rotary roller may be rotated in a direction opposite to the direction to pull the glass ribbon. 
     The non-contact support member in another mode of the present invention comprises an inclined plate, the inclined plate including a plate base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, wherein a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state into the plate base member, wherein the vapor-film forming agent is evaporated by high heat from the glass ribbon, wherein the glass ribbon is supported through a thin gas layer, which is formed from the evaporated portion of the vapor-film forming agent, and wherein the inclined plate has a supporting surface inclined so as not to be perpendicular to the direction to pull the glass ribbon. By employing the inclined plate as the non-contact support member, the pulled-down glass ribbon is supported by the inclined plate, and then the glass ribbon slides down along the inclined plate and is pulled in a downward direction again or is pulled in a horizontal direction. By this arrangement, the glass ribbon can smoothly move without staying on the noncontact support member. 
     As another mode of the present invention, the apparatus may comprise a non-contact horizontally pulling device, the non-contact horizontally pulling device being configured to horizontally pull, in a non-contact state through a thin gas layer, the glass ribbon, which is pulled down via the non-contact support member. By this arrangement, the horizontally pulled glass ribbon has one side brought into contact with air and the other side brought into contact with the thin gas layer formed by the non-contact support member. Even if the glass ribbon is horizontally pulled, the glass ribbon can be gradually cooled since the air and the thin gas layer both have a small thermal conduction. Accordingly, it is possible to reliably remove distortion remaining in the glass since it is possible to ensure a sufficient annealing time without need of a tall building and an operation space extending long in a vertical direction as in the prior art employing the overflow fusion process. 
     The non-contact horizontally pulling device in a specific preferred mode of the present invention comprises a direction-changing roller and a conveying supporter. The direction-changing roller includes a roller base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, wherein a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state into the roller base member, wherein the vapor-film forming agent is evaporated by high heat from the glass ribbon, and wherein the pulled-down glass ribbon is turned in a horizontal direction by the direction changing roller through a thin gas layer, which is formed from an evaporated portion of the vapor-film forming agent. The conveying supporter includes a supporter base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, wherein a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state into the support base member, wherein the vapor-film forming agent is evaporated by high heat from the glass ribbon, and wherein the glass ribbon, which has been turned by the direction-changing roller, is conveyed and supported by the conveying supporter through a thin gas layer, which is formed from the evaporated portion of the vapor-film forming agent. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of the entire structure of an apparatus for manufacturing sheet glass, according to the present invention; 
         FIG. 2  is a front view of a forming body; 
         FIG. 3  is a perspective view of the forming body; 
         FIG. 4  is a schematic view of a rotary roller, which is an embodiment of a non-contact support member; 
         FIG. 5  is a cross-sectional view of the rotary roller; 
         FIG. 6  is a schematic view of an inclined plate, which is another embodiment of the non-contact support member; and 
         FIG. 7  is a schematic view of an embodiment of a conveying support member. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Now, preferred embodiments of the apparatus for manufacturing thin sheet glass, according to the present invention, will be described in detail in reference to the accompanying drawings. 
       FIG. 1  is a schematic view of the entire structure of an apparatus for manufacturing thin sheet glass, according to the present invention,  FIG. 2  is a front view of a forming body, and  FIG. 3  is a cross-sectional view of the forming body. 
     The apparatus for manufacturing thin sheet glass  10 , s according to the present invention comprises the forming body  12 ; a non-contact support member  14  for supporting a glass ribbon GR over the entire width in a non-contact state, the glass ribbon being pulled down from the forming body  12 ; and a horizontally pulling device  16  for horizontally pulling the downwardly pulled glass ribbon GR in a non-contact state. 
     As shown in  FIG. 2  and  FIG. 3 , the forming body  12  comprises a main body  18  having a wedge-shaped form converging downwardly in section, and lateral end members  20  and  21  disposed both side edges of the main body  18  to restrict the width of molten glass G. The lateral member  20  has a molten glass introduction hole  22  formed therein. Although not shown in the drawings, the introduction hole  22  is connected to a melting furnace with refined molten glass G contained therein, and the introduction hole is used for supplying the molten glass G from the melting furnace to a channel  24  formed in the main body  18 . 
     The molten glass G supplied into the channel  24  flows along the channel  24  from a left side to a right side in  FIG. 3  and overflows upper edges  26 . The molten glass G that has overflowed flows downward along both surfaces  28  of the main body  18  as shown in  FIG. 4 , and streams of the molten glass converge into a ribbon-shaped glass ribbon GR at a lower edge portion  30 . A downwardly pulling force is applied to the glass ribbon GR by conveying driving rollers  60  stated later, whereby the glass ribbon GR is formed into desired thin sheet glass. The main body  18  is made of, e.g., a refractory material, such as an alumina material or a zirconia material, or a refractory material coated with refractory metal, such as platinum or a platinum alloy. 
     The non-contact support member  14  may have any structure as long as the support member can support the glass ribbon GR over the entire width in a non-contact state through a thin gas layer  32  and as long as the glass ribbon GR can downwardly move in a smooth way without staying on the non-contact support member  14 . For example, a rotary roller  34  as shown in  FIG. 4 , or an inclined plate  36  as shown in  FIG. 6  may be advantageously applicable. 
       FIG. 4  shows a case wherein the non-contact support member  14  comprises the rotary roller  34 . The rotary roller  34  is configured so that the roller includes a roller base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, that a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature (about 20 to 30° C.) but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state, that the vapor-film forming agent is evaporated by high heat from the glass ribbon GR, and that the glass ribbon GR is supported through a thin gas layer, which comprises a vapor film formed from an evaporated portion of the vapor-film forming agent. As shown in  FIG. 5 , the rotary roller  34  has a peripheral portion  37  of a roller cylinder made of such a roller base member capable of containing the vapor-film forming agent therein, and both roller edge portions  38  and  38  are made from a base material with no vapor-film forming agent contained therein. The vapor-film forming agent introduced into the roller base member is evaporated from a roller surface  40  by high heat from the glass ribbon GR. The rotary roller  34  is rotated by an unshown electric motor. By this arrangement, the glass ribbon GR can be supported in such a state that the thin glass layer  32  made of the vapor film is stably formed between the glass ribbon GR and the rotary roller  34 . Although the rotary roller  34  may be rotated in either direction, it is more preferred that the rotary roller be rotated in the direction opposite to the direction to pull down the glass ribbon GR. 
     As the method for supplying the vapor-film forming agent to the roller base member, the vapor-film forming agent may be spread into the entire roller base member by being supplied in a central hollow portion  46  defined between a rotary shaft  44  of the rotary roller  34  and the peripheral portion  37 , or a wet roller (not shown) may be disposed so as to have contact with a side of the rotary roller  34  remote from the glass ribbon GR so that the vapor-film forming agent supplied to the wet roller is transferred onto the rotary roller. It is also acceptable to adopt a spray system wherein the vapor-film forming agent is sprayed on a surface of the rotary roller  34  by a nozzle. The point is that any supply method is acceptable as long as it is possible to supply the vapor-film forming agent so that the vapor-film forming agent can be sufficiently contained in the roller base member of the rotary roller  34 . 
     The roller base member is made of such a base material capable of containing a liquid therein or has such a structure capable of containing a liquid therein. For example, a porous member or a fibrous material is advantageously employed. In the case of a porous member, it is preferred that the porous member have through holes. The porous material has a surface formed with fine holes, which have a hole diameter of preferably 5 mm or below, more preferably 1 mm or below, or further preferably 100 μm or below. It is preferred that the roller base member be made of a material having a high affinity with the vapor-film forming agent. 
     As the basic material of the roller base member, porous hydrophilic carbon is particularly appropriate. Other materials, e.g., a polymer material derived from a natural product, such as cellulose, paper, wood or bamboo, a synthetic polymer material, such as a thermoplastic resin, a thermosetting resin or rubber, and a carbon material are advantageously applicable. A metal material, such as iron, stainless steel or platinum, a metal oxide, such as an aluminum oxide, a zirconium oxide, silicon carbide or silicon nitride, metal carbide or metal nitride are also applicable. 
     As the vapor-film forming agent, it is possible to employ various kinds of substances, which is an organic substance or inorganic substance that is liquid at room temperature and a gas at a temperature of not lower than the glass transformation point. From the viewpoint of supply operation to the forming body  12 , it is preferred that the vapor-film forming agent have a melting point of not higher than 40° C. and a boiling point of from 50 to 500° C., more preferably not higher than 300° C., at atmospheric pressure. It is additionally preferred that the vapor evaporated from the vapor-film forming agent do not react chemically with the glass or the rotary roller to such a degree to have an adverse effect on the glass or the rotary roller, and that the vapor have a low toxicity and be incombustible at a service temperature. Water may be employed as a typical example of the vapor-film forming agent. As explained, it is necessary to employ, as the vapor-film forming agent, a liquid that can be instantly evaporated by high heat from the glass ribbon GR to form a stable vapor film. Since the thin glass layer  32  of the vapor film that is formed from a liquid instantly evaporated by high heat has a s significantly lower thermal conductivity than a liquid or a solid material, the thin glass layer can effectively form a heat-insulated circumference for the glass ribbon GR. 
       FIG. 6  shows a case wherein the non-contact support member  14  comprises the inclined plate  36 . The inclined plate  36  is configured so that the inclined plate includes a plate base member, which is made of a base material capable of containing a liquid therein or has a structure capable of containing a liquid therein, that a vapor-film forming agent, which is not in a gas state at a temperature in the vicinity of room temperature but is in a gas state at a temperature of not lower than the glass transformation point, is introduced in a liquid state, that the vapor-film forming agent is evaporated by high heat from the glass ribbon GR, that the glass ribbon GR is supported through a thin gas layer  32 , which comprise a vapor-film formed from an evaporated portion of the vapor-film forming agent, and that the inclined plate has a supporting surface  36 A inclined so as not to be perpendicular to the direction to pull the glass ribbon GR. The plate base member forming the inclined plate  36 , the basic material of the plate base member, the vapor-film forming agent, and the method for providing the inclined plate  36  with the vapor-film forming agent are similar to those in the rotary roller  34  stated earlier. By this arrangement, since the vapors film forming agent introduced into the plate base member is evaporated from the inclined plate  36  by high heat from the glass ribbon GR, the glass ribbon GR can be supported in such a state that the thin glass layer  34  of the vapor film is stably formed between the glass ribbon GR and the inclined plate  36 . In connection with to what degree the inclined plate  36  should be inclined with respect to the horizontal direction, it is sufficient that the inclined plate is inclined at such an angle that the glass ribbon GR supported by the inclined plate  36  can smoothly move without staying on the inclined plate. In this case, it is preferred to make the inclination angle of the inclined plate  36  variable according to a viscosity and a thickness since whether or not the glass ribbon GR can smoothly move on the inclined plate or not is affected by the viscosity and the thickness of the glass ribbon GR. It is preferred from the viewpoint of easily supporting the glass ribbon GR that the supporting surface  36 A of the inclined plate that supports the glass ribbon GR is curved in a concave shape. 
     As shown in  FIG. 1 , the non-contact horizontally pulling device  16  is a device wherein the glass ribbon GR, which is pulled down via the non-contact support member  14 , is horizontally pulled in a non-contact state through a thin glass layer  32 . For example, the non-contact horizontally pulling device comprises a direction changing roller  48  for directing the pulled-down glass ribbon GR to a horizontal direction in a non-contact state, and a conveying supporter  50  for conveying and supporting the horizontally-pulled glass ribbon GR in a non-contact state. Since the structure of the direction changing roller  48  is similar to that of the rotary roller  34  stated earlier, explanation of the direction changing roller will be omitted. 
     The conveying supporter  50  of  FIG. 1  is of fixed bed type. In the fixed bed type conveying supporter SO, a plurality of rectangular supporting members  52  are disposed so as not to be movable at least in a direction parallel with the direction to convey the glass ribbon GR, and adjacent supporting members  52  have a groove  54  formed therebetween so as to escape vapor evaporated from the vapor-film forming agent. Each of the grooves  54  is important for releasing vapor of the thin glass layer  32 , which comprises a vapor film formed between the supporting members  52  and the glass ribbon GR. By this arrangement, the thin glass layer  32  of a vapor film can be stably formed between the glass ribbon GR and the supporting members  52 . The respective supporting members S 2  are supported by a base  58  through posts  56 . The supporter base member forming each of the supporting member  52 , the basic material of the supporter base member, the vapor-film forming agent, and the method for providing the vapor-film forming agent to the supporting members are similar to those of the rotary roller  34  stated earlier. The pulling tension that pulls the glass ribbon GR above the supporting members  52  in the conveying direction is created by contact resistance between the plural conveying driving rollers  60  and the glass ribbon GR. 
     The conveying supporter  50  of  FIG. 7  is of conveyor type, wherein a plurality of supporting members  52  are fixedly disposed in a longitudinal direction at intervals on a surface of an endless belt  62  of a belt conveyor  61 . Adjacent supporting members  52  have a groove  54  formed 15 therebetween so as to extend in a direction perpendicular to the direction to convey the glass ribbon GR. The endless belt  62  extends between a pair of rollers comprising a driving roller  64  and a driven roller  66 , and the endless belt moves in a clockwise direction  68  or in a counterclockwise direction  70  in  FIG. 7 . The moving speed of the endless belt  62  is set so as to be different from the conveying speed of the glass ribbon GR above the supporting members  52 . By this arrangement, the supporting members  52  and the glass ribbon GR slide through a thin glass layer  32  made of a vapor film. The belt conveyor  62  is provided with guide plates  72  for guiding the endless belt  62  on an upper moving path, and a portion of the endless belt  62 , which is moving in the upper moving path, moves in a stable way, being guided by the guide plates  72 . The supporter base member forming each of the supporting members  52 , the basic material of the supporter base member and the vapor-film forming agent are similar to those of the rotary roller  34  stated earlier. In the case of this conveyor type, the endless belt  62  has a supply vessel  74  for the vapor-film forming agent provided in a lower moving path. When supporting members  52  are moving through the lower moving path, the supporting members  52  are supplied with the vapor-film forming agent, passing through the supply vessel  74 . 
     Explanation will be made with respect to a case wherein glass having a high devitrification temperature (having a viscosity of less than 30,000 poises, in particular from 10,000 to 20,000 poises) is formed by the apparatus for forming thin sheet glass  10 , thus constructed. 
     After the glass ribbon GR, which has been formed at the lower edge portion  30  of the forming body  12 , is pulled down via the non-contact support member  14 , the glass ribbon is pulled in a non-contact state and in a horizontal direction by the non-contact horizontally pulling device  16  to be formed into thin sheet glass. 
     In the forming of the thin sheet glass, the glass ribbon GR, which has been formed by the forming body  12 , is supported over the entire width in a non-contact state through the thin glass layer  32 . Thus, as shown in  FIG. 4  and  FIG. 6 , the own weight of a portion GR 1  of the glass ribbon upstream from the non-contact support member  14  is supported by the non-contact support member  14 , and the own weight of a portion GR 2  of the glass ribbon downstream from the non-contact support member and the pulling force are restricted from being applied to the upstream portion GR 1  of the glass ribbon. As a result, the forces that are applied to portions of the glass ribbon having a low viscosity, in the vicinity of the lower edge portion of the forming body  12  are minimized. Even if glass that has a viscosity of less than 30,000 poises at the lower edge portion of the forming body  12  is formed by the overflow fusion process, the glass ribbon GR is prevented from being abruptly extended by the own weight or the pulling force. Although the glass ribbon GR needs to be pulled down, being gradually cooled, in order to remove distortion in the glass ribbon GR with good precision, the present invention can prevent the glass ribbon GR from being abruptly cooled since the glass ribbon GR is supported through the thin glass layer  32  having a small thermal conduction. Additionally, the glass ribbon GR cannot be damaged since the glass ribbon GR is supported over the entire width through the thin glass layer  32  to prevent the glass ribbon GR from being brought in direct contact with the non-contact support member  14 . 
     Additionally, the apparatus is configured so that even when the glass ribbon GR is horizontally pulled, the glass ribbon GR, which is pulled down via the non-contact support member  14 , is turned by the direction-changing roller  48  through the thin glass layer  32  having a small thermal conduction, and the glass ribbon GR thus turned is conveyed arid supported by the conveying supporter  50  through the thin glass layer  32  having a small thermal conduction. By this arrangement, it is possible to remove distortion in the glass ribbon GR with good precision since the glass ribbon GR can be gradually cooled while the glass ribbon GR is horizontally pulled. 
     INDUSTRIAL APPLICABILITY 
     In accordance with the apparatus for forming thin sheet glass, of the present invention, it is possible to drastically ease known limitations to the viscosity of glass that glass, which is likely to be devitrified, cannot be formed by the overflow fusion process, and to easily form even special glass having a high devitrification temperature, such as glass to be used as a substrate for a flat panel display or information recording medium. Additionally, in accordance with the present invention, it is possible to reliably remove distortion remaining in glass since it is possible to ensure a sufficient annealing time without need of a tall building and an operation space extending long in a vertical direction.