Abstract:
A vacuum degassing apparatus for molten glass includes a vacuum housing which is evacuated to be depressurized therein; a vacuum degassing vessel which is provided in the vacuum housing to vacuum-degas molten glass as the molten glass flows therein; an uprising pipe which connects to the vacuum degassing vessel, and sucks and draws up undegassed molten glass to introduce the undegassed molten glass into the vacuum degassing vessel; and a downfalling pipe which connects to the vacuum degassing vessel and draws down the degassed molten glass from the vacuum degassing vessel to discharge the degassed molten glass. The cross sectional area of the path at the upper end portion of the uprising pipe is larger than the cross sectional area of the path at the lower end portion of the uprising pipe.

Description:
TITLE OF THE INVENTION 
   The present application is a Divisional application of U.S. patent application Ser. No. 10/256,014, now U.S. Pat. No. 7,007,514, filed on Sep. 27, 2002, the entirety of which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a vacuum degassing apparatus for molten glass, which removes bubbles from molten glass continuously supplied. 
   In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass that has been molten in a melting tank is formed by a forming apparatus, as shown in  FIG. 3 . 
   The vacuum degassing apparatus  110  shown in  FIG. 3  is used in a process wherein molten glass G in a melting vessel  112  is vacuum-degassed and is continuously supplied to a subsequent treatment vessel. In the vacuum degassing apparatus  110  are provided a vacuum housing  114  which is evacuated to be depressurized therein for vacuum-degassing the molten glass, a vacuum degassing vessel  116  which is provided in the vacuum housing  114  and is depressurized together with the vacuum housing, and an uprising pipe  118  and a downfalling pipe  120  which are connected to respective end portions of the vacuum degassing vessel in a downward and vertical direction. The uprising pipe  118  has a lower end immersed in the molten glass G in an upstream pit  122  in communication with the melting vessel  112 . Likewise, the downfalling pipe  120  has a lower end immersed in the molten glass G in a downstream pit  124  in communication with the subsequent treatment vessel (not shown). 
   The vacuum degassing vessel  116  is substantially horizontally provided in the vacuum housing  114  which is evacuated through a suction port  114   c  by a vacuum pump, not shown, to be depressurized therein. Since the inside of the vacuum degassing vessel  116  is depressurized, through suction ports  116   a  and  116   b  in communication with the inside of the vacuum housing  114 , to a pressure of 1/20-⅓ atm together with the inside of the vacuum housing  114 , the molten glass G in the upstream pit  122  before degassing is sucked and drawn up through the uprising pipe  118 , and is introduced into the vacuum degassing vessel  116 . Then, the molten glass is vacuum-degassed as it flows through the vacuum degassing vessel  116 , and the molten glass is drawn down by the downfalling pipe  120  to be discharged into the downstream pit  124 . 
   The vacuum housing  114  may be a casing made of metal, such as stainless steel and heat-resisting steel. The vacuum housing is evacuated from outside by e.g. a vacuum pump (not shown) and the inside is depressurized, so that the inside of the vacuum degassing vessel  116  provided therein is depressurized and maintained under a prescribed pressure, e.g. under a pressure of 1/20-⅓ atm. In the vacuum degassing vessel  116 , an upper space  116   s  is formed above the molten glass G filled to a certain depth in the vacuum degassing vessel. The upper space  116   s  is a space depressurized by a vacuum pump (not shown) so that gas components from bubbles which have risen to the surface of the molten glass G and broken up, are sucked from the upper space being the depressurized space, through a suction port  114   c  by a vacuum pump (not shown). Therefore, the larger the area of the molten glass G in contact with the upper space  116   s  is, the more remarkable the vacuum degassing effect becomes. 
   Around the vacuum degassing vessel  116 , the uprising pipe  118  and the downfalling pipe  120  in the vacuum housing  114  is provided thermal insulation material  126 , such as refractory bricks, to cover these members for thermal insulation. 
   Further, with a conventional vacuum degassing apparatus  110  as illustrated in  FIG. 3 , it is conceivable to enlarge the apparatus in an attempt to increase the flow rate i.e. the throughput capacity for degassing by constituting the vacuum degassing vessel  116  by dense refractory bricks, particularly by electro-cast refractory bricks, as disclosed in JP-A-11-240725 filed by the present applicant. 
   However, in order to increase the flow rate of the molten glass and to perform the desired vacuum degassing treatment, it is necessary to increase the width and the total length (namely, the bottom area) of the vacuum degassing vessel  116 , and the diameters of the uprising pipe  118  and the downfalling pipe  120 , by taking into consideration, changes in various factors (for example, a change in the flow rate of the molten glass G to be degassed, a change in the concentration of gas components dissolved in the molten glass G due to a temperature drop of the molten glass G in the melting furnace, or a change in the pressure in the vacuum degassing vessel which is depressurized). 
   However, by increasing the width and the total length of the vacuum degassing vessel  116 , and the diameters of paths in the uprising pipe  118  and the downfalling pipe  120 , the apparatus will be large-sized, and necessary refractory bricks, etc. will inevitably increase, thus leading to a problem of an increase of the costs. 
   Further, when the number of bubbles contained in the molten glass G rapidly increase, there will be a problem such that non-removed bubbles will remain in the molten glass G, which will flow into the downfalling pipe, and the bubbles are likely to remain in the glass as a product. Further, due to the increase of the number of bubbles, unbroken bubbles may build up on the surface of the molten glass G, and stick to the ceiling of the vacuum degassing vessel  116 . Consequently, a volatile matter present at the ceiling, which has been solidified in the form of crystals, may be included in the molten glass G. As a result, small opaque matters may remain in the glass product and form defects which are so-called “stones”. Further, even if the volatile matter is dissolved in the high temperature molten glass G, it will not be diffused uniformly in the molten glass G, and consequently, the molten glass G may have local changes in the composition. Due to the changes, the product glass obtained from the molten glass G, may have local changes in the refractive index and the see-through image of the glass may be distorted, which is so-called deterioration of leam. 
   Further, in order to increase the bottom area of the path in the vacuum degassing vessel  116 , a method of increasing total length of the path of the vacuum degassing vessel  116  may be conceivable. However, there is a problem such that as the size of the apparatus increases, the apparatus becomes long as compared with the melting vessel  112 . Consequently, it becomes necessary to change the relative position between the melting vessel  112  being the existing facility, and the downstream pit  124 , whereby there will be a demerit that the existing facility can not effectively be utilized. Further, if the vacuum degassing vessel is made linearly long, the expansion of the vacuum degassing vessel  116  by a heat, increases in proportion thereto, and there will be a change in the center to center distance between the uprising pipe  118  and the downfalling pipe  120 , which creates a distortion of the apparatus and thus may deteriorate the safety. 
   Otherwise, in order to increase the bottom area of the vacuum degassing vessel, a method of increasing the width of the path may also be conceivable. However, it is difficult to sufficiently improve the vacuum degassing performance only by increasing the width of the path. The reason will be described with reference to  FIGS. 8 and 9 .  FIG. 8  is a schematic cross-sectional view of the vacuum degassing apparatus  110  taken along line B-B′ in  FIG. 3 , and illustrates the cross-sectional shape of the path for molten glass in the vacuum degassing vessel  116 . As illustrated in  FIG. 8 , the path for molten glass in the vacuum degassing vessel  116  in the vacuum housing  114 , is formed by assembling path members  116   c , and the bottom portion  116   d  of the path for molten glass is flat. 
     FIG. 9  shows a flow rate distribution of molten glass in the transverse direction of the path. As shown in  FIG. 9 , it is evident that the flow rate of molten glass is highest at the center in the transverse direction (hereinafter referred to as the center of the path), and on the contrary, the flow rate of molten glass is lowest at the ends in the transverse direction (hereinafter referred to as the sides of the path). For this reason, there is a possibility that the molten glass flowing in the center of the path reaches the downfalling pipe without undergoing sufficient degassing, and bubbles are thereby included in the glass product. Namely, there has been a problem such that, even if the width of the path is simply increased, the molten glass still has a low flow rate and hardly flows at the sides of the path. Therefore, the increase of the width does not remarkably contribute to improvement of the vacuum degassing performance. 
   SUMMARY OF THE INVENTION 
   Under these circumstances, it is an object of the present invention to provide a vacuum degassing apparatus, whereby the costs can be minimized, the vacuum degassing performance can be improved, and it is possible to produce molten glass free from such problems as bubbles, stones or deterioration of leam. 
   The present invention provides a vacuum degassing apparatus for molten glass, comprising: 
   a vacuum housing which is evacuated to be depressurized therein; 
   a vacuum degassing vessel which is provided in the vacuum housing to vacuum-degas molten glass as the molten glass flows therein; 
   an uprising pipe which connects to the vacuum degassing vessel, and sucks and draws up undegassed molten glass to introduce the undegassed molten glass into the vacuum degassing vessel; and 
   a downfalling pipe which connects to the vacuum degassing vessel and draws down the degassed molten glass from the vacuum degassing vessel to discharge the degassed molten glass; 
   wherein the cross sectional area of the path at the upper end portion of said uprising pipe is larger than the cross sectional area of the path at the lower end portion of said uprising pipe. 
   The present invention further provides the above-mentioned vacuum degassing apparatus, wherein the cross sectional area of the path at the upper end portion of said uprising pipe is from 1.1 to 9.0 times the cross sectional area of the path at the lower end portion of said uprising pipe, and the above-mentioned vacuum degassing apparatus, which is a vacuum degassing apparatus having an uprising pipe having a structure wherein a critical portion is provided at an intermediate position of the path in said uprising pipe, and the cross sectional area of the path at said upper end portion is larger than the cross sectional area of the path at said critical portion of said uprising pipe, and the distance from said upper end portion to said critical portion is from 0.05 to 0.5 times the distance from said upper end portion to said lower end portion. 
   The present invention still further provides a vacuum degassing apparatus for molten glass, comprising: 
   a vacuum housing which is evacuated to be depressurized therein; 
   a vacuum degassing vessel which is provided in the vacuum housing to vacuum-degas molten glass as the molten glass flows therein; 
   an uprising pipe which connects to the vacuum degassing vessel, and sucks and draws up undegassed molten glass to introduce the undegassed molten glass into the vacuum degassing vessel; and 
   a downfalling pipe which connects to the vacuum degassing vessel and draws down the degassed molten glass from the vacuum degassing vessel to discharge the degassed molten glass; 
   wherein at least a part of the path for molten glass in the flow direction in said vacuum degassing vessel, a bottom portion at the center in the transverse direction of the path, has a ridge shape and bottom portions at both ends in the transverse direction of the path, which are located on both sides of the center bottom portion, have valley shapes. 
   The present invention still further provides the above-mentioned vacuum degassing apparatus, wherein when the shortest distance from the top of the ridge shaped bottom portion at the center in the transverse direction of the path for molten glass in said vacuum degassing vessel, to the surface of the molten glass, is designated as the center depth D 1 , and the shortest distance from the bottom of the valley shaped bottom portion at each of both ends in the transverse direction, to the surface of the molten glass, is designated as the side depth D 2 , then the center depth D 1  is from 20 to 500 mm, and the side depth D 2  is from 1.1 to 5.0 times the center depth D 1 , and the above-mentioned vacuum degassing apparatus, wherein a bubble blocking means is provided in the path for molten glass in said vacuum degassing vessel, to prevent bubbles formed by the vacuum-degassing from flowing out to the downfalling pipe side. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a schematic cross-sectional view illustrating an embodiment of the vacuum degassing apparatus of the present invention. 
       FIG. 2  is a schematic view illustrating the degassing mechanism for removal of bubbles in the uprising pipe in  FIG. 1 . 
       FIG. 3  is a schematic cross-sectional view illustrating an embodiment of a conventional vacuum degassing apparatus. 
       FIG. 4  is a cross-sectional view illustrating the degassing mechanism for removal of bubbles in the uprising pipe in  FIG. 3 . 
       FIG. 5  is a view illustrating the change of the bubble size under reduced pressure. 
       FIG. 6  is a schematic cross-sectional view of the vacuum degassing apparatus taken along line A-A′ in FIG.  1 . 
       FIG. 7  is a view illustrating the distribution in the transverse direction of the flow rate of molten glass flowing in the path of the vacuum degassing vessel in  FIG. 1 . 
       FIG. 8  is a schematic cross-sectional view of the vacuum degassing apparatus taken along a line B-B′ in  FIG. 3 . 
       FIG. 9  is a view illustrating the distribution in the transverse direction of the flow rate of molten glass flowing in the path of the vacuum degassing vessel in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Now, the vacuum degassing apparatus for molten glass according to the present invention will be described in detail with reference to a preferred embodiment shown in the accompanying drawings. 
     FIG. 1  shows a schematic cross-sectional view illustrating an embodiment of the vacuum degassing apparatus of the present invention.  FIG. 1  shows, as compared with  FIG. 3 , that the cross-sectional area of the path at the upper portion of the uprising pipe is larger than the cross-sectional area of the path at the lower portion of the uprising pipe. As illustrated in  FIG. 1 , the vacuum degassing apparatus  10  is used in a process wherein molten glass G in a melting vessel  20  is vacuum-degassed and the degassed molten glass is continuously supplied to a subsequent treatment vessel, not shown, such as a forming treatment vessel for plate material like a floating bath, or a forming treatment vessel for bottles. The vacuum degassing apparatus  10  essentially comprises a vacuum housing  12 , a vacuum degassing vessel  14 , an uprising pipe  16  and a downfalling pipe  18 . 
   The vacuum housing  12  is one for securing airtightness of the vacuum degassing vessel  14 , and it is formed in a gate-like-shape and has a main body portion  12   a , an uprising pipe accommodating portion  12   b , and a downfalling pipe accommodating portion  12   c . There is no particular limitation as to the material and the structure of the vacuum housing  12  as long as the vacuum housing has sufficient airtightness and strength required for the vacuum degassing vessel  14 . However, the vacuum housing is preferably made of metal, in particular stainless steel. The vacuum housing  12  is evacuated from outside by e.g. a vacuum pump (not shown) and the inside is depressurized, so that the inside of the vacuum degassing vessel  14  provided therein is maintained under a prescribed pressure, for example, in a depressurized state of from 1/20 to ⅓ atm. 
   In the main unit portion  12   a  of the vacuum housing  12 , a vacuum degassing vessel  14  is provided. An uprising pipe  16  is connected to the left end portion of the vacuum degassing vessel  14 , and a downfalling pipe  18  is connected to the right end portion of the vacuum degassing vessel  14 . Here, the uprising pipe  16  and downfalling pipe  18  are provided so that their main portions are respectively accommodated in the uprising pipe accommodating portion  12   b  and the downfalling pipe accommodating portion  12   c  in the vacuum housing  12 , and the lower end portions of the uprising and downfalling pipes  16  and  18  extend out of the vacuum housing  12 , respectively. 
   It is preferred to use a dense electro-cast refractory material for the vacuum degassing vessel  14 , the uprising pipe  16  and the downfalling pipe  18  of the present invention. Namely, the essential part of the vacuum degassing apparatus  10 , which is directly in contact with the molten glass, is formed by assembling electro-cast refractory bricks being dense electro-cast material, whereby the cost can be reduced to a large extent as compared with the essential part made of a noble metal such as platinum or a platinum alloy such as platinum-rhodium, which has been used heretofore. Further by this cost reduction, the vacuum degassing apparatus  10  can be designed to have a desired shape and a desired thickness. As a result, not only a large capacity of the vacuum degassing apparatus is realized, but also vacuum degassing treatment at a higher temperature becomes possible. 
   The electro-cast refractory bricks are not particularly restricted so long as they are molded into a prescribed shape by casting after the raw refractory material is melted by an electric melting process. Various types of conventional electro-cast refractory bricks may be used. Among them, alumina (Al 2 O 3 ) type electro-cast refractory bricks, zirconia (ZrO 2 ) type electro-cast refractory bricks and alumina-zirconia-silica (Al 2 O 3 —ZrO 2 —SiO 2 ) type electro-cast refractory bricks may be mentioned as preferred examples. Particularly, it is preferred to use MARSNITE (MB) when the temperature of molten glass G is at most 1,300° C., and to use ZB-X950 or ZIRCONITE (ZB) when it is more than 1,300° C. (all manufactured by Asahi Glass Company, Limited). 
   Although a dense electro-cast refractory material is used in this embodiment, the material is not limited thereto, and a dense burned refractory material may also be used. 
   Dense burned refractory bricks to be used as the dense burned refractory material, are preferably dense alumina type refractory bricks, dense zirconia-silica type refractory bricks or dense alumina-zirconia-silica type refractory bricks. 
   Further, there are boundaries between the molten glass G and the atmosphere at the lower end portion of the uprising pipe  16  where the pipe is immersed in the molten glass G in the upstream pit  22  downstream from the melting vessel  20 , and at the lower end portion of the downfalling pipe  18  where the pipe is immersed in the molten glass G in the downstream pit  24 . Accordingly, the vicinity of these boundaries is highly reactive, and particularly, deterioration of the electro-cast refractory bricks tend to advance at said boundary portions or at the joint portions between the bricks. Therefore, the lower end portion of the uprising pipe  16  and the lower end portion of the downfalling pipe  18  are preferably made of platinum or a platinum alloy. 
   Around the vacuum degassing vessel  14 , a heat insulating material  26  is provided to cover the vacuum degassing vessel  14 . Also around the uprising pipe  16  and the downfalling pipe  18 , the heat insulating material  26  is provided to cover them respectively. 
   As the heat insulating material  16 , various known standard shaped bricks or castable bricks may be used, and there is no particular restriction. The insulating material  26  thus provided, is covered by the vacuum housing  12  from outside, and is thereby accommodated in the vacuum housing  12 . 
   Here, the temperature of the outer wall of the vacuum housing  12 , is preferably made as low as possible, e.g. about 100° C., by insulating the heat conducted to the vacuum housing  12  as much as possible by the heat insulating material  26 . 
   Now, the degassing mechanism of the vacuum degassing apparatus  10  and the path shape of the uprising pipe  16 , which are the characteristics of the present invention, will be described. 
   A bubble included in the molten glass has a certain bubble size under the atmospheric pressure. When the pressure applied to the molten glass is lowered (depressurized), the bubble size increases inversely proportional to the pressure according to the Boyle-Charles&#39; law. However, the present inventors have discovered that when the pressure is further depressurized beyond a certain pressure, the bubble size departs from the Boyle-Charles&#39; law and rapidly increases. This phenomenon will be described with reference to  FIG. 5 . 
     FIG. 5  is a schematic view illustrating the change of the bubble size in molten glass by the decrease of the pressure applied to the molten glass, when molten glass having the composition as identified in Table 1 is used as the molten glass. The temperature of the molten glass is 1,320° C. 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Chemical 
                 
             
             
                 
               composition 
               Mass % 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               SiO 2   
               72.5 
             
             
                 
               Al 2 O 3   
               2.0 
             
             
                 
               MgO 
               4.0 
             
             
                 
               CaO 
               8.0 
             
             
                 
               Na 2 O 
               12.5 
             
             
                 
               K 2 O 
               0.8 
             
             
                 
               SO 3   
               0.2 
             
             
                 
                 
             
           
        
       
     
   
   In  FIG. 5 , the ordinate axis represents the bubble size, and the abscissa axis represents the vacuum degree which is the difference between the pressure applied to the molten glass and the atmospheric pressure, and “vacuum degree is 0 (zero) (mmHg)” means that the pressure applied to the molten glass is the atmospheric pressure. Further, “vacuum degree is 700 (mmHg)” means that the pressure applied to the molten glass is depressurized from the atmospheric pressure by 700 (mmHg). The solid line in  FIG. 5  indicates the relationship between the theoretical bubble size and the vacuum degree when they follow the Boyle-Charles&#39; law. In  FIG. 5 , it is evident that by depressurizing a bubble having an initial bubble size of 0.2 (mm) under the atmospheric pressure, the bubble size gradually increases. 
   Whereas, the black triangular marks in  FIG. 5  indicate the relationship between the actual bubble size of a bubble in the molten glass and the vacuum degree, and it is evident that a bubble having an initial bubble size of 0.2 (mm) departs from the Boyle-Charles&#39; law after the vacuum degree exceeds 300 (mmHg) and becomes larger than the bubble size calculated by the Boyle-Charles&#39; law. The vacuum degree where the bubble size departs from the Boyle-Charles&#39; law is referred to as the critical pressure, and the critical pressure differs depending on the type of molten glass. Such a phenomenon that a bubble in the molten glass departs from the Boyle-Charles&#39; law and becomes larger by increasing the vacuum degree, is considered to occur since some gas component in the molten glass diffuses into the bubble. Further, the bubble size increases, as the bubble is subjected for a longer time to a pressure below the critical pressure, and as the bubble size increases, the rising speed increases and the bubble becomes more likely to break on the surface of the molten glass. 
   When this principle is applied to the vacuum degassing vessel, it is evident that by making the cross sectional area of the path at the upper portion of the uprising pipe larger than the cross sectional area of the path at the lower portion of the uprising pipe, a bubble is subjected for a long time to a pressure below the critical pressure, and the improvement of degassing performance can be achieved. Here, the cross section of the path means a cross section of the path perpendicular to the direction in which the molten glass flows, the upper portion of the uprising pipe means a portion where the uprising pipe and the bottom surface of the vacuum degassing vessel are connected, and the lower portion of the uprising pipe means the lowest portion of the uprising pipe. Now, a vacuum degassing vessel to which this principle is applied, will be specifically described. 
     FIG. 4  is a schematic view illustrating a conventional degassing mechanism for removal of bubbles in the uprising pipe  118  in  FIG. 3 , and it illustrates the state of the path where the molten glass G flows in the uprising pipe  118  of a conventional vacuum degassing apparatus  110  in  FIG. 3 . 
   In  FIG. 4 , the molten glass G flows together with bubbles  140   a  from the upstream pit  122  to the uprising pipe  118 , and according to the principle of siphoning, rises in the path in the uprising pipe  118 . Meanwhile, the bubbles  140   a  also rise in the path in the uprising pipe  118  along with the flow of the molten glass G. Since the pressure applied to the molten glass G lowers upwards from the lower end portion  16   d  of the uprising pipe  16 , the bubble size increases from the bubbles  140   a  to bubbles  140   b  and then to bubbles  140   c  according to the Boyle-Charles&#39; law. Further, as the molten glass G rises, the pressure applied to the bubbles further decreases, whereby the bubbles further expand departing from the Boyle-Charles&#39; law. 
   However, as illustrated in  FIG. 4 , when the cross sectional area of the uprising pipe  118  is constant from its upper end portion  118   c  to the lower end portion  118   d , there is no space in the path for bubbles to expand, and as shown by a bubble  140   g  and a bubble  140   h , bubbles can not expand sufficiently. Accordingly, when the number of bubbles flowing into the vacuum degassing vessel  116  increases rapidly, the bubbles will not be completely removed and will remain in the molten glass, and unbroken bubbles will build up on the surface of the molten glass in the vacuum degassing vessel  116 . In such a case, as shown by a bubble  140   n  in  FIG. 4 , bubbles may stick to the ceiling of the vacuum degassing vessel  116  and may produce “stones” or the deterioration of leam. 
   Further, it has become evident that when bubbles thus expanded are combined together to form a huge bubble, the huge bubble has a very large buoyancy, and rises at a high speed as compared with bubbles around it. This creates a problem that the flow rate per unit time of the molten glass G is not stable. 
   Whereas,  FIG. 2  is a schematic view illustrating the degassing mechanism for removal of bubbles in the uprising pipe  16  in  FIG. 1  according to the present invention, and it illustrates the state of the path where the molten glass G flows in the uprising pipe  16  of the vacuum degassing vessel  14  in  FIG. 1 . In  FIG. 2 , a critical portion  16   b  is provided at an intermediate position of the path in the uprising pipe  16 , and the cross sectional area of the path is constant from the lower end position  16   d  of the uprising pipe  16  to the critical portion  16   b . However, the structure is such that the cross sectional area of the path from the critical portion  16   b  to the upper end portion  16   c , gradually increases from the critical portion  16   b  towards the upper end portion  16   c . Here, the pressure applied to the molten glass G lowers upwards from the lower end portion  16   d  of the uprising pipe  16 . Therefore, there is a point where the pressure applied to the molten glass G becomes the critical pressure at an intermediate position of the path, and the critical portion  16   b  is preferably locate at a lower position than such a point, since the vacuum degassing performance can thereby be improved. 
   As illustrated in  FIG. 2 , when the molten glass G flows in the path in the uprising pipe  16 , the pressure applied to the molten glass G lowers as the molten glass G rises in the path in the uprising pipe  16 . Therefore, bubbles  40   a  in the molten glass G expand to bubbles  40   b  and then to bubbles  40   c  according to the Boyle-Charles&#39; law. Then, after the bubbles pass the critical portion  16   b , the bubble size departs from the Boyle-Charles&#39; law and rapidly increases. However, since there is a space where the bubbles can expand in the path in the uprising pipe  16  in  FIG. 2 , the bubbles can easily expand there and readily break on the surface of the molten glass G. Thus, the vacuum degassing performance can be improved, and bubbles become less likely to remain in the molten glass G. Further, bubbles become more readily breakable as they become larger, which brings about such an effect that the bubbles are less likely to stick to the ceiling of the vacuum degassing vessel  14 , and thus, formation of defects such as “stones” or deterioration of leam can be suppressed. 
   Further, by employing the structure as shown in  FIG. 2 , the flow rate of the molten glass G flowing from the critical portion  16   b  to the upper end portion  16   c  can be decreased. It is thereby possible to subject bubbles for long time to a pressure below the critical pressure, and thereby to improve the vacuum degassing performance. Further, since there is a large space for the molten glass G to flow, the expanded bubbles are less likely to combine together to form a huge bubble, and the flow rate per unit time of the molten glass G can be stabilized. 
   The cross sectional shape of the path in the uprising pipe  16  may not necessarily be a circular shape, and may be an elliptical shape or a rectangular shape. Further, the cross sectional area of the path in the uprising pipe  16  may be increased from the lower end portion  16   d  gradually or stepwisely. 
   Further, the distance from the upper end portion  16   c  of the uprising pipe  16  to its lower end portion  16   d  is preferably from 2 to 5 m, and the distance from the upper end portion  16   c  to the critical portion  16   b  is preferably from 0.05 to 0.5 times the distance from the upper end portion  16   c  to the lower end portion  16   d , since the vacuum degassing performance can thereby be improved. 
   The cross sectional area of the path at the upper end portion  16   c , may vary also depending on the width of the path in the vacuum degassing vessel  14 . However, in order to improve the vacuum degassing performance, it is preferably from 1.1 to 9.0 times, particularly preferably from 1.5 to 4.0 times, the cross sectional area of the path at the lower end portion  16   d . Further, the flow rate of the molten glass per unit time in the vacuum degassing vessel of the present invention, may vary depending on the size of the vacuum degassing vessel, but it is from 1.5 to 350 ton/day. 
   Further, the cross sectional shape of the vacuum degassing vessel  14 , which is a characteristic of the present invention, will be described as follows. 
     FIG. 6  is a schematic cross-sectional view of the vacuum degassing apparatus  10  taken along line A-A′ in  FIG. 1 . As illustrated in  FIG. 6 , it is characterized that at at least a part of the path of molten glass in the flow direction in the vacuum degassing vessel  10  of the present invention, a bottom portion  14   a  at the center in the transverse direction of the path, has a ridge shape and the bottom portions  14   b  at both ends in the transverse direction of the path, which are located on both size of the center bottom portion  14   a , have valley shapes. (Hereinafter, this shape will be referred to as a ridge-valley shape.) 
   By making the bottom portion of the path in the vacuum degassing vessel  14  have a ridge-valley shape as illustrated in  FIG. 6 , it is possible to let more glass flow at the areas (namely at both sides of the path) where longer time can be taken for vacuum degassing, as compared with a case where, as shown in  FIG. 8 , the bottom portion  116   d  of the path is flat. Such an effect is thereby obtained that both sides of the path which have not been very useful for vacuum degassing (namely, the sides of the path which have had an extra room in the vacuum degassing capacity, because through there, the time for the molten glass G to flow from an inflow port  16   a  to an outflow port  18   a  becomes longest), can now effectively be used for vacuum degassing, and the vacuum degassing performance is thereby be improved. These results are also confirmed by a simulation test of the vacuum degassing at the same time. As described above, by making the bottom portion of the path of the molten glass in the vacuum degassing vessel  14  have the above-mentioned ridge-valley shape, the vacuum degassing performance can be improved, vacuum degassing treatment of molten glass in a larger flow amount becomes possible, and molten glass free from residual bubbles can be obtained. 
   Further,  FIG. 7  shows a diagram illustrating the distribution in the transverse direction of the flow rate of molten glass flowing in the path having the ridge-valley shaped bottom portion in  FIG. 6 . As shown in  FIG. 7 , the flow rate of molten glass at the center of the path is evidently lower than in the case of  FIG. 9  where the bottom portion  116   d  of the path is flat. For this reason, molten glass flowing at the center of the path is less likely to reach the downfalling pipe before being sufficiently degassed, whereby the vacuum degassing performance is improved, and vacuum degassing treatment of molten glass in a larger flow amount becomes possible. 
   Further, in the path for molten glass in the vacuum degassing vessel  14  as illustrated in  FIG. 6 , rising of bubbles is active not only at the center of the path but also at both sides of the path. Further, the molten glass G present at the center bottom portion  14   a  flows into the side bottom portions  14   b , and consequently the flow rate of the molten glass G at both sides of the path increases as illustrated in  FIG. 7 . Thus, it is possible to obtain a flow of the molten glass G having a constant flow rate in the transverse direction of the path without stagnation. Namely, there is no portion where the flow rate of the molten glass G is locally low, and the molten glass G is discharged smoothly from the downfalling pipe  18 . Therefore, a glass product such as a glass sheet being a final product, will have a uniform composition, and will be free from deterioration of leam (a problem that the see-through image is badly distorted due to a local change in the refractive index) caused by non-uniformity of the composition. Thus, an improvement of the quality of a glass product such as a glass sheet can be achieved. 
   The bottom portion of the path for molten glass in the vacuum degassing vessel  14 , may not necessarily have the ridge-valley shape as shown in  FIG. 6  over its entire length, namely over the entire length in the flow direction of molten glass, so long as a part of it has the ridge-valley shape. Further, it is preferred that the bottom portion of the path in the area from the flow-in port  16   a  from the uprising pipe to the flow-out port  18   a  to the downfalling pipe, is made to have the ridge-valley shape. Further, the width of the ridge shaped portion at the center of the bottom portion of the path, or the width of the valley shaped portions located on its both sides, are adjusted so that the effective degassing can be performed. Namely, the width of the ridge shaped portion or the width of the valley shaped portions may not necessarily be constant in the flow direction of molten glass, and may gradually increase or gradually decrease. The path for molten glass having the ridge-valley shape, preferably has a smooth structure at the portion where the molten glass is in contact, in order to prevent occurrence of deterioration of the leam. 
   Further, by making the bottom portion of the path in the vacuum degassing vessel  14  have the ridge-valley shape as illustrated in  FIG. 6 , it is possible to decrease the center depth D 1  being the shortest distance from the top of the ridge shaped bottom portion at the center in the transverse direction, to the surface of the molten glass, and it is possible to increase the side depth D 2  being the shortest distance from the bottom of the valley shaped bottom portion at each of both ends in the transverse direction, to the surface GS of the molten glass. By such a structure, the vacuum degree at the center bottom portion of the path, can be increased when the vacuum degree at the surface of the molten glass is constant, whereby bubbles tend to readily rise, and the vacuum degassing performance can further be improved. When the bottom portion of the path has a flat shape as shown in  FIG. 8 , if the depth of the molten glass is entirely reduced, the flow rate of the molten glass increases at the same time. Consequently, the time for molten glass to flow from the flow-in port  16   a  to the flow-out port  18   a  becomes shorter, thus bringing about a demerit that the degassing becomes difficult, at the same time, and it is thereby difficult to improve the vacuum degassing performance. 
   Further, it is preferred that the center depth D 1  of the path of molten glass in the vacuum degassing vessel  14  is from 20 to 500 mm, and the side depth D 2  is from 1.1 to 5.0 times the center depth D 1 , whereby the vacuum degassing performance can further be improved, and the deterioration of leam can effectively be prevented. Further, as shown in  FIG. 6 , the side depths D 2  exist at both sides in the path of the molten glass, but these may not necessarily have the same value, and may be different from each other. 
   Further, as illustrated in  FIG. 6 , it is preferred that a bubble blocking means  28  is provided in the downstream portion in the vacuum degassing vessel  14 , to prevent bubbles formed by the vacuum-degassing from flowing out to the out flow port  18   a  connected to the downfalling pipe. This is to prevent bubbles which are not broken on the surface GS of the molten glass while molten glass G flows in the upstream portion of the vacuum degassing vessel  14 , from passing, as they float, through the out flow port  18   a  and being discharged from the downfalling pipe  18 . 
   An embodiment of the present invention has been described in detail above. However, the present invention is by no means restricted to the above-mentioned embodiment, and it is a matter of course that various improvements and modifications may be performed without departing from the gist of the present invention. 
   As described above, according to the present invention, by making the cross sectional area of the path at the upper end portion of the uprising pipe, larger than the cross sectional area of the path at the lower end portion of the uprising pipe, it is possible to have a space in the path where bubbles can expand. Thus, bubbles can easily expand, and the vacuum degassing performance can be improved. Further, as the bubbles expand, they become more likely to break, whereby such an effect is obtained that bubbles are less likely to stick to the ceiling of the vacuum degassing vessel, and it is thereby possible to obtain molten glass which is not likely to produce defects such as bubbles, “stones” and the deterioration of leam. Further, since there is a space in the path where bubbles can expand, it is less likely to expanded bubbles will combine together to form a huge bubble, and thereby, the flow rate per unit time of the molten glass G, can be stabilized. 
   Further, according to the present invention, the cross sectional shape of the path for molten glass in the vacuum degassing vessel, is characterized that at at least a part of the path in the flow direction, a bottom portion at the center in the transverse direction of the path, has a ridge shape and bottom portions at both ends in the transverse direction located on both sides of the center bottom portion, have valley shapes. Accordingly, it is possible to let more glass flow at the areas (namely the side of the path) where longer time can be taken for vacuum degassing, and to increase the vacuum degree at the center bottom portion of the path. Further, it is possible to decrease the flow rate of molten glass at the center of the path, whereby such an effect can be obtained that the vacuum degassing performance is improved. As a result, the vacuum degassing treatment of molten glass in a large flow amount becomes possible, and it is thereby possible to obtain molten glass free from residual bubbles. 
   Further, both sides of the path which have not been very useful for vacuum degassing, can now be effectively used for vacuum degassing, whereby the vacuum degassing performance can be improved, and further, the deterioration of leam can be prevented. 
   The entire disclosures of Japanese Patent Application No. 2001-299213 filed on Sep. 28, 2001 and Japanese Patent Application No. 2001-334106 filed on Oct. 31, 2001 including specifications, claims, drawings and summaries are incorporated herein by reference in their entireties.