Vacuum refining of glass or the like with enhanced foaming

Foaming of molten glass or the like as it enters a vacuum refining vessel is enhanced by altering the incoming stream so as to increase its surface area and/or retarding the passage of the stream through the vacuum headspace so as to increase its exposure to the vacuum.

BACKGROUND OF THE INVENTION 
This invention relates to enhancing foam generation in an operation for 
refining molten glass under vacuum. In commonly assigned U.S. patent 
application Ser. No. 894,143 filed Aug. 7, 1986, there is disclosed a 
method and apparatus wherein the creation of foam in a vacuum refiner is 
encouraged by introducing molten glass into the upper headspace of the 
vacuum chamber. The molten glass immediately foams as it encounters the 
reduced pressure, and it has been found that foaming the glass is not a 
problem as had been thought previously, but is highly beneficial in 
removing dissolved and entrained gases from the melt. Moreover, it has 
been found advantageous to generate the foam above the liquid level in the 
refiner vessel, and preferably above the foam layer, so as to subject the 
foam to the lowest pressure in the vessel. The removal of gases from the 
liquid phase is greatly enhanced by the large surface area provided to the 
liquid in the bubble membranes of the foam. Thus, very thin liquid layers 
are subjected to low pressure in the foam within the vacuum chamber. 
Under certain conditions, however, not all of the incoming molten glass 
stream foams immediately upon entering the vacuum space. In those cases, 
surface portions of the stream may foam quickly, but a central portion of 
the stream may penetrate unfoamed into the foam layer or even to the 
underlying molten pool. If the molten glass does not foam above the foam 
layer, it may not be exposed to the lowest pressure in the system and thus 
would be subjected to less than the optimum refining conditions, because 
even in the relatively light foam layer, pressures increase substantially 
at lower elevations. Any incoming glass that penetrates to the underlying 
liquid runs a considerable risk that it will not be exposed to sufficient 
vacuum to be refined and may be carried into the outgoing stream where it 
would degrade the quality of otherwise good product. Foaming of the 
incoming stream of glass may be improved by providing lower pressure in 
the vacuum chamber and/or reducing the glass flow rate, but it would be 
desirable to improve the degree to which the stream foams without altering 
such predetermined parameters as throughput and pressure. 
Prior art proposals to refine glass by vacuum have generally avoided 
foaming the glass rather than enhancing it. Prior to the aforesaid 
co-pending application, molten glass was not injected into the upper 
headspace of a vacuum chamber. Therefore, there is no guidance in the 
prior art as to enhancing foam formation in such an operation. 
SUMMARY OF THE INVENTION 
The present invention has as its primary objective the enhancement of the 
degree to which molten glass is foamed as it enters a vacuum refining 
chamber. This is achieved by increasing the amount of time that the molten 
glass stream spends in the vacuum headspace, and/or reducing the thickness 
of the molten glass stream in the vacuum headspace at a given vacuum level 
and molten glass temperature. Conversely, the foam enhancement means of 
the present invention may permit maintaining a given degree of foaming 
with less vacuum or lower temperatures. Two approaches are taken in the 
present invention which need not be mutually exclusive: (1) the entering 
molten glass stream may be separated into a plurality of smaller streams 
or reshaped into a thinner stream, or (2) the passage of the molten glass 
stream through the vacuum headspace may be retarded. 
In the first approach, the thickness of molten glass in the stream entering 
the vacuum headspace is reduced by subdividing or spreading the stream. 
Since foaming of the liquid stream progresses from the exterior surface 
toward the interior, reducing the thickness permits the entire thickness 
to foam in less time. Optimally, the time for complete foaming is reduced 
to less than the time required for the stream to fall to the level of the 
retained molten material in the vacuum chamber, preferably less than the 
time required to fall into the foam layer resting on the molten material. 
The particular thickness desired will be a function of glass temperature, 
vacuum pressure, and mass flow rate. The number of subdivisions of the 
stream will, in turn, depend upon the thickness requirement. Reducing the 
thickness of the stream is also advantageous even if it is completely 
foamed because gases from bursting foam bubbles may be more readily 
expelled from the foam when the bubbles are near the surface rather than 
within the interior portion of a relatively thick stream. 
In the second approach, the incoming molten glass stream is obstructed 
temporarily from falling directly through the vacuum headspace. Thus, the 
molten glass stream is retained for a longer period of time in the region 
of lowest pressure, whereby more time is provided for the foaming to 
progress throughout the stream before it falls into the pool of molten 
material or the foam layer. The retardation is effected by providing an 
obstacle in the path of the stream within the vacuum headspace. This 
obstacle can achieve its purpose by creating a longer flow path for the 
molten glass, by creating a drag on the glass flow to decelerate it, or by 
providing a reservoir in the headspace to temporarily hold a volume of the 
incoming stream. In many cases, several or all of these effects can be 
achieved simultaneously, and in preferred embodiments subdividing and 
retarding the flow are both achieved. 
The means for subdividing the stream of glass may comprise a plurality of 
inlets to the vacuum refining chamber, but because the inlets generally 
involve costly refractory metal construction, valving means, and sealing 
arrangements, it is preferred to employ a single inlet and to subdivide 
the stream within the vacuum chamber. Therefore, the apparatus for 
subdividing or retarding the stream of molten glass may be suspended 
within the upper portion of the vacuum chamber in the path of the molten 
glass discharging from the inlet orifice. It is preferred to mount the 
foam enhancing device as high as possible so as to maximize the time that 
the falling foam is exposed to the low pressure in the vacuum headspace. 
The device is preferably fabricated of a non-contaminating refractory 
material such as platinum and may be in the form of a plate, bowl, screen, 
or basket, as examples. Perforations to subdivide the stream are 
preferably present in the device. Two or more foam enhancing devices may 
be provided one below the other to act on the glass stream in sequence. 
These and other aspects of the invention will become more apparent from the 
drawings and the detailed description of the preferred embodiments which 
follows.

DETAILED DESCRIPTION 
The present invention is described herein as relating to the making of 
glass. It should be understood that the term glass is intended to be used 
in its broadest sense to include materials that are "glassy" or 
"glass-like" since the final state of vitrification is not a critical 
factor in the present invention. On the other hand, the most likely 
application for the present invention is in the making of transparent 
glass products of high quality for which the elimination of bubbles and 
seeds from the glass is important. Moreover, the advantages of the present 
invention are particularly relevant to the continuous, large scale (e.g., 
greater than 10 tons per day, 9 metric tons per day), commercial 
production of glass, especially glass for vision glazing. 
In the preferred embodiment, an apparatus is provided by which vacuum 
refining may be employed in a commercial scale, continuous glassmaking 
process. Molten glass is admitted to the vacuum refining chamber after the 
majority of the thermal energy required for melting has been imparted to 
the melt so that little or no thermal energy need be supplied to the 
molten material contained within the vacuum chamber. Preferably, no more 
heat is added at the vacuum stage than is necessary to compensate for heat 
loss through the vessel walls. At sufficiently high throughput rates, the 
vacuum chamber may be completely unheated by other than the incoming 
molten glass itself. In preferred embodiments of the present invention, 
batch materials are first liquefied at a stage specifically adapted for 
that step of the process, and the liquefied material is transferred to a 
second stage where dissolution of solid particles is essentially completed 
and the temperature of the material may be raised to a temperature to 
provide a viscosity suitable for refining. Subsequently, the molten 
material is passed to the vacuum chamber. As a result, a large portion of 
the gaseous by-products of melting are driven off before the material is 
subjected to vacuum, and the region of greatest gas evolution is separated 
from the refining zone, whereby materials undergoing the early stages of 
melting cannot become mixed with portions of the melt undergoing refining. 
Because most or all of the thermal requirement for melting has been 
satisfied before the material enters the vacuum refining stage and heating 
of the refining stage can therefore be substantially avoided, excessive 
convection of the melt in the refining zone can be avoided. As a result, 
vessel erosion is reduced and the probability of incompletely refined 
portions of the melt becoming mixed with more refined portions is reduced. 
The assistance provided by the vacuum to the refining process enables lower 
temperatures to be used for refining. Lower temperatures are advantageous 
not only for less energy consumption, but also for the sake of reduced 
corrosive effect on the vessel. Glass normally refined at peak 
temperatures on the order of 2800.degree. F. (1520.degree. C.) can be 
refined to the same extent at temperatures no greater than about 
2600.degree. F. (1425.degree. C.) or even 2500.degree. F. (1370.degree. 
C.) or lower, depending upon the level of vacuum employed. 
It is theorized that the creation of foam in the vacuum refining chamber 
significantly enhances removal of gases from the melt. The thin film and 
large surface area presented by the foam increases exposure to the low 
pressure conditions and expedites transport of the gases out of the liquid 
phase. This contrasts to conventional refining where residence time must 
be provided to permit bubbles to rise to the surface and escape from the 
viscous melt, which entails retaining a large pool of the melt. Thus, 
vacuum refining can achieve a given degree of refining in a considerably 
smaller space. The beneficial effects of exposing foamed melt to vacuum 
are enhanced by foaming the material as it enters the vacuum vessel, 
before it enters the body of molten material retained therein, and 
preferably before the entering stream penetrates into the foam layer. 
The preferred configuration for the vacuum refining chamber is a vertically 
elongated vessel, most conveniently in the shape of an upright cylinder. 
Liquified material is introduced into the headspace above the molten 
material held in the vessel. Upon encountering the reduced pressure in the 
headspace, at least a substantial portion of the material foams due to 
evolvement of gases dissolved in the material and due to enlargement of 
bubbles and seeds present in the material. Creating a foam greatly 
increases the surface area exposed to the reduced pressure, thus aiding 
the removal of gaseous species from the liquid phase. Producing the foam 
above the molten pool held in the vessel rather than from the molten pool 
is advantageous for collapsing foam and aiding the escape of gases. 
Another advantage of the vertically elongated geometry is that 
stratification occurs due to the less dense foam or bubble containing 
material remaining at the upper end, so that the overall mass transport is 
away from the foam region, thereby rendering it unlikely that any of the 
unrefined material would become included in the product stream. Stripping 
gases from the melt at reduced pressure reduces the concentration of gases 
dissolved in the melt to below their saturation points at atmospheric 
pressure. As the molten material progresses downwardly toward an outlet at 
the bottom, the increasing pressure due to the depth of the melt in the 
vessel induces any residual gases to remain in solution and decreases the 
volume of any small seeds that may remain. Dissolution of gases may also 
be aided by permitting the temperature to fall as the material progresses 
toward the outlet. Moreover, the low concentration of gases remaining 
after vacuum refining reduces the probability of nucleation of bubbles in 
subsequent stages of the glassmaking process, as is frequently a problem 
with conventional refining. 
In commercial melting of glass, especially soda-lime-silica glass, sodium 
sulfate or calcium sulfate or other sources of sulfur are usually included 
in the batch materials to aid the melting and refining process. The 
presence of refining aids such as sulfur in the melt has been found to be 
a problem when refining with vacuum because of the large volumes of foam 
induced and because of attack on the ceramic refractory walls of a vacuum 
refining vessel. But heretofore, effective melting and refining of glass 
have been difficult to achieve without the refining aids. In accordance 
with preferred embodiments of the present invention, glass is melted and 
refined to a high standard of quality with the use of little or no 
chemical refining aid. This is feasible in the present invention because 
the melting and refining steps are carried out in discrete stages, whereby 
each stage may be carried out by a process adapted to minimize or avoid 
the use of chemical refining aids. It is generally believed that chemical 
refining aids serve to expedite the accumulation and rise of bubbles from 
within a molten pool, but such a mechanism is believed to play no more 
than a minor role in the refining process of the present invention. 
Therefore, no significant effect on quality results from eliminating or 
substantially reducing the amount of refining aids used. Elimination or 
reduction of the refining aids is also desirable for the sake of reducing 
undesirable emissions into the environment. In the float process of 
manufacturing flat glass, reducing or eliminating sulfur from the glass is 
additionally advantageous for the sake of avoiding defects caused by the 
formation and volatilization of tin sulfide in the flat forming chamber 
that leads to condensation and drippage onto the top surface of the glass. 
Sulfur in combination with iron has a coloration effect on glass, so the 
avoidance of sulfur for refining permits more precise control of the color 
of some glass. 
Referring to FIG. 1, the overall melting process of the present invention 
preferably consists of three stages: a liquefaction stage 10, a dissolving 
stage 11 and a vacuum refining stage 12. Various arrangements could be 
employed to initiate the melting in the liquefaction stage 10, but a 
highly effective arrangement for isolating this stage of the process and 
carrying it out economically is that disclosed in U.S. Pat. Nos. 4,381,934 
and Re. 32,317 which are hereby incorporated by reference for details of 
the preferred liquefaction stage embodiment. The basic structure of the 
liquefaction vessel is a drum 15 which may be fabricated of steel and has 
a generally cylindrical sidewall portion, a generally open top, and a 
bottom portion that is closed except for a drain outlet. The drum 15 is 
mounted for rotation about a substantially vertical axis, for example, by 
means of an encircling support ring 16 rotatably carried on a plurality of 
support wheels 17 and held in place by a plurality of aligning wheels 18. 
A substantially enclosed cavity is formed within the drum 15 by means of a 
lid structure 20 which is provided with stationary support by way of a 
peripheral frame 21, for example. The lid 20 may take a variety of forms 
as may be known to those of skill in the art of refractory furnace 
construction. The preferred arrangement depicted in the figure is an 
upwardly domed, sprung arch construction fabricated from a plurality of 
refractory blocks. It should be understood that monolithic or flat 
suspended designs could be employed for the lid. 
Heat for liquefying the batch material may be provided by one or more 
burners 22 extending through the lid 20. Preferably, a plurality of 
burners are arranged around the perimeter of the lid so as to direct their 
flames toward a wide area of the material within the drum. The burners are 
preferably water cooled to protect them from the harsh environment within 
the vessel. Exhaust gases may escape from the interior of the liquefaction 
vessel through an opening 23 in the lid. Advantageously the waste heat in 
the exhaust gases may be used to preheat the batch material in a 
preheating stage (not shown) such as that disclosed in U.S. Pat. No. 
4,519,814. 
Batch materials, preferably in a pulverulent state, may be fed into the 
cavity of the liquefying vessel by means of a chute 24, which in the 
embodiment depicted extends through the exhaust opening 23. Details of the 
feed chute arrangement may be seen in U.S. Pat. No. 4,529,428. The batch 
chute 24 terminates closely adjacent to the sidewalls of the drum 10, 
whereby batch material is deposited onto the inner sidewall portions of 
the drum. A layer 25 of he batch material is retained on the interior 
walls of the drum 10 aided by the rotation of the drum and serves as an 
insulating lining. As batch material on the surface of the lining 25 is 
exposed to the heat within the cavity, it forms a liquefied layer 26 that 
flows down the sloped lining to a central drain opening at the bottom of 
the vessel. The outlet may be fitted with a ceramic refractory bushing 27. 
A stream of liquefied material 28 falls freely from the liquefaction 
vessel through an opening 29 leading to the second stage 11. The second 
stage may be termed the dissolving vessel because one of its functions is 
to complete the dissolution of any unmelted grains of batch material 
remaining in the liquefied stream 28 leaving the liquefaction vessel 10. 
The liquefied material at that point is typically only partially melted, 
including unmelted sand grains and a substantial gaseous phase. In a 
typical soda-lime-silica melting process using carbonate batch materials 
and sulfates as a refining aid, the gaseous phase is chiefly comprised of 
carbon oxides and sulfur oxides. Nitrogen may also be present from 
entrapped air. In the present invention, the need to use sulfates is 
greatly reduced, so that sulfur oxides may not be present to a significant 
extent as part of the gas content of the melt. 
The dissolving vessel 11 serves the function of completing the dissolution 
of unmelted particles in the liquefied material coming from the first 
stage by providing residence time at a location isolated from the 
downstream refining stage. Soda-lime-silica glass batch typically 
liquefies at a temperature of about 2200.degree. F. (1200.degree. C.) and 
enters the dissolving vessel 11 at a temperature of about 2200.degree. F. 
(1200.degree. C.) to about 2400.degree. F. (1320.degree. C.), at which 
temperature residual unmelted particles usually become dissolved when 
provided with sufficient residence time. The dissolving vessel 11 shown is 
in the form of a horizontally elongated refractory basin 30 with a 
refractory roof 31, with the inlet and outlet at opposite ends thereof so 
as to assure adequate residence time. The depth of molten material in the 
dissolving vessel may be relatively shallow in order to discourage 
recirculating of material. 
Although the addition of substantial thermal energy is not necessary to 
perform the dissolving step, heating can expedite the process and thus 
reduce the size of the dissolving vessel 11. More significantly, however, 
it is preferred to heat the material in the dissolving stage so as to 
raise its temperature in preparation for the refining stage to follow. 
Maximizing the temperature for refining is advantageous for the sake of 
reducing glass viscosity and increasing vapor pressure of included gases. 
Typically a temperature of about 2800.degree. F. (1520.degree. C.) is 
considered desirable for refining soda-lime-silica glass, but when vacuum 
is employed to assist refining, lower peak refining temperatures may be 
used without sacrificing product quality. The amount by which temperatures 
can be reduced depends upon the degree of vacuum. Therefore, when refining 
is to be performed under vacuum in accordance with the present invention, 
the glass temperature need be raised to no more than 2700.degree. F. 
(1480.degree. C.), for example, and optionally no more than 2600.degree. 
F. (1430.degree. C.) prior to refining. When the lower range of pressures 
disclosed herein are used, the temperature in the refining vessel need be 
no higher than 2500.degree. F. (1370.degree. C.). Peak temperature 
reductions on this order result in significantly longer life for 
refractory vessels as well as energy savings. The liquefied material 
entering the dissolving vessel need be heated only moderately to prepare 
the molten material for refining. Combustion heat sources could be used in 
the dissolving stage 11, but it has been found that this stage lends 
itself well to electric heating, whereby a plurality of electrodes 32 may 
be provided as shown in the figure extending horizontally through the 
sidewalls. Heat is generated by the resistance of the melt itself to 
electric current passing between electrodes in the technique 
conventionally employed to electrically melt glass. The elecrodes 32 may 
be carbon or molybdenum of a type well known to those of skill in the art. 
A skimming member 33 may be provided in the dissolving vessel to prevent 
any floating material from approaching the outlet end. 
A valve controlling the flow of material from the dissolving stage 11 to 
the refining stage 12 is comprised of a plunger 35 axially aligned with a 
drain tube 36. The shaft 37 of the plunger extends through the roof 31 of 
the dissolving vessel so as to permit control over the gap of the plunger 
35 and the tube 36 to thereby modulate the rate of flow of material into 
the refining stage. Although the valve arrangement is preferred, other 
means could be provided to control the flow rate of molten material to the 
refining stage as are known in the art. An example would be the use of 
heating and/or cooling means associated with the drain tube so as to 
modulate the viscosity, and thus the flow rate, of the molten material 
passing therethrough. 
The refining stage 12 preferably consists of a vertically upright vessel 
that may be generally cylindrical in configuration having an interior 
ceramic refractory lining 40 shrouded in a gas-tight water-cooled casing. 
The refractory may be an alumina-zirconia-silica type well known in the 
art. The casing may include a double walled, cylindrical sidewall member 
41 having an annular water passageway therebetween and circular end 
coolers 42 and 43. A layer of insulation (not shown) may be provided 
between the refractory 40 and the sidewall 41. The valve tube 36 may be 
fabricated of a refractory metal such as platinum and is sealingly fitted 
into an orifice 44 at the upper end of the refining vessel. The tube 36 is 
shown extending vertically through the top of the refining vessel 12, 
which is preferred for the sake of introducing the incoming stream of 
glass as high as possible within the vacuum space. But it should be 
understood that the present invention would also be applicable to inlet 
arrangements that do not maximize the height factor. Thus, the inlet could 
be horizontal and could extend through the side of the vessel, but would 
be above the level of molten material and preferably above the normal 
level of foam in the vessel 12. 
As the molten material passes through the tube 36 and encounters the 
reduced pressure within the refining vessel, gases dissolved and occluded 
in the melt expand in volume, creating a foam layer 50 resting on a body 
of liquid 51. As foam collapses it is incorporated into the liquid body 
51. Subatmospheric pressure may be established within the refining vessel 
through a vacuum conduit 52 extending through the upper portion of the 
vessel. As used herein, "foaming" can be considered to be characterized by 
at least a doubling of the volume of the molten material. If the material 
is fully foamed, the volume increase is usually much greater than double. 
Foam has also been defined as being characterized by a gas phase of at 
least 90 percent of the total volume, or as the condition in which bubble 
membranes touch each other. 
Distributing the molten material as thin membranes of a foam greatly 
increases the surface area exposed to the reduced pressure. Therefore, 
maximizing the foaming effect is preferred. It is also preferred that the 
foam be exposed to the lowest pressures in the system, which are 
encountered at the top of the vessel in the headspace above the liquid, 
and therefore exposure is improved by permitting newly introduced, foamed 
material to fall through the headspace onto the top of the foam layer. 
Also, it is more consistent with the mass transfer in the vessel to 
deposit freshly foamed material onto the top of the foam layer rather than 
generating foam from the surface of the liquid pool beneath the foam 
layer. 
The heat content of the molten throughput material entering the refining 
vessel 12 can be sufficient to maintain suitable temperatures within the 
vessel, but at lower throughput rates energy losses through the walls may 
exceed the rate at which energy is being transported into the vessel by 
the molten material. In such a case, it may be desirable to provide 
heating within the refining vessel for the sake of avoiding undue 
temperature reduction. The amount of heating could be relatively minor 
since its purpose would be merely to offset heat losses through the walls, 
and may be carried out by conventional electric heating arrangements 
whereby electrodes extend radially through the side wall and electric 
current is passed between the electrodes through the glass. 
Regardless of the throughput rate, the space above the molten body 51 in 
the vessel 12 can tend to be cooler than desired because of the absence of 
the molten mass and because radiation from the molten mass is insulated by 
the foam layer 50. As a result, the upper portion of the foam layer can 
become cooler, which in turn increases the viscosity of the foam and slows 
the rate at which gases are expelled. In that case, it has been found 
advantageous to provide means for heating the headspace above the liquid 
and foam. For this purpose, it has been found feasible to provide a burner 
53 and to sustain combustion within the vacuum space. A conduit 54 may be 
provided at the upper end of the vacuum vessel whereby a small amount of 
water may be sprayed onto the foam periodically. The water spray has been 
found to assist the foam to collapse. 
In the embodiment depicted, refined molten material is drained from the 
bottom of the refining vessel 12 by way of a drain tube 55 of a refractory 
metal such as platinum. It would also be feasible to locate the drain in a 
side wall of the vessel in the region of the bottom. The drain tube 55 
preferably extends above the surface of the refractory bottom section 56 
within which it is mounted to prevent any debris from entering the output 
stream. Leakage around the tube is prevented by a water cooler 57 under 
the bottom section 56. The flow rate of molten material from the drain 
tube 55 may be controlled by a conical throttle member 58 whereby 
adjusting the gap between the throttle member and the tube 55 controls the 
flow rate therefrom. A molten stream 60 of refined material falls freely 
from the bottom of the refining vessel and may be passed to a forming 
station (not shown) where it may be shaped to the desired product. Refined 
glass, for example, may be passed to a float glass forming chamber where 
the molten glass floats on a pool of molten metal to form a flat sheet of 
glass. 
Melting and fining aids such as sulfur compounds (e.g., sodium sulfate, 
calcium sulfate) are conventionally included in glass batches, but produce 
a substantial portion of the undesirable emissions in exhaust gas from 
glass melting operations. Sulfur compounds are responsible for a 
significant portion of the gas phase found in glass products, and thus 
their removal is an important part of a refining process. In a vacuum 
refining process, the presence of amounts of sulfur that would 
conventionally be considered very small are more than desired because of 
the large contribution sulfur compounds make to the atmosphere in the 
vacuum chamber and because of the accelerated erosion of the vacuum 
chamber walls caused by the present of sulfur. Typically, flat glass batch 
includes sodium sulfate in the amounts of about 5 to 15 parts by weight 
per 1000 parts by weight of the silica source material (sand), with about 
10 parts by weight considered desirable to assure adequate refining. When 
operating in accordance with the present invention, however, it has been 
found preferable to restrict the sodium sulfate to two parts by weight, 
most preferably, no more than one part per 1000 parts sand, with one-half 
part being a particularly advantageous example. These weight ratios have 
been given for sodium sulfate, but it should be apparent that they can be 
converted to other sulfur sources by molecular weight ratios. Complete 
elimination of refining aids is feasible with the present invention, 
although trace amounts of sulfur are typically present in other batch 
materials so that small amounts of sulfur may be present even if no 
deliberate inclusion of sulfur is made in the batch. 
The attainment of complete foaming is easier at lower pressures, and 
therefore lower pressures in the refining vessel are preferred. The foam 
enhancement devices of the present invention permit the use of higher 
pressures than would otherwise be possible, thus saving on vacuum pump 
costs. But even when using these devices, exceeding certain pressure 
levels can make it very difficult to achieve complete foaming. These 
maximum pressure levels depend upon the viscosity of the glass stream 
entering the vacuum chamber, which, in turn, depends upon the temperature 
and composition of the glass. For a conventional soda-lime-silica flat 
glass composition at 2500.degree. F. (1370.degree. C.) to 2700.degree. F. 
(1480.degree. C.) it has been found to be desirable to provide pressure 
below 100 torr absolute, preferably below 70 torr, in the headspace of the 
refining vessel 12 to achieve complete foaming. How much lower than these 
maximum pressures the pressure in the vacuum space should be maintained 
depends upon the mass flow rate of glass and the particular foam 
enhancement device used. At relatively high glass flow rates pressures 
below 40 torr may be preferred even with extensive foam enhancement 
devices. But at about 20 torr, only modest foam enhancement devices may be 
required even at high flow rates. 
In FIGS. 2 through 12 there are illustrated several embodiments of flow 
enhancement devices in accordance with the present invention. Each of 
these devices is intended to be mounted within the vacuum refining vessel 
12 shown in FIG. 1, in association with the lower end of tube 36 so as to 
be impinged by the stream of molten material passing from the tube 36. The 
foam enhancement device may be affixed to or be integral with the tube 36 
itself, or it may be independently mounted to the adjacent lid structure 
of the vessel 12 or on a support arm extending into the vacuum vessel 
through an access opening in a wall portion. The latter is preferred for 
the sake of adjusting the position and orientation of the device as well 
as permitting relatively easy replacement. The elevation of the device is 
above the molten body 51, preferably above the foam layer 50. 
Except where noted, the device of each embodiment is preferably fabricated 
from platinum, and in particular the platinum/rhodium alloys commonly used 
for molten glass contact applications. Theoretically, any refractory 
material could serve the purpose of the present invention, but durability 
problems render most other materials impractical. Ceramic refractories 
would be subject to considerable erosion due to contact with the flowing 
stream. The atmosphere in the headspace of the vessel 12 has been found to 
be sufficiently oxidizing to deterimentally affect the durability of 
molybdenum. Water-cooled stainless steel or the like can be used, but it 
is preferred to minimize extraction of heat from the glass by cooled 
members. Some water-cooled elements may be employed in some of the 
embodiments without undue heat loss, provided that they are limited in 
their area of exposure. 
In its simplest form, the foam enhancement devices of the present invention 
may involve modifications to the outlet end of the tube 36. Thus, in the 
embodiment of FIG. 2, the tube 36 is provided with a closed lower end 62 
and a plurality of side orifices 63, whereby the stream of molten material 
passing through the tube is divided into a plurality of smaller streams, 
each of which is more readily acted upon by the low pressure environment. 
The orifices 63 are shown as rectangular slots, but could be any shape. 
The size of the orifices is chosen in accordance with the number of stream 
subdivisions desired and the amount of flow resistance that can be 
tolerated. The end member 62 may also be provided with one or more 
orifices, and if provided with a plurality of orifices, may serve as the 
stream dividing means instead of side orifices. 
Along these lines, FIG. 3 depicts an embodiment that includes an insert in 
the end of the tube 36 for subdividing the molten glass stream. The insert 
may take the form of a grid of rods or wires 65, as shown, or a wire mesh, 
perforated plate, or other foraminous member. Close spacing of the 
subdivided stream portions may cause the stream portions to recombine 
subsequently, but that is not detrimental if the stream portions have been 
acted upon by the vacuum before they recombine. Depending upon the mass 
flow rate and the vacuum level, the foaming of the stream portions may 
occur almost immediately upon encountering the reduced pressure and before 
recombining. At higher throughput rates, however, it may be preferred to 
use some of the other embodiments disclosed herein that provide a 
subdivided or thinned stream for more extended periods of time. 
The embodiments of FIGS. 2 and 3 are not the preferred embodiments, 
particularly at high flow rates, because of the added resistance to flow 
through the tube 36. This resistance not only reduces flow through the 
tube, but also complicates regulating the flow rate by valve means 35 or 
the like. Therefore, it is preferred to permit the molten material to be 
discharged from the tube 36 and then to be engaged by a foam enhancing 
device of the present invention a sufficient distance from the tube to 
avoid imparting a significant back-pressure within the tube. Accordingly, 
a wire mesh or the like is employed in the embodiment of FIG. 4 in the 
form of a basket-like receptacle 67. Instead of a mesh, the receptacle 67 
could be fabricated from perforated sheet material or other foraminous 
material. The openings in the receptacle subdivide the incoming molten 
stream at least momentarily, and the receptacle provides resistance to 
prevent the stream from falling immediately into the foam layer or below. 
The receptacle 67 is shown generally cylindrical in configuration, but it 
should be apparent that it could be provided with virtually any shape that 
would retard the velocity of the stream. Some advantage is provided by 
enclosing the outlet end of the tube 36 with the receptacle 67 in that no 
portion of the stream can escape being acted on by the receptacle. The 
upper end of the receptacle 67 may be affixed around the tube 36 by means 
of a compression band 68 as shown in FIG. 4. The receptacle 67 may be 
somewhat elongated to provide sufficient interior volume to accommodate 
any build-up of foam or molten material therein so as not to affect flow 
from the tube 36. 
A variation of the basket type foam enhancer of the present invention is 
shown in FIG. 5. There, an enclosure resembling a cage is fabricated from 
a plurality of rods 70. The cage could be provided with a wide variety of 
shapes, but some advantage may be found for the conical configuration 
shown in FIG. 5 in that it presents openings of varying width, with wide 
openings at the top for passing foamy material and narrow openings at the 
bottom for restricting the size of subdivided liquid streams passing 
through. Additionally, the conical shape presents little obstruction to 
the passage of the stream and thus is not prone to create a back-pressure 
in the tube 36. As illustrated, the rods 70 may be joined at their bottom 
ends on a disk 71 and their upper ends may be affixed to the tube 36 such 
as by welding. Optionally, the rods may be reinforced by one or more 
circumferential rings 72 at an intermediate elevation or elevations. 
In FIGS. 6 and 7 there is shown another embodiment of the present invention 
wherein the foam enhancing receptacle is in the form of a perforated cup 
75. The cup 75 may be formed of a side wall portion 76, bottom 77, and 
optionally a flared rim portion 78, all of which are preferably fabricated 
of platinum alloy. The side wall and bottom of the cup are provided with a 
plurality of holes through which the molten material may pass. The number 
and size of the holes may vary considerably, depending upon the effect 
desired in a particular case. Holes in the range of 0.5 to 3 centimeters 
in diameter have been found to be satisfactory in typical cases, with the 
number of holes being maximized without unduly reducing the structural 
strength of the cup. Although holes of a uniform size may be used, it is 
preferred to provide larger holes near the top so that at high flow rates 
any surplus flow will pass through the large holes rather than over the 
rim of the cup. The cup embodiment of FIGS. 6 and 7 could be suspended 
from the tube 36 as in the embodiments described heretofore, but a 
different support arrangement is shown wherein the flow enhancement device 
is mounted on a support arm 80 independent from the tube 36 and preferably 
extending to the exterior of the vacuum refining vessel 12 so as to permit 
adjustment or replacement of the device. In order to provide structural 
rigidity along a substantial horizontal distance in the high temperature 
environment of the refining vessel, it is preferred that the support arm 
80 be cooled by circulation of cooling fluid such as water therethrough. 
To this end, the arm 80 shown in FIGS. 6 and 7 includes concentric coolant 
conduits 81 and 82. In order to provide rigid support of the cup 75 
itself, the illustrated embodiment includes a conduit loop 83 through 
which the coolant is circulated and within which the cup 75 rests. As 
shown, the loop 83 comprises an extension of the inner arm coolant conduit 
81 and terminates with a connection to the outer arm coolant conduit 82. 
Accordingly, coolant may circulate from conduit 81 through the loop 83 and 
out through conduit 82. The direction of flow may, of course, be reversed. 
Because of the cooling, the conduits 81 and 82 and the loop 83 need not be 
fabricated of platinum, but may be made of stainless steel or the like. In 
order to reduce the cooling effect of the cooled support arm 80 on the 
interior of the refining vessel, the arm may be provided with an 
insulating cover such as the refractory tube 84 shown in FIGS. 6 and 7. 
Additional structural strength may be provided to the cup 75 by 
reinforcement members 85 which may comprise platinum alloy tubes welded to 
the loop 83 and extending down the side and under the cup. It should be 
understood that the shape of the cup may vary considerably from the 
frusto-conical embodiment shown in FIGS. 6 and 7. In particular, 
horizontally oblong cups may be advantageous for the sake of increasing 
the volume of the cup of while permitting passage into the vessel through 
an access opening of limited size. The arm 80 may extend substantially 
horizontally through an opening in the side of the vessel, or it may 
include a bend so as to extend vertically through the top of the vessel. A 
swivel connection may be provided at the location where the arm passes 
through the vessel wall to provide a gas-tight seal while permitting 
adjustments to the position of the foam enhancing device. 
In any of the embodiments of the invention, heavy flow rates can cause the 
capacity of the foam enhancement device to be exceeded. In such cases it 
may be desirable to provide a two stage foam enhancement device. In other 
words, if the stream is not foamed to the extent desired after passing a 
first foam enhancement device, it may be passed to one or more additional 
foam enhancement devices positioned to receive the partially foamed stream 
from the first device. The secondary devices may be substantially the same 
design as the first device, or combinations of different designs may be 
employed. Generally, each secondary or subsequent stage would preferably 
be larger than the preceding stage. An example of a two stage device is 
shown in FIG. 8 where a first apertured bowl 90 is supported above a 
second apertured bowl 91, both being mounted in vertical alignment with 
the outlet of tube 36 so as to receive the incoming stream of molten 
material. Material that may overflow bowl 90 or otherwise be 
insufficiently treated by bowl 90 is received by bowl 91 where the flow is 
again retarded, subdivided, and reduced in thickness. Two stages appear to 
be adequate for most purposes, but three or more stages may be resorted to 
if necessary to achieve a desired degree of foaming under particular 
conditions. The structure for supporting bowls 90 and 91 illustrated in 
FIG. 8 is merely a preferred example. It should be apparent that many 
variations of such a structure are possible. A plurality of tubes 92 
support the upper bowl 90 and another group of tubes 93 supports the lower 
bowl 91. Both sets of tubes may extend upwardly to an attachment to a ring 
94, which in turn may be supported from the tube 36 as shown, or from a 
laterally extending arm as in FIGS. 6 and 7. In the particular arrangement 
shown in FIG. 8, the tubes 92 angle inwardly below the bowl 90 in order to 
provide additional support to bowl 90 and extend through the lower bowl 91 
to join with tubes 93 in attachment to a bottom support ring 95, thereby 
providing additional rigidity to the structure. The rim of bowl 91 is 
reinforced with a tube 96 as an optional feature. 
Subdividing the stream in order to reduce its thickness and thus expose all 
portions of the stream to the effects of the vacuum is a feature of each 
of the embodiments discussed hereinabove. Instead of subdividing, it is 
also possible to reduce the stream thickness by spreading. This approach 
is shown in the embodiments of FIGS. 9 through 12. A falling stream of 
liquid attains a cylindrical shape under the influence of surface tension 
because that shape has the least surface area. Unfortunately, that shape 
is the least desirable for the sake of vacuum foaming the stream. Any 
modification to the shape of the stream will advantageously increase the 
surface area per unit volume of the stream. Therefore, one approach to 
reducing the stream thickness to enhance foaming is to provide a 
non-cylindrical stream shape, such as by using a non-circular inlet 
opening (e.g., a slot). A solid body placed in the path of the stream that 
alters its shape is also a beneficial approach and has the additional 
advantage of reducing the velocity of the stream, whereby its residence 
time in the headspace is greater. Therefore, the preferred stream 
modifying shapes are those that also provide substantial retardation. 
Maximum retardation can be achieved with horizontal surfaces, and a simple 
embodiment of that type is shown in FIG. 9. 
FIG. 9 shows a solid tray 100 that may, for example, be generally circular 
in shape, supported horizontally below tube 36 by a plurality of brackets 
101. Alternatively, the tray could be carried by a laterally extending 
support arm. Molten material flowing from the tube 36 spreads on the tray 
100, is reduced in thickness as it is momentarily detained on the tray, 
and flows over the edges of the tray in a generally annular shape having 
considerably greater surface area than the original stream. The tray may 
be provided with upwardly projecting edge portions if it is desired to 
retain a larger amount of material on the tray. The edge portions may also 
be notched to produce subdivided streams flowing from the tray. 
A variation of the FIG. 9 embodiment is shown in FIG. 10 where the flow 
spreading device is not horizontal but has substantial horizontal 
dimensions. Instead of a tray, an upwardly pointed conical member 105 is 
provided, supported by brackets 106 from tube 36. It should be apparent 
that various other shapes could be used to spread the stream, such as a 
hemisphere, wedge, or tetrahedron. 
In the embodiment depicted in FIGS. 11 and 12, some spreading of the stream 
may occur, but the primary effect is to lengthen the flow path so as to 
increase the time that the melt is exposed to the low pressure in the 
headspace. This embodiment entails means to divert the falling stream from 
its vertical path to a more tortuous path. An example of a relatively 
simple flow diverting means is a trough 110 as shown in FIGS. 11 and 12. 
The trough may be supported within the headspace of the refining vessel 12 
at a slight angle from horizontal, and the incoming stream may impinge on 
the high end of the trough and flow off the low end. In order to provide 
stiffness to the horizontally elongated trough the use of cooling means is 
preferred. Therefore, a conduit 111 for circulation of coolant (preferably 
water) may be provided in the trough with a hair pin configuration for 
example. The coolant conduit also may serve as the support arm for the 
trough. The conduit 111 may be fabricated from stainless steel or the 
like. To reduce heat loss from refiner to the cooling conduit 111, the 
conduit may be encased in refractory material 112. The refractory 112 is 
preferably sheathed in a platinum alloy cover 113 to provide durability to 
flowing molten glass. As shown in FIG. 12, the upper surface of the trough 
may be concavely contoured to help guide the flow along its length. 
Although the primary intended flow path is along the length of the trough, 
at high flow rates some material may overflow the sides of the trough. The 
trough 110 is depicted as being linear, but it should be understood that 
the flow diverting means could include bends and curves. It may also be 
desirable for the molten stream to flow from one flow diverter to another, 
thereby additionally extending its residence time in the headspace. 
Other variations and modifications as are known to those of skill in the 
art may be resorted to without departing from the scope of the invention 
defined by the claims which follow.