Forehearth homogenization method and apparatus

A forehearth for feeding molten glass to a flow feeder, a set of blenders, and a set of homogenizers are spaced from each other along the length of the forehearth with three evenly spaced vertically positioned elongated plates located between the set of blenders and the set of homogenizers. The particular arrangement provides a system for eliminating, to a great extent, cords at the feeder.

BACKGROUND OF THE INVENTION 
It has long been known that stirring can markedly improve the quality of 
glass, especially when done when the glass is in a relatively viscous 
state just before it is cast. With the recent emphasis on reduction of 
weight and wall thicknesses of many mass produced glass products such as, 
for example, television envelopes, non-returnable beverage bottles, 
improvement in the quality of the glass is becoming increasingly 
important. Striae, cords and other imperfections constitute localized 
weaknesses in the glass products, rendering them unmerchantable. Reduction 
of striae and other imperfections permits the achievement of lighter 
weight glass products because it enables a reduction in wall thickness 
without loss of strength relative to similar products in which account 
must be taken of considerable localized weakening due to the presence of 
cords and striae. 
Thus, even for products which, heretofore, have been made of glass of 
relatively poor quality, there is a growing demand for glass of relatively 
high quality, of a quality even approaching that ordinarily required in 
the manufacture of optical ware. 
Although the practice of the present invention is expected to have 
relatively wide application, its immediate utilization on a practical 
commercial scale is thought to have its greatest potential in connection 
with the standard type of industrial equipment on which substantially all 
of the mass produced glassware in the United States is made. The standard 
equipment includes a melting furnace where the initial batch ingredients 
of the glass are melted and heated to a relatively high temperature to 
form a glass of relatively low viscosity, a forehearth where the glass 
discharged from the melting furnace is cooled to make it more viscous, and 
a discharge bowl, or spout which receives the glass from the forehearth 
and discharges it through an orifice in the form of gobs. The apparatus of 
the present invention is especially suitable for substitution in place of 
the final section of the standard forehearth for stirring the glass just 
before it is delivered to the spout. 
The standard forehearth comprises, in the order of flow of the glass from 
the melting furnace to the spout, first, a cooling section, and then a 
conditioning, or equalizing section. Both heating fires and cooling wind 
are provided in the cooling section, and heating fire is provided in the 
conditioning section but basically the design is intended to accomplish 
substantially all of the cooling in the cooling section, and to render the 
temperature of the entire body of flowing glass uniform throughout its 
thickness and width in the conditioning section. Heretofore, some stirring 
has been done at the entrance of the conditioning section by placing 
rotating paddles or turbines in the molten stream of glass at that point. 
Such stirring as heretofore carried out has been effective to improve the 
uniformity of temperature of the molten stream of glass and to reduce 
thermal gradients in it, but has failed in significantly reducing the 
amount of striae and cords in the glass and for physically homogenizing 
it. 
SUMMARY OF THE INVENTION 
Method and apparatus for mixing and blending molten glass in a forehearth 
in which a pair of helical blenders mounted upstream of the feeder end of 
the forehearth are rotated such that the blenders raise the glass in the 
zone of their influence. Downstream toward the feeder end of the 
forehearth, a plurality of transversely spaced helical homogenizers are 
rotated to effectively push the glass downward in their zone of influence 
and intermediate the blenders and homogenizers are positioned a plurality 
of elongated vertical plates with their lower edges resting on the bottom 
of the forehearth and extending the full depth of the molten glass such 
that movement of the glass from the blenders to the homogenizers will be 
such that cords or other stria will be attenuated by the plates.

DETAILED DESCRIPTION OF THE DRAWINGS 
With particular reference to FIGS. 1 and 2, there is shown what is normally 
termed the conditioning section 10 and feeder bowl 11 of a typical 
flow-type forehearth. Within the conditioning section 10, there is 
positioned two blenders 12 and 13. In the particular configurations shown, 
the conditioning section 10 has a width of approximately 26 inches with 
the glass therein being at a depth of about 12 inches. Each of the 
blenders 12 and 13 may be approximately 7 inches in diameter. The vertical 
axis of the blenders 12 and 13 is positioned approximately 61/2 inches 
inwardly from the respective sidewalls of the conditioning section. It 
should be understood that the flow of glass in the apparatus shown will be 
from the left to the right with the glass issuing in the form of gobs from 
an orifice 14 positioned below an opening formed in the bottom of the 
forehearth. Immediately above the orifice 14 is a plunger 15 which serves 
to control the formation of gobs in the usual manner. A tube 16 surrounds 
the plunger 15 and is rotated to maintain the glass in the vicinity of the 
orifice relatively even in temperature. 
Upstream from the tube 16 are four homogenizers 17, 18, 19 and 20. As a 
particular example, the homogenizers are 4 inches in diameter. The 
homogenizers 17-20 are rotated in the direction indicated by the arrows in 
FIG. 1. The blenders 12 and 13 are also rotated in the direction indicated 
by the arrows in FIG. 1. Intermediate the blenders and homogenizers are 
positioned three 8-inch long plates 21, 22 and 23. The plates are 
supported by a bar 24 which extends across the width of the forehearth, as 
can best be seen in FIG. 1. The plates are generally rectangular in shape 
and evenly spaced from each other and are equally spaced from the blenders 
and homogenizers. The lower edge of the plates is in contact with the 
bottom of a forehearth 25 and, in conjunction with the adjacent sidewalls 
of the fourhearth, provide shear planes for the moving glass. These plates 
are stationary and formed of a material that is capable of withstanding 
the glass temperature and erosion characteristics of the moving glass. The 
blenders are homogenizers, as can be seen from the drawings, are 
essentially helical mixers which are spaced from the bottom of the 
forehearth an amount, 2 inches - 3.5 inches, sufficient to avoid excessive 
shearing of the glass on the bottom thereof. 
Applicants have found that the use or positioning of stationary vertical 
plates in the forehearth increases the shear stress and attenuate cord. 
The low level of shear stress with the arrangement described does not 
generate seeds. With the particular arrangement shown in the drawing, it 
can easily be seen that equal width flow channels between the plates are 
used and this is so as to minimize non-symmetric side-to-side variations 
of mixing effectiveness. Several other configurations were tried by 
applicants in a model forehearth with plates of 4 inches in length, 8 
inches in length and 12 inches in length being tried. All plate 
configurations when positioned between blenders and homogenizers had some 
effect in mixing cord. It was found, however, that 4-inch plates were less 
effective for cord which would appear at the top and middle center of the 
forehearth flow channel than either the 8-inch or 12-inch long plates. For 
all of the plates, mixing effectiveness is improved when they are 
positioned between the blenders and homogenizers for all top and middle 
center cord. There is not much change in middle side and bottom cord; 
however, these are affected by the mixing systems anyway. The 8-inch 
plates where found to be more effective in mixing cord than the 12-inch 
plates, primarily because the 8-inch plates created less restriction to 
the flow patterns around the stirrers than the 12-inch plates. 
It should be noted that the introduction of stationary plates within the 
forehearth does increase the glass flow resistance and for this reason, in 
order to maintain a 12-inch glass level at the feeder end of the 
forehearth, in the feeder bowl, it would be necessary to increase the 
glass level in the refiner up to about one-half inch, depending upon the 
actual flow rate through the forehearth. 
With the particular arrangement where the blenders move the glass upward 
and the monogenizers are moving the glass downward, much of the glass will 
be moving from the blenders to the homogenizers in the upper zone thereof 
within which the plates are positioned. In this manner, the plates will 
effectively attenuate cords which might be present in the glass. 
Several other factors should be considered. The 7-inch diameter blenders, 
when operated at 20 rpm and because of their large size, must be carefully 
monitored to be effective. The mixing effectiveness of the 7-inch diameter 
blenders with 4-inch homogenizers results in an increased radius of 
influence and the circulation of fluid around the blenders is such that 
chances of short circuiting the influence of the blenders is greatly 
diminished. As might be expected, the 7-inch diameter blenders do entrap 
air at a lower angular velocity than would 4-inch diameter blender. These 
blenders, however, compensate for this adverse effect by being capable of 
operating at a lower angular velocity and yet achieving improved mixing 
effectiveness. 
In general, the radius of influence is the farthest radial distance from 
the stirrer which the rotation of the stirrers significantly influences 
the fluid. For all fluid to be mixed, the stirrer operation must 
incorporate significant interaction among the radii of influence and the 
stationary forehearth surface. If sufficient overlapping of the radii of 
influence does not occur, then significant short circuiting around the 
stirrers will permit cord to flow past the stirrers without being mixed. 
Applicants have found that to reduce undesirable mixing variations, the 
stirring should be symmetrical about the longitudinal centerline of the 
forehearth. Non-symmetrical stirring produces regions of short circuiting. 
A symmetrical stirring set-up minimizes side-to-side variations and 
reduces short circuiting. To reduce the possibility of one stirring 
arrangement passing unmixed fluid to the weak region of another stirring 
arrangement, consecutive stirring set-ups should be different. Perhaps the 
easiest way to obtain different stirring set-ups and still maintain 
symmetry, is to have a different even number of stirrers at consecutive 
stirring locations. The operation of these stirring set-ups must be 
symmetric with respect to the longitudinal centerline of the forehearth. 
Another consideration is the possibility of producing shear induced seeds 
by operation of a stirrer. It is known that seed generation is related to 
shear stress, shear strain rate or other shear related function. With this 
in view, care should be taken with regard to the amount of shearing 
involved with the mixing of the glass. Furthermore, with helical mixers of 
the type disclosed herein, there are limits to the velocity which the 
stirrers may be rotated to avoid both shear induced seeds as well as air 
entrapment. Another important aspect of the operation of a successful and 
acceptable homogenization method must avoid glass level fluctuation at the 
feeder, and yet maintain an adequate glass level above the stirrer. 
As set forth in the appended claims, applicants' invention relates 
specifically to the combination of blenders, homogenizers and stationary 
plates which result in removing cord from moving in a feeder forehearth.