Patent Application
20070089460
The invention concerns a method for conditioning and homogenization of glass melts that flow in flow channels with a vertical central longitudinal plane and side walls, using the effect of alternating cross-section changes and stirrers with vertical stirrer shafts, installed in the flow direction.
The Problems that Occur During the Homogenization of Glass:
A glass with certain transmission properties is produced by the addition of concentrates that influence the transmission, such concentrates being preferably added to a flint or light base glass in a forehearth or flow channel filled with glass. The concentrates, being carriers of components such as polyvalent ions that influence the transmission, are added and must first be melted. This results in differences in the concentration and the density in the glass. These must be equalized to the extent that no differences in concentration or transmission can be determined in the end product. Diffusion, assisted by convection currents in the glass bath, plays a major role in this.
However, as on the one hand very long residence times are required for these processes, and on the other hand the complete bulk of the glass is not involved in equal measure, an attempt is made to speed up these processes by means of intensive forced mixing. The aim is to mix in such a way that glass with higher and lower concentrations of components that influence the transmission are repeatedly brought together and that if possible the whole of the glass bath is included in the mixing process. These considerations also apply to processes other than coloring. During the actual mixing process the glass is pulled apart to produce the longest possible volume components with large reaction surfaces, and consequently to increase the natural diffusion processes. To this end, normally one or even several stirrers are used, which should produce optimum glass mixing.
The present invention concerns the location of the stirrers in conjunction with the design of the flow channel.
State-of-the-Art:
U.S. Pat. No. 3,328,150 and U.S. Pat. No. 3,723,084 teach the installation of several banks of spiral stirrers in a flow channel or forehearth. The number of stirrers in a bank, the number of banks, the stirrer types and rotation direction can differ and depend on the tonnage to be produced and the channel geometry.
U.S. Pat. No. 3,328,150 describes equipment for mixing molten glass, where in a flow channel, either two cuboid raised areas on the bottom and a skimmer block that projects into the melt from above, or alternatively, cuboid deflector walls that project into the melt from the sides, produce strongly meandering glass flows. Pairs of stirrers are installed between these bottom raised areas or the deflector walls perpendicular to the longitudinal axis of the flow channel. This creates right-angled stagnant areas in front of and behind the raised bottom areas or deflector walls, whereby the contents of these stagnant areas are at best barely affected by the stirring process. This results in long cleaning periods and a great loss of glass when the self-cleaning method is used after a glass change.
U.S. Pat. No. 3,723,084 teaches the mixing of glass melts in a flow channel, where two cuboid raised areas on the bottom and a skimmer block that projects into the melt from above produce a strongly vertically meandering glass flow. Stirrers are located between these bottom barriers along the longitudinal axis of the flow channel. This creates right-angled stagnant areas in front of and behind the raised bottom areas or deflector walls, whereby the contents of these stagnant areas are at best barely affected by the stirring process. This results in long cleaning periods and a great loss of glass when the self-cleaning method is used after a glass change.
The advantages of these concepts lie in their relatively simple construction. The disadvantages of these concepts lie in their limited mixing capacity. The specific load of the coloring channels is therefore limited by the fact that the base glass flows along the channel walls towards the outlet, without being influenced by the stirrers.
The known solutions should prevent unimpeded flow of the base glass. As an example, the strongly meandering current in the channel fitted with stirrers is designed to achieve this. The strongly meandering current is created by the obstacles described. It is further recommended that the glass is diverted from horizontal to vertical flow in order to increase the stirring effect. This technology is preferentially applied for the homogenization of small flow volumes in the special glass sector.
On the one hand, the diversion of the glass into a vertical flow is advantageous with regard to the homogenization effect produced. On the other hand, it also has the disadvantage that the channel bottom is not flat. When a change to a new set of coloring agents is made this means that the colored glass treated with the old coloring agents must be removed from the channel. This is achieved as a natural progression by self-cleaning by the flow through the channel. The flow through the channel is less intensive in front of and behind the obstacles, and especially in front of and behind those in the bottom that are used to divert the flow into a vertical direction. This in turn means that longer cleaning periods and high colorant and glass losses must be expected on such installations.
The problems described are also not solved satisfactorily by the state-of-the-art described below.
U.S. Pat. No. 3,352,659 discloses a method of mixing a glass melt inside a flow channel by the installation between two channel sections of a single vertical and partly cylindrical passage, in which at least two stirrer elements are installed one above the other on a vertical shaft so that the glass melt is transported along a Z-shaped path. The glass flow is concentrated by wedge-shaped projections installed in both vertical channel walls with stirrer elements located between their vertical edges. However, despite the complicated multi-level installation the mixing effect is limited and restricted to the area of the only passage.
German patent DE 1 471 832 A1 describes a method for the homogenization and coloring of glass melts by installing at least one row of stirrers across a flow channel with flat parallel walls, in order to create upward currents in the upper area of the melt. However, to enable the formation of return currents in the melt, the stirrers should not influence the bottom layer. This separation is enforced by the fact that stationary circular discs are installed below the stirrers at a distance above the flat channel bottom. This publication presents a controversial view of the problem that in the case of known stirrer systems there should be no return currents (column 2, lines 43 to 48).
U.S. Pat. No. 3,463,627 describes a method for mixing colorants into a glass melt in a flow channel with a flat channel floor and flat channel walls by the use of vertical, baffle plates independent of the side walls and at an angle to the longitudinal axis of the channel, with vertical stirrers installed between in an alternating arrangement. The baffle plates and/or the stirrers are the actual colorant transporters and therefore gradually wear away. With this method the glass melt is forced to flow in large meanders with the successive addition of colorants, whereby the final colorant is stirred the least. Furthermore the stirrer paddles are surrounded by large areas with so-called “stagnant areas”, which contain large glass quantities, which leads to long cleaning times and high glass losses at a color change. Most importantly, for a color change, the complete installation must be taken apart and the most important operating components exchanged.
German patents DE 25 52 116 A1 and DE 26 47 673 A1 describe a method for mixing in coloring agents or homogenizing the glass melt in a flow channel by the installation of two stirrers in vertical stirrer casings, these being installed as barriers across the flow channel and with vertical wall surfaces in the direction of flow. In the first stirrer casing the melt is transported from top to bottom and in the second stirrer casing the melt is transported from bottom to top. The two stirrer casings are connected to one another at the lower end. As a result of the stagnant areas, a large part of the melt volume is not affected by the stirring, so that long cleaning periods and high glass losses occur when either the color or the glass is changed.
German patent DE 31 19 816 A1 discloses a high performance forehearth for conditioning a glass current, with a flow channel in which a stirrer zone with a single vertical stirrer is installed between two cooling zones on the one hand and an equalization zone and a bowl and extraction zone on the other. This stirrer is installed in a vertical casing over a descending stepped bottom of the flow channel. The glass flows downwards in this casing, but the flow is retarded by the sense of rotation and construction of the stirrer.
In order to extend the flow path in the two cooling zones, obstacles in the form of plates are installed opposite one another on alternate sides of the strongly meandering path. Heating electrodes are installed in the stagnant areas that are created on both sides of these obstacles. Such stagnant areas also lead to long cleaning times and high glass losses during a change of glass. The equalizing zone is a pipe with an unchanging cross-section. In order to maintain glass homogeneity in the equalizing zone the design and control of the heating system is complicated. The glass content in this zone also leads to long cleaning times and high glass losses during a glass change.
German patent DE 102 53 222 A1 is concerned with the problem of glass refining by enforcing repeated changes in the glass flow direction and frequent diameter reductions by the use of special obstacles built into the channel and a reduction in the surface tension of the glass. If a stirrer unit is provided, then it should also be used to assist the mixing and homogenization of the melt. The stirrer unit consists of a stirrer in a vertical tank, into which the melt enters on one side about half way up and from which it leaves through the bottom. The gap between the stirrer and the channel wall should be less than 20 mm and the stirrer rotation speed higher than 10-20 revolutions per minute. Up to 20 baffle plates are installed as flow deflectors. The examples show flat, angled and conical baffle plates with numerous openings with relatively small cross sections. However, direct interaction between the baffle plates and the stirrer is not mentioned.
U.S. Pat. No. 5,862,169 describes a method of connecting a working end and a float bath by means of a channel with a right-angled bend for thermal conditioning and homogenization. In order to prevent stagnant areas a swan-neck is created at the bend location by reducing the channel width from two meters to one meter using two cambered wall elements installed opposite one another. On the one hand, it is disclosed that stirrers can also be installed in this channel, but on the other hand, it is not disclosed that they should be installed in the immediate vicinity of the bend and the projections. The swan-neck therefore is not used to constrict the flow in the area immediately around each stirrer.
Document No. WO 2004/070251 A1 discloses a method of vacuum degassing for expelling gas bubbles from glass melts. This process is carried out while the glass flows through metal tubes with round or oval cross-sections with sequential convex and concave longitudinal sections that successively increase and decrease the diameter of the tube and which allow variation of the pipe length as a result of vibrations. In the corresponding published U.S. Patent Application No. 2005/0268663 A1, at paragraph [0074] it is disclosed that the convex sections 20 can be pushed together or extended in an axial direction, in order to compensate for thermal expansion or shrinkage of tube 10, without changing the total length. This is not possible with ceramic materials. Where the wall thicknesses are specified they should be between 0.1 and 1.5 mm. If stirrers are mentioned they should be installed in separate chambers at a distance from such areas of variable tube diameter (
Japanese Patent No. JP 61-006133 A discloses the installation of five groups of stirrers in a forehearth or feeder channel split into different sectors for mixing in lead compounds for the production of table or crystal glass. The axes of each stirrer group are perpendicular to the longitudinal axis of the symmetrical feeder channel. The second to fourth stirrer groups each have five stirrers, i.e. a total of fifteen stirrers and the fifth stirrer group has five stirrers. To accommodate the second to fourth stirrer groups the feeder channel is widened and symmetrical extensions are built opposite one another, without any narrowing. The plan view of these protrusions shows a trapezoid cross-section with angled walls. In this way the tendency to create stagnant corners in the flow is suppressed. Only the inner stirrers do not interact with the angled walls. The extensions serve only to accommodate so many stirrers. These extensions result in significant volume increases, which is contrary to the object of the invention, which is to reduce both the time and amount of glass needed for a change in the glass type. It deals rather with the cladding with resistant platinum plates.
An object of the invention is, on the one hand, to achieve effective homogenization of the main glass current in the flow channel, and on the other hand, to attain high flexibility in the complete installation. As part of this aim, a change to another type of glass should take place with the shortest possible cleaning time and the lowest possible glass losses. In particular, with coloring processes the addition of coloring agents should be reduced and, when changing from one color to the next, the shortest possible cleaning times and lowest coloring agent and glass losses should be attained.
Achievement by Means of the Inventive Process:
This object of the invention is achieved with the method described initially in that
On the one hand, the invention achieves effective homogenization of the main glass flow in the channel and on the other hand, achieves a high flexibility of the total plant. As a result, when the change from one glass type to the next is made, cleaning times and glass losses are reduced. In particular, with the coloring processes fewer coloring agents must be added and when the change is made to different coloring agents, shorter cleaning and shutdown times and fewer glass losses are the result.
For further embodiments of the invention it is particularly advantageous if, either singly or in combination:
The invention also concerns an apparatus for conditioning and homogenizing glass melts with at least one flow channel with a vertical central longitudinal plane and side walls, between which, viewed in the flow direction, alternating changes in the cross-section and stirrers with vertical stirrer shafts are installed in the direction of flow.
In order to achieve the same object and attain the same advantages according to the invention, it is recommended that
For further embodiments of the invention it is particularly advantageous if, either singly or in combination:
The method and the apparatus are particularly suitable for the mixing of coloring agents, for the supply of molten glass to float glass tanks and for the processing of high quality glasses.
In the following, examples of the object of the invention and their effectiveness and other advantages are explained in detail on the basis of FIGS. 1 to 5. The figures show:
The flow channel 1 has a vertical central longitudinal plane E-E, in which the axes of the stirrer shafts 6 lie. The side walls 3 and 4 have protrusions 7 and 8, that are installed opposite to one another in pairs and symmetrical to the central longitudinal plane E-E, and reduce the clearance “A” of the cross-section to dimension “Q”. The ratio Q:A is between 0.5 and 0.9. This produces so-called pockets, in which the stirrers rotate concentrically. When viewed laterally as per
According to
In the case of the stirrers 5 so-called paddle stirrers are preferred. However, the stirrers 5 may also be fitted with several layers of paddles in pairs. Furthermore the stirrers 5 can also have single paddles or stirrers with several stirrer arms that are fitted vertically above one another. The stirrer arms may have a round or flattened cross-section, and be positioned at an angle to the vertical. The arrangement, geometry, rotation speed and direction of the stirrers 5 can be further optimized. The stirrers can, for example, rotate in the same direction or alternately in opposite directions, provided that the advantageous upward currents are created.
The flow channel 1 is heated from the side by burners that are not shown. The burners are typically supplied with a pre-mix of combustion gas and air, oxygen enriched air or oxygen. The flow channel 1 can also be heated electrically by electrodes that are not shown. The electrodes protrude laterally into the melt in the flow channel 1.
The protrusions 7 and 8, that are designed to match the contours of the stirrer movement are located both upstream and downstream of the stirrers 5. It has been shown that the horizontal protrusions 7 and 8 produce flow restrictions, which are very advantageous for homogenization in comparison with a simple configuration of rows of stirrers in a flow channel with a constant width “A”.
The same numbering system is continued in
It is significant here that the curved area 11 can be flattened at the sides or rounded, in particular to prevent the formation of stagnant corners in the flow at this location. It can also be seen here that the principle of protrusions 7 and 8 creates pockets, in which the stirrers 5 are installed. The flow direction is shown by the arrow 12, and it can also be seen that a transverse row 13 of so-called spiral stirrers is installed in front of the row with alternating stirrers 5 and protrusions 7 and 8. Another transverse row with between 2 to 6 stirrers can also be installed immediately before the exit from the flow channel 1.
It should also be noted that such flow channels 1, 10 and 14 are normally installed between a melting furnace with melting, refining and conditioning areas and possibly a working end on the one hand and an extraction area for container and flat glass or rolled plate glass on the other. When coloring agents are to be added, which, for example, can be added in the form of a frit, this is done according to known principles, e.g. the addition to a flow channel near the entry from the melting installation.
However the invention is certainly not limited to coloring processes. Its use is also envisaged for special glasses that are not colored, such as, for example, for the manufacture of LCD and/or TFT glass, as this type of homogenization is also advantageous here, as here good glass homogeneity is of prime importance.
From the above description, it is apparent that the objects of the present invention have been achieved. While only certain embodiments have been set forth, alternative embodiments and various modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit of scope of the present invention. It should be understood that we wish to embody within the scope of the patent warranted heron all such modifications as reasonably and properly come within the scope of our contribution to the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102005050871.5-45 | Oct 2005 | DE | national |
