Note: Descriptions are shown in the official language in which they were submitted.
CA 02125929 1998-02-16
"METHOD AND APPARATUS FOR
CONDITIONING AND HOMOGENIZING A GLASS STREAM"
BACKGROUND OF THE INVENTION
The invention concerns a method for conditioning and homogenizing a
continuously flowing stream of colored glass, in particular of amber or green
glass,
in a conditioning stretch in a forehearth, which extends from an inlet to an
outlet,
and which consists of at least one cooling zone and a subsequent homogenizing
zone for the glass temperature, whereby the temperature in the conditioning
stretch is reduced from the inlet temperature T1 to the working temperature
T2,
preferably for the production of molded glass articles such as containers and
pressed glass articles.
Whereas the temperatures necessary for melting glass depend on the .
composition, on the production process and on other factors, the temperatures
required for processing the glass are normally lower than the melting
temperatures
of the glass. Consequently the glass must be cooled between the melting and
working processes. Cooling of the glass is a part of the so-called
"conditioning",
during which the glass is prepared for processing. The achievement of the
level of
thermal homogeneity necessary for the particular working process is also part
of
the conditioning of the glass.
Conditioning of the glass can only take place when the glass has left the
:.,, .,
actual melting unit. Certain developments in the recent past have radically
changed the situation concerning the cooling of glass. Various improvements
have
been made in the melting furnaces which have resulted in a significant
increase in
the specific melting capacity; i.e. the melting capacity related to the area
of the .~
melting zone. Consequently the temperature of the glass leaving the furnace
has
increased. Other melting aids, such as bubblers or bottom heating, which have
the
1
CA 02125929 1998-02-16
r~~~
~~ect of increasing the glass temperature on the bottom of the melting tank,
have
also led to an increase in the temperature of 'the glass leaving the melting
tank.
Continual improvements have also been made to glass processing machines,
amongst other things, to increase the throughput. Whereas in the 1960's and
1970's, machines for the mass production of containers were equipped with 6, 8
or 10 stations each for two gobs, nowadays 12 to 16 stations each for two gobs
or 10 stations each for three or four gobs are used. The throughput capacity
of
individual machines has therefore been greatly increased.
As a result of the factors mentioned above, significantly more heat must
now be removed from the glass after it has left the melting tank and before it
is
worked than in the past. The increase in the throughput of the individual
machines
has also reduced the residence time of the glass in those parts of the system
where the glass conditioning takes place. Thus, a greater amount of heat must
be
removed in a shorter time. This results in the fact that the productivity of
the
complete production line depends to a large extent on the cooling capacity
along
the conditioning stretch. However, numerous technical problems must also be
taken into consideration.
As a result of the relatively high viscosity of the glass, the flow of glass
in
forehearths, the basic form of which is normally a channel) is laminar. It is
usual
for a velocity profile to be established in the glass bath, in which the
maximum lies
on the glass surface approximately in the center of the channel. As the
viscosity
depends on the temperature of the glass, there is an interaction between the
glass rr--
temperature, the heat losses and velocity of the glass. Wherever the velocity
in a 3--w~
particular area is lower, the resulting increase in the residence time leads
to higher
heat losses. Thus, the temperature sinks even further, and the increased
viscosity
leads to an additional decrease in the velocity.
At a constant throughput, a reduction of the velocity in one area
automatically leads to an increase in the velocity in other areas with higher
glass
temperatures. This results in a reduction of the residence time in the higher
temperature areas and so reduces the effective cooling capacity. For this
reason
2
CA 02125929 1998-02-16
the area of the glass bath affected by a cooling system must be clearly
dyefined,
and, as far as possible, this cooling area must avoid areas in which there are
low
flow velocities.
Areas of low temperatures and higher viscosity produce an effective
reduction in the flow cross-section, which in turn leads to an increased drop
in the
glass level between the melting tank and the extraction point. This can also
result
in production disturbances.
Furthermore, when glass of a certain composition is cooled below a specific
temperature limit, which is dependent on the glass composition, crystals can
be
formed, a process known as "devitrification". ~ This process can also cause
significant disturbance in the production. Therefore the cooling of the glass
bath
to temperatures below the devitrification temperature should be avoided. As
crystal formation depends on both the temperature and time, the residence time
of
the glass in the critical temperature range is also an important factor.
The transport of heat within the glass bath itself is almost completely by
radiation, whereby the transport velocity depends on the glass composition.
For
'example, the presence of ferrous iron or chromium, which are used as coloring
agents in green glass, reduces the rate of heat transport in the glass bath in
comparison with a colorless glass. This results in a delay in the heat
transport
from the lower areas of the glass bath. However, the lower areas of the glass
bath must be cooled. If the cooling is applied too late, then no effective
cooling
effect can be observed in the lower areas of the glass bath.
Numerous cooling systems for glass conditioning are known, in most of
which the heat transport is primarily by radiation. This type of heat removal
is
advantageous because the heat is not removed directly from the glass surface,
but
from a layer of the glass bath, the thickness of which depends on the
radiation
transmission of the glass. The Stefan-Boltzmann Law is used to calculate the
amount of heat transported by radiation. An important factor in this
mathematical
function is the temperature difference between the radiator and receiver.
Applying
this function to a typical case for the glass industry, the temperature of the
3
1~
CA 02125929 1998-02-16
radiator is the temperature of the glass, Therefore the temperature of the
receiver
s
determines the amount of heat which is removed.
U.S. Patent No. 3,645,712 describes the installation of water cooled plates
in a forehearth, whereby the width of, and distance of, such plates from the
melt
is so low that only the center of the glass stream and only the surface
thereof are
intensively cooled. The distance quoted can be varied between 1.27 and
15.24cm. Measurements made across the glass bath 7.62cm below the bath
surface, and made in conjunction with supplementary air cooling, show a clear
wave shaped temperature variation, with the absolute minimum point in the
middle
of the channel, whereby temperature differences of between 6 and 18°C
were
a;.~.
found between the maximum and minimum. This temperature profile extends
along at least part of the subsequent homogenizing zone. At the measuring
point
the glass bath is approximately 40cm deep, so that the area of melt near the
bottom is not effectively cooled and a significant temperature gradient from
the
bath surface to bottom is to be expected. This situation is not significantly
changed by the raised bank installed at the beginning of the forehearth, as
this
covers only about half of the cooling zone length and in which area a glass
bath
depth of approximately 25cm can be assumed. Furthermore, it is stated that the
bath must have a considerable depth.
When the distance between the cooling plates and the glass bath surface is
increased the temperature profile straightens out, but an increasing amount of
energy is removed from the heating gases, which is detrimental to the thermal
efficiency of the forehearth.
U.S. Patent No. 2,888,781 describes a forehearth with basically a single
depth, except for a deepened area situated approximately half way along a
forehearth, in which a cooling coil, stirrers and electrodes are installed,
which are
all designed to homogenize the glass temperature. Even if this is successful,
the
homogeneity is reduced again in the following cooling zone, which includes a
long
radiation opening in the forehearth roof. The glass temperature is reduced
from
1316-1371 °C to 1 1 16°C. The length of the following
homogenization zone from
4 ,
CA 02125929 1998-02-16
the beginning to the extraction point is shorter than the length of the
radiation
opening.
U.S. Patent No. 4,029,488 describes cooling units installed in the bottom of
the channel at the forehearth entry, such that the glass flows over the
cooling
units, which are thereby said to exert an intensive cooling effect. At the
beginning
of the forehearth two cooling units are installed in line along the center
line of the
channel, followed by two more cooling units installed side by side. However, a
strong cooling effect is only exerted at some distance from the forehearth
entry.
This type of cooling unit extracts heat only from the layer of glass directly
in
contact with it, which is of necessity on the bottom of the channel. In
practice it
is difficult to move this layer, even when stirrers are used. Furthermore the
depth
of glass in the channel is relatively high, so that it is difficult to achieve
a more or
less homogeneous temperature distribution. The stirrers cannot be installed
deep
enough to pick up the cold bottom layer as this would cause too much corrosion
to
the refractories.
German Patent DE-PS 25 07 015 describes the use of water cooled stirrers
in the melting tank itself, between a melting and refining section with a high
temperature on the one hand and a refining zone with a lower temperature on
the
other, in order to increase the homogenization and to improve the quality of
the
glass. However, this requires a longer melting tank, and the problems
connected
with further cooling and temperature homogenization before the processing of
the
glass are not solved.
Finally, it is known from German Patent Application DE-OS 31 19 816 that it
is possible to divide a forehearth into five zones, the first two of which are
a rapid
cooling zone and a fine cooling zone. The glass is mechanically stirred in the
third
zone, and the fourth zone is an equalizing section for homogenizing the
temperature before the glass enters the fifth zone, in which the normal gob is
formed. Enclosed channels are pro4ified in the roof and the bottom of both the
rapid cooling and fine cooling zones for the selective or simultaneous flow of
a
cooling fluid. However, the heat removal per unit length of the two cooling
zones
._ ......w:,.
., . ,
F~-
CA 02125929 1998-02-16
is still limited, so that the glass must flow through a zigzag-shaped channel,
in
which additional electrodes are installed to heat the glass in the so-called
"dead
corners". Cooing and additional heating of the glass must therefore be carried
out
simultaneously, so that large quantities of heat are passed from the
additional
heating to the cooling zones.
In "GLASS-MAKING TODAY" (Doyle, 1979, Portcullis Press) Redhill/GB,
pages 199/200) a forehearth is described with air cooling for the glass and
with a
glass bath depth of 152mm, whereby the glass bath width should be 91.4cm for a
throughput of 90 tons per day and 122cm for a throughput of 150 tons per day.
It can be assumed that with flint glass these parameters are sufficient to
achieve
the necessary level of homogenization; there is no information about colored
glass.
In "Increased Conditioning Time Leads to Improved Thermal Homogeneity",
(Sims, "GLASS INDUSTRY", November 1991, pages 8-15), a forehearth is
described with a throughput of 70 tons per day of green glass, in which the
~,~~~"w...__ _
temperature in the first third of the forehearth is reduced from 1290 to
1180°C,
i.e. a reduction of 1 10°C, and in which the glass is homogenized in
the following
homogenizing zone as far as the bowl with the extraction point, to a level
where
the maximum temperature difference is 4°C, measured in the center of
the stream,
from top to bottom at depths of 25mm, 75mm and 125mm. This result must be
considered to be very good for the given throughput of green glass. All other
measurements were carried out at only 25mm below the glass surface.
Subsequent measurements and calculations have shown that the
temperature of the melt entering the cooling zone 20mm above the bottom is
approximately 1220°C, and the temperature of the glass just above the
bottom can
only be reduced to about 1210°C in the cooling zone, i.e. only by about
10°C.
Therefore only a relatively low amount of energy is removed on average.
~._-
With larger throughputs, and/or when it is necessary to reduce the glass , ,
Y~ f'
temperature by a large amount, it is neither possible to remove the larger
amounts
of energy involved, nor is it possible to reach the necessary level of thermal
homogeneity, simply because the lower layers of the glass stream do not
6
CA 02125929 1998-02-16
r'
participate sufficiently in the heat transfer by radiation. The temperature
curve
measured along the length of the forehearth becomes flatter and the minimum
point moves along towards the extraction point to the detriment of the
homogenization zone.
It would be possible to increase the channel width in proportion to the
increase in throughput, whilst maintaining a given bath depth of 15cm.
However,
this approach is limited by constructional problems. In addition, the
temperature
difference between the two side streams and the central stream (the horizontal
temperature profile and the horizontal velocity profile are both parabolic)
will
increase without additional side heating. This decreases the thermal
efficiency, as
this heat must also be removed later.
As already. explained, the bath depth cannot be increased to more than
15cm) and therefore it is logical at a given bath depth to increase the flow
velocity, at least for a oars, and to increase the length of the cooling zone,
in order
to increase the residence time of the melt in the cooling zone.
As described above, an increase in the surface cooling alone is not
sufficient, as the temperature gradient from top to bottom increases: a hotter
and,
as a result of its lower viscosity, faster moving bottom layer flows below a
relatively cold, high viscosity surface layer. This effect cannot be
compensated by
any cooling zone, regardless of length. A compromise must therefore be sought.
The conditions are particularly difficult in the case of amber or green
glasses,
which absorb a significant proportion of the longer wavelength radiation. In
"Glass
Furnaces" (German - "Glasschmelzwannen"), published by the Springer Publishing
Company in 1984, Trier, shows in a diagram on pages 211 and 212 that the
radiation transmission of amber and green glasses at a temperature of
1300° is
only approximately 15-25% of the transmission of white flint glass (for
example
for tableware or window glass). Increasingly poor cooling conditions therefore
exist with both increasing glass bath depth and increasing glass color. This
leads
to increasingly large temperature differences between the glass su~ face and
the
bottom parts of the glass bath. Particularly long homogenization zones are
then
7
t
CA 02125929 1998-02-16
necessary to compensate for these conditions, whereby such zones requite
significant amounts of energy. Furthermore, the space requirements for such
zones poses a further problem. As already indicated, these problems also
increase
in severity by a factor of 4-6 as the glass color becomes darker.
The problem of cooling in the deeper areas of the glass bath could
conceivably be solved by reducing the glass bath depth in the forehearth or
feeder. '
However, this solution would lead to the establishment of a glass level loss
as a .
result of the temperature dependance of the glass viscosity and the typical
flow
pattern which occurs in highly viscous liquids, whereby the extent of the
glass
level loss would increase with increasing throughput. High throughput levels
are
exactly what is required for modern glass production units. However, a
significant
glass level loss must be avoided in the forehearth or feeder, as this would
make it
impossible to apply the same production parameters at each outlet.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide a method of the '
type described initially, whereby with a high throughput of colored glass
through
the forehearth and when it is necessary to reduce the glass temperature by a
large
amount, both a strong cooling effect and a good homogenization are achieved.
As
already demonstrated, these objectives are diametrically opposed to one
another to
a certain extent.
The object is achieved according to the invention by the application of the
method described initially in which in the at least one cooling zone, with a
throughput of at least 70, preferably 90 tons per day, a maximum bath depth
Dmax of
120mm is achieved by means of a raised section of the bottom along the length
of the
cooling zone, and in which the cooling capacity is chosen so that the
temperature 20mm
above the bottom of the forehearth or feeder is reduced by at least
40°C, preferably by at
least 50°C, between the beginning of the cooling zone and the entry to
the
homogenization zone.
Expressed more simply, the aim of the invention is to create an area at the
beginning of the forehearth in the cooling zone(s1, with a very shallow glass
bath, by
8
CA 02125929 1998-02-16 '_.
t_
t
means of at least one raised section of the bottom, and to extract as much
heat as
possible from this shallow area, so that, depending on the length of the total
conditioning
stretch, the longest possible homogenizing zone is available, in which the
remaining
temperature differences can be equalized. However, outside the shallow cooling
zones)
the bath should be deeper in order to avoid greater glass level losses.
A comparison with the state of the art can demonstrate the amounts of energy
which can be led off per hour with green glass, whereby a forehearth entry
temperature
T1 of 1280°C is used as the basis:
TABLE 1:
Example Throughput Temperature Energy Max. .
tons/day reduction per hour deviation
0°C Kcal/h °C
1 *) 70 80 82,000 4 - 5
2 *) 70 100 102,000 4
3 * *) 90 120 160,000 4
4 **) 160 160 ***) 375,000 4
*) - comparative examples from "Sims" bath depth DB - 150mm
* *) - Bath depth DB = 120mm
* * *) - for champagne bottles, 700m1
TW (working temperature) = 1 120°C
portion weight (glass glob) = 950g
An increase in the cooling capacity solely by increasing the size of the
radiation
openings, i.e. with no change in the bath depth, neither produces the
necessary .
temperature reduction nor does it lead to the desired thermal homogeneity.
These
requirements were only met when the bath depth was limited locally by the
raised section
of the bottom in accordance with the subject of the invention.
It is obvious the situation improves further as the depth is reduced, when,
for
example, the maximum depth Dmax is further reduced to 100mm, 80mm or less. All
intermediate values are also advantageous.
9
,,
.... _ . _
CA 02125929 1998-02-16
The extremely shallow glass bath depths mentioned here are not sglf-evident at
the
high throughputs required today. The reason for this is that a drop in the
glass level
occurs along the length of the cooling zone and the remaining channel, the
extent of
.,
which depends on the flow velocity. The flow velocity, and therefore the glass
level .
drop, increases significantly with shallower glass baths.
It is obvious that the glass depth cannot be reduced below a certain minimum
value in each specific case. These values can be determined by experiment.
It is also the intention of the invention to utilize a shallow bed cooling,
with which
it is possible to use known cooling systems to remove, in a relatively short
distance, an
extremely large proportion of the heat equivalent to the temperature
difference between
T1 and T2.
The temperature T1 is that at the outlet of the distribution channel or that
at the
forehearth entry, 20mm below the bath surface, whereas temperature T2 is the
temperature at the first or only extraction point. Temperature T1 is a result
of the
operating conditions required in the melting tank and typically lies in the
range of 1250 to
1400°C. Temperature T1 is normally higher when the throughput or
melting tank
capacity is higher. Temperature T2 is determined by both the type of glass and
by the
forming conditions for the glass gobs which are normally produced, and
typically lies in ;'~°""'"'"'~'
the range of 1120 to 1 180°C. If possible the local variation of T2
should be kept within
the tolerance range of +/-4°C, preferably +/-2°C.
The use according to the invention of a wide and shallow channel (for a given
throughput or a given flow velocityl permits an extremely high level of
cooling in the .
shortest possible distance with a simultaneous reduction in the differences in
the flow
velocity and in the temperature, and therefore also in the viscosity. It is
advantageous if
at least 30% of the amount of energy equivalent to the temperature difference
between
T1 and T2 is removed from the glass bath per meter of length at the beginning
of the
cooling zone. However it is particularly advantageous if the cooling effect is
intensified
and at least 40% or if possible, at least 50% of the amount of energy
equivalent to the
temperature difference mentioned is removed from the glass bath per meter of
length in
the cooling zone. r~__
CA 02125929 1998-02-16
As already mentioned, it is possible to use known measures or methods for
removing heat. Bottom cooling can be achieved by means of cooling channels
installed in
the bottom of the conditioning stretch andlor by means of reduced insulation
in the
bottom area. Furthermore it is possible to install surface cooling, either as
an alternative
or an addition, by means of variable openings in the roof, and/or by blowing
in a gas,
such as air and/or by means of cooling units which are installed above the
glass surface
without touching the glass bath. Finally it is also possible to cool in the
glass itself, for
example with cooling units which are submerged in the glass and which can also
have
the same effect as stirrers.
Prior art methods can also be used for the subsequent temperature
homogenization
in the glass. Such methods include a homogenization zone of the appropriate
length with
the best possible thermal insulation to the surroundings, uncooled stirrers or
deflector
plates, and also gentle heating of the glass from above by means of radiation
or within
the glass by means of direct electrical resistance heating and appropri~.ie
heating
electrodes.
The use of shallow bed cooling in accordance with the invention brings all the
colored glass under the influence of the cooling system for a short time. In
particular, the
bottom of the glass bath is also subject to cooling, for example, by means of
significant
radiation upwards. This effectively suppresses the negative effects of the
interactive
relationships between temperature, viscosity, residence time and glass flow,
which are
otherwise found.
The installation of the cooling zone right at the beginning of the
conditioning
stretch is of particular importance, as the temperature at this location is
very high and the
viscosity of the glass is very low.
The application of shallow bed cooling to the shortest possible length
according to
the invention allows the use of a basically linear channel, so that so-called
"dead corners"
can be avoided, as can additional heating of the glass in these dead corners.
Furthermore
the design of the cooling zone is made much more simple and the temperature
distribution is homogenized to a large extent in the cooling zone.
The invention also concerns an apparatus for the implementation of the process
11
CA 02125929 1998-02-16
with a conditioning stretch integrated in a forehearth between an inlet point
and an
extraction point and which consists of at least one cooling zone and a
subsequent
homogenization zone for the glass temperature, whereby a raised area is
provided in the
bottom of the flow channel in the cooling zone area, said raised area
therefore being
shallower than neighboring areas.
In order to achieve the aim of the invention, the apparatus according to the
invention is characterized by the fact that the raised area covers the
complete length of
the cooling zone, that the maximum depth Dmax of the channel in the area of
the cooling
zone is 120mm, and that the channel in the homogenization zone, which is
adjacent to
and follows the cooling zone, is at least 30mm deeper.
The explanations given above in connection with the reduction of these values
is
still valid.
BRIEF DESCRIPTION OF THE DRAWINGS
Examples of the invention are explained in more detail below, with the aid of
FIGS.
1 to 12.
FIG. 1 is a schematic plan view of a melting tank, distribution channel and
three
forehearths, each with one extraction point.
FIG. 2 is a longitudinal section through a cooling zone with two radiation
openings
t
in the roof.
FIG. 3, on the left hand side, is a plan view of the object shown in FIG. 2,
and on
the right hand side, a section along the line III-III in FIG. 2 is shown.
FIG. 4 is a cross section through the object shown in FIG. 3 along line IV-IV.
t
FIG. 5 is a longitudinal section through a cooling zone similar to that shown
in FIG.
2, but with an enclosed cooling channel in the roof.
FIG. 6 is a schematic plan view of a forehearth)
FIG. 7 is an enlarged section through left hand side of FIG. 6.
FIG. 8 is a longitudinal section through a cooling zone, which is extended to
include an immersed cooler in the form of a barrier.
FIG. 9, on the left hand side, is a plan view of the object shown in FIG. 8,
and, on
the right hand side, a section along the line IX-IX in FIG. 8 is shown.
12
CA 02125929 1998-02-16 '
i
FIG. 10 is a cross section along line X-X through the object shown in FIG. 9.
FIG. 11 graphically shows temperature values at several measuring points along
a
the complete length of the conditioning stretch according to the state of the
art and
according to the invention.
FIG. 12 shows viscosity values at several measuring points along the complete
length of the conditioning stretch. '
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Unless otherwise stated the parts shown in section consist of the normal
ceramic
or refractory furnace and insulation materials.
FIG. 1 shows a melting tank 1, with a charging opening 2, which is normally
constructed as a "doghouse", and the throat 3, through which the glass is
removed from
the melting tank. The glass formers, which can consist of the usual batch and
cutlet, and
the resulting glass, flow through the melting tank in the direction of arrow
4. It is also
possible to install several charging openings along both long sides of the
melting tank,
which produces a different flow pattern in the melte.r. A so-called riser can
also be part
of the throat 3. Such details are state of the art, so that no further
particulars must be
given here.
The distribution channel 5, which can also be constructed as a working end, is
connected to the throat 3. In a first example of the invention, two cooling
zones 6 and
7, which are indicated by hatching and are enclosed within a dotted line, are
located one
on each side of a central plane M-M. It is also possible to join these two
cooling zones
into a single cooling zone, in which case the cross hatched area 8 is then
added.
Two conditioning stretches 9 and 10 are formed on both sides of the center
line
M-M, to which cooling zones 6 and 7 and, when applicable, area v, belong. The
remaining parts of the flow path of the glass melt then form the homogenizing
zones 11
and 12. Homogenization of the temperature distribution is achieved as far as
possible in
the homogenizing zones.
In a second example of the invention three forehearths or feeders 13, 14 and
15
are connected to the distribution channel 5, each ending in an extraction
point E. The
total lengths of the forehearths 13, 14 and,15 in this example each constitute
a
13
JrYi:
CA 02125929 1998-02-16
conditioning stretch 16. In each of these conditioning stretches 16 there is a
cooling
zone 17, 18 or 19, which is marked by hatching and enclosed within a dotted
line. It is
emphasized that the diagram has been kept very simple in order to aid
understanding. A
homogenization zone 20, 21 and 22 follows each cooling zone in each of the
conditioning stretches 16.
In the example shown it is possible to omit the cooling zones 6, 7 and 8 in
the
distribution channel 5, so that the glass enters the forehearths at a very
high
,.
temperature.
FIGS. 2, 3 and 4 show different views and sections of a cooling zone K, which
can
be installed as the cooling zones 17, 18 and 19 in the forehearths 13, 14 and
15. The
cooling zones concerned represent a length of channel with a bottom 23, side
walls 24
and a roof 25. The glass enters from the inlet side 26 and exits on the outlet
side 27. In
the example shown in FIGS. 2 - 4 two rectangular shaped openings 28 are
provided in
the roof 25, which allows loss of heat by radiation from the glass. In order
to be able to
control the flow of energy to the surroundings, covers 29 are located on top
of the
openings, by means of which the effective sizes of the openings 28 can be
varied. The
opening and closing movement is indicated by arrows. In both side walls 24
there are
rows of burners 30 (FIG. 4), the nozzles of which are installed in the burner
blocks 31.
The burner blocks have openings 31 a for the outlet of the combustion gases,
which are
shown in FIG. 2 as semicircles as the upper half is more or less hidden by
ribs 31 b which
reach down from the roof 25, as approximately shown in FIG. 4.
FIGS. 2 and 3 show that the bottom 23 has a raised area 23a over a length
which ~"~~' ~~
roughly corresponds to the length of the openings 28 plus the intermediate
roof section
25a. This raised area extends across the complete width B of the channel. The
raised
area 23a is connected to the lower bottom level 23c at both ends by means of
sloping
areas 23b, whereby the height difference is at least 30mm. It can be clearly
seen that .
the glass 32 is reduced to a very shallow cross section across this raised
area 23a, which
makes the high cooling capacity possible. The homogenizing zone begins at the
outlet ,
side 27.
14
'__
v
t:;
CA 02125929 1998-02-16
FIGS. 2 and 4 show dotted diagonal lines which cross each other within the
roof
openings 28, and continue outside these openings 28. These lines characterize
the
radiation absorption effect that the background to these openings has on the
glass melt
32. As a result of the varying radiation permeability of the glass melt and of
the angular
distribution of the radiation which emanates from every location on the glass
surface, it is '~
,., -w
clear that there are no clearly defined limits. However, the diagonal lines
give sufficient
information to allow a calculation of the cooling effect of the roof openings
28 to be
made. It can be seen that a relatively large surface area of the glass melt 32
is reached
by the cooling effect of the cover openings 28.
FIG. 5 shows a further version of the cooling zone K, in which a single hole
33 in
the roof 25 is covered with a plate 34, which in this case forms the radiation
receiver for
the heat radiated by the glass bath 32. A U-shaped channel 36 for the passage
of
cooling air is provided above the plate 34 within the superstructure
refractory 35. The
entry 36a and exit 36b of the channel 36 point upwards; the flow direction as
indicated
by arrows. The temperature of the plate 34 is influenced by the quantity of
cooling air
used, and the cooling effect on the glass bath is thereby varied.
FIG. 6 shows that the length of the cooling zone K or 17 covers almost exactly
the
first third of the forehearth, and the remaining length is formed by the
homogenization
zone H or 20. The following temperature metering points are indicated in the
middle of
the glass batch:
M1 - directly in front of the forehearth entry VE to measure the entry
temperature T1;
M2 - right at the end of the cooling zone K or at the entry to the
homogenization zone H;
M3 - in the middle of the homogenization zone H; and
M4 - shortly before the extraction point E, which is located in the
bottom of a bowl, to measure the exit temperature T2.
Temperature sensors are installed 20mm below the glass surface and 20mm above
the bottom at the measuring points M1 and M2. At measuring points M3 and M4
there
are 3 temperature sensors installed vertically above one another, 20mm, 60mm
and
..
CA 02125929 1998-02-16
100mm below the glass surface, to measure the vertical temperature
homogeneity.
FIG. 7 shows the cooling zone K from FIG. 6 in a larger scale, the four dotted
rectangles R3, R4, R5 and R6 show the cross sections of four conventional roof
openings
which correspond to position 28 in FIGS. 2-4. The sum of the opening cross
sections in
existing installations (also according to Sims) amounts to a maximum of
approximately
20% of the area of the bottom of the forehearth cooling zone.
The two fully drawn rectangles R1 and R2 represent the cross sections of the
openings F1 and F2 of the roof openings 28, which, according to the invention,
amount
to at least 30%, preferably at least 40) 50, 60 or even 70% of the cooling
zone area as
defined by the surface FB of the raised bank on the bottom (23a) of the
forehearth. The
surface FB, which is calculated from the length and width of the channel in
the area of
the raised bottom, can also be described as the plateau surface.
As already mentioned, success is not achieved solely by an increase in the
cooling t
s
capacity, as there is a hotter bottom layer which flows faster than and below
the slower
flowing surface layer, and which is cooled less as the residence time is
further reduced,
whereby the negative result is amplified by an even steeper temperature
gradient.
The example shown in FIGS. 8, 9 and 10 is different from FIGS. 2-4 in that,
instead of two openings 28, only a single larger opening 28 is provided, and
while
otherwise maintaining similar geometric proportions to those in FIGS. 2, 3 and
4, a
further area is provided, and while otherwise maintaining similar geometric
proportions to
those in FIGS. 2, 3 and 4, a further area is provided after the raised area
23a of the
bottom 23. in this further area a submerged cooling device 27 is installed
above the
lower bottom level 23c, the cooling device being supplied with a cooling
medium via two
vertical pipes 38. The supply pipes 38 can also be designed as concentric
pipes, but this
version is not shown in detail. The cooling effect in the upper and/or lower
regions of the
glass bath 32 can be varied by raising or lowering the immersion cooling
device 37 in the
direction of the double arrow 39. The cooling effect can also be increased by
periodic
crosswise movement (perpendicularly to the glass flowl. Combined movements of
the
immersion cooler are also possible. It.is also very easy to install the
immersion cooler 37
in the form of a stirrer, in order to achieve additional homogenization of the
temperature
16
CA 02125929 1998-02-16
U_.
in the glass bath. The supply pipes 38 pass through an opening 40 in the roof
25.
Lines 41 a and 41 b in FIG. 1 1 show the temperature profiles 20mm below the
bath
surface and 20mm above the bottom along the distance "L" of a conditioning
stretch
with a constant bath depth of 150mm, according to the state of the art. The
measuring
points M1 to M4 as described above are shown on the x-axis. It can be seen
that the
temperature in the cooling zone K can be reduced by about 100°C, i.e.
from 1290°C to
1190°C just below the surface (line 41 a), whereas the temperature just
above the bottom
is only reduced by about 10°C, i.e. from 1220°C to
1210°C, whereby the difference is
about 30°C at the measuring point M2. This difference could be
significantly reduced by
the extraction point E at the end of the homogenization zone, so that a
reasonably
homogeneous temperature distribution around 1 205°C was achieved.
If an attempt is made in the same forehearth to reduce the glass temperature
by
170°C, to a level of 1120°C, the temperature near the surface
will only reach a value of
about 1135°C, although the homogenization zone is also being used (line
42a). The
temperature near the bottom can only be reduced to about 1185° (line
42b), so that the
temperature distribution at the extraction point 3 is very inhomogeneous) with
a dT of
approximately 50°C, which cannot be reduced further in the
homogenization zone H.
The lines 43a and 43b show the analog temperature profiles of the object of
the
invention with a shallower bath depth (120mm) above a raised bank and with
increased
cooling capacity. The temperature near the surface at the end of the cooling
zone has
already been reduced by 165°C to 1 125°C, and the temperature
near the bottom is
reduced from 1220°C to 1135°C, i.e. by 85°C.
The difference of 10°C at the end of the cooling zone K can be reduced
by 2 to
3°C around the set point value of 1 120°C by the end of the
homogenization zone H.
Comparable conditions are valid for other inlet and outlet temperatures. A
characteristic
of the method according to the invention is therefore the significant
temperature
reduction near the bottom of the feeder and/or forehearth.
With a wide, shallow glass bath, the average temperature is reduced
significantly
along the relatively short cooling zone K, even with a high throughput of
colored glass, as
indicated by the steep temperature curve. In this way, with a given total
length, there is
17
. _..,,
,._.. _
CA 02125929 1998-02-16
a relatively long stretch available for the homogenization zone H, along
w#~ich the average
temperature remains largely unchanged. Despite the steep temperature drop in
the
cooling zone K and the even average temperature in the homogenization zone H,
the edge
zones on both sides of the wide glass bath may be heated additionally, as
shown for the
cooling zone in FIGS. 2-5 and FIGS. 8-10 (burner arrangement). Attention must
merely
be paid to the achievement of the best possible temperature distribution, by
the
combined effect of shallow bath cooling, edge and, where necessary, surface
heating.
However a relatively long current path is available in the homogenization
zone, and in the
cooling zone conditions have been established which ensure that the
temperature
differences at the individual points of the glass bath cross section are not
too great at the
entry to the homogenizing zone. As the glass depth is small compared with the
width B,
the heat losses through the side walls 24 are reduced, which also improves
homogeneity
and saves energy.
The lines 44 and 45 in FIG. 12 depict the variation of the average viscosity
with a
state of the art system (line 44) and according to the invention (line 45).
The use of the .r....---
shallow bath cooling increases the average viscosity very much earlier,
whereby a more
even flow velocity is achieved across the complete flow cross section,
together with a
better temperature homogeneity. The intensive shallow bath cooling
systematically
counteracts high flow velocities in areas of high temperature and low
viscosity, as the
cooling directly affects virtually all areas of the glass bath.
When the complete situation is considered it is clear that it is particularly
important
to install the section with the highest cooling capacity as near to the
beginning ~of the
conditioning stretch as possible.
By consideration of the invention special attention should be paid to the high
throughputs, average flow velocities and temperature differences.
~~.~T......~ _.::.,._..._
Forehearths normally have small cross sections and so high flow velocities
occur.
The average flow velocity of at least 8 m/h, which can be achieved with the
invention, is
a very good value if the required thermal homogeneity is achieved at the same
time. In
accordance with the invention the average flow velocity in a forehearth can
even be
increased to 17 m/h and above, without affecting the required average
temperature T2
18
,."
CA 02125929 1998-02-16
and/or the thermal homogeneity of the glass melt.
As is apparent from the foregoing specification, the invention is susceptible
of
being embodied with various alterations and modifications which may differ
particularly
from those that have been described in the preceding specification and
description. It
should be understood that we wish to embody within the scope of the patent
warranted
hereon all such modifications as reasonably and properly come within the scope
of our
contribution to the art.
,r .
19
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