Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an electrically-heated,
melting furnace having a cooling trough located in the bottom
thereof.
Electrically-heated, glass melting furnaces have been
known in the art to a limited extent since the early 1900's.
One problem with the earlier furnaces was the lack of effective
electrodes by means of which the electric heating could be
accomplished. More recently, satisfactory electrodes have been
developed. At the same time, electric heating of glass melting
lQ furnaces has become more desirable since the electrically-
heated furnaces have a particular advantage in substantially
eliminating pollution at the glass melting site~ An additional
advantage results when the electric power is generated by coal
or nuclear fuel, in contrast to burners in conventional glass
melting furnaces utilizing relatively scarce gas or oil as the
fuel. However, the electric heat also has other advantages,
including making it possible to obtain higher quality glass and
a hlgh degree of melting efficiency. Better control over the
final glass composition can also be achieved, especially when
there are more volatiles in the glass batch.
With conventional glass melting furnaces employing
combustion burners firing over the molten glass, as the through-
put of the furnace increases, the temperature of the glass tends
to decrease. With electrically-heated furnaces, however, with
the electrodes immersed in the molten glass, an increase in the
throughput has been Eound -to result in an increase in the
temperature of the molten glass exiting from the furnace.
This invention seeks to provide an electrically-
heated glass melting furnace which facilitates achieving a more
uniform temperature of the exiting glass even under conditions of varying
throughput.
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Accordinc3 to one aspect of this invention there i~
provided a furnace for melting a heat-softenable material,
the furnace comprising a tank for holding molten material
formed by a bottom, side walls, and end walls, means for
supplying batch material over molten material in the tank,
means extending into the molten material below the batch
material for supplying heat to the molten material, wall
means forming a cooling trough centrally located in the
bottom of the tank and extending beyond an end wall theréof
to discharge molten material from the tank, the wall means
being exposed to ambient conditions below the bottom of
the tank to dissipate heat from molten material in the
trough.
According to another aspect of this invention there
is provided a method of operating an electrically~heated
melting furnace for melting a heat-softenable material,
comprising establishing a pool of molten material in a tank,
supplying batch material onto the surface of the pool to ~ :~
establish a layer of batch thereover, heating the molten
material below the batch layer, collecting molten material ;~
in a trough in the bottom of the tank, directing the molten
material in the trough toward a location beyond the pool, and
cooling the molten material in the trough.
A furnace in accordance with an embodiment of the
invention has a cooling trough centrally located in the
bottom of the furnace
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tank and extending beyond a discharge end wall of the furnace
to a riser. The cooling trough is designed so as to project
below the main bottom of the tank, whereby both the sides and
bottom oE the trough are exposed -to ambient conditions below the
glass melting furnace. The cooling trough thereby presents a
substantial surface area from which heat can be dissipated. The
cooling trough preferably has a width not exceeding about one~
fourth the width of the furnace bottom, and preferably has a
depth at least equal to the wldth to assure substantial protru-
sion of the trough below the hottom o~ the tank and a corres-
; pondingly substantial degree of heat loss. The trough preferably
extends not more than about one-half the distance between the end
walls of the tank since the glass near the forward end of the
tank has a minimal convection current, and a cooling trough at
this portion of the tank would be less effective. The extra
length of such a trough would increase the cost of the tank and
increase general heat loss without having any real benefit on
the cooling of the exiting glass through the trough at the dis-
charge end of the tank.
Cooli~g means also extend along the cooling trough
and provide a cooling medium in heat exchange relationship with
the outer surfaces of the trough. More specifically, the cool-
ing means can be in the form of an air duct extending along the
trough and having a plurality of outlets directing air into con-
tact with an outer surface of the cooling trough. The volume
of air then can be controlled to control the extent of cooling
of the trough. Temperature-sensing means can be located down~
stream in the trough to sense the temperature of the glass
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flowing therethrough, with a control unit controlling the air
volume in response to the temperature. Hence, as the throughput
increases and the glass temperature rises, the volume of the cool-
ing medium can be increased to correspondingly increase the amount
of heat dissipated from -the surfac~sof the trough.
Short electrodes can also be provided for the cooling
trough with these electrodes extending through a wall of the
trough, preferably the bottom, and having exposed portions below
the furnace. The electrodes, being heat conducting, can thereby
conduct heat from the molten glass in the trough to the ambient
conditions below the furnace. The extent to which the electrodes
protrude into the trough can also be controlled, with the degree
of cooling by the electrodes there~y regulated. If desired, a
power supply, preferably an emergency power supply, can be pro-
vided for the trough electrodes. In the event of a main power
failure, power can then be supplied to the electrodes to heat
the glass in the trough and prevent solidification.
The invention, its objects, and its advantages will be
further understood from the following detailed description of a ;~ `
preferred embodiment thereof, reference being made to the accom~
panying drawings, in which:
Fig. 1 is a schematic, fragmentary view in longitudinal
cross section of a furnace embodying the invention;
Fig. 2 is a schematic, fragmentary, plan view of the
furnace of Fig. 1, without glass in the furnace;
Fig. 3 is a schematic view in transverse cross section
taken through the furnace of Figs. 1 and 2; and
Fig. 4 is a greatly enlarged view of a portion of the
furnace of Fig. 3, and showing certain additional details.
Referring to the drawings, and particularly to Figs. 1
and 2, an overall melting furnace embodying the invention is
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indicated at 10. The furnace is illustrated in connection with
glass melting operations, although the furnace according to the
invention can also be used to advantage in the melting of other
materials. The furnace supplies molten ylass to a riser 12,
located beyond the discharge end, and to a conditioning chamber
14 and a forehearth 16. From the forehearth, the molten glass
can be supplied through openin~s 18 to suitable fiber-for~ing
devices 20 located therebelow. Of coursel the furnace according
to the invention is not limited to supplying glass for fiber-
forming operations.
The furnace 10 includes a glass melting tank 22 formedby side walls 24, a foxward end wall 28, a rear or dis-
charge end wall 30, and a main bottom 32. A suitable roof tnot
shown) can be supported above the tank 22. Heating means for
melting glass or other batch in the furnace 10 include a plural-
ity of electrodes 34 extending upwardly into the tank 22 from
a lower level through the bottom 32. The electrodes 34 are
preferably positioned in a symmetrical manner with respect to a
center line extending longitudinally through the tank 22, with
electrodes toward the forward half of the tank being substan-
tially uniformly spaced apart, and with those toward the dis-
charge half of the tank being spaced farther apart at the center
of the tank than at the sides. The portions of the electrodes
34 exposed below the tank bottom 32 can be protected by suitable
sleeves containing an inert gas, and the electrodes can also be
water cooled, as is well known in the art. Power is supplied to
the electrodes 34 through suitable leads from a suitable power
source (not shown).
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The portions of the electrodes 34 within the tank 22
are totally immersed in a pool of molten glass 36, with the upper
ends of the electrodes typically being about ten inches below the
surface of the molten glass 36. Additional glass batch is c~n~
tinually supplied over the molten glass 36 to form a batch layer
38 thereon which also serves as an effective in~ulating layer on
the molten glass. The batch can be supplied ~rom a batch car-
riage 40 which extends across the entire wldth of the tank 22
between the side walls 24 and is reciprocated back and
forth substantially over the length of the tank. A carriage of
this nature is disclosed more full~ in U.~ patent No.3,877,917
issued April 15, 1975 in the name of Charles M. Hohman. The car-
riage can include one or more vibratory feeders to regulate the
rate of feed of the batch from the carriage 40 to the layer 38.
When the output of the fiber-forming units 20 or other
glass forming devices is increased, the throughput of the
furnace 10 also must be increased. To accomplish this, more
electrical power is supplied to the electrodes 34 and a larger
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quantity of batch is fed from the carriage 40. Since the
20- heating means or electrodes are entirely immersed within the
molten glass 36, the increased throughput with the increased
power to the electrodes 34 results in a higher temperature in
the molten glass supplied to the forehearth 16. This can be
detrimental to the fiber-forming operation or other operation,
particularly where the forehearth glass temperature is critical,
as is true of fiber-forming operations. To achieve a more uni-
form glass temperature in the forehearth 16, in spite of
increased glass pull or throughput, a cooling trough 42 in
accordance with the invention is provided in the bottom of the
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tank 22. The trough 42 is formed by wall means indicated at 44
protruding below the bottom 32 of the tank 22, the wall means 44
constituting two side walls 46 and 48 (see ~igs. 3 and 4) and a
bottom wall 50. These walls present three surfaces exposed to
ambient conditions below the tank 22 to provide substantial heat
loss to the ambient from the trough 42. The width of the trough
42 preferably is from one-sixth to one-fourth the width of the
tank 22, with the depth of the trough preferably being at least
equal to the width to provide adequate heat loss through the
walls 46 and 48 and the bottom 50. The trough 42 is of a length
sufficient to extend to the riser 12 past a throat 52 located
between the tank and the riser. At the other end, the trough 42
prefexably extends no more than about half the distance between
the discharge end wall 30 and the forward end wall 28. Any
shorter distance does not enable sufficient cooling to occur in
the trough 42. If the trough extended further toward the
forward end wall 28, it would not have any substantial beneficial
cooling effect, particularly since convection currents in the
- molten glass toward the forward end are minimal. Consequently, -
the coolest glass would not necessarily collect in the trough to
any noticeable extent toward the forward end. Thus, an extra
length of the trough toward the forward end wall would only
generally increase heat loss and result in higher construction
costs for the furnace. Toward the discharge end of the tank 22,
convection currents are more pronounced ~Fig. 3) with the coolest
glass collecting in the trough 42, thus reducing the cooling
requirements for the glass flowing in a stream through the
trough 42.
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Cooling means are provided for the trough 42 to enable
the cooling effect on the glass flowing through the trough to
change as the throughput changes. In a preferred form, the
cooling means in accordance with the invention are indicated at
54 and include a main duct 56 extendiny longitudinally of the
trough 42 and a plurality of branch outlets 58 communicating
therewith and directing a cooling m~dium, specifically air,
toward the bottom 50 of the wall means 44. The cooling medium
or air can be supplied by a suitable blower 60 (Fig. 1), with
the volume of the air controlled by a damper or valve 62~ The
flow-control damper 62 can be opened more as the throughput oE
the furnace 10 increases and the ~lass discharged therefrom
tends to increase in temperature. A higher volume of the air
is then directed through the outlets 58 toward the bottom 50 of
the trough thereby to increase the dissipation of heat from the
trough and produce a greater cooling effect on the molten glass
flowin~ therethrough. ~?
The damper 62 can be automatically controlled, if
desired. For this purpose, a temperature-sensing device or
thermocouple 64 is located in the trough 42 toward the down-
stream end thereof with the device 64 being connected to a
temperature controller 66. This, in turn, operates a control
motor 68 which is operatively connected to the damper 62 so as
to tend to close the damper 62 as the temperature sensed by the
thermocouple 64 decreases and to open the damper 62 as the
temperature sensed by the thermocouple 64 increases. The com-
ponents 64, 66 and 68 are commerclally available devices and -~
are not discussed in detail.
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Also, in accordance with the invention, a plurality of
short electrodes 70 can be loca-ted in the trough 4 , extending
upwardly through the bottom 50 of the wall means 44. By them-
selves, the electrodes 70 are heat-conducting elements which can
serve to cool the glass by conducting heat in the glass in the
trough 42 to ambient conditions below the tank 22. If desired,
each of the electrodes 70 can ba provided with a gear drive 72
(Fig. 4) and a motor 74, by means of which the electrodes 70
can be raised so as to protrude further into the trough 42, or
lowered so as to have a shorter length immersed in the trough.
With this arrangement, the extent of the cooling effect of the
electrodes 70 can be controlled. As the extent of protrusion
of the electrodes into the trough increases, the cooling effect
of the electrodes on the glass flowing through the trough also
increases, as long as a sufficient portion o the length remains
exposed so as to dlssipate heat adequately. The streams of
cooling air supplied by the outlets 58 can be directed-at the
exposed portions of the electrodes 70 below the bottom 50, if
desired, to increase the rate of conduction of heat away from
the glass in the trough by means of the electrodes.
If desired, the electrodes 70 can also be connected
to a power supply as indicated in Fig. 4, preferably an emer-
; gency power supply. In the event of a main power failure for
the electrodes 34, the emergency power supply can be used to
heat the electrodes 70 and thereby maintain the glass in the
trough 42 in a molten condition under emergency conditions.
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