Download The Glass Industry - Electric Melter Design

DesignConSlderO[IOnS
rorAn-e~ec[rIC
Me~[ers
A review of basic considerations,
appropriate power systems and electrode types
By WilLIAM R. STEITZand CARLW. HIBSCHER
Toledo EngineeringCo., Inc.
Toledo, Ohio
N THE SURFACE, it might
furnace heat losses. Furnace aging
the melting area and the furnace
appear as though the engimust not be forgotten in considering
dimensions have been determined, it
the maximum demand load.
neering aspects of electric
becomes necessary to establish the
After the power requirement has
glass melting are merely simple apspecifications
for the electrical
been established, and for the time
plications of Ohm's Law, Joule's
power equipment. Total power rePrinciple, and a few other rudimenbeing we will assume that the type of
quired is a straight-forward calculatary electrical engineering principles.
electrical system and the electrode
tion based on the energy required to
It is, however, a more complicated
placement have also been establishmelt and refine the glass, plus the
subject, and this paper will deal with
only three aspects of it: (1) some
basic considerations, (2) appropriate
power systems, and (3) electrode
types.
BASIC CONSIDERATIONS
O
A. Joule's Principle-Raw
glass
batch is an electrical insulator, but
when the batch becomes molten, it
becomes a good conductor of electricity. When a voltage is applied
across a circuit in which molten glass
is the resistive element, heat is
generated by the Joule Principle; i.e,
P=PR, ALSO P=EI or P
Where,
R
P = Power in Watts
E = Voltage in Volts
I = Currents in Amps
R = Resistance in Ohms
B. Determination of Glass Resistance- The physical sizing of the
electric melter is not considered in
this paper. After the pull capacity,
L,
Parameters of Furnace Resistance
Fig. 1
Phasing Vector
A,
3C1>Voltage
Regulator
..+..
A,
L.
Scott-T
Transformer
x,
Fig. 2
A,
x,
Single PhasePlate Electrode System
Fig 3.
A,
B,
B,
Single Electrode Square (Two Phase System)
Fig. 4 Double ElectrodeSquare(Two (2)-Two Phase Circuits)
ed, it becomes necessary to specify
the transformer's secondary characteristics; i.e., the voltage range and
current capacity. To do this with any
degree of accuracy, the operating
phase resistances must be determined. This is where problems can
begin.
Resistance is determined by the
basic equation:
R = P Le
Where, Ae
R = Resistance in Ohms
P = Resistivity in Ohm-Ft.
or Ohm-Cm
Le = Electrical Length in Ft.
or Cm
Ae = Electrical Area in Sq. Ft.
or Sq Cm
Fig. 5
Let's examine each variable.
Glass resistivity can be determined
by laboratory testing techniques.
Because few commercial laboratories have the equipment to properly
conduct this testing and since a standard test method has not been established, one must accept published
results on similar glass, or set up
one's own testing facility. Accepting
published data, while reasonable to
do with most soda-lime container
glass, can be very risky with other
more specialized glass. As a furnace
designer dealing in many varied
types of glass, we have found it
essential to have our own testing
facility with a known reliability.
Once the resistivity curve is
known, the operating temperature
Vertical Electrodes
HorizontalElectrodes
Electrical Length (Le)- Two(2)Electrodes Per Pole
POWER SYSTEMS
A
A
B
Fig. 6
Iii'"
.
Throat
C
c
B
===
Unsymmetrical Three Pole - Three Phase System
A
A
B
Fig. 7
-
SymmetricalThree Pole-Three Phase System
range must then be predicted so the
resistivity range of the glass can be
determined.
Electrical length (Le) is usually,
but not always, a simple measurement once the electrode locations are
determined. Electrical area (Ae) is
another story. The area in question is
the effective electrical area through
which the current will flow. A quick
look at a simple two-electrode single
phase system, which is shown in Fig.
1, will readily point out the problem.
It becomes obvious that the effective
area of current flow will not be the
total length of the furnace times the
depth. The width of the effective
electrical path (We) is some fraction
of the actual furnace length.
Mathematical approaches for determining resistance have been
attempted
by such experts as
Edouard Borel' as far back as 1955.
Since the formula developed is based
on very specific conditions and is not
generally applicable to most designs,
the designer has four options: (1)
guess, (2) overspecify the transformer equipment, (3) have sufficient design experience to have
developed empirical formulas to
cover the specific design, (4) develop
modeling techniques to predict the
characteristics
of each specific
design. Obviously, the latter, coupled with experience, is the preferred
approach.
c
The electrical power systems used
in conjunction with electric melting
are dependent on the physical size
and shape of the melter. Conversely,
the shape of the melter is dependent
on the electrical power system used.
The shape and dimensions are also
dependent on a number of other
variables. Among them are: glass
composition, resistivity, type of electrodes and number used, voltage
limitation as related to personnel
safety, acceptable electrode current
densities, and charging method.
Discussion of the three basic types
of electrical power systems will help
to explain the designer's considerations.
A. Single Phase Systems-Normally, when single-phase power is
employed, current passes from one
side of the tank to the opposite wall
as shown in Fig. 2. For this reason, it
is ideally suited to a square or rectangular shape.
Single phase systems are most frequently applied to low capacity furnaces; i.e., 30 tons per day or less.
Power requirements for these units
might range up to 1500KW demand,
which normally will not pose a plant
phase load balance problem. If it
does, phase balancing equipment
resistance or conductance control.
Manual control is a straightforward
and simple process. The operator
needs only to monitor the voltage
and amperage of one phase. Multizone judgements are unnecessary
since all phase legs are equal and
identical and the entire melter acts as
a single zone.
Another significant advantage of
this system includes the ability to use
multiple electrodes at each pole with
equal and uniform current distribution to all electrodes as shown in Fig.
5. All electrical lengths (Le) are equal
as indicated by (x). This permits the
optimum utilization of electrodes.
Current to the electrodes can be
equalized to within plus or minus
SOlo.
C. Three-Phase Systems-The
three-phase systems generally used
today fall into two distinct design
philosophies; for simplicity, they can
be referred to as the symmetrical and
the unsymmetrical types. Since many
different arrangements exist for both
types, it is impractical to discuss all
possible variations. Therefore, the
examples chosen are only to il-
AIII-Elec[fIC
Mell[ers
can be employed.
All types of electrodes are applicable to this type of application.
On small units, however, we have
found plate electrodes to be particularly well suited. They provide
the ability to maximize the electrical
length and to effectively utilize the
molybdenum to create both uniform
and low current densities.
A single phase system such as
depicted in Fig. 2 will produce
uniform melting and fining conditions throughout the entire melter
chamber.
B. Two-Phase Systems-These
systems utilize two phases whose
phase angle is at 90°. To accomplish
this phase shift and to provide a
balanced three-phase primary load,
a Scott 'T' transformer connection is
used.3 (See Fig. 3.) The voltage that
exists between phases is equal to
0.707 of the phase voltage.
To take advantage of the 90° phase
relationship and to equalize the load
on both phases, electrode placement
must be in a square. The square configuration can be used singularly or
in multiples.
Since each phase is symmetrical
and equal and the cross phase relationships are also symmetrical and
equal, uniform power release is accomplished within the melter. Thus,
these furnaces will practically always
be square, or a rectangle with whole
number multiples of width to length
as shown in Fig. 4.
The two-phase system is particularly applicable to most furnace
sizes, except those that are very
small. It is, however, limited to the
use of bottom rod electrodes when
more than one electric square is
utilized.
Power regulation of this system,
whether single or multiple squares
are employed, is easily accomplished
by a single primary three-phase
regulating transformer,
or other
three-phase regulating devices. This
single control is only possible by virtue of the uniform and totally symmetrical energy release that can be
accomplished through this circuitry.
Also, with these two-phase
systems,
current
and voltage
measurements are the same as single
phase measurements and are truly
representative of phase operating
characteristics. The symmetry of the
system makes each phase equal and
identical to the other phases. Thus,
this system is ideally suited to glass
-
A
E
D
c
-
F
F
SymmetricalSix Pole-Three Phase System
Fig. 8
B,
B,
.:'. ..
,'".
1
..."
,
.
...' \;
1
'..'.. \
r
:
B,
":.f...""
r""",
A,
1
:
;; ,
A,
:
' \;.
",,"Y"
,.
c,
A,
"""
A,
A,
C,
A,
~...",
,:'j
',:
Fig. 9
Unsymmetrical Nine PoleThree Phase System
.'."
",
"" ..'
".'
.: .~
..'
~.', '"
r
c,
;
:
".."
"','.;.:
'..
~",
,~
'-,
""
~'
"'--,-,
,
c,
c,
",.""
',.,'
B,
Unsymmetrical Nine PoleFig. 10
Three Phase System
Note:
Fig. 11
:~~
.'..';;
0,1""""""'"~~~:~""""""'" 'f'
t~", ...,
""" ,,/\
Glass
Glass
\
""..\
,;'
:
Low
Energy
Area
B,
:
"
""""'..."
:",
""""l
,.,.,,\
.'... ,
.:""'.
\:
""'~.~
B,
Thermocouple
Optional
Horizontal Electrode Assembly
lust rate
the
basic
design
philosophies.
1. Symmetrical Design-In
a
three-phase system, the voltage is at
1200phase angle, phase to phase, as
is illustrated in Fig. 6. If a prerequisite of the design is uniform and
symmetrical power release, as is
desirable in a cold top melter, it can
readily be seen that square or rectangular construction is undesirable.
Perhaps the most used method of
accomplishing symmetrical threephase control is to use the hexagon
configuration with electrodes placed
on alternate sides as shown in Fig. 7.
The electrodes can be either bottom
rod (vertical) or sidewall (horizontal)
type.
Multiple
three-phase
power
systems can also be incorporated,
such as the double delta illustrated in
Fig. 8. This arrangement is also used
on the "vertical melter" series of
furnaces, except electrodes A, Band
C are displaced vertically from D, E
andF.
Use of these symmetrical type
three-phase
systems has been
primarily limited to small and
HorizontalElectrode
Current Pattern
HorizontalElectrode
Erosion Pattern
Fig. 12
MultipleElectrodes,
Unsymmetrical Three Pole - Three Phase System
Fig. 13
Note: Electrical
Lengths for Phase
A-C are Unequal
(y& z).
Throat
Two Phase
Fig. 14
Symmetrical
Three Phase
Multi-Phase Systems
medium-size furnaces; i.e., approximately 100 tons per day capacity or
less.
Power regulation
can be accomplished in the same manner as
the single phase or two-phase
systems.
2. Unsymmetrical Design-Unsymmetrical three-phase design can
be defined as a furnace in which the
power release is not symmetrical
about either or both the longitudinal
and transverse centerlines and thus
does not create a uniform energy
release. The simplest form might be a
rectangular furnace with a single
three-phase delta system, as shown
in Fig. 6. The obvious disadvantage
of this system is the low energy
release areas created in two corners.
In attempts
to eliminate
or
minimize low energy areas, multiple
delta systems have been used, such as
represented by Fig. 9 and Fig. 10.
These types of systems compromise
the requirements of symmetry about
center lines and uniformity of power
release. They do tend to minimize
low energy areas, but do not
eliminate them. Balanced loading of
the three-phase primary circuit is difficult to achieve, due to the unequal
resistive loads per phase. Also,
uniform or nearly uniform current
loading on all electrodes per pole
cannot usually be attained without
the use of artificial means, such as inductive chokes.
Unsymmetrical schemes also have
energy release patterns that are different than any of the schemes mentioned heretofore. The previous
symmetrical schemes, including the
vertical melter, endeavored to produce a uniform release of energy
within the melter and promote a vertical melting pattern. The unsymmetrical schemes, however, develop
unsymmetrical energy release patterns and tend to develop variable
temperature
zoning within the
melter. This zoning makes these
systems applicable only to melters
that have a greater length than
width. Irregular zoning patterns are
not conducive
to a uniformly
distributed batch cover as melting
rates will vary over the melter surface. Thus, these systems are
generally associated with fixed position batch charging and semi-cold
top furnaces (i.e., furnaces that have
a variable percentage batch cover).
Power control of an unsymmetrical system is more complex.
Resistance or conductance control or
current control are difficult to apply
as there is no sensing point or points
that adequately represent the whole.
Additionally, there is no distinct
power zone. As a result, most of
these applications utilize automatic
AU-Elec[fIC
Mel[ers
power (Kilowatt) control. This
works well WIder steady state circumstances with all conditions in
equilibrium. However, should the
batch cover vary, and thus the furnace thermal losses vary, power control does not automatically compensate and the glass will either go cold
or hot as the batch cover dictates,
creating unstable melting and fining
conditions. It can also be responsible
for varying color conditions of a
glass as a varying batch cover makes
the glass susceptible to a changing
Electrical
Isolation
Material
Electrical
Isolation
Material
Legend
_Air
IJEilljWater
~
~
redox condition.2
ELECTRODE TYPES
After having determined
the
melter parameters that point up the
power system to employ, we must
now determine the most appropriate
electrode type for applying the electrical energy to the body of molten
glass. There are two basic electrode
types in use: sidewall and bottom entry. The predominant
electrode
material used for. most glass is
molybdenum, and this paper will
deal only with molybdenum types.
A. Sidewall Entry Electrodes
1. Rod Electrodes-Horizontal
rod electrodes are used in both new
electric melters andin boost applications. Fig. 11 is a drawing of a typical
sidewall rod (horizontal) electrode
highlighting the electrode, coolant
tube, and terminals. The coolant
tube delivers water to the electrode at
the point where the molybdenum rod
enters the atmosphere. The rod at
this point must be kept below 300°C
(572OF)to prevent oxidation of the
molybdenum.
The holder often has a thermocouple mount in the head to provide an
early warning in the event of coolant
loss or if the electrode becomes too
short and develops a heat concentration near the head.
Fig. 12 shows a typical electrical
current envelope for horizontal rod
electrodes. This pattern is basically
an ellipsoid. The greatest energy concentration is obviously at the tips
when the electrodes are opposing as
is the general case. Sometimes electrodes will be angled to reduce the
severity of this concentration. In
continuous use, these electrodes
erode away at the ends, assuming the
tapered shape shown, and require
periodic advancement. The judgement as to how far to "push" the
electrodes is made by trying to match
an initial set of conditions; i.e., a
reference amperage for a given
InertGas
Electrical
Isolation
Material
Bottom Entry Electrode
Fig. 16
voltage, and glass temperature or
resistance. Precise insertion is difficult,
however,
because the
reference factors are not easily
duplicated.
Utilization of multiple rod electrodes per electrical pole can result in
different current densities and consequently different erosion rates. This
can be caused by different electrical
lengths, which create unbalanced
current loading. Referring to Fig. 13,
it can be seen that the inside electrodes have a shorter electrical length
(y) than the outer electrodes (z) in
phase A-C. This unbalance does not
necessarily exist for single phase
systems, and might not exist for symmetrical multiple-phase applications
(see Fig. 14). Each design should be
evaluated for this condition, as the
electrode current densities can easily
vary as much as two to one, or more.
2. Plate Electrodes-The
plate
electrode is another form of a
sidewall electrode that is primarily
used in single phase melters of
relatively low melting capacity.
These electrodes are also ideal for
certain furnaces and glasses requiring low current densities and very
uniform energy release. Fig. 15 illustrates a typical application. The
primary disadvantages of plate electrodes are that they must be installed
cold and they cannot be replaced or
supplemented during the campaign.
Also, practical design application
limits the plate electrodes to smaller
furnaces.
B. Bottom Entry Electrodes-Fig.
16 shows a vertical rod (bottom entry)-type electrode.4 The electrode is
mounted in a special holder that accommodates an inert gas purge at the
glass/atmosphere
interface. The
purpose of the purge is to prevent
any possible oxidation
of the
molybdenum. This holder also provides a coolant jacket which surrounds the electrode to reduce the
electrode temperature to a safe level
before the molybdenum is exposed
to the atmosphere.
~
An-E~~c[rICMell[ers
CONCLUSION
It is readily apparent that electric
glass melting is a many faceted subject with many interrelated
variables. We hope that this paper
has furthered the understanding of
some engineering aspects of this subject. Additional topics concerning
cold top, all-electric glass melting
will be discussed in a subsequent
paper.
(Editor's Note: This article is
based on a paper presented by Mr. .
Hibscher at the 40th Annual Conference on Glass Problems held Nov.
13-14, 1979, at the University of Illinois.
REFERENCES
Fig. 17
Vertical Electrode
Current Pattern
Vertical Electrode
Erosion Pattern
When rods are placed in a vertical
position, the electrical length (Le)
between any two rods is always equal
from tip to base of the electrode as
seen in Fig. 17. This condition
creates a near uniform current flow
in the main body of the glass. Point
concentrations
of current are
eliminated and the electrodes erode
in a near uniform manner over their
full length.
The bottom installation has little
or no restriction on placement of
electrodes. This permits the designer
to achieve a very close balance of
current loading on all electrodes.
This same ability exists whether
single or multiple electrodes are used
per pole.
This uniformity of current density
and consequent erosion over the
electrode length make it possible to
obtain optimum utilization of the
molybdenum rod. In nearly all applications, adequate rod material
can be provided in the initial installation to last the furnace campaign
life. No electrode "pushing" is encountered in the vast majority of installations.
However,
should
replacement be required during the
campaign, it can usually be accomplished without furnace shutdown.
Some of the glass industry has
been reluctant to utilize the bottom
rod electrodes as shown in Fig. 18
because of the fear of bottom leaks.
However, our experience with this
method does not support such concern. We have installed over 600 vertical rod electrodes in the United
States and Canada and many more
throughout
the world this past
decade, which represents well over
3,000 electrode years of operation,
and only one has been associated
with a bottom leak. In no instance
has the electrode system been
responsible for a leak. Experience
has shown that only normal diligence
to simple maintenance procedures is
required to assure that bottom leaks
do not occur.
Electric Melter with Vertical Electrodes
Main
Support
Beam
JackFig. 18
Forehearth
(1) E. Borel, Electric Melting of Glass, Neuchatel:
Allinger (1958); P. LaBurthe, E. Borel, and G. de
Piolehc, "Contributions
of Electric Melting and
Boosting to Glass Technology,"
Am. Ceram. Soc.
Bull. 36, 18-25 (1957).
(2) W. H. Manring and R. E. Davis, "The Role of the
Raw Material Supplier in Energy Conservation for the
Glass Industry," Cant. on Glass Problems, 38th,
141-163 (1977); "Controlling Redox Conditions in
Glassmelting," Glass Ind. 59 (5) 13 (1978).
(3) P. A. M Gell and T. H. Waterworth, "Furnaces
for Heating
Glass Electrically,"
U.S. Patent
3,440,321
(April 21,1969).
(4) W R. Steitz, R. 0 Bradley, and T. H. Waterworth, "Electric Glass Furnace," U.S. Patent No.
3,634,588
(Jan. 11, 1972); P. A. M. Gel!, F. M.
Merrill, and W. R. Steitz, "Protection
of Glass
Melting
Furnace
Electrode,"
U.S. Patent
3,777,040
(Dee 4,1973)
About
the Authors
William Steitz, who joined Toledo
Engineering Co., Inc., in 1966, has
been responsible for the design and
commissioning of 32 electric glass
melters.
He is presently
vice
president-technical director and is
responsible
for the company's
research,
development,
and
technical activities.
A graduate of Northwestern
University, where he received a B.S.
degree in mechanical engineering,
Mr. Steitz started his professional
career working for Owens-Corning
Fiberglas as an engineer-manager at
various locations. He was also
employed by the Ferro Fiber Glass
Division as chief engineer of the
Nashville, Tenn., operation.
As manager
of commercial
development for Toledo Engineering, Carl W. Hibscher is responsible
for market research and development of new market areas. He joined
the firm in 1970after a 20-year caI~eer
at Toledo Scale, during which he
worked in the Systems Division and
served as engineer, sales engineer,
chief engineer, and marketing
manager.
A registered professional engineer,
Mr. Hibscher attended Toledo
University, where he received a B.S.
degree in electrical engineering. He is
a member of the IEEE and is currently serving on its Glass Industry Group
controls subcommittee.
DESIIGIN
COINSIIDEIATIIOINS
FOR
ALL=ELECTRIIC
MELTEIS
A presentation made at the 41st Conference of Glass Problems
By WILLIAM R. STEITZ and CARL W. HIBSCHER
Toledo Engineering Co., Toledo, Ohio
Review of Part I
D
ETERMINING glass resistivity
and furnace resistance are important considerations in resolv
ing the parameters of an electric
melter, and these factors are best determined by laboratory tests and models. After the melting and electrical
characteristics of the glass are determined, the selection of the type of electrical power system can be made. The
final shape and geometry of the melter
is then coordinated with the appropriate electrical scheme.
Single-phase application of power
to an electric melter is normally used
for small melters up to about 30 tons
per day (TPD) as a practical design
limitation. Two-phase and threephase power is used for larger capacity
melters.
Multiphase melters can be either
symmetrical or unsymmetrical from
both a geometrical and an electrical
aspect. The symmetrical system has a
uniform energy release that is conducive to vertical melting and utilizes a
distributor type batch charger to create
a uniform batch cover or blanket over
the entire melter surface. Symmetrical
three-phase-type melters have been
used to about 90 TPD capacity, whereas the two-phase type has been used up
to 240 TPD.
The unsymmetrical system has a
nonuniform energy release pattern,
which creates zonal temperature differences in the melter. Unsymmetrical
furnaces usually have a length to width
ratio of more than one, and are generally associated with fixed position
batch chargers at the backwall. These
types of melters can have a broad turndown capability, but it willbe at the expense of excessive power utilization
when operating with a partial batch
cover. Also, the Redox condition of
the glass is dependent on pull rate and
percent batch cover.
Control of an unsymmetrical system is complex since generally no one
leg of the electrical power circuitry is
truly representative of the resistance or
the condition of the body of glass.
Part I also discussed sidewall entry
and bottom entry electrodes. The sidewall type can be easily installed in operating furnaces without shut down.
The tip current density is normally
high relative to the body of the electrode, and accelerated erosion of the
molybdenum rod is encountered.
The bottom rod-type electrode has
a very uniform current density over its
full length, and electrodes usually last
~campaign without the need for push~
mg.
The Semicold Top Furnace
The semicold top furnace illustrated
in Fig. I operates 100070on electric energy. It incorporates fixed position
batch feeding that results in a varying
percent batch cover, depending on the
rate of pull. This type melter can incorporate either symmetrical or un-
AlLlL=ElLECfIIIC
MElLfEIS
symmetrical application of electrical
energy.
A symmetrical application is illustrated in Fig. 2 by a single-phase furnace with one or more screw chargers
positioned in the backwall and projecting over the melt. Three-to-two
phase systems could also be applied,
with either screw or pusher-type
chargers.
An unsymmetrical system is illustrated in Fig. 3, again with a screw
charger and with a basically squareshaped furnace.
When furnaces in the capacity range
of 100 tons per day or more are required, designs generally take on a
length to width ratio similar to the conventional fuel-fired furnace. This is
usually done to facilitate the batch
feeder design and sometimes because it
more readily fits the building space. In
some instances,it is done mistakenlybe-
cause fossil fuel furnaces are designed
in this manner, even though the principles of melting are quite different.
When the furnace assumes a shape
with the length greater than the width
(Fig. 4), it becomes nearly impossible
to have a symmetrical electrical system
with a three-phase secondary power
application. Although symmetry
could be created with either singlephase or multiple three-to-two phase
electrical squares (Fig. 5), this is not
necessarily a desirable feature due to
the method of batch charging and the
flow forward of the batch blanket, the
glass, and the heat within the glass.
Therefore, even these systems should
be installed in such a manner as to
allow zone control of electrical energy
input.
Melting Characteristics
With fixed position batch charging,
the batch is floated out on the glass
surface in the same manner as a fuelfired furnace. The hope is that it will
cover 100% of the melter surface to
create a condition similar to a cold top
furnace in an effort to achieve the most
efficient, lowest energy input per unit
of melt. However, this condition is seldom realized, and then only when the
melter is near its maximum pull rate.
More commonly, a red or "slush"
area of varying size will exist at the
throat end of the melter as illustrated
in Fig. 6. The reason for this relates to
Semi-Cold Top Electric Melter
Fig. 1
x,
X,
Symmetrical Single Phase Melter
with Screw Charger
Fig. 2
the basic principles of electric melting
and the melting requirements and conditions that must be met to make acceptable glass quality.
These basic requirements for acceptable quality can be summed up as
follows:
a. Controlled and consistent raw
materials and accurate batch weighing;
b. Mixed batch homogeneity;
c. Uniform melting conditions
where each increment of homogeneous batch receives the same timethermal treatment, or convection
and/or mechanical mixing;
d. The glass must reach some minimum temperature at which it will
refine.
Only Item a is not a function of the
furnace design, while b, c and d can all
be influenced by both furnace design
and operation.
Item b is normally assumed to be a
function of the batch mixing and delivery systems; however, in electric melting, this is not always the case. Batch
homogeneity can be lost within the
B
1
--- --- --- --- ---
A,
I
I
I
----1---II
A,
c
UnsymmetricalThree Phase Melter
with Screw Charger
Fig. 3
melter during tte melting process (Fig.
6). When the batch is floated out on
the molten glass, the melting process
begins. In electric melting, this occurs
only on the bottom or interface area
between the batch and the hot glass.
With fixed position charging, we must
accept the fact that the batch pile is
moving and that once any given unit of
batch is deposited within the furnace,
no further batch material additions
can be made to that unit of batch.
Since the batch is composed of a
multiplicity of various mineral and
chemical ingredients blended together
in an uncombined state and as these
various materials melt and combine at
different temperatures, it stands to
reason that they do not all melt and
combine into the glass at the same
time. This phenomenon is referred to
as "differential melting."
As the batch begins its travel down
00.0
"
0
Fig. 5
A,
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, .,;:
.
';)""""""""""""""'O"""""""""""","""'"
c,
Unsymmetrical Three Phase Melter
with Screw Chargers
Fig. 4
"
00
.If
0
0
Double Electrode Square
Two Phase Circuits with Screw Chargers
the furnace, as it must from a fixed
point of entry, the lower temperature
melting and reacting ingredients will
become liquid first and separate from
the batch. During a given period of
time, as the heat penetrates up through
the batch, these lower temperature
melting ingredients will replenish
themselves from the batch above.
However, at some point in the melting
process only the higher melting temperature ingredients will remain to be
melted, and when this occurs, the
batch and melting homogeneity has
been destroyed. This often results in
seed, blisters, stones, and silicacord as
this melting condition is always downstream in the furnace and usually very
near the frontwall or throat end of the
furnace. This suggests that strong convection currents are a requirement in
this type of furnace to achieve mixing
and homogenization of the glass.
c,
c,
the control of the furnace and the glassmaking process.
When a melter area is fully covered
with a batch blanket, the furnace heat
losses remain virtually unchanged
throughout its range of operation (Fig.
7A). Also, the batch blanket losses are
uniform over each individual square
foot of the blanket. True, there is some
variation in losses due to crust thickness and glass temperature changes,
but these are minimal. More importantly, these losses are constant at any
given time.
In the semicold top operation, a
slight overpowering or underpowering
can cause the crust to change its degree
of hearth coverage from full to partial
and vice versa (Fig. 7B). As this crust
condition changes, it will significantly
change the heat losses and heat balance
of the furnace. In the event that the
crust recedes, the newly exposed surface of glass givesup heat by radiation
to the superstructure.
If the glass conditions are to remain
constant, this additional energy loss
must be supplied by increasing the input energy. However, the area of receding crust is always localized and not
Electric Melter using Backwall Charging
Differentialmelting occurs
as batch movesforward
Operation
The effect of semicold top operation, with its variable crust, goes beyond just the melting phenomena. It
also creates some interesting aspects in
8,
8,
.If
-
A
" '.
B3
XA
Two (2)
A,
A3
Fig. 6
AlLlL=ElLECfAIIC
Heat Loss Characteristics
Constant Loss
+
MElLfEAS
uniform over the entire hearth. There, fore, it is giving up energy, with the
glass going cold locally and with generally no means of selectivelycorrecting
this condition. When this occurs at the
frontwall, cord and seeds are usually
close behind the change. The result of
all this is that careful and frequent operator monitoring of the melter is required and long runs at very constant
pull rates are highly desirable.
When the above described condi~
tion occurs, the correct operator action is opposite from what might be expected. Normal reaction would be to
increase energy input to restore the
energy being lost. Since this cannot be
done locally at the point required, it
Constant
Loss
A. Complete
Cover
(cold top)
+
Constant Loss
BATCH
B. Partial
Cover
(semi-cold top)
FURNACE
BATCH
STORAGE
VIBRATORY
---
I---
MELTER
-1~HRO\t-
must be done by raising the overall
energy input to the furnace. This added energy will result in further opening
up of the glow area. This, in turn, increases the losses, and the cycle is reinitiated. Thus, the correct move must
be to reduce the input energy, which
will reduce the glass temperature, slow
down the melting rate, and reestablish
the crust cover.
Only in very small furnaces, operating with automatic resistance control,
has this phenomenon been successfully
coped with, and then at the expense of
wide swings in power input and high
overall power consumption.
Turn-Down
Requirement d for making glass indicated that "the glass must reach
GLASS
Decreasing Temp.
Increasing Temp.
Variable Loss
+
.
Fig. 8
-
Constant Loss
Fig. 7
.
f[ mCR
Increasing Temp.
Decreasing Temp.
Constant
Loss
Cold Top Melter
FURNACE
CULLET
STORAGE
SUPERSTRUCTURE
COVER
Receding
Increasing
RISER
1
FOREHEARTH
--
x,
X2
Symmetrical Power
Systems for
Symmetrical Electric
Melters
Fig. 9
some minimum temperature at which
it will refine." This means that a furnace operator cannot continuously reduce the melt temperatures in an effort
to maintain a high percentage batch
cover. Therefore, if the cover is to be
maintained, the semicold toR, melter
has virtually the same turn-down limitations as the cold top-type melter.
However, if the open (batch free)
hearth surface is allowed to expand to
the equilibrium point, it should be possible to achieve 100070turn-down.
The negative side of this condition is
that the power consumption will be
high. Also, a variable Redox situation
will exist at the exposed glass surface.
This could require different batch
compositions for different pull rates.
The Cold Top Furnace
The cold top electric glass furnace illustrated in Fig. 8 operates on 100070
electric energy and has a continuously
Three-Phase
.
distributed uniform batch cover over
the entire melting surface.
These melters nearly always employ
the symmetrical-type power designs
described in our previous paper and illustrated in Fig. 9. They can be single
phase, two phase or three phase. The
shape can be square, rectangular,
round, or hexagonal to suit the type
power system chosen.
The symmetrical systems are chosen
to create the most uniform below-crust
condition possible so that the melting
conditions will be as uniform as possible over the entire melter surface.
Batch is uniformly deposited over the
melter surface by several different
means. A commonly used scheme is a
traveling boom charger with belt conveyor (Fig. 8) that lays down a triangular, overlapping, nonrepeating pattern, as illustrated in Fig. 10. The actual patterns are wider than shown in
Fig. 10, and a complete layer of batch
is placed on top of the hearth with several traversing cycles of the charger.
COLDTOP Electric Melting.
The state-of-the-melt-art.
Assuring highest glass quality. Least energy consumption. And largest pull capacities.
TOLEDO ENGINEERING in conjunction with
ELEMELT, LTD., has brought the science of electric
glass melting closer to perfection. With the electric
melter that's a true COLD TOP utilizing complete
and uniform batch cover:
And by providing maintenance free heat recovery,
maintenance free emission control, consistent control of REDOX and excellent glass quality.
And we can build all those advantages into furnaces
that represent the largest capacities possible in
the glass industries today. Up to 350 tons per day.
With melter areas up to 1,000 sq. ft.
We'd like to discuss the science of electric glass
melting with you. And introduce you to our state-ofthe-art electric melter:
Write or call for more information.
Melting Characteristics
In cold top electric melting, as defined above, each unit of mixed batch
is melted in place (Fig. 11). No horizontal movement of the batch takes
place on the surface of the melter. It
can be said, therefore, that the batch
melts vertically.
As described in the discussion of the
semicold top furnace, differential
melting does occur within the mixed
batch; however, there is a significant
and important distinctionbetweenthe
cold top and semicold top melting process. In the case of cold top melting,
batch materials are constantly being
added, one unit on top of the previous
one, thus always replacing those ingredients that have melted out from the
unit below. In this manner, a melting
equilibrium is reached and a uniform
homogeneous melt is achieved across
the entire furnace surface.
Experience has shown that when the
above melting conditions are met, convection mixing in the furnace is no
longer required, nor is it necessarily
desirable. Very uniform temperature
conditions, both horizontally and vertically, can then be designed into the
furnace. These uniform temperature
conditions are illustrated in Fig. 12,
which shows actual temperature
probes of a cold top furnace with bottom rod electrodes operating on a flint
soda lime glass. Uniform conditions
are accomplished by low current or
watt densities evenly distributed over
the electrode surfaces and also with
equal densities on all electrodes.
When these uniform temperature
conditions are achieved, it then follows that vertical or "plug flow" melting will result, and little convection
.
\
TECD
TOLEDO ENGINEERII:'G CO
3400 Executive Parkway
Toledo, Ohio 43606
(418) 537-9711
TWX 810 442-1627 TECOGlAS
TOl
AlLlL=ElLECfIIIC
MElLfEIS
Batch Cover Pattern of
Distributive Charger
mixing is present. Fig. 13 shows a
batch and glass composition change as
it occurred with time in an operating
furnace. The theoretical changeover
time of the furnace was 42 hours. Note
that the glass composition shift basically started 41 hours after the batch
formulation was changed. The glass
change was essentially completed 12
hours later, with 90070of the change
occurring within eight hours. The glass
volume of this particular furnace was
just over 300 tons, and the melter area
was approximately 900 square feet.
Fig. 10
of the electric system are equal in all
characteristics, and only a single effective melting zone exists, the resistance
of any phase can be taken as representative of the whole. This resistance is
easily sensed and provides extremely
accurate, repeatable and reliable measurement of the entire glass bath thermal condition.
With automatic resistance control,
the operator need only monitor the
batch crust condition for required adjustment to set point. Since a changing
crust thickness is somewhat self-compensating by virtue of changing heat
losses, adjustment in resistance set
point for any given pull rate is seldom
required. As the thermal losses on a
cold top furnace are nearly constant
throughout the operating range,
power requirements are easily predicted and will be close to the theoretical heat-toe glass value.
Cold Top Electric Melter
.'-,--'
~_.l........
Note: Vertical Melting
Fig. 11
Operation
Because uniform melting conditions
are a prerequisite of good cold top
melting, it is always desirable to have
the furnace react as a single zone in the
horizontal plane. In some very deep
cold top melters, vertical energy zoning has been applied. However, analysis of the melting phenomenon suggests that this may be unnecessary, and
does not contribute to glass quality. In
fact, if higher temperatures occur in.
the lower zones, unwanted convection
will result. If lower temperatures are
desired, this can be better accomplished
through design balances of glass depth,
elevation of electrodes, and insulation
so that there is no requirement for additional energy.
With the features of single zone operation, a symmetrical and totally balanced electrical system, and a fulr'
batch cover, the furnace control becomes simple (Fig. 14).Sinceall phases
0
5
1--1- -1- -1- -I
CRUST
10
Electric Melter Probes
15
,
20
1"""'21
I
25
~ -T'15
-
fif
T~- - }:1~rc-:--
i
i
I
L---
0
40
45
17
...........
15
21
"""""'""""""""""
55
LOC
CODE
CHANNEL
BOTTOM
65
2400
Fig. 12
MELTER
BOTTOM
50
60
LOC CODE
14
.
35
~
w
Q CRUST TC-1 TC-2"
14
2559 2555
6"
LOC~ .0287
2559 2557
.0288
6"
15
2559 2555
.0287
6"
17
2559 2557
.0288
6"
21
------
30
~
u
::r
BACK WALL
I
~
2450
2500
2550
TEMPERATURE-oF
2600
Glass
composition change
with cold top electric melter
I-
41 HR.
-1--12
X
I-
1-8 HR~
HR J
this by the sizing of the melter. The
glass technologist can influence this
through composition and batch formula. The furnace operator has practically no influence on this capability
of the melter.
Conclusion
I!.
Z
W
'i.
.,""
C
w
a:
CJ
~
..
:::
s.
~-~~"
% ~ '
- ,:" I
;J!.
FINAL
COMPOSITION
0
6
12
18
24
6
12
TIME
Fig. 13
18
24
6
I
I
12
18
24
(HR.)
It can be seen that all-electric, Joule
effect glass-melting furnaces are not
all the same. The mechanisms of melting and refining for the semicold top
and the cold top melter have significant differences and requirements,
and it is important that these differences be recognized by management
when selecting the type of furnace to
use. The designer must be cognizant of
these differences if he is to provide the
Turn-Down
The requirement to maintain a
minimum glass temperature for refining establishes the degree of turndown of a cold top furnace. The melt
rate of the batch must always be in
close equilibrium with the pull on the
furnace to maintain a stable crust.
When the glass temperature must be
dropped below the limit for acceptable
glass quality, the turn-down limit is
reached.
For most common glasses, turndown will be approximately 50070of
full capacity. For glass with less stringent quality requirements, primarily
seeds, a 60070turn-down or slightly
better may be possible. For some special glasses with stringent quality requirements, tutn-down might be as little as 20070.
The furnace designer can influence
Double Electrode Square (Two(2)
RA
Fig. 14
engineer of the Nashville, Tenn.,
operation.
As manager of commercial development for Toledo Engineering, Carl W.
William R. Steitz, who joined
Hibscher is responsible for market
Toledo Engineering Co., Inc., in 1966,
research and development of new
has been responsible for the design and
market areas. He joined the firm in
commissioning of 32 electric glass
1970 after a 20-year career at Toledo
melters. He is presently vice presidenttechnical director and is responsible
Scale, during which he worked in the
for the company's research, developSystems Division and served as engineer, sales engineer, chief engineer,
ment, and technical activities.
and marketing manager.
A graduate of Northwestern UniA registered professional engineer,
versity, where he received a B.S.
degree in mechanical engineering, Mr. . Mr. Hibscher attended Toledo UniverSteitz started his professional career
sity, where he received a B.S. degree in
electrical engineering. He is a member
working for Owens-Corning Fiberglas
of the IEEE and is currently serving on
as an engineer-manager at various loits Glass Industry Group controls subcations. He was also employed by the
committee.
Ferro Fiber Glass Division as chief
About the Authors
-
TwoPhase Circuits)
= Rs = Rc = RD
best furnace for a particular application. It is also important that the plant
personnel operating a 100070electric
furnace understand the principle of the
specific design utilized. The correct
thing to do for one type of melter
might be wrong for another type.
Selecting the appropriate melter,
implementing a sound design, and
employing the appropriate operating
practices can make a significant difference in the glass quality, the melter efficiency, and the operating costs of an
all-electric glass melter.
(Editor's Note: The above paper was
presented by Mr. Hibscher at the 41st
Conference on Glass Problems held
Nov. 18-19, 1980 at Ohio State University.)
REFERENCES
1. William H. Manring and R. Eugene Davis, "The Releof
the Raw Material Supplier in Energy Conservation
for the
Glass Industry,"
in Collected
Papers
of the Annual
Conference
on Glass Problems.
38th, Univ. of Illinois.
Urbana-Chmlpaign,
pp. 141-163.
1978.