Note: Descriptions are shown in the official language in which they were submitted.
1~87Z36
me invention relates to furnaces for heating steel
to forging temperatures and particularly to electric heating
element batch furnaces for same.
It is a familiar experience that ba~ch-type furnaces
are used for heating steel to forging temperatures and are
usually loaded and unloaded by hand, the heating time being
determined by the furnace operator. Typically, batch furnaces
used for heating forging stock are of the slot type, and instead
of a door, these furnaces have a horizontal opening across the
front through which the forging stock is received into the heat-
ing chamber. These furnaces generally have a refractory hearth
and fire brick inner side walls and an inner arched roof and may
be oil-fired or gas-fired with burner units extending into the
heating chamber through the side walls of the furnace. Because
combustion occurs within the chamber, a flue is provided and there
may be a gas curtain to minimize entry of oxygen into the cham-
ber.
During combustion of gas-fired and oil-fired furnaces,
oxidizing agents are either present or formed as a result of
the combustion of the fuel. Furthermore, other products are
formed that contaminate the air and may require the use of
costly pollution control devices to meet clean air standards.
Oxidizing agents, such as carbon dioxide and water vapor, cause
- decarborization and scaling of the forging steel. Carbon mon-
oxide also may be present and is a carborizing agent. There
exists a delicate balance in the ratio of fuel to air that will
burn to produce an atmosphere substantially neutral to steel,
i.e., that is neither carborizing or decarborizing. Excessive
air reduces heating efficiency and allows excessive scale forma-
tion on the forging stock. Insufficient air also reduces heat-
ing efficiency, but results in a thin scale on the steel that
is difficult to remove. Moreover, in the instance of oil-fired
10~7Z3t~
burners, an improper mixing of oil and air may result not only
ln poor combustion, but in soot deposits on the furnace hearth
and walls. Where the forging operation includes a die impres-
sion in which the stock is forged close to final dimensions
and decarborization cannot be tolerated, a controlled atmosphere,
such as a full muffle type furnace, may be used to protect the
forging stock.
Because of the special emphasis currently on conserv-
ing energy, especially gas and oil, it has been proposed to
convert existing gas and oil-fired batch furnaces to electrical
heat because of the reasonable forecast that through nuclear
power electricity is expected to become more available than
gas or oil in the future. AR gas and oil become scarce, their
prices are expected to escalate at a more rapid rate than elect-
ricity.
It has been found, however, that merely convertingexisting furnaces to electrical heating is not a solution to
the problem. The construction of existing furnaces are not
such that forging stock can be heated to forging temperatures
rapidly and uniformly throughout the heating zone of a slot
furnace with the ki~ds of efficiencies needed to effectively
utilize electricity as a power source vis-a-vis gas and oil.
As a means of effecting electrical conversion, however, it has
also been proposed to close flue openings in the furnace and
other kinds of circulation openings typically used in muffles
to eliminate some of the heat losses and to improve efficiencies
as a result. Such steps, however, likewise have not proved
to be a desirable solution to the problem.
While various forms of electrically heated furnaces
are known for heating steel to forging temperatures, it is an
object of this invention to provide an improved batch-type
-2- -
1~il7236
furnace in the form of a slot furnace heated by energy other
than gas or oil for heating steel to forging temperatures and
be at least as economical to operate as gas or oil-fired fur-
naces.
It is a further object of this invention to provide
a slot furnace of the foregoing type that has substantially
no need for air purification devices.
It is yet another object of this invention to provide
~ slot furnace of the foregoing type that is efficient, safe,
and has a rapid response to production demands.
It is yet another object of this invention to provide
a slot furnace of the foregoing type having a firing chamber
that has a minimum circulation of air to reduce scaling of
the forging steel.
The above objects are met with the present invention
which provides a slot furnace for heating steel to forging temp-
eratures, comprising: a housing; a chamber in the housing for -
receiving forging stock, a refractory lining in the housing
the inner surface of which generally defines the chamber; a ~ ~
20 refractory hearth; a slot at the front of the housing adjacent `!
the hearth and having communication through the lining to
the chamber through which forging stock is inserted and with~
drawn; an electrical heating element at each side of the cham-
ber inwardly adjacent the respective side walls of the lining;
a refractory ceiling defining the top of said chamber and pro-
truding inwardly thereof between the electrical heating ele-
ments and above the slot; means for connecting the heating
elements to an electrical power source; a temperature sensor dis-
posed adjacent the hearth; and means connected to the sensor
for controlling the electrical power applied to the elements
in response to the sensor.
The inventive aspects of the invention will become
1o~7236
apparent and the in~ention better understood by reference
to the following detailed description read in conjunction
with the accompanying drawings, in which:
FIGURE 1 is a perspective view of a slot furnace con-
structed in accordance with the principles of this invention;
FIGURE 2 is a sectional view taken along the line 2-2
of FIGURE l;
FIGURE 3 is a side sectional view taken along the
line 3-3 of FIGURE 2;
FIGURE 4 is a plan sectional view taken along the
line 4-4 of FIGURE l;
FIGURE 5 located on same sheet as FIGURE 1 is a
schematic diagram of the power and control. circuit used in
the furnace of FIGURE l;
FIGURE 6 located on same sheet as FIGURE 1 is a
graph illustrating the effect of the power and control cir-
cuit of FIGURE 5 in the operation of the furnace of
FIGURE 1.
FIGURE 7 is a sectional view similar to FIGURE 2
-3a-
kg/ !~
1~7Z3~;
~llustrating an embodiment having an alternative interior struc-
ture;
FIGURE 8 is a partial side sectional view similar
to FIGURE 3 illustrating the alternative interior structure
of the furnace of FIGURE 7;
FIGURE 9 is a partial plan sectional view similar
to FIGURE 4 illustrating the alternative lnterior structure
of the furnace of FIGURE 7; and
FIGURE 10 is a sectional view taken along the line
10-10 of FIGURE 9 to illustrate the rear fire wall in the alter-
native interior structure of the furnace of FIGURE 7.
Briefly, a slot type batch furnace for heating steel
to forging temperatures in accordance with the invention is
one which avoids the use of gas or oil for heating and avoids
lS the need for a muffle to control or substantially eliminate
scale forming on the forging stock. Rather, it employs electri-
cal heating elements within a newly configured heating chamber to
heat steel placed on its hearth uniformly to forging tempera-
tures, and such ope~ation compares favorably economically with
present furnaces using gas or oil as the heating source. The
effect is to not on:Ly provide for an alternative source of heat
c~t/c~l/
when energy is er~6~ aL, but also for a more simple furnace con-
struction while enhancing the ~uality of the surface of the
heating forging stock.
Referring now to FIGURES 1 and 2 for a brief descript-
ion of the illustrated construction, a slot furnace 11 is pro-
vided with a metal outer shell housing 13, a heating chamber 15,
and a heat insulating lining 17. A hearth 19 is provided in
the chamber 15 and serves as a floor on which bars or rods 37
of forging stock are placed for heating. A slot 21 extends
hori20ntally in an elongated manner at the front of the housing
13 adjacent the hearth and is in communication through the lining
-- 10~7236
17 with the chamber 15~ The forging stock is inserted and with-
drawn through the slot 21.
An electrical heating element 23 is disposed at each
side of the chamber~l5 on the inside of and next to inner side
walls 25 and 27 respectively of the lining. In the illustrated
embodiment, three such electrical heating elements 23 are pro-
vided at each side of the chamber. These elements extend para-
llel with the side walls 25 and 27 and are at right angles
to the plane of the slot 21, i.e., the elements extend generally
parallel to the direction of forging stock insertion. The
arrangement and location of these elements are described in
further detail in the following paragraphs.
A ceiling 29 defines the top of the chamber 15, and
protrudes into the chamber from the side walls 25 and 27
re8pectively to a low portion or line 87 paralleling the elec-
trical heating elements 23 and disposed between these
heating elements and in particular centered with respect to
the slot 21 as described in detail hereinafter. The extent
of the protrusion of the ceiling 29 into the chamber 15 (the
line 87) is above the slot 21. Thus, the ceiling does not
extend into a volume 35 bounded by lines projecting the slot
21 to a projection 21' thereof on the back inner wall. A tem-
perature sensor 31 (FIGURES 3 and 4) is provided adjacent
the hearth to detect the temperature of the hearth. In FIGUR~
5, there is shown a schematic diagram illustrating schematically
a power circuit with control, generally referred to as 33, for
operating the furnace 11. The temperature sensor 31 is connected
into this circuit as will be described in detail hereinafter.
The volume 35 defined by the projection lines of the
slot 21 to 21' illustrates the relationship between the opening
of the slot, the chamber 15 generally and various other members
to be described hereinafter. The forging stock, such as the
1~7236
steel rods 37, is inserted to a position on the hearth 19 within
the boundaries of the volume 35. Heat from the elements 23
is reflected from the side walls 25 and 27 and the ceiling 29 to
the hearth 19 to heat the rods 37. The slot 21 is the only
opening into the chamber 15, and little air from the outside of
the furnace enters the chamber to circulate therein. Accord-
ingly, little convection occurs, and most of the heat is trans-
mitted through radiation. As will be seen hereinafter, the
fire walls bounding the chamber 15 radiate heat efficiently
with little absorption of the heat.
When the bars or rods 37 are first placed on the hearth
19~ they are cold relative to the hearth and immediately draw
heat from it, reducing the hearth temperature, which is imme-
diately detected by the sensor 31. Heat is called for by
the sensor and power is applied to the elements 23 in response
until the sensor is satisfied. At such time the power is re-
moved from the elements 23 or at least modulated to maintain
the temperature at a given predetermined level.
- Because little additional oxygen is added to the
mixture within the chamber, relatively little scaling occurs
on the rods 37. Several advantages accrue to the bars or rods
having less scale, such as a finer surface finish, a greater
fo~ging die life, and use of smaller forging stock because the
bars or rods are sized closer to that required for the finished
product.
More specifically, because a furnace, such as the
slot furnace ll, is located close to a forging hammer it is
subject to the impact of the hammer. To diminish the effect
of this impact, the illustrated embodiment of the furnace 11
is mounted on an angle and channel iron support table 39, which
in turn has a provision for shock absorbing mountings to the
floor, such as by a plurality of coil springs 41 suitably inter-
--Ç--
- ~0~7Z36
ceding between pads 43 and the floor 45 to provide cushioned
mountings at anchor bolts 47. The height of the table 39 is
selected to place the slot 21 at a level convenient for the
operator to insert and withdraw the xods 37.
The details of construction of the furnace 11 are best
seen in FIGURES 2, 3 and 4. Referring first to FIGURE 2, the
heating chamber 15 is defined by the heat insulating lining 17.
such heat insulation is refractory material capable of withstand-
ing operating temperatures up to about 3000F.
As used herein, the term "refractory" is intended to
refer to material which will resist change of shape, weight,
or physical properties at high temperatures.
The outer shell housing 13 constitutes the basic
structural support for the fùrnace 11. As seen in FIGURE 1,
the outer shell 13 includes corner angles 49, wall plates 51,
53, 55 and 57, top plate 59, and base plate 61 (FIGURE 2).
( F~C ~R ~
A ring of outer foundation plates, exemplified by a plate 62
is also provided. This outer shell 13 is of a suitable metal
and serves as a structural support for the furnace as well as
a means of grounding the structure (FIGURE 5).
Inside the metallic shell housing 13 is the heat in-
sulating lining 17 which includes several layers of material.
Preferably, this material is of an asbestos-free, high tempera-
ture ceramic fiber insulation. One source of supply of such
material is Carborundum Co., Niagara Falls, New York. Such high
temperature insulation produced by this company is made from
high-alumina ceramic fiber and inorganic bonds. Such ceramic
fiber insulation is available in a flexible blanket form, in
flat or curved boards, and as loose fill. Furthermore, vacuum-
formed special shapes can be made to order. One source of supplyfor the fire wall material, i.e., the material forming all inner
surfaces of the heating chamber 15 except the hearth, is
_ 7_
10~7Z36
Refractory Products, Inc. of Carpentersville, Illinois This com-
pany's material is known as WRP-XA and is usable to 3000F. The
following illustrates its typical physical properties:
Average DensitY: 15#/cu~ ft.
Approvimate Chemical Analysis:
Na O tTotal sodium) .06%
2 -;
Na2O (Leachable sodium) .005%
Fe23 . 0019~ -j
23 .005%
All other Trace Inorganics
(Includes CaO, MgO, Nio, CrO) .15%
AL2
) Balance
si2
Permanent Linear Change Permasized for
15 After Firing for 24 Hours At:Minimum Shrinkage
2000F. . .6%
2200F. .8%
2300F. . 1.25%
2400F. 1.75%
2500F. 2.0
2600F. 2.0
2700F. 2.0
3000F. No pe~ceptable further
. shrinkage
25 Compressive Strength: .1240#/Sq. Ft.
(10% Deformation)
~4 A~ ~ f ~
A~psYwi~Hce "K" Value
At Mean Temperature Of:
500F ---
1000F. . .55
1500F. .81
2000F. 1.26
-8-
:~087Z36
Referring once again to FIGURE 2, the side walls 25
and 27 of the lining 17 includes fire walls 63 and 65 respec-
tively which, in the illustrated embodiment, are made of cer-
amic fiber board having characteristics described above. These
fire walls are in turn bonded by high temperature mortar to
backing walls 67 and 69 respectively for added strength. The
A Spa~c e.S
s~ between the backing walls 67 and 69 and the outer shell
wall plates 51 and 55 respectively are packed with loose ceramic
fiber fill 71 to complete the side walls 25 and 27.
As best seen in FIGURE 4, a high temperature front
fire wall 73 and a high temperature rear fire wall 75 complete
the four-walled enclosure of the chamber 15. In the illustrated
embodiment, these front and rear fire walls also are high temp-
erature ceramic fiber board. Between these front and rear fire
- 15 walls and the front and rear wall plates 53 and 57 respectively
of the housing 13 are packed multiple layers of ceramic fiber
77
blankets~to form a tight fit for the lining in the housing while
allowing for expansion and contraction without excessive mechan-
ical strain. In this connection, it will be noted that the
fire chamber walls are held together without metallic or other
kinds of rods or connectors. An interlocking relation between
the four walls is provided by shallow channels 79, 81 and 83,
85, respectively, formed in the inner faces of the front and
rear fire walls 73 and 75 which are used to interlock with the
side fire walls 63 and 65 which slide in the channels in engag-
ing relation with the front and rear fire walls. During expan-
sion and contraction, this interlocking relation continues and
provides integrity to the fire wall of the lining. Alternatively,
a unitized box of four fire walls can be moulded or otherwise
formed and set in place during assembly of the furnace. In
this connection, attention is drawn to FIGURE 3, wherein the
lapped relationship is seen between the top blankets 77 and the
_g_
1087236
blankets of the front and rear walls of the lining 17.
Returning once again to FIGURE 2, it will be noted
that the ceiling 29 extends from side to side in the form of
an undulation. Thus, in the illustrated embodiment, the
ceiling 29 that defines the top of the chamber 15 protrudes
inwardly of the chamber from either side wall 25 and 27 to a
lower line portion or line 87 that extends front to rear, is
substantially parallel to and in between the heating elements
23 and is substantially centered above the volume 35 defined by
the lines projecting the slot 21 to the rear fire wall 75.
Such centering of the inmost portion of the ceiling over the
center of the volume 35 affords concentration of heat uniformly
to the hearth 19 and is desirable even if, for some reason,
the slot and hearth are not centered with respect to the furnace
proper. The line 87, which represents the most inwardly extent
o the ceiling 29 into the chamber 15, is spaced above the vol
ume 35. Such a ceiling form is in sharp contrast to the typical
form of sprung arches made of fire brick that shape the ceiling
of known batch-type furnaces. The form of the ceiling 29 set
forth in this invention i9 advantageous over such arch-type
ceiling because it decreases the volume of the chamber 15, and
hence the amount of air to be heated; it allows space for the
heating elements 23 to be located at the sides of the chamber,
as is discussed in detail hereinafter; and it concentrates the
heating effect of the elements on the hearth 19. It is believed
that the precise protruding form that the ceiling 29 takes is
not as important as the fact that at least a portion of it pro-
trudes inwardly toward the center of the hearth, although spaced
above it. Thus, in addition to the broadened sine-wave type
curve undulated form illustrated, the form may be more of a
broad based triangle projecting inwardly so that either its apex
forms a single line 87 or a truncated apex forms two spaced
--10--
101~7236
apart lines (not shown) to which the ceiling protrudes. A var-
iation of this last suggested form is illustrated in an alter-
native interior structure in FIGURES 7-10 and discussed herein-
after.
In the FIGURE 2 illustrated embodiment, the shape of
the ceiling 29 is defined by a specially formed curved board
89 of ceramic fiber insulation. Above the board 89, all recesses
are filled with the loose ceramic fiber fill 71 to form a level,
and above that a plurality of ceramic fiber blankets 77 are
packed between the top plate 59 of the housing and the ceramic
fiber fill 71 to provide a tight fit.
The structure supporting the hearth 19 is also best
illustrated in FIGURE 2. Forms or molds are utilized to con-
struct two "L" shaped (in cross section) hearth supports 91
and 93 that are poured of a castable insulating aggregate,
much like concrete is poured in a form. Such an insulating
aggregate is both insulative and supportive. The supports 91
and 93 are spaced apart back to back to support the side edges
of the hearth 19. The space between the supports 91 and 93
is filled in with loose ceramic fiber fill 71, and the whole
combination is overlayed by a platform 95, preferably also
poured of the castable insulating aggregate as used for the
supports 91 and 93. Although the platform 95 is both
supportive and insulative, preferably a heavy duty fireclay
brick is laid over the platform 95 to form the hearth 19. Such
fireclay brick is capable of withstanding physical abuse the
hearth receives from the forging steel.
As a safety precaution for personnel, a plurality of
guard bricks 97 are provided to partially enclose the working
area for the forging stock as represented by the volume 35.
As best seen in FIGURE 2, these guard bricks include a verti-
cally extending body 99 and an overhead projection member 101,
1087236
these members 101 being directed inwardly toward the center
of the hearth 19, but extending only part of the way. A purpose
of these guard bricks and their particular structure is to limit
the rods 37 generally to the volume 35 as defined by the in-
wardly extending projection lines of the slot 21. Such restric-
tion serves to inhibit direct contact of the steel rods 37 with
the heating elements 23. As a further precautionary measure,
however, the entire surface of the portal or input side of the
slot 21 is covered with a conductive metal frame 103 tied to
the grounded housing 13 to insure grounding of any rod 37 that
might accidentally touch a heating element 23. By means of a
ground detection unit provided in the circuitry, as discussed
hereinafter in connection with FIGURE 5, if such a ground fault
occurs the power to the furnace is instantly shut off.
- Preferably, the guard bricks 97 are made of a highly
heat conductive material such as silicon carbide, or they may
have a high percentage of alumina (AL2O3) in their content to
make them highly conductive to heat. The guard bricks 97 are
incorporated on each side of the hearth floor primarily to act
as a block to the steel bars and prevent them from hitting the
heating elements 23. Because they are in the line of sight
r~diation from the lower heating elements they are preferably
mad~e of a hard-highly conductive brick to permit fast flow of
heat through them. A suitable instrument, such as a diamond saw,
may be used to cut the desired shape of the guard bricks from
rectangular material. It will be noted that when the guard
bricks are in position, their location and shape is such as
to encompass the volume 35, which means the inner edges of op-
posing guard bricks are separated by a distance substantially
coextensive with the height of the slot 21.
The guard bricks 97 are set into position on the
feet of the supports 91 and 93 and their positions secured by
-12-
1~37236
a row each of insulating fire bricks 105 and 107 outwardly
adjacent the supports 91 and 93 respectively. Filler walls
109 and 111, preferably poured of a castable insulating aggregate,
are provided between the rows of insulating bricks 105 and 107
and the side fixe walls 63 and 65 respectively. A plurality
of suitable anchor members 113 may be provided in the areas
where the castable aggregate is poured to anchor the furnace
through its base plate 61 to the support table 39.
The slot 21 preferably is made as small as possible
consistent with the need to accommodate the forging stock.
The smaller the opening, the less heat loss through this open-
ing. Such a small slot also minimizes the possibility of an op-
erator directly contacting the laterally mounted electrical
elements with the forging stock. Heat loss through the slot
21, however, cannot be completely eliminated, and as best seen
in FIGURE 1, provision is made on the front of the furnace 11
for dissipating the heat that does escape from the slot and
to maintain a working temperature on the front of the outside
-shell of the furnace that is safe for operating personnel.
For this purpose, a front cover 115 is provided that
has perforated side body portions 117 and 119, a sloping solid
deflector 121, a front panel 123 vertically extending upwardly
from the front edge of the deflector 121, and a top panel 125
extending across the entire cover. The perforated side body
portions 117 and 119 have perforated bottoms also. The perfor-
ated body portions 117 and 119 also serve the purpose of enclos-
ing the protruding front ends of the heating elements 23 (FIGURE
3) wherein the needed electrical connections are made. Prefer-
ably, the deflector panel 121 is made of stainless steel or
a suitable alloy thereof.
Returning once again to FIGURE 2, at least one elec~
trical element 23 is provided inwardly adjacent each of the
-13-
1~7Z36
lining side walls 25 and 27 but outwardly adjacent the volume
35, which represents the work area for heating the forging
stock~ In the illustrated embodiment, three such heating ele-
ments 23 are provided at each side location. It is, of course,
well known to pass electric current through resistance elements
to transform electrical energy into heat energy, and one form
of such resistance element in which the efficiency of this
energy transfer is high is a rod or bar made up of silicon
carbide crystals. Such silicon carbide elements are capable
of operating at the temperatures needed for forging steel while
yet having the desirable characteristics of providing stable
and precise heating over a relatively long useful li~e. Further-
more, such elements are substantially non-corrosive under such
elevated temperature conditions. The silicon carbide rods 23
in the illustrated embodiment extend through the refractory
lining 21 to the outside of the furnace where the electrical
connections (not shown) to the power circuit 33 (FIGURE 5) are
made. Adequate clearance holes are made for the elements in
the front and back housing plates 53 and 57, respectively, and
close fitting holes are made through the refractory lining 17.
Loose ceramic fiber fill, such as that used at 71, is utilized
to pack the elements into the lining walls, and this allows
easy removal of an element when the element must be replaced.
After replacement, the element should be repacked in,the open-
ings through the linings with the loose ceramic fiber fill.
It is noted that the heating elements 23 in the il-
lustrated embodiment are stacked three outwardly of one side
and three outwardly of the other side of the hearth 19 and
the volume 35. Secondly, it is noted that the shape of the
members below'the electrical elements 23, i.e., the guard bricks
97, the rows of insulating fire brick 105 and 107, and the fil-
ler walls 109 and 111 form a trough along each side of the
-14-
10~7Z36
hearth 19 and directly below the heating elements 23. Thus,
in the unlikely event that a heating element is broken, it or
portions of it, will fall into the trough below at the side of
the hearth rather than on the hearth itself. Furthermore, the
disposition of the heating elements 23 on either side of the
hearth also serves to inhibit direct contact of the forging
steel with the elements.
Referring now to FIGURE 5, electrical power is applied
to the heating elements 23 by means of the power circuit 33.
A suitable power source 127 provides the power, and generally,
the kinds of power required for such heating will be supplied
from a three-phase 60 Hz source of a suitable voltage, such
as 480 v. The source is connected directly to the input side
of a suitable three-phase circuit breaker 129 having an integral
shunt trip 131. Thus, the circuit breaker will trip not only
from a current overload through its main contacts, but also at
- any time the shunt trip circuit 131 is energized, which in this
instance is when a ground fault is detected on any phase of
the circuitry as described hereinafter.
The output of the circuit breaker 129 is connected
to a solid state controller 133 which in turn controls the
voltage applied to the primary of a three-phase isolation type
transformer 135. The heater elements or furnace resistors 23
are equally divided for connection to each phase of the three-
phase circuit. In the illustrated embodiment, there are six
resistors, two per phase connected in series with the phases
connected in a wye. As indicated previously, the furnace outer
shell housing 13 is made of metal and is grounded. There is
also provided in the circuit 33, a conventional three-phase
resistor type ground detection circuit 137, which includes a
relay to energize the solenoid of the shunt trip 131 should a
ground fault occur on any of the three phases. If a ground
~15-
. ~
10~7Z3~
fault occurs and the shunt trip is activated, the circuit breaker
129 immediately opens and removes power from the furnace resis-
tors 23. An example of such a circuit is generally given in
EEE Transactions on Industry Applications, Vol. lA-8, No. 3,
May/June, 1972, pp. 231-236.
One segment of the circuit 33 is a temperature control
circuit 139 which includes the temperature sensor 31 (FIGURES 3
and 4). This temperature sensor 31 is disposed adjacent the
hearth 19, and preferably, it is embedded in the fireclay bricks
that make up the hearth 19. As is seen in FIGURES 3 and 4, the
sensor extends to the approximate center point of the hearth
19. This thermal sensor 31 preferably includes a thermocouple
located inside the tip of its tubular structure. A small con-
tinuous current signal is generated between the thermocouple
and a reference voltage that is inversely related to the tempera-
ture of the hearth. This current signal is coupled into the
solid state controller 133, which operates in response to the
signal. In the preferred embodiment, an SCR phase shift control-
ler utilizing conventional circuitry and known technology is
used to accurately control in smooth infinite steps the power
applied to the furnace resistors 23 to maintain a desired fur-
nace temperature for forging steel within +10F. The SCR cir-
cuit is capable of completely shutting off power, applying
full power, or applying modulated power through the transformer
135 to the resistors 23 in response to the signal from the sen-
sitive thermocouple (not shown) in the tip of the sensor 31.
Because the silicon carbide rods used as the heating
elements 23 age, i.e., experience a persistent increase in
resistance with use, it is desirable to provide taps on the
output of the voltage transformer 135 for adjusting the full
secondary voltage that can be applied to the resistors 23.
For example, taps may be provided on each phase at 25 volt
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1~7236
increments for a range in line to line RMS output voltage of
from about 100 to 240 volts tSix taps per phase).
As a result of the structure described in accordance
with this invention, the heat insulating lining 17 and the
hearth 19 along with its foundation provides the characteristics
of very low heat loss and low heat absorption. This allows
rapid and efficient heat transfer from the resistors 23 to the
steel to be forged. Further, the disposition of the sensor 31
which locates the thermocouple in the center of the hearth 19
causes an immediate response to the cold steel entering the
furnace and measures the temperature continuously to bring
about a power application to the resistors through the solid
state control as needed to hold the temperature within the
tolerance required.
Further in accordance with this invention, FIGURE
6 illustrates the rapid increase in temperature of the furnace
to the desired working temperature that the illustrated power
and control circuit 33 brings about in the furnace 11 without
an overshoot of power. Thus, the desired temperature is reached
efficiently, rapidly, and with only the minimum of power suf-
ficient to maintain the desired temperature thereafter.
Because all of the heat is generated entirely within
the chamber 15, a minimum of air circulation exists. Scaling
of the steel thus is minimi~ed, and this allows a finer surface
on the finished product, a greater forging die life, and the
use of smaller sizes of steel rods or bars for the product to
be forged. The combination of the high temperature heating
- elements 23, the extremely low heat loss through heat absorption
of t~e lining 17, the solid state power supply which causes the
furnace temperature to increase very rapidly while being precise
enough to modulate the power to not overshoot the temperature,
and the small volume that must be heated of the chamber 15 pro-
1087236
vides an electrically powered slot furnace in accordance withthe invention tha~ is able to compete economically with present
day furnaces that are either gas or oil-fired.
One example of a furnace that has been constructed
S in accordance with this invention includes a furnace 11 (FIGURE
1) having external dimensions of 48 inches wide x 35 inches
deep x 34 inches high. The furnace is suitably anchored to
a support table 39 that mounts the base of the furnace 11 ap-
proximately 40 inches above the floor 45. The pads 43 are 6
inch square, one-quarter inch thick steel plates and the springs
41 are four inches OD and three and one-half inches ID. Anchor
bolts 47 are provided.
The slot 21 is two inches high x twenty-two inches
wide x seven inches deep.
lS With reference to FIGURE 2, the blankets 77 are each
one inch thick, and the fire walls 63, 65 and the ceiling board
89 are each about two inches thick. The distance from the line
87 to the top of the hearth 19 is approximately 11 inches~ and
the distance between the side walls 25 and 27 through the cham-
ber 15 is approximately 34 inches. The depth of the working
area of the hearth, i.e., the distance between the front and
rear fire walls 73 and 75 respectively (FIGURE ~) is approximatel
18 inches. $here are six heating elements 23, and each are
silicon carbide rods having a diameter of 2-1/8" and a length
of 37 inches. The rods used are known as GL~BAR type LL as
manufactured by Carborundum Company, Niagara Falls, New York.
In this example, the top two resistors (FIGURE 2j are connected
in one phase, the two lower resistors on the left in the figure
are connected in a second phase, and the two lower resistors
on the right are connected in the third phase of the circuit.
The desired forging temperature for the steel is
app~oximately 2250F, and for this a 50 KW power supply, i.e.,
~,LoB~2 ~S o,
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.. . . ..
:~0~7236
FIGURE 5 circuit elements 129-135, from a power source of 480
A volt~three-phase 60 Hz is utilized to supply energy to the
furnace. The controller 133 i5 an SCR phase-shifting type
manufactured by Magnetics, Inc. of Sandy Lake, Pennsylvania.
This example furnace provides 300 pounds of forging
steel per hour at an electrical usage of ap2roximately 43 KWHR.
To prolong resistor life, it has been found desirable
to maintain a temperature of 1500F during all non-production
hours. During this time, the slot 21 is plugged to absolutely
minimize losses and amount of power needed to maintain the
resistors at 1500F. On production days, 10-15 min. is required
to elevate the fire chamber of the furnace from 1500F to 2250F
and to allow the fire chamber to stabilize.
An alternative interior structure of the furnace
just described is shown in FIGURES 7-10 where reference numbers
with subscripts indicate like parts. Thus, a furnace lla is
Qhown which differs from the furnace 11 principally in the
structure of the top of the chamber 15a and the front and rear
fire walls of the refractory lining 17a. For ease of illustra-
tion, only the rear fire wall 75a of these two walls is shown,
but it should be understood that the front fire wall is similar
in structure to the illustrated rear fire wall 75a.
.Referring now to FIGURE 7, a ceiling 141 is shown
that has a portion protruding inwardly between the heating
elements 23a, the portion being in the form of a beam 143.
This beam is generally rectangular in cross section and imme-
diately underlies the ceiling 141 and extends from front to
rear of the chamber 15a, i.e., from the front fire wall having
the access slot (positionally the same as front fire wall 73 of
the lining 17 through which is the slot 21 of FIGURES 1 and
4) to the opposite or rear fire wall 75a. The beam 143 includes
a plurality of slabs 145 that preferably are stiff and flat
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and generally rectangular in cross section. These slabs are
placed broad face to broad face and secured together by means
of plugs 147 formed of a moldable ceramic fiber and cured after
molding to a rivet-like structure. The slabs themselves are
preferably of an alumina ceramic fiber composition as previously
disclosed herein. Their widths are disposed vertically in the
structure to provide supportive strength to the ceiling 141.
In the illustrated embodiment, the beam has been made from alumina
ceramic fiber boards that are approximately two inches thick and
approximately six inches wide. The lower side edges of the
beam 143 are beveled at 149 by an angle of approximately 45.
The lower surface of the beam is disposed above the projection
21'a of the slot in a manner similar to the lowest extent 87 of
the ceiling 2~ in the furnace 11 (FIGURE 2). In this connec-
tion, the ratio of dl to d in FIGURE 7 may be 2:1. The config-
uration as shown in cross section for the beam extends between
the fire walls, but each end of the beam is formed into a rec-
tangular shaped stud 150 (FIGURE 10) so as to interlock with
a respecti~e adjacent fire wall in a conforming slot, such as
a slot 151 in the rear fire wall 75a as illustrated in FIGURE
10 .
The width of the beam 143 is deter~ined by the number
of slabs 145 comprising the Iamination, but the width can be
furth~r adjusted by interleaving blankets 153 intermediate the
faces of the rectangular slabs. The material of the blankets
also is preferably of an alumina ceramic fiber composition in
the soft blanket form as previously described. In such instance,
the plugs 147 secure both the blankets and the slabs. The illus-
trated beam 143 is approximately nine and one-half inches wide
and six inches high, i.e., dl plus d2 equals six inches.
The ceiling 141 in this embodiment comprises a pair
of two inches thick boards 155 that meet in the center of the
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furnace lla over the beam 143. Thereover, five one-inch blanks
77a are applied to complete the top of the lining 17a. As seen
in both FIGURES 7 and 8, the ceiling boards 155 and blankets
77a on the top extend across the full length and width of the
furnace within the housing plates 51a, 53a, 55a and 57a, and
thus across the top of the front, side and rear fire walls.
The front and rear walls of the refra~tory lining
17a each comprise layers of horizontally oriented material
vis-a-vis vertically extending blankets and boards of the fur-
nace 11 (FIGURE 3). The rear fire wall 75a is best seen in
FIGURES 9 and 10. In particular, a stack of horizontal slabs
or ribs 157 are illustrated in FIGURE 10. These ribs may be,
for example, two inches high by four inches deep, and preferably
are also of an alumina ceramic fiber composition. Some adjacent
ribs in the stack are interconnected by plugs 159, which, like
the plugs 147 in the beam 143, are formed of a moldable ceramic
fiber and have been cured after molding, resulting in a rivet-
like structure. Such interconnection is desirable in the inter-
face, for example, of adjacent members through which the heating
elements 23a extend as illustrated (FIGURES 7 and 10). The
ribs 157 preferably are stiff and flat, but they may also be
interleaved with varying thicknesses of blankets 161 of an
alumina ceramic fiber composition in the soft, flexible form
to cause the stack of ribs 157 to reach the desired height as
needed to form the rear fire wall 75a. To enhance the connec-
tion between the adjacent "locked" ribs where the plugs 159 are
utilized, a suitable high temperature ceramic seal material
163 may also be provided. As mentioned previously, the front
fire wall of the furnace lla is constructed in a similar manner
to that just described for the rear fire wall 75a.
While such stack of horizontally disposed ribs com-
prises the rear fire wall 75a, combinations of vertically
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.
7Z36
disposed boards and blankets may be used to fill the space
between this stacked rib construction and the rear plate 57a
of the housing to complete this portion of the lining 17a, and
the front portion of the lining 17a (not shown) may be con-
structed similarly.
In FIGURE 9 it will be seen that the side walls 25a
and 27a are constructed in the furnace lla similar to the walls
25 and 27 in furnace 11 (FIGURES 2 and 4). Shallow channels
83a and 85a in the rear fire wall 75a and similar channels op-
posing in the front fire wall (not shown) are provided to in-
terlock with the side fire walls 63a and 65a. To provide in-
creased strength in these side fire walls, a plurality of plugs
165 formed of a moldable ceramic fiber and cured after molding
are used to interconnect the side fire walls 63a and 65a with
the backing walls 67a and 69a respectively. Loose ceramic fiber
fill can be used to fill the space between the backing plates
and the housing wall plates 51a and 55a respectively.
There has been supplied in accordance with this in-
vention a slot furnace for heating steel to forging temperatures,
such as temperatures approximating 2300F, that operates econom-
ically from electrical energy without costly air purification
equipment. Although operating costs are comparable at this
time, there appears to be an economic advantage in utilizing
electrical power in the future because of the scarcity of gas
and oil and the availability, on the other hand, of nuclear
` power for electricity.
The furnace of this invention utilizes high tempera-
ture, high power silicon carbide resistance elements as heating
elements. The furnace also utilizes high temperature ceramic
fiber insulation made of high-alumina ceramic fiber and inorganic
bonds as the refractory material to insulate the firing chamber
of the furnace. This insulation is in the form of boards, soft
blankets, and loose fiber and provides low heat loss and low
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heat absorption that is necessary for rapid and efficient ~eat
transfer from silicon carbide resistors to the steel to be
forged. The furnace of this invention further utilizes solid
state electronic control systems to accurately control the power
flow to the furnace resistors for maintaining an extremely close
furnace working or firing temperature. Power is turned on or
shut off or modulated in response to a very small eleçtric
current signal provided by a thermocouple and sensing circuit.
The thermocouple is embedded in the center of the hearth to
rapidly sense the temperature of the steel to be forged that
is placed on the hearth.
In the furnace structure of the invention, the resis-
tors are located on either side of the hearth, the sides of
the hearth incorporate guard bricks, the slot is small and
its portal is lined with metal connected to a ground, and the
circuitry includes a ground detection relay circuit to shut
off power if a ground fault develops, all to maximize safety to
operating personnel.
The ceiling of the fire chamber protrudes inwardly
of the chamber to a region spaced above the hearth. Such struc-
ture both reduces the volume within the chamber and serves to
concentrate the heat from the electrical elements to the working
area of the hearth where the forging steel is heated. The fire
walls and ceiling of the chamber are interlocked without metal-
lic rods or other connectors, and the insulation lining is packedtightly within the metal shell housing of the furnace while
allowing the needed expansion and contraction without excessive
mechanical strain. The small slot minimizes heat loss and the
amount of air circulation within the chamber. The reduced air
circulation within the chamber reduces scaling on the steel,
and a heat dissipating cover is provided on the front of the
furnace to dissipate the heat without increasing the temperatures
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of the housing to that which is dangerous for operating person-
nel. The furnace of this invention rapidly comes up to the de-
sired operating temperature without overshoot and maintains
temperature accurately and with an efficient use of power.
It is intended that the term "hearth" as used herein
include the entire lower or bottom inner structure of the fur-
nace 11 generally betwesn the guard bricks 97 as well as the
floor surface of the furnace chamber 15, particularly that por-
tion below the volume 35 (FIGURE 2).
While the invention has been described in connection
with a preferred embodiment with an alternative interior, other
alternatives, modifications and variations may be apparent to
those skilled in the art in view of the foregoing description.
Accordingly, it is intended to embrace all such alternatives,
lS modifications and variations as fall within the spirit and scope
of the appended claims.
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