Scrap preheating system for an electric furnace

This disclosure relates to a system for preheating scrap before charging in an electric furnace and for preventing the release of malodorous or noxious gas. The system includes a first duct leading from the furnace exhaust gas outlet to a chimney; a branching duct is connected in the first line for passing part of the gas to the chimney and for diverting the remainder of the gas to a scrap preheater. After preheating the scrap, the diverted gas is recirculated back to the first line at a junction which is adjacent the furnace outlet. The system further includes gas combustion means connected in the first line between the junction and the branching duct, and a heat equalizer-accumulator in the first line between the combustion means and the chimney. The system may further include a heat exchanger connected in the first line downstream of the branching duct and a second line for passing the diverted gas through the heat exchanger after it leaves the scrap preheater. A temperature responsive device may be connected in the first line for adjusting the amount of the diverted gas.

This invention relates to a system for preheating scrap metal before it is 
processed in an electric furnace, employing the hot exhaust gas from the 
furnace. 
Conventional scrap preheating apparatus of the present type releases 
objectionable exhaust gas into the air. This gas contains malodorous and 
noxious vapors of organic substances such as oils, paints and rubber that 
are mixed with the scrap. Because this gas is a source of environmental 
pollution, some apparatus of this character have been prohibited. 
FIG. 1 shows a prior art preheating system that attempts to overcome the 
above pollution problem, wherein the objectionable gas from a scrap 
preheater 1 is fed back through a duct 2 to a high temperature duct 4 at 
an exhaust outlet of an electric furnace 3. Thus, the gas is reburned and 
thereby cleansed before being passed through a dust remover 5 and a 
chimney 6 and to be released into the air. 
However, the temperature of the exhaust gas from the furnace 3 varies 
greatly as shown by the curves A in FIG. 2, as air enters the furnace each 
time the furnace is charged with preheated scrap. If the furnace gas is 
too hot, scrap in the preheater will fuse and the preheating basket(s) 
will be deformed; on the other hand, if the gas is too cool, the scrap 
will not be sufficiently heated in the preheater. If the exhaust gas 
immediately leaving the furnace has a temperature which is too low to burn 
objectionable components of the gas from the preheater, these components 
will not be reburned and the gas will not be cleansed, and the bad odors 
will be released into the air. 
In addition, some dust contained in the exhaust gas from the furnace will 
stick to the scrap in the preheater, and some of this dust will enter the 
furnace together with the scrap, thereby lowering the quality of the 
steel. The remainder of the dust on the scrap will spread around the 
furnace area when the furnace is charged with the scrap, thus 
necessitating the use of dust collectors, etc. 
It is an object of this invention to provide a scrap preheating system for 
use with an electric furnace, which enables the temperature of the exhaust 
gas from the furnace, the temperature of which gas normally varies 
greatly, and the cooler gas from the preheater, to be maintained at 
substantially constant temperature which is high enough to burn 
objectionable components of the gas, in order to deodorize the gas from 
the preheated scrap, without necessitating the use of other fuel. 
It is another object of the invention to provide such a scrap preheating 
system, which prevents the steel quality from being lowered because of 
dust which has adhered to the scrap in the preheater and entered the 
furnace, and which prevents dust on the preheated scrap from spreading 
around when the furnace is charged with the scrap. 
It is a still further object of the invention to provide such a scrap 
preheating system, which effectively recovers a quantity of the excess 
heat possessed by the portion of the gas mixture which is eventually 
released into the air. 
In accordance with this invention, a system is provided for preheating 
scrap using the exhaust gas from an electric furnace, said system 
comprising a first gas line or duct for passing the hot and dust-laden 
exhaust gas from the furnace through a dust remover and eventually into 
the surrounding air, branch means provided in said first line before or 
upstream from said dust remover for diverting a portion of the gas 
entering said branch means, and a second gas line for recirculating said 
portion of the gas through a scrap preheater to said first line at a 
junction between said furnace and said branch means, said system further 
including combustion means connected in said first line between said 
branch means and said junction of said first and second lines for burning 
objectionable components of the gas, and a heat regenerator or 
equalizer-accumulator provided in said first line between said combustion 
means and said dust remover and either before or after said branch means. 
This system may further comprise a heat exchanger connected in said first 
line following said branch means and said heat accumulator, and means for 
directing the gas from said preheater to pass through said heat exchanger 
before being fed back to said first line. 
This system may further comprise temperature-responsive means provided in 
said first line between the junction where the forward end of said second 
line is connected to said first line and the output of said heat 
accumulator, and means provided in said second line and operated by said 
temperature responsive means to control the quantity of gas being 
recirculated through said second line. 
This system may further comprise a preheating burner provided in said first 
line between the junction where the forward end of said second line is 
connected to said first line and said regenerator.

The equivalent parts of the various embodiments are given the same 
reference numerals to avoid the need for repeating the explanation. 
With reference first to FIG. 3, an electric furnace 11 for refining scrap 
has an outlet or gas vent 11a for exhausting gas. A high temperature gas 
duct or line 16 extends from the outlet 11a through a gap adjusting ring 
12, a combustion tower 13 and a high temperature dust remover 14, to the 
inlet of a branching duct 17, all of which are connected in series. 
Connected to one outlet branch 17a of the branching duct 17 is a scrap 
preheater 18 having two or more scrap baskets 18a and 18b disposed in 
parallel or in series with each other. Switching valves for the gas are 
connected at opposite sides of each basket. A gas return line 20 extends 
from the outlet of the preheater 18 through a flow inducing fan 19 and 
back to the exhaust duct 16, between the furnace outlet 11a and tower 13, 
for example, between the ring 12 and the tower 13, as illustrated, or to 
the tower 13. 
Connected to the other branch 17b of branching duct 17 is a gas line 23 
which extends through a regenerator or temperature equalizer-deodorizer 15 
(better shown in FIG. 4), a dust remover 21 and a chimney 22. 
The ring 12 interconnects the outlet 11a and the line 16, and varies the 
gap between the ducts at its opposite sides to mix a desired amount of air 
with the exhaust gas leaving the furnace, in order to control the 
combustion of the components of the gas which have not been burned and the 
gas temperature. 
The combustion tower 13 may be a dust remover of the gravity setting type, 
which lowers the flow rate of the gas, deflects the gas flow by a 
partition 13a to remove dust by its mass or gravity, and burn the 
substances which have not burned by the heat possessed by the gas and the 
lowered gas flow rate. 
The hot dust remover 14, which may be omitted from the system, may be a 
cyclone-type for removing dust which is not removed in the combustion 
tower 13. 
As shown in FIG. 4, the equalizer-deodorizer 15 comprises a housing loaded 
with a number of heat accumulating elements 15a such as pieces of steel 
and bricks for heat exchange with the gas passing therebetween. 
The equalizer-deodorizer 15 is provided to substantially smooth out or 
equalize the temperature of the gas passing through it, as shown by curves 
B in FIG. 2, even though the temperature of the exhaust gas from the 
furnace 11 changes due to charging of the furnace with scrap at 
predetermined time intervals. The heat accumulator elements 15a absorb 
heat when the gas is at temperature peaks, and they release heat when the 
gas is at temperature valleys, thereby evening the gas temperature. This 
enables the temperature of the gas passing through the 
equalizer-deodorizer 15 to always be kept sufficiently high to burn the 
malodorous components. A temperature of at least 600.degree. C. is desired 
in order to eliminate the malodorous components of the gas. Without the 
unit 15, the temperature periodically falls below the 600.degree. C. level 
as shown by the valleys between the temperature peaks of Curve A. The 
equalizer-deodorizer 15 smoothes out the temperature variations and 
maintains the gas temperature in it above this level even though the 
temperature of the gas leaving the furnace varies greatly. Since the 
temperature in the unit is maintained above approximately 600.degree. C., 
any malodorous components of the gas will be burned out in the unit 15. 
In operation, the valves are opened at both ends of a basket 18a which is 
charged with scrap, and the valves of the other basket 18b are closed. The 
basket 18b may at that time either discharge preheated scrap, be charged 
with new scrap, or be on standby. 
While the electric furnace 11 is working, the hot exhaust gas passes 
through the gap adjusting ring 12 to the combustion tower 13, where 
substances in the gas which have not been burned are burned, and the 
coarse dust is removed. After additional dust is removed in the hot dust 
remover 14, the gas enters the branching duct 17. 
A portion of the gas then passes from the duct branch 17a to the preheater 
basket 18a in order to preheat the scrap to a desired temperature. After 
heat exchange with the scrap, the gas will have a temperature in the order 
of, for example, 100.degree.-200.degree. C., and consequently it will have 
a bad odor due to evaporation of organic substances contained in the 
scrap. 
The malodorous gas is recirculated by the fan 19 through the duct 20 to the 
hot gas duct 16, and mixed with the hot exhaust gas from the furnace. The 
other portion of the gas passes from the duct branch 17b to the 
regenerator or deodorizer 15 for heat exchange with the accumulating 
elements 15a to raise or lower the temperature the gas. 
In the equalizer-deodorizer 15, the substances which have not been burned 
in the superfluous portion of the gas containing the malodorous 
components, burn to deodorize the gas. These components are burned in the 
unit 15 because the temperature of the elements 15a is above the level 
needed to burn these components, and consequently these components are 
burned out as the gas flows through the unit 15. After further dust is 
removed in the remover 21, the deodorized gas is released from the chimney 
22 into the air. Thus, the bad smell produced by preheating the scrap is 
removed and is not released into the air. 
Also, the dust which would otherwise stick to the scrap is removed in the 
combustion tower 13 and in the hot dust remover 14 in order to reduce the 
amount of dust entering the scrap preheater 18. This prevents dust from 
spreading around when the furnace 11 is charged with preheated scrap, and 
it also improves the steel quality. 
FIG. 5 shows the second embodiment, which can effectively utilize a greater 
amount of heat possessed by the hot exhaust gas from the electric furnace. 
In the line 23 between the equalizer-deodorizer 15 and the dust remover 21 
is provided a heat exchanger 24, which is passed by both the gas from the 
equalizer-deodorizer 15 and the malodorous gas from the preheater 18 
through a line 26, the gases exchanging heat with each other. 
As a result, the temperature of the malodorous gas leaving the scrap is 
raised by a superfluous amount of heat of the gas from the 
equalizer-deodorizer 15, which amount would otherwise be released into the 
air. The heated malodorous gas is recirculated through the line 20 to the 
hot gas line 16. This promotes the combustion in the tower 13 of the 
substances which have not burned, and increases the amount of heat which 
is accumulated by the equalizer-deodorizer 15. 
The heat exchanger 24 should preferably be of the indirect or multitubular 
type, but may otherwise be of direct type having heat accumulating 
elements, such as does the equalizer-deodorizer 15. 
FIG. 6 shows the third embodiment having the arrangement of FIG. 3, but 
with the equalizer-deodorizer 15 located in the line 16 between the hot 
dust remover 14 and the branching duct 17, rather than after tne duct 17 
as shown in FIG. 3. 
Even if the exhaust gas from the electric furnace 11 is too hot, this 
arrangement enables the gas to be cooled to a temperature suitable to 
preheat scrap so as to prevent the scrap from fusing. If the gas is too 
cool, suitably heated gas can be supplied to the scrap preheater 18. 
FIG. 7 shows the fourth embodiment having, in addition to the components 
shown in FIG. 6, a heat exchanger 24 provided in the line 23 between the 
duct branch 17b and the dust remover 21, similar to the arrangement of the 
exchanger 24 shown in FIG. 5. The malodorous gas passes from the scrap 
preheater 18 to the heat exchanger 24 to raise its temperature before 
returning to the connection between the furnace 11 and the combustion 
tower 13. 
FIG. 8 shows the fifth embodiment having, in addition to the arrangement of 
FIG. 3, a temperature detector 27 provided, for example, at the outlet of 
the equalizer-deodorizer 15, and a valve or damper 28 provided in the 
return line 20, for example, between the outlet of the scrap preheater 18 
and the fan 19. The damper 28 is controlled by the detector 27 to control 
the rate of the gas being recirculated. 
The control varies the rate of the gas flow through the preheater 18 in 
response to the temperature variation of the gas at the outlet of the 
equalizer-deodorizer 15 across of predetermined value, at or above which 
the malodorous components of the gas are burned. When the exhaust gas from 
the furnace 11 is too hot, the quantity of the recirculated gas is 
increased to cool the hot gas, thereby preventing the scrap in the 
preheater 18 from fusing and the baskets from deforming. When the exhaust 
gas is too cool, the quantity of the recirculated gas is reduced to keep 
the gas temperature at the deodorizer 15 high enough to burn the 
malodorous components. 
Instead of the damper 28, means may instead be provided for changing the 
speed of the fan 19 to control the rate of gas flow through the return 
line 20. Such a temperature detector 27 may otherwise be provided at a 
location in the hot gas duct 16 between its junction with the forward end 
of the return duct 20 and the outlet of equalizer-deodorizer 15, to detect 
the temperature of the gas or the heat accumulating elements 15a. 
The temperature range within which the gas can be deodorized in the 
equalizer-deodorizer 15 depends on its malodorous components. Experiments 
have proved that a temperture of at least 600.degree. C. is necessary to 
deodorize the gas from scrap of the sort processed by an electric furnace, 
and that a higher temperature speeds up the deodorization. 
Consequently, the control 27 should be preset so that, when the temperature 
of gas at the outlet of deodorizer 15 exceeds, for example, 650.degree. 
C., the damper 28 is opened wider to increase the quantity of hot gas 
flowing into the preheater 18, giving the scrap more heat. The increased 
quantity of gas cooled in the preheater 18 returns to the duct 16 to be 
mixed with the hot furnace gas and then enters the deodorizer 15. When the 
temperature at the equalizer-deodorizer outlet drops below 650.degree. C., 
the damper is closed somewhat. This increased damping continues until the 
temperature is approximately restored to the predetermined value, as 
graphically shown in FIG. 9. 
If the rate of gas flow through the preheater 18 were constant as shown by 
the line K, the temperature of the exhaust gas from the furnace 11 would 
change and become high as shown by the curves Ll, L2, L3, raising the 
temperature of equalizer-deodorizer 15 as shown by the curves Ml, M2, M3. 
Now, if the flow rate in the return duct 20 were raised as shown by lines 
N2 and N3, the temperature of the gas at the deodorizer outlet would lower 
as shown by the lines 02 and 03. 
Thus, the temperature variation of the mixed gas is reduced in amplitude to 
thereby preheat the scrap always at a suitable temperature, and to 
maintain the burning of the malodorous components of gas from the 
preheated scrap. 
FIG. 10 shows the sixth embodiment which is similar to the arrangement of 
FIG. 8, but wherein the branching duct 17 is located between the 
equalizer-deodorizer 15 and the dust remover 21, similar to the FIG. 6 
embodiment. This allows the temperature of the gas to be adjusted 
substantially to a predetermined value in the equalizer-deodorizer 15 
before it is supplied to the scrap preheater 18, thereby improving the 
heat recovery by the scrap. 
FIG. 11 shows the seventh embodiment having, in addition to the components 
of FIG. 5, a temperature detector 27 provided in the outlet of the 
equalizer-deodorizer 15, and a damper 28 provided in the line 26 between 
the scrap preheater 18 and the heat exchanger 24. The damper 28 is 
controlled by the detector 27 to control the rate of the gas recirculation 
through the preheater 18 and the heat exchanger 24 to the hot gas duct 16. 
FIG. 12 shows a specific form of regenerator 115 adapted for use as the 
heat equalizer-deodorizer 15 such as the unit shown in FIG. 3. The 
regenerator 115 has a gas inlet 30 connected to the duct leading from the 
duct branch 17b, and an outlet 32 connected to the duct leading to the 
dust remover 21. The regenerator 115 has a shell 34 lined with refractory 
matter 36, to function also as a regenerative furnace. 
The body of the regenerator 115 is formed with an upper opening 38 that 
extends through the shell 34 and the liner 36, which is normally closed in 
gas-tight fashion by a lid 40 that is also lined with refractory matter 
36a. Fixed to the inside of the regenerator body are a pair of laterally 
spaced supports 42 (see also FIGS. 13 and 14), which extend longitudinally 
of the regenerator 115, parallel to each other and just below the opening 
38. 
As shown in FIGS. 13, 14, the supports 42 support both ends of pairs of 
removable spaced steel angles 44, which extend perpendicularly to the 
supports 42. Their vertical sides 46 face each other and their horizontal 
bottom sides 47 rest on the supports 42. 
Each pair of angles 44 supports between the vertical sides 46 the upper 
ends of a number of vertical steel bars 115a that form heat-accumulating 
elements, the bars 115a extending vertically at spaced intervals. The bars 
115a and the angles 44 thereby form a separate unit 48. Thus, a bundle of 
elements 115a extend in spaced parallel relation to each other, both 
transversely and longitudinally of regenerator shell 34. The elements 115a 
may be secured to the angles 44 by bolts and nuts 49 (FIG. 13) on the 
vertical sides 46, or welded to the sides 46. 
Each unit 48 comprising the angle members 44 and the elements 115a can be 
removed from the interior of the regenerator 115 through opening 38 when 
the lid 40 is removed, for replacement. 
Alternatively, as shown in FIG. 15, the elements 115a may be arranged to 
extend horizontally in a ladder configuration. The horizontal elements 
115a are supported at their ends by vertical supports, and the upper ends 
of the supports are secured to the angles 44. It is possible to combine 
the two types of heat accumulating units shown in FIGS. 14 and 15, e.g., 
by alternately placing one type upstream of the other with respect to the 
gas flow. 
The heat-accumulating elements 115a may otherwise take the form of long, 
deformed or flat bar steel, shaped steel or other rolled steel, or pipe 
steel, which can effectively absorb a high amount of heat energy from the 
exhaust gas and later release it. 
Round bars 115a having a smooth surface pervent dust from adhering thereto, 
and are preferable when the exhaust gas contains a large amount of dust. 
Round bars having a rough surface, deformed bars and shape steel are 
effective in heat transfer, and are suitable when the gas contains a 
smaller amount of dust. 
The elements 115a in the form of bar steel can be washed in water to remove 
the dust adhering thereto, and may be reused as scrap steel with which to 
charge the electric furnace 11, without the need to set them aside or 
discard them even when they can no longer be used as heat accumulators. 
As shown in FIG. 12, the regenerator inlet 30 may be provided with a burner 
50 therewithin, the nozzle of which is directed in the direction of gas 
flow, to preheat the regenerator 115 and the gas upstream of the steel 
elements 115a to a temperature that is high enough to burn the malodorous 
components when the regenerator 115 is not yet hot enough to operate as a 
deodorizer, e.g., just after the furnace 11 is started. 
As shown in FIG. 16, the steel elements 115a may be divided into two or 
more bundles to form a space 52 therebetween, which is a gas path greater 
in cross-section than between the elements in the bundles. The gas flow 
speed is lower in the space 52. By varying the width 52a of the space 52, 
the time for the gas to pass through the regenerator 115 may be changed in 
order to adjust the degree of deodorization. 
FIG. 17 shows a modification of the regenerator, which includes additional 
heat-accumulating units 54 provided at least upstream of the units 48, and 
may include further units 56 downstream of the units 48. Each additional 
unit 54, 56 includes steel bars 115a in crossed or latticed form and 
defining a number of small gas paths. Alternatively the elements may be 
formed of refractory bricks. These units 54 and 56 not only serve as heat 
accumulators, but rectify the gas flow so that it is distributed over the 
whole area of the units 48, thereby improving the heat exchange 
efficiency. 
The regenerator 115 may be provided with a cleaning hopper (not shown) at 
the bottom of the shell 34 below the heat accumulators 115a, or in its 
bottom of inlet 30, to permit removal of the accumulated dust. 
The upper opening 38 may instead be formed in the side wall of shell 34 of 
the regenerator 115, and the units 48, 54, 56 may be adapted to laterally 
slide on the supports and be inserted or removed through the opening.