High velocity billet heater

A billet is transported through an improved furnace by convention means and is heated in the furnace by an external burner unit in which fuel and air are combusted together and then injected at a high velocity into a semi-cylindrical chamber of the furnace tangentially to the billet so as to create a hot vortex of gases circulating around the billet along the length of the furnace. The temperature of the hot gas vortex within the furnace is sensed by means of a thermocouple placed close to the billet surface but not in contact with it so as to sense the temperature of the billet as a function of the temperature of the circulating gas. The temperature of the injected gas is thereby controlled to heat the billet to a desired temperature and then maintain it at that temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to a billet heater and more particularly to a 
furnace for pre-heating a metal billet for a metal extrusion press. 
In the extrusion of aluminum billets it is desirable to pre-heat and thus 
soften the metal to some extend. Prior art devices for carrying out this 
step have relatively poor thermal efficiency. This is due to the design of 
the interior contour of the chamber and the method of thermal energy 
transfer which is limited by a short contact time between the billet and 
the combustion gases and by the low velocity of the hot gases. In such 
prior art furnaces the burners are situated within the furnace chamber on 
both sides of the billet and direct their flames non-tangentially against 
the underside of the billet. The flames pass only partially around the 
billet before being exhausted to the furnace exterior. Thus, the flames, 
although of high temperature, have a low mass velocity and only contact 
the billet for a short period of time. Thermal efficiency, however, is a 
function of both the mass velocity of the combusted gases, the length of 
time the gases are in contact with the billet, and the gas temperature. 
Another problem of some prior art aluminum billet heaters is that the 
temperature maintained within the furnace is controlled by means of a 
thermocouple which is inserted through the side of the furnace and which 
contacts the billet as it moves through the length of the furnace. This 
thermocouple probe develops an aluminum oxide coating from the billet 
which acts as a thermal insulator and miscalibrates the probe. Still 
another problem of some prior art billet furnaces is that they are not 
readily adaptible between oil and gas fuels, but must be specially 
converted for each one. 
SUMMARY OF THE INVENTION 
The above and other disadvantages of prior art billet heaters are overcome 
by the present invention of an improved billet heater of the type having 
an elongated, heated furnace and means for transporting an elongated metal 
billet through the length of the furnace wherein the improvement comprises 
a plurality of combustion units located exteriorly of the furnace chamber 
and spaced along the length for injecting high velocity, combusted, hot 
gas into the furnace chamber in a direction generally tangential to the 
exterior surface of the billet and non-parallel to its length. The 
interior surface of the furnace chamber has a generally concave shape in 
cross section taken perpendicular to the length of the chamber so as to 
create an annular space surrounding the length of the billet whereby a 
vortex of hot gas is created around the billet to heat it. Because the gas 
is injected with a high velocity, the billet and the interior chamber wall 
act as a venturi nozzle to induce and maintain the flow of existing gases 
in a circumferential flow around the billet. The combined effect of high 
mass velocity of the gases and extended contact time with the billet 
provides a rapid and highly efficient method of heat transfer. Exhaust 
gases are removed from the chamber through ports located at the base of 
the chamber. 
In order to control the temperature of the hot gases injected into the 
chamber, the temperature of the hot gas circulating around the billet and 
closely spaced from it is sensed and a control signal is thereby produced 
to control the temperature of the injected gas. It has been determined 
that for a given billet size and temperature requirement, the difference 
in temperature between the billet and the closely surrounding vortex of 
hot gas is relatively constant. Thus, the temperature of the hot gas can 
be used to determine the cut-back point for entering a modulated control 
period to maintain the billet at a specified temperature. By sensing the 
temperature of the envelope of gases around the billet, the actual billet 
temperature can thus be sensed without the necessity of contacting it. The 
thermocouple, therefore, does not develop the oxide coating which would 
interfere with its function as happens in some prior art devices. 
In the preferred embodiment, the hot gases from the combustion chamber are 
injected into the furnace at velocities of between 100-400 feet per second 
at temperatures of 800.degree. to 2,000.degree. F. The hot gases are 
injected tangentially against the billet surface to initiate the vortex of 
hot gases around the billet. 
The hot gas which is injected into the furnace chamber is produced by 
separately combusting the fuel and air under pressure in a cylindrical 
chamber situated outside of the billet furnace chamber. In this way, the 
fuel and air are mixed together in a rapid vortex within the combustion 
chamber to produce very high temperature gas at a relatively high pressure 
of 15 - 40 ounces per square inch, gauge. The resulting gas injected into 
the billet furnace chamber at 100 - 400 feet per second is thus injected 
at a much higher velocity than the 20 - 60 feet per second for prior art 
systems. 
The temperature of the injected gas can be controlled by a by-pass system 
which mixes cooler air with the combusted gas prior to its injection into 
the furnace chamber so as to reduce the temperature of the injected gases 
below the combustible range of the burner fuel-air system. 
It is therefore an object of the present invention to provide a billet 
heater having improved thermal efficiency. 
It is another object of the present invention to provide a billet heater 
capable of readily adapting to liquid or gaseous fuels. 
It is still another object of the present invention to provide a billet 
heater having improved temperature control sensing means. 
The foregoing and other objectives, features and advantages of the 
invention will be more readily understood upon consideration of the 
following detailed description of certain preferred embodiments of the 
invention, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now, more particularly, to FIGS. 1A and 1B and 2, elongated 
five-zone billet heater 10 is depicted for receiving and heating an 
elongated, cylindrical aluminum billet 12 advancing in the direction of 
the arrow 14, that is, from the right to the left of FIGS. 1A and 1B. The 
billet 12 is transported by conventional mechanisms which therefore will 
not be described in detail. On the left side of the oven 10, taken with 
respect to the direction of travel of the billet 12, are located five in 
line combustion burner units 16, each of which is connected by means of a 
flanged, coaxial pipe 18 with a nozzle unit 40 in a separate zone of the 
oven 10. An electric motor powered air blower 20 at one end of the line of 
burner units 16 feeds all of the burner units 16 with a source of air 
under pressure. At one side of one of the burner units 16, the left side, 
as viewed in FIGS. 1A and 1B, air is supplied to the burner unit through a 
large diameter pipe 22 which is connected through a manifold (not shown) 
with the main blower 20. The burner unit is also alternatively supplied 
with liquid fuel in the form of oil through a pipe 24 or with gas through 
a pipe 26, both of which connect into the burner unit 16 through a 
bell-shaped member 28 which is attached to the air supply pipe 22 and the 
input port 30 of the burner unit 16 (see FIG. 3). 
The alternative use of fuel oil through the pipe 24 or gas through the pipe 
26 is dependent primarily on economic considerations as to which of the 
two fuels is more readily available and is more economical. The fuel and 
air mixture is ignited within the bell-shaped member 28 either by an 
ignition spark mechanism or by a pilot flame and the complete combustion 
process is carried out within a horizontally cylindrical chamber 32 within 
the burner unit 16. The pipes 22, 24 and 26 all have flexible sections 
allowing the burner unit 16 to be moved slightly. 
Referring now more particularly to FIG. 2, it can be seen that the side of 
the burner unit 16 which faces the oven 10 has a conically shaped portion 
34 tapering to an exhaust port 36. The exhaust port 36 of the burner 16 is 
connected by means of the pipe 18 to a hot gas injection nozzle unit 40 
situated within the furnace chamber 38 of the oven 10. 
The nozzle unit pipe 18 is extended through the side of the wall of the 
oven 10 through a port 42. The nozzle 40 may be moved laterally with 
respect to the interior of the furnace chamber 38 to accommodate different 
sized billets by moving the burner unit 16 relative to the oven 10 as 
indicated by the arrow 39. This is made possible by mounting the burner 
unit 16 on a horizontal support 68 which is wider than the base of the 
burner unit 16. As can be seen in FIG. 2, the billet 12 is supported in 
the oven 10 by means of a plurality of rollers 44 mounted on supports 46 
at the bottom of the oven chamber 38. The rollers 44 are spaced along the 
length of the oven 10. 
The oven 10 is comprised of an outer steel frame 48 which surrounds and 
supports an inner furnace wall 50 made of refractory material. The upper 
interior surface 52 of the furnace chamber 38 has a generally 
semi-cylindrical cross-section taken perpendicular to the length of the 
furnace 10. Thus, a semi-annular space 54 is formed between the surface of 
the billet 12 and the upper interior surface 52 of the oven 10. The 
portion of the furnace wall 56 extending below the midline of the billet 
12 to the jack 46 is generally straight and slopes inwardly. This furnace 
wall 56 is on the left side of the billet as viewed in FIG. 2. The 
corresponding opposite side of the chamber has a vertical wall 58 which is 
straight and slopes downwardly towards the nozzle unit 40. In this way, 
the actual space around the billet 12 assumes an almost tapered, oval 
cross section with the nozzle 40 being in the narrowest part of the oval. 
Beneath the nozzle 40 is a downwardly extending pipe 60 leading to an 
enlarged horizontal manifold 62 which together carry away the exhaust 
gases from the furnace interior 38. 
Referring now more particularly to FIGS. 5 and 6, it can be seen that the 
nozzle unit 40 is actually a T-shaped pipe with the short, stem pipe 18 
being connected to the midpoint of a long horizontal pipe 64 extending 
along the length of one of the oven sections. Projecting upwardly from the 
pipe 64 are a plurality of nozzle jets 66 at spaced intervals along the 
length of the horizontal pipe 64. The jets are inclined upwardly from the 
horizontal by approximately sixty eight degrees to point toward the billet 
12 and direct the hot gases exiting from the jet tangentially to the 
billet surface 12. 
The pipe 18 connects the nozzle unit 40 to a vertical flange 80 which, in 
turn, is bolted to a corresponding flange 82 of the burner unit 16. The 
pipe 18 is made of a pair of coaxial, schedule 40 alloy pipes 70 and 72 so 
that space 74 exists between the two pipes 70 and 72. The inner pipe 72 
communicates with the horizontal pipe 64 so that the hot combustion gases 
from the burner unit 16 are thus conveyed to the nozzle jets 66. It should 
be noted that the nozzle jets 66 have an oval shaped opening with the long 
axis of the oval being parallel to the axis of the billet 12. 
A pipe 76 communicates with the space 74 between the pipes 70 and 72. The 
inner pipe 72 is supplied with ports 78 which allow communication between 
the space 74 and the inner pipe 72. Additional, by-pass air can be 
introduced through the pipe 76 from the main blower to thereby further 
control the temperature of the gases exiting from the nozzle jets 66. This 
air can be used to further reduce the temperature of the injected gas into 
the oven 10 to a temperature below the combustion temperature within the 
burner unit 16. As will be explained further hereinafter, the temperature 
of the injected gas can also be controlled by controlling the amount of 
the fuel-air mixture in the burner unit 16. 
Referring now more particularly to FIGS. 2, 3 and 4, the main supply of 
fuel is supplied through one or more lines 84 which connect with all of 
the burner units 16 through an assortment of individual, servo valves 86. 
The servo valves 86 are controlled, in part, by a master controller 88. 
The details of the master controller 88 will not be discussed since such 
controllers are well known in the art. A separate thermocouple probe 90 is 
inserted through each oven section wall to project into the furnace 
chamber 38. The ends of the thermocouple probes are spaced just slightly 
away from the surface of the billet 12 and are thus in a position to sense 
the temperature of the hot gases circulating in a vortex around the 
surface of the billet 12. By sensing the temperature of the hot gas 
circulating around and close to the billet, the temperature of the billet 
can also be determined. This is possible because the actual temperature of 
the billet differs from the temperature of the hot gas by a relatively 
constant amount during the time in which the billet is being heated. This 
can be seen from FIG. 4 where the upper curve 92 indicates the rise in 
temperature of the hot gas with respect to the length of time the billet 
is in the oven. The lower curve 94 parallels the upper curve 92 and 
represents the actual temperature of the billet as it is being heated. It 
can be seen that the two curves 92 and 94 are roughly parallel. This 
sensed gas temperature represented by the curve 92 may be used to control 
the temperature of the injected gases through the nozzle 66 by metering 
the amount of fuel and/or air to the combustion unit 16 or by metering the 
amount of air which is added through the pipe 76 to be mixed with the hot 
injected gases, or both. The actual metering is done by servo valves 86 
under the control of the master control unit 88. The thermocouple 90 is 
electrically connected to the master control unit 88. 
It it is desired to heat the billet to a temperature of, for example, 
800.degree. to 825.degree. F., then the combustion burner unit 16 is 
activated to inject hot gas into the oven at a rate of at least 100 feet 
per second until the sensed gas temperature circulating close to the 
billet surface reaches a temperature of approximately 1200.degree. F. At 
this point, it is known that the billet temperature is approximately 
800.degree. F. The master control unit 88 thus automatically modulates the 
fuel and air supplied to the burner unit 16 to cut back the temperature of 
the gas injected into the oven to maintain the air temperature at 
approximately 800.degree. F. This will cause the billet to maintain its 
temperature at 800.degree.-825.degree. F. It should be noted that all of 
this control is accomplished without directly contacting the billet 12, 
thus preventing the thermocouple probe 90 from becoming coated with 
aluminum oxide which would otherwise interfere with its operation, as 
happens in some prior art devices. 
In practice, in the modulated state, the master controller 88 controls the 
temperature of the injected gas simply by switching the combustion unit 16 
between a high and a low state and by simultaneously controlling the 
amount of by-pass air supplied to the furnace nozzles through the pipes 
76. Once the cut-back temperature is reached at a particular oven section, 
the controller 88 reduces the fuel-air mixture to the burner unit 16 
associated with that oven section from a high burn to a low burn state. 
The amount of by-pass air through the pipe 76 of that burner unit 16 is 
then proportionately controlled to maintain the cut-back temperature 
within the oven section. The fixed by-pass settings and fuel-air settings 
are such that in the low state with maximum by-pass air the burner unit 16 
will just barely not be able to maintain the cut-back temperature within 
the oven section. In other embodiments, the fuel-air ratio could be 
proportionally varied to control the temperature. 
Because of the high velocity mass transfer of the injected gases together 
with their high temperature and the venturi effect created by the annular 
space between the billet 12 and the interior wall 52 of the oven 10, the 
thermal efficiency of the billet heater according to the invention is 
quite high compared to conventional units. For example, with a standard 
billet heater having an oven length of approximately 25 feet and a burner 
rating of approximately 9 million B.T.U.s per hour, and a production rate 
with 7 inch diameter billets of 2,100 pounds per hour, then to heat the 
billets requires 200 B.T.U.s per pound or 420,000 B.T.U.s per hour. A 
typical fuel consumption for such a furnace would be approximately 5 
million B.T.U.s per hour. This works out to a thermal efficiency of 
approximately 8.4 percent. With the high velocity billet heater of the 
present invention, for a similarly dimensioned oven and billet, the burner 
need only have a rating of 3.5 million B.T.U.s per hour and a fuel 
consumption of only 1,680,000 B.T.U.s per hour. This has a thermal 
efficiency of approximately 25 percent which greatly exceeds the 
conventional unit. 
In an actual test with an experimental high velocity billet heater 
according to the invention, a 9 inch diameter billet weighing 250 pounds 
was heated from a cold start of 50.degree. F. to 850.degree. F. in 26 
minutes. The actual fuel consumed was 1.25 gallons. It can be calculated 
that this required 43,000 B.T.U.s of heat and the fuel consumed was 
175,000 B.T.U.s, yielding a thermal efficiency, therefore, of 
approximately 24.6 percent. With a hot furnace, the efficiency could be 
expected to be 30-35 percent and the time required to heat the billet 
would be reduced to about 20 minutes. 
It can thus be seen that the high velocity billet heater, according to the 
invention, is far more efficient than conventional billet heaters and thus 
uses far less fuel than such conventional billet heaters. A further 
advantage of the invention is that because of the construction of the 
combustion burner unit 16, the furnace is easily adaptable to either gas 
use or oil fuels with no special modifications being necessary. 
While in the above described embodiment the thermocouple probes 90 are 
advantageously spaced from the billet surface, in other less advantageous 
embodiments the prior art method of using contacting thermocouples may be 
utilized; however, such thermocouples can be expected to suffer from the 
problem of becoming coated with aluminum oxide over a period of time. 
The terms and expressions which have been employed here are used as terms 
of description and not of limitations, and there is no intention, in the 
use of such terms and expressions, of excluding equivalents of the 
features shown and described, or portions thereof, it being recognized 
that various modifications are possible within the scope of the invention 
claimed.