Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2032652
AN INDUSTRIAL FURNACE WITH
IMPROVED HEAT TRANSFER
This application is related to U.S. Patent 4,963,091 entitled "Method
and Apparatus for Effecting Connective Heat Transfer in a Cylindrical,
Industrial Heat Treat Furnace".
BACKGROUND OF THE INVENTION
In the heat treat field, metal work is to be heated and
cooled in accordance with known, time-temperature-atmosphere
composition heat treat processes. Simplistically, the work
is heated, held and cooled at specific rates and times while
the gaseous or furnace atmosphere surrounding the work is
controlled to impart desired metallurgical and mechanical
properties to the work.
In the furnace art, cooling of the work (except for
furnace cool heat treat processes) always occurs by convec-
tion, while heating by convection is typically limited to
low temperature furnace applications, about a maximum of
1400° F. Connective heat transfer is typically accomplished
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in batch furnaces by either baffle arrangements which divert
and direct the flow of furnace atmosphere about the work or,
alternatively, by high speed jets which are used to impinge
the work to establish high heat transfer rates. All baffle
arrangements require adjustment and are thus "sensitive" to
performing different processes on different furnace loads.
In addition, the cost to construct the baffles is expensive.
Jet nozzle arrangements are generally used only for cooling
the work and are specifically designed as a predetermined
nozzle configuration for impinging cooled atmosphere against
a specific workpiece shape. or are of a general config.urati.oz~
which directs multiple streams of jets against the work. In
either instance, separate longitudinally-extending plenum
chambers are built within the furnace to develop high pres-
sure jets or alternatively an external heat exchanger is
used which then pumps the cooled air into a plenum or mani-
fold distributor within the furnace. This is also an expen-
sive furnace construction.
There are, however, numerous, convective heat transfer
arrangements in the prior art and it is known to use the
intake of a fan as a centrally positioned under-pressure
zone to establish closed loop, pump type recirculation
schemes in the sense that atmosphere is drawn into the fan,
pressurized by the fan in a plenum chamber and then directed
through a manifold to impinge the work. A variation of this
theme is disclosed in U.S. Patent 4,789,333 to Hemsath where a free-
standing, longitudinally moving circular jet is aeveloped through an ori-
fice and expanded into turbulent~contact with a cylindrical
shell member as the jet travels the length of a cylindrical
shell. At the end of the shell, the high speed jet is redi-
rected by a special diverter plate to impinge the work and
after impingement the atmosphere is collected through the
under-pressure zone to be pressurized again into a jet.
This arrangement is limited to the thin shell furnace of the
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'333 patent which can be placed into heat transfer contact
with a high velocity jet travelling along its length. U.S.
Patent 4,395,233 to Smith et al illustrates the use of a central under-
pressure zone to cause recirculation of flat sheets of forced air in a
baking oven by causing the forced air to assume a torroidal
shape as it travels to the fan under-pressure zone. Howev-
er, Smith's oven is rectilinear in configuration and Smith
uses the same prior art concept of pressurizing the wind in
a plenum chamber which is directed from the plenum chamber
through rectangular slots which orifice the forced air into
his oven. None of the recirculation arrangements is suffi-
cient to develop the "wind" pattern required in the heat
treat furnace applications to which the present~invention is
concerned.
Also, it often occurs that the work, when heated, emits
toxic gases or fumes. For example, powdered or sintered
metal parts when heated, even at low temperatures, emit
smoke which contains hydrocarbons and require separate af-
terburners or incinerators to burn the hydrocarbons and oth-
er pollutants which increases furnace cost. Heat is typi-
cally recovered from the incinerators through heat exchanges
and typically used to heat boilers or preheat the combustion
air used in the furnace burners. This is inefficient be-
cause heat must first be developed to incinerate the
volatiles and the recovery of the heat is limited to secon-
dary processes.
Apart from specific furnace design considerations, in
general, furnace construction is typically divided between
low temperature and high temperature applications. As indi-
cated previously, heat transfer efficiencies dictate that
low temperature furnaces be heated by convection while high
temperature furnaces heat the work by radiation although
convection/radiation heat transfer is employed to heat the
work through the lower temperature ranges. For high
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temperature applications, the furnace construction is fur-
ther divided between those furnaces which operate at slight
positive pressure, or standard atmosphere furnaces, and
those furnaces which operate under vacuum such as vacuum
furnaces, ion "glow discharge" furnaces, etc. Traditional-
ly, high and low temperature, positive pressure, batch fur-
naces were typically distinguished in their construction by
the type of insulation used in the furnace. Low temperature
applications in many instances use an "oven" panel con-
struction where low grade insulation is simply sandwiched
between metal skins to form panels which are welded together
to form a box into which a burner is placed. In contrast,
high temperature, standard atmosphere furnaces typically
were constructed about a steel liner or casing to which re-
fractory was bricked. With improvements in ceramic, fibrous
furnace insulation which replaced refractory brick linings,
the physical distinctions between the two furnace construc-
tions began to dissipate although the low temperature fur-
nace, because of insulation prices, remains a low cost fur-
nace while high temperature applications use a more expen-
sive insulation construction technique. Finally, certain
metallurgical processes require that the heat treating gas
be diffused into the case of a heated metal part under a
vacuum. Typically, vacuum furnace constructions use a dou-
ble wall or double casing construction which is spherically
or cylindrically shaped to withstand collapse when a vacuum
is pulled therein and water is typically circulated between
the walls to provide a cold wall design so that the furnace
door can have an elastomer seal to vacuum seal the enclo-
sure. Insulation is provided on the inside of the inner
casing. There are, however, heat treat applications which
fall short of the high vacuum levels that would demand a
double walled, vacuum vessel construction.
47hile a high temperature furnace can be used to perform
either high or low temperature heat treat processes, the use
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of a high temperature furnace t,o perform low temperature
heat treat processes is not cast effective to the heat
treater. Further, it is not cost effective to the heat
treater to use a vacuum furnace to perform a high tempera-
s ture heat treat process such as carburizing when the
carburized case tolerances are such that the process could
be performed in a standard atmosphere, high temperature
batch furnace. Basically, the furnace throughput coupled
with furnace cost dictate the heat treater's charge and
heretofore precluded small heat treaters who did not have a
range of furnaces from competing with large heat treaters
who could afford to purchase a number of different furnaces
to perform different heat treat processes.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the invention
to overcome the difficulties of the prior art noted above by
providing a system, arrangement, method and/or apparatus for
improved heat transfer within an industrial furnace.
This object along with other features of the invention
is achieved in an industrial furnace for effecting heat
treat processes on metal work disposed therein which in-
cludes a cylindrical, insulated furnace section with a seal-
able door at one axial end of the casing and an end plate at
the opposite axial end thereof to define a closed end cylin-
drical chamber therein. A circular fan plate centrally po-
sitioned within the chatrcber divides the chamber into a fan
chamber extending between the circular plate and the end
plate and a heat treat chamber extending between the circu-
lar plate and the door. Importantly, the circular plate
defines an annular space which is non-orificing and which
extends between the circular plate's outer diametrical edge
and the interior or inside surface of the cylindrical fur-
pace section to provide fluid communication between the fan
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chamber and the heat treat chamber. The plate also has a
centrally positioned under-pressure opening which extends
therethrough to also provide fluid communication between the
heat treat chamber and the fan chamber. A heat transfer
arrangement in the form of a plurality of circumferentially
spaced, tubular elements longitudinally extend from the end
plate through the fan chamber, through the annular space and
into the heat treat chamber. The tubular elements provide a
temperature source which is initially different than the
temperature of the work within the heat treat chamber. A
fan centrally positioned within the fan chamber directs the
furnace atmosphere as a wind mass in a direction perpendicu-
lar to the longitudinal center of the chamber and normal to
the cylindrical furnace section and swirling with a high
circumferential velocity about the interior of the cylindri-
cal furnace section. The wind mass gradually moves longitu-
dinally through the non-orificing space toward the door end
to effect rapid heat transfer by circumferential velocity
impingement with the tubular elements. The under-pressure
opening causes the wind mass after heat transfer contact
with the work in the heat transfer chamber to return to the
fan chamber.
In accordance with another aspect of the invention, the
tubular elements include a first plurality of tubular heat
ing elements and a second plurality of cooling tubes. The
first plurality of heating elements are heated to a temper-
ature which is initially hotter than the temperature of the
work. A coolant is provided to the interior of the second
plurality of cooling tubes for cooling the second plurality
of cooling tubes to a temperature initially colder than the
temperature of the work. The first and second plurality of
tubular elements are selectively heated and cooled so that
the wind mass arrangement described above can be utilized to
direct high circumferential velocity flow to effect both
improved connective heating and cooling of the work.
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Significantly, the first plurality of tubular heating ele-
ments, by extending within the heat treat chamber at circum-
ferentially spaced increments about the work, provides a
uniformly distributed source of radiant heat to the work to
achieve uniform high temperature heating of the work re-
quired for certain heat treat processes and the connective
heat transfer arrangement assures rapid heating of the work
at least through the low-end of the temperature range of the
heat cycle.
In accordance with another feature of the method of the
invention, a paddle bladed fan in the fan chamber develops a
wind mass which circumferentially rotates in a non-turbulent
manner about the smooth interior surface of the cylindrical
furnace section at high velocities. Significantly, the fan
does not impart axial force components to the wind mass and
the non-orificing annular space prevents axial pressure from
developing within the fan chamber so that as the wind mass
builds within the fan chamber, Ghe wind gradually travels
axially towards the door end of the chamber. The high cir-
cumferential velocity of the wind mass directly impacts the
tubular heating elements and cooling tubes which are circum-
ferentially spaced and diametrically sized so that the wind
mass annulus remains substantially intact. The under-pres-
sure opening establishes a centripetal force tending to
gradually strip tt~e inner portion of the wind mass annulus
as the wind mass axially travels to the door end of the fur-
nace. However, the circumferential velocity of the wind
mass is established at a sufficiently high speed to permit
most of the wind mass to swirl in heat transfer contact with
the tubular elements until contact with the door end at
which time the swirl is broken and the under-pressure zone
pulls the now heated or cooled wind past the work into the
fan chamber.
In accordance with a significantly important feature of
the invention, especially when the furnace is used to heat
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treat powdered. metal or sintered metal parts which emit
volatiles or hydrocarbons when heated, the cylindrical fur-
nace section is provided with an incineration track formed
as a circumferentially extending groove adjacent to and in
heat transfer contact with the interior surface of the cy-
lindrical furnace section. At least a portion of the track
is situated to extend longitudinally about a portion of the
fan chamber and the track need not circumferentially extend
about the entire circumference of the furnace section. The
track has an inlet in fluid communication with the fan cham-
ber which receives a portion of the wind mass drawn by the
fan into the fan chamber through the under-pressure zone and
an outlet extending from the exterior of the cylindrical
furnace section for exhausting the furnace atmosphere. A
heater is provided for thermally cleaning or incinerating
the furnace atmosphere in the incineration track which in
turn provides additional heat to the swirling wind mass de-
veloped by the fan so that the incineration heat may be di-
rectly inputted to the furnace section.
In accordance with yet another feature of the inven-
tion, the cylindrical furnace section, the end wall and the
sealable door are constructed as a typical steel casing to
which a conventional fibrous, high density ceramic insula-
tion is affixed in tight abutting relationship through the
chamber so that a vacuum at at least slight negative pres-
sure can be drawn into the chamber. A gas inlet is provided
in the cylindrical furnace section in communication with the
chamber for purposes of selectively drawing a vacuum in the
chamber and/or admitting a furnace atmosphere gas therein so
that the furnace can be selectively operated as a draw fur
nace, or a high temperature furnace or a vacuum furnace.
It is another object of the invention to provide an
industrial heat treat furnace with enhanced heat transfer
capabilities with a heat source and/or a heat sink.
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It is still yet another object of the invention to pro-
vide a furnace with improved heat transfer with a heat
source and/or heat sink and from the heat source and/or heat
s ink to the caork .
It is still yet another object of the invention to pro-
vide a baffle-free and nozzle-free industrial heat treat
furnace which can operate as a vacuum furnace with improved
connective heat transfer characteristics.
It is still yet another object of the invention to pro
vide an industrial heat treat furnace which can heat treat
work at high heat treat temperatures and which can rapidly
and uniformly heat the work to the high temperatures.
A still further object of the invention is to provide
an economical furnace which can function as either a batch
vacuum furnace or a standard atmosphere batch furnace capa
ble of processing work at low or high temperatures.
Still yet another object of the invention is to provide
a furnace which can thermally clean, within the furnace, the
fumes exhausted therefrom.
Yet another object of the invention is to provide a
furnace which incinerates the fumes exhausted therefrom and
uses the heat from the incineration to heat the work.
Still another object of the invention is to provide a
furnace with internal incineration of the flue gases and
which can heat the work in the absence of significant quan
tities of air so that the furnace can pyrolyze toxic and/or
hazardous wastes.
These and other objects and advantages of the invention
will become apparent from a reading and understanding of the
Detailed Description of the Invention set forth below taken
together with the drawings which will be described in the
next section.
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BRIEF DESCRIPTION OF THE DRAHINGS
The invention may take physical form in certain parts
and arrangement of parts, a preferred embodiment of which
will be described in detail herein and illustrated in the
accompanying drawings which form a part hereof and wherein:
FIGURE 1 is a front end elevation view partially broken
away of a furnace illustrating concepts of the present in-
vention;
FIGURE 2 is a sectioned, side elevation view of the
furnace shown in FIGURE 1 taken along line 2-2 of FIGURE 1;
FIGURE 3 is a schematic side elevation view schemati-
cally illustrating the wind mass developed in the present
invention;
FIGURE 4 is a schematic end elevation view of the fur-
nace of the present invention showing schematically the wind
mass pattern developed and shown in FIGURE 3;
FIGURE 5 is a further schematic end view showing sche-
matically the formation of the swirling wind mass;
FIGURE 6 is a sectioned, side elevation view of an al-
ternative embodiment of the invention similar to the view
shown in FIGURE 2; and
FIGURE 7 is an end elevation view of the furnace shown
in FIGURE 6 taken along line 7-7 of FIGUP,E 6.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings wherein the showings are
for the purpose of illustrating a preferred embodiment of
the invention only and not for the purpose of limiting the
same, there is shown in FIGURES 1 and 2 a heat treat furnace
10 which will be described as functioning as a vacuum fur-
nace. Furnace 10 comprises a cylindrical furnace section 12
closed at one end by a spherically shaped end furnace sec-
tion 13 and at its opposite axial end by a spherically
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shaped sealable door 14 through which work in the form of
loose metal parts placed in a tray shown by dot-dash lines
15 is loaded into and out of furnace 10. The furnace sec-
tions 12, 13 (and for that matter door 14) comprise standard
3/8" plain carbon steel plate (i.e. 1012) or a casing 16 to
which is secured a vacuum-formed ceramic fiber insulation 17
of a relatively high density, i.e. 10-15 lbs./ft3. The sur-
face of the insulation is sprayed with a conventional silica
sand mixture, i.e.* Kaowool rigidizer, to make it hard and
rigid. Insulation 17 is conventional and is secured to cas-
ing 16 in a conventional manner which is not shown or de-
scribed herein in detail. More specifically, insulation 17
is formed into preshaped blocks individually secured to cas-
ing 16 by studs extending therefrom and fitted together like
pieces of a jigsaw puzzle into tight, compressive contact
with one another which when sprayed with the rigidizer pre-
vents gas leakage therethrough. While insulation 17 is con-
ventional, it is to be noted that an inner metal lining is
not applied to the face or interior of insulation 17 al-
though a high velocity wind mass flow is developed in fur-
nace. 10. The wind mass flow is not turbulent at the exposed
surfaces of insulation 17 and this is the reason why interi-
or metal lining plates are not required. Further, it is or
should be noted that furnace 10, while capable of function-
ing as a standard batch type furnace, will be described as a
vacuum furnace and that the casing-insulation construction
shown is not typical for a vacuum application. There is no
water jacket in furnace 10. At the same time, it is to be
noted that furnace 10 will not withstand the vacuum levels
which a double casing, water jacket vessel is able to with-
stand.
A hinge (not shown) on one side of furnace 10 connects
to trunnions 18 on door 14 to rotate door 14 from an open to
a closed position and a plurality of clamps 20 circum-
ferentially spaced about the outer periphery of door 14 are
~ TRADEMARK
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employed to vacuum seal door 14 to cylindrical furnace sec-
tion 12. With door 14 sealed, cylindrical furnace section
12, end furnace section 13 and door 14 define a smooth
walled closed ended cylindrical chamber 21 symmetrical about
the longitudinal centerline 22 of furnace 10.
Concentrically positioned relative to centerline 22 as
by fasteners (not shown) secured to end furnace section 13
is a circular fan plate 25. Fan plate 25 divides chamber 22
into a fan chamber 26 defined as axially extending from fan
plate 25 to end furnace section 13 while that portion of
chamber 21 axially extending from fan plate 25 to door 14 is
defined as a heat treat chamber 27. Fan plate 25 has a cen-
tral under-pressure opening 29 concentric with centerline 22
and which, as described hereafter, produces an under-pres-
sure zone developing a centripetal force tending to collapse
the wind mass into heat treat chamber 27 and effective to
cause recirculation of the furnace atmosphere. Importantly,
the outside diameter of fan plate 25 is sized relative to
the inside diameter of cylindrical casing 12 to establish an
annular space 30 which has a sufficient radial distance to
be non-orificing in nature as will be explained hereafter.
In the preferred embodiment, the outside diameter of fan
plate 25 is about 52 inches and the inside diameter of cy-
lindrical casing 12 is about 68 inches establishing the ra-
dial distance of annular space 30 at about 8". These dimen-
sions are established for a furnace sized to process a work
tray 15 having a dimension of 36 x 48 x 36". At this dimen-
sion and at the circumferential speeds of the wind mass,
annular space 30 will not develop a pressure or any signifi-
cant pressure in fan chamber 26 which could adversely dimin-
ish the rotational speed of the swirl or otherwise act to
straighten the swirl and in the process thereof impart lon-
gitudinal velocity to the swirl. It is for this reason that
fan chamber 26 is designated a fan chamber and not a plenum
chamber which is conventional in the art.
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A fan bung 32 mounted to end casing 13 journals a fan
shaft 33 concentrically positioned on centerline 22 driven
by an external motor 34. Mounted on fan shaft 33 is a pad-
dle bladed impeller 36. Importantly, a paddle blade 36 is
chosen since it directs its wind mass normal to cylindrical
casing 12 and perpendicular to centerline 22. There is no
axial component of wind mass developed by paddle blades 36.
Within heat treat chamber 27 is a hearth 37 supported
by a plurality of longitudinally spaced, vertically extend
ing supports 38 affixed to casing 16 of cylindrical furnace
section 12. Hearth 37 supports the work which typically
comprise loose metal pieces placed in conventional trays or
baskets 15 which may have either open mesh sides or closed
sides and which as noted above are all shown by dot-dash
lines. As noted above, for the specific furnace 10 illus-
trated in the drawings, work trays or baskets can have a
height of 36 inches, and a length of 48 inches and a width
of 36". Those skilled in the art will recognize that fur-
nace 10 is sized by the tray dimension which can fit within
the furnace and the tray size quoted for the preferred em-
bodiment is typical. Also within heat treat chamber 27 is a
vacuum port or opening 40. A conventional vacuum pump (not
shown) and a conventional regulated valve train (also not
shown) are connected to port 40. When furnace 10 is operat-
ed as an atmosphere furnace or when furnace 10 is operated
as a vacuum furnace in its standard atmosphere "mode", the
valve train emits an inert gas such as nitrogen into chamber
21 and when furnace 10 is operated as a vacuum furnace an
inert gas such as nitrogen is also emitted by the valve
train through port 40 with or without hydrogen addition in
accordance with conventional practice. It is to be under
stood that when furnace 10 is operated as a standard atmo
sphere batch furnace, port 40 will control furnace atmo
sphere exhaust and regulate furnace pressure in a conven
tional manner.
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Furnace 10 is provided with a heat source and a heat
sink. The heat source can take the form of any longitudi-
nally extending tubular heating elements 42 which provides
heat, somewhat uniformly, along its length. In furnace 10
shown in FIGURES I and 2, tubular heating elements 42 are
shown to take the form of a single ended radiant tube which
extends through end wall section 13, through fan chamber 26,
through non-orificing annular space 30 and into heat treat
chamber 27 whereat radiant tube terminates after extending
longitudinally within heat treat chamber 27 a distance which
is preferably equal to that of work 15. While a radiant
tube is illustrated in the drawings, tubular heating ele-
ments 42 in practice take the form of tubular, electric rod
bundle elements. These are simply carbon or graphite elec-
trodes which generate heat when electric current is applied
to the electrode. However, radiant tubes, which are conven-
tionally known in the art and will not be described in fur-
ther detail herein, can be used with either electric heating
elements within the tube or a fuel fired burner firing its
products of combustion into the open end of the tube. All
tubular heating elements 42 disclosed generate somewhat con-
stant heat output incrementally along this length. In the
illustrated furnace 10, there are four radiant tubes 42
which are spaced in equal circumferential increments about
non-orificing annular space 30.
The heat sink provided for furnace 10 takes the form
of an internal heat exchanger tube 44 which like the radiant
tube extends from outside furnace 10 through end wall sec-
tion 13, through fan chamber 26, through non-orificing
annular space 30 and into heat treat chamber 27 where heat
exchange tube 44 terminates after extending a longitudinal
distance at least equal to that of the length of work 15.
These internal heat exchange tubes are available from the
assignee of this invention, Surface Combustion, Inc., under
the brand name *Intra-Kool. Basically, the tubes are pipes
~CTRADEMARK
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through which a coolant such as water is piped when the work
is to be cooled and over which hot furnace atmosphere is
blown to become cool by contact therewith prior to contact-
ing and cooling work 15. However, the same, improved con-
s vective heating concept employed in the present invention
for uniduely heating the work can likewise be employed vis-
a-vis internal heat exchanger tubes 44 to cool work 15. As
best shown in FIGURE 1, a plurality of heat exchanger tubes
44 are provided spaced in circumferential increments about
an orificing annular space 30 and in between radiant tubes
42. It should be noted that the circumferential spacing
between tubes taking into account the four radiant tubes 42
and the eight heat exchanger tubes 44 together is equal. It
should be noted that tubular heating elements 42 as well as
heat exchanger tubes 44 extend through only end wall section
13 which limits the number of openings which have to be made
in casing 16.
FURNACE OPERATION
The heat transfer aspects of furnace 10 will now be
explained with reference to the diagrammatic flows shown in
the schematic illustrations of FIGURES 3 to 5. It is to be
appreciated that the flows schematically correspond to what
has been witnessed in streamer tests conducted on the inven-
tion. What is set forth below represents what is believed
to occur to produce the flow streams discussed.
As best shown in FIGURE 5, when paddle blades 36 rotate
in the direction of arrow 50, the wind leaves blades 36 with
a radial direction component indicated by arrow 51 and a
tangential direction component indicated by arrow 52 to ac-
tually produce a wind vector 55 which is displaced from the
tangential vector a slight angle designated as "X" which in
practice is no more than about 8°. Since the wind vector 55
is almost tangential to blades 36, a swirl is created within
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fan chamber 26. If under-pressure opening 29 were blocked
off, the wind would simply wipe the interior surfaces of
cylindrical furnace section 12 in fan chamber 26. Because
of the presence of under-pressure opening 29, furnace atmo-
sphere is drawn into fan chamber 26 and the swirling mass of
furnace atmosphere ejected from paddle blades 36 is dis-
placed into heat treat chamber 27. Now it is important and
critical to the working of the invention that space 30 be
large enough, considering fan sizing, fan rotation and OD/ID
spacing, so that a pressure drop or a significant pressure
drop of the wind mass through space 30 does not occur, i.e.
the space 30 is by definition a non-orificing annular space.
If space 30 was shortened, a pressure would build in fan
chamber~26 which~would then function as a plenum 'chamber and
the wind would be injected longitudinally into heat treat
chamber 27 with a longitudinal velocity correlated to the
pressure drop through the reduced space. More significant-
ly, if annular space 30 was reduced in size to result in a
pressure buildup in fan chamber 26, the wind mass would be-
gin to straighten as it Ieft the reduced annular space and
the circumferential velocity of the swirl would be reduced.
For example, if annular space 30 was reduced to the orifice
s i z a s p a c i f i ed i n Hems a th U,S. Patent 4,789,333 to develop a high
speed
jet, the jet emanating from annular orifice space would be,
straight and without a swirl. It is also critical to the
invention that furnace section 12 be cylindrical and smooth
so that the wind mass can swirl about the section without
breaking up or becoming turbulent. That is, cylindrical
section 12 restrains the outward expansion of the swirl re-
sulting from the slight radial component 51 and nut only
insures, but promotes the swirl 'configuration of the wind
mass. In this connection, it is to be appreciated that if
the radial component of the wind mass was significant, the
fact the cylindrical furnace section 12 is cylindrical may,
in itself, produce a swirl in the wind mass, but the exposed
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furnace insulation 17 would deteriorate, in time, under the
impact. This does not occur in the present invention and
the wind mass flow at the interface with insulation 17 is
non-turbulent in both fan chamber 26 and heat treat chamber
27.
In the present invention and in contrast to U.S. Patent
4,963,091, tubular heating elements 42 extend into heat treat
chamber 27 as opposed to burners located in fan chamber 26.
Tubular heating elements 42 thus provide an extensive surface
area to which the wind mass can be placed into heat transfer
contact therewith. Importantly, the circumferential velocity
of the wind mass developed by blades 36 is maintained high
enough so that the swirl is not broken up or made turbulent by
contact with tubular heating elements 42 or heat exchange
tubes 44. That is, the high circumferential velocity of the
swirl is providing wind impact at speeds approaching Reynolds
numbers associated with jet velocities. Yet, because of the
way the wind mass is developed and because of the smoothness
and shape of insulation 17 over cylindrical furnace section
12, the wind mass is developed and because of the smoothness
and shape of insulation 17 over cylindrical furnace section
12, the wind mass is not turbulent within cylindrical furnace
section 12 adjacent insulation 17. This is schematically
illustrated by wind streamers 45 which passes radially
inwardly of tubular heating elements 42, wind streamers 47
which passes radially outwardly of tubular heating elements 42
and wind streamers 46 which impact tubular heating elements 42
and heat exchanger tubes 44. When the wind reflected by
streamer 46 hits tubular heating elements 42 and heat
exchanger tubes 44 it, of course, must break up. However, the
speed of the wind reflected by streamers 45 and 47 is so high
that the wind reflected by streamer 46 forms eddy flows
indicated by reference numeral 60 downstream of tubular
heating elements 42 and heat exchange tube 44 which dissipates
to permit the wind deflected by streamer 46 to reform prior to
impacting the next tubular heating element 42 or heat exchange
tube
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44. This reformation is shown as the wind circumferentially
traverses tubular heating element 42a, then heat exchange
tubes 44a, 44b, etc. in FIGUP.E 4.
The wind enters heat treat chamber 27 as a swirling
wind mass shaped as an annulus with a ring diameter approxi
mately equal to the radial distance of non-orificing space
30. Because of the non-orificing characteristics of space
30 and because paddle blades 36 are used to develop the wind
mass, there is little, if any, spiral or helical twist im
parted to the wind mass as it travels the length of heat
treat chamber 27. Within heat treat chamber 27 under-pres-
sure opening 29 exerts a centripedal force on the wind mass
annulus tending to strip inner portions of the annulus into
work 15 as the wind mass longitudinally travels towards door
14. Because of the high circumferential speed of the wind
mass in this invention, the wind mass that gets peeled off
the inner portion of wind mass annulus at longitudinal dis-
tances indicated in FIGURE 3 as L-1, L-2, L-3 is believed
not that significant. A significant portion of the wind
mass travels to door 14, shown as distance L-4, where it is
then broken up, i.e, made turbulent, and drawn back at rela-
tively low speed through work 15 into under-pressure open-
ings 29. Thus, within heat transfer chamber 27 about its
inner portions surrounding work 15 is Iow velocity, and tur-
bulent wind which is returning to under-pressure opening 29.
Heat transfer is effected between work 15 and this low ve-
locity turbulent wind by the high volumetric wind flow which
contacts work 15.
As thus far explained, a swirling wind mass in excel
lent heat transfer contact with longitudinally extending
heat sinks and heat sources is developed. The wind mass is
non-turbulent in its flow because of the smooth cylindrical
shape of furnace section 12. The high circumferential ve
locity is sufficient to overcome any turbulence which would
destroy its swirl characteristics by contact with the heat
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20~2f ~~
source and heat sink obstructions. The wind retains its
aerodynamic swirl characteristics,developed by paddle blades
36 because of non-orificing space 30. The under-pressure
opening 29 causes the wind to be drawn, at low velocity,
through work 15 back into fan chamber 26 and in the process
thereof establish heat transfer contact with work 15. In
this arrangement, high volumes of wind are circulated when
compared to jet nozzles and the like of the prior art.
Thus, the differential temperature between wind and work is
not as great as tht~t which would exist between wind and work
in jet nozzles. However, the significantly greater volumet-
ric flow produces good heat transfer from the swirl to the
work and the circumferential velocity produces excellent
heat transfer to the wind from the heat sink or heat source.
At low temperature applications where heating occurs princi-
pally by convection, the arrangement described imparts good
temperature uniformity to the work. Furthermore, and as
noted at the outset, final high heat temperatures are
achieved by radiation and the cylindrical shape of furnace
section 12 and positioning of tubular heating elements 42
circumferentially about the work imparts uniform heat to
work 15 at the upper temperature ranges which permits excel-
lent temperature uniformity with work 15.
Furnace 10 is ideally suited for batch vacuum tempering
operations. In this heat treat process, work 15 previously
subjected to a heat treat process is transferred to furnace
10 for a vacuum tempering cycle. With door 14 sealed and
furnace 10 at standard atmosphere pressure, the chamber is
evacuated to approximately 50 microns (.05 mm Hg). A nitro
gen backflow (i.e. nitrogen gas admitted through opening 40)
takes place which raises the pressure to .5 - 2 psig posi-
tive pressure and tubular heating elements 42 are heated and
fan motor 34 actuated to transfer the heat from tubular
heating elements 42 to work 15 in the manner described.
Work 15 approaches and is controlled at the desired
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2032652
tempering range of 800 to 1400° F. Furnace 10 is held at
this soak temperature for a specified time. After the soak
time is completed, tubular heating elements 42 are turned
off and fan motor 34 is shut off for a short period of time.
At that time, coolant is introduced into heat exchange tubes
44 and fan motor 34 activated to produce a specified cooling
rate of work 15. Other vacuum heat treat processes can be
accomplished and while the operation of furnace 10 has been
described with respect to a vacuum process, it should be
clear that furnace 10 can also operate as a standard atmo-
sphere type furnace and it is a particular feature of the
low cost construction of furnace 10 that the furnace can be
economically manufactured and commercially sold for either
or both applications.
ALTERNATIVE EMBODIMENT
FIGURES 6 and 7 illustrate an alternative embodiment of
the invention and reference numerals used to describe fur-
pace 10 in FIGURES 1 through 5 will likewise be used to de-
scribe the same components and parts of furnace 10 in FIG-
URES 6 and 7 where applicable. The construction of furnace
10 illustrated in FIGURES 6 and 7 bears a closer resemblance
to the furnace described in the parent patent than the fur-
nace construction shown in FIGURES 1 through 5. Since this
invention is related to U.S. patent 4,963,091, the furnace
construction shown in FIGURES 6 and 7 was chosen to better
illustrate common elements of the inventions. As will be
explained below, furnace 10 shown in FIGURES 1 through 5 can
be modified to incorporate concepts shown in FIGURES 6 and 7.
Furnace 10 of FIGURES 6 and 7 was specifically devel-
oped in the course of heat treating powdered metal parts
(and by analogy sintered metal parts). Powdered metal parts
are conventionally heated in a positive pressure, standard
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203~:fa~2.
atmosphere, batch furnace. When powdered metal parts are
heated, heavy smoke containing hydrocarbons and volatiles
from the resin binder is emitted. Heretofore, smoke was
vented from the furnace to an afterburner or incinerator
which would heat the volatiles in the presence of oxygen to
a sufficiently high temperature to incinerate the fumes pri-
or to discharging the cleansed fumes to the stack. While
heat recovery is typically used in conjunction with the ex-
ternal afterburner or incinerator, the heat recovered is
used only as a secondary heat source in the sense that com-
bustion air to the furnace burner is preheated or boiler
feed is heated, etc. Thus, additional energy in the form of
fuel must be provided to the incinerator to heat furnace
fume and this additional energy is only partially recovered
as secondary heat. Furnace 10 of the alternative embodiment
shown in FIGURES 6 and 7 provides, for internal incineration
of the fumes so that the furnace atmosphere can be directly
exhausted to the stack without a separate afterburner or
incinerator requiring its own source of fuel and the heat
generated by the incineration of the fumes is directly used
in the primary sense to heat work 15 in furnace 10.
Cylindrical furnace section 12 is modified over that
portion of the furnace enclosure which substantially encom
passes fan chamber 26. More specifically, and as best shown
in FIGURE 7, an inner steel casing 80 coaxial with the in
side diametrical surface of insulation 17 for cylindrical
furnace section 12 is provided. As shown in FIGURE 7, inner
casing 80 extends circumferentially through an included an-
gle of about 270° around cylindrical furnace section 12 al-
though inner casing 8G could extend almost completely around
the interior of cylindrical furnace section 12 or could spi-
ral about cylindrical furnace section 12. As shown in FIG-
URE 6, inner casing 80 longitudinally extends a distance at
leant equal to the length of fan chamber 26 and in fact is
slightly in excess thereof. Concentric with inner casing 80
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\ 2 0 3 2 6 5 2 sF-8387
and extending the length of inner casing 80 is an outer cy-
lindrically shaped steel casing 81 which has a diameter
greater than that of casing l6.for cylindrical furnace sec-
tion 12. Inner casing 80 (and for an included angle of 90°
the inside surface of cylindrical furnace section 12) and
outer casing 81 define an annular casing space into which
insulation 83 of the same type as that previously described
is placed. Insulation 83 is affixed within the annular cas-
ing space either by packing or by studs and fasteners to
form an annular groove or incineration track 85. As shown,
incineration track 85 extends from inner casing 80 radially
outwardly a fixed distance which is approximately half of
the radial distance of the annular casing space between cas-
ings 80, 81 so that the wall thickness of the insulation can
be made consistent throughout cylindrical furnace section
12. Incineration track 85 is thus in heat transfer contact
with fan chamber 26 vis-a-vis inner casing 80.
An opening 86 is provided in inner casing 80 to place
incineration track 85 into fluid communication with fan
chamber 26. An outlet opening 87 is provided in outer cas
ing 81 and insulation 83 to place incineration track 85 in
fluid communication with ambient atmosphere vis-a-vis the
conventional stack (not shown). A baffle 89 shown in outlet
opening 87 but typically located in the duct leading to the
stack controls the rate at which flue products leave outlet
opening 87 and establishes (through back pressure) the pos-
itive pressure at which furnace 10 operates. As an aside,
those skilled in the art will recognize that gas outlet
opening 40 in furnace 10 shown in FIGURES 1 through 5 func-
tions for the same purpose as that of baf f le 89 and outlet
opening 87 in furnace 10 shown in FIGURES 5 and 6.
In the alternative embodiment, a tangentially positioned
gas fired burner 90 similar to that described in U.S. patent
4, 963, 091 is located within fan chamber 26 and directs its
products of combustion generally tangential to the
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interior surface of fan chamber 26. In U.S. patent 4,963,091
two burners were required whereas in the alternative
embodiment only one burner 90 fires into fan chamber 26. A
second burner defined as incineration track burner 91 is
provided adjacent inlet opening 86 and is orientated to fire
its products of combustion into incineration track 85. Both
burners 90, 91 can be sized to have the same capacity, i.e.
typically 0.50 mm Btu/hr but incineration track burner 91 is
operated at a high percentage of excess air, i , a . 1000% excess
air.
Operating fuel fired furnaces at excess air is well
knocm to those skilled in the art and need not be explained
herein. However, in order to appreciate the far reaching
and diverse applications of the alternative embodiment, some
discussion may be in order. Air and fuel are typically sup-
plied to a gas fired burner at a stoichiometric ratio (i.e.
for natural gas an air-fuel ratio of about 9:1) which as-
sures complete combustion of the fuel. The hot gas or prod-
ucts_ of combustion produced by a burner fired
sto~chiometrically does not contain oxygen. It is known
that when the burner is operated at excess air or air in
excess of that required to produce a stoichiometric ratio of
fuel and air, burner flame temperature increases slightly
and oxygen appears in the products of combustion. Thus,
operating incineration track burner 91 at very high percent-
ages of excess air produces high levels of oxygen in the
burner's product of combustion. This oxygen, at elevated
temperature, will react with the volatiles in the smoke in
an exothermic reaction to cause incineration thereof and
thus generate additional heat which will further heat incin-
eration track 85. Thus, inner casing 80 will be hotter than
the wind mass in fan chamber 26 from heat in incineration
track 85 and as the wind mass swirls about fan chamber 26,
it will be heated from contact with inner casing 80. In
fact, heat calculations indicate that the heat output
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203~65~
produced by the two tangentially fired burners in, the parent'?
p~'at~n will not be greater than the heat output produced by
furnace burner 90 plus incineration track burner 91 (which
are identically sized in burner output) even though inciner-
ation track burner 91 does not fire its products of combus-
tion directly into fan chamber 26. Next, it is to be appre-
ciated that the fumes within incineration track 85 are not
co-mingled with the furnace atmosphere in fan chamber 26
even though fluid communication is provided vis-a-vis incin-
eration track opening 86. Incineration tack burner 91 is
orientated to tangentially fire its products of combustion
relative incineration track 85 just at the incineration
track opening 86. However, it is contemplated that incin-
eration track burner 91 could be positioned just upstream of
incineration track opening 86. The point is that when in-
cineration track burner 91 is fired tangentially, it will
pull, suck, draw or aspirate furnace swirl mass through in-
cineration track opening 86 into incineration track 85 but
the burner's products of combustion will not enter into fan
chamber 26. Thus, the fume composition in incineration
track 85 has no effect on the composition of the furnace
atmosphere. This feature significantly expands the various
industrial applications which furnace 10 can perform. Spe-
cifically, it is known that certain materials such as scrap
fiberglass waste and plastic coated wires can be thermally
reclaimed and in the process thereof will emit hydrocarbons
and volatiles which must be thermally disposed of. It is
also known that certain hazardous and/or toxic material can
be thermally disposed of. In both instances, thermal clean-
ing and/or destruction of hazardous wastes can be accom-
plished by heating the material in the absence of oxygen at
controlled rates which would drive off hydrocarbons or
volatiles which heretofore were incinerated by an external
incinerator. An example of such process is set forth in
Surface Combustion's United States Patents 4,913,069 dated
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2032652 ~v
April 13, 1990 and 4,924,785 dated May 15; 1990. Furnace 10 of the
alternative embodiment can function to operate as a pyrolyzer furnace.
First, burner 90 can be fired at various temperatures and
can produce a "rich" gas to assure that work 15 (which now
is a waste material c~r a non-metallic material such as scrap
fiberglass) is heated in the absence of oxygen at controlled
rates. Then incineration track burner 91 will incinerate
the volatiles and because of its tangential firing and per-
haps upstream position will not produce any gas mingling
with the furnace atmosphere. Furnace 10 is thus not limited
in application to heat treat processes.
The operation of alternative embodiment furnace 10 is
believed obvious from the foregoing. The swirling wind mass
within fan chamber 26 is developed as described above. The
swirling wind mass does not short circuit from inlet opening
86 to exit opening 87 because inlet opening 86 is a discrete
spot of finite size which will not trap a significant pro-
portion of the swirling wind mass and baffle 89 regulates
the rate at which the wind mass is tapped by incineration
track 85. Further, the position of incineration track burn-
er 91 and its firing velocity determines entrainment of fur-
nace wind mass. Controller 100 regulates exhaust through
baffle 89 and controls the firing rates of burners 90, 91
for temperature control of the process. Temperature is
sensed through a centrally positioned thermocouple near work
15 (not shown) and a thermocouple in incineration track 85
(not shown) which are outputted to controller 100. As al-
ready noted, when the volatiles are combusted in incinera-
tion track 85, inner casing 80 is heated and the heat trans-
ferred to the wind mass swirling about inner casing 80.
For simplicity, incineration track 85 is shown as ex-
tending only about fan chamber 26. If furnace 10 as shown
in FIGURES 1 through 5 were modified to include an incinera-
tion track, then incineration track 85 could be formed as a
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SF-8387
20320~~
groove which would completely spiral around furnace section
12 and extend the length of radiant tube heaters 42. This
would extend the residence time of the gases within inciner-
ation track 85 and assure that complete incineration of the
gases occurred within furnace 10 and not outside the furnace
in some insulated duct.
The invention has been described with reference to a
preferred and alternative embodiment. Obviously, further
modifications and alterations to the invention will become
apparent to those skilled in the art upon reading and under-
standing the Description of the Invention set forth above.
It is intended that all such modifications and alterations
are included herein insofar as they come within the scope of
the present invention.
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