Note: Descriptions are shown in the official language in which they were submitted.
t~3~
Background of the Invention
This invention is in the field of furnaces; more particularly,
the invention relates to furnaces having positive pressure at-
mospheres, such as positive pressure carburizing furnaces.
Furnace atmospheres are used for surface treatment of metals.
One of the more common types of treatment is gas carburizing
of steel. Vacuum carburizing furnaces operate at lower then
atmospheric pressures while positive pressure carburizing furnaces
operate at or above atmospheric pressure. In either vacuum
carburizing furnaces or positive pressure carburizing furnaces,
the carburizing atmosphere is injected into the treatment chamber
and circulated. An informative reference on gas carburizing
is the Metals Handbook, Eighth Edition, Volumn 2, Heat Treating,
Cleaning and Finishing, prepared under the direction of the
ASM Handbook Committee and published by the American Society
for Metals, Metals Park, Ohio. A discussion of gas carburizing
begins at page 93 and a discussion of gas nitriding begins
at page 149. This reference reviews various types of furnaces,
furnace atmospheres and metal surface properties.
Presently, most carburizing furnace applications require
some kind of atmosphere circulation within the furnace. ~ether
the furnace is a positive pressure or a vacuum furnace, the cir-
culated atmosphere must penetrate the workload for uniform car-
burizing. Most positive pressure furnace applications requiring
circulation use either an axial or radial fan system. The fan
system has its limitations, especially side fan carburizers
where additional furnace height and width are required to
accommodate the fan and its attendant equipment.
A typical furnace for heat treating of ferrous articles
with controlled atmospheric compositions is shown in U.S. Patent
No. 4,049,472, Atmosphere Compositions and Methods of Using Same
for Surface Treating Ferrous Metals, by Arndt. The furnace shell
has numerous atmospheric ports through which the atmosphere is
introduced and maintained in the furnace. The furnace includes
a fan blade which is driven by a fan motor to circulate the
atmosphere within the furnace and to help equalize the furnace
for uniform heat treatment of the parts continuously moving along
in the furnace. The circulation and penetration of the con-
tinuously moving load is achieved by the fan within the furnace
chamber. There is no indication of the designed arrangement or
location of the atmosphere ports in any way to cooperate with
the entrainment of furnace atmosphere into the stream of freshly
injected gas, and the circulation and penetration of the furnace
atmosphere into the area of the load.
U.S. Pate~t No. 3,950,192, Continuous ~arburizing Method,
by Golland et al, is an improved method for continuously car-
burizing low carbon cold rolled coil stock. The carburizing
gases introduced and flows counter to the pa~h of the strip of
metal coil exiting through a discharge pipe in the vicinity of
the coil entrance. The atmosphere system in this particular
furnace is designed for continuous carburizing of very thin
ribbon-like coils.
Other types of positive pressure furnaces, such as soaking
pit furnaces, achieve uniform treatment of the load with gases
which are circulated by high pressure into and/or have recircu-
lation systems to circulate the hot gases in the soaking pit.
Examples of this are U.S. Patent No. 2,991,832, Recirculation
System for Heat Treating Furnace, by Dailey, and U.S. Patent No.
2,849,221, Heat Treating Furnace, by ~one et al.
Jet injection has been used in vacuum carburizing furnaces
in the past. It is desirable to achieve similar entrainment of
furnace atmosphere, and circulation and penetration of atmosphere
into the load area in positive pressure furnaces by jet injection
3Z
without the use of a circulation means such as a fan. Jet
injection into positive pressure furnaces has not been used
because it was believed that the jets would have to operate
at too high of a pressure to achieve the desired entrainment
of carburizing gases in the jet stream and at the same time
provide the desired circulation and penetration of the load
area by the carburizing atmosphere.
The present invention is an improvement in posi-tive
pressure furnaces. It is an apparatus and method for the
injection of atmosphere into a positive pressure furnace
treatment chamber.
The method of the present invention therefore
is for injection of atmosphere into a positive pressure
furnace chamber containing a load supported on a long support
means the method including the steps of injecting an
atmosphere at a high pressure and low momentum into a chamber
of the furnace from a plurali-ty of ]ow pressure nozzles to
achieve circulation of atmosphere in the chamber and penetration
of the load while entraining the chambered atmosphere within
the jets to achieve uniform temperat~ure treatment of the load.
In the apparatus of the invention the walled
treatment chamber is sealed from the ambient atmosphere. The
chamber has a hearth on which the load is supported. The gas
to be injected into the furnace is brought to a manifold
along the outer wall of the furnace. A plurality of tubes
are connected to the manifold and passed through the wall of
the furnace into the treatment chamber. Jet nozzles at the
ends of the tubes are straitegically located within the furnace
chamber so as to cooperate with the geometry of the furnace
3Q and the load so that atmosphere injected from the tubes into
the furnace penetrates the load area and circulates to
uniformly treat the load.
More specifically the jet nozzles are sized to
-- 3
pc/" ~,~
52~2
achieve entrainment of the atmosphere in the chamber within
their jet streams for uniform treatment of the load and to
quickly reach furnace temperature. There must be the minimum
necessary jet stream momentum to achieve -the desired
circulation and penetration of the load area. This invention
is particularly useful in the continuous carburizing furnace.
It is a general object of the present invention
to use jet injection of atmosphere in a positive pressure
furnace.
It is the object of the present invention to inject
atmosphere into the chamber of a furnace through a plurality
- 3a -
pc/
of jet nozzles so as to achieve the necessary circulation and
penetration of the load without the use of internal circulation
means such as fans. It is a further object of the present in-
vention to achieve the necessary entrainment of furnace atmosphere
in the jet stream to quickly bring the gases in the jet stream
to the conditions within the furnace. It is another object of
the present invention to eliminate circulation equlpment and
reduce the furnace size by use of the jet system which is
equivalent to the fan system.
It is an object of this invention to obtain one or more
of the objects set forth above. These and other objects and
advantages of this invention will become apparent to those
skilled in the art from the following specification and claims,
reference being had to the attached drawings.
Brief Description of the Drawing
Figure 1 is a perspective view of a section of a furnace
with the present invention.
Figure 2 is the top view of a continuous carburizing furnace
showing the piping plan of the present invention.
Figure 3 is a section along lines 3-3 of Figure 2 showing
a cross-section along the length of a continuous carburizing
furnace with the present invention.
Figure 4 is a section along lines 4-4 of Figure 2 across
the width of a continuous carburizing furnace with the present
invention.
Figure 5 is a detailed view of the manifold piping and
nozzle of the present invention as shown in Figure 4.
Figure 6 is a detailed cross-section of the nozzle.
Figure 7 is a plot of the log of jet height vs. load width
for various load densities.
Figure 8 is a plot of pressure drop across the nozzle vs.
flow per jet in standard cubic feet per hour.
Figure 9 is a plot of percent surface carbon uniformity
vs. the pressure drop across the nozzle.
Description of the Pre~erred Embodiments
The present invention will be understood by those skilled
in the art by having reference to Figures 1 through 4 which are
views of one embodiment of the present invention installed in a
continuous carburizing furnace 10. Although the present inven-
tion will be illustrated as used i.n a multi-zone, continuous
carburizing furnace, it is understood that it can generally be
used in positive pressure continuous or batch furnaces in which
there is a need to inject atmosphere which will circulate within
the furnace chamber and penetrate into the load area for uniform
treatment and contact with the load.
The present invention in its most basic form is an apparatus
and method for the injection of atmosphere into a positive pres-
sure furnace through a plurality of strategically located at-
mosphere inlet means.
Structure
The multi-zoned continuous carburizing furnace lO used to
illustrate the present invention has a walled furnace chamber 11.
The furnace lO has a hearth 13, walls 14,15 and a ceiling 16.
The furnace is supported by a structural frame or housing 18.
There is a suitable hea~ing means within the furnace, such as
radiant tube heaters 20 disposed near the ceiling 16 and the
hearth 13.
The furnace 10 is a continuous pusher tray furnace. In
this furnace there are load support means, such as piers 24,25
and rails 21. Load trays or boxes 22 are supported and contained
in an aligned position by the rails 21. The furnace rails 21 can
be on piers 24,25 which are on the hearth 13. In a preferred
embodiment, the rails 21 sit on piers having as many checkered
openings as design considerations allow. The piers 25 at the
ends of each zone should be closed. The load trays can be merely
trays or boxes as shown in Figures 1 and 4. When boxes are used
they can have corrugated steel sides or screened sides. The
load 23 is placed on the ~rays or in the boxes 22. The trays or
boxes 22 are pushed through the furnace on the rails 21 from
zone to zone during operation.
The furnace 10 illustrated in Figures 1 through 4 is a
four-zoned carburizing furnace. Zone 1 is the heat zone.
In this zone the atmosphere is purged and a reducing environment
created. No carburizing is expected in this zone and the work
is heated to an operating temperature of from about 1700F to
1850F. The work is then pushed from zone 1 to zone 2. A drop
arch 26 separates the two zones. Zone 2 is the carburizing zone.
In this zone it is desired to get a maximum of surface carbon
onto the work. An endothermic carrier gas containing methane is
the atmosphere which is typically usecL. It is desired to have
maximum penetration and circulation of the carburizing atmosphere
within this zone. This zone is held at about 1700F to about
1850F. The load trays continue to be pushed through drop arches
27 and 28 to zone 3. Zone 3 is the diffusion zone where the
carburizing atmosphere is maintained as well as the temperature
at about 1700F to 1850F. The work proceeds through another
set of drop arches 29 and 30 to zone 4 which is a cooli.ng and
equalizing zone. Equalizing is the final diffusion state in
which the carbon case depth achieves its final value. The
atmosphere is adjusted for final diffusion control and the
temperature is cooled to about 1550F. The work must be at
this temperature for quenching. From zone 4 the work is finally
pushed out of the furnace to suitable vestibules or other exit ~:
means (not shown).
In prior art furnaces of this type, atmosphere has been
injected through a suitable inlet port and circulated to the
load by a suitable circulation means, such as a fan. In this
invention7 the atmosphere is brought to a manifold 33 which runs
along the outer wall of the furnace. The manifold can be sup-
ported on the furnace housing 18 as shown in Figure 1. As can
be seen in Figure 2, each zone can have a separate manifold 33,
and each manifold can have its own controlled source of gas.
There are a plurality of tubes 34 with each tube connected at
one end to, and in communication with, each manifold 33. The
tubes pass from the manifold into the furnace chamber 11 where
the opposite ends of the tubes are located. In the preferred
embodiment, the manifold 33 runs along the furnace on the outside
of the ceiling 16 although it can run along either of the sides.
The tubes 34 pass from the manifold 33 through the ceiling wall
16 into the furnace chamber 11. The manifold is in communication
with the furnace chamber through the tubes 34. The opposite ends
of the tubes must be strategically located so as to cooperate
with the geometry of the furnace chamber and load so that
atmosphere injected from the tube into the furnace penetrates the
load area and circulates so that the load is uniformly treated.
The tube 34 length can be changed depending on how far into the
furnace it is desired to extend the nozzle end of the tube.
The injected atmosphere must entrain the gases in the furnace
chamber 11 for achieving uniform treatment temperatures and
properties. Means as are known in ~he art, can be used to
remove spent furnace atmosphere from the furnace chamber 11.
Figures 5 and 6 are detailed cross-sectional drawings of
the present invention showing the manifold 33, the tube 34 and
nozzles 36 connected to the opposite end of the tubes 34. As
seen in Figures 5 and 6, the nozzle is preferably a venturi-type
jet nozzle. In the preferred embodiment of this invention the
tube 34 passes through the ceiling 16 into the furnace chamber
and is directed to inject atmosphere from the top of the chamber
down onto the load.
--7--
5~32
D~
When designing a positive pressure furnace having a p-lurality
of nozzles to injec~ atmosphere into the furnace chamber with
the same total momentum flux as if a fan system~ere used, and the
nozzles being high velocity low momentum jet nozzles, the
following design criteria must be considered: jet height above
~he load, the number of jets, jet size, jet placement and furnace
geometry.
Figure 7 is a plot of the log of the nozzle height H above
the load as a function of load width W for two different load
densities KL. For various load densities a family of straight
lines can be plotted. This family of curves can be described
by the equation J = eMW + KL where J is the nozzle height above
the load; W is the load width; and M is a function of the
particular furnace system. KL is a function of load density and
can be further broken down where K is a function of resistance
to flow and L is the depth of the load. A better term for KL
than load density could be porosity o the load. It is a
function of the geometry of the parts that make up the load and
also the arrangement or placement of the parts within the load.
The curves shown in Figure 7 are for the continuous pusher
tray carburizing furnace used to illustrate the presen~ invention;
however, similar sets of curves can be generated for any batch
or continuous furnace in which the present invention is to be
used.` The KL factor is a measuremen~ of resistance to flow
which depends upon the density of the load within the furnace.
A dense load might have a Kl. factor of .1 while a less compact
load might have a KL factor of .01 to .05.
For the furnace used to illustrate the present invention,
the tray width W is about 22 inches. At the denser of the two
loads with KL equal to 0.05, the jet height above the load
required for a single row of jets along the length of the furnace
would be 12 inches (see Figure 7). In the example furnace, two
rows of trays are used with one row of jets above each row of
trays. The jet height above each row of -trays is 12 inches.
The optimum jet arrangement for maximum recirculation and pene-
tration has been found to be jet rows about the load. The
furnace can be sized for one or two rows of jets, above each
row of trays, with one row above each load of jets shown in the
illustrations. The jet height above the load is 7.5 inches when
two rows of jets are used. In this case an effective load width
of 11 inches is used.
Test work has determined that equal distribution of flow
and maximum circulation and entrainment coupled with maximum load
penetration are exhibited when co-linear jet streams just impinge
upon one another at the load surface. The worst cases are
exhibited with the jet spacing approaching a slot jet and the
opposite extreme when the co-linear jets are spaced too far apart.
The slot jet arrangement has the greatest penetration with the
poorest entrainment. When the co-linear jets are spaced too :Ear
apart, the reverse is true, high entrainment and low penetration.
It is important to have both the proper amount of entrainment
of the atmosphere within the furnace for uniform treatment of
the load as well as uniform and complete penetration of the load
by the jet streams. To meet the above maximized state the jets
are tentatively spaced so that the jet streams just impinge
upon one another at the top surface of the load with a jet
angle spread of about 20. This is ideal for batch furnaces
where this maximized state was initially established and where
the number of jets can be kept to a minimum. II1 a continuous
carburizer, as shown in Figures 1 through 5, not only will this
require a large number of jets but also because of the roof
geometry and upper radiant tubes the space available does not
accommodate all the jets necessary. In a continuous carburizer,
_9_
~ 3~,
however, there can be a free space 31 over at least one tray
position in several of the zones. This free space 31 should have
jets spaced as shown in Figures 2 and 3 for maximum recirculation
and penetration. The number of jets must be reconciled with the
momentum the system is designed for, the flow per jet and ~e
jet size.
In the furnace shown in Figures 1 through 4, there are
radiant heating ~ubes 20 along the ceiling 16. Jets are
preferably arranged in one or ~ore rows parallel to the longi-
tudinal axis of the furnace. The row or rows of jets above the
radiant tube must be designed so that the jet streams do not
impinge on the radiant tubes. Therefore, there is a compromise
between the optimum jet height and radiant tube location. The
jet spread can range from 20 to a maximum of 30. The angle A
of jet spread is shown on Figure 4. With a jet to load distance
of 12 inches and a tube to load distance of 5 inches, the minimum
perpendicular distance from the jet a~is to the tube is (tangent
15) ti.mes (12-(5~ radius of the tube)) or approximately 1 inch.
Thus, the jets can be placed between the legs of the U-shaped
radiant tube with no problems. This illustration shows the
compromises and calculations which must take place in the design
to assure a practical jet height above the load.
Once the number of rows of jets, the jet height above the
load, the number of jets and the spacing are established, the
sizing of jets can be determined. The sizing of jets depends
on the required flow rates. In a continuous carburi~ing furnace
as illustrated in Figures 1 through 5, the flow is about 500
standard cubic feet per hour in zone 1, about 625 (125 of methane
and 500 of reducing gas) standard cubic feet per hou-r in æone
2, about 500 standard cubic feet per hour in zone 3, and about
~00 standard cubic feet per hour in zone 4.
- 10-
~5~32
The jets are sized to have a total momentum equivalent to
a fan's m~m~tum flux at the exit of the plenum to the fan. The fan's
momentum flux is not uniform throughout the zone. The optimum
momentum is measured at ~he an plenum exit and the zone
momentum is taken as the average of all the plenums of the zone.
The rails are connected to the fan plenum by checkered openings
in the piers. This ducting provides a path for atmosphere
coming from the loads to be diverted back along the free flow
area between the load and the wall.
It has been experimentally determined that the design
parameter for op~imum momentum per square foot per tray area
should be greater than .016 pounds per square foot in a tray with
the minimum zone momentum flux greater than .007 pounds per square
foot in a tray. Thus, the jets are spaced so that in the free
area themomentum flux is greater than .016 pounds per foot squared.
The free area above the load is where there are no radiant tubes
or other e`quipment. To insure equal clistribution throughout the
zone, the remaining jets are positioned between the legs of
U-shaped radiant tube.
The total flow per zone is usually predetermined and fixed.
The flow per jet is dependent on the number of jets. An
important consideration is to achieve the entrainment, penetration
and treatment of the work within the furnace with a minimum flow.
T~is is important in conserving ~reatment atmosphere as well as
conserving energy. The pressure drop across each jet, although
somewhat arbitràry, is usually kept small. Figure 8 is a plot
of pressure drop per jet vs. flow per jet for four momentum fluxes
A, ~, C and D. The mo~entum fl~ of A is less than B, which is
less than C, which is less than D. The minimum flow asympto~e
is approached for a given l~mentum flux regardlessof pressure. Thus,
low pressures are generally sufficient.
Figure 9 is a plot of percent of surface carbon uniformi~y
vs. pressure drop per jet. This curve based on experimental
~ 3'~
data shows that the optimum pressure drop per jet for the
greatest uniformity of surface carbon is between 3 and 6 psig.
The curve indicates that the uniformity decreases if the pressure
drop is too low, i.e. insufficient penetration of the jet into
the chamber, or if the pressure drop is too high, i.e. in-
sufficient entrainment of furnace atmosphere in the jet. There
must be a balance of penetration and entrainment. The entrain-
- ment of furnace atmosphere is important to bring the injected
atmosphere to the proper composition and temperature. Penetration
~ important to attain the necessary surface contact of the
furnace chamber atmosphere wit~ the work to be treated~
- The jet diameter must be determined considering the
momentum flux and pressure drop limits discussed above. Additionally,
.
to size the je~s the temperature at the jet exit must be known.
The atmosphere for carburizing cannot be greater than about
1000F so as not to break down and form carbon. The exit
temperature can be approximated from 1:he equation:
Q Cp (Tf-Ti) = hc A(Tamb - Tavg~
where Q = gas flow per jet în s.c.f.h.
Cp = specific heat in Btulscf/F.
hc = heat transfer coefficient in Btu/hr/ft.2/F.
A = internal surface area of the pipe
Ti = initial gas temperature upon entering the tube
Tf = jet exit temperature
TaVg = (Tf + Ti)/2
Tamb = furnace temperature
The momentum flux per jet is determuned by the following
equation:
-i2-
~ 3
G(lbs) = ~ Q~
40573 P/a PTS
where G = momentum flux in lbs .
Q = flow per jet s.c.f.h.
C = discharge coefficient
Y = adiabatic expansion factor
Y = 1-.863(X/K~ where X - ~P~P and K =
. . ratio of specific heats Cp/Cv
P = upstream pressure (PSIA)
a P = pressure drop (psig)
T = temperature (R)
S = specific gravity (air = 1.0)
. .
The jet diameter is then calculated by the following
equation:
d =I G
~ 1.57 (c24 py2)
where d = iet dlameter inches
G = momentum flux ~er je~ irl lbs.
. C = discharge coefficient
Y = adiabatic expansion factor (defined above)
~ P = pressure drop per jet psig
The above techniques are therefore used to design the
height of the jet nozzles above the load, the number of jet
-nozzles, the spacing of the nozzles and the diameter of the
nozzles. The construction of the continuous carburizing
furnace as illustrated in Figures 1 through 5 uses these design
techni~ues. Table I summarizes key parameters used and the
design results which can be used in this furnace to achieve
continuous carburization wi.th circulation of the carburizing
atmosphere, pene~ration of the atmosp~ere into the load area
-13-
-- , . ...... ~ .. .
and entrainment of the residing furnace atmosphere within the
jet streams so as to achieve uniform treatment of the work. In
this furnace, there are two rows of trays with a tray width of
22" and the furnace will operate at 4 psig pressure drop per jet.
The furnace will operate with U-shaped radiant tube heaters. It
is important to note that where there is free space 31, jets
are equally spaced within the free space to achieve optimum
impingement. Optimum jet number and location is based on an
angle of the jet or jet spread of about 20 so that the impinge-
ment of each jet on the surface of the load does not interferewith adjacent jets. Because radiant tubes are along the ceiling
and it is not desired to have the jets impinge on the radiant
tubes where there are tubes the jets are placed between the tubes
or between the legs of a U-shaped tube.
-14-
~5~ 2
O H cr~ ~1 1``
~1 ~O oo
N
~ ~ O O O
O ,1
æ
~ ~ ~ ~ o
~ o oo ~o
.,, ~ ~
_
o O U~
a~ ~,a o o o o
. . . .
~o o o o o
. ~ o ~ U~ ~ ~J
~ C Ln ~ ~ ~ ~D
E~
H c~l
~ ~ tn o
X a~
~,~ C`JC~
H H ~1 ~ ra N
H a~ rl
U7 U~
~ a~ ~ .
1- ~.
~ O ~ O ~O .,~ ~
U~ U~ ~ ,.;,
O
O C~ ~ O ~ O ~
U ~ OC~ I:d 1~ 1~ 1--
Q~ ~1 --1~1~1 ~1 ~ u~ ~S\ O
æ ~ o
a~ r~U~ r~ rl
~ ~ ~ ~
O U~ O O U~
~ X o ~ o o ~ ~ ,~
O ~ ~ oo c~
~ ~ l l l l ~ $~ ~ ~ ~
u~ ,~ ~ ~ ~ ~ ~ ~ ~ n
N
-15-
In furnace systems where atmosphere within the furnace
chamber is circulated or must otherwise be brought into contact
with a workpiece being treated, there are geometric requirements
so that the necessary flow of atmosphere can occur. This
generally is accomplished by certain free areas where flow can
take place. There can be a free flow area around the work as
well as areas below and/or above the work depending upon where
the generation of the flow is located.
Generally, using the high velocity, low momentum jet system
of the present invention, it has been determined that the optimum
free-flow area ratio was about 47% with an ideal range from
about 34% to about 54%. The free-flow area ratio is defined as
the plan view open cross-sectional area around the load divided
by the plan view cross-sectional area of the load. Thus, for
one tray position the area ratio is the length of the tray times
the total width o:E the chamber minus t:he area of the tray then
divided by the area of the tray. Knowing the ideal area ratio
and the tray dimensions, the total wiclth required can be
determined. For example, using 22 inch by 22 inch trays, which
are used in the furnace illustrating the present invention, and
.47 as the optimum free-flow area ratio, the furnace chamber
width, C, can be determined as follows for each tray:
.47 = ~(22 x C) - (22 x 22)] + (22 x 22) or C = 32"
This indicates that 32 - 22 = 10 inches of available free space
on one side of the tray or 5 inches of free space on each side
of the tray is optimumly required.
To conserve space and still be in the ideal area range,
the minimum value of 34% can be used for furnace design. Solving
for the total width as illustrated the minimum total width needed
is approximately 29~1/2 inches for the illustrated furnace. Thus,
the available free space on one side of the tray is 7-1/2 inches.
-16-
This value is preferred to be used rather than the optimum to
conserve on furnace width. This value can be used on one side
of the tray rather than dividing it into two equal areas on
both sides of the tray particularly in the furnace of Figures
1-5.
When considering a continuous pusher tray-type furnace used
to illustrate this invention where the jet noz~les pass through
the ceiling 16, the space below the load can be optimized in
terms of the pier ratio. It has been determined that the
optimum pier ratio is approximately 18%. Referring to Figures
3 and ~, the pier ratio is defined as the percent openness times
the pier height divided by the width of the load or
R = P x (H~W)
where R = the pier ratio
H = the height of the pier
W = the load width
P = the percent openness
where P is essentially equal to the open area under a load divided
by the total area under the load. More specifically, the
percent openness is defined as the pier width D times the tray
width W minus the plan view area of any obstructions to the
pier such as rails, radiant tubes, etc., divided by the pier width
D times the tray width W.
The basic operation of the present invention is the injec-
tion of atmosphere into a positive pressure furnace chamber of a
furnace as described above. The main step is the injection of
atmosphere into the chamber of the furnace from a plurality of
low pressure nozzles to achieve a circulation of atmosphere in
the chamber and penetration of the load while entraining chambered
atmosphere within the jets to achieve uniform temperature and
treatment of the load. The atmosphere is injected from each jet
at a relatively high velocity and low momentum. This method can
5~,~2
be used in most positive pressure batch and continuous furnaces
in which an atmosphere is important for treatment in the load
and circulation of that atmosphere, penetration of the load
and uniform treatment and temperature are necessary. A plurality
of low pressure, low momentum, high velocity jets, s-trategically
located within the furnace chamber in cooperation with the
furnace and load geometries can be used to achieve this desired
treatment and circulation. The furnace used to illustrate the
present invention and described in Table I can be operated with
the flow, momentum and temperatures indicated for the four zones.
This furnace is operated without a fan at the same gas momentum
in achieving the same or better circulation penetration and
uniformity as a prior art continuous carburizing furnace where a
fan is used to achieve the circulation penetration and uniformity
of the atmosphere within the furnace chamber. Additionally, by
using the jet injection system of the present invention, the
furnace can be made smaller by removal of the fan equipment.
This is especially true on side fan carburizers where additional
height and width are required, not for the process but for the
fan hardware.
Modifications, changes, improvements to the preferred forms
of the invention herein disclosed, described and illustrated may
occur to those skilled in the art who come to understand the
principals and precepts thereof. Accordingly, the scope of the
patent to be issued herein should not be limited to the particular
embodiments of the invention set forth herein, but rather should
be limited by the advance of which the invention has promoted
the art.
-18-
SUPPL_MENTARY ~ISCLOSURE
It has been noted that it is desirable to
maintain an optimum height of the nozzles or jets above
the load. To accomplish this, it is sometimes necessary
to use a less desirable tube size to keep the carburizing
gas exiting the nozzles 36 from becoming overheated.
For example, the carburizing gas entering the tubes 34
at ambient temperature, è.g. 60-70F., should be kept
below 1000~F. as it circulates through the tu~es, to
minimize the breakdown of methane and formation of soot
within the tubes and nozzles. The increase in temperature
experienced by the carburizing gas as it passes through,
a tube 34, is dependent on the inside area of the tube
and the temperature of the atmosphere surrounding the
tube. Since the leng-th of the tube to achieve optimum
jet height is fixed, it is sometimes necessary to use a
' smaller or larger diameter tube than desired to prevent
! the carburizing gas Erom being overheated before it exits
the nozzle attached to the tube.
The tubes in the high tempera-ture areas of the
furnace should be made of ma,terial which,is resis;tant,to'
oxidation and does not promote the breakdown of methane
and the forma-tion of soot which acts to block the tubes
and nozzles. It has been found desirable to provide an
aluminum diffusion coating on the outside of the high alloy
steel tubes and nozzles that are normally used in the
furnace to prevent, for example r the nickel in ~he alloy
from being exposed to the methane and carburizing
atmosphere, since it has been found that the nickel acts
as a catalyst in the promotion of the breakdown of
methane and formation of soot.
-- 19 --
~, sb/ 1`'~!,
'i ;' I
