CA1145232A - Atmosphere injection system - Google Patents
Atmosphere injection systemInfo
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- CA1145232A CA1145232A CA000354092A CA354092A CA1145232A CA 1145232 A CA1145232 A CA 1145232A CA 000354092 A CA000354092 A CA 000354092A CA 354092 A CA354092 A CA 354092A CA 1145232 A CA1145232 A CA 1145232A
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Abstract
ATMOSPHERE INJECTION SYSTEM
Abstract of the Disclosure This invention is an apparatus and method for the injection of atmosphere into a positive pressure furnace treatment chamber.
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 strategically located within the furnace chamber so as to cooperate with the geometry of the furnace and the load so that atmosphere injected into the furnace penetrates the load area and circulates to uniformly treat the load. This invention is particularly useful in the continuous carburizing furnace.
Abstract of the Disclosure This invention is an apparatus and method for the injection of atmosphere into a positive pressure furnace treatment chamber.
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 strategically located within the furnace chamber so as to cooperate with the geometry of the furnace and the load so that atmosphere injected into the furnace penetrates the load area and circulates to uniformly treat the load. This invention is particularly useful in the continuous carburizing furnace.
Description
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.
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 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 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.
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.
~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 -- , . ...... ~ .. .
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.
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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.
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
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 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 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.
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.
~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 -- , . ...... ~ .. .
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.
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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.
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.
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.
~, sb/ 1`'~!, 'i ;' I
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.
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.
~, sb/ 1`'~!, 'i ;' I
Claims (26)
1. An apparatus for injection of atmosphere into a positive pressure furnace, having a walled treatment chamber sealed from the ambient atmosphere, the chamber having a hearth on which a load is supported comprising:
a manifold along the outer wall of the furnace;
a plurality of tubes with each tube connected at one end to the manifold and passing from manifold through the furnace wall into the treatment chamber, the manifold being in communi-cation with the furnace chamber through the tubes and where the opposite end of the tubes are strategically located so as to cooperate with the furnace geometry and load geometry whereby atmosphere injected from the opposite end of each of the tubes into the furnace penetrates the load area and circulates so the load is uniformly treated.
a manifold along the outer wall of the furnace;
a plurality of tubes with each tube connected at one end to the manifold and passing from manifold through the furnace wall into the treatment chamber, the manifold being in communi-cation with the furnace chamber through the tubes and where the opposite end of the tubes are strategically located so as to cooperate with the furnace geometry and load geometry whereby atmosphere injected from the opposite end of each of the tubes into the furnace penetrates the load area and circulates so the load is uniformly treated.
2. The apparatus as recited in claim 1, wherein the manifold passes along the top of the furnace and the tubes pass through the top of the furnace, which further comprises venturi jet nozzles connected to the opposite end of the tubes, the nozzles being directed to inject atmosphere from the top of the chamber down onto the load, and the nozzles being sized to feed a high velocity low momentum jet of atmosphere.
3. The apparatus as recited in claim 2, the furnace further having a load support means and a load tray removably set on the load support means for supporting the load, wherein:
the nozzle height above the load is related to the load width, the load density, and the furnace geometry and is substantially equal to:
J = eMW + KL
where J = the tube nozzle height above the load W = the load width KL = the function of load density M = the function of the particular furnace system.
the nozzle height above the load is related to the load width, the load density, and the furnace geometry and is substantially equal to:
J = eMW + KL
where J = the tube nozzle height above the load W = the load width KL = the function of load density M = the function of the particular furnace system.
4. The apparatus as recited in claim 3 wherein the jets are spaced so that the jet streams just impinge upon one another at the top of the load with the jet having a jet spread angle in the range of 20 to 30 degrees.
5. The apparatus as recited in claim 4 wherein the jet spread angle is 20 degrees.
6. The apparatus as recited in claim 4 wherein the jets are arranged in at least one colinear row parallel to the longitudinal axis of the furnace.
7. The apparatus as recited in claim 4 wherein the free-flow area around the sides of the load is between about 34% and about 54% of the hearth, where the free-flow area is the space between the load tray and the furnace chamber walls.
8. The apparatus as recited in claim 5 wherein the free-flow area around the sides of the load is about 47%.
9. An apparatus for injection of atmosphere into a continuous, positive pressure carburizing furnace having a plurality of zones, and each zone having a walled treatment chamber, the chamber having a hearth on which a load is supported, which comprises:
a manifold along the outer wall of each zone of the furnace;
a plurality of tubes with each tube connected at one end to one of the manifolds and passing from the manifold into the furnace chamber, each manifold being in communication with the furnace chamber through the tubes, and each tube having an opposite end;
venturi jet nozzles connected to the opposite end of the tubes, the nozzles being strategically located within the furnace chamber so as to cooperate with the furnace geometry and load geometry, being directed to inject atmosphere from the top of the chamber down onto the load, and being sized to feed a high velocity low momentum jet atmosphere, whereby injected atmosphere into the furnace chamber penetrates the load area and entrains furnace chamber atmosphere, so that the load is uniformly treated.
a manifold along the outer wall of each zone of the furnace;
a plurality of tubes with each tube connected at one end to one of the manifolds and passing from the manifold into the furnace chamber, each manifold being in communication with the furnace chamber through the tubes, and each tube having an opposite end;
venturi jet nozzles connected to the opposite end of the tubes, the nozzles being strategically located within the furnace chamber so as to cooperate with the furnace geometry and load geometry, being directed to inject atmosphere from the top of the chamber down onto the load, and being sized to feed a high velocity low momentum jet atmosphere, whereby injected atmosphere into the furnace chamber penetrates the load area and entrains furnace chamber atmosphere, so that the load is uniformly treated.
10. The apparatus as recited in claim 9, the furnace further having a load support means and a load tray removably set on the load support means for supporting the load, wherein:
the nozzle height above the load is related to the load width, the load density, and the furnace geometry and is substantially equal to:
J = eMW + KL
where J = the tube nozzle height above the load W = the load width KL = the function of load density M = the function of the particular furnace system.
the nozzle height above the load is related to the load width, the load density, and the furnace geometry and is substantially equal to:
J = eMW + KL
where J = the tube nozzle height above the load W = the load width KL = the function of load density M = the function of the particular furnace system.
11. The apparatus as recited in claim 9, the furnace further having piers upon the hearth and rails supported on the piers, there being a plurality of load trays removably set on the rails for support of the load, wherein:
the pier ratio is greater than 18%, the pier ratio being substantially equal to:
R = ? x P
where R = the pier ratio H = the height of the pier W = the load width P = the percent openness where the percent openness is substantially equal to the open area under the load divided by the total area under the load.
the pier ratio is greater than 18%, the pier ratio being substantially equal to:
R = ? x P
where R = the pier ratio H = the height of the pier W = the load width P = the percent openness where the percent openness is substantially equal to the open area under the load divided by the total area under the load.
12. The apparatus as recited in claim 11 wherein the jets are spaced so that the jet streams just impinge upon one another at the top of the load with the jet having a jet spread angle in the range of 20 to 30 degrees.
13. The apparatus as recited in claim 11 wherein the jets are arranged in at least one colinear row parallel to the longi-tudinal axis of the furnace.
14. The apparatus as recited in claim 13 wherein the furnace has four zones; Zone 1, Zone 2, Zone 3 and Zone 4, each zone having two rows of tubes with the venturi jet nozzles of each tube about 12 inches above the load and, Zone 1 having at least 5 jets in each row, Zone 2 having at least 8 jets in each row, Zone 3 having at least 5 jets in each row, and Zone 4 having at least 6 jets in each row.
15. A method for injection of atmosphere into a positive pressure furnace chamber containing a load supported on a load support means, comprising the steps of:
injecting atmosphere at high velocity and low momentum into the chamber of the furnace from a plurality of low 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 temperature treatment of the load.
injecting atmosphere at high velocity and low momentum into the chamber of the furnace from a plurality of low 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 temperature treatment of the load.
16. The method as recited in claim 15 wherein the furnace is a continuous carburizing furnace and a load tray is removably set on the load support means and the atmosphere is injected with a minimum momentum of .007 lbs/ft2-tray and the minimum momentum in the free area is greater than .016 lbs/ft2-tray where the free area is the space near the ceiling of the furnace chamber free of obstructions.
17. The method as recited in claim 15 wherein the pressure drop across each nozzle is between 3 and 6 psig.
18. A method of injection of carburizing atmosphere into the furnace chamber of a continuous positive pressure carburizing furnace having four zones; Zone 1, Zone 2, Zone 3 and Zone 4, comprising the steps of:
injecting carburizing gas into Zone 1 through 10 jets with a momentum per jet of about .020 lbs. and a flow of about 50 scfh per jet;
injecting carburizing gas into Zone 2 through 16 jets with a momentum per jet of about .015 lbs. and a flow of about 39 scfh per jet;
injecting carburizing gas into Zone 3 through 10 jets with a momentum per jet of about .019 lbs. and a flow of about 50 scfh per jet;
injecting carburizing gas into Zone 4 through 12 jets with a momentum per jet of about .025 lbs. and a flow of about 66.7 scfh per jet.
injecting carburizing gas into Zone 1 through 10 jets with a momentum per jet of about .020 lbs. and a flow of about 50 scfh per jet;
injecting carburizing gas into Zone 2 through 16 jets with a momentum per jet of about .015 lbs. and a flow of about 39 scfh per jet;
injecting carburizing gas into Zone 3 through 10 jets with a momentum per jet of about .019 lbs. and a flow of about 50 scfh per jet;
injecting carburizing gas into Zone 4 through 12 jets with a momentum per jet of about .025 lbs. and a flow of about 66.7 scfh per jet.
19. The method as recited in claim 18 wherein the pressure drop across the jets are between 3 and 6 psig.
20. A gas carburizing furnace, comprising:
(a) a horizontally elongated chamber defined by a vertically lowermost hearth spaced from an opposing vertically uppermost ceiling and a pair of vertically upstanding sidewalls connecting the hearth and ceiling;
(b) a plurality of upstanding piers extending from the hearth in the direction of the ceiling for supporting a plurality of rails which extend longitudinally of the chamber in a generally horizontal plane, the rails designed to support a load in the chamber;
(c) a manifold disposed outside the chamber and extending longitudinally, thereof adjacent the ceiling only, the manifold including a plurality of tubes communicating therewith and extending therefrom downwardly through the ceiling and terminating at a plurality of venturi jet nozzles which are vertically spaced above the rails and designed to direct high velocity, low momentum jets of carburizing gas against a load position on the rails, the nozzles being designed such that the jets of gas directed therefrom have a spread angle in the range of from about 20° to about 30°, and the gas pressure drop across each of the nozzles is about 3 to about 6 psig;
(d) means for circulating a carburizing gas from the manifold through the tubes and jet nozzles such that the carburizing gas impinges against a load, positioned on the rails and generally uniformly penetrates the load to effect a mass transfer of carbon to the load; and (e) means disposed in the chamber for heating the gaseous atmosphere therein.
(a) a horizontally elongated chamber defined by a vertically lowermost hearth spaced from an opposing vertically uppermost ceiling and a pair of vertically upstanding sidewalls connecting the hearth and ceiling;
(b) a plurality of upstanding piers extending from the hearth in the direction of the ceiling for supporting a plurality of rails which extend longitudinally of the chamber in a generally horizontal plane, the rails designed to support a load in the chamber;
(c) a manifold disposed outside the chamber and extending longitudinally, thereof adjacent the ceiling only, the manifold including a plurality of tubes communicating therewith and extending therefrom downwardly through the ceiling and terminating at a plurality of venturi jet nozzles which are vertically spaced above the rails and designed to direct high velocity, low momentum jets of carburizing gas against a load position on the rails, the nozzles being designed such that the jets of gas directed therefrom have a spread angle in the range of from about 20° to about 30°, and the gas pressure drop across each of the nozzles is about 3 to about 6 psig;
(d) means for circulating a carburizing gas from the manifold through the tubes and jet nozzles such that the carburizing gas impinges against a load, positioned on the rails and generally uniformly penetrates the load to effect a mass transfer of carbon to the load; and (e) means disposed in the chamber for heating the gaseous atmosphere therein.
21. The furnace of claim 20, wherein the carburizing gas circulating means and jet nozzles cooperate to produce a momentum flux per square foot of area on which a load is positioned, of at least .007 pounds.
22. The furnace of claim 21, wherein the momentum flux per square foot at the load is at least .016 pounds.
23. The furnace of claims 20 or 22, which includes means for dividing the chamber longitudinally into a plurality of zones, each of which zones has its own manifold and nozzle arrangement which is specifically designed to carry on a certain treatment of the load within a particular zone.
24. The apparatus of claims 20 or 22, which includes means for dividing the chamber longitudinally into four successive zones each of which zones has its own manifold and nozzle arrangement, and the means for circulating carburizing gas from the manifold to the nozzle includes:
(I) means for circulating carburizing gas into zone 1 through ten venturi jet nozzles at a momentum flux per jet of about .02 pounds and at a flow rate of about 50 scfh;
(II) means for circulating carburizing gas into zone 2 through sixteen venturi jet nozzles at a momentum flux per jet of about .015 pounds and at a flow rate of about 39 scfh;
(III) means for circulating carburizing gas into zone 3 through ten jets at a momentum flux per jet of about .019 pounds and at a flow rate of about 50 scfh; and (IV) means for circulating carburizing gas into zone 4 through twelve venturi jet nozzles at a momentum flux per jet of about .025 pounds and at a flow rate of about 66.7 scfh.
(I) means for circulating carburizing gas into zone 1 through ten venturi jet nozzles at a momentum flux per jet of about .02 pounds and at a flow rate of about 50 scfh;
(II) means for circulating carburizing gas into zone 2 through sixteen venturi jet nozzles at a momentum flux per jet of about .015 pounds and at a flow rate of about 39 scfh;
(III) means for circulating carburizing gas into zone 3 through ten jets at a momentum flux per jet of about .019 pounds and at a flow rate of about 50 scfh; and (IV) means for circulating carburizing gas into zone 4 through twelve venturi jet nozzles at a momentum flux per jet of about .025 pounds and at a flow rate of about 66.7 scfh.
25. The apparatus of claim 20 or 22 wherein the gas heating means (e) includes a plurality of radiant heaters disposed adjacent the ceiling and hearth.
CLAIM SUPPORTED BY SUPPLEMENTARY DISCLOSURE
CLAIM SUPPORTED BY SUPPLEMENTARY DISCLOSURE
26. The apparatus of claims 20 or 22, wherein the tubes and nozzles within the chamber have an aluminum diffusion coating on the outer surfaces thereof exposed to the surrounding gaseous atmosphere within the chamber.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5632679A | 1979-07-10 | 1979-07-10 | |
| US056,326 | 1979-07-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1145232A true CA1145232A (en) | 1983-04-26 |
Family
ID=22003671
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000354092A Expired CA1145232A (en) | 1979-07-10 | 1980-06-16 | Atmosphere injection system |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS5616666A (en) |
| CA (1) | CA1145232A (en) |
| MX (1) | MX154174A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102352478B (en) * | 2011-10-31 | 2013-02-20 | 北京机电研究所 | Automatic telescopic carburizing gas nozzle device of vacuum low pressure carburizing device |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS50159807A (en) * | 1974-06-18 | 1975-12-24 |
-
1980
- 1980-06-16 CA CA000354092A patent/CA1145232A/en not_active Expired
- 1980-07-09 JP JP9378280A patent/JPS5616666A/en active Pending
- 1980-07-10 MX MX18311180A patent/MX154174A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| MX154174A (en) | 1987-06-01 |
| JPS5616666A (en) | 1981-02-17 |
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