US3779532A

US3779532A - Convection heating system for a tempering furnace

Info

Publication number: US3779532A
Application number: US00128978A
Authority: US
Inventors: H Mescher
Original assignee: Pacific Scientific Co
Current assignee: Pacific Scientific Co
Priority date: 1971-03-29
Filing date: 1971-03-29
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18
Also published as: JPS5140850B1; GB1381667A

Abstract

Within an insulated furnace chamber, a blower circulates heated gasses axially through a cylindrical rotary retort by way of large holes in opposite ends of the retort. A baffle wall near the blower directs the gas flow in the proper direction. An inlet in the entrance end of the retort opens to the exterior of the furnace for introducing the parts to be heated, and a guide within the retort prevents the parts from falling through the forward gas circulation holes. The holes in the discharge end of the retort cooperate with a chute in the wall of the furnace to serve as discharge holes for the heated metal parts.

Description

1111, 3,779,532 Dec. 18, 1973 CONVECTION HEATING SYSTEM FOR A TEMPERING FURNACE [75] Inventor: Harold E. Mescher, Pico Rivera,

Calif.

[73] Assignee: Pacific Scientific Company, Commerce, Calif.

22 P111311; Mar. 29, 1971 21 Appl. NO.Z 128,978

Blasier 266/5 R X 2,877,562 3/1959. Krantz 263/33 R X 3,441,259 4/1969 Heyer 263/34 1,841,625 l/l932 Musso 266/18 X 255,925 4/1882 Breer 263/34 1,314,849 9/1919 Bassett 263/34 Primary ExaminerRobert D. Baldwin Assistant Examiner John S. Brown AltorneyFowler, Knobbe & Martens 57 ABSTRACT Within an insulated furnace chamber, a blower circulates heated gasses axially through a cylindrical rotary retort by way of large holes in opposite ends of the retort. A baffle wall near the blower directs the gas flow in the proper direction. An inlet in the entrance end of the retort opens to the exterior of the furnace for introducing the parts to be heated, and a guide within the retort prevents the parts from falling through the forward gas circulation holes. The holes in the discharge end of the retort cooperate with a chute in the wall of the furnace to serve as discharge holes for the heated metal parts.

11 Claims, 3 Drawing Figures PATENTEDDEB 18 ms 3.779.532 SHEET 2 UP 2 MW lr P v M INVENTOR, HAEOL D .5. MESCMEE BY FOWL 5?, M0555 a MAETEA/S flTTOENEVS.

CONVECTION HEATING SYSTEM FOR A TEMPERING FURNACE This invention relates generally to heat treating apparatus, and more particularly to a rotary retort heat treating furnace having a novel construction for recirculating convection heating.

To attain efficient and uniform heat treating of small and odd shaped parts such as nuts, bolts, and similar work, it is desirable that the parts be fed continuously into the furnace and be moved towards a discharge point while being heated. One way of accomplishing this has been to utilize a cylindrical rotary retort positioned within a heating chamber and having a spiral conveyor for moving the work from an entrance end of the retort to a discharge end. Various systems have been developed for sealing the junctions between the retort and the surrounding furnace housing so that a controlled'atmosphere can be maintained within the retort. Such an atmosphere may require readings as high as 1,250 F, and typically the lowest maximum temperature specified is800 F.

It is mandatory that temperature be exact and consistent in that the desired hardness characteristics of the finished work can be obtained only by a precise temperature control. A'toler'ance of plus or minus 25 F for the discharge work is a minimum requirement, and plus or minus F is commonplace.

In continuous processing furnaces this can be accomplished in at least two ways. One way is to provide multiple temperature control zones along the length of the furnace, each with its own control instruments and combustion or electricheating zones. Such an arrangement requires large amounts of heat at the entrance to the furnace to bring cold work up to temperature, and much smaller amounts of heat to maintain an exact temperature in subsequent zones. Due to the duplication of control systems, this approach is relatively expensive.

An alternate system which heretofore has been utilized in nonrotating retort type furnaces utilizes axial convection so that the heated circulated gasses are passed over the work load with the direction of flow contrary to that of the work. Thus the hottest gasses are at the discharge end. The temperature of the circulated gasses is precisely controlled as it leaves the heating system and then passed'over the work closer to the discharge end first. This insures that the correct discharge temperature can be accurately maintained. As the gasses progress toward the cold incoming work load, they give up their heat to the work load. The gasses then are drawn through the heating system and are brought back to the precise discharge temperature once again before being returned to the discharge end. Such a design is commonplace in nonrotating conveyor furnaces and other similar furnaces but heretofore this principle has not been applied to a rotary retort type furnace because of the difficulties of trying to recirculate the heated gasses and because of the cost of the system if the heated air were not recirculated.

Convection heating has been used in previous rotary retort tempering furnaces with perforated retorts and with convection heating occurring vertically through the perforations in the retort. While this system has advantages concerning circulation of the heated gasses, there are several disadvantages. Perforated alloy materials are very expensive. Small holes tend to obstruct the circulationof the heated gasses while large holes prohibit processing small parts. Also, this design requires multiple zoning at extra cost.

In accordance with this invention, a novel rotary retort furnace is provided utilizing recirculation of heated gasses and axial flow convection heating of the parts within the retort. This is accomplished by utilizing imperforate walls on the retort except for large holes near the ends. The holes in the discharge end of the retort also serve as holes for discharging the heated parts from the retort. The work is introduced into the retort through a separate inlet in the entrance end, with a guide being provided to prevent the parts from falling through the air circulation holes in the side wall of the entrance end. The rotary retort if supported by means positioned outside of the insulated walls of the furnace housing while the means for heating and circulating the air are positioned within the insulated furnace housing. The circulating means includes a blower and baffle wall which directs the air to move in the proper direction.

For a more thorough understanding of the invention, refer now to the detailed description and drawings in which:

FIG. 1 is an elongated cross-sectional view of the furnace of the invention;

FIG. 2 is a cross-sectional view of the furnace on line 22 of FIG. 1 on the axis of the blowers; and

FIG. 3 is an elevational view of the entrance end of the furnace.

Referring now to the drawings, it may be seen that the furnace comprises an elongated housing 10 having a generally rectangular cross-section defining a similarly shaped chamber 14. The housing wall includes an exterior metal casing 11 and a heavy insulated section 12. Within the upper portion of the chamber 14 is an elongated substantially cylindrical hollow retort 16 having an entrance end wall 16a, a discharge end wall 16b, and a cylindrical side wall with a spiral conveyor formed on its interior surface for moving parts to be heat treated.

The entrance end wall 16a, which is heavily insulated, extends through the entrance wall 10a of the furnace housing and is rotatably supported by a pair of rollers 19 mounted on the furnace exterior, as may be seen from'FIGS. l and 3.'The central portion of this end wall 10a defines an inlet 20 through which extends an entry chute'2l from adjacent apparatus for feeding the parts to be heat treated into the retort. A gravity operated door 21a on the end of the chute remains closed except when parts are entering. Surrounding the chute is a seal 22 which restricts air leakage into or out of the retort.

The imperforate discharge end wall 16b has a shaft 23 attached thereto and extending outwardly through the discharge or rear end wall 10b of the furnace housing. The shaft 23 is supported by a bearing 24 attached to the furnace housing wall 10b and is connected to a drive means 26 for rotating the retort l6.

Near the entrance end of the retort 16, in the cylindrical wall 160, is formed a series of circumferentially spaced large diameter holes 28. A frusto-conical shaped shield or guide 32 is positioned in the entrance end of the retort with its smaller end 32a facing the inlet 20 but spaced rearwardly from the entrance end wall 16a. The larger diameter end 32b of the guide 32 is attached to the cylindrical wall 160 of the retort immediately to the rear of the holes 28. As a result the shield side wall is spaced inwardly fromthe circulation holes so that parts to be heated are introduced through the inlet chute 21 directly into the forward end of the shield and thus can not drop through the circulating holes 28.

A series of circumferentially spaced large diameter holes 30, similar to the forward holes 28, are formed in the cylindrical wall 160 near the rear end wall 16b. A discharge chute 34 extends through the lower wall c of the housing near the discharge end of the retort. Note that this lower wall 100 is spaced upwardly from the main portion 10d of the lower wall of the retort which rests on a supporting surface. The discharge chute 34 is axially aligned with the discharge holes 30 so that as the holes pass the discharge chute, any heat treated parts arriving at that point can fall through a discharge hole and into the chute 34. A small hopper 36 is attached to the exterior wall 10c of the furnace, aligned with the chute 34, and a flapper door 38 is mounted to normally close an outlet from the hopper. An actuator 40 for opening the flapper door 38 is shown mounted on the furnace housing wall 10c. A vertically oriented tubular extension 42 is positioned beneath the outlet of the hopper 36 to guide the parts downwardly into a waiting tray or dolly (not shown), which'may be conveniently placed beneath the extension 42. V I

Within the lower portion of the furnace front wall 10a is positioned a burner 43 which receives a fuel and air mixture through a conduit 44. The flame from the burner extends in the chamber 14 and is protected from adjacent air currents by a surrounding tubular shield 45.

Towards the rear of the chamber 14 adjacent the vertical wall 10c are a pair of centrifugal blowers 46 provided for recirculating the heated combustion gasses within the furnace chamber. As may be best seen from FIG. 2, the blowers 46 are spaced from each other and located adjacent the side walls 10f and g of the housing, with the drive shaft 47 for the blowers 46 extending outwardly to be driven by a belt 49 to a pair of motors 50 so that the drives themselves are not subjected to the high temperatures within the furnace. Each blower 46 includes an axially directed inlet 46a and a centrifugal outlet 46b which is directed upwardly toward the retort. A

A thermostat 56 for controlling temperature within the chamber is positioned in the outlet 46b of the blower and a second thermostat 58 is positioned near the intake 46a of the blower.

A baffle wall 52 surrounds the rotary retort 16 extending between the cylindrical wall 160 of the retort and the furnace housing 10. The lower edge of the baffle wall 52 engages the casings 46c surrounding the blowers. A forward baffle wall 54 is positioned near the entrance end of the retort positioned slightly to the rear of the circulating holes 28. The forward baffle wall 54 surrounds the rotary retort 16 and terminates spaced upwardly from the flame shield 45.

From the foregoing, the operation of the furnace is probably apparent, but it will be summarized at this point. The work, typically comprising small metal parts, to be heat treated is introduced through the inlet chute 21 in the inlet of the retort into the small end 32a of the shield 32 so that the parts are fed into the conveyor portion of the retort. The retort is slowly rotated, causing the conveyor spiral 18 to slowly move the parts from the entrance end to the discharge end where the parts fall through the discharge holes 30. The parts fall through the chute 34 in the furnace housing wall and collect in the hopper 36 beneath the chute. Periodically, the flapper door 38 is automatically opened by the actuator 40, such as for each revolution of the retort, so that the heat treated parts fall from the furnace into a suitable receptacle.

While the parts are being slowly moved through the furnace, they are being subjected to convection heating by the heated gasses as they are moved by the blowers 46 in the direction indicated by the arrows. As can be seen, the heated gasses are drawn from the forward end of the chamber 14 into the inlet 46a of the blower and directed centrifugally outwardly and upwardly through the chamber space between the retort and the housing walls and into the discharge holes 30 where they are directed axially through the retort in a direction opposite to that of which the parts are moving. Upon reaching the forward end of the retort, the gasses are drawn outwardly through the circulation holes 28 and recirculated past the flame shield 45 to the blower 46. Note that the rear baffle wall 52 directs the blower output to the holes 30 and prevents the blower output from being recirculated directly back to its inlet 46a. The forward baffle wall 54 directs the recirculated gasses back towards the flame shield 45 to improve the heat transfer.

There are several advantageous features of the furnace which should be carefully noted. Providing the circulation holes and the baffles enables the desired recirculation of heated gasses to be obtained. The mounting means for the retort means must of necessity be outside the heated chamber since the temperatures within the chamber 14 typically are at least 800 F and often times are considerably higher. The temperature of the circulated gasses is precisely controlled by the thermostat 56 as they leave the blower and are then passed over the work closer to the discharge end first. This insures that the correct discharge temperature for the heated parts can be accurately maintained.

Because of the precision temperature control requirements, that is of maintaining temperatures of plus or minus-10, itis essential that the tempering furnace be sealed against the unwarranted invasion of outside room air which might chill the work. Thus, the dis-' charge end of the furnace is normally sealed, the flapper door 38 controlling the discharge chute being open only for a few seconds per revolution of the retort. Since the chute 36 is in a positive pressure area, close to the discharge of the circulating blowers 46, any leakage is outward rather than inward. Thus no cold air is drawn in, and the critical temperature can be maintained within the rotary retort.

Gas flow is also restricted in the forward end of the retort since the entrance chute 21 is open only when parts are entering the retort. The chute provides considerable equalization of pressure without appreciable leakage. Any pressure build-up caused by the introduction of combustion products through the burner and not handled by the chute 21, is vented through a flue 60 in the forward position of the top wall 10f of the furnace. A gravity controlled flapper 62 allows flow in through the flue 60 only the outward direction.

I claim: A

1. A heat treating furnace comprising:

an insulated furnace housing defining a heating chamber;

means defining holes in the exterior walls of the retort near opposite ends of the retort, the holes being within the heating chamber and arranged to interconnect the interior of the retort with the space in the chamber surrounding the retort, the retort being substantially imperforate between the holes in its opposite ends;

means for heating said chamber;

blower means in the chamber adjacent the retort arranged to continuously, positively recirculate the heated gasses within the chamber in'only one direction through said retort by way of said holes;

an inlet for permitting the continuous introduction of work to be heated into the entrance end of the retort; conveyor means within the retort for moving the work from the entrance end to the discharge end of the retort; and

discharge means for permitting the withdrawal of heated work from the retort and the furnace, the heated gasses being directedthrough the retort in a direction opposite to that of the work.

' 2. The furnace of claim 1 wherein the conveyor means comprises a spiral conveyor formed on the inner wall of the retort, the spiral commences to the rear of the holes in the forward portion of the retort.

3. The furnace of claim 1 wherein the discharge means for permitting the work to be withdrawn from the retort comprises the holes in the rear of the retort which also permit the circulation of the heated gasses into the retort.

4. The furnace of claim 1 wherein said recirculating means comprises a blower within the chamber and baffle means for directing the blower output towards the holes in one end of the retort so that the input to the blower must be drawn through the holes from the other end of the retort.

5. A heat treating furnace comprising:

an insulated furnace housing defining a heating chamber;

a hollow rotatably mounted retort positionedwithin the housing for receiving parts to be heat treated,

means defining holes in the walls of the retort near' opposite ends of the retort, the holes being within the heating chamber, the retort being substantially imperforate between the holes in its opposite ends;

means for heating said chamber;-

means for continuously, positively recirculating the heated gasses within the chamber in only one direction through said retort by way of said holes;

the retort has substantially a cylindrical shape and the forward end of the retort extends through the wall of the housing, with the holes in the forward end of the retort being in the cylindrical wall and positioned within the housing, and said holes near the rear end of .the retort comprise a series of circumferentially spaced holes formed-in'the cylindrical wall of the retort, said latter holes being sufficiently large to permit the heated parts to fall from the retort;

means defining an inlet formed in the forward end wall of the retort for introducing into the retort the work to be heated; and

means defining an outlet in the wall of the housing which cooperates with the holes in the rear end of the retort for permitting the heated parts to be removed from the furnace.

6. A heat treating furnace comprising:

an insulated furnace housing defining a heating chamber;

a hollow rotatably mounted retort positioned within the housing for receiving parts to be heat treated, means defining holes in the walls of the retort near opposite ends of the retort, the holes being within the heating chamber, the retort being substantially imperforate between the holes in its opposite ends;

means for heating said chamber;

said retort is substantially cylindrical and said housing has a substantially rectangular cross-section, the width of the housing being slightly larger than the diameter of the retort whereas the height of the housing is considerably greater than the diameter of the retort and the retort is positioned in the upper portion of the heating chamber;

said heating means is located in the lower forward end of the furnace; and

said recirculating means includes a blower located in the lower portionof the chamber to the rear of said heating means, and said recirculating means includes a baffle wall extending between the cylindrical retort and the adjacent housing walls, the baffle wall further cooperating with the blower so that the blower output is directed toward the rear end of the retort while the blower input is drawn from the forward end of the retort passed the heating means, with the result that the heated gasses are circulated axially through the retort in one direction and circulated through the lower portion of the chamber adjacent the retort in the opposite direction.

7. A heat treating furnace comprising:

an insulated furnace housing defining a heating chamber;

a hollow substantially cylindrical retort extending within said chamber, the forward end of the retort extending through a wall of the housing, rneans positioned exterior of the housing for rotatably supporting the forward end of the retort;

a shaft attached to the rear of the retortand extending through the wall of the housing, means positioned exterior of the housing rotatably supporting the shaft; v

means defining aninlet in the forward end wall of the means defining a' series of circumferentially spaced holes in the cylindrical wall of the retort adjacent the rear end of the retort for allowing the heat treated parts to fall from the retort;

chute means defining an outlet in the wall of the housing cooperating with the retort outlet holes to allow the parts to fall "from the furnace chamber; a door closing the outlet chute to restrict air flow through the chute, the door to be periodically opened to permit a quantity of parts to fall from the chute;

means defining circulation holes in the cylindrical wall of the retort near the forward end of the retort but within the chamber, the cylindrical wall of the retort being substantially imperforate between the circulation holes and said holes near the rear end of the retort;

an inlet guide attached to the wall of the retort cooperating with the inlet in the forward wall of the retort to guide the parts into the retort towards the spiral conveyor while preventing parts from falling through the circulation holes, the guide being mounted to permit gas circulation between the interior of the retort and the surrounding chamber by way of the circulation holes;

means for heating the chamber;

blower means in the chamber for circulating the heated gasses within the chamber; and

baffle wall means extending between the retort and the chamber walls and cooperating with the blower means to direct the circulation of the heated air from the outlet of the fan to the holes in the rear of the retort, through the retort in a direction opposite to the movement of the parts through the retort, and out of the retort through its forward circulation holes back to the blower means inlet.

8. The furnace of claim 7 wherein said means for heating the chamber is positioned beneath the forward end of the retort, and said blower means is positioned beneath the retort rearwardly from the heating means, and the means for driving the blower means is located to the exterior of the furnace housing.

9. The furnace of claim 7 including thermostat means mounted in the blower output and connected to the heat source for maintaining the output temperature precisely controlled.

10. The furnace of claim 7 wherein said means for heating the chamber and said blower means are located in the lower portion of said chamber beneath the retort, the housing wall surrounding said chute means being spaced above the housing wall beneath the blower means to permit convenient access to the outlet of the chute means.

11. The furnace of claim 7 wherein said guide has a frusto-conical shape having its larger end attached to the cylindrical wall of the retort and its smaller end extending towards but spaced from the forward end of the retort, and the circulation holes nearthe forward end of the retort comprise a series of circumferentially spaced openings that are positioned radially outwardly from the frusto-conical guide so that parts being introduced into the retort may be fed into the guide and will not fall through the circulation holes in the retort.

Claims

1. A heat treating furnace comprising: an insulated furnace housing defining a heating chamber; a hollow rotatably mounted retort positioned within the housing for receiving parts to be heat treated, means defining holes in the exterior walls of the retort near opposite ends of the retort, the holes being within the heating chamber and arranged to interconnect the interior of the retort with the space in the chamber surrounding the retort, the retort being substantially imperforate between the holes in its opposite ends; means for heating said chamber; blower means in the chamber adjacent the retort arranged to continuously, positively recirculate the heated gasses within the chamber in only one dirEction through said retort by way of said holes; an inlet for permitting the continuous introduction of work to be heated into the entrance end of the retort; conveyor means within the retort for moving the work from the entrance end to the discharge end of the retort; and discharge means for permitting the withdrawal of heated work from the retort and the furnace, the heated gasses being directed through the retort in a direction opposite to that of the work.

2. The furnace of claim 1 wherein the conveyor means comprises a spiral conveyor formed on the inner wall of the retort, the spiral commences to the rear of the holes in the forward portion of the retort.

5. A heat treating furnace comprising: an insulated furnace housing defining a heating chamber; a hollow rotatably mounted retort positioned within the housing for receiving parts to be heat treated, means defining holes in the walls of the retort near opposite ends of the retort, the holes being within the heating chamber, the retort being substantially imperforate between the holes in its opposite ends; means for heating said chamber; means for continuously, positively recirculating the heated gasses within the chamber in only one direction through said retort by way of said holes; the retort has substantially a cylindrical shape and the forward end of the retort extends through the wall of the housing, with the holes in the forward end of the retort being in the cylindrical wall and positioned within the housing, and said holes near the rear end of the retort comprise a series of circumferentially spaced holes formed in the cylindrical wall of the retort, said latter holes being sufficiently large to permit the heated parts to fall from the retort; means defining an inlet formed in the forward end wall of the retort for introducing into the retort the work to be heated; and means defining an outlet in the wall of the housing which cooperates with the holes in the rear end of the retort for permitting the heated parts to be removed from the furnace.

6. A heat treating furnace comprising: an insulated furnace housing defining a heating chamber; a hollow rotatably mounted retort positioned within the housing for receiving parts to be heat treated, means defining holes in the walls of the retort near opposite ends of the retort, the holes being within the heating chamber, the retort being substantially imperforate between the holes in its opposite ends; means for heating said chamber; means for continuously, positively recirculating the heated gasses within the chamber in only one direction through said retort by way of said holes; said retort is substantially cylindrical and said housing has a substantially rectangular cross-section, the width of the housing being slightly larger than the diameter of the retort whereas the height of the housing is considerably greater than the diameter of the retort and the retort is positioned in the upper portion of the heating chamber; said heating means is located in the lower forward end of the furnace; and said recirculating means includes a blower located in the lower portion of the chamber to the rear of said heating means, and said recirculating means includes a baffle wall extending between the cylindrical retort and the adjacent housing walls, the baffle wall further cooperating with the blower so that the blower output is directed toward the rear end of the retort while the blower input is drawn from the forward end of the retort passed the heating means, with the result that the heated gasses are circulated axially through the retort in one direction and circulated through the lower portion of the chamber adjacent the retort in the opposite direction.

7. A heat treating furnace comprising: an insulated furnace housing defining a heating chamber; a hollow substantially cylindrical retort extending within said chamber, the forward end of the retort extending through a wall of the housing, means positioned exterior of the housing for rotatably supporting the forward end of the retort; a shaft attached to the rear of the retort and extending through the wall of the housing, means positioned exterior of the housing rotatably supporting the shaft; means defining an inlet in the forward end wall of the retort for introducing the parts to be heated; a spiral conveyor formed on the inner wall of the retort for moving the parts from the forward end of the retort to the rear end; means defining a series of circumferentially spaced holes in the cylindrical wall of the retort adjacent the rear end of the retort for allowing the heat treated parts to fall from the retort; chute means defining an outlet in the wall of the housing cooperating with the retort outlet holes to allow the parts to fall from the furnace chamber; a door closing the outlet chute to restrict air flow through the chute, the door to be periodically opened to permit a quantity of parts to fall from the chute; means defining circulation holes in the cylindrical wall of the retort near the forward end of the retort but within the chamber, the cylindrical wall of the retort being substantially imperforate between the circulation holes and said holes near the rear end of the retort; an inlet guide attached to the wall of the retort cooperating with the inlet in the forward wall of the retort to guide the parts into the retort towards the spiral conveyor while preventing parts from falling through the circulation holes, the guide being mounted to permit gas circulation between the interior of the retort and the surrounding chamber by way of the circulation holes; means for heating the chamber; blower means in the chamber for circulating the heated gasses within the chamber; and baffle wall means extending between the retort and the chamber walls and cooperating with the blower means to direct the circulation of the heated air from the outlet of the fan to the holes in the rear of the retort, through the retort in a direction opposite to the movement of the parts through the retort, and out of the retort through its forward circulation holes back to the blower means inlet.

11. The furnace of claim 7 wherein said guide has a frusto-conical shape having its larger end attached to the cylindrical wall of the retort and its smaller end extending towards but spaced from the forward end of the retort, and the circulation holes near the forward end of the retort comprise a series of circumferentially spaced openings that are positioned radially outwardly from the frusto-conical guide so that parts being introduced into the retort may be fed into the guide and will not fall through the circulation holes in the retort.