US3860221A

US3860221A - Direct fired furnace

Info

Publication number: US3860221A
Application number: US356402A
Authority: US
Inventors: Klaus H Hemsath; Harry Dean Schmidt; Arvind C Thekdi
Original assignee: Midland Ross Corp
Current assignee: Surface Combustion Corp; Grimes Aerospace Co
Priority date: 1973-05-02
Filing date: 1973-05-02
Publication date: 1975-01-14
Anticipated expiration: 1992-01-14
Also published as: FR2228205A1; DE2420758A1; JPS5220166B2; CA1021529A; JPS5014533A

Abstract

A direct fired furnace having two or more confluent chambers with means for segregating the atmosphere in the respective chambers. Each chamber has high velocity burners placed above the work to create a strong convection pattern. A gas passageway is located beneath the work with a central flue located intermediate the chambers, which central flue may lead to an incinerator.

Description

Unite States atet r191 Hemsath et al.

[ Jan. 14, 1975 4] DIRECT FIRED FURNACE [75] Inventors: Klaus H. Hemsath; Harry Dean Schmidt, both of Sylvania; Arvind C. Thekdi, Toledo, all of Ohio [73] Assignee: Midland-Ross Corporation,

Cleveland, Ohio [22] Filed: May 2, 1973 [21] Appl. No.: 356,402

[52] US. Cl. 266/5, 432/128 [51] Int. Cl C2ld 1/08 [58] Field of Search 266/5 R, 5 F, 5 T;

[56] References Cited UNITED STATES PATENTS 2,886,303 5/1959 Rusciano 266/5 3,637,198 1/1972 Knaak 432/128 Primary ExaminerRoy Lake Assistant Examiner-DeWalden W. Jones Attorney, Agent, or FirmPeter Vrahotes; Frank J. Nawalanic [57] ABSTRACT A direct fired furnace having two or more confluent chambers with means for segregating the atmosphere in the respective chambers. Each chamber has high velocity burners placed above the work to create a strong convection pattern. A gas passageway is located beneath the work with a central flue located intermediate the chambers, which central flue may lead to an incinerator.

7 Claims, 2 Drawing Figures DIRECT FIRED FURNACE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to directly heated furnaces and, more specifically, to furnaces wherein confluent chambers are present which require a segregation of atmospheres. An example of such a furnace is one which is used to dewax and cure metal casting molds.

2. Description of Prior Art In certain furnace operations, it is desirable to exercise atmosphere control over adjacent and confluent zones. As an example of such furnaces, in U.S. Pat. No. 3,189,336, issued to .I. Montagino, a furnace is shown wherein a double arch construction yields a buffer zone that results in segregation of adjacent confluent zones. Also, in U.S. Pat. No. Re. 26,935, issued to O. E. Cullen, a furnace is shown wherein zone control is exercised again through use of double arches and selected flue locations. Neither of these furnaces, however, is a directly heated furnace, each one being heated by radiant tubes. Additionally, each of these furnaces requires the use of fans for the purpose of circulating the atmosphere within each of their treating zones. To the applicants knowledge, no direct fired furnace has heretofore successfully accomplished the segregation of furnace zones. A particular application in which such a directly heated furnace having zone control is applicable is in the field of dewaxing and curing of metal casting molds. Prior art furnaces would burn the wax in a first zone and pass the work into a second zone wherein the work would then be cured, there being an interchange of atmosphere between such zones. There were two disadvantages to this prior art method in that the burned waxes would accompany the work into the second chamber, thereby interfering with the results achieved, and the off-gases would be extremely smokey, a condition that is no longer permissible under antipollution regulations.

SUMMARY OF THE INVENTION This invention is demonstrated by its application with regard to a dewaxing furnace, but it will be understood that the invention has application with other types of furnaces as well. A standard method for fabricating metal castings is to make a wax model of the part to be cast, then dip the wax model into a slurry of ceramic material. After the slurry is cured, the wax model is eliminated in a dewaxing furnace. The dewaxing furnace illustrated herein has two chambers, a low temperature chamber and a second or high temperature chamber. There is zone separation without internal doors through the use of a drop arch and burner and flue placement. Proper operation of the furnace results in atmosphere control to eliminate uncontrolled burning of wax within the furnace. Instead, the wax is melted and vaporized, as opposed to being melted and burned, and the wax is removed from the furnace without entering into the second zone. After the wax is removed, the molds are conveyed into the second, or higher temperature, zone wherein the heat treating and curing takes place. Circulation of the atmosphere within each of the zones is accomplished by burners which exercise a high degree of entrainment. The burners are located above the work and a flue system is located below the work, thereby creating circulation of the atmosphere within each zone which yields uniform gas temperatures. The circulating flow pattern thus achieved results in uniform heat treatment of the work. Obviously, another advantage to having segregated zones is that greater temperature control is achieved by preventing the gas in the first zone from entering the second zone.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, a direct fired heat treating furnace is shown generally at 10 and has a housing comprising a refractory roof l2, refractory side walls .14, front and back end walls l6, 18, respectively, and a floor 20. The front end wall 16 has a charge opening 22 therein and a slidable door 24 is disposedadjacent this opening. At the upper end of the door 24 is an attaching means 26 which provides communication between the door and door actuating means 28 which includes a piston 30, pulley 32 and chain 34, said actuating means being operative to lift and lower the door over the opening 22. Dispersed along the floor 20 of the furnace 10 are a plurality of bricks 38 which support platform member 40 that receives laterally spaced rows of longitudinally spaced rollers 42 therein which define roller conveying means. The platform member 40 has openings 37 at each end thereof. The combination of the openings 37, open brickwork 38 and the platform 40 yields a gas passageway 41 located beneath the conveying means of the furnace which is part of an overall flue system. workpieces 43, or work trays, may be placed upon the rollers and conveyed from one end of the furnace 10 to the other.

Forming a part of the roof 12 is a drop arch member 46 having a bottom wall 48 and side walls 50. Each of the drop arch side walls 50 has at least one opening 52 therein and the drop arch 46 segregates the furnace into a first chamber 54, a buffer zone 56 located immediately below the drop arch and a second chamber 58. The first and second chambers have thermocouples 55 and 57 located therein respectively. Received within each opening of the side walls 50 is a direct fired burner 59. Each burner 59 has an annular burner housing 60 with a central opening 62 therein that defines a combustion chamber. Secured to the outside of each side wall 50 and confluent with the combustion chamber 62 is a gas mixing housing 64, there being a gas pipe 66 and an air pipe 68 secured to the housing. Ignition means 70, such as a spark plug, is provided to ignite the gas-air mixture within the burner 59.

Immediately adjacent the furnace 10 is an incinerator 72 having annular side walls 73 and a stack 75 extending from the upper end thereof. The base 76 of the incinerator has an opening 78 therein which receives a burner 80. Wall means 82 define a passageway 84 that is confluent with that portion of the passage 41 immediately below the drop arch 46, to form a flue system therewith.

As can be seen by the drawing, the direct fired burners 59 fire directly into their respective chambers 54 and 58. As the burners fire into the chambers 54 and 58, their discharging gases entrain the atmosphere gases therein so that a portion of the atmosphere gases is carried along by entrainment thereby creating a circulation of the furnace atmosphere. The amount of entrainment is determined by the ratio of the length of the furnace chamber 54 or 58 to the diameter of the burner 59, each furnace chamber having a length L and each burner having a diameter D as seen in FIG. 1. Preferably, the L to D ratio will vary from 20:1 to 80:1, It will be observed that the burners are located above the work and fire over the top of the same towards the end walls 16, 18. This results in high entrainment which assures best temperature uniformity in each chamber 54 and 58.

It has also been found that proper choice of the relative dimension of the furnace is helpful in obtaining the desired results, i.e., gas temperature uniformity, high circulation of atmosphere gases, wax vapor transportation and the like. The furnace may be so designed that the ratio of the height H of the chambers 54 and 58 to the distance from the arch 46 to the burner center line h is from 4:1 to 10:1. Another pair of dimensions whose relative sizes can be varied for better results is the ratio of the height H to the distance from the chamber ceiling to the bottom of the drop arch a. It has been found advantageous to design the furnace so that the ratio H/a varies from 3:1 to 8:1.

Beneath the work is the passageway 41 which is confluent with the passageway 84 leading to the incinerator 72. This combination results in a circular motion being created in the chamber 54 and 58 atmospheres as illustrated by the arrows in FIG. 1. As the gases move within their respective chambers 54 and 58 and the circulation motion is set up, the space 56 immediately under the drop arch 46 becomes a stagnation zone that segregates chamber 54 from chamber 58. Additionally, a portion of the atmosphere, an amount equivalent to the products of combustion from the burners 59 plus the wax vapors, is continually discharged through the flue system comprising gas passageway 41, passageway 84 and incinerator 72.

The temperatures within the chambers 54 and 58 are maintained at different levels, thermocouples 55 and 57 being provided in each chamber to control these temperatures. When such a furnace is used for the purpose of dewaxing the curing metal casting molds, the temperature in the first chamber 54 is only raised to approximately l,000F., the normal range being between 800l,600F. Preferably, the temperature of the first chamber 54 is raised sufficiently high to allow the wax to vaporize. Additionally, the burners 59 which fire into chamber 54 are fired with a stoichiometric proportion of air and fuel so that no excess air is present in the chamber 54 which would tend to ignite or burn the wax vapors. Since the wax vapors are at a relatively high temperature, they are carried away by the

flue system

41 and 84 and are kept from entering the second chamber 58 by the balanced counterflow motion of the gases. Within the second chamber 58, the temperature is increased to approximately 1,800F. so that the curing of the molds may take place. Again, because of the means in which the two systems are kept segregated, the temperature within chamber 58 will have very little influence on the temperature of chamber 54.

In order to avoid pollution, the effluents from the

flue system

41 and 84 are incinerated in an incinerator 72. The incinerator 72 is provided with an air rich burner wherein the wax vapors and other effluents may be completely reacted before being discharged into the 1 environment.

What is claimed is:

1. A longitudinally extending furnace comprising a housing having a roof, floor, sidewalls and end walls, opening means for charging work through one end wall, opening means for discharging work from the other end wall, means for conveying work from the charge end of the furnace to the discharge end, said roof having a drop arch located intermediate said end walls thereby dividing said furnace into two treating chambers, direct fired burner means located within the upper portions of each chamber, said direct fired burner means including at least one direct fired burner located in'each chamber and received within said drop arch and directed to fire longitudinally in the direction of its respective end wall, each of said burners having an annular housing defining a combustion chamber, means for supplying combustible gas and air to said combustion chamber and means for igniting the gas mixture, gas passageway means confluent with each of said treating chambers and located adjacent said floor, flue means confluent with said passageway means, and the ratio of the furnace treating chamber longitudinal length to burner diameter ranging from 20:1 to 80:1.

2. The furnace of claim 1 wherein the ratio of the height of a furnace treating chamber to the'distance from a burner center line to the top of said treating chamber varies from 3:1 to 10:1.

3. The furnace of claim 2 wherein said flue means is located below said drop arch and the ratio of the height of one of said chambers to the distance from the cham ber top to the arch bottom varies from 3:1 to 8:1.

4. A longitudinally extending furnace comprising a housing having a roof, floor, sidewalls and end walls,

opening means for charging work through one end wall, opening means for discharging work from the other end wall, means for conveying work from the charge end of the furnace to the discharge end, wall means located intermediate said end walls to divide said furnace into two confluent treating chambers, direct fired burner means located within each chamber and including at least one direct fired burner having an annular housing defining a combustion chamber located at each chamber end adjacent said wall means and being directed to fire towards its respective end wall, gas passageway means confluent with each of said treating chambers, flue means confluent with said passageway means and the ratio of a furnace treating chamber length to the diameter of a burner'ranging between 20:1 to 80:1.

5. A furnace for heat treating a plurality of workpieces therein at least two different heat treating operations comprising:

a roof, floor, sidewalls and end walls defining a furnace enclosure;

opening means for charging said workpiece through one end wall and discharging said workpiece through the opposite end wall;

said roof having a drop arch in between said end walls to divide said enclosure into first and second heat treat chambers;

burner means including at least one direct fired burner for each chamber received within said drop arch, each burner orientated to discharge its products of combustion towards its respective end wall,

6. The furnace of claim 5 wherein said passage means further includes said floor having an opening in each chamber adjacent said end wall thereof; a plurality of gas passages beneath said floor in communication with said opening at one end, and flue passages adjacent said arch communicating the opposite ends of said gas passages with said atmosphere.

7. The furnace ofclaim 6 wherein said passage means further includes incinerator means within said flue means to provide a positive draft within said gas passages while incinerating unburnt fumes from said work-

Claims

1. A longitudinally extending furnace comprising a housing having a roof, floor, sidewalls and end walls, opening means for charging work through one end wall, opening means for discharging work from the other end wall, means for conveying work from the charge end of the furnace to the discharge end, said roof having a drop arch located intermediate said end walls thereby dividing said furnace into two treating chambers, direct fired burner means located within the upper portions of each chamber, said direct fired burner means including at least one direct fired burner located in each chamber and received within said drop arch and directed to fire longitudinally in the direction of its respective end wall, each Of said burners having an annular housing defining a combustion chamber, means for supplying combustible gas and air to said combustion chamber and means for igniting the gas mixture, gas passageway means confluent with each of said treating chambers and located adjacent said floor, flue means confluent with said passageway means, and the ratio of the furnace treating chamber longitudinal length to burner diameter ranging from 20:1 to 80:1.

2. The furnace of claim 1 wherein the ratio of the height of a furnace treating chamber to the distance from a burner center line to the top of said treating chamber varies from 3:1 to 10:1.

3. The furnace of claim 2 wherein said flue means is located below said drop arch and the ratio of the height of one of said chambers to the distance from the chamber top to the arch bottom varies from 3:1 to 8:1.

4. A longitudinally extending furnace comprising a housing having a roof, floor, sidewalls and end walls, opening means for charging work through one end wall, opening means for discharging work from the other end wall, means for conveying work from the charge end of the furnace to the discharge end, wall means located intermediate said end walls to divide said furnace into two confluent treating chambers, direct fired burner means located within each chamber and including at least one direct fired burner having an annular housing defining a combustion chamber located at each chamber end adjacent said wall means and being directed to fire towards its respective end wall, gas passageway means confluent with each of said treating chambers, flue means confluent with said passageway means and the ratio of a furnace treating chamber length to the diameter of a burner ranging between 20:1 to 80:1.

5. A furnace for heat treating a plurality of workpieces therein at least two different heat treating operations comprising: a roof, floor, sidewalls and end walls defining a furnace enclosure; opening means for charging said workpiece through one end wall and discharging said workpiece through the opposite end wall; said roof having a drop arch in between said end walls to divide said enclosure into first and second heat treat chambers; burner means including at least one direct fired burner for each chamber received within said drop arch, each burner orientated to discharge its products of combustion towards its respective end wall, said burner means effective to entrain fumes emanating from said workpieces within said products of combustion; passage means within said floor and in fluid communication with each chamber at one end and with atmosphere at the opposite end, said passage means effective to a. cause said products of combustion to circulate about chamber to heat said workpieces primarily by convection, and b. to establish a stagnation zone underneath said arch to separately maintain and prevent the atmosphere in each chamber from commingling with one another.

7. The furnace of claim 6 wherein said passage means further includes incinerator means within said flue means to provide a positive draft within said gas passages while incinerating unburnt fumes from said workpieces.