US2619341A - Heat-treating furnace - Google Patents

Description

NOV- 25, l952 .1. WALLERIUS HEAT-TREATING FURNACE 3 Sheets-Sheet 2 Filed Sept. 26. 1947 IN V EN TOR.

l3am Haga/w31 yan1 Hago/w31 NOV. 25, 1952 J WALLERHJS 2,619,341

HEAT-TREATING FURNACE Filed Sept. 26, 1947 3 Sheets-Sheet 3 I U l l IN V EN TOR.

John UQZZQTuS,

Patented Nov. 25, 1952 HEAT-TREATING FURNACE John Wallerius, Glen Ellyn, Ill., assignor to Sun- `beam Corporation, Chicago, Ill., a corporation of Illinois Application September 26, 1947, Serial No. 776,213

3 Claims.

The present invention relates to heat treating furnaces and more particularly to recirculating hot air or gas furnaces particularly useful for low temperature heating in many types of work. Such furnaces are very satisfactory for tempering carbon and alloy steels, for annealing brass between drawing operations, for stress relieving castings, welding structures and cold worked metals, and for aging all types of castings. In addition almost all aluminum heating or heat treating and all heating operations and heat treating of magnesium can be carried out in such furnaces.

The recirculating heat treating furnace has been extensively used in recent years due to its eilicient and high speed operation. With the advent of high temperature alloys it was possible to build fans which were trouble free and which could produce the large capacity for satisfactory heating. It is a well-known fact that in a recirculating furnace perhaps the most important factor is fan capacity. It is necessary to get high velocities of circulation to take full advantage of the convection principle for rapid uniform heating. vThe second important requirement of welldesigned recirculating furnaces is an even distribution of the flow of gases or air through the work. The widespread use of recirculating furnaces has resulted in the following accepted advantages; uniformity of temperature throughout the furnace, high speed heating at relatively low temperatures, high speed heating through considerable depths of work, ability to maintain low temperatures without difficulty, operation through wide temperature ranges, capability of using oil as a fuel as well as gas or electricity, greater eniciency due to recirculation, and increased tonnage per furnace.

In the recirculating furnaces now in extensive use it has been found that the work which is being heated at the air or gas entrance to the heating chamber initially attains a much higher temperature than the work at the air or gas exit and a substantial length of time is required until the temperatures of the work at each end of the heating chamber are substantially the same. It has been discovered by extensive tests that the fastest and most uniform heating is achieved when the temperature drop of the gases through the work isa minimum. It is obvious that the temperature drop will be a minimum when the gases take the shortest and most direct path through the work. The greater the number of particles of work with which each particle of air or gas comes in contact the coolerl the gas becomes and therefore the less heat can be imparted to each succeeding particle of work. The greater the amount of turbulence, or stirring action, of the gases, the more the air or gas is cooled and the greater the variation of temperature of the work itself. The reduction of turbulence to a minimum has resulted in improvement of all types of recirculating furnaces. Additional tests have shown that when a load of work was heated in one of these furnaces the parts next to the inlet of the hot gases heated much faster than those near the outlet. This caused an ever increasing temperature variation through the work until a maximum point with respect to temperature variation was reached. Then, as the differential of temperature between the incoming hot gases and the nearest parts became less, more heat was left in the gases resulting in a relatively faster heating rate for the parts near the outlet, This gradually reduced the temperature variation through the load to an amount considered satisfactory for most work by the end of a reasonable heating period. It would be desirable to provide means for substantially reducing the temperature variation with the consequent reduction in heating time and the resulting increase in furnace capacity.

Accordingly it is an object of the present invention to provide a new and improved recirculating type of furnace with greatly reduced heating time for the same satisfactory service and with a substantial increase in furnace capacity.

It is another object of the present invention to provide a new and improved recirculating furnace for heat treating purposes including means for periodically reversing the direction of flow of gas or air through the work.

Still another object of the present invention is to provide a new and improved recirculating furnace including numerous improved features from the standpoint of accessibility, speed of operation, simplicity, ease of operation and the like.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a, part of this application.

For a better understanding of the present invention reference may be had to the accompanying drawings in which:

Fig. 1 is a sectional elevational view of a furnace embodying the present invention, the sectional view lbeing taken substantially on line I-I of Fig. 2, assuming Fig. 2 shows a complete structure;

Fig. 2 is a sectional view taken on line 2-2 of Fig. 1, assuming Fig. 1 shows a complete structure;

the furnace taken in the direction of the arrows 3-3 of Fig. 2, with the front wall removed to Fig. 3 is a side elevational view of a portion of show certain control and operating mechanism of the furnace;

Fig. 4 is a sectional view taken on line 4 4 of Fig. 1 assuming Fig. 1 shows a complete construction;

Fig. 5 is a sectional View taken on line 5-5 of Fig. 2 assuming Fig. 2 shows a complete structure;

Fig. 6 is a schematic diagram of the furnace of the present invention illustrating the electrical control circuits therefor; and

Figs. 7 and 8 are curve diagrams to aid in understanding the operation of the recirculating furnace of the present invention.

Referring now to the drawings there is illustrated a recirculating furnace generally designated at I comprising a unitary structure formed of a frame of angle and channel members and provided with an exterior wall generally designated at I I comprising parallel side walls I I a and IIb, a straight end wall llc, a curved end wall Ild, and a top wall IIe. The furnace I0 is specifically illustrated as a so-called batch or basket type of furnace comprising a heating chamber generally designated at I2 which is deiined by a suitable enclosing wall structure I3 preferably formed of an alloy steel or the like to withstand the high temperatures prevalent in the heating chamber I2. As illustrated the wall structure I3 is of circular configuration and is spaced from the curved outer wall l Id of the furnacel by a substantial amount to denne a space which is filled with suitable heat insulating material generally designated at I4 which heat insulating material may comprise rock wool or any other suitable similar material, many varieties of which are available on the market today.

For the purpose of providing a furnace unit which is movable as a unitary structure a base generally designated at I5 defined by suitable channel irons and the like is provided which supports the furnace structure. The bottom of the heating chamber I2 is defined by a substantial layer of an insulating cement generally designated at I6, which insulating cement provides a hard surface. This insulating cement bottom I6 is also supported by the base I5'.

For the purpose of providing a support for a work enclosing container structure there is provided at the bottom of the heating chamber I2 a ring I8 preferably formed of an insulating concrete similar to the material I6 which is circumferentially disposed around the heating chamber I2 so as to define a ledge I1. This ledge I1 has a discontinuous portion indicated at the lower left hand part of the heating chamber I2 so as to define an air or gas passageway I9 which interconnects the bottom of the heating chamber I2 with the other parts of the furnace to be described hereinafter.

In order that a suitable work containing structure may be centrally supported in the heating chamber I2, the walls I3 of the heating chamber are provided near the ledge I1 with suitable guide members 20 arranged in spaced relationship around the heating chamber just above the ledge I1. 'Ihese guides 20 provide a centering arrangement so as to tend to center a work containing structure inserted into the heating chamber I2.

It will be understood that any suitable Work containing structure may be employed for insertion and removal from the heating chamber I2. As a matter of fact if large pieces are to be heat treated they might be inserted directly into the heating chamber I2. However since the type of furnace illustrated is particularly adapted to heating batches of small parts there is illustrated a work containing structure 22 in the form of a cylindrical container preferably formed of an alloy steel or the like to withstand the high temperatures in the heating chamber I2. The bottom of the cylindrical work containing structure 22 is designed to permit the passageway of gas or air therethrough and is comprised of a grid structure generally designated at 23 or any other grill work capable of supporting the work pieces in the container 22 and still permitting substantially unimpeded ow of gas or air in either direction. The work enclosing structure 22 may be open at the top or it may be provided with a suitable cover 24 having the necessary passageways therethrough for air or gas. If a cover 22 is employed it is generally employed for the purpose of insuring the even distribution of such air or gas throughout the area defined by the cover 24. Also, in order that the work containing structure 22, with work for heat treating purposes, may read'- ily be removed or inserted with reference to the heating chamber I2 it is provided with suitable handles 25 engageable by a crane or other suitable manipulating equipment. With this arrangement the basket 22 may readily be loaded with work to be heat treated which of course may subsequently be unloaded to permit other batches of material to be treated. Preferably a. single heat treating furnace such as IU would employ several work containing structures 22 so that the furnace may be employed to its maximum capacity without delays due to loading or unloading new batches or permitting relativelyV slow cooling of treated batches.

It will be apparent that to permit insertion and removal of the work containing structure 22 from the heating chamber I2 that access to the heating chamber from the exterior must readily be afforded. In addition the closure means provided for any opening to the heating chamber for the removal and insertion of the work containing structure 22 must be an insulating cover structure. As illustrated the heating chamber I2 is closed by a suitable cover 28 which is of generally hollow wall construction so that the hollow space may be lled with a suitable heat insulating material designated at 29 and preferably comprising rock wool or the like. For the purpose of providing a sealing surface around the opening to heating chamber I2 there is preferably provided a metal plate such as 30. In order to make a proper seal between the cover 28 and the metal plate 30 the lower surface of the cover 28 is illustrated as being provided with four rings 3| arranged in spaced concentric relationship. These rings are fastened to the bottom of the cover as by welding or the like and are arranged in pairs so as to define a relatively narrow space between each pair of rings. This space is filled with a suitable high temperature packing material generally designated at 32 so as to provide two spaced concentric seals between the cover 28 and the plate 30 surrounding the entrance to the heating chamber I2 in a manner which is clearly apparent from an examination of Fig. 1 of the drawings.

It will be apparent that the cover structure described above to provide the desired insulating features and the like has substantial mass. In order that this cover may readily be removed or replaced when access to the heatingA chamber I2 is desired there is provided in accordance with the present invention a suitable cover maparticularly when filled' 5, nipulating mechanism generally designated at 35 and best shown in Figs. 3 and 4. This mechanism includes a suitable cover support 36 which is fastened in any suitable manner to the cover 28 and includes a portion 36a which extends beyond the circumferential edge of the cover 28. The portion 36a includes a vertically disposed opening for receiving a vertically movable piston rod 38 which has its lower end connected to a suitable piston not shown, disposed within the cylinder 4I. To insure straight line motion, the piston rod 38 passes through a pair of aligned sleeves 39 and 58 suitably and rigidly supported from a vertically disposed channel member 42 positioned for also supporting the wall I Ib. The rod 38 is removably related to the support 36 as by the pin 43 and the notch 44 best shown in Fig. 3.of the drawings. It will be apparent that if air under pressure is supplied to the cylinder 4I below the piston fastened to the piston rod 38, upward movement of the piston rod 38 results with the consequent upward movement of the cover 28. Such opening movement of the cover 28 still does not provide satisfactory access to the opening to the heating chamber I2. In accordance with the present invention means are provided to produce subsequent pivotal movement of the cover 28 about the axis of the connecting rod 38 following initial upward movement thereof. As illustrated the sleeve 4I] which is provided with a suitable opening for the piston rod 38 so as to permit both axialA and rotational movement thereof relative thereto, includes a cam-like slot 46 for receiving a pin 41 fastened to the piston rod 38. The slot 46 is so shaped that upward movement of the piston rod 38 and consequently the pin 41 causes rotation of the shaft or piston rod 38 and consequently pivotal movement of the cover 28 away from the opening to the heating chamber I2.

The remainder of the space defined by the walls II is divided into several chambers including a control and mechanism chamber 58, a heat source chamber I, a fan or blower generally designated at 52 and various ducts and mixing chambers to be described hereinafter. The fan or blower 52 preferably comprises a suitable fan housing 53 and is illustrated in Fig. 2 of the drawings, as substantially disposed on the longitudinal central axis of the furnace I8. Mounted for rotation in the fan housing 53 is the fan or impeller 54 mounted on a suitable shaft 55 which extends out of the fan housing 53 into the mechanism or control chamber 58. As is best shown in Figs. 2 and 4 of the drawings, the fan housing 53 is supported by a suitable support 56 from channel members 51 in turn supported by the base I5. The fan chamber or fan housing 53 is provided with an air or gas intake opening generally designated at 58 which causes the gas to flow into the housing in the direction of the axis of the shaft 55 so as to provide an axial inlet 58. On the other hand the fan has a circumferential outlet generally designated at 5S which outlet is connected toducts 68 and 5I arranged in the form of a Y with the juncture between the ducts 60 and 6I connected to the circumferential outlet 59. Each of the ducts 68 and 5I is connected by means of openings 68a and Ia respectively with a return gas chamber 62 which is defined in the space within the Y portion of the ducts 68 and 6I and the wall I3 of the heating chamber I2. The return gas chamber 62 as is best shown in Fig. 2 of the drawings extends in the horizontal direction between the Y duct arrangement comprising the ducts 60 andIiI and a mixing chamber to be described hereinafter. It will be understood that the fan parts are constructed of suitable high temperature alloys to withstand the high temperature air or gases to which they are subjected. It might be mentioned at this time that in many heat treating operations air is employed as the heating medium. However the present invention is equally applicable where some gas other than air is employed and it should be understood that the term gas as employed in this specication and claims includes air or any other gases which might be employed in the heat treating operation.

For the purpose of preventing substantial loss of heat from the fan housing 53 or the air or gas ducts 58 or 6I, the space surrounding these members is iilled with a suitable heat insulating material generally designated at 64 which heat rsulatlng material may be rock wool or the Although the source of heat employed in vthe furnace I0 of the present invention might comprise a gas burner, an oil burner, lor electric heater the invention has been specifically illustrated as comprising a gas burner generally designated at 66 which is connected to a suitable source of air and gas by a conduit means generally designated at 61. (See Fig. 6 of the drawings.) The gas burner is adapted to extendvinto the heat source chamber 5I which, as isA best shown in Fig. 4 of the drawings, is dened by walls 68, 69 and 18 of an insulating nre brick and wall 1I of re brick not insulating in character. Preferably also a layer of ordinary insulating brick 12 is provided above the insulating fire brick layer 69. Upon operation of the gas burner 66 a gas flame is produced in the heat source chamber 5I which causes hot gases to be supplied at the exit of the heat source chamber 5I. As is best shown in Fig. 1 of thedrawings the wall 59 terminates in a curved portion 69a so as to direct the hot gases from the heat source chamber 5I in a downward direction into a mixing chamber generally designated at 14. This mixing chamber 14 is dened by the duct 15 which extends between the axial fan inlet 58 and the return gas chamber 62, the term mixing chamber 'I4 being appended thereto since it provides the chamber where the return gases from the chamber 62 are mixed with the hot gases from the heat source chamber 5I which are guided into the mixing chamberv14 by the curved wall portion 68a.

It will be understood that due to the burning of hot gases which are introduced into the furnace, means must be provided for bleeding off an amount of gas equal to that continually being introduced by the gas burner 66, if one is employed. It will be understood that if the heat source is electrical then this problem is not the same. In accordance with the present invention a positive gas pressure exists in the return gas chamber 62 at the entrance to the mixing chamber 14 and at this point there is provided a conduit 19 connecting the return gas chamber 62 to atmosphere outside the furnace whereby a predetermined amount of returned gas from the mixing chamber 14 may be vented to the atmosphere and no loss of hot newly burned gas results,

since the newly burned gases are supplied to the mixing chamber 14 between the fan inlet 58 and the portion of the return gas chamber I62 vented to atmosphere by the conduit 18. A

suitably adjustable damper 88 may be providedv 7 in the conduit 19 to control the amount of gas venting, which may be substantially zero when an electric heat source is employed.

To further insulate the working parts of the heating chamber there preferably is provided a layer of asbestos` board 8| as is best shown in Fig. 4 of the drawings between the insulating material described above and the exterior walls II of the furnace I0.

Thev mechanism and control chambei` 59 essentially comprises a space defined alongside the fan housing 53 between the channel member 42 and the enclosing portions of the walls, IIb and IIc adjacent thereto. Disposed within this chamber is the cover-manipulating mechanism 35 described above. Also a suitable motor 82.is suitably mounted within this chamber and drivingly connected with the fan shaft as by means of cooperating pulleys 03 and 84 and a belt 85. Preferably a suitable opening 96 in the insulating wall structure is defined immediately adjacent the fan outlet 59 for the purpose of permitting connections to be made with a suitable temperature measuring device designated at 81, and schematically indicated as a suitable pyrometer or the like in Fig. 6 of the drawings.

As has been mentioned above if the heating chamber I2 is supplied with hot gases through operation of the fan 54, say for example in the direction of the arrows shown in Fig. 1 of the drawings, the heating of the work at three different points therein is represented by the three curves A, B, and C shown in Fig. 7 of the drawings Where the temperature thereof is plotted with reference to time. The curve A represents the top of the work which is first engaged by the hot circulating gases indicating that it is clearly the hottest portion of the work until after a substantial time has elapsed. The curve C represents the point of the work adjacent the hot gas exit which heats at a much slower rate. while the curve B represents the work in the middle of the container 22 which heats at an intermediate rate. As sucient time elapses the temperature differential between the top. bottom and middle of the work gradually disappears and substantially the same temperature is approached. If the flow of gases were in the opposite direction curve C would remain the same and curves A and B would be interchanged. accordance wlth the present invention the gas circulation is periodically reversed throughout the heating cycle to reduce to a minimum the dilerential in temperature between various parts of the work, and in Fig. 8 there are illustrated curves A', B', and C', corresponding respectively to the curves A, B and C, of Fig. '1, for anarrangement where periodic reversal of the air flow through the work occurs. It is apparent from these curves that not only is the temperature differential reduced but the time required to arrive at the same end temperature is also greatly reduced.

For the purpose of providingV a simple means for reversing the flow of air there is provided a damper 90 as shown in Fig. l of the drawings which is disposed at the junction point between the ducts and 6I. The damper 90 furthermore controls two paths of flow. When in the position shown in Fig. 1 of the drawings the damper closes the opening 60a vbetween the duct 60 and the return gas chamber 62 and also closes the opening between the fan outlet 5'9 and the duct 6| thereby requiring air or gasV to ow through the work fromV the top to the bottom.

If the damper 90 were rotated in a counterclock- Wise direction as viewed in Fig. 1 through an angle of substantially 90% the opening IiIa would be closed and the passageway between the fan outlet 59 and the duct 60 would also be closed so that the flow of air through the heating chamber I2 would be from the bottom to the top. The now of air in allv other parts of the device would be unchangedr however, since the air would 'flowfrom the return gas chamber 62 to the mixingy chamber 14 and from the heat source chamber 5I to the mixing chamber 14 in exactly the same manner regardless of the'direction offlow through the heating chamber I2. Obviously the flow from the mixing chamber 14 through the axial fan inlet 58 is also unchanged regardless of the position of the damper 90.

For the purpose of manipulating the damper 90 it is mounted on a suitable shaft 9| connected by a crank 92, a link 93, and a crank 94v to av unidirectional control motor 95. As illustrated in the drawings the unidirectional control motor 95 is preferably of the impulse type extensively available on the market today which rotates only through an angle of upon the reception of an impulse and then rotates through a successivev 180 angle in the same direction upon the reception of a different impulse. Such a motor 95 generally is provided with four terminals as indicated best in Fig. 6 of the drawings.

Any suitable control circuit for the furnace' Il may be provided. As illustratedin Fig. 6 of ther drawings, the control circuit comprises a. source of potential |00, The pyrometer 81 is connected to a temperature control device IOI for controlling a valve |02 associated with the gas burner 66 thereby varying the temperature in the heat source chamber 5|, in accordance with the operation of the temperature control device IUI in a manner which will be understood by those skilled in the art.

For the purpose of providing the desiredperiodic reversal of gas ow through the heating chamber I2 the damper motor 95 has two terminals thereof connected directly by the conductors |93 and |04 with the power source |00. The other two terminals of the motor 95 are connected by conductors |05 and |06, to one line of the power source |00 through the contacts |01a and I01b respectively of a relay |01, the contacts I01a being indicated as normally open contacts and the contacts |011) as normally closed contacts. It will be apparent that with this arrangement the conductors |95 and |06 are alternately connected to one conductor of the power source |00 in response to the alternate energiaation and deenergization of the relay |01. The relay |01 forms a part of a timer unit generally designated by the reference numeral |08.

In order to provide a time control cycle, the control apparatus includes a timer motor generally designated at |09 including the winding |09a and |091) and the rotor |09c which is adapted to rotate in either a forward or a reverse direction depending upon whether the winding |09a or the winding I09b is energized. The rotor |09c includes a pair of spaced control arms III and II2 for actuating the switches II3 and ||4 respectively when the rotor |09c reaches its limiting positions. The control arm III is adapted to close normally open switch II3, in one extreme portion of the rotor |09c while the arm I'I2 is adapted to open a normally closed switch ||4 when the rotor |09c assumes its other extreme position. One terminal of each of the windings 9 |09a and |095 of the motor |09 is connected directly to the same conductor |04 and thence to the power line |00. The other terminals of the windings |09a and |091; are connected through the contacts |`|c and |01d respectively of the relay |01 to the other terminal of the power line |00 through a suitable manual switch I 5 and the conductor |03. The winding |01 of the relay |07 is connected in parallel with the winding |0912 of a timer motor |09. With this arrangement when the switch ||5 is closed and the parts of the mechanism are in the position indicated in Fig. 6 of the drawings, power is supplied to the Winding |09a of the motor |09 causing it to rotate in a direction to close the switch ||3. As soon as the switch ||3 is closed, which occurs after the expiration of a predetermined time the relay |01 is energized to close its contacts |01a which supply the motor 95 through the conductor |06 with the necessary impulse to produce 180 rotation of the control motor 95, with the consequent movement of the damper 99 to reverse the flow of air to the heating chamber 22. Opening of the contacts |0`||d furthermore deenergizes the winding |0902. Also closing of the contacts |0'lc energizes the winding |09b to reverse the direction of the rotation of the rotor |090 of the timer motor |09. Subsequent opening of the switch ||4 when the rotor |09c reaches its other extreme position causes deenergization of the relay |01 and the winding |0919, with a further 180 movement of the motor 95 and reversal of the position of the damper 90. It will be apparent that with this arrangement alternate positions of the damper 90 are assumed successively during a heating cycle in dependence upon any predetermined desired timing cycle.

In view of the detailed description included above the ope-ration of the recirculating furnace I0 will be apparent to those skilled in the art and no further discussion is included herewith.

Although the present invention has been described in connection with a so-called basket or batch type of recirculating furnace it should be apparent that it might equally well be employed with other types of furnaces such as the oven type of furnace, car or conveyor type, and the like and the illustrated embodiment is by way of example only.

While there has been shown and described a particular embodiment of the present invention it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention in its broader aspects and it is therefore aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A recirculating furnace comprising a heating chamber adapted to receive a removable container for the material to be heated, said container being provided with suitable openings in the ends thereof to permit the circulation of a gas therethrough, the combination of a blower disposed adjacent said heating chamber and having a gas inlet and a gas outlet, duct means for connecting said outlet with both ends of said heating chamber, a source of hot gas, a mixing chamber disposed adjacent said inlet to said blower, means connecting said source to said mixing chamber, a return gas chamber connected to said duct means, means for connecting said inlet to said mixing chamber, means connecting said return gas chamber with said mixing chamber, means for venting to atmosphere gas from said return gas chamber, a single damper at said outlet for controlling the direction of flow of gas through said heating chamber, and control means for periodically actuating said damper to reverse the flow of gas through said heating chamber.

2. In a recirculating furnace comprising a heating chamber adapted to receive material to be heat treated, the combination of a blower disposed adjacent said heating chamber and having a gas inlet and a gas outlet, duct means adapted to connect said outlet with both ends of said heating chamber, a source of hot gas, a mixing chamber adjacent said inlet to said blower, means connecting said source to said mixing chamber for injecting said hot gas into said furnace, a return gas chamber connected to said duct means, means for connecting said inlet to said mixing chamber, means connecting said return gas chamber with said mixing chamber, means connected to said return gas chamber for bleeding off an amount of gas comparable to that injected by said source, a single damper at said outlet for controlling the direction of flow of gas through said heating chamber, and control means for periodically actuating said damper to reverse the flow of gas through said heating chamber.

3. In a recirculating furnace comprising in combination, a unitary structure including a heating chamber adapted to receive material to be heat treated, a blower disposed adjacent said heating chamber and having an axial inlet and a gas outlet, duct means adapted to connect said outlet with both ends of said heating chamber, a hot gas chamber, a mixing chamber disposed adjacent said axial inlet, means connecting said hot gas chamber to said mixing chamber for supplying newly heated gas to said mixing chamber, a return gas chamber connected to said duct means, means connecting said axial inlet to said mixing chamber, means connecting said return gas chamber with said mixing chamber, means connected to said return gas chamber for venting a predetermined amount of gas to atmosphere, an adjustable damper in said last mentioned means, a single pivotal damper disposed at said blower outlet for controlling the direction of flow of gas through said heating chamber, and control means for periodically actuating said damper to reverse the flow of gas through said heating chamber.

JOHN WALLERIUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,324,667 Hanson Dec. 9, 1919 1,392,317 Fisher Oct. 4, 1921 1,542,954 Puening June 23, 1925 1,799,702 Puening Apr. 7, 1931 1,923,145 Harsch Aug. 12', 1933 2,163,727 Dean June 27, 1939 2,228,564 Guthrie Jan. 14, 1941 2,277,592 Keener et al. Mar. 24, 1942 2,338,965 Parsons Jan. 11, 1944 2,414,312 Lee Jan. 14, 1947 2,469,740 Moore May 10, 1949 2,474,505 West June 27, 1949

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