US2246322A - Gas atmosphere in electric furnaces - Google Patents

Description

June 17, 1941. w, ROTH 2,246,322

GAS ATMOSPHERE IN ELECTRIC FURNACES Filed sept. 21, 1938 s sheets-sheet* 1 June 17, 1941. w RO'TH 2246,322

GAS ATMOSPHERE IN ELECTRIC FURNACES WITNESS S: INVENTOR ATTORNEY June 17, 1941. w. ROTH 2,246,322

GAS ATMOSPHERE IN ELECTRIC FURNACES WITNESSES: lNVENTOR Ml w1' am; Pof/z.

ATTORNEY Patented June 17,V 1941 GAS ATMOSPHERE IN ELECTRIC FURNACES Willard Roth, Wilkinsburg, Pa., assigner to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., `a corporation of Pennsyl- Vania Application September 21, 1938, Serial No. 230,940

(Cl. 26S-5i 7 claims.

This invention relates, generally, to that type of furnace in which a charge, preferably a metallic substance, is to be heat treated while enveloped by a protective gaseous atmosphere of a composition appropriate to the particular charge and the particular heat treatment process involved.

Protective gases are employed in many different heat treatment processes as a blanket to prevent deleterious effects upon the charge undergoing treatment. Depending on the heat treatment process, protective gases are employed to prevent scaling, decarburization or carburization, tarnishing, or some other objectionable result as the case may be, on the metal treated. The composition of the protective gases depends largely on the function it is to perform in the furnace and many methods are known by which different compositions are produced. Invariably, however, the method involves the removal of undesirable and obnoxious constituents from the gases before they can be delivered to the-furnace. All traces of water vapor, sulphur vapor and sometimes carbon dioxide, as examples, must be removed for many heat treating processes in order that the resultant gases will be able to perform a really protective function, since the specific gases enumerated constitute the chief offenders in blemishing the charge being heat treat/ed. The removal of these, the objectionable constituents, is, therefore, imperative and methods now known for accomplishing their removal depend upon apparatus operated at relatively low temperatures, much lower than that within the furnace to which the purified gases are conveyed.

Common practice has been to feed the protective gaseous atmosphere after it has been purified and while in its cool state directly to the heating chamber of the furnace. Many-fold objections exist to the feeding of the protective gaseous atmosphere into the furnace while in a relatively much cooler condition than the furnace proper. The charge treated may be chilled, or the temperature distribution within the furnace disturbed, or the temperature within the furnace lowered, or all of these effects may occur, making the control of the heating process frequently a difficult matter. To offset these effects, the amount of heat generated by the heating units of the furnace can be materially increased, but this method is attended by greater heat losses and a correspondingly greater increase in operating costs. A preferred manner of offsetting the undesirable and unfavorable chilling effects aforementioned is to preheat the protective gaseous atmosphere before it envelops the charge in the heating chamber of the furnace and my invention is particularly directed to the construction of a heat treating furnace for deliberately preheating protective gases before their entry into the heating chamber.

In one form of my invention the furnace is constructed with passages in the re brick comprising the walls of the furnace. In a preferred construction, I form certain of the bricks with notches or grooves so that when assembled to form a wall the grooves of the bricks' can be aligned to form complete passages through which the protective gases may be forced.

If the furnace has but a single door for the insertion and removal of the charge, I prefer to some degree by the sizes of th'e passages which.

can be suitably designed. Moreover, by having the gases discharge at a pressure slightly above atmospheric, a plenum system results whereby the gas will tend to leak out of the/heating chamber at the loose joints generally present Y between the furnace door and its frame or guides.

.In this way the protective gas will sweep out of the heating chamber any air, moisture, or other harmful gases that may be present in the heating chamber due to their previous occlusion in the furnace walls or the charge itself, or perhaps that have entered the furnace chamber with the insertion or the removal of the charge. In furnaces with the doors at both ends, it is ad-. visable to have the protective gases discharge into the furnace chamber at approximately the center thereof in order that an efficient plenum system be created, although with the protective gases at a sufficiently high pressure within the furnace, a plenum system can generally always be attained regardless of where the gases enter the furnace chamber.

In another form of my invention, I make use of a characteristic of the more common fire brick and refractory materials suitable for furnace walls, and hereinafter embracedly designated as flre brick for brevity. The characteristic I utilize is the porosity or semi-porosity of fire brick. By discharging the protective gases about the fire brick wall at the side away from the furnace chamber, and permitting the gases to seep through the walls to the interior of the chamber, I achieve the desired preheating of the gases and I also clean the iire brick of any unwanted gases, this latter feature incidentally permitting a more rapid purginglof theA chamber at the beginning of a heat treating process or the like. Moreover, occluded gases in the walls can be rapidly removed by the second embodiment and the at mosphere within the heating chamber can be kept clean and pure.

It is, accordingly, among the objects of my invention to preheat the protective gaseous atmosphere entering a heat treatment furnace and thereby avoiding chilling the charge therein while making the temperature distribution more uniform throughout the chamber and economizing in the operating costs ofthe furnace.

It is the important object of my invention to provide a furnace capable of preheating a protective gaseous atmosphere and which is simple in construction, economical to build, and involves little departure from the construction of customary furnaces without provision for such preheating.

It is a further object of my invention to admit the protective gases to the chamber at a pressure somewhat higher than atmospheric, whereby gas 1r between the joints in the furnace walls will be from the inside out rather than from the outside in. In this way the gases tend toiiush orsweep out of the furnace chamber unwanted gss or vapors, that may be given up or occluded in the re brick or otherwise find their way into the heating chamber. l

It is still another object of my invention i provide in my furnace structure simple, easily traversed ages for preheating the protective gaseous atmosphere emitted under a plenum system, as described in the previous paragraph.

Other objects and novel features oi my invention will be apparent from a perusal oi' the iollowing description, taken in conjunction with the accompanying drawings, in which:

Figure i is a dia f tic view in vertical, longitudinal section of a furnace of one embodiment of my invention;

Iilg 2 is a transverse, vertical, sectional view along the line l-II of the furnace of Fis. l:

Fig. 3 is a side view of a re brick employed in the furnace;

Fig. 4 is a plan view of part of a fire brick employed in the furnace:

' Fig. 5 is a view similar to that of Fig. 1 of a second embodiment oi' my invention:

Fig. 6 islaL transverse, vertical, sectional view thereof along the line VI-VI of Fig. and

Fig. '1 is a fragmentary horizontal view of a vertical wall of a furnace showing a further expedient in accordance with my invention.

Referring more particularly to the first embodiment, the furnace shown in Figs. 1 and 2 Y, comprises a heating chamber defined by a top wall, a bottom or charge-supporting wall, side walls, a. rear wall and a front wall, the latter having an opening controlled by a door. The furnace is encased in a gas impervious shell l of metal, as is customary in heat treating furnaces employing protective atmospheres. The top, side and rear walls of the particular embodiment shown comprise an outside layer of mineral wool l and an adjacent layer of re brick 8. Juxtaposed to the last layer is a second layer oi' fire brick I0, preferably of a refractory material, which defines the inner lining walls of the heating chamber 2. The bottomwall of the furnace in this particular embodiment is substantially the saine as the otherwalls with the 'exception that instead of the mineral wool, an outside layer of ilre brick i2 is employed so that a substantial support is provided for the furnace structure as well as the charges.

, Heating units, which may be either electrical or of the radiant gas-tube type, Serve to raise the temperature of the heating chamber 2, and in the particular embodiment shown, I prefer to employ electrical resistance heating elements i4 for the purpose, supported on appropriate hooks IB set in grooves formed in the iire brick i9 of the side walls, and in suitable trenches in the bottom wall. The front wall of the furnace has the usual opening it through which the charge may be admitted or withdrawn, and which is closed by a door 2G sliding in guides 22.

'I'he bricks comprising the top, bottom and side walls of the inner lining of the heating chamber are specially formed to provide the necessary preheating passages for the gaseous atmosphere. A header supply passage, such as 24, is formed at the forward end of the heating chamber and from this header passage extend longitudinal passages 25 and Z8 in the top wall, 3B in one of the side walls, 32 in the other side wall, and 34 and 38 in the bottom wall. Discharge outlets for each of the longitudinal passages are constructed in a er to be subsequeiritly described, and Fig. 1 shows one of the discharge passages 38 for a longitudinal passage in the top wall, SQ for a longit passage of a side wall, and 62 for a longitudinal pge in the bottom wall.k

In order to form these passages, the bricks of the lining walls deiining the heating chamber are appropriately shaped. In Fig. 3 I show a side view of the iront brick of the top wall lining bricks, a sectional view of which may be deemed to be shown in Fig. 2. This brick M has upper grooves forming part of the passages 2S and 28, respectively, extending longitudinally thereof, that is, longitudinally with reference to the heating chamber 2, and which open into a groove I8 which is part oi the passage 24. One upper longitudinai groove is shown at 2e', Fig. 3, the other, of course, being parallel thereto. The other top lining bricks, such as Il and 48, are similar in all respects to the brick 4l except that they do not have a groove such as 46, inasmuch as asingle supply pe is suiilcient in the embodiment disclosed. The top lining bricks are assembled with the faces of their grooves adjacent the layer 8 of iire brick and in this way form continuous passages for preheating the gaseous atmosphere ilowing therethrough.

I prefer to so design the furnace that the total overall length of the top lining brick is somewhat less than the length of the heating chamber, whereby the discharge passage 38 may be formed with the rear wall. Spacers (not shown) may be employed to maintain the top lining d rear walls in proper spaced relation. However, ether ilarly formed with suitable grooves and properly assembled. This construction o! the bricks in this respect is relatively simple and obvious, and for the sake of brevity and simplicity the details of the grooves for the particular bricks of the lining layer are not further shown or described.

An inlet pipe Il passes through one of the walls of the furnace, in this instance, the bottom wall, and leads to the supply passage or header 2l. The header 2l extends substantially com.n pletely around the furnace and gas from the pipe 52 tends to ll this header from whence the gas may flow longitudinally through the passages 26, 28, l0, I2, 3l and Il to the apropriate discharge openings at the rear of the heating chamber oi which Il, Ill and 42 may be deemed representative.

A uniform distribution of the gases discharging into the heating chamber may obviously be obtained by a suitable proportioning of the cross sectional area of the respective passages. In the particular embodiment, a distribution of flowing gases is desired which requires that the top passages 28 and 2l be larger in cross sectional area than the side passages il and 82 which, in turn, are larger than the bottom passages 3l and II. However, it is obvious that the cross-sectional area of any groove may be formed as desired, whereby the volume oi' gas flow through the in dividual passages may be suitably proportioned.

In most heat treating furnaces, and particularly in high temperature furnaces, there is usually sumcient leakage of gas around the door to permit of a continuous supply of fresh gas, but in the event that the leakage is insufficient to maintain the atmosphere pure, an additional outlet pipe 54 may be provided, controlled by a valve Si. l

'I'he operation of the preheating system of the furnace of Figs. l and 2 is obvious. Gas under pressure is' supplied to the pipe l! and may be controlled by valve B8. The gas from the pipe discharges into the header passage 24 and completely fills the same. The longitudinal es 28, 2t, 3|, 32 3l and Il! extend from the header passage 26 and provide a path by which the gases in the header may travel longitudinally to the discharge openings represented by Il, Il, i! and B0. It may be observed that the path provided for the gases prior to their discharge into the heating chamber is subjected to the relatively high temperature gradients existing in the lining walls of the chamber, and will naturally absorb heat which otherwise might be lost.

'Ihe furnace shown in Figs. 5 and 6 diagrammatically represents one in which a charge is admitted at one end and removed at the other. This furnace also includes the customary outer metallic shell 8| adjacent to which is mineral wool 83. The porosity of mineral wool, in this embodiment, is utilized to distribute gases about the outside surface of the fire brick walls. The gases under pressure seep through the nre brick wall Il to the heating chamber Ii. 'The fire brick of this furnace is deliberately chosen to have a marked or high porosity and magnesium or zirconia bricks are adapted for this purpose, although any appropriate fire brick may be em- Jployed so long as it has some degree of porosity. In this furnace, as in the prior embodiment, I prefer to omit the mineral wool from the bottom wall for the purpose of obtaining the necessary strength for the structure. One or more inlet pipes l1, controlled by valves 6I, extend into the lower fire brick layer 1I in the base. Branch 'l5 pipes 18 and 1l convey the gas upward in the side walls to somewhat below the center of the furnace and terminate in open elbows 11 and 1l, respectively. As many inlet pipe systems similar to 01, Bl, etc. may be provided as the length of the furnace may require, and two are depicted in Fig. 5.

Outlet pipes Il controlled by valves may also be provided when the lea age of gas about the closure is insufilcient or hen a large volume of protective atmosphere must be passed through the heat treatment chamber.

In operation, the gas admitted under pressure through the pipes l1 will divide, some o! it going to the branch pipe 13 and discharging at the open end 11, while another portion will flow through the branch pipe 15 and discharge at the open elbow 1l. Manifestly, the outer metallic shell will prevent the flow of gas outwardly of the furnace and the great porosity of the minerai wool 8l will distribute the gas flowing out of the open elbows 11 and 1! completely about the side and top walls. Because the gas is under pressure, it will be forced through the porous fire brick walls into the interior of the chamber Il and absorb heat in this passage. Moreover, any obnoxious gases within the pores of the fire brick are flushed out by the flowing protective gas, and after a short period oi such purging, the protective lgaseous atmosphere is relatively pure and. clean. As further obnoxious gases might be passed out of the fire brick that may have been `occluded in their structure, the continued seepage ofthe protective gaseous atmosphere through the wall.; will tend to sweep them out of the furnace chamber, and in this way a furnace structure is provided by which possibleharmful effects of contaminating gases are obviated.

A third gas discharge, such as represented by numeral Il, may be provided in the bottom wall of the furnace in which case the gas will seep through the bottom wall nre brick also deliberately chosen of a porous variety for the purpose. Manlestly the number and disposition of the gas outlets is largely s. matter o! choice and design for a particular furnace, and does not form a maior feature of my invention, it being only necessary that a sufficient number be provided and so located as to discharge the necessary volume of the protective atmosphere.

The protective gases also may flow into the heating chamber by way of the customary expansion joints 01 provided in the furnace walls which are purposely staggered so that no direct path for the gas from the mineral wool to the heating chamber will be present. However, as shown in Fig. 7, the expansion joints can be deliberately more or less aligned to provide discharge passages for the gaseous atmosphere from the mineral wool Il to the interior of the furnace, with the gases being preheated while flowing therethrough. However. I prefer to stagger the expansion joints to insure a seepage oi' the protective gases through the fire brick for the purpose of quickly removing occluded gases in the nre brick as well as having the gas contact the heated fire brick for a greater period oi' time.

It is manifest that the furnace of the first embodiment of my invention may employ nre brick which is either porous or non-porous, and by porous I intend to include any fire brick which has the property of permitting gas to seep through to either a greater or lesser extent. If the fire brick of the embodiment in Figs. l and 2 is porous, there will naturally be some seepage through them through the interior of the furnace,-but it is obvious that such seepage would be an advantage since it would provide additional passages for preheating the gaseous atmosphere.

While I have shown my invention in forms which I believe to be the best embodiments thereof it is obvious that other forms or embodiments utilizing the general principles of my invention may/be employed. I, therefore, do not desire the appended claims to be limited except as required by the prior art.

I claim as my invention:

l. A heat treatment furnace of the type in which a charge is to be heat treated while enveloped in a protective gaseous atmosphere, said furnace comprising walls defining a heat treating chamber, said walls being formed of fire brick, certain of which are formed with aligned grooves forming a network of passages within the walls, and means for delivering protective gases to said network, said brick being formed and positioned to provide outlet passages from said network to said chamber.

2. A heat treatment furnace of the type in which a charge is to be heat treated whiley enveloped in a protective gaseous atmosphere, said furnace comprising walls defining a heat treating chamber, said walls comprising top, bottom, and side walls formed of fire brick, certain of said brick being formed with aligned grooves forming a network of passages within the walls, the network comprising a supply passage running along the said top, bottom and side walls, individual communicating passages extending from said supply passage for some distance in said walls, and outlet passages from said communicating passages to said chamber.

3. A heat treatment furnace of'the type in which a charge is to be heat treated while enveloped in a protective gaseous atmosphere, said furnace comprising walls defining a heat treating chamber, said walls comprising top, bottom, and side walls formed of fire brick, certain of said brick being formed with aligned grooves forming a network of passages within the walls, the network comprising` a supply passage running along the said tcp, bottom, and side walls, individual communicating passages extending from said supply passage for some distance in said walls, and outlet passages from said communicating passages to said chamber, said walls also comprising a front wall having an opening and door therefor, and a rear wall, said supply passage being located near said front wall and said outlet passages being located near said rear wall whereby` a plenum system for the protective gases may be had.

4. A heat treatment furnace comprising an outer layer and a juxtaposed inner layer of re brick forming a heating chamber, individual bricks of said inner layer having aligned grooves along the surfaces thereof adjacent said outer layer, forming therewith extended passages, means for delivering the gases to said passages, and outlet means from said passages to said chamber.

5. The structure of claim 4 characterized by said passages extending longitudinally of the furnace, and said inner layer having open spaced portions extending from said passages to said heating chamber. v

6. A heat treatment furnace of the type in Which a charge is to be heat treated in a heat treatment chamber of the furnace whileA enveloped in a protective gaseous atmosphere, walls defining said heat treatment chamber, heating means in said chamber, certain of said walls com,

prising a gas impervious outer section, and an inward porous fire brick section at said chamber, and means for discharging protective gas at the outer part of said fire brick whereby the gas may seep through said fire brick and be preheated thereby, said fire-brick section including expansion joints along its length through which gas passes4 for preheating, and discharges into said chamber.

'7. A heat treating furnace of the type in which a charge is to be heat treated While enveloped in a protective gaseous atmosphere, said furnace comprising walls having a layer of fire brick defining a heat treating chamber with a closed end, heating means in said chamber, said walls providing passage means in said fire brick through which gases may iiw in heat exchange relation to said walls, inlet means for delivering protective gases to said passage means, the said passage means terminating in outlets for discharging the gas into said chamber, said walls including a charge-opening at the end opposite the said closed end, and door means for said charge-opening, said passage means having inlets near the end of said furnace having said charge-opening, said outlets being in proximity to said closed end whereby gas delivered to said chamber under pressure tends to flow toward said opening.

WILLARD ROTH.

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