US 3293406 A
Description (OCR text may contain errors)
Dec. 20, 1966 G. A. BAIN 3,293,406
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Dec. 20, 1966 G. A. BAIN 3,293,406
METHOD AND APPARATUS FOR BAKING CAROINACEOUS LININGS Affomey United States Patent O 3,293,406 METHOD AND APPARATUS FOR BAKING CARBONACEOUS LININGS Gordon Alexander Bain, Arvida, Quebec, Canada, as-
signor to Aluminium Laboratories Limited, Montreal,
Quebec, Canada, a corporation of Canada Filed Dec. 14, 1964, Ser. No. 418,086 16 Claims. (Cl. 219-279) This is a continuation-impart of my copending application Serial No. 111,317, filed May 19, 1961, for Method of Baking Linings for Aluminum Reduction Cells, now abandoned.
This invention relates to new and improved procedure and apparatus for forming monolithic or like linings of carbonaceous material, especially in an upwardly facing cavity of a vessel, i.e. chiefiy on an upwardly facing surface therein, and notably in vessels or like structures to be used in electrolytic operations at high temperature, as where the lined vessel is to contain molten electrolyte material. More specifically, this invention is concerned with methods and apparatus for forming monolithic linings in aluminum reduction cells or so-called pots, involving the use of infrared heating elements.
In the past, there has been great difficulty associated with conventional methods of forming a monolithic carbonaceous lining on a body especially in the early stages of the baking process. Tests have shown one source of this difficulty to be that green pot lining mixtures exhibit a strong tendency to swell in what is often called the plastic zone. The plastic zone is that in which the temperature ranges between 300 C., or below, and 500 C. and the mass becomes plastic, expands and decomposes. In many cases, the plastic condition arises at 150 C. or -at least 200 C. The swelling is succeeded by a period when the carbonaceous mass congeals into a hard solid coke body. It has been found that this expansion and contraction .phenomenon depends not only on the composition of the mixture, but to a great extent upon the rate of heating in that critical zone, Iand the phenomenon is most pronounced when the rate of heating is high. Sudden changes in temperature yare likely to have an adverse effect on the ultimate durability as well as enhancing the likelihood of the lining cracking. This same phenomenon can produce disastrous results where a large monolithic body is subjected to varying temperatures producing uneven degrees of expansion and shrinkage in different parts at the same time. Accordingly, one of the principal requirements for obtaining better pot linings is to apply a uniform and precisely regulated rate of heating in this critical plastic zone.
Under the conventional methods, gradual and even heating at low temperatures is almost impossible. It has been determined empirically that a two stage baking process works out very well. Thus during the first stage, that which includes the plastic zone, the :baking is carried `out under conditions where the temperature control is relatively flexible and precise. The second stage then can be performed by ordinary resistance baking. In fact, since -a carbon body staked to 600 C. already has sub.- stantial electrical conductivity, the second stage baking can be carried out during regular pot startup procedures. Although the first stage of the baking process, wherein the plastic phase is passed and a solid, ycoherent lbody of coke is reached, is the part of the complete baking operation to which this invention is primarily directed, it will be understood that the duration of baking by the means and methods herein disclosed can be carried to any further extent desired, or alternatively, the second stage or later portions of it can be effected in conventional pot startup procedure.
According to the new and valuable process of this invention, finely Icontrolled uniform heating can be achieved through the use of infrared heating panels disposed in close proximity to the lining to be baked. Such panels consist of ordinary infrared heating elements and reflectors. In preferred embodiments, these panels are placed just above a thin protective layer of divided solid material, which may be coke or may very advantageously be a layer of non-combustible material such as alumina or cryolite, i.e. materials used in the molten electrolytes of aluminum reduction cells. This arrangement provides a space for air to ycirculate between the panels and the thin layer of particulate material, which itself aids in preserving the carbonaceous lining body against oxidation. A- major advantage of this arrangement is that it permits the evolved fumes which normally infest the air above the baking operation to be burned off at the surface of the layer. Such fumes are very obnoxious to personnel, and where present to any appreciable degree, fume removal means must be provided. Indeed although it may nevertheless be desirable to utilize means for removing the gaseous products of fume combustion, the fumeless baking operation simplifies the nature and use of any gas removal equipment and may indeed in some cases avoid it entirely. This new fumeless baking process thus eliminates the necessity of providing elaborate fume removal equipment. On theory as to why this method of baking has this fumeless feature is that where the heating elements are very close to the volatile matter lbeing given off, the radiation is so intense that it actually ycracks the hydrocarbons in the fumes.
Such fumeless baking has a distinct `advantage over conventional methods in the matter of power consumption, for fumes tend to obstruct the penetration of heat energy. Thus since the fumes are eliminated under this new baking process, the heating elements can be used more efficiently 'and with less cost than under conventional baking processes. A further and very significant advantage of this invention, in its provision for burning the fumes just above the surface of the carbonaceous lining, is that significant savings in power consumption are achieved, the heat of such combustion being effectively utilized in aid of the baking operation.
Where the thin layer of material over the green carbo-naceous lining is coke, the latter has some advantage in protecting the surface of the lining material from oxidation. That is to say, the coke is consumed, and the resulting ash remains to protect the lining.
A presently preferred arrangement, however, is to utilize a thin layer of finely divided solids of the sort ultimately employed for `the molten electrolyte in the cell. Such solid material not only cooperates in inhibiting oxidation at the surface of the lining, as well as in aid of providing a bed where the escaping fumes are burned, but also remains as a noncontaminating substance when the reduction cell is later put in operation. It is further found that by limiting or controlling the access of air to the lining, as with special hood arrangements, appreciable oxidation of the lining material is effectively avoided, i.e. in cooperation with this layer of electrolyte compounds. In contrast to these modes of operation, conventional resistance baking methods have been such that coke is added from time to time to the surface of the lining during baking, and after baking, any residual coke has to be removed while it is still extremely hot; in consequence, with the methods of the present invention there are considerable savings in coke, time, manpower and supervision. e
A major advantage of the present, improved method of baking potlining, using infrared heating, is the sub stantial elimination of oxidation of the lining material, in
contrast to conventional resistance baking where such oxidation is very considerable and practically impossible to avoid.
Another special feature, as mentioned above, is that the heat of combustion of the volatile matter, i.e. fumes, from the lining is employed to furnish a significant part of the heat required for baking, with corresponding saving of electrical energy.
Accordingly, it is one object of this invention to provide an improved and more efficient method for baking a monolithic lining on a body to be used in electrolytic operation.
It is a further object to provide an improved method for baking carbonaceous linings by heating the lining material in a fully controlled and uniform manner (which may be slow if desired), with infrared heating elements. Indeed with the procedure of the invention, the rate of heating may be selected as desired, and can be slow or rapid as a matter of choice. In contrast, conventional methods of baking potlining afforded practically no control oVer the rate of temperature rise, in part because there was no way to regulate or attenuate the localized burning of gas and pitch or to utilize efficiently, or to balance off, the heat of excess burning.
It is a further object to provide a method of baking carbonaceous linings wherein a thin layer of coke or very preferably, of finely divided, solid electrolyte material is used.
' It is a further object to provide a method of fumeless baking of carbonaceous linings for electrolytic elements.
It is a still further object to provide apparatus whose function is to carry out the objectives mentioned above.
In a preferred embodiment of this invention the above objectives are best carried out by first covering the green pot lining mixture with a thin layer of solid particulate material. In most instances, `this material is added only 'once and its main function is to coact in protecting the surface of the lining from oxidation. Where the material is coke, the layer of ash formed from its oxidation serves the stated function.
The main source of heat consists of a series of panels each of which contains several heat producing elements whose principal emission is within the infrared range. The elements are combined with parabolic reflectors or other backing means above the elements to produce more uniform heat dispersion. The rate of heating is controlled by a variac (e.g. a variable auto-transformer) or any other suitable (power) control device for regulating the input current to the resistors or otherwise controlling the supply of power as explained below.
In -order to achieve fumeless baking, the heating elements are placed in close proximity to the lining material, that is, less than four inches from the surface of the lining and preferably about two inches from the lining. For proper placement, the heating panels are supported by suitable means which may be appropriately constructed or preset to dispose the elements at the desired height, or may involve adjustable means, such as adjusting screws, for attainment of such height. For instance, if a layer of coke on the lining is applied to a thickness of one inch, or a layer of other material to a thickness of one-half inch, and if the elements are then held above the coke or other material to the extent of another inch or inch and onehalf respectively, by the means mentioned above, then the distance between the infrared source and the lining 4material is the desired two inches.
Since it is necessary for there to be a steady supply of oxygen so that the burn off of the volatile matter is complete, the heating panels should be spaced far enough creased, and to that end it has been found lchat lsuch control may be attained with unusual success by relating the operation of the heater elements to the temperature over (rather than in) the carbon bed, e.g. the ambient ternperature as at the level of the elements themselves. Thus the operation can be conveniently such that the ambient temperature is progressively raised in accordance with a predetermined characteristic or plotted line, as may be achieved by progressive increase 1of power supply to the heating elements, for instance by stepwise lmodification of such supply during the cycle. In such case, the elements may be identical and identically powered across the surface of the lining, `but it has also `been found that in colaction with the provision for surface burning of evolved fumes, there is unexpected advantage, in maintaining uniform conditions within and along the lining bed, by the employment of temperature sensing devices at a multi- -plicity of localities above the carbonaceous surface (to measure local ambient temperature) and by correspondingly selective control of porwer supply to each adjacent element or group of elements.
As indicating above, the burning `of the evolved fumes from the lining (just on top of the protective layer) can be employed to account for considerable heat yin effectuation of the baking, thus reducing the electrical power needed. At the same time, it has been discovered that localized intensification of the operation may occur from time to time, manifested by a greater rate of fume evolution and considerably higher flame of burning at the place of such occurrence, all of such nature as to be representative of localized overheating, and in particular to tend to promote such overheating by the energy released from the greatly intensified combustion. It has been discovered that with the localized sensing of temperature at a multiplicity of places and corresponding individual control of the heating elements, the local disturbances can be obviated or compensated, indeed simply by turning off the current supply to the heating elements entirely at the particular place until the ambient temperature returns below the governing maximum for the then stage of the cycle. This control is preferably effected in an automat-ic manner, as by means described below, with the overall result that entirely uniform heating of the mass of pot lining is achieved in agreement with the predetermined or programed schedule.
In coaction, especially with the control of the heating elements as just explained, means are also provided for regulating the 'amount of air supplied to the Ivicinity of the lining surface, and also simultaneously for withdrawing and exhausting the gaseous products of combustion. Novel hood arrangements to this end are employed, whereby there is an avoidance of any tendency to promote undue co-mbustion of fumes at central areas of the lining surface or the like, such provisions of air s'upply and exhaust control being thus conductive to maintenance of burning at a uniform low level just above the lining, at all localities.
In the drawings:
FIG. 1 shows a vertical sectional View in perspective of one embodiment -of the invention for the baking of a reduction cell or furnace lining; l
FIG. 2 shofws a plan view of a group of heating panels arranged on the reduction cell or pot of FIG. 1;
FIG. 3 `shows a view in part Schematic of one heating panel, employed in FIGS. l and 2, as seen from underneath the panel;
FIG. 4 is 1an expanded view in perspective and in vertical section of one heating element, its parabolic reflector, and a portion of the panel frame;
FIG. 5 is a sectional view of a heating element;
FIG. 6 is a vertical section of a heating panel showing a modification in which the heating elements areused without reflectors;
FIG. 7 is a plan View of another reduction cell, showing (as a further and presently preferred embodiment) an arrangement of heating panels disposed over the lining, with temperature sensing means, the reflective or other :backing means -for such panels being removed in this view for clarity;
FIG. l8 is a plan View of a reduction cell or pot having a hood and exhaust structure disposed thereon, -i.e. as employed with the pot shown in FIG. 7;
FIG. 9 is a view, chiefly in longitudinal vertical section as if on line 9-9 of FIG. 8, but with portions of the hood and exhaust structure shown in elevation;
FIG. 10 is an end view of the apparatus of FIGS. 7, S and 9, with the pot shown tfragmentarily in vertical section and likewise parts of the structure above;
FIG. 11 is a diagram of an automatic control system for the heating means of FIGS. 7 to 10; and
FIG. 12 is a graph showing the overall heating operation and its control line, as with apparatus of FIGS. 7 -to 11.
FIG. 1 and FIG. 2 show two different views of one form of apparatus used in a process of baking .a carbonaceous lining 4on 1an aluminum reduction cell, in accordance with certain aspects of the invention. The pot or furnace, generally designated at 10, is usually an opentopped, moderately shallow, rectangular vessel, and consists essentially of a 'base 11 and pot lining material 12. In the usual Icase, this lining material consists of a mix of fine carbon particles and binder, such as pitch, tar and the like.
This mixture is tamped into the approximate shape of the lining, for example, to provide a horizontal, planesurfaced bottom portion (in which the conventional oonductor bars, not here shown, may be embedded) and side and end walls which may be vertical, or as shown, may slope downwardly and inwardly toward the bottom. Then in accordance with this example of the process, a thin layer of metallurgical coke 13 is placed on top of the pot lining material. The primary function of the coke is to protect the surface of the carbon lining from oxidation during baking. This coke is consumed during the baking process. Without a covering layer there is usually at least some pitting of the lining surface, but in some instances the pitting may be sufficiently shallow to be tolerable and thus permit omission of the layer.
After the coke has been laid on the pot lining body, heating panels 14 are arranged in a manner such as shown in FIGS. 1 and 2, i.e., so that they collectively cover substantially all of the exposed surface of the lining.
As indicated in FIG. 1, the panels 14 are rigid structures shaped to conform to the contours of the cell. Panel height adjusting screws 15 resting on cover plates 22 of pot 10 are used to support the heating panels 14. The desired height of the panels 14 above the coke 13 may be attained by the appropriate setting o-f the adjusting screws 15.
The panels consist of a `frame 18 and a series of heating elements 16 which run parallel to each other and to the plane of the lining material. The heating elements respond to a flow of electricity to transmit infrared heat energy. It has been found that infrared heat energy in the wavelength range of .3 to 4 microns is the most effective for baking cell linings. Each heating element 16 is associated with a reflector element 17 which roughly approximates the shape of a parabola as seen in FIGS. 1 and 4; thus where the heating elements are long straight rods as shown, the reflectors are correspondingly elongated, inverted troughs parallel to the rods and having a parabolic cross-section. In the embodiment as shown in FIG. 4, frame 1S holds the heating elements in a position corresponding to the focal point of the parabolic shape reflect-or elements. The purpose of this is to direct the reflected rays of heat in a parallel beam through the pot lining body.
FIG. 2 shows an arrangement by which the heating panels 14 are disposed so -as to provide maximum coverage of the lining material with heating elements, yet at the same time afford sufficient air circulation to encourage complete combustion of the volatile matter. It is an important feature of this invention that this volatile matter is burned off at the interface of the lining and surrounding atmosphere. If the source of heat energy were more remote from the surface, then the volatile matter would be likely to obstruct the penetration of the heat energy.
After the heating panels are set at the desired height above the surface layer of coke, electrical energy is supplied to the heating elements from a suitable power source. FIG. 3 shows a schematic of the electrical distribution system for the heating elements 16 in a heating panel 14. The amount of electrical energy supplied to the heating elements can be controlled by an adjustable transformer or other suitable power cont-rol device (not shown).
Since the proximity of the heating elements to the lining and the fumeless feature of this method -of baking produce such an efficient baking process, a direct relationship exists between power input and heat energy transmitted to the lining. Because of this direct relationship, the rate of heating can be precisely controlled by regulating the power input to the radiant elements. It has been found too that the heat penetration by infrared heating in this manner is so uniform throughout the mass of pot lining material that the variations in temperature within the same plane can be kept to plus or minus 10 C.
Although one embodiment involves the use of high reflectance reflectors such as highly polished parabolic shaped aluminum reflectors, it is not essential that these reflectors be of that shape or of such high reflectance. It is true that, in some cases, optimum efficiency may depend -on the geometry and reflectance of the reflectors; however, good results may still be obtained with more economical but poorer quality reflectors. In fact, it has been found that using suitably effective heating elements, for instance units where the conductors are sheathed in quartz, Iresults have been excellent even with rough corrugated stainless steel. FIG. 5 shows an element generally indicated at 16 consisting of conduct-or 19 enclosed in quartz she-ath 20. The unusually good results obtainable where the conductors are sheathed in quartz have been theoretically explained by the fact that the quartz sheath prevents the cooling of the elements by convection currents. Any other type of heating element may be used, however, which will give the required radiation and will withstand the conditions of the work.
Indeed, it has been found that with heating elements of the above type or of equivalent effectiveness, it is not always necessary to use reflectors at all, or to use anything but a simple backing structure having relatively little reflecting power. Thus, FIG. 6 shows a row of heating elements 16, for example of the kind in which the conductors are sheathed in quartz, and a plain asbestos sheet 21 instead of a row of reflectors. The elllciency achieved in Ia process involving this combination is of the same order as that involving stainless steel reflectors.
The particular physical disposition of the panels shown in the embodiment of FIGS. 1 and 2 is not an essential feature of this invention. The same advantages may be achieved through any of a variety of arrangements incorporating the key features of the invention. For instance, instead of spacing the panels as shown in FIG. 2, the panels might be provided with slots to produce the necessary circulation. However, it should be clear that as the distance between the heating elements and the lining increases, the fumeless feature and the efficiency of the process drop off sharply. This is particularly true where that distance is greater than four inches.
It is not necessary that heating panels 14 be rigid in structure. If circumstances made it desirable, these panels could be hinged at appropriate places in which case the same panel could be used for baking linings on cells of a variety of different contours.
It may be further explained here that in baking a mass of carbonaceous pot lining or the like the progressive stages at any locality of heating are as follows: First, the binder (by which the carbon particles are held together) becomes progressively softer, to the point of melting, rendering the material plastic and relatively impermeable to gases. As heating progresses, the binder begins to decompose and gases -begin to evolve, forming capillary channels through which they escape, in the form of the fumes described above. After some time the lbinder is converted, in effect, to coke and the entire plastic mass or zone congeals into a coke, i.e. a hard, coherent state. It has been found that a desirable mode of heating is such that this sequence of changes occurs progressively deeper in the linin-g, from the surface. Thus as heat progresses deeper, the plastic zone in effect moves downwards leaving a bakedout, ooked mass at the top, characterized by vertical orientation of the above-described capillary structure.
A particular feature of the present invention is its susceptibility to careful control, so that the rate of heating can follow a gradual curve or increase. It has thus been discovered to be extremely desirable that travel downward of the baking operation (from the upper, outer surface of the carbonaceous body) be appropriately gradual and uniform; where the heat penetrates downward at a very fast rate, the period of gas evolution is correspondingly short but of greater intensity, occurring over `a greater depth, with the result of undesirably larger pores and indeed usually the formation of vertical cracks in the ultimately solidified body. With the careful control attained in the present invention by infrared heating, the above difficulties can be readily avoided. Thus it is believed that the infrared emission heats the surface regions of the lining body by radiation, with further penetration of the heat downward by conduction. Inasmuch as evolved gases at any level are blocked by the unbaked or plastic material (or other structure) below, they can only depart through the porosity, preferably of a very fine capillary nature, that is developed in regions nearer the surface. Since the present process takes account of this condition, by providing a controlled, progressive baking of the body downwardly from the surface, the result is a more solid, stronger and more uniform baked body of truly monolithic character,
As indicated above, the combustion of the fumes adjacent the surface o-f the lining is advantageous in a number of respects, including the contribution of such burning to the requirement of heat energy for baking. Where, however, as sometimes occurs, combustion may intensify at certain localities or at certain times in the cycle of operation, there can be a tendency to overheating, usually localized, which may interfere with accurate control of the baking operation by simple regulation of the power supply to the heaters. Thermocouples can be embedded in the lining, but difficulty of control in such situation may still be experienced, for example in that overheating may progress too far before it significantly affects an embedded temperature-sensing element.
It has been discovered, however, that extraordinarily effective heating control is attainable by programming the ambient temperature above the lining surface, e.g. in the proximity of the infrared heating elements, a particularly effective process involving (as explained above) the sensing of the ambient temperature at a multiplicity of localities distributed over the surface and subjecting the power supply to control, both as an entirely in accord with a desired temperature curve and also individually, again with reference to such curve, in the separate localities of temperature sensing.
T/hus as indicated diagrainmatically in FIG. 12, an overall heating or maximum temperature curve for the baking of the lining of a typical pot is indicated at 30, such temperature (left hand vertical axis) being the ambient ternperature within a few inches of the surfa-ce and the desired characteristic being that it rise gradually over an interval of many hours (measured along the horizontal axis), i.e. a baking time which may extend, for example, up to hours or more. It is found that this overall characteristic may generally be maintained by progressively increasing the power or rate of energy supply (right hand vertical axis) to the infrared heaters, as by stepwise increase represented by the line 31, plotted against the same horizontal axis of time.
In general it is therefore desired to maintain the target ambient temperature, of line 30, throughout the region just above the lining surface, so that both the average temperature and the temperature at individual localities is kept close t-o but not appreciably above this curve. The plotted curve 32 is a representative illustration of temperature measurements (e.g. an average over the surface) in a satisfactorily controlled baking cycle. Despite a programed regulation of the power supply, but with no other control, it was discovered that the ambient temperature occasionally arose above the curve 30 to a substantial degree, e.g. as indicated at the dotted line 33, and that this condition meant an overheating, usually localized at one place or another, with some corresponding detriment t0 the properties or structure of the baked lining. It at first appeared difficult to detect this overheating, especially in itslocalized nature, and thus even to be aware that it was occurring, but with the preferred procedure of the present invention, which includes sensing the ambient temperature over the surface at a large multiplicity of\ places, the difficulties are overcome. Indeed it is found that by simply cutting off the power supply entirely at the sensed localities of such excess ambient temperature, the heating curve, even for those localities, is maintained essentially as represented at 32 and without rise above the target line 30.
When the power is cut off, the temporarily intense combustion at the locality of difficulty is not then supplemented by the infrared radiation, so that the net heating effect is correspondingly limited. This limitation of heating reduces the tendency to high burning, and upon sufficient reduction of the condition the infrared power supply can be restored or increased if it had only been reduced), for resumption of normal conditions.
Referring now to FIGS. 7 to 10 inclusive, especially FIG. 7, an aluminum reduction cell 40 having an outer casing 41 and a thick lining 42 of carbonaceous material in the so-called green state, is subjected to baking by a multiplicity of horizontal panels 44 covering the large bottom area of the cavity, i.e. the flat upwardly facing surface 45 which constitutes most of the lining surface. In FIG. 7 the shielding or reflecting covers 47 (FIG. 9) for the panel assemblies are removed for clarity of illustration. Conveniently the sloping side faces of the pot cavity, constituted by lining material, are faced with narrower panels or units as indicated at 48, i.e. both along the sides and ends. In this arrangement, the infrared heating elements S0 are of elongated U-shape, providing electrical terminals at their ends, which may be suitably connected, e.g. in effect in parallel, for supply of electrical power thereto.
At a considerable number of spaced localities over the bottom and preferably also at the sides and ends, there are temperature-sensing elements, i.e. thermocouples as indicated at 52a, 52b, 52C, generally designated 52. Although other dispositions can be employed, preferably in the plane of the heating elements, which are disposed within about four inches and usually at about two inches, from the carbon surface 45, the thermocouples 52 are here shown as situated within the arms of the U of alternate heating elements 50, disposed toward the outer curved ends of these elements. Similar thermocouples 52 are located at center regions of the side and end walls of the cavity, again preferably in the plane of the heating elements, all as shown.
Referring to FIGS. 9, 10 and 11, the cavity is covered by la shallow upwardly enclosed inner hood 60, e.g. in the shape of a large inverted pan having downward skirts or walls 61 at the sides and ends, which may be arranged .9 to rest, with a little spacing by projecting portions (not shown) or natural unevenness, on the upper borde-ring faces of the lining body as at 62. Supported above the hood 60, `a further, and similarly upwardly enclosed hood 64 is disposed, having an outwardly flaring skirt 65 at all sides, that extends laterally beyond the skirt 61 of the hood 60 and overlaps it downwardly to a substantial extent, of the order of half the height of the skirt 61. For promoting removal of gases, the upper hood 64 may be of domed or similar raised configuration at its center, with suitable exhaust means, as including the exhaust pipe `arms 67, 68, such conduits leading to a blower or exhaust fan 70, which thus draws the gases from beneath the hood 64 and propels them outwardly to a suitable stack or other discharge (not shown), via the conduit 72.
The heating panels 44 may be appropriately connected together for mutual support and can be -arranged to seat in spaced relation on and above the lining, or can be suspended from the hood structure as by appropriate means indicated at 74, extending between the panel assembly and the lower hood 60. Similar supporting or suspending means 75 may extend yfrom the upper hood 64 to the lower hood 60, and thus with the exhausting ductwork Vand fan carried on the upper hood, the entire assembly constitutes a baking unit which may be readily applied to a line pot 40, either by lowering the suspended structure over a stationary pot, or by bringing the freshly lined pot shell to a locality where it may be raised up against the underside of the assembly.
The hood system of FIGS. 8, 9 and 10 serves: to reduce heat losses above the panels; to prevent discharge of the combustion products into the working area of the plant; and to afford control in the access of air and thus in the combustion of the evolved fumes during the heating operation. Because the lower skirt 61 affords only a narrow space between it and the walls of the lined pot, i.e. at 77, an unduly large ow of air into the heating region is prevented; indeed even an extremely limited opening is found to afford sufficient air for desired burning. Moreover, the only path of escape from the combustion products, i.e. the gases of combustion, is outwardly through this same region, so that there is no tendency for the gases which leave the burning area to draw in large quantities of air, as might be the case if the combustion gases were exhausted centrally and upwardly of the pot cavity. By virtue of the outwardly flaring and depending skirt 65 of the upper hood 64, the Laction of the exhaust fan 70 is to draw air into the upper hood in the space below the skirt 65. At the same time this upward draft beneath the skirt 65 pulls the exhaust gases that escape at the region 77 al-ong with it, so that they too are carried out, by the fan, through the exhaust duct system 67, 68 and 72. In this fashion, the products of combustion are effectively removed from the pot area without centralized drafts or the like which would promote or intensify combustion as at the middle of the pot cavity, while the leakage space 77 through which the gases depart, in relatively gentle How, is also adequate for inlet of all the air necessary to support the burning of the fumes along or just above the lining surface 45.
The arrangement constituted Iby the lower hood 60 and the upper hood 64 thus provides for the controlled removal of gases from the region above the pot lining sur- Iface, so as to minimize excessive localization or nonuniformity in the burning of fumes. Although some release of gas may be provided at other localities as by a few small holes (not shown\ 1'n the roof of the lower hood 60 if better distribution of discharge gas flow requires, the construction and arrangement are even in those cases, advantageously such that substantially all of the gas is withdrawn sidewise from the region, or lat least the major quantity of gas is so withdrawn.
Although the process of control of Ithe heating operation as explained above can be performed manually or semi-automatically, for instance as by frequently noting the .reading of conventi-onal meters connected to the thermocouples 52 and turning olf the supply of current to the adjacent pair of U-shaped heater elements 50 when and so long as the :reading is above the predetermined value for the then existing time or stage of the baking cycle, and by progressively raising the overall power supply to all the elements 50l at successive times pursuant to the same predetermined curve (as curve 30` in FIG. l2), the operations are adapted to fully automatic control by an appropriate and novel combination of conventional instrumentalities as illustrated in FIG. ll. Referring to the latter, it will be seen that three of the pairs of infrared heater elements 50l .are indicated at 50a, 50b and 50c, with their associated thermocouples 52a, 52b and 52C, in accordance with the physical arrangement of FIG. 7. It will be understood that the circuit of FIG. l1 is applicable to all of the thernocouples 52, erg.' a total of 18 as shown in FIG. 17, the inclusion of same being merely indicated by the representation of a thermocouple 521' with the set of heater elements 50r. The manner of extension of the system of FIG. l1 to all of these other thermocouples and to the corresponding control of the other heater elements will be readily apparent from what is shown in FIG. 1l and described hereinbelow, so that it will suffice to show and describe specifically only the three thermocouples 52a, 5212 and 52C and the related control devices therefor.
For further simplicity, most of the circuit connections in FIG. l1 are shown as single lines rather than complete electrical circuit, it being understood that such circuits and their usual means of energization are constructed and completed in accordance with well known practice of electrical control systems. The infrared elements 50 are arranged to be energized in parallel (this circuit being shown somewhat more fully) from an alternating current source 80, which may include a transformer or other device (not shown) whereby the necessary current is supplied at appropriate voltage. Specifically, the elements 50a, 50b and 50c are arranged in pairs, with the paired circuits to the line separately controlled by power relays 82a, 8212 and 82C respectively, which may be closed or open under appropriate control as described below. While other modes of adjustment or change of the power are usable, a particularly convenient arrangement, for realization of a progra-m -as at 31 in FIG. l2, involves a group of selectively operated percent timers such as the timers y84, `85, 86 and 87, these being conventional devices, each adapted, by continuous cam control or otherwise, to energize the power relays 82a to 82C periodically, with corresponding off periods, whereby current flows to the heater elements 50 during a stated percentage of the time, in accordance with the time that is selected to operate in governing relation. These timers are known, conventional devices, having the described function.
Thus for example, the timer 84 may be adapted to effect closure of the power relays for 30% of each minute of time, the timer 85 may be similarly adapted to maintain the relays closed for 40% of each minute, the timer 86 for 50% of each minute and the timer 87 for 60% of each minute. Hence by operating the timer 84 for the first 30 hours of the baking cycle, a relatively low power input to the heaters is achieved. Then by switching the timer 85, a somewhat greater power input is had for a succeeding interval of 30 hours, with correspondingly larger supply in successive periods of hours with the sequential use of timers 86 and 87 in the further parts of the cycle. This sequential operation is indicated by the steps 84a, 85a, `86a and 87a -of the power input line 31 in FIG. l2. It will be unldestood that although the selected percent timer such as 85 effects supply of current to the heaters for only a certain fraction of each minute (say a 24 second on period and a 36 sec-ond olf period), the resulting effect may be measured and considered as a control of power to the infrared elements. For convenience the measure of power so controlled can be expressed in special terms, such as kilowatt-hours per hour, but it is still properly a measure of power (equivalent to kilowatts), i.e. as in effect a measure of the rate of delivering energy. As will now be readily apparent, the stated units, such as kilowatt-hours per hour, or similar units, can be utilized for power delivery over intervals of less .than -one hour as well as longer times.
For control of the percent timers, the thermocouples may be connected to an appropriate potentiometer-recorder device generally designated 90, which may be of known type of embody known instrumentalities, including scanning means as for successively noting the te-mperature readings of the several thermocouples and delivering control signals over individually corresponding control circuits. Such control circuits may extend to the latch relays 92, specifically latch relays 92er, 92b and 92C, which correspond respectively to the thermocouples 52a, 52b and 52C and likewise to the power relays 82a, 82h and 82o. The recorder-scanner instrument 90' is also controlled by a programer 95,-which may likewise comprise conventional instrumentation, including appropriate rotary or Ilinear cam means designed for delivering a temperature level signal, as by connection 96, to the recorder-scanner 90, `and also for delivering a sequence or stepping control signal through a line 97 for programing the percent timers 84 to 87 inclusive. Thus it will be understood that the device 95 is set to control the entire program of the baking cycle as over a desired total period of 104 hours, and to that end delivers a maximum temperature signal through the line 96, in accordance with the curve 30 of FI-G. 12, and likewise through the line y97 delivers stepping signals in .accordance `with the line 31 of FIG, l2, as to a stepping switch 98 of conventional character, which is connected to the group of percent timers so ythat the sequentially selected timer is brought into and maintained in operation or in controlling function, during its desired portion of the programmed cycle.
As will be noted, the control function of the selected percent timer is exerted, as through the multiple connector 99, to the power relays 82a, 82h, 82C in ganged relation, so that basically the latter relays are energized and deenergized together in the periodic fashion described above with relation to the timer. The individual control lines 100a, 100b and 100C extending to the several power relays are under control respectively, however, by the latch relays 92a, 9212 and 92C, these being all normally closed for maintenance of control to the power relays, i.e. so long as the temperatures read from the thermocouples, in the device 90, are all at or below the maximum or target temperature then signaled by the connection 96 from the programer 95. Where as indicated above, the scanner device periodically over a short cycle, reads the temperature signals from all the thermocouples in succession, and when and as one of such temperatures exceeds the target or maximum temperature signaled at 96 from the programer 95, the corresponding one of the relays 92a, 92b, 92C is operated to break the connection between the percent-timer and the related power relay.
Thus for example if thermocouple 52b is found to sense an overheating condition, so that the ambient temperature is higher than the programed target temperature, the corresponding latch relay 9Zb is operated, opening the control circuit to the power relay 82h, which then does not close under the control of the currently selected percent timer, such as percent timer 85, and no power is delivered to the elements 50h. Although other types of relays may be used, the devices 92a, 92b, 92e are conveniently latch or locking relays as stated, having conventional means (not shown) for holding them in the open-circuit position until released by appropriate signal for restoration of their closed function in the circuits to the corresponding power relays. It will therefore be understood that the device 90 may include appropriate conventional means, similarly functioning on the scanning cycle, to release any previously operated or latched relay of the 12 set 92a, 92b, 92C when and as the temperature reading from the corresponding thermocouple is found to have dropped below the target or maximum temperature signaled by the programer.
summarizing, the procedure effectuatedby the system in FIG. l1 thus maintains the rate of delivery of energy, i.e. the power supply, to all of the infrared heater elements 50 at successive values in accordance with a predetermined program, e.g. represented by the line 31 in FIG. l2 and brought about by the sequential functioning of the timers 84 to 87. At the same time, the` ambient temperature is in effect continuously sensed at a large number of localities, and to the extent that the temperature at one or more localities is found to exceed the then desired value (pursuant to the line 30 in FIG. 12), as signaled at 96 from the programer 95, appropriate modiction of power to the heating elements in the noted area or areas is effected, e.g. conveniently by turning off such power at the related power relay. When and as the ambient temperature at any given area is-restored, usually in a very short time, to the programed target value or below, power supply is resumed to the related heating elements, as by reestablishment of control of the corresponding power relay from the percent timer. As explained above, the regulation thus afforded is found to provide effective maintenance of uniform temperature conditions in the baking operation, throughout the lateral extent of the lining, and to provide the desired gradual increase of heating as progress is made through the complete baking cycle.
Although the indicated arrangements are such that if a large number or even all of the regions of temperature sensing are found to be overheated, the current supply to all elements is duly interrupted (e.g. as the scanning sequence of the device is carried out), it has been found that this seldom if ever occurs. Tests have indicated that the overall target curve, as at 30 in FIG. 12, may be very easily determined for a given set of conditions, and especially with properly controlled burning of the fumes, there are usually only a few individual areas, at any one time, that require the described operation for suppression of overheating. A thoroughly reliable heat control is achieved, indeed to the extent that the baking operation is far more reproducible than has been heretofore possible with conventional resistance baking methods or the like. As a result, when successive pots in a given line come up for relining, the baking operation can be essentially identical in every case, thus promoting uniformity and ease of control of the aluminum reduction operation for the entire series of working pots.
The system may include appropriate recording elements (not specifically shown) for registering the temperature conditions at the several localities of sensing, and likewise other indicia representative of operating or non-operating conditions of the several relays and other parts, for instance as indicated by the signal lights 104, 105, 106 and 107 associated with the percent timers 84 to 87 inclusive, and individually lighted, as at 105, to indicate which timer is currently in use. Although in some instances the infrared heating elements along the side walls and end walls of the lining cavity may be omitted, and with them the thermocouples in such places (the large number of heating panels across the floor of the cavity being sometimes sufficient for the whole baking operation)l and although in other situations the control with respect to ambient temperature at such side and end walls may be exercised at a value which is constantly lower than the target ambient temperature for the large upwardly facing bottom area, the drawings for simplicity show arrangements wherein heating units are employed at all localities, and the temperatures are all controlled alike.
In all cases, the surface of the lining is preferably covered by a layer 110, say one-fourth inch thick, of finely divided material such as alumina, cryolite or mixtures thereof.
`rise above the curve.
l -at the bottom and upwards of 6 inches at the sides.
formed body of carbon and binder. 'to infrared baking in accord-ance with the present invenasse/toe 13 By way of example, a lining for a pot cavity (which may or may not have an outer, insulating refractory layer, not shown in FIGS. 7 to 10) composed of con- Ventional carbon particles and binder and shaped to provide a cavity inside of said lining which is about 10 'feet wide and 18 feet long at the top, and 15 inches deep,
lwas found to be served, for purposes of installation and subsequent breaking-in on the line of aluminum reduction cells, by a total baking cycle of 104 hours. Specifically, with a lay-out of heater units substantially as 35% on for 30 hours,
45% on for 30 hours,
50% on for 24 hours, and
65% on for the remaining hours.
The ambient temperature was maintained at or closely below an approximately straight line characteristic rising from about 20 C. to 690 C., with no appreciable The product was a hard, well baked monolithic body of pot lining, free of cracks or the like and ready for startup.
A particular advantage of the air and gas control as in FIGS. 8 to 10 is that lwhen a few heating panels are in off condition and the remainder are supplied with power, the heat distribution is not disturbed by the draft.
As will be appreciated, the procedure is effective for relatively thick layers of carbonaceous material; for instance in the above example, the carbon lining across the bottom of the pot was 14 inches thick, and l0 to 14 inches thick along the sides. The process is generally Y applicable to like linings having substantial thickness of at least several inches, such as thicknesses ranging upward from four inches, and indeed usually 12 to 18 inch-es As will also be appreciated, the invention is primarily concerned `with long baking intervals, over which the controlled infrared heating may be applied, e.g., times exl tending upward Ifrom 10 hours, or more commonly 50 vhours or longer, and indeed under present contemplated practice, periods of the order of 100 hours.
While the foregoing description has been specifically related to linings formed by a continuous mass of a mixture of carbon particles and binder of conventional character, the operation is also applicable to linings constructed of preformed, green blocks or bricks laid up with a mortar or joining paste of carbon and binder, the blocks being themselves essentially unbaked, i.e., a pre- When subjected tion, a lining of this block or brick type is completed in a thoroughly uniform manner, baking the blocks and the mortar together and resulting in an essentially monolithic structure of hard, strong, fully coked nature.
While I have shown and described preferred embodiments -of my invention, other modifications thereof will readily occur to those skilled in the art and I therefore intend my invention to be limit-ed only by appended claims.
1. A method of baking a carbonaceous lining which is disposed, to a thickness of at least several inches, in an upwardly facing cavity of a vessel and which is composed essentially of carbon and binder, adapted to release combustible fumes on heating, and is capable of being baked to a hard, coherent state, comprising radiating infrared heat energy substantially downwardly to and throughout the surface of said lining for a sufficient length of time from a multiplicity of sources of said energy disposed above and distributed over said surface,
to cause evolution of said fumes and to bake said lining to said hard, coherent state, and while admitting air to the vicinity of said lining for combustion, burning said fumes `in a r-egion closely above said lining by maintaining said sources, during said radiation, sufficiently close to said lining, within a spacing therefrom of not more than four inches, to effectuate such burning.
2. A method as defined in claim l, wherein the heating Iis effected by radiating said infrared heat energy in the range of wavelengths of 0.3 to 4 microns, said heating being -continued to bake the lining to a temperature yabove 500 C.
3. -A method of baking a carbonaceous lining which is disposed, to a thickness of at least several inches, in an upwardly facing cavity of a vessel and Iwhich is composed essentially of carbon and binder, adapted to release combustible fumes on heating, and is capable of being baked to a hard, coherent state, comprising radiating infrared heat energy substantially downwardly to and throughout the surface of said lining from a multiplicity of electrically powered, infrared heating elements disposed above and distributed in proximity to each other over said surface, to cause evolution of said fumes and to heat said lining to a temperature above 500 C., said infrared heating being continued to bake the lining to a vhard, coherent state, and while admitting air to the vicinity of said lining for combustion, burning said fumes in a region closely above said lining by maintaining said elements at a level sufficiently close to said lining, within a spacing about two inches therefrom, to effectuate such burning.
4. A method of baking -a carbonaceous lining which is disposed, to a thickness lof at least several inches, in an upwardly facing cavity of a vessel and which is cornposed essentially of carbon and binder, adapted to: release combustible fumes on heating, and is capable of being baked to a hard, coherent state, comprising radiating infrared heat energy substantially downwardly to said lining Iand substantially uniformly through-out the surface of said lining, form -a multiplicity of electrically powered sources of said energy disposed above and distributed over `said surface, to cause evolution of said fumes and to bake said lining, and while |admitting air to .the vicinity of said .lining for burning said fumes, maintaining said sources, during said radiation, sufficiently close to said lining, within a spacing therefrom of not more than four inches, to effectuate combustion of said fumes adjacent the lining, said radiation of heat energy being continued for a sufficient time, while controlling the supply of power to said sources to maintain the temperature of the lining substantially uniform over its surface, to bake said lining to a temperature above 500 C. and to convert it to `a hard, coherent state.
S. A method as defined in claim 4, in which said control of power supply includes detecting the ambient temperature Iabove the lining 4at a multiplicity of localities `distributed over said surface within the same aforesaid spacing therefrom of not more than four inches, Vand reducing the supply of power to each individual source when and so long as the ambient temperature detected at la locality adjacent said individu-al source is higher than a desired temperature, lto prevent overheating of s-aid lining.
6. A method 'of baking -a carbonaceous lining that is` composed essentially of carbon and binder and is capable of being baked to a hard, coherent state, in an upwardly facing cavity of a vessel, comprising radiating infrared heat energy substantially downwardly to said lining from a `multiplicity of individually controllable sources of said energy disposed closely above and distributed over said surface within ya spacing of no-t more than four inches therefrom, ldetecting the ambient temperature above s-aid lining at a multiplicity of localities distributed over said surface within said same spacing therefrom, .and controlling said energy sources to prevent overheating of said lining, by reducing the delivery of energy of each individual source when and so long as the ambient temperature detected at a locality adjacent said individual source is higher than a desired temperature.
7. A method as defined in claim 6, for baking such carbonaceous lining having a thickness of at least several inches and providing the substantially flat bottom of an aluminum rreduction cell that comprises such lined vessel, which includes maintaining a multiplicity of electrically powered infrared heating elements constituting said sources, over the surface of s-aid lining within said spacing of not more than four inches therefrom, Iand while continuing the heating of said lining by `said elements over a period of many hours to bake the lining to a hard, coherent state, controlling the supply of electrical power to all of said elements together for maintenance of -ambient temperature conditions in accordance with a predetermined characteristic throughout the surface, the aforesaid control of the individual energy sources by Ireducing energy delivery thereof being effected by interrupting power supply to individualheating elements when and so long as the ambient temperature adjacent such elements is higher than a temperature that accords with said predetermined characteristic.
8. A method as dened in claim 7, wherein said lining releases combustible fumes during said heating by the infrared elements, and which includes admitting air to the vicinity of said lining for combustion, burning said fumes in a region closely above the lining, said burning being eifectuated by maintaining said elements close to the lining surface, and limiting drafts of Iair and burning fumes over the lining, to inhibit localized excess heating, by conning the space above the cavity, and restricting passa-ge of `said air inward and gaseous combustion products outward substantially to regions adjacent the lateral boundaries of the cavity, and withdrawing gaseous combustion products from localities immediately outside said last-mentioned regions, for exhaust of said products.
9. A method of baking an upwardly facing carbonaceous lining having a thickness of at least several inches, that is composed essentially of carbon and binder and is capable of being baked to a hard, coherent state, comprising radiating infrared heat energy substantially downwardly to and throughout the surface of said lining, from a multiplicity of individually controllable sources of said energy disposed above and distributed over s-aid surface and having a close spacing therefrom of not more than about two inches, and while continuing said radiation of heat to said lining to bake it to a temperature Ia-bove 500 C. and to convert it to said hard, coherent state, detecting the ambient temperature above said lining at a multiplicity of localities distributed over s-aid surface and within about the same aforesaid spacing therefrom, and controlling said energy sources to maintain a lsubstantially uniform temperature over said surface, by reducing the delivery of energy of each individual source when and so long as the ambient temperature detected at a locality adjacent said individual source is higher than a desired temperature,
10. A method as defined in claim 9, wherein said lining i releases combustible fumes during heating, and which includes admitting air to the lining for combustion, burning said fumes in a region closely above the lining by maintaining said elements within said spacing of two inches from the lining surface, and withdrawing gaseous combustion products of the burning fumes substantially only laterally, and outwardly at side boundaries of the lining, to inhibit localized excess burning.
11. A method as dened in claim 10, which includes maintaining a layer of finely divided inert solid material over the surface of the lining during said infrared heating, to inhibit deterioration of said surface.
12. A method of baking a carbonaceous lining having a thickness of at least several inches, that is composed essentially of carbon and binder and is capable of being baked to a hard, coherent state, in a cavity of a vessel,
comprising radiating infrared heat energy toward ,and throughout the surface of said lining, from a multiplicity of individually controllable sources of said energy disposed close to and distributed over said surface and having a spacing therefrom of not more than four inches, detecting the ambient temperature adjacent said lining at a multiplicity of localities distributed over and close to said surface within said same spacing therefrom, and controlling said energy sources to provide heating of said lining in accordance with a predetermined program of temperature during the course 'of baking the lining, said control of the sources including reducing the delivery of energy of each individual source when and so long as the ambient temperature detected at a locality adjacent said individual source is higher than a desired temperature under said program.
13. A method as dened in claim 12, in which said lining is adapted to release combustible fumes on heating and constitutes the upwardly facing lining of an aluminum reduction cell and said infrared energy sources are electrically powered, said method including admitting air to the lining for combustion, and while maintaining the spacing of the sources and of the detection of ambient temperatures within about two inches of the lining surface, burning fumes .adjacent the surface of said lining under the influence of said energy sources, and said reduction of energy of individual sources being effected by interrupting electrical power supply to each individual source during excessive ambient temperature conditions adjacent such source as aforesaid.
14. A method as defined in claim 1, which includes confining the space above the cavity and the energy sources, restricting passage of air int-o said space, for said admission of air to the lining, and passage of gaseous combustion products out of said space, substantially `to regions adjacent the lateral boundaries of the cavity, and withdrawing gaseous combustion products from localities immediately outside said last-mentioned regions, for eX- haust of said products.
15. Apparatus for baking a carbonaceous lining in an upwardly facing cavity of a vessel, such lining being cornposed essentially of carbon and binder and being capable of being baked to a hard, coherent state, comprising an assembly of a multiplicity of substantially coplanar infrared heating elements arranged in a lateral array and havv ing means for supporting said elements, said means comprising structure adapted to hold the assembly in proximity to the aforesaid lining so that the elements are disposed close to the lining surface in distribution thereover, to radiate infrared heat energy toward said lining, cover means having lateral dimensions at least substantially coextensive with said assembly of elements and disposed over said assembly in enclosing relation thereto, to confine gases above said assembly, said cover means including supporting means therefor adapted to hold the cover means in covering relation to said lined cavity of the vessel, said cover means being constructed and arranged to provide gas communication between the cavity covered thereby and the exterior thereof substantially only at side regions of the cover means, and gas withdrawal means in cluding structure disposed over said side regions of the cover means for receiving gas passing from beneath said cover means at said side regions, and exhausting means communicating with said structure, said gas withdrawal means being constructed and arranged so that said lastrnentioned gas is drawn up into said structure by said exhausting means, for removal.
16. Apparatus for baking a carbonaceous lining in an upwardly facing cavity of a vessel, such lining being composed essentially of carbon and binder and being capable of being baked to a hard, coherent state, comprising an assembly of a multiplicity of substantially coplanar infrared heating elements arranged in a lateral array and having means for supporting said elements, said means comprising structure adapted to hold the assembly in proximity to the aforesaid lining so that the elements are disposed close to the lining surface in distribution thereover, to radiate infrared heat energy toward said lining, a hood disposed over said assembly in enclosing relation thereto to conne gases closely above said assembly, said hood having means for seating it in covering relation to the cavity of the vessel and being constructed and arranged for passage of air into and gas out of the space under said hood substantially only through limited areas at side regions of said hood, a second hood disposed above and covering the rst hood, to provide a conned space thereover, said second hood having depending skirt means surrounding and spaced outwardly from the sides of the first hood, `and means for exhausting gases from the second hood, said exhausting means and said rst and second hoods being mutually constructed and arranged so that gases departing from said side regions of the rst hood 18 are drawn up into the second hood for removal by said exhausting means.
References Cited by the Examiner UNITED STATES PATENTS 2,891,297 6/ 1959 Klemm 264-29 3,107,212 10/1963 Landucci 204-243 3,148,272 9/1964 Aitken et al. 219-347 3,149,223 9/ 1964 Zimmerman 219-347 OTHER REFERENCES Modern Plastics, vol. 30 (August 1953), pages 109-111, 114 and 115.
TOI-IN H. MACK, Primary Examiner.
H. S. WILLIAMS, Assistant Examiner.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,293 ,406 December 2O 1966 Gordon Alexander Bain It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column l line 60 for "staked" read baked column 2 line 27 for "On" read One line 39 for "this" read the column 4 line 22 for "indicating" read indicated line 55 for "conductive" read conducive column 7 line 65 for "entirely" read entirety column l0 line 30, for "circuit" read circuits line 39, for "the" read their column l3 line 63 before "appended" insert the column 14, line 40, for "form" read from Signed and sealed this 19th da)7 of November 1968.
Edward M. Fletcher, Jr. EDWARD j. BRENNER Attesting Officer Commissioner of Patents