US2512442A - Solid material heating apparatus - Google Patents

Description

June 20, 1950 C. L. NORTON, JR

SOLID MATERIAL HEATING APPARATUS Filed 001;. 31, 1945 INVENTOR Char/es Norton Jr:

BY I

ATTORNEY 09w M A2 x 3 Patented June 20, 1950 SOLID MATERIAL HEATING APPARATUS Charles L. Norton, In, New York, N. Y., assignmto The Babcock & Wilcox Company, Rockleigh, N. J., a corporation of New Jersey Application October 31, 1945, Serial No. 625,776

2 Claims. (Cl. 263-19) The present invention relates to the construction and operation of furnaces or kilns of the vertical shaft type for the direct heating of a fluent mass or column of solid material to a big temperature.

Industrial processes lIlVOlViIlg the heating of solid materials to a high temperature have usually been characterized by a very low thermal efficiency, despite efforts to conserve heat by the use of large amounts of heat insulating material in the heating apparatus and various methods and apparatus for recovering sensible heat from the outgoing heating gases. Such low thermal efficiencies, even in continuous heating apparatus, have been mainly due to the large heat radiation losses from the heating apparatus during the relatively long heating cycle, heat storage losses in the charge and in the mechanism for conveying the material to be heated through the heat zone, and the sensible heat in the outgoing heating gases. The addition of heat recovery devices to recover heat from the outgoing gases increases the thermal efiiciency, but does add to the initial apparatus investment and apparatus space requirements. Many such industrial installations have thermal efiiciencies as low as 5%. The term thermal efliciency is used in this art to define the ratio of t e net heat put into the charge to the potential heat in the fuel. I J

The general object of this invention is the provision of an improved apparatus for continuously heating a fluent mass of solid material which is particularly cliaraterized by a substantially continuous movement of the solid material to be heated through a furnace or kiln of the vertical shaft type having a special arrangementof the inlets for complementary fluid combustion constituents relative to the moving column of material and to each other providing an extremely high thermal efliciency, a, resulting low fuel consumption, and the continuous production of furnace temperatures in a range whose upper limit is dependent only upon the physical properties of the refractory materials employed in the furnace or kiln construction. A further and more specific object of my invention is the provision of a furnace or kiln of the character described which is constructed and arranged to continuously effect a successive heating of a fluent solid material to an elevated temperature,

soaking the heated material at a substantially uniform elevated temperature for a predetermined period, and then cooling the material to a low temperature before its discharge from the kiln. An additional object of my invention is 2 v to the heating apparatus or the material to be heated.

The various features of novelty which characterize my invention are pointed out with particularity-in the claims annexed to and forming a part of this specification. For a. better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to'the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodimerit of my invention.

Of the drawings: v

Fig. 1 is a somewhat diagrammatic sectional elevation of a vertical shaft kiln constructed in accordance with my invention;

Fig. 2 is a horizontal section taken on the line 22 of Fig. 1, and I Fig. 3 is a horizontal section taken on the line 3-3 of Fig. l.

While in its broader aspects this invention is adapted for the continuous heating of a wide range of solid materials to a wide range of final temperatures, it is particularly designed and especially useful for the continuous heating of a fluent mass of solid material to a wide range of high temperatures, such as 2000 F. and over;

In the drawings I have illustrated my invention as embodied ina vertical shaft kiln for the burning or vitrification of ceramic refractory pellets intended for use as heat transfer material in fluid heating apparatus of the general type disclosed in the copending application of E. G. Bailey and R. M. Hardgrove, Serial No. 502,580, filed Sept. 16, 1943, now Patent 2,447,306 of Aug. 17, 1948. Pellets of this character may be formed, for example, from a plastic mixture of precalcined Georgia kaolin, raw Georgia kaolin, water and a binder. The mix is extruded from the dies of an extrusion press and cut into small cubes, which are passed through a rotating cylinder to shape the same into substantially sperical pellets having a diameter in the size range of 4-1". The pellets after being dried in a drying ovenmust be fired to a temperature in the range of 2500-3000 to secure the high strength, hardness, and resistance to thermal shock necessary for their described use. refractory compositions applicable for this purpose require still higher burning temperatures.

The vertical shaft kiln of my invention consists of a vertically elongated generally cylindrical gas tight metal casing in enclosing a ver-- tically elongated chamber of circular cross-section which is functionally divided into three superposed zones or. sections, namely, a heating section I I in the upper portion, a cooling section l2 in the lower portion and an intermediate soaking section I 3 therebetween. Each sectionv of the kiln chamber is defined by a wall of suitable re- Other fractory material, such as an inner wall section of high temperature fire brick l4 and an outer wall section of insulating brick l5. While the heating and cooling chamber sections are of substantially the same uniform diameter throughout their height, the intermediate soaking section is of reduced cross-sectional area for a portion of its height as hereinafter described. The upper end of the kiln is closed by a top l6 having a heating gas outlet pipe l1 containing a control damper 8. The pellets to be heat-treated are supplied to the upper end of the chamber I I from a hopper or receptacle (not shown) through a suitable fluid sealing feeder, such as a variable speed rotary pocket feeder 20, and a solid material inlet pipe 2| opening into the top of the chamber section II.

The bottom of the kiln at the lowermost portion of the hamber section I2 is defined by an inverted imam-conical metal screen 22 surrounded by a reduced diameter cylindrical section 23 of the casing l which has a bottom discharge opening 24 spaced below the open lower end of the screen 22 to permit any solid material passing outwardly through the screen to reach the outlet 24 and be discharged. The members 22 and 23 cooperate to define an annular fiuid inlet chamber 25 therebetween to which one or more valve controlled fluid supply pipes 26 are connected for the admission of a fluid under pressure. The discharge of solid material through the outlet 24 and discharge pipe 21 is controlled by a suitable fluid sealing discharge mechanism, such as a variable speed rotary pocket feeder 28 similar in construction to the feeder 2|). With the chamber sections l2 and I3 filled with a fluent mass or column of pellets, the rate of descent of the pellets through the kiln chamber can be effectively controlled by regulating the speed of the feeder 28, while the fluid sealing characteristics of the feeders 20 and 28 permit the chamber to be maintained under a positive pressure which can be regulated by operation of the control damper l8.

As shown in Fig. 1 the rotary pocket feeder 28 has an adjustable inclined gate 30 at the entrance end of the feeder housing 3| which limits the level of solid material flowing into the pockets formed by the feeder blades 32 during their rotation. The feeder shaft 33 is driven by an electric motor 34 through suitable variable speed transmission means as indicated.

In the heating of various solid materials, such as ceramic refractory articles, for example, it is important that the articles be progressively heated to the desired maximum heating temperature, held at approximately this maximum temperature for a predetermined period of time and then progressively cooled to a low temperature to avoid thermal shocks sufficient to fracture the articles. It is also desirable to have the solid material, when discharged, at a 'relatively low temperature to facilitate its handling. Such results are attained by the improved kiln construction of my invention with an accompanying high thermal efliciency by introducing the complementary combustion constituents into the kiln at different levels, one adjacent the pellet discharge end-0f the kiln and the other at a level intermediate its height, whereby the high temperature combustion zone will be located at or above the intermediate level and the combustion constituent entering at the discharge end of the kiln will be preheated to a high temperature while flowing upwardly through the cham- 4 ber 2 in intimate heat exchange contact with the descending pellets.

While in its broader aspects, my invention contemplates that either combustion constituent, i. e. fuel or oxygen, can be introduced at either level of the kiln, it is more advantageous to introduce all or a substantial part of the oxygen required for combustion in the form of air entering the lower end of the kiln chamber section l2 through the supply pipe 25, annular chamber 25 and screen 22, because of the greater amount of air required and the correspondingly greater heat transfer attainable between the descending pellets and the ascending air stream. I have also found that it is desirable to burn the combustion constituents in a combustion chamber or chambers separated from the solid material descending through the kiln so as to obtain complete combustion conditions and a thorough mixing of the gaseous products of combustion before their contact with the solid material. Such an arrangement advantageously avoids local over and/or under heating of portions of the descending solid material which is particularly undesirable in the high temperature heat treatment of the described pellets wherein the treating temperature of those pellets may closely approach their softening temperature. If any of the descending pellets are heated to their softening temperature, clusters of pellets tend to form and obstruct the free gravity flow thereof through the kiln; It is also particularly advantageous to subject the pellets being heat treated to a high temperature soaking for a predetermined period.

In accordance with the present invention the upper end of the cooling chamber section I2 is so constructed as to provide an increased crosssectional area which, in conjunction with the reduced cross-sectional area or throat forming the adjoining lower portion of the chamber section l3, will divert substantially all or at least a major portion of the preheated air ascending through the chamber section l2 into a plurality of vertically elongated furnace chambers or combustion tubes 35 formed in the refractory walls surrounding the chamber section l3. Four angularly spaced symmetrically arranged furnace chambers 35 are used in this embodiment. As shown in the drawings, the cylindrical wall of the cooling chamber section I2 is constructed with an upwardly and outwardly flaring upper end 36. Thus an annular manifold chamber 31 is formed at the.

upper end of the chamber section l2 with its upper end defined by the horizontal lower end 38 of a portion of the inner kiln wall section l4. The furnace chambers 35 have their lower ends opening to the chamber 31 adjacent its outer circumference.

Each furnace chamber is provided with an outlet 40 at its upperend opening into a corresponding radial tapering passage 4| connected to the lower end of the heating chamber section II. Since the kiln chamber is normally filled with a downwardly moving mass of pellets, each passage 4| is constructed to prevent flow of pellets into the corresponding furnace chamber 35 by having a downward offset in the passage 4| as shown. The lower portion of each furnace chamber is constructed with a Venturi-type throat 42. A suitable fluid fuel, preferably gaseous, is introduced under pressure directly beneath the throat through a radially arranged fuel nozzle 43 extending through the kiln wall and casing Hi from an annular supply duct (not shown) to which a valve-controlled supply pipe 44 is connected.

section I3 is of substantially reduced cross-sec-v tional area as compared with the cross-sectional area of the kiln chamber above and below this portion.- For example, as indicated in the draw- -ings, the throat area may be approximately 4 that of the remainder of the chamber section l3. The height and cross-sectional area of the throat 45- will be dependent upon and designed in accordance with the desired ratio ofthe total air passing through the cooling chamber section I2 to the air required for combustion in the furnaces 35, except that the diameter must not be below a predetermined minimum dependent on the pellet size, to prevent bridging of the pellets in the throat. With the relative proportions illustrated, the amount of air diverted to the furnace chambers will be from 80 to 90% of the total air supplied, while the remaining air will pass up through the throat 45 and mingle with the products' of combustion entering the lower end of the chamber section II.

In the construction shown, the portion of the refractory wall 14 between the furnace chambers 35 and the soaking section chamber I3 is thick enough to be structurally sound while affording a flow path of minimum heat resistance between the chambers 35 and the soaking section 13. The external diameter of the casing I is increased for the central portion of the kiln to provide a thick layer of insulating brick l5 surrounding the soak-' ing section l3 and the adjoining portions of the sections H and [2. Thus the radiation losses from the chambers 35 are minimized by this heat insulation and a substantial portion of the heat generated therein will be transmitted through the intervening refractories to the pellets passing through chamber section l3. The intermediate or soaking section l3 of the kiln chamber extends from the level of the chamber 31 to the level of the passages 4 I, this height being dependent upon the desired high temperature soaking period at the normal pellet flow rate. The soaking period, as well as the heating and cooling periods, can be varied by varying the speed of the feeders 23 and 28.

In the normal operation of the described kiln construction and arrangement, air is supplied through the pipe 26 at a predetermined pressure and approximately room temperature and enters the chamber section l2 through the screen 22. The stream of air flows upwardly through the interstices in the descending mass of heated solid material and the resulting intimate counterflow contact of the air and solid material effects a high rate of heat transfer, cooling the solid material to a low temperature by the time the solid material reaches the outlet 24 and heating the combustion air above the ignition temperature of the fuel by the time the air reaches the level of the fuel inlet nozzles 43. The fluid fuel, or a combustible mixture of fluid fuel and an oxygencontaining gas, such as air, in relative quantities depending upon the maximum kiln temperature desired, is supplied through the nozzles 43 under pressure. A rapid and intimate mixture of the ascending air and entering fluid fuel streams then occurs passing through the throat 42 of each tube and a rapid and intense combustion of the fuel takes place, creating a high temperature zone in the chamber section I I at the level of the heating gas entry from the passages 4|.

The highest temperatures attained in the heating section II with fuel gas'and air are secured with only fuel gas entering through the nozzles 43 and all of the combustion air supplied through the pipe 26, and more particularly, when the air supply through the pipe 26 approximates the theoretical combustion air requirements. For example. temperatures above 3200 F. have been easily reached in the furnace chambers 35 with the described apparatus under such conditions. The maximum temperatures attainable under such conditions would be such as to cause the fusion of most refractories now used commercially. Accordingly, it is preferable under existing conditions and the described fields of use to have the air supply substantially in excess of the theoretical combustion air requirements, such as of the order of 100-200% excess air. The use of such large amounts of air results in a reduction in the combustion zone temperature, and the consequent higher mass flow of the air through the chamber section l2 provides a lower exit temperature for the solid material to the feeder 28 with a given height of the chamber section l2.

The heating gases generated in the furnace chambers 35 and introduced to the chamber section II flow upwardly through the mass of solid material in that chamber in an intimate counterflow contact therewith, effecting a high rate of heat transfer to the solid material, progressively heating the descending solid material to a high temperature, and cooling the heating gases to a relatively low temperature before they reach the heating gas outlet I1. The solid material preferably enters the chamber at a low temperature and is thus progressively heated during its descent to the desired final high temperature, held for an appreciable length of time substantially at that temperature, and then progressively cooled to the desired outlet temperature.

In starting up the unit after a normal shut down, the fluid fuel or combustible mixture entering the fuel nozzles 43 and the combustion air entering the pipe 26 are supplied in a predetermined readily ignitable ratio and ignition is initially effected by a torch or other suitable means inserted through a normally closed ignition passage 46 adjacent the upper end of each of the furnace chambers 36. The feeders 20 and 28 are operated to create a substantially continuous downward movement of t e solid material through the kiln. As the ascending air stream comes into contact with heated solid material. the air is progressively preheated before reaching the furnace chambers 35 and when mixed with the entering fuel, its sensible heat correspondingly increases the combustion chamber temperature. The solid material thereafter is heated atul'e and thus the temperature of the material in chamber section II continuously increasing to an extent that a reduction of the ratio of the entering fuel to air is usually necessary to avoid overheating of the solid material and/or chamber refractories. Flexibility of control is secured by controlling the amounts of fuel and/or air to stop the described cumulative effect when the desired maximum fluent solid material temperature is reached. The desired temperature is maintained by establishing a rate of supply of the combustion constituents in a predetermined fuelair ratio at which the heat input will provide the desired heating as temperature and balance the heat losses from the unit. A suitable means for the regulation of temperatures is provided. such as the thermocouples 41, which may be used as a visual device for manual adjustment of the valves in the pipes 26 and 44 or to actuate an automatic valve control of the fluid flows through those pipes. Any solid material insufliciently burned during starting up is returned to the feeder 20 in any suitable manner. In View of the low exit temperatures of the solid material and heating gases and the absence of any heat absorbing conveying means for carrying the solid material through the heating zone and the nearly complete recovery of heat in the heated solid materials, which is normally lost in the usual types of furnaces, the fuel consumption is almost entirely used to balance the heat radiation losses of the unit. Consequently an extremely high thermal efliciency will be attained.

By way of example and not of limitation, in one run of apparatus of the character described for burning dried pellets of the composition described, the pellets, fuel gas and combustion air were supplied to the kiln at room temperature with the air supply substantially 82% in excess of the theoretical requirements. A temperature of 3000 F. was maintained in the chamber section H at the level of the heating gas inlets through the passages 4| and the heating gases had an outlet temperature of 250 F. The pellets were heated to a burning temperature of approximately 3000 F. and were discharged at a temperature of 100 F. Under these conditions the total heat input to the furnace chambers 35 was 2825 B. t. u. per minute and 2.8 pounds of pellets were treated per minute. Thus, with an average specific heat for the pellets of .27, the thermal efliciency of the operation was:

The apparatus of my invention is adapted for the heat treatment of solid materials other than the refractory pellets described. such as for the annealing of small metallic articles or the calc ning of crushed limestone. Where the mass of solid material to be heat treated is not sufficient- 1y fluent alone to provide the desired rate of movement through the kiln chamber, the fluidity of the mass can be increased by mixing the material to be heated with previously fired pellets of incombustible refractory material of the character described herein, which can be subsequent l v separated from the other solid material when discharged, and reused.

I claim:

1. Apparatus for the heat treatment of a fluent mass of solid material comprising refractory walls defining a vertically elongated kiln chamber having a heating gas outlet at its upper end and a solid material outlet at its lower end, means for maintaining a fluent gas-pervious mass of solid material to be heat treated moving downwardly as a continuous column through, said chamber to said solid material outlet, said kiln chamber having an upper material heating seccent portion of said heating section and a lower throat portion of substantially reduced crosssectional material flow area and sufficient height to substantially restrict gas flow therethrough, a combustion chamber having a heating gas discharge connection to said kiln chamber immediately above said soaking portion, means for supplying a fluid combustion constituent to said combustion chamber, means for introducing a complementary fluid combustion constituent into the lower part of said cooling section and in a position to flow upwardly therein in heat absorbing relation with thedescending solid material, and means for conducting said heated complementary fluid combustion constituent to said combustion chamber.

2. Apparatus for the heat treatment of a fluent mass ofsolid material comprising refractory walls defining a vertically elongated kiln chamber having a heating gas outlet at its upper end and a solid material outlet at its lower end, means for maintaining a fluent gas-pervious mass of solid material to be heat treated moving downwardly as a continuous column through said chamber to said solid material outlet, said kiln chamber having an upper material heating section, a lower material cooling section, and an elongated section intermediate said heating and cooling sections, said intermediate section consisting of an upper soaking portion of substantial height and substantially the same cross-sectional material flow area as the superjacent portion of said heating section and a lower throat portion of substantially reduced cross-sectional material flow area and suificient height to substantially restrict gas flow therethrough, a combustion chamber having a heating gas discharge connection to said kiln chamber immediately above said soaking portion and a heat conducting wall common with at least a portion of and substantially coextensive in height with said soaking portion, means for supplying a fluid combustion constituent to said combustion chamber, means for introducing a complementary fluid combustion constituent into the lower part of said cooling section and in a position to flow upwardly therein in heat absorbing relation with the descending solid material, and means for conducting said heated complementary fluid combustion constituent to said combustion chamber.

' CHARLES L. NORTON, JR.

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

UNITED STATES PATENTS Number Name Date 1,148,331 Olsson July 27, 1915 1,614,387 Pereda Jan. 11, 1927 2,115,586 McFarland Apr. 26, 1938 2,199,384 Azbe May 7, 1940 2,345,067 Osann Mar. 28, 1944 2,399,450 Ramseyer Apr. 30, 1946 FOREIGN PATENTS Number Country Date 501,765 Great Britain Mar. 6, 1939

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