US2869849A - Rotary furnace - Google Patents

Description

Jan. 20, 1959 A. 1 IFOLLI'OT ET AL ROTARY FURNACE Filed April 11, 1955 IIIIIIIIIYIIKWIIRWAWAWWm relatively small up tothe present time.

2,869,849 Patented Jan. 20, 1959 2,869,849 ROTARY FURNACE AlbertE. J. Folliot, Limay, and Louis-Charles Kenzinger, Albi, Tarn, France, assignors to Ciments Lafarge, Paris, France, a company of France Application April 11;, 1955, Serial No. 500,502" Claims priority, application France May 17, 1954 3 Claims. (c1. 263 -33 The invention relates to rotary furnaces and 'more particularly to an improvement which has-forits object to increase the exchanges of heat in these furnaces.

It is known that a rotary furnace is essentially constituted by a cylinder the length of which may attain 150 metres and which is mounted so as to rotate about its axis, the latter being slightly inclinedwith respect to the horizontal. The material to be treated enters at theupstreamend of the cylinder and progresses by virtoe of the rotation of the furnace, down to the lower extremity, at which a longitudinally-directed flame produces the heat required for-the treatment to be effected. This flame results from the combustion of pulverized coal, of coal gas-or water-gas, fuel oil, etc.

The calorific energy resulting from the combustion is transmitted to the material by radiation and by convection, this material being heated progressively from one end of the furnace to the other and being subjected to the physical and chemical changes which it is desired to be produced.

It is convenient for the purposeof studyingthe transmission of heat to divide the furnace into three zones:

(1) In the zone DC (seeFig. 1) situated in the downstream portion, the flame is very hot and powerfully emitting; the transmission of heat takes place by radiation and convection, either. directly from the flame to the material to be fired, or through the medium of refractory walls which are heated by the flame and then transmit this heat to thematerial. v

(2) The gases leaving, this zone have a temperature which progressively decreases and in the upstream zone BA (zone extending substantially from a transverse section at which the temperature isof the order of 700 C., to the feed end of the furnace );it is relatively simple to take out the remaining heat of these gases. The devices adapted to transmit the available heat from the gases to the material are not, in fact, subjected in this zone to an excessively high temperature which might be liable. to affect their mechanical strength. It is for this reason that, in this zone BA, are employed usually chains used for drying the material, mechanical scoops,

etc.

(3) In the central zone CB, which will hereinafter be termed the heating up zone-f the transmission of the heat from the gases to the material has always been On the one hand, the gases resulting from the combustion of the flame, although having a high emissivity, are at a temperature which decreases from the temperature of the flame down to 700 C., at which the heat exchanges by radiation have .slowed down, While on the other hand,

the transmission ofheat by convection is not facilitated by the relatively small areas of contact between the gases and the material and between the gases and thewalls. In particular, the gases are still too hot for the installation at this point of devices, generally made of steel, and aimed atassisting in the transmission, which are normally provided in the zone BA. The temperature of the gasesin the heatingupzone CB, thus, decreases.

slowly from the temperature of the flame (section C) to about 600 C. to 700 C. (section B), and the tendency is therefore to give this zone a considerable length so as to increase the contacts between gases and material and between gases and Walls. The result of this is that these installations are heavy and costly, having a correspondingly heavy supporting equipment and having also a high heat loss through the external furnace walls which have therefore a very large surface area.

The invention has for its object to increase the ex- T changes of'heat between the hot gases and the material being treated in the, heating up zone of a rotary furnace, this enabling the length of the said zone to be decreased and resulting in'a reductionof the cost of the installation. In accordance with the invention, the heating up zone of a rotary furnace comprised between the extremity of the flame and-the section of the furnace at'which the temperature of the gases is about 600 C. isdivided into compartments by means of longitudinal partitions, made of a suitable refractory material and arranged so as to take part in the-rotational movement of the furnace and retained so as to prevent any longitudinal displacement.

In an advantageous form of embodiment of the invention, which is more especially capable of resisting the relatively high temperatures obtained in the downstream portion of this heating up zone, the dividing partitions are made of silicon carbide which, by virtue of the mechanical strength and high thermal resistance of this material, enables relatively thin dividing walls to be used which do not reduce to an excessive extent the cross-. section area of passage for the gases and for the material to be treated.

The dividing of the furnace into compartments by means of longitudinal'partitions, divides the descending mass of material to be treated into a number of banks and the rising flow of gases is similarly divided into a number of individual streams; the surfaces of direct contact between gas and material and walls and material are thus very substantially increased with a corresponding increase in the transmission of heat by convection and by radiation. It has, in fact, been established by measurements of radiation carried out in furnaces, that the transmission of heat by radiation from a cylindrical flow of gas is an exponential function of its diameter such that a division of the internal cylindrical space of the furnace into four cylinders increases the transmission by radiation from the gaseous flow by 1.6 times, and division stood, reference is hereby made to the accompanying drawing, in which:

Fig. 1 is a schematic view in perspective of a rotary furnace of which a part of the casing is broken away so as to show the internal partitions in the heating up zone.

Fig. 2 is an axial cross-section to a larger scale of a. portion of the zone BC of Fig. 1.

Fig. 3 is a transverse cross-section taken along the line: IIL-HI of Fig. 2.

As shown in Fig. 1, a rotary furnace comprises essen-- tially an elongated tubular chamber of large size and inclined at a slope of about 4% to the horizontal, the.- chamber being constituted by an external casing 1 of steel sheets, the inside of which is lined with refractory mate-- rial, the casing being supported by means of steel bands 2 rolling on rollers (not shown) which rest on solid foundation blocks. In the downstream portion of the furnace a burner 3 of any particular type suitable for the fuel employed enables a long, flame to be produced, extending over the zone DC.

I In a standard type of cement furnace, the upstream por tion AB, in which the drying of the slurry and its heating upto a temperature of about 150 C. take place, extends approximately over one quarter of the length of the furnace, and the gases in the section B are at a temperature in the -vicinity of 600 C. to 700 C. The heating of the material and its decarbonatation take place in the second and third quarters, and the material at the end of this third quarter is then situated in the very hot zone of the flame, its temperature being of the order of 1400 0., sufficient for starting the exothermal reaction known as clinkerisation.

. The heating up zone CB thus has a considerable length, of the order of half the total length of the furnace. In this zone, the temperature of the gases decreases from the temperature of the flame at C down to about 600 C. to 700 C. at B.

In the case of ordinary cement furnaces, the process I of heat exchange in this heating up zone is as follows:

The gases give heat to the material and the walls both by convection and by radiation; the walls in their turn heat up the material by direct conduction and by radiation through the gases.

It will be understood that if the superficial area of the surfaces coming into contact with the material to be treated and with the hot gases of combustion is increased, the exchanges of heat in the Zone BC may be increased to a very considerable extent. The convection of the gases on the material and on these surfaces increases in proportion with the increase in superficial area of the surfaces with which the gases are in contact, and the radiation of the gases is increased-when the column of gas is split up in individual streams.

In the form of embodiment of the invention shown in Figs. 1 to 3, the internal cylindrical space of the furnace is divided into compartments in the heating up? zone BC by means of a central body 4 which divides this space into six external channels 5 and one axial conduit 6. The central body 4 is formed by placing and joining together a number of sections 7 (see Fig. 2) each consisting of six radial arms 8 connected to a central ring 9. The arms 8 have their outer ends built into the refractory layer 10 which forms the internal lining of the furnace and have their inner ends formed into parts of an arc of a circle 80 forming a kind of arch-stone ending lateral- 1y by a pair of planeradial surfaces. The central ring 9 of each section is completed by means of intermediate members 9a forming other arch-stones having a curved cross-section ending laterally by a pair of plane radial surfaces mating with the corresponding radial surfaces of the curvedportions of arms 8. For accurately and firmly securing the intermediate members 9a to the arms 8, a groove is provided in each radial surface of the curved portions of said arms parallely to the axis of the furnace, and the intermediate members 9a are provided on each of their radial surfaces with one projecting tongue 12 parallel to the axis of the furnace and engaging in the groove provided in the adjacent arm. This fitting ensures at the same time the fixation of the various segnients in position and a good tight seal between the external channels 5 and the axial conduit 6. All the sections of the central body 4 are provided on one of their faces at right angles with respect to the axis of the furnace with grooves 13 intended to receive a cement joint during the building in position and fixing of the central body 4 in the furnace.

When the sections 7 are axiallyjuxtaposed and sealed to the adjacent ones by appropriate cement joints, the central body 4 which they form together comprises a cylindrical structure encompassing the axial conduit 6,

and radial partitions extending in an axial directiondelimiting between said cylindrical structure and the inner lining of the furnace the said external channels 5.

The arms 8 and the intermediate arch pieces 11 may be of any suitable refractory material, but they are preferably composed of silicon carbide which combines a high resistance at high temperatures (in the vicinity of 1400 C.) with a good mechanical strength. This enables these parts to be of relatively small dimensions in a cross section of the furnace and in consequence, the crossscction area of passage available for the gases and the material to be treated is not reduced to any great extent.

During the rotation of the furnace, the material which forms a single heap in the upper part BA is divided up at the entrance to the central body 4 between the external channels 5 at which it forms six banks, each of which comes into contact with a stream of gases which passes through the same channel, and as the material passes down it comes successively into contact with the four walls of this channel. The hot gases which pass through the axial conduit 6 heat-up the wall of this conduit and the heat supplied to these walls is rapidly transmitted to the material in the external channels by reason of the relatively good conductivity of the silicon carbide.

This division of the interior space of the furnace by means of a central bodyof silicon carbide is particularly well suited for the downstream part of the heating up Zone in the vicinity of the section C, where the temperature is relatively high, and in which therefore feasible to mount metallic bodies.

The increase in transmission of heat which results from the application of the arrangements in accordance with the invention, enables the relative length of the heating up zone to be reduced for the same exit temperature of the gases from that zone. This constitutes a considerable advantage which leads to a large reduction in the cost of erection and manufacture resulting from the substantial reduction in the overall dimensions of the furnaces and, in a subsidiary manner, in the losses by radiation from the walls.

It will, of course, be understood that modifications may be made to the forms of embodiment which have just been described, without thereby departing from the spirit or from the scope of the present invention. In particular, the number of arms of each section of the central body 4 shown in Fig. 3 is not limited to six and may be any other number, this number being especially a function of the internal diameter of the furnace.

What we claim is:

1. A rotary furnace working at high temperature Wherein a divided material flows continuously in direct contact and in counter-current with heating gases comprising a cylindrical casing of steel sheet, an internal lining of firebricks and a device for increasing heat exchanges between said gases and said material in a portion of said. furnace, wherein said device is provided along the second and the third fourths of the length of said furnace and comprises a cylindrical hollow body coaxial with said casing connected to radial partitions dividing the inner section of the furnace in peripheral channels and one axial conduit, said body and said partitions being formed of sections, made of a refractory material having a high heat transmission characteristic, juxtaposed in the axial direction of the furnace by their faces at right angles thereof and sealed to the adjacent sections; each of said sections comprising radial arms extending on the whole axial length of said section and having at their outer extremity in the circumferential direction a thickness materially equal to the thickness of the bricks of said internal lining and providing at their inner end curved portions forming arch-stones ending laterally by a pair of plane radial surfaces in each of which one groove is provided parallelly to the axis of said furnace, and intermediate members forming other arch-stones for said cylindrical body having a curved section ending laterally by a pair it is not of plane radial surfaces each having one projecting tongue parallel to the axis of said furnace, said tongues engaging in the grooves of a pair of adjacent arms; said arms having a length such that they end just a short distance from said cylindrical casing, their outer ends thus taking the place of a row of bricks in said internal lining, said arms thereby forming part of said lining and being firmly secured to said cylindrical casing, whereby the structure formed by these arms and the intermediate members are arranged so as to take part in the rotative movement of the furnace and retained against any longitudinal displacement.

2. A rotary furnace as claimed in claim 1 wherein said arms and said intermediate members are provided in one of their faces at right angles with respect to the axis of the furnace with grooves for receiving sealing and securing cement.

6 3. A rotary furnace as claimed in claim 1 in which said arms and said intermediate members are made of silicon carbide.

References Cited in the file of'this patent UNITED STATES PATENTS 945,498 Doherty Jan. 4, 1910 996,070 Edison June 27, 1911 1,097,177 Jones May 19, 1914 1,222,244 Prindle Apr. 10, 1917 2,061,741 Rosenfeldt Nov. 24, 1936 2,177,551 Perkins Oct. 24, 1939 FOREIGN PATENTS 614,453 Great Britain Apr. 4, 1949

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