US3764258A

US3764258A - Device for heating or drying dust-like material

Info

Publication number: US3764258A
Application number: US00212323A
Authority: US
Inventors: J Brandt
Original assignee: Apparatebau Rothemuehle Brandt and Kritzler GmbH
Current assignee: Apparatebau Rothemuehle Brandt and Kritzler GmbH
Priority date: 1970-12-28
Filing date: 1971-12-27
Publication date: 1973-10-09
Anticipated expiration: 1990-10-09
Also published as: DE2141825B2; GB1333319A; DE2141825A1; FR2120937A5

Abstract

In a rotating drum heater for fluent solid material in which the material is repeatedly lifted by the motion of the drum and allow to fall through a nest of stationary heating tubes, with a slight component of motion along the drum and tubes, lifting blades have two or more material-retaining elements to assist uniformity of distribution of material over the nest of tubes. Also described are, inter alia, improved sealing arrangements between stationary end-caps and the drum, a self-contained transporting chassis for the heater and an improved arrangement for recycled and mixing heating gas.

Description

llnited States Patent 1 [111 3,764,258 Brandt Get. 9, 1973 DEVICE FOR HEATING 0R DRYING 3,076,270 2/1963 Madsen 34/128 DUST-LIKE MATERIAL Inventor: Joachim Brandt, Rothemuhle,

Germany Assignee: Apparatebau Rothemuhle Brandt &

Kritzler, Rothemuhl, Germany Filed: Dec. 27, 1971 Appl. No.: 212,323

Foreign Application Priority Data Dec. 28, 1970 Germany P 20 64 132.1 Aug. 28, 1971 Germany P 21 41 825.7

US. Cl. 432/107 Int. Cl. F27b 7/10 Field of Search 263/32, 33; 432/107 References Cited UNITED STATES PATENTS 12/1966 Hollefriend 263/33 R Primary Examiner-John J. Camby Attorney-Lackenbach & Lackenbach ABSTRACT 14 Claims, 18 Drawing Figures PATENTEU BET 9 I975 SHEET 02 0F 13 PATENTED 9 SHEET 0 0F 13 PATENTEDBBT 91% 3,764,258

SHEEI 05 0F 13 PATENTED BET 9 sum 10 hr 13 PATENTED 9 SHEET 12 0F 13 PATENTEDUET elm 3.764.258

SHEET .13 0F 13 DEVICE FOR HEATING OR DRYING DUST-LIKE MATERIAL The invention relates to apparatus for the recuperative heating or drying of pulverulent or granular fluent solid material. Known apparatus of this type has a stationary bundle of heating tubes with lenticular or thembic cross-sections, which lie at a small angle to the horizontal, and, in the middle portion of their length, lie within and parallel to the axis of a rotating drum. This rotating drum is equipped at its inner circumference with lifting blades, which continuously transport the fluent material upwards and then pour it over the heating tubes. The ends of the heating tubes are fastened in stationary end caps, which have short cylindrical parts surrounding the tube ends at both ends of the drum and which meet the end flanges of the rotating drum through sliding seals. An inlet or outlet duct for the heating medium passes to the heating tubes through the end wall of the end caps and within the short cylindrical part of the end caps the feeding in of the heating material is effected from the top at one end of the drum, whilst the discharge of the heating material is effected at the bottom of the other end.

These devices are used, for example, for the heating up of the stone-flour filler used for bituminous roadconstruction.

The known apparatus have had the characteristic that since the lifting blades were simple metal strips which projected into the drum the material they were lifting would always be deposited from them as they reached a particular level in the drum, and would be deposited from them while they executed only a small range of travel. Thus the material would tend to be deposited onto a particular portion only of the array of heating tubes.

This is not disadvantageous in apparatus such as that seen in German Patent No 160 406, which is for the drying of corn and wherein the tubes are surrounded by a stationary container within the rotating drum. There, the object is to keep the container full (there is a controlled outlet at its bottom) so that there is a compact mass of corn around the heating tubes. Obviously the fact that the container is filled by deposition from one small range of angles around the drum is immaterial. The exact range of angles depends on the angle of repose of the material.

But in arrangements such as that shown in German Patent 491 182, where there is no internal container, this is a disadvantage because either the array of pipes is not fully used (if the material deposited at one position does not spread across all of them) or, to achieve that efficient use, the inside of the drum must be kept almost as full as the stationary container was in the construction shown in German Patent 160 406. For some materials, this compaction and slow movement past the tubes is undesirable.

We find, for example, that for materials with a low moisture content and particularly when they are stone materials, which have very low thermal conductivity, the most efficient way to use such an apparatus is with the material kept in continuous free motion over the tubes; that is to say when the particles of material fall at their free-fall velocity through the passages between the tubes. In general, with such materials, we find that the heat transfer coeffient increases proportionally with the speed at which particles pass through the array of tubes. It is therefore apparent that the mode of operation which was desired in German Patent 160 406, and which is imposed on the user by the construction shown in German Patent 491 182 is not efficient or desirable in the case of many materials, and in particular in the case of stone-flour filling materials.

The construction which will be described later has the object of transporting the greatest possible quantity of material to be heated in a distribution which as equal as possible over all passages between the heating-tubes, so that an optimum heat transfer shall be achieved by a continuous falling movement of the particles. There may also be turbulent mixing of the particles with the air in the apparatus.

In this invention, the rotatable drum is equipped uniformly distributed on the circumference, with a plurality, preferably about 25 to 30, of two-step or multi-step lifting blades. These are formed by two unequal legs which are at or more to each other and on the longer leg of which one or more intermediate or distributing legs are fixed, parallel to the shorter leg. The longer leg of this is set in the radial direction of the internal wall of the rotary drum, or has, in the sense of rotation of the drum, an inclination of up to 30 to the radial. In either case the shorter legs and the intermediate or distributing legs are tangents to circles which are concentric with the drum.

The ratio of the lengths of the legs is preferably about 1:2, and the radial height of the longer leg is preferably approximately 1.2 times the distance between the radially outer ends of those longer legs. Preferably also the width of the intermediate or ledges amounts to about the 0.4 to 0.6 times the radial height of the longer legs.

It is preferred, particularly for improvement of the flow-properties of the tubes relative to the fluent material, and for reduction of possible accumulated deposits on the tubes, to provide a truly aligned arrangement of the tubes with equal dividing distances in lieu of the staggered tube arrangement used hitherto.

In order to provide further improvements in the setup and operation of the heater, which allow a simpler and more compact type of construction and lead to substantial cost savings, a mixing chamber for circulating and fresh combustion gases is provided coaxially with the rotating drum and adjoining the outside of the end wall of the outlet-side end cap.

Particular embodiments of the invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 is a side diametrical sectional view of a first embodiment showing heating tubes and their support FIG. 2 is an end view to show the arrangement of the heating tubes FIG. 3 is a view on an enlarged scale of part of FIG. 2

FIG. 4 illustrates in end view, one arrangement of lifting blades in the rotating drum in this embodiment FIGS. 5 and 6 show alternative such arrangements FIGS. 7 and 8 are side views of the embodiment showing mounting and transporting elements in different dispositions FIG. 9 is an end view and FIG. 10 is a plan view with parts removed both showing a drive arrangement for the drum FIG. 11 is a section, on one radius, to show some detail of an end-cap sealing arrangement FIG. 12 is a side view of the end-cap,

FIG. 13 is a section on the line A-A, of FIG. 2

FIG. 14 is a detail of one part of the end cap sealing arrangement FIG.'I5 is a side view of a second embodiment FIG. 16 is a cross-section on the line 3-8 of FIG. 15, and

FIGS. 17 and 18 are sections on one radius through end cap sealing arrangements at an inlet end and an outlet end, respectively.

in the drawings, the embodiments have the elements such as heating tubes, rotating drum, stationary end caps which have been mentioned in the discussion of the prior art. These will not be generally described further, since such further description would be unnecessary to one skilled in the art, and the following description details only the novel or inventive features of the embodiments.

The lenticular-sectioned tubes 1 are comparatively long (e.g. about 4 m) and a support is provided only for every alternate row of tubes. These supports are arranged at about one-third up to two-thirds of the total length of the tubes, and are horizontal flat bar crosspieces 70. As shown in FIG. 1, the horizontal crosspieces 70 are in this case arranged in a staggered formation relative to each other in such a way that seen in cross-section, they lie in a line at an inclination of about 45 to the axis of the rotatable drum 4.

The pitch of the axes of these tubes in the array is preferably 1.3 to 1.5 times the largest cross-sectional dimension of the tubes.

Due to the rotation of the rotatable drum bending vi brations of the heating tubes are incited. This is desired, because the tubes of the rows which are between the rows supported by the crosspieces 70, can vibrate freely along their entire length, they being supported only at their extreme ends, in the end walls 2 of the end caps 3. This reduces the chances of the fluent material accumulating and promotes the trickling flow of the material between the tubes.

To the same end the tubes 1 are arranged in a rectangular lattice pattern, shown in FIGS. 2 and 3, which offers vertical (and horizontal) passages for the fluent material to pass between them.

FIG. 4 shows an arrangement of 27 two-step lifting blades 19a with F-shaped cross-section, which are uniformly distributed around the inner circumference of the rotatable drum. This blade-shape is formed by two unequal legs, to the longer leg of which a medium intermediate or distributing leg Z is welded parallel to the smaller leg.

The longer leg S of each blade is welded by its outer end onto the inner drum wall to lie in the radial direction of the rotatable drum. These blades retain material in them over a wider arcuate angle of travel than would a single lifting blade, thus improving the distribution of material over the tubes 1.

The distribution of the material to be heated is still further improved in the arrangement shown in FIG. 5 where the blades have an F profile 19b which is inclined to the radial towards the rotation of the rotatable drum. The most favourable profile angle is fixed in such way, that all the lifting blades below the uppermost level of the drum and above the horizontal centre line through axis of the rotatable drum have their planar rear faces inclined below the angle of repose of the material to be heated. The angle of repose of stone-flour is about 55.

In this form of execution the last portion of the material to be heated will fall completely out of its lifting blade and between the heating tubes without any portion being able to deposit itself on the back of the preceding blade. The inclination of the longer leg of each blade 19b to the radius of the rotatable drum amounts in this arrangement to about 25 to 30.

The height h of both these profiles is measured in the radial direction and is preferably of the order of 1.2 times the distance between the outermost ends of the longer legs, and the width of the intermediate or distributing leg Z amounts to about 0.4 h to 0.6 h.

A form of blade 19c with more than one intermediate or distributing leg is shown in FIG. 6.

With these constructions it is possible with speeds of rotation of the drum between 5 and 9 r.p.m. to obtain an optimum distribution of material over the entire heating tube system.

It is convenient to arrange the whole drum 4 on an undercarriage frame 80, which serves for the transport of the device as a semitrailer. FIGS. 7 and 8 show this arrangement, together with devices for the support and adjustment of inclination of the drum to the horizontal. A rear wheel bogie 81 is fixed on the undercarriage frame 80 in a detachable manner. Rack-driven jacks 82 or other telescopic members are pivotally arranged on two positions one each side of the frame below the front drum end and swingable supports 83 are similarly arranged below the rear drum end.

By means of these adjustable supports the plant can be immediately placed on a foundation provided for this purpose, as shown in FIG. 8. This feature of the heater allows for the alteration of the angle of inclination of the drum to the horizontal (usually between 3 and 8) to increase or decrease of the number of times the material to be heated passes over the heating tubes between its entry to and outlet from the drum. The optimum number of times depends mainly on the moisture content of the material, and the drum inclination is in such a way that a material to be heated (which has the lowest heat transfer coefficient at maximum moisture content) can with the maximum flue gas temperature (about 400C) still be brought to the required end temperature (about 200C).

FIGS. 9 and 10 show the drive of the rotatable drum 4 by means of a polyphase induction motor via a gear unit 76. Motor and gear unit are arranged in suitable manner on the undercarriage frame below the rotatable drum 4. The transmission of the rotational movement from the motor to gear unit is effected by means of V-belts, and a chain drive is provided from gear unit 76 to drum 4.

The drum speed can be changed between 5 and 9 r.p.m. by exchanging various V-belt pulleys within the V-belt drive. By this means it becomes possible to adapt the speed to the different flow characteristics of e.g. the rock flour. If the angle of inclination of the drum under full load conditions is also adjusted, optimum heating up of the material is obtained using an approximately constant flue gas temperature.

A further feature of the present heater is the arrangement of axially glidingly movable sealing elements between the rotatable drum 4 and the stationary front caps 3 shown in FIGS. 11 to 14.

The sealing devices consist of seals 52, which are axially movable and touch, under spring pressure, rotating sealing rings 51 replaceably fixed to end flanges 50 of the rotatable drum 4. The seals 52 are fixed via flange rings 53 to short cylinders 54, which glide over and are guided along the radially inner wall of the stationary end cap 3 and are therefore axially movable on the end cap.

The flange ring 53 is gastightly connected with the end cap 3 by means of an expansion sleeve 65.

To secure the axially movable but

non-rotating sealing elements

52,53,54 against the rotational movement of the drum 4, each flange ring 53 is retained by swinging hinge rods 56, attached at three equidistant circumferential points of the end cap 3. These hinge rods are movably journalled by means of ball-and-socket joints 57 to connecting elements 58 at the circumference of the end cap 3 and at the circumference of the flange ring 53.

The sealing rings 51,52 are replaceably fixed on the non-rotating, axially movable, flange rings 53 and on the flanges 50 of the rotating drum. The flange rings 53 have adjustable stops, which limit the wear of the sealing rings under

abrasion

51,52 to a specific thickness. Pins 60 are fixed on end flanges 61 of the end caps 3 at equal circumferential distances and the compression springs 62 which urge the

rings

52 and 51 into contact are kept adjustable as to their spring length by means of screw threaded collars 63 and exert their spring pressure against the studs 64 on the flanges 53.

The advantages obtained by this embodiment consist in particular in that, in comparison with previously known devices, a considerably bigger quantity of material to be heated can be transported continuously and uniformly distributed between the heating tubes whereby an optimum heat transfer is not only effected by contact with the heating surfaces, but to a substantial part also by convective heat transfer from the heated air, which is actively participating in the turbulent mixing of the tiller particles during the falling of the material between the tubes.

The setting of the drum on the plane base frame on which the heater is both supported and transported contributes substantially to the simplification and cost reduction of the total design achieved by the embodiment.

' These advantages are also presented by the second embodiment, which will now be described, but which has certain additional features of interest.

This second embodiment of heater is equipped with a two-stage oil-burner 87 with fully automatic regulation. Said oil-burner is installed at the outer end of the burner muffle 88 which is itself mounted coaxially with the drum. The burner muffle 88 has an external mantle of normal steel and an internal lining consisting of tirebricks. It is enclosed by a coaxial sleeve 91 thus forming a duct 92 around it. At one end the duct is con nected via a mixing chamber 89 with a gas inlet opening of the heater and which connects at its other end to gas recirculation ducts 93 (see also FIG. 16). The mixing chamber 89 for recirculated gas and freshly combusted gases is mounted outside the end cap 3 coaxially with the drum.

The freshly combusted gases flow from the right (as shown in FIG. into the axially central portion of mixing chamber 89. In the said chamber they are mixed with the recirculated gases which have cooled during their passage through the tubes and by means of the recirculation fan 95 the mixed gases are delivered continuously through the stationary heating tubes within the rotary drum 4.

The thorough mixing of the freshly combusted gases with the recirculated cooled gases is helped by a perforated metal plate 4 to 5 mm thick which is twothirds to three-fourths along the length of the mixing chamber, in the direction of flow. It serves at the same time as protection against radiant heat, by dissipating the radiant heat of the flame gases from the central region of the heating tube system and distributing it to the mixed gases which will be passing into all of the crosssection of the heating tube system.

The perforations in the plate 90 are holes 30 to 35 mm diameter with their centres uniformly pitched at distances of 50 to 60 mm.

The recirculation fan is mounted at the gas outlet end of the drum 4, in prolongation of the axis of the rotary drum 4. It is mounted in continuation of the gas outlet duct 97 with interposition of an expansion sleeve 96, from there the cooled gases flow, for the major part, into the recirculation ducts 93. However, a short vertical chimney flue 94 is arranged above the outlet connection of the recirculation fan 95, and allows a small portion of the cooled gases to pass to the outside atmosphere.

The complete heater is supported on a plane frame 85, which acts as a chassis-frame with a detachable bogie. This can be of a type suitable either (for any one heater) for road transport or for rail transport. The heater as a whole can be dimensioned on this frame so that the whole may be transported by rail.

On site, the heater is set up as described for the first embodiment.

In use, the material to be dried e.g. stone-flour filler material, is fed in through the upper inlet opening 15 of the end cap 3 which is at the gas outlet end of the rotary drum 4 and which is, when the axis of the drum is inclined to the horizontal, at a higherlevel than the other end. As described above, the material reaches the outlet opening 16 at the bottom of the material outlet (and gas inlet) end cap after a number of passages over and between the heating tubes determined by the angle of inclination to the horizontal of the axis of the drum.

The improved material delivery achieved by the multi-stage lifting blades is further aided by a better shielding of the sealing elements 51 to 54 at the end flanges of the rotating drum 4 against possible ingress of particles of material. For this purpose coaxial short cylinder surfaces 100,101 are arranged to lie radially inside seals, penetrating into both ends of the rotatable drum and having a diameter about 2 to 3 cm smaller than the inner diameter of the rotatable drum.

FIG. 17 shows the cylinder surface 100 at the material inlet end. It penetrates about 100 to mm into the rotatable drum 4 to the end wall 2 of the'adjoining end cap 3 to which it is made fast by welding.

Within the length that this cylinder 100 penetrates into the drum one or two rings or spirals of about 12 mm square profiles 103 are fixed to the wall of the drum in the small annular gap between the inner wall of the rotatable drum and this cylinder to leave only a narrow air gap of about I to 2 mm open outside the cylinder surface 100.

The arrangement at the material outlet end is shown in FIG. 18. About 15 to 20 mm behind the end flange of the rotatable drum 4 a cylinder 101 is welded by means of a thin annulus 102 lying normal to the axis of the drum. The cylinder 101 is welded fast to this annulus and penetrates about 230 to 250 mm into the length of the adjoining end cap 3.

In a manner analogous to that shown in FIG. 17, one or two rings or spirals 104 of about 12 mm square profile are fixed in the gap outside the cylinder, but this time it or they are secured to the cylinder to lie within the front cap 3 in the gap between the guide cylinder 54 of the sliding sealing elements 51 to 53. The cylinder rotates with the rotatable drum, so that these rings or spirals rotate in the gap at this end of the drum and, in the case of a spiral, continuously urge any filler particles which enter this gap out of the region of the sliding seals toward the material outlet 16. At the material inlet side as shown in FIG. 17, such a spiral again rotates with the rotatable drum but in this case is arranged to urge material into the drum.

The advantages obtained by this embodiment consists in a more compact and simpler design as well as a technically more advantageous and reliable regulation obtained by the installation of a two-step regulated oil or gas burner with its combustion chamber within a ring duct for the leading in of the recirculated gases into the adjoining coaxial mixing chamber.

This arrangement of burner allows for shorter duct lengths and hence smaller incipient heat losses so that less thermal insulation is required.

The protection against radiant heat installed in front of the burner flame gives a higher service life of the heating tube system.

Shielding the sliding seals at the ends of the rotary drum and the arrangement of rotating spirals in the gaps outside these shields mean that the ingress of particles between the sealing surfaces of the sliding sealing elements is almost completely prevented whereby their life is increased to a considerable extent.

What is claimed is;

1. Apparatus for the heating or drying of pulverulent or granular fluent solid material comprising, in combination, a rotatable drum, an array of heating tubes of generally oval cross-section having a pitch between the centers of the tubes within the range of about 1.3 to 1.5 times the major cross-section dimension of the tubes, said rotatable drum with the major sectional axes of said tube extending in a generally vertical direction, a plurality of supports for at least some of said tubes disposed at distances about one-third of the way therealong, and a plurality of lifting blades extending generally longitudinally within the drum secured along its inner periphery for lifting pulverulent or granular fluent solid material within said drum continuously upwards during rotation of said drum to drop it generally uniformly over the array of tubes, the lifting blades each comprising in turn, two unequal legs, the longer of which is fixedly attached at one end portion thereof with the inner circumferential wall of the rotatable drum and at the other end portion thereof with the shorter leg, said shorter leg being a tangent to a circle inscribed about the axis of rotation of the drum, the longer leg extending at an angle to the radial direction of the drum which is in the approximate range 0 to in generally the same direction and parallel to said shorter leg.

2. Heater according to claim 1 wherein the ratio of the length of the longer leg to the shorter leg is about 2:1.

3. Heater according to claim 1 wherein the radial distance between the inner circumferential wall of the drum and the shorter leg is about 1.2 times the distance apart of the radially outer ends of adjacent longer legs.

' 4. Heater according to claim 1 wherein the length of the intermediate distributing leg is about 0.4 to 0.6 times the radial distance between the inner circumferential wall of the drum and the shorter leg.

5. A heater according to claim 1 wherein only every alternate row of the array is so supported.

6. Heater according to claim 5 wherein the array comprises of a square lattice with vertical free passages between the tubes.

7. Heater according to claim 1 further comprising a stationary end cap, a seal between the rotatable drum and said stationary end cap a telescopic guide construction associated with the end cap, a stationary wear ring carried by said telescopic guide construction, a rotating wear ring secured to the end of the rotatable drum, means for urging the wear rings together and means for restraining the wear ring associated with the end cap from rotation relative to that end cap.

8. Heater according to claim 7 wherein the means for restraining the rotation of the wear ring comprises a swingable hinge rod attached by respective ball and socket joints on one end portion thereof to the telescopic guide construction on the other and portion thereof to the end cap. 7

9. Heater according to claim 7 wherein there is additionally provided radially within the telescopic construction a cylindrical sleeve fastened with one of the end caps and the rotating drum and penetrating axially within the other to lie radially within the other, and a screw element in the gap between the said cylindrical sleeve and the one of the rotatable drum and the end cap into which it penetrates to urge material penetrating into that gap in an axial direction away from said wear rings.

10. Heater according toclaim 7 wherein the seal further comprises an adjustable stop to limit the distance of the closest approach of the end cap and the drum upon abrasion of the wear rings.

11. Heater according to claim 7 mounted on a chassis which has a detachable bogie adjacent at least one end and hinged telescopic supports adjacent at least the other end to enable movement to a construction site and mounting thereat at a desired angular elevation.

12. Heater according to claim 1 wherein the drum has a gas inlet end and gas outlet end, further comprising a stationary end cap at each end thereof a mixing chamber provided on the end cap at the gas inlet end, a burner for producing heated combustion gases, a burner muffle connected between said burner and said mixing chamber for conducting the heated combustion gases from said burner to said mixing chamber, and recirculating gas means comprising, in turn, an annular duct surrounding said burner muffle, for conducting combustion gases from the gas outlet end to said mixing chamber, the mixing chamber comprising a central portion for receiving freshly combusted gas from said burner via said burner muffle, and a radially outer portion for receiving recirculated gas via said annular duct outside the drum, together with means for controlledly causing a portion of the circulating gas to escape to the outside atmosphere.

14. Heater according to claim 12 further comprising a shield against radiant heat penetration provided in the mixing chamber.

Claims

1. Apparatus for the heating or drying of pulverulent or granular fluent solid material comprising, in combination, a rotatable drum, an array of heating tubes of generally oval cross-section having a pitch between the centers of the tubes within the range of about 1.3 to 1.5 timeS the major crosssection dimension of the tubes, said rotatable drum with the major sectional axes of said tube extending in a generally vertical direction, a plurality of supports for at least some of said tubes disposed at distances about one-third of the way therealong, and a plurality of lifting blades extending generally longitudinally within the drum secured along its inner periphery for lifting pulverulent or granular fluent solid material within said drum continuously upwards during rotation of said drum to drop it generally uniformly over the array of tubes, the lifting blades each comprising in turn, two unequal legs, the longer of which is fixedly attached at one end portion thereof with the inner circumferential wall of the rotatable drum and at the other end portion thereof with the shorter leg, said shorter leg being a tangent to a circle inscribed about the axis of rotation of the drum, the longer leg extending at an angle to the radial direction of the drum which is in the approximate range 0* to about 30* so that the angle between the two legs is between approximately 90* to about 120*, and further compressing at least one intermediate distributing leg secured with the longer leg between the inner circumferential wall of the drum and the shorter leg, extending in generally the same direction and parallel to said shorter leg.

4. Heater according to claim 1 wherein the length of the intermediate distributing leg is about 0.4 to 0.6 times the radial distance between the inner circumferential wall of the drum and the shorter leg.

8. Heater according to claim 7 wherein the means for restraining the rotation of the wear ring comprises a swingable hinge rod attached by respective ball and socket joints on one end portion thereof to the telescopic guide construction on the other and portion thereof to the end cap.

10. Heater according to claim 7 wherein the seal further comprises an adjustable stop to limit the distance of the closest approach of the end cap and the drum upon abrasion of the wear rings.

12. Heater according to claim 1 wherein the drum has a gas inlet end and gas outlet end, further comprising a stAtionary end cap at each end thereof a mixing chamber provided on the end cap at the gas inlet end, a burner for producing heated combustion gases, a burner muffle connected between said burner and said mixing chamber for conducting the heated combustion gases from said burner to said mixing chamber, and recirculating gas means comprising, in turn, an annular duct surrounding said burner muffle, for conducting combustion gases from the gas outlet end to said mixing chamber, the mixing chamber comprising a central portion for receiving freshly combusted gas from said burner via said burner muffle, and a radially outer portion for receiving recirculated gas via said annular duct portion for receiving recirculated gas via said annular duct.

13. Heater according to claim 12 wherein said recirculating gas means comprises a recirculating fan mounted in prolongation of the gas outlet end of the drum and adapted to recirculate gas to the mixing chamber and gas recirculating ducts connected thereto extending generally parallel to the axis of the drum and outside the drum, together with means for controlledly causing a portion of the circulating gas to escape to the outside atmosphere.