US3713225A

US3713225A - Pneumatic driers

Info

Publication number: US3713225A
Application number: US00133841A
Authority: US
Inventors: A Mark
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-04-28
Filing date: 1971-04-14
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: FR2088855A5

Abstract

In a pneumatic drier contact of the particles with the wall of the drying duct is eliminated by creating an air screen along the zones of the duct wall which are to be protected. In the portion of the duct wherein the material to be dried is introduced, the duct may be surrounded by a pressurized chamber from which air may flow into the duct under a high velocity through louver-like slits. Pressurized air may be forced into a frustro-conical chamber within which the drying duct is interrupted. A mere blade-like jet obliquely directed within the drying duct below the material inlet passage so as to strike the duct wall in front of the latter may be sufficient in some cases. In the bends the outer wall of the drying duct may also be protected by a bladelike air jet.

Description

Appl. No.: 133,841

Ulllted States Patent 1 [111 3,713,225

Mark Jan. 30, 1973 [54] PNEUMATIC DRIERS Primary Examinerl(enneth W. Sprague [76] Inventor. tpgrrl'eFlrlaaxil, 54 Cours Lafayette, A'wmey Alexander & Dowel] [22] Filed: April 14, 1971 [57] ABSTRACT ln a pneumatic drier contact of the particles with the wall of the drying duct is eliminated by creating an air screen along the zones of the duct wall which are to be protected. In the portion of the duct wherein the material to be dried is introduced, the duct may be surrounded by a pressurized chamber from which air may flow into the duct under a high velocity through louver-like slits. Pressurized air may be forced into a frustro-conical chamber within which the drying duct is interrupted. A mere blade-like jet obliquely directed within the drying duct below the material inlet passage so as to strike the duct wall in front of the latter may be sufficient in some cases. In the bends the outer wall of the drying duct may also be protected by a blade like air jet.

16 Claims, 9 Drawing Figures PATENTED JAN 30 I975 SHEET 1 UF 3 I 4 INVENTOR n 4 BY 1'- I W ATTORNE,

PATENTEDJAN 30 1973 SHEET 2 UP 3 ENTOR ATTORNE PNEUMATIC DRIERS This invention relates to pneumatic driers of the kind wherein a fractionated bulky material to be dried is introduced in the crystalline, pasty or powdery state into a duct through which flows a hot and dry gaseous stream which entrains the material while absorbing its moisture content, the dried material thus reaching an appropriate separator such as a cyclone, from which it may be collected.

The apparatus of the kind above referred to may ensure quite good results for a large number of materials. It is always possible to increase the length of the drying duct between the point of introduction of the material and the separator, and/or to recycle at least a fraction of the material issuing from this duct, so as to obtain any value for the residual moisture content.

It has however been found that the operation was unsatisfactory with some materials due to the contact of their particles with the walls of the drying duct.

A first class of materials which cannot be satisfactorily treated in pneumatic driers includes those which have in the wet state marked adhesive properties, their particles sticking to the walls as soon as they strike them. It is admittedly possible to reduce this disadvantage by recycling into the feeding device of the drier a portion of the material issuing therefrom, but this solution cannot be applied in all cases, and more particularly in the case of fragile material (such as crystals) which are affected by abrasion phenomena within the drier and which therefore should not be submitted to a plurality of drying cycles.

Another class of materials which cannot be satisfactorily dried in pneumatic driers comprises hard and crystalline products which either erode the drying duct or are broken by impact against the wall of the latter. In the first case the useful operating life of the drier is reduced and metallic particles are entrained with the dried product. In the second case the dried product is degraded, which is unsuitable for many subsequent purposes.

It is an object of the present invention to eliminate these inconveniences of the prior art and to avoid, or at least to minimize, contact between the walls of a drying duct and the particles of the material to be treated.

In accordance with the present invention there is interposed between the material and the walls of the drying duct a gaseous screen which moves along the wall with a velocity substantially higher than the flowing velocity of the main drying gas stream. Under such conditions the particles of the material to be treated, which due to their random displacements within the drying air stream tend to strike the wall of the drying duct, are deflected by the aforesaid screen and are returned into the central portion of the duct.

In an embodiment adapted to the treatment of sticking products which have a tendency to settle on the portion of the wall of the drying duct adjacent to the point of introduction of the materiaLthis portion is formed with slits opening substantially in the direction of the main drying gas stream and the said zone is surrounded by a chamber into which a gas is supplied under an appropriate pressure. With such an arrangement the slits determine the formation within the drying duct of an annular sleeve-like gas screen which flows with a high velocity along the wall and which protects the latter against any contact with the particles to be dried. The slits may be conveniently disposed between inwardly projecting portions of the conduit wall, so as to form louvers.

In another embodiment the drying duct is arranged in the form of an injector, i.e. it is interrupted for a short distance within a closed chamber. A pressurized gas is injected into the said chamber and here again this gas forms within the duct a high velocity annular sleeve-like gas screen which surrounds the central portion of the drying duct into which the material to be dried is introduced. In order that this annular gas screen should not interfere with the introduction of the material into the duct, there is preferably provided below the point of introduction of the material a small deflector which determines downstream of itself a dead zone through which the material may easily pass. The formation of the annular gas screen may further be im proved by imparting to the chamber a frustro-conical shape with the apex of the geometrical cone situated downstream of the chamber itself. It is also possible to provide means whereby the distance which separates the facing ends of the drying duct within the chamber may be varied at will.

In a third embodiment the arrangement is simplified and the gas screen is obtained by merely injecting obliquely into the drying duct, immediately below the inlet of the material to be dried, a high velocity bladelike gas jet which prevents the material from reaching the portion of the duct wall which faces the point of introduction of the material. With such an arrangement the gas screen admittedly only protects a quite limited portion of the duct wall, but this is sufficient in many cases, since the zone in which the wet particles have the greatest tendency to stick to the said wall of the drying duct is generally situated in front of the point of introduction of the material, slightly above the latter.

Concerning the bends of the drying duct, they are preferably equipped with a device similar to the third embodiment above described. In such a case the bladelike gas screen should be injected into the drying duct in such a manner as to flow along theportion of the duct wall farthest from the center of curvature of the bend in order to deflect the particles which would otherwise tend to strike the said portion under the action of centrifugal force.

In a further embodiment the gaseous screen formed along a portion of the wall of the drying duct has a high velocity rotational motion component about the axis of the duct itself. This may be obtained by injecting the pressurized gas tangentially into the drying duct through appropriately directed slits. Owing to this rotational component the injected gas sweeps the duct wall and dislodges any particle which 'may'have reached the latter. Y

In the annexed drawings FIG. 1 is a general view in elevation of a drier according to the invention.

FIG. 2 is an enlarged side view with parts in section, illustrating the material inlet device of this drier.

FIG. 3 diagrammatically shows a development of the inner side of the wall of the drying duct in the zone corresponding to the material inlet of the drier.

FIG. 4 is a section of another embodiment of the material inlet device of a pneumatic drier according to the invention.

FIG. 5 is an enlarged section of the seal provided for the sliding tube in the embodiment of FIG. 4.

FIG. 6 is a section of a simplified embodiment of the material inlet device of a drier according to the invention.

FIG. 7 is an enlarged vertical section of the upper bend of the drying duct in the pneumatic drier of FIG. 1.

FIG. 8 is a cross-section of an embodiment wherein the gaseous screen is imparted a rotational motion component about the axis of the drying duct.

FIG. 9 is a longitudinal section thereof.

The pneumatic drier illustrated in FIG. 1 comprises a drying duct 1 which opens into a separating cyclone 2 the upper outlet of which is connected with the suction side of a fan 3, the blowing side of this fan communicating with an exhaust chimney 4. Cyclone 2 comprises as usual a lower outlet 5 equipped with an appropriate airlock and through which it is possible to collect the dried product separated from the air stream which circulates through the drying duct 1 under the action of fan 3.

The inlet of the drying duct 1 communicates with an appropriate air heater 6. The material to be dried is introduced into duct 1 downstream of this air heater 6. There is provided for this purpose a hopper 7 and an air-lock 8. The details of the feeding device 9 through which the material passes before reaching the drying duct will be described below.

The drier of FIG. 1 generally operates in the conventional manner The wet material introduced through the feeding device 9 is entrained by the air stream which circulates through the drying duct 1. The moisture of the wet particles is absorbed by the relatively hot and dry air in which they are in suspension. The dried product is separated from the air stream within the separating cyclone 2 and it may be collected through the lower air-lock 5. It is possible to provide means for re-cycling into the duct 1 a portion of the product issuing from the air-lock 5.

FIG. 2 illustrates the details of the material feeding device 9 of FIG. 1. The drying duct 1 is surrounded by a wall 10 which defines an annular chamber 11 connectedby a conduit 12 with the blowing side of a fan 13. The suction side of this fan is in turn connected with a conduit 14 which communicates with a point of the drying duct 1 situated upstream of the material feeding device 9. This conduit 14 is also'provided with an air inlet 15. Vanes l6 and 17 (FIG. 2) permit of adjusting at will the relative proportions of hot air from duct 1 and of fresh airfrom inlet which are supplied to fan 13, in order to control as desired the temperature within the said chamber 11.

Within the annular chamber 11 the wall of the drying duct is embossed and slitted in such manner as to determine a series ofstrips 18 arranged in the manner of louvers, the intermediate spaces or slits 18a opening upwardly into the drying duct. Owing to this arrangement the'air supplied by fan 13 exhausts from the annular chamber into the drying duct in the form of an annular sleeve-like screen which flows with a high velocity along the inner side of the wall of the said duct. The flowing velocity of this screen should be substantially higher than the velocity of the main drying air stream as determined by the fan 3' associated with the cyclone. For instance for a given material the velocity of the main drying air stream being about 35 meters per second, the velocity of the screen should be approximatively meters per second. Since the dynamic effects of an air stream are proportional to the square of its velocity, the effect of the annular screen is then four times higher than for the main air stream.

The outlet of the air-lock 8 is connected with the drying duct 1 through an inclined passage 19. This passage comprises an intermediate longitudinal partition which defines two superposed spaces 19a and 19b, this partition being arranged as the wall of the drying duct 1 within the annular chamber 11, i.e. in the form of a series of strips or louvers 20 separated by intermediate slits which open towards the duct 1, starting from the lower space 19b. This lower space communicates directly with the blowing conduit 12 of fan 13.

It will further be remarked that the arrangement illustrated in FIG. 2 includes a vertical air conduit 21 which opens in the blowing conduit 12 of fan 13. The object of this conduit 21 will be discussed later with reference to FIG. 7.

In operation the fan 3 generates in the drying duct 1 an air stream of sufficient velocity for entraining the bigger particles of the material to be dried. This velocity should not however be too high in order to ensure the residence time required for the evaporation of the moisture from the particles in suspension in the air stream. The material which is introduced into the apparatus through the air-lock 8 reaches partition 20 on which it tends to flow downwardly in the form of a more or less uniform layer. This layer is fluidized and therefore homogenized by the airforcedunder pressure into the lower space 19b, so that it flows regularly and reaches the inside of the drying duct where it is entrained in suspension in the main drying air stream. This fluidization of the layer of the material to be dried on partition 20 regularizes the thickness of the layer and more particularly avoids any sticking of moist particles to the walls of the passage. It further permits of reducing the slope of partition 20 and therefore the height of the feeding hopper 7, which is generally an advantage.

But at the same time the air forced into the chamber 1 l by fan 13 flows through the slits which separate the strips 18. This determines within duct 1' the formation of an annular sleeve-like air screen which surrounds the main air stream determined'by' fan 3. The characteristics of fan 13 and the dimensions of the slits 18a which separate the strips from each other are so determined that the velocity of this annular air screen is substantially higher than that of the main drying air stream. Under these conditions experience demonstrates that the particles of material supplied from hopper 7 through air-lock 8 are at all times maintained spaced from the wall of the drying ductsin the whole zone of this duct which is surrounded by the annular chamber 11 and along a certain length above the latter. It results therefrom that any sticking of still very-wet particles to the walls is fully avoided. It is to be noted in this respect that in the zone adjacent to the material feeding device 9, the particles are superficially dried and that therefore any risk of sticking may be considered as non-existent somewhat above device 9.

Vanes

16 and 17 permit of adjusting to the most favorable value the temperature of the air supplied by fan 13.

It is obvious that in order to obtain the best results the annular air screen which is thus formed within duct 1 should be quite uniform. This may be realized by disposing the slits 18a in a staggered pattern, as illustrated in FIG. 3, this figure corresponding, if desired, to a development of the wall of duct 1 within chamber 11.

The drying duct 1 may be of circular or rectangular cross-section, as desired.

In the modified embodiment of FIGS. 4 and 5, the material feeding device comprises a mere inclined passage 22, it being understood that the latter could also be divided by an intermediate partition provided with slits in order to fluidize the material, as described with reference to FIG. 2. Duct 1 is interrupted below the outlet of this passage 22 so as to determine two facing ends. The higher end has a downwardly directed frusto-conical extension 23 which delimits around the lower end of duct 1 an annular chamber 24, the latter being closed by a horizontal partition 25 through which the said lower end is passed. The blowing conduit 12 of fan 13 opens into this chamber 24.

Here again the air supplied by fan 13 flows in duct 1 above chamber 24 in the form of a high velocity annular sleeve-like air screen, which prevents the sticky particles of the material to be dried from coming into contact with the walls of the drying duct, at least up to a certain height starting from the outlet of the passage 22.

FIG. 4 shows a small deflector 45 disposed against the inner side of the wall of duct 1 immediately below the outlet of the passage 22 through which the material is introduced into the duct. The object of this deflector is to prevent the high velocity air screen from hindering the introduction of the material issuing from passage 22. It is understood that this deflector determines on its downstream side a dead zone or an underpressure which on the contrary tends to draw the material from passage 22.

It is of advantage to provide means for adjusting the length of the portion la of the drying duct which is situated within the annular chamber 24, or in other words the cross-section of the annular space provided between the end of this portion 1a and the frustro-conical wall 23 of chamber 24. This enables to obtain the most favorable air velocity in the annular space for a given material to be dried. It is possible for this purpose to insert in the duct a removable annular stay 26 of any desired height. The length of portion 1a may thus be varied by selecting an annular stay having the proper height. The seal between the said portion hand the transverse wall 25 may be realized as illustrated in FIG. 5. In this arrangement a deformable gasket 27 is clamped between a fixed annular flange 28 secured to the partition 25 and a movable flange 29 which is acted upon by screws 30 carried by brackets 31 secured to the said partition.

FIG. 6 shows a simplified embodiment of a material feeding device according to the invention. As in the embodiment of FIG. 4 the material issuing from the airlock 8 falls into an inclined passage 32 which may comprise the fluidizing device of FIG. 2. Here the blowing conduit 12 of fan 13 ends in the form ofa flat nozzle 33 adapted to determine within the drying duct 1 and across the full width of the latter a high velocity bladelike air jet (indicated by the arrows 34) which is directed obliquely so as to constitute a screen in front of the outlet of passage 32. It will be understood that the particles of material which fall from the said passage are thrown upwardly as soon as they reach the blade-like jet 34 and thus undergo an intense preliminary superficial drying before being entrained towards the zone of the wall of duct 1 which faces the zone against which the jet is directed. The former (righthand zone in FIG. 6) is thus protected. With a number of materials such an arrangement is sufficient for avoiding any substantial building-up of sticky particles on the duct wall.

Referring again to FIG. 1, it will be understood that the above-described embodiments effectively avoid any sticking of the material to the wall of the drying duct 1. It will also be grasped that the above explanations concerning the case of sticky materials also apply to abrasive or fragile materials with which it is desirable to limit as far as possible any contact of their particles with the duct wall. However the upper portion of the drying duct 1 must comprise at least one bend such as 35, in order to be connected either with a separating cyclone such as 2, or with further portions of the said duct directed horizontally or in any other direction. Whatever may be the case under consideration there appears in such a bend an effect of centrifugal separation which tends to throw the bigger particles against the portion of the bend wall farthest from the center of curvature. In the case of sticky materials this may lead to the building up of material against the wall, while with very abrasive particles, the said portion is submitted to an intense erosion, or in the case of quite fragile crystalline particles, the impact against the conduit wall may brake the crystals and lower the value of the finally dried product.

In accordance with the present invention, in order to avoid these disadvantages, the above-mentioned conduit 21 is so arranged that it opens into the drying duct 1 at the inlet of the bend 35 in the form of a preferably flat nozzle 36 which determines within the vend the formation of a high velocity air layer (arrows 37) adapted to form a screen against the portion of the wall of the bend opposed to the center of curvature. Here again this layer or screen prevents the particles from reaching the said portion of the duct wall.

The embodiment of FIGS. 8 and 9 may be considered in some respect as a modification of the construction illustrated in FIGS. 1 to 3. Here again the drying duct 1 is surrounded by a wall which 'delimits an annular chamber 111 to which pressurized air is supplied by a conduit 112. The portion of the wall of duct 1 situated within chamber 111 is embossed and slitted so as to form a series of louver-like strips 118 with intermediate spaces 118a. But here these strips are longitudinal with respect to the axis of the duct (i.e. vertical); the annular chamber 111 is arranged as a volute and conduit 1 12 opens tangentially thereinto.

In this embodiment the air which issues at a high velocity from spaces 1180 into duct 1 forms therein a screen which has two motion components, namely a first component of rotation about the axis of the duct and a second component directed longitudinally since finally this air must exhaust through chimney 4. The resultant movement is thus helicoidal, which has two important results. In the first place the rotational component sweeps the wall of duct 1 and dislodges immediately any particle tending to stick thereto. In the second place the rotational motion of the air sleeve is in part transferred to the main drying air stream and therefore the particles in suspension therein follow a helicoidal path the length of which is greater than the length of the drying duct itself, whereby the residence time is increased.

The invention therefore permits of establishing a pneumatic drier wherein screens formed of highvelocity air layers prevent the particles from contacting the wall of the drying duct, at least in the portions thereof wherein such a contact is undesirable.

Of course while the present description refers to air as the gaseous medium used in the drier, it is obvious that the latter may operate with any other drying gas other than air proper, as for instance inert gases in the case of explosive or oxidable materials, combustion gases, superheated steam, etc.

I claim:

1. In a pneumatic drier comprising a drying duct having a wall with an inner side, means to introduce a fractionated bulky material to be dried into said duct in a zone thereof situated in the vicinity of one end thereof, and means to cause a stream of a drying gas to flow through said duct to entrain said fractionated bulky material, with said wall having apertures through which a gas is introduced into said duct, the improvement which consists in means to create against at least a zone of the inner side of the wall of said drying duct a gas layer introduced through said apertures and moving along said wall substantially in the same direction as said drying gas stream but at a higher velocity so as to form a screen between said wall zone and the particulated material entrained by said drying gas stream to prevent said particulated material from coming into contact with said wall.

2. In a pneumatic drier as claimed in claim 1, said zone extending across the whole inner side of said wall, and said last-named means creating in said zone of said drying duct a gas layer which forms a sleeve wholly surrounding said main drying gas stream.

3. In a pneumatic drier as claimed in claim 2, said last-named means comprising a chamber which surrounds said drying duct in said zone and means to supply gas under pressure to said chamber and the portion of the wall of said duct situated within said chamber being formed with passages which direct said gas under pressure into said duct along the inner side of the wall thereof in the direction of flow of said drying gas stream.

4. In a pneumatic drier as claimed in claim 3, said passages being in the form of successive slits provided in the wall of said drying duct, substantially transversely to same, with said wall being embossed adjacent each slit so as to determine a series of louvers which open into said duct in the direction of flow of said drying gas I stream.

5. In a pneumatic drier as claimed in claim 3, said passages being in the form of an annular space determined by an interruption of said drying duct, with said interruption determining for said duct within said chamber an upstream end and a downstream end, and said chamber being defined by an outer wall which converges towards said downstream end so as to form a converging nozzle which surrounds said upstream end.

6. In a pneumatic drier as claimed in claim 5, means to adjust the length of the interruption of said drying duct within said chamber.

7. In a pneumatic drier as claimed in claim 1,

the wall of said drying duct having an aperture for passage of the material introduced into said duct by said material introducing means said portion of the inner side of the wall of said duct being situated in front of said aperture and said gas layer creating means being formed of a nozzle opening into said drying duct upstream of said aperture and so arranged as to produce a sheet-like gas layer, obliquely directed with respect to said drying duct in the direction of flow of the said drying gas stream so as to strike said wall portion in front of said aperture and along said wall portion in the same direction as said drying gas stream.

8. In a pneumatic drier as claimed in claim 1,

said material introducing means including an aperture in the wall of said drying duct for passage of said material therethrough said portion of the inner side of the wall of said duct being situated in the zone thereof adjacent to said aperture and downstream of same said gas layer creating means being situated upstream of said aperture with said layer circulating along said portion of said duct in the form of a sleeve and a deflector on the inner side of the wallof said drying duct immediately upstream of said aperture to prevent said gas layer from forming in front of said aperture a gas screen hindering introduction of said particulated material into the central portion of said drying duct.

9. In a pneumatic drier as claimed in claim 1, said drying duct having a bend and said portion of the inner side of said duct extending in a bend thereof, in the zone of the bend farthest from the center of curvature.

10. In a pneumatic drier as claimed in claim 1, said material introducing means comprising an inclined passage having an upper end and a lower end, means to supply particulated material to the upper end of said passage, and means to fluidize said particulated material in said passage and said wall having an aperture forming a communication between said passage and said drying duct. i

11. In a pneumatic drier as claimed in claim 1, said last-named means creating in said zone of said drying duct a gas. layer which forms a sleeve wholly surrounding said main drying air stream, and said last-named means also imparting to said gas layer a rotational motion component about the axis of said duct.

12. In a pneumatic drier as claimed in claim 11, said last-named means comprising a chamber which surrounds said drying duct in said zone and means to supply gas under pressure to said chamber and the portion of the wall of said duct situated within said chamber being formed with passages which direct said gas under pressure into said duct along the inner side of the wall thereof and substantially tangentially to said wall.

13. In a pneumatic drier as claimed in claim 12, said passages being in the form of slits provided in the wall of said drying duct, substantially longitudinally to same, with said wall being embossed adjacent each slit so as chamber being in the form of a conduit opening tangentially into same.

15. A method to prevent a wet particulated material from sticking to a zone of the inner side of the wall of the drying conduct in a pneumatic drier in which said material is entrained by a main drying gas stream,

which consists in creating along said inner side in the said zone a gaseous layer moving along said inner side in the same direction as said main drying gas stream, but at a higher velocity so as to form a screen preventing said particulated material from coming into contact with said inner side in said zone. 1

16. In a method as claimed in claim 15, the step of creating said gaseous layer in the form of a sleeve whollysurrounding said main drying gas stream.

Claims

3. In a pneumatic drier as claimed in claim 2, said last-named means comprising a chamber which surrounds said drying duct in said zone and means to supply gas under pressure to said chamber ; and the portion Of the wall of said duct situated within said chamber being formed with passages which direct said gas under pressure into said duct along the inner side of the wall thereof in the direction of flow of said drying gas stream.

4. In a pneumatic drier as claimed in claim 3, said passages being in the form of successive slits provided in the wall of said drying duct, substantially transversely to same, with said wall being embossed adjacent each slit so as to determine a series of louvers which open into said duct in the direction of flow of said drying gas stream.

7. In a pneumatic drier as claimed in claim 1, the wall of said drying duct having an aperture for passage of the material introduced into said duct by said material introducing means ; said portion of the inner side of the wall of said duct being situated in front of said aperture ; and said gas layer creating means being formed of a nozzle opening into said drying duct upstream of said aperture and so arranged as to produce a sheet-like gas layer, obliquely directed with respect to said drying duct in the direction of flow of the said drying gas stream so as to strike said wall portion in front of said aperture and along said wall portion in the same direction as said drying gas stream.

8. In a pneumatic drier as claimed in claim 1, said material introducing means including an aperture in the wall of said drying duct for passage of said material therethrough ; said portion of the inner side of the wall of said duct being situated in the zone thereof adjacent to said aperture and downstream of same ; said gas layer creating means being situated upstream of said aperture with said layer circulating along said portion of said duct in the form of a sleeve ; and a deflector on the inner side of the wall of said drying duct immediately upstream of said aperture to prevent said gas layer from forming in front of said aperture a gas screen hindering introduction of said particulated material into the central portion of said drying duct.

10. In a pneumatic drier as claimed in claim 1, said material introducing means comprising an inclined passage having an upper end and a lower end, means to supply particulated material to the upper end of said passage, and means to fluidize said particulated material in said passage ; and said wall having an aperture forming a communication between said passage and said drying duct.

11. In a pneumatic drier as claimed in claim 1, said last-named means creating in said zone of said drying duct a gas layer which forms a sleeve wholly surrounding said main drying air stream, and said last-named means also imparting to said gas layer a rotational motion component about the axis of said duct.

12. In a pneumatic drier as claimed in claim 11, said last-named means comprising a chamber which surrounds said drying duct in said zone and means to supply gas under pressure to said chamber ; and the portion of the wall of said duct situated within said chamber being formed with passages which direct said gas under pressure into said duct along the inner side of the wall thereof and substantially tangentially to said wall.

13. In a pneumatic drier as claimed in claim 12, said passageS being in the form of slits provided in the wall of said drying duct, substantially longitudinally to same, with said wall being embossed adjacent each slit so as to determine a series of louvers which open into said duct substantially tangentially to the wall thereof.

14. In a pneumatic drier as claimed in claim 12, said chamber being volute-shaped around said drying duct and said means to supply a gas under pressure to said chamber being in the form of a conduit opening tangentially into same.

15. A method to prevent a wet particulated material from sticking to a zone of the inner side of the wall of the drying conduct in a pneumatic drier in which said material is entrained by a main drying gas stream, which consists in creating along said inner side in the said zone a gaseous layer moving along said inner side in the same direction as said main drying gas stream, but at a higher velocity so as to form a screen preventing said particulated material from coming into contact with said inner side in said zone.