FR2563327A1 - Installation for drying products by heat exchange with a drying fluid - Google Patents

Abstract

Installation for drying products, comprising a central vertical shaft mounted rotatably in an enclosure 10 and supporting superposed levels of annular plates 26, on each of which there is deposited a layer of products to be dried, a fan 24 provided at the inlet of the shaft to make circulate there a constant flow of drying fluid such as superheated steam which passes through the layers of products supported by the annular plates and is returned on the outside of the central shaft towards the fan 24. The invention makes possible the drying of agro-alimentary products and divided products.

Description

Installation for drying products by heat exchange with a drying fluid.

The invention relates to an installation for drying products by heat exchange with a drying fluid such as superheated steam or humid air.

Industrial dryers are known comprising a thermally insulated tubular enclosure, with vertical central drum rotatably mounted in this enclosure around its vertical axis and carrying superimposed stages of annular plates which are each intended to support a layer of the products to be dried. Means are provided for continuously bringing the products into the enclosure; and for depositing them on the upper tray carried by the central fit, and for passing the products from a tray to the lower tray at each turn of the central barrel. lower part of the enclosure is crossed by means for removing the dried products, which are poured from the lower plate carried by the central barrel.

In these known dryers, the drying of the products takes place by circulation of hot air horizontally sweeping the layers of products deposited on the annular plate, this air circulation being ensured by a series of fans carried by the internal wall of the enclosure and blowing the air radially towards the central Àt. The upper part of the enclosure forms a chimney for evacuating outside the hot air charged with humidity after having swept the layers of products. As a result, such an industrial dryer, with an open circuit, consumes a relatively very large amount of energy for its operation.

The subject of the invention is in particular a heating installation also comprising a rotating central fAt carrying the stages of the plates, but having a relatively very low energy consumption.

The drying installation according to the invention is characterized by a closed circuit of drying fluid - (for example superheated steam), comprising an impulse member placed at the inlet of the central fAt to circulate in that -a drying fluid flow, fluid passage openings formed in the wall of the central fAt at each tray stage to bring the fluid in contact with the products to be dried, the annular space between the enclosure and the stages of trays outside the central barrel forming a conduit for the return of the fluid to the impulse member, a conduit for sampling in this space a small fraction of the fluid flow corresponding to the amount of water evaporated from the products, and a heat exchanger forming a condenser-heater, through which the fluid flow upstream of the impulse member. The constant flow of fluid which circulates in a closed loop in the enclosure is thus brought into contact with the layers of products deposited on all the annular plates and ensures their drying by taking charge of a mass of water vapor corresponding to the quantity of water evaporated from the products. This water vapor is extracted from the enclosure while the constant flow of drying fluid is recycled towards the impulse member after passing through a heat exchanger bringing it back to the temperature level. desired.

Advantageously, when the drying fluid is superheated steam, the abovementioned sampling duct is connected to the inlet of a compression member, the outlet of which supplies the exchanger with recompressed water vapor at a level of sufficient temperature to ensure overheating of the constant flow of water vapor circulating in the enclosure.

The energy consumption of such an installation is therefore reduced essentially to that of the impulse member circulating the constant flow of vapor in the enclosure and to that of the member for compressing the vapor flow which is extracted. of the enclosure.

According to another characteristic of the invention, the condensed water outlet of the heat exchanger feeds means for reheating the wall of the enclosure, which makes it possible to limit the heat losses of the installation and to reduce the parasitic condensations.

This condensed water outlet of the exchanger can also be connected, outside the enclosure, to a water-air heat exchanger for heating atmospheric air supplying a product pre-drying enclosure.

According to yet another characteristic of the invention, the openings formed in the wall of the central post at each level of the stage open into radial boxes carried by the central post and whose upper walls are perforated floors supporting the layers of products to be dried, so that the drying fluid passes through these layers of products.

Each annular plate carried by the central barrel is provided with a plurality of these radial crisson, which are substantially juxtaposed in the same horizontal plane, and the perforated sole of two successive radial caissons are connected to each other by a view. radial jet with inverted V section which contributes to the confinement of products on the floors of the caissons.

Advantageously, each of these flaps is pivotally mounted on an edge of a box around a horizontal radial axis and is associated with return means in the closed position of the gap between the two radial boxes, and means bringing by pivoting in the open position of this interval, these means acting on the flap at each turn of the central barrel for the transfer of the products on the annular plate of the lower stage.

In the description which follows, given by way of example, reference is made to the appended drawings, in which FIG. 1 is a schematic view in axial vertical section of a drying installation according to the invention; Figure 2 is a schematic view of a product pre-drying enclosure associated with the drying installation; Figure 3 is a partial schematic view, developed, of the two upper stages of annular plates; Figure 4 is a partial schematic view in axial vertical section of an alternative embodiment of the drying installation; Figure 5 is a schematic view in axial vertical section of another alternative embodiment of the installation.

Reference is first made to FIGS. 7 to 3 showing a first embodiment of the invention.

The drying installation comprises a vertical enclosure 10, preferably insulated, which is of cylindrical tubular shape with vertical axis 12 and <contains in the lower part a heat exchanger 14 forming a condenser-superheater, in annular arrangement, which is traversed radially by a constant flow Q of water vapor circulating in the enclosure 10.

A vertical central barrel 16, coaxial with the enclosure 10, is rotatably mounted in this enclosure around the axis 12 and is driven in rotation around this axis by suitable motor means 17. The lower end of the vertical barrel 16 is supported, for example, by the upper end of the heat exchanger 14, by means of sealed bearings 18, while its upper end, closed substantially at the level of the upper wall 20 of the enclosure 10 is guided by a pivot 22.

An impulse member 24, such as an axial fan, is placed at the open lower end of the barrel 16, to circulate the flow rate Q of water vapor inside this barrel, from bottom to top. figure 1.

 I, was 16 door, externally, a number of stages of annular plates 26, regularly distributed over substantially its entire height between a hopper 28 for supplying products which opens inside the enclosure 10 by an opening of the upper wall 20 thereof, and a product outlet chute 30, open under the lower annular plate 26 and connected to a conduit 32 passing through the enclosure 10 and provided with means such as an airlock 34 allowing the products to exit and preventing steam leaks and air inlets. The feed hopper 28 is itself connected to a feed pipe 36, for example of the worm type 38 bringing the products to the hopper 28 from a pre-drying enclosure 40 (FIG. 2) .

 your operation, the hopper 28 is permanently filled, at least partially, with relatively cold products on which condenses a small amount of the water vapor circulating in the enclosure, so that a watertight waterproof plug is permanently formed at the outlet of the hopper 28 in the enclosure 10 and prevents the exit of steam and the entry of air through the hopper. The hopper 28 can be connected in a substantially leaktight manner to the supply duct 36, being provided in the upper part with holes or vent ducts, allowing the escape of the air (noncondensable) which is brought with the products into the hopper.

Each annular plate 26 is formed. (Figures 1 and 3) of a plurality of radial boxes 42 integral with the outer wall of the central barrel 16 and substantially juxtaposed in a mime horizontal plane, each radial box having the shape of the annular sector with vertical section, for example trapdzoidal, the lower, lateral and radially outer walls of the box being full while its upper wall 44 is constituted by a perforated sheet or a grid (called sole) intended to support the layer of products to be dried.

Two successive radial boxes 42 of a stage of the plate 26 are separated from each other by a radial gap which can be of constant width and which is normally closed by a flap 46 having a triangular or V-shaped vertical section overturned, pivotally mounted about a substantially radial horizontal axis 48 along a radial edge of one of the boxes 42. In the closed position of this gap between two boxes, the flap 46 extends over a certain height above the upper walls or perforated floors 44 "of the boxes 42 and participates in the confinement of the products deposited on the floors 44. At the radially external end of each box, the confinement is ensured by a vertical rim 50 of the box, which extends above the perforated sole 44.

Each flap 48 is held in its position for closing the gap between two successive radial caissons 42 by suitable means, for example by spring or by counterweight, and can be brought at each turn of the vertical drum 16 into a position d opening of this gap by pivoting down around the axis 48, for example by means of a fixed cam system 52 carried by the internal wall of the enclosure 10 and cooperating with an element such as an arm or a lever integral with the flap 46 and ensuring the pivoting of this flap downwards during a rotation of the order of 20 to 400 of the vertical shaft 16, corresponding substantially to the angular extent of the perforated hearth 44 of a radial box. Scrapers are fixedly mounted in the enclosure 10 above each tray stage 26, being carried by a plate or sheet 53 and include an equalizing scraper 54, disposed after a supply of products on the annular tray to form on that here a layer of thickness predetermined product to be dried, and a transfer scraper 56, placed just before this supply of products and intended to sweep the perforated sole 44 of each radial box 42 to transfer the layer of products on the inner plate 26, by the radial intervals between the successive radial wells 42.

Each radial box 42 communicates with the internal space of the central vertical barrel 16 by calibrated openings 58 of the wall of this barrel, which can be formed by perforated plates or grids and which open inside the boxes 42 under their perforated floors 44.

The calibrated openings 58 form steam distributors in the boxes 42 of the different stages of trays and are dimensioned so as to ensure the desired flow of steam in each radial box of each stage.

A vapor withdrawal duct 60 is externally connected to the enclosure 10, preferably at the top of the latter, and is connected for example to the intake of a vapor compression member 62, the outlet is connected at 64 to the steam inlet of the heat exchanger 14 forming the condenser-superheater. The outlet of the condenser is connected by a conduit 66 to means for heating the wall of the enclosure 10, for example of the type with coils 67 wound around the enclosure 10, and / or to the inlet of a heat exchanger of heat 68 water-air crossed by atmospheric air supplying the pre-drying enclosure 40, conne shown schematically at 71 in FIG. 2. The liquid outlet of exchanger 68 is provided with an expansion valve 70 , allowing to evacuate the water.

The hot air supplying the enclosure 40 passes through a perforated conveyor belt conveyor 72 which supplies products with the lower part of the duct 36 connected to the hopper 28 supplying the enclosure 10.

The hot air which has passed through the layer of products carried by the perforated conveyor belt 72 is evacuated outside the enclosure 40 by a chimney 74.

The installation which has just been described operates in the following manner
The products to be dried undergo a pre-drying in the enclosure 40 and are brought to the hopper 28 by the conduit 36 and the worm 38 to be deposited in a layer of uniform thickness, thanks to the scraper 'equalizer- 54 , on the perforated floors 44 of the radial boxes 42 of the upper annular plate 26. By rotation of the central drum 16 around the axis 12, the products to be dried are transferred step by step from an annular plate 26 to the lower annular plate, each flap 46 between two radial boxes pivoting down when it is located: opposite the fixed cam 52 to allow the passage of the products on the lower floor. In the lower part of the enclosure, the products carried by the lower annular plate 26 are poured, in the same way, into the chute 30 and are conveyed outside by the conduit 32 and the outlet airlock 34. The fan 24 maintains in the enclosure 10 a constant flow Q of superheated water vapor, which circulates inside the central barrel 16 and passes, through the calibrated openings 58, inside all the radial wells 42 to pass through the layers of products carried by the perforated floors 44 of these boxes. In contact with the products, the flow rate Q of steam is responsible for an additional flow rate q of water vapor corresponding to the quantity of water evaporated from the products, and this additional flow rate q is extracted from the enclosure by the conduit 60 and is preferably compressed by the compressor 62 and sent to the heat exchanger 14, where the vapor condenses and gives up its latent heat of condensation at the flow rate Q of vapor which passes radially 1'8changeur 14 pour rev enir towards the fan 24
In an exemplary embodiment, the flow rate Q of steam circulating in the enclosure 10 is of the order of 106 m3 per hour, the steam being at atmospheric pressure and at a temperature of the order of 130-150 C and being able to reach 200 C, the flow q of the steam extracted from the enclosure and corresponding to the amount of water evaporated from the product is of the order of 20 tonnes per hour, the steam extracted from the enclosure being at a temperature of the order of 110 ° C. and at a pressure equal to atmospheric pressure, the steam being at a temperature of the order of 150 ° C. and at a pressure of 5 bars at the outlet of the compressor 62 and at the inlet of the exchanger 14, l the water leaving the condenser being at a temperature of the order of 150 ° C. at a pressure of approximately 5 bars for supplying the water-air heat exchanger 68, the water leaving the exchanger 68 being at a temperature of about 50 C and at atmospheric pressure while the atmospheric air which leaves the exchanger 68 to supply the enc einte 40 of pre-exchange is at a temperature of the order of 100115 C.

In another exemplary embodiment, where the water vapor extracted from the enclosure is not compressed by the compressor 62 before being brought to the heat exchanger 14, the flow
Q of steam circulating inside the enclosure is of the order of some 108 m3 per hour with overheating which can range from thirty to several hundred degrees C, the flow rate q of steam extracted from the enclosure being l 'order of 20 tonnes per hour and supplying the exchanger 14 at atmospheric pressure.

The speed of rotation of the central barrel 16 can vary quite widely depending on the nature and the flow rate of products to be dried, the central barrel 16 being able to complete a complete revolution in a period of, for example, between one and six minutes, corresponding to stay of the products on each stage of the tray, the time of stay of the products in the enclosure can be between 10 minutes and one hour approximately.

Since the flow rate Q of steam circulating in the enclosure is relatively large, it is advantageous for the steam passage sections to be arranged so as to reduce the pressure losses. For this reason, the outside diameter of the annular plates 26 progressively decreases from one stage to the lower stage, as shown in FIG. 1.

This reduction in diameter of the plates also results in the thickness of the layer of products deposited on an annular plate progressively increasing downwards, as the product dries, which, in combination with an appropriate determination of the dimensions of the calibrated openings 58 for the passage of steam formed in the wall of the central drum 16, ensure the most efficient drying of the products.

Furthermore, as) the water vapor passing through the product layers is liable to partially condense, the lower walls of the boxes 42 can be inclined slightly downwards towards the central drum 16, to promote the recovery of condensates.

FIG. 4 shows a drying installation of the same type as that described with reference to FIGS. I to 3, but which also comprises a vertical central mast 80 mounted fixed in rotation inside the central barrel 16, coaxial with the latter , the mat 80 carrying radial tubular arms 82 ending with scraper members, brushes 84 or the like for cleaning the openings 58 for the passage of steam through the wall of the central barrel 16, to avoid obstruction of these openings by product particles.

The fixed central mast 80 also carries one or more radial arms 86 ending in curved plates 88 intended to close the calibrated openings 58 of the barrel 16 at the loading and transfer zones of the products, to avoid entrainment of fine particles. of products by the steam flow passing through the perforated floors 44 of the radial boxes in these loading and transfer zones.

The upper end of the mat 80 crosses watertight @ the upper wall of the enclosure 10 in order to be connected to a suction duct 90 fitted with a filter 92 and reinjecting the vapor into the upper part of the enclosure 10, at the outside the upper annular plate 26. The fine particles of products which are liable to obstruct the calibrated openings 58 are thus released from these openings by the brushes or the scraper members 84 and sucked in by the tubular arms 82, the mast 80 and the suction duct 90 to be conducted. to filter 92.

It will also be noted that in FIG. 42 the; central barrel 16 is of frustoconical shape with section gradually reducing towards its upper end
FIG. 5 shows an alternative embodiment of the drying chamber of FIGS. 1 to 3, in which the heat exchanger 14 is placed in the upper part of the chamber 10, above the vertical central drum 16 carrying the stages of trays, the fan or the like 24 being at the open upper end of the barrel 16, the lower end of which is closed. In this case, the constant flow rate Q of water vapor circulates from top to bottom in the barrel 16, passes through the calibrated openings 58 of the wall of the barrel, crosses the layers of products carried by the perforated floors of the radial caissons 42, circulates from bottom to top in the enclosure 10 outside the barrel 16 and stages of plates and passes radially through the heat exchanger 14 to return to the fan 24
By this arrangement, the speed of the vapor circulated by the fan 24 is the most important at the level of the upper stages of trays which. carry the wettest products, and is weakest at the level of the lower stages of trays where the products are practically dry, which reduces the entrainment of fine particles of products by the flow of vapor.

The drying installation according to the invention can operate with a constant flow rate Q of superheated water vapor, as described with reference to the appended drawing, the water vapor extracted from the enclosure may or may not be compressed before supplying - lie on the exchanger 14 forming a condenser-superheater, and it can also operate with a flow Q of air charged with humidity, the flow q which is extracted from the enclosure then not being compressed before passing into the exchanger 14.

The installation according to the invention allows the drying of very diverse divided products, such as agro-food products (beet pulp, bagasse, fodder plants, for example alfalfa, etc.), mineral or vegetable grain products ( sand), powder, chips, crystals, etc.

Claims (19)

 Claims  1 Installation for drying products by heat exchange with a drying fluid, comprising a tubular enclosure (10) with vertical axis (12), a vertical central drum (16) rotatably mounted in the enclosure around this axis and carrying superimposed stages of annular plates (26) each intended to support a layer of products to be dried, means (28, 36, 38) for continuously bringing the products into the upper part of the enclosure (10) and depositing them on the plate upper (26), means (46, 52, 56) for passing products step by step from one tray to the lower tray, and means (30, 32, 34) for output of the products in the lower part of the enclosure, characterized by a closed drying fluid circuit, comprising an impulse member (24) placed between the central barrel (16) to circulate therein a flow (Q) of drying fluid, openings (58) fluid passage formed in the wall of the central barrel (16) at each stage of p slat (26) for bringing the fluid into contact with the products to be dried, the annular space between the enclosure (10) and the stages of trays (26) outside the central barrel (16) forming a return duct from the fluid to the impulse member (24d, a conduit (60) for sampling in this space a small fraction q) of the fluid flow (Q) corresponding to the amount of water evaporated from the products, and a heat exchanger ( 14) forming a condenser-heater, crossed by the flow (Q) of fluid upstream of the impulse member (14). 2. Installation according to claim 1, characterized in that the openings (58) open into radial boxes (42) outside the barrel (16) and carried by the latter, the upper walls of these radial boxes being perforated floors ( 44) supporting the layers of products to be dried, so that the drying fluid passes through these layers of products. 3. Installation according to claim 1 or 2, characterized in that, the drying fluid being superheated steam, the sampling conduit (60) is connected to the inlet of a compression member (62) the outlet of which supplies the exchanger (14) with recompressed water vapor at a temperature level sufficient to ensure the superheating of the flow rate (Q) of steam circulating in the enclosure. 4. installation according to one of claims 1 to 3, characterized in that the outlet (66) of condensed water from the exchanger (14) feeds means (67) for reheating the wall of the enclosure (10 ). 5. Installation according to one of the preceding claims, characterized in that the outlet (66) of condensed water from the exchanger (14) is connected, outside the enclosure (10) to a heat exchanger (68) water-air for heating atmospheric air supplying an enclosure (40) for pre-drying products. 6. Installation according to claim 5, characterized in that the enclosure (40) for pre-drying comprises a perforated conveyor belt (72) supporting a layer of products and traversed by the flow of hot air leaving the exchanger ( 68). 7. Installation according to one of the preceding claims, characterized in that the outer diameter of the plates (26} progressively decreases from one stage of the plate to the lower stage.  8. installation according to one of the preceding claims, characterized in that, each annular plate (26) being formed of a plurality of radial boxes (42) substantially juxtaposed in the same horizontal plane, two successive radial boxes (42) are connected to each other by a radial flap (46) of triangular or inverted V section, participating in the confinement of the products on the upper walls or perforated floors (44) of the radial boxes (42). 9. Installation according to claim 8, characterized in that each flap (46) is pivotally mounted on an edge of a radial box (42) around a horizontal radial axis (48) and is associated with return means in closed position of the gap between the two radial boxes, and to means (52) bringing it by pivoting into the open position of this gap, these means acting on the flap (46) at each turn of the radial barrel ( 16) for the transfer of products to the lower floor. 10. Installation according to claim 9, characterized in that the aforementioned means comprise a cam or the like (52) carried by @ the internal wall of the enclosure (10) and located on the circular path of an element integral with the flap ( 46). 11. Installation according to one of the preceding claims, characterized in that the lower walls of the radial wells (42) are inclined downwards towards the central drum (16) for the evacuation of condensates. 12. Installation according to one of the preceding claims, characterized in that the openings (58) of the central barrel (16) are formed by perforated sheets or rilles. 13. Installation according to one of the preceding claims, characterized in that it comprises a fixed vertical mast (80) extending axially in the central barrel (16) over the entire height thereof and carrying fixed members ( 84) for cleaning the openings (58) of the barrel (16), as well as means (88) for closing these openings during the periods of loading of the products on the upper shelf and of transfer of the products from a shelf to the shelf inferior. 14. Installation according to claim 13, characterized in that the mast (80) is tubular and is connected to a duct (90) for suction and to a filter (92), the members (84) for cleaning the openings (58 ) being carried by tubular arms (82) opening into the mat (80) for the suction of fine particles of products. 15. Installation according to one of the preceding claims, characterized in that the heat exchanger (14) is placed in the lower part of the enclosure (10) under the central barrel (16), the constant flow (Q) of drying fluid flowing from bottom to top in the central barrel (16) and from top to bottom in the enclosure (10) outside the central barrel (16). 1 16. Installation according to one of claims 1 to 14, characterized in that the heat exchanger (14) is placed in the upper part of the enclosure (10) above the central barrel (16), the constant flow (Q) drying fluid flowing from top to bottom in the central drum (16) and from bottom to top in the enclosure (10) outside the central drum (16). 17. Installation according to one of the preceding claims, characterized in that it makes it possible to evaporate, at atmospheric pressure, a water flow of approximately 20 tonnes per hour corresponding to a flow (Q) of steam from water in the enclosure of the order of 106 m3 per hour superheated at a temperature of about 130-1500C, the flow of steam extracted from the enclosure being recompressed before being brought to the heat exchanger (14) . 18. Installation according to one of claims 1 to 16, characterized in that it makes it possible to evaporate, at atmospheric pressure, a water flow rate of approximately 20 tonnes per hour corresponding to a flow rate (Q) of steam in the enclosure of the order of a few t m3 per hour, the superheating of this steam being able to range from 300C to several hundred degrees C. 19. Installation according to one of the preceding claims, characterized in that it allows the drying of divided products such as beet pulp, bagasse, fodder plants, for example alfalfa, products in grains, for example sand, powder, chips, crystals, such as mineral or vegetable products.