METHOD AND DEVICE FOR THERMAL DRYING OF A SOLID PRODUCT IN SMALL PIECES

Abstract: The layer (2) of the product in small pieces is brought between a first contact element (4) and a second contact element (5) which are both in contact with the layer (2), whereby at least one of the contact elements (4 and 5) is heated and whereby at least one of the contact elements (4 and 5) is gas-permeable. The device applied therefore comprises a first contact element (4) and, opposed thereto, a second contact element (5), between which a layer (2) of the product in small pieces can be brought into contact with these contact elements (4 and 5), and comprises a heat source forming part of means for heating these contact elements (4 and 5) directly or indirectly.

Description: Method and device for thermal drying of a solid product in small pieces. This invention relates to a method and device for thermal drying of a solid product in small pieces, whereby the small pieces are brought in form of a layer onto a contact element and subjected to the influence of heat. The invention particularly relates to a method for drying food products such as meals, vegetables, meat or fish, pharmaceutical products, minerals and so on, as far as they are in small pieces and preferably are somewhat deformable. These products can be cut into pieces beforehand, but they can also naturally have the shape of small pieces, such as, for example, peas. In known methods for thermal drying, the so-called conveyor drying, a stream of rather hot air is supplied over the products on a conveyor. These known methods, however, are relatively slow, whereas the applied high temperatures exert a disadvantageous influence upon the quality of the products. Although thermal drying always is preferred in respect to freeze-drying, as it is cheaper, it, anyhow, requires a lot of energy and has a low thermal efficiency. The invention aims at a method for thermal drying of a product in small pieces, whereby these and other disadvantages are excluded and which is relatively fast and economical and can provide dried products of a good quality. According to the invention, this aim is achieved by bringing the layer of the product in small pieces between the aforementioned contact element and a second contact element which are both in contact with the layer, whereby at least one of the contact elements is heated, and preferably both contact elements are heated, and whereby at least one and preferably both contact elements are gas-permeable . The contact elements may exert a certain pressure upon the small pieces. The heating may take place either directly or indirectly, for example, by supplying hot air. Preferably, the heating is performed by supplying hot air with a temperature below 70°C, or even below 60°C, which is significantly lower than the temperature with classical drying by hot air. In a particular form of embodiment, the drying comprises deep-freezing as a first step. It is also possible to dry not fully to the end in the manner described above, but to perform drying at the end by means of hot air which is supplied over or through a layer of small pieces during their transportation, or, in other words, in the classical manner of thermal drying. The invention also relates to a device which is particularly suitable for the application of the method according to one of the preceding forms of embodiment. The invention thus relates to a device for the thermal drying of a product in small pieces, comprising at least one contact element and at least one heat source, characterized in that it comprises a second contact element opposite to the aforementioned contact element, between which a layer of the product in small pieces can be brought into contact with these contact elements, whereas the heat source forms part of means for heating these contact elements directly or indirectly. One or both contact elements preferably are gas-permeable and thus provided with openings. Preferably, the device comprises means for moving these contact elements in respect to each other in order to come into contact with the small pieces situated in between and possibly exert a light pressure upon them. The contact elements can be formed by a transport conveyor which is endless and preferably is provided with openings. The contact elements are preferably arranged in a closed chamber which is provided with an inlet and an outlet for the product in small pieces. The small pieces may be pieces of root- or bulbous vegetables, and the device may comprise means for bringing the products in a single layer onto one of the contact elements. In order to shorten the total drying cycle, the drying device in front of the contact elements may comprise a deep-freezing device. It may also comprise an ordinary conveyor drying device following behind the contact elements. With the intention of better showing the characteristics of the invention, hereafter, as an example without any limitative character, preferred forms of embodiment of a method and device for thermal drying of a product in small pieces is described, with reference to the accompanying drawings, wherein: figure 1 shows a basic scheme illustrating the method for thermal drying according to the invention; figure 2 schematically shows a cross-section of a practical form of embodiment of a device according to the invention; figure 3 schematically shows a cross-section analogous to the one of figure 1 , but in respect to another form of embodiment of the device according to the invention. In order to dry a product in small pieces, for example, a vegetable which is cut into small pieces or naturally has the shape of small pieces, according to the invention, these small pieces first of all are deep-frozen in a deep-freezing device 1 to, for example, -4 to -8°C. This deep-freezing may be performed in the classical manners and mostly takes about twenty minutes. This deep-freezing allows a considerable shortening of the total drying cycle and, thereby, an enhancement of the quality of the dried product. By this deep-freezing, the structure of the product is almost not damaged, contrary to the actual freeze drying. If the products are vegetables, they need less blanching. Of course it is also possible, instead of deep-freezing fresh products at the beginning of the drying cycle, to start with products already deep-frozen before, so that, for example, drying is also possible out of the harvest season of the products . In a following and most important step, the small pieces are provided in a thin layer 2 in a contact dryer 3 between a lower contact element 4 and an upper contact element 5, in such a manner that the layer 2 is in contact with both contact elements 4 and 5. Hereby, provisions must be made to maintain this contact even when the small pieces shrink as a result of drying. If the small pieces are three-dimensional and, for example, blocks or discs of bulbous vegetables, a single layer 2 of small pieces is provided between the contact elements 4 and 5. These small pieces then will have a thickness of, for example, 2 to 5 mm. If the product consists of, in first instance, two- dimensional pieces, for example, leaf products, either cut into pieces or not, then a layer 2 of several pieces on top of each other is formed, with a thickness which preferably is between 2 and 15 mm. A light pressure may be exerted upon the layer 2 in order to realize a good contact of the small pieces with the contact elements 4-5 even as shrinkage occurs. This pressure has to be limited in order not to cause an overheating or mechanical damage of the small pieces. This pressure may be between 1 g/cm 2 and 36 g/cm 2 for small pieces of bulbous or tuberous vegetables and smaller than 1 g/cm 2 for leafy vegetables. Both contact elements 4 and 5 are gas-permeable and, for example, provided with openings, such as perforations. Up to 80% of their surface may consist of openings. Both contact elements 4 and 5 are also heated. This heating may be performed directly, for example, by means of a built-in electric resistance, but preferably is performed indirectly by means of heated air which is supplied over or through the contact elements. This air is somewhat heated, namely, up to a temperature which is below 70 °C and preferably below 60°C and is mostly situated 15 to 30 "c above room temperature. The small pieces themselves must not be heated above these temperatures and preferably are heated in such a manner that the temperature in their core does not rise above approximately 58 °C. The heating of the air can be performed electrically as well as by means of gas, oil or other fuels. The heat which is set free during deep-freezing may be recovered as well. The speed of the air which does not only provide for the heating of the contact elements 4 and 5, but also carries off the humidity set free by the small pieces, also determines the speed of drying and is relatively large and larger than in normal conveyor drying, which is possible as the small pieces are retained between the contact elements 4 and 5 and cannot be blown off. This air speed preferably is situated between 1 and 10 m/s and is, for example, approximately 3,8 m/s. The pressure of the air is regulated in such a manner that the wind speed is almost the same everywhere on the contact elements 4-5. With such air speeds, the small pieces are kept a relatively short time, preferably less than 40 minutes and, for example, for an average time of approximately 15 minutes, between the contact elements 4 and 5 in the contact dryer 3. In order to save space and costs, the retention of the small pieces in the contact dryer 3 can be stopped before they are completely dried, in fact as soon as the product is stable and does not shrink any further. For the final drying, the small pieces are brought into an end dryer 6 where they are brought onto a plate or conveyor belt in a much thicker layer than the layer 2, and where a heated air stream is supplied over this layer. Normally, the temperature of this air is higher than in the contact dryer 3, and the air speed is lower than in this contact dryer 3. As a final dryer 6, a classic conveyor dryer can be used. This final drying has almost no influence upon the quality. The dried end product is not only obtained rapidly and in a relatively cheap manner, but is of an excellent quality which approaches the taste obtained with freeze-drying, but without structural changes and with preservation of colours. The drying may also take place without preceding deepfreezing and/or end drying, in the classical manner by means of hot air. It may, thus, take place exclusively by means of contact drying, this is the second step, as described heretofore. The drying time, however, will be longer, for example, up to 2 hours, which is still shorter than exclusively the classic drying with hot air, whereas the quality of the end product is significantly higher than the one obtained by means of this classic drying. The complete drying or at least the contact drying may take place discontinuously or batchwise as well as continuous . In figure 2, a device is represented with which drying according to the invention can be performed discontinuously, which device exclusively comprises a contact dryer 3. This device substantially comprises two horizontal plates situated above each other and provided with openings, respectively forming the aforementioned contact elements 4 and 5 and means for heating these contact elements 4 and 5. These means comprise a fan 7 the speed of which is regulated by means of a frequency control 8 and a heating unit 9 connected to the exit thereof which is controlled by a temperature-regulating unit 10 and which forms a heat source. This heating unit 9 may get the thermal energy out of electricity, gas, oil or other fuels. The fan 7 and the heating unit 9 are provided in an air- inlet channel 11 which, over a flexibly compressible part 12, is connected to the upper housing part 13 of a two- part housing 13-14. This upper housing part 13 may be moved by means of lifting cylinders 15 in respect to the lower housing part 14 of the housing which is fixedly mounted on a stand 16 and with its underside gives out to an air-outlet channel 17. The air-outlet channel 17, together with the air-inlet channel 11, forms part of the means for heating the contact elements 4 and 5 by means of hot air. The plates forming the contact elements 4 and 5 are, for example, perforated stainless steel plates. They close off the upper housing part 13 and the lower housing part 14, respectively. Above the upper contact element 5 and beneath the lower contact element 4, temperature meters 18 for measuring the temperature of the air stream are installed in the housing 13-14. In the air-inlet channel 11, in front of the fan 7, and in the lower housing part 14, airspeed meters 19 are provided. The temperature meters 18 and the airspeed meters 19 are connected to an operating unit 20 which is provided at the stand 16 and which controls the lifting cylinder 15, the frequency control 8 and the temperature regulating device 10. Whereas the upper housing part 13 is kept in its highest position by means of the lifting cylinders 15, a layer 2 of the product in small pieces to be dried is provided on the perforated plate forming the lower contact element 4. Afterwards, this housing part 13 is moved downward by means of the lifting cylinders 15, until the perforated plate which forms the upper contact element 5, is pressing against the layer 2 with the desired force. In order to facilitate and to accelerate the supply, this layer 2 can be provided beforehand between two mats which, during drying, have to be seen as forming part of the two contact elements 4 and 5, respectively. In order to avoid during drying the lateral leakage of air from between the contact elements 4 and 5, the layer 2, after the provision thereof, may be surrounded by a sealing strip 21. Finally, by means of the fan 7 and the heating unit 9, air is sent through the contact elements 4 and 5, with the desired speed and at the desired temperature. These contact elements 4 and 5 and the layer 2 caught in between warm up, whereas the hot air carries off the humidity which has been set free out of the product, due to the heat. As the product is drying and, therefore, the small pieces are shrinking, the distance between the contact elements 4 and 5 can be regulated by means of the lifting cylinders 15. After the product has dried sufficiently, the upper housing part 13 is moved back upwards, and the layer 2 is removed from the housing 13-14. Instead of the upper housing part 13, the lower housing part 14 may be movable in respect to a fixed upper housing part 13. It is also possible that both housing parts 13 and 14 are movable. In figure 3, a device for continuous drying is shown, whereby this device comprises a contact dryer 3 which eventually may be used alone, for drying the product in a continuous manner. The device shown in this figure 3 consists of three major parts placed one after the other, namely a freezing device 1 , a contact dryer 3 and an end dryer 6 , whereby between the freezing device 1 and the contact dryer 3 a vibration device 22 is provided in order to supply a uniform layer 2 of small pieces of leafy vegetables or one single layer 2 of small pieces cut root- or bulbous vegetables to the contact dryer 3 , and whereby between this contact dryer and the end dryer 6 an endless transport conveyor 23 with a scraper 24 above is provided. The freezing device 1 comprises a closed cooling chamber and an endless transport conveyor 25 placed therein for moving the product to be dried in small pieces through this cooling chamber. This may take place in a classic manner with a cooling circuit 26 or by means of liquid nitrogen. The continuous contact dryer 3 differs from the above- described contact dryer substantially in that the housing 27 does not consist of two parts which are movable in respect to each other, but of only one unit and in that the contact elements 4 and 5 are endless transport conveyors provided with openings, for example, perforated conveyors, which are situated above each other and are driven separately by means of motors 28 or by means of a transmission together by one motor, in such a manner that the belts situated above each other between which the layer 2 is provided, move in the same sense of movement, away from the vibration device 22. Instead of a perforated conveyor belt, the belt may be a woven belt or consist of a flexible grid. One of the transport conveyors, for example, the uppermost, is adjustable in height by means of, for example, cylinders 29, in respect to a stand 16 which also carries the housing 27, in such a manner that the distance between the transport conveyors situated above each other can be adapted to the thickness of the layer 2 and in such a manner, that eventually a light pressure may be exerted onto this layer 2. The air-inlet channel 11, in which the fan 7 and the heating unit 9 are situated, gives out in the housing 27, either above the upper transport conveyor forming the contact element 5, or inside this upper transport conveyor, or still beneath the lower transport conveyor forming the contact element 4 , or inside this lower transport conveyor, as shown in figure 3. In the first two cases, the air-outlet channel 17 is connected to the underside of the housing 27 and the hot air stream flows downwards from above. In the latter cases, the air-outlet channel 17 is connected upwards to the housing, and the air flows upwards from below, through the transport conveyors. These transport conveyors are heated by the heated air which also takes along the humidity which is set free by the product. The carried-off air may wholly or partially, for example between 40 and 60 vol.%, be re-used after drying and supplied at the inlet. At the front, the housing 27 is provided with an opening for bringing in the layer 2 through the vibration device 22. At the back, the housing 27 is provided with an opening through which the transport conveyors protrude. The aforementioned scraper 24 cooperates with the protruding extremity of the lower transport conveyor. After scraping, the transport conveyors eventually might be cleaned or brushed off. The end dryer 6 is a classic conveyor dryer with a closed housing 30 and a horizontal endless transport conveyor 31 which is driven by a motor 32. To this housing 30 are also connecting an air-inlet channel 33 and an air-outlet channel 34, in an analogous manner as with the contact dryer 3. The major difference, however, consists in that the end dryer 6 does not comprise an upper transport conveyor, and in the working conditions. The layer of small pieces which is brought onto the transport conveyor 31 by the aforementioned transport conveyor 23, is thicker than the layer 2, and the temperature of the air is higher, up to 75°C. The end dryer 6 may comprise several transport conveyors which are provided behind each other or below each other and onto which the smal pieces successively drop. Constructively, the end dryer 6 can be provided below the contact elements 4 and 5 in a common housing. The discontinous as well as the continous drying, as described heretofore, allows a faster drying with a significantly better quality than the usual conveyor drying with hot air. The present invention is in no way limited to the forms of embodiment described heretofore and represented in the figures, on the contrary may such method and device for thermal drying be realized in different variants without leaving the scope of the invention.

Claims: Claims . 1.- Method for the thermal drying of a solid product in small pieces, whereby the small pieces are brought in a layer (2) onto a contact element (4) and are subjected to the influence of heat, characterized in that the layer (2) of the product in small pieces is brought between this first contact element (4) and a second contact element (5) which both are in contact with the layer (2), whereby at least one of the contact elements (4 and 5) is heated and whereby at least one of the contact elements (4 and 5) is gas-permeable. 2.- Method according to claim 1, characterized in that both contact elements (4 and 5) are heated. 3.- Method according to claim 1 or 2, characterized in that both contact elements (4 and 5) are gas-permeable. 4.- Method according to any of the claims 1 to 3, characterized in that with the contact elements (4 and 5) a certain pressure is exerted onto the layer (2) with small pieces. 5. - Method according to any of the claims 1 to 4 , characterized in that the contact elements (4 and 5) are heated indirectly by the supply of hot air. 6.- Method according to any of the claims 1 to 5, characterized in that the layer (2) is heated up to a temperature which is below 70 °C and preferably below 60°C. 7.- Method according to the claims 5 and 6, characterized in that the heating is performed by supplying hot air at a temperature below 70°C and preferably below 60°C. 8.- Method according to any of the claims 1 to 7, characterized in that the thermal drying of the product by means of contact of the contact elements (4 and 5) is preceded by deep-freezing of this product. 9. - Method according to any of the claims 1 to 8 , characterized in that drying is not fully performed to the end by means of the contact elements (4 and 5), but that in the end, drying is performed by means of hot air which is supplied over or through a layer of small pieces during their transportation. 10.- Device for the thermal drying of a product in small pieces, comprising at least one contact element (4) and at least one heat source, characterized in that it comprises a second contact element (5) opposite to the aforementioned contact element (4), between which a layer (2) of the product in small pieces can be brought into contact with these contact elements (4 and 5), whereas the heat source forms part of means for heating these contact elements (4 and 5) directly or indirectly. 11.- Device according to claim 10, characterized in that the contact elements (4 and 5) are installed in a housing (13-14 or 27) and the means for heating the contact elements (4 and 5) comprise an air-inlet channel (11) which ends up in the housing (13-14 or 27), a fan (7) which is provided in this air-inlet channel (11), and an air-outlet channel (17) connected to the housing (13-14 or 27), whereas the heat source is a heating unit (9) which is provided in an air-inlet channel (11). 12.- Device according to any of the claims 10 to 11, characterized in that at least one of the contact elements (4 and 5) and preferably both contact elements (4 and 5) are gas-permeable. 13.- Device according to any of the claims 10 to 12, characterized in that it comprises means (15) for moving these contact elements (4 and 5) in mutual respect in order to come into contact with the small pieces situated in between and possibly to exert a light pressure upon them. 14.- Device according to any of the claims 10 to 13, characterized in that the contact elements (4 and 5) are formed by a transport conveyor which is endless and preferably provided with openings . 15.- Device according to any of the claims 10 to 14, characterized in that it comprises means for bringing small pieces of root- or bulbous vegetables in a single layer or small pieces of leafy vegetables in a uniform thin layer onto one of the contact elements (4). 16.- Device according to claim 15, characterized in that the aforementioned means comprise a vibration device (22). 17.- Device according to any of the claims 10 to 16, characterized in that it comprises a deep-freezing device (1) preceding the contact elements (4-5). 18.- Device according to any of the claims 1 to 17, characterized in that it comprises an end dryer (6) following in the moving direction of the product behind the contact elements (4 and 5). 19.- Device according to claim 18, characterized in that this end dryer (6) is a conveyor dryer with an endless transport conveyor (34) and means for creating a hot air- stream thereover.

Inventors: CRUL GUIDO, CRUL, GUIDO
Applicants: CRUL GUIDO, D S P N V, CRUL, GUIDO, D.S.P. N.V.
Application year: 1997
Main classification: F26B3
IPC codes: F26B3, F26B17