Vacuum dryer with an evaporation chamber working in a continuous process

Abstract: The invention discloses a vacuum dryer, particularly suited for the drying of leather hides, in which the evaporation chamber (10) works continuously and is confined between a conveyor belt (2), on which the hides to be dried are spread and a counter-belt (5), arranged at a close distance from the conveyor belt, at least one intermediate belt (7,47,48) being arranged between them, said intermediate belt keeping the hides to be dried adhering to the conveyor belt. Each intermediate belt (7,47,48), which is formed by a plurality of spirals (8) joined together without interruption by gudgeons (14), defines the volume of the evaporation chamber which is formed essentially by the free volume between the spirals forming it. Means for the creation of the vacuum within the evaporation chamber (10) and for the removal of the condensate from the evaporation chamber itself are also foreseen.

Description: The invention concerns a vacuum dryer comprising the features according to the first part of claim 1. Such a dryer is known, for example, from DE-A-26 57 041. It is a well-known fact to those skilled in the art that there are on the market different types of dryers for drying hides, after they have undergone the wet treatments in the classical drum-tumblers or in equivalent machines. The US-A-3 224 109 discloses an apparatus for vacuum-drying of leather having an heatable plate for receiving leather, a movable cover adapted to form a sealed vacuum chamber and means for creating the vacuum and a water vapour condenser. The US-A-3 316 655 discloses an apparatus for drying tanned skin which comprises a heat transmitting conveyor belt, a lower hollow plate member located under said conveyor belt and an upper hollow plate member divided into a number of separate drying zones, wherein a vacuum is established. After a predetermined time the conveyor belt moves until each skin passes from a certain drying compartment to the following drying compartment. The DE-A-26 57 041 discloses a method of drying continuos paper strip where the strip is carried by a supporting band between upper and lower belts. The belt in contact with the paper strip is heated with steam, while the belt in contact with the supporting band is cooled. In such a way the water in the paper condenses in the supporting band. Drying installations, called 'PASTING' are widely known and used. Through these the hides to be dried are spread over cold glass plates, which are connected with each other so as to form a closed ring. The plates progress with a continuous or a discontinuous motion and take the hides arranged thereon into an oven, where hot air circulates, so that the hides are dried. When they come out of the oven, the hides are removed from the plates on which they had previously been spread and made to adhere by means of a bonding agent, which prevents them from falling down because of gravity at the end of the drying process. The inconveniences of said installations are the high cost, the need for the use of the bonding agent which cause the hides to adhere to the plates, and the high cost due to the thermic losses of the dryer working with the ventilation system. However, one of the main inconveniences is the space between the plates on which the hides are spread, which causes a considerable loss in the development of the closed ring formed by the plates, which, since it cannot be used for the spreading of the hides causes high production losses. Drying installations under vacuum are also known. They consist of a number of horizontal or vertical plates where the drying of the hides arranged on the plates is obtained by creating a depression in correspondence with the area where said hides are spread. Said installations, which are generally called "VACUUM DRYERS" also present some inconveniences. One of these inconveniences consists in that said dryers need a rather long drying cycle, a good part of the time being required for the removal of the steam which forms in the drying chamber during the leather-drying process. This steam must be removed by means of pumps and, particularly when the evaporation chambers are rather large, this removal process requires a rather long period of time. It is easy to understand how this entails waiting periods which cause a decrease of production of the dryer. Another inconvenience arises from the fact that during the drying process the steam is not immediately removed from the evaporation chamber and, therefore, it partially saturates it. As a consequence, the drying performance decreases. An attempt to overcome this inconvenience consists in increasing the leather heating temperature, but this entails an increase in the operating costs of the dryer, because of the increased heating cost. Moreover, the hides con not be overheated, otherwise they will be damaged. With the purpose of eliminating the above-mentioned inconveniences, the dryer according to the invention has been designed with the following purposes: to obtain a drying installation which, in comparison with equivalent dryers of the known type, permits the decrease of the cycle periods by obtaining a higher degree of vacuum while the temperatures are lower; to obtain a drying installation which, in comparison with equivalent drying installations of the known types, is able to remove the steam formed during the drying process in shorter periods of time; to obtain a drying installation which, in comparison with equivalent installations of the known types, permits increased productions by optimizing the continuous exploitment of the entire surface available for the loading of the hides to be dried; to obtain a drying installation which, in comparison with equivalent drying installations of the known types, presents reduced overall dimensions, the production performance remaining the same. The described purposes are reached by a vacuum dryer, particularly suited to dry leather hides, which, in accordance with the main claim, comprises a conveyor belt on which the hides to be dried are spread, said conveyor belt being arranged without interruption and with a substantially horizontal development between the external surfaces of at least two cylinders, so as to form, an evaporation chamber, confined between the mentioned conveyor belt and a counter-belt placed at a close distance from the conveyor belt along a common stretch comprising said conveyor belt, the hides to be dried being kept adhering to the conveyor belt by means of at least one intermediate belt placed between the conveyor belt and the counter belt, said vacuum dryier being characterized in that said intermediate belt is formed by a plurality of spirals connected with each other without interruption by means of gudgeons and in that the inner volume of said evaporation chamber is defined essentially by the free volume between the spirals forming each intermediate belt, means for the heating of the hides, air-tightening means between the belts forming the evaporation chamber, means for the creation of the vacuum in the evaporation chamber and, finally, means for the removal of the steam from said evaporation chamber being also present in said dryer. According to a preferred embodiment of the invention, the evaporation chamber of the dryer, which is comprised between the conveyor belt and the counter-belt, presents in its interior a single intermediate belt which keeps the hides to be dried adhering to the mentioned conveyor belt. The air tightness in the evaporation chamber at the edges is obtained by the adherence of the conveyor belt and of the counter-belt against the listings of the intermediate belt, said listings being obtained by impregnating the edges of the intermediate belt with water-proof plastic material. According to a different embodiment, within the evaporation chamber there are two intermediate belts facing each other, eventually separated from each other by the interposition of a permeable dividing belt and which, therefore, create a larger volume in the interior of the evaporation chamber. Advantageously a vacuum drying installation is obtained, which offers the advantage of a decreased operating cost, while yielding higher production rates than are possible in dryers of the known types presenting equivalent overall dimensions and characteristics. Moreover, said drying installation is suited to be inserted in production lines working in continuous process beginning from the wet phase to the phase when the trimming begins. Further applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. Fig. 1 shows the sectional side view of the vacuum dyer according to the invention; Fig. 2 shows a top view of the dryer of Fig. 1 seen along the section II - II; Fig. 3 shows the perspective view of the dryer of Fig. 1; Fig. 4 shows a magnified detail of a part of the edge of the evaporation chamber of the dryer represented in Fig. 3; Fig. 5 shows a magnified detail of a part of the edge of the evaporation chamber in a different embodiment of the evaporation chamber itself; Fig. 6 shows the magnified detail of the transmission rollers of the conveyor belt, of the counter-belt and of the intermediate belt at the spot wherein said belts join together; Fig. 7 shows the magnified detail of the counter-belt, of the suction valves applied to it and of one of the suction openings creating the vacuum within the evaporation chamber. As can be observed in Fig. 1 and Fig. 2 the vacuum dryer according to the invention, which is indicated as a whole with 1, comprises a conveyor belt 2 on which the hides to be dried 4 are arranged, said conveyor belt presenting no interruption and having an essentially horizontal development between the external surfaces of the two cylinders 3. Facing the conveyor belt 2 in correspondence with its bottom part and with a section of the area where it winds around the cylinders 3, there is a counter-belt 5 arranged without interruption and with an essentially horizontal development between the outer surfaces of the other cylinders 6 having their axes co-planar with the axes of the cylinders 3. Between the conveyor belt 2 and the counter-belt 5, along the entire stretch where they face each other, there is an intermediate belt 7 consisting of a plurality of spirals 8, joined together without interruption through the gudgeons 14 , which are better visible in the detail of Fig. 6. This intermediate belt is arranged without interruption and with an essentially horizontal development between the outer surfaces of the other cylinders 9, the axes of which are parallel with the axes of the already mentioned cylinders 3 and 6. Other driving cylinders 60 and 90 guide the belts 5 and 7 respectively and keep them stretched. Therefore, when the intermediate belt 7 is comprised between the conveyor belt 2 and the counter-belt 5, it keeps the hides adhering to the conveyor belt 2. At the same time, it also keeps the counter-belt 5 apart from both the hides and the conveyor belt 2 and creates a chamber 10, which is visible in Fig. 4, the volume of said chamber consisting of the free volume of the spirals 8 forming the intermediate belt 7. Said chamber 10, having a width corresponding essentially with the width of the cylinders 3 and 6, which are in contact with each other, and a length equal to that of the conveyor belt and of the counter-belt, is the evaporation chamber, within which, as will be seen further on, the steam issuing from the hides during the drying process is first generated, collected and then condensed. The drying process of the hides within the evaporation chamber 10 occurs by heating with suitable means, for instance heating coils or similar means, the conveyor belt 2, while, at the same time, the vacuum is created within the evaporation chamber 10, through the openings 11, visible in Fig. 1 and also in Fig. 3, which suck the air from the interior of the evaporation chamber 10 through the single-acting valves 12, which are visible in the magnified detail of Fig. 7. It is a known fact that, when a certain degree of vacuum is created within chamber 10, the hides 4, heated at a suitable temperature, dry up because of the evaporation of the water permeating them. It is, however, indispensable that the generated steam be condensed, so that it can be removed from the evaporation chamber as water. For this purpose the counter-belt 5 is suitably cooled, for instance by means of cooling coils or, more simply, by running over its outer surface a cold fluid. It is very important for the condensation of the steam to occur as quickly as possible, so as not to damagingly saturate the evaporation chamber. The essential condition for the evaporation process of the water impregnating the hides 4 and its subsequent condensation to occur is for the condensation chamber 10 to be under vacuum and hermetically insulated from the outside environment. This is achieved by impregnating the side edges of the intermediate belt 7 with suitable plastic resins, so as to create at each of its edges a listing 13, which is visible also in Fig. 3. Said listing 13 adheres with its upper face to the conveyor belt 2 and with its bottom face to the counter-belt 5 and thus achieves the tightness along the edges. Moreover, as can be observed in Fig. 6 and also in Fig. 1, in correspondence with the points of tangency of the winding cylinders 3 of the conveyor belt with the cylinders 6 winding the counter-belt 5, each of the gudgeons 14 connecting the spirals 8 forming the intermediate belt 7, creates the tightness both at the inlet and the outlet of the hides 4 into and out of the evaporation chamber 10. The operation process of the dryer is simple. First the conveyor belt 2 is heated, the counter- belt 5 is cooled and the vacuum is created in the evaporation chamber 10, by means of the suction openings 11 and the valves 12. The cylinders 3 for the winding of the conveyor belt 2, 6 for the winding of the counter-belt 5 and 9 for the winding of the intermediate belt 7 are made to rotate in the respective directions 15, 16 and 17, indicated in Fig. 1 and also in Fig. 5, while, at the same time, the hides 4 to be dried are spread on the upper surface of the conveyor belt 2. While progressing in the direction 19, the hides 4 heat up because of their adherence to the conveyor belt 2, which is hot and, when they enter into the evaporation chamber 10, they release the humidity they contain which escapes in the shape of steam into the evaporation chamber. It has already been mentioned that the volume of the evaporation chamber 10 corresponds essentially with the free volume between the spirals 8 forming the intermediate belt 7, which also acts as a separating element between the conveyor belt 2 and the counter-belt 5. The conveyor belt and the counter-belt then face each other and are separated by a small distance, since the thickness of the intermediate belt 7 is small, so that, as soon as the steam issues from the hides, it immediately comes into contact with the counter-belt 5, which, being cold, causes the condensation of the steam touching it, as can be observed in Fig. 4. The condensed steam takes the form of water drops 18 at the bottom of the counter-belt 5. The removal of the condensate 18 from the evaporation chamber 10 occurs because of its passage through the single-acting valves 12 during the suction phase and it is separated by means of suitable means which are present in the suction openings 11 before it reaches the suction pumps. As can be observed in Fig. 3, in order to make easier the conveyance of the condensate 18 through the valves 12, the evaporation chamber 10 is arranged at a slanted angle 80 in relation to the horizontal plane in a direction which is orthogonal as compared with the progress direction of the belts. The hides coming out of the evaporation chamber are dry and they are removed from the top surface of the conveyor belt 2 on which they had been spread. Other hides are spread on the conveyor belt 2 and the cycle is repeated without interruption. It is important to observe that, with reference to Fig. 4 and also to Fig. 3, that the height 20 of the evaporation chamber 10 corresponds essentially with the thickness 21 of the hides to be dried plus the thickness 22 of the intermediate belt 7, said thicknesses being very small and comprised, at any rate, within a range of a few millimeters. Therefore, as has already been mentioned, the steam issuing from the hides into the evaporation chamber 10 because of the combined action of the vacuum and the temperature, is immediately condensed. This fact is extremely important, since the immediate condensation of the steam permits, first of all, the advantage of obtaining within the evaporation chamber 10 a higher degree of vacuum than can be obtained in equivalent dryers, wherein the ensuing steam is not condensed in the evaporation chamber , but rather it is removed by means of suction pumps or through lateral external condensers. In fact, while in the dryer according to the invention the just produced steam is immediately turned into water, in the dryers under vacuum belonging to the known technique, on the other hand, the steam produced remains for a certain period of time within the evaporation chamber and it saturates it, thereby decreasing the degree of vacuum. As a consequence the period of time necessary for drying the hides is lengthened. In an attempt to overcome this inconvenience in the dryers belonging to the known technique, the temperature for heating the hides is increased, thereby attempting to make up for the decrease in the degree of vacuum by a higher degree of heating, in order to keep constant the evaporating capacity in the dryer. This, however, entails an increase in the heating costs, said costs not being necessary in the dryer according to the invention, since the immediate condensation of the steam does not generate any saturation in the evaporation chamber. As a consequence, in comparison with the dryers belonging to the known technique, it is possible to create the same evaporating effect, by operating with lower heating temperatures to the advantage of the quality of the hides. Moreover, since the removal of the steam occurs during the entire drying cycle, dead times due to the necessity of removing the steam from the evaporation chamber, as is the case in the vacuum dryers belonging to the known technique, are also reduced. The dryer according to the invention reaches, therefore, also the purpose of obtaining shorter drying cycles, as compared with the equivalent dryers belonging to the known technique. It has been observed that the distance between the conveyor belt 2 and the counter-belt 5 is very short, therefore, the only relevant overall dimensions of the dryer consist of the cylinders for the winding of the conveyor belt and the counter-belt. Therefore, as compared with the vacuum dryers belonging to the known technique, which present plates and counter-plates for the formation of the evaporation chamber, the dryer according to the invention presents much smaller overall dimensions and, as a consequence, lighter weights. The conveyor belt 2 presents an uninterrupted development, so that its entire upper surface, comprised between the cylinders 3, between which it is stretched and on which it winds itself, is available to receive the hides in a continuous way, contrary to the vacuum dryers belonging to the known technique, wherein the plates placed side by side or on top of each other, are loaded in a discrete way and, therefore, not in an optimum way. Fig. 7 shows by way of illustration only a possible variation of embodiment of the single-acting valve 12 through which the suction opening 11 achieves the vacuum within the evaporation chamber 10. Said single-acting valve 12 comprises a body 30 belonging to the counter-belt 5, and it presents in its central part an opening 31 communicating on one side with a seat 32, arranged within the body 30 and, on the opposite side, with the evaporation chamber 10. A shutter 33 slides within the seat 32 and, when the pressure within the evaporation chamber 10 is lower than the environmental pressure, said shutter is pushed upwards by the environmental pressure itself, so that its upper surface 34 closes the opening 31. This is the situation represented in Fig. 7, when the valve 12 is not communicating with the suction opening 11. When the valve 12 comes into correspondence with one of the suction openings 11, the depression created underneath the shutter 33 wins the depression existing within the evaporation chamber 10 and causes it to drop, thereby freeing the opening 31. Through the ducts 35 the suction opening 11 sucks from the interior of the evaporation chamber 10, thereby maintaining the desired degree of vacuum within its interior. The projections 37 of shutter 33 act as a stroke stop by contrasting against the ledges 38 of the valve 30. It is obvious, however, that the described single-acting valve 12 can be replaced by any other type of single-acting valve. It may even be constituted by an area made of permeable material acting in an single-acting way, attached and applied to the counter-belt 5. On the basis of what has been described it can be understood how the vacuum dryer according to the invention reaches all the proposed purposes. In Fig. 5 it can be observed that between the conveyor belt 45 and the counter-belt 46 are comprised two intermediate belts - 47 and 48 respectively - which are separated from one another by a porous surface 49, which allows the passage of the ensuing steam as well as of the condensate. Thus it can be observed that the evaporation chamber 50 presents a volume resulting from the sum of the free volumes comprised between the spirals 51 and 52 belonging to the intermediate belts 47 and 48. At any rate, this variation of embodiment also permits to obtain a vacuum dryer which reaches all the proposed purposes and also permits to obtain all the previously described advantages. Both the cylinders for the winding for the conveyor belt and those for the winding of the counter-belt and also those of the intermediate belt can be arranged in a different way, and the development of the belts and counter-belts can acquire any shape. The number of the intermediate belts can also be varied.

Claims: A vacuum dryer, particularly suited to dry leather hides, comprising a conveyor belt (2) on which the hides to be dried are spread, said conveyor belt being arranged without interruption and with a substantially horizontal development between the external surfaces of at least two cylinders (3), so as to form an evaporation chamber (10), confined between the mentioned conveyor belt and a counter belt (5) placed at a close distance from the conveyor belt along a common stretch of said conveyor belt, the hides to be dried being kept adhering to the conveyor belt by means of at least one intermediate belt (7,47,48) placed between the conveyor belt (2) and the counter belt (5), characterized in that said intermediate belt (7) is formed by a plurality of spirals (8) connected with each other without interruption by means of gudgeons (14) and in that the inner volume of said evaporation chamber (10) is defined essentially by the free volume between the spirals forming said at least one intermediate belt, means for the heating of the hides, air-tightening means between the belts forming the evaporation chamber, means for the creation of the vacuum in the evaporation chamber and means for the removal of the steam from said evaporation chamber being also present in said vacuum dryer. A vacuum dryer according to claim 1, characterized in that the counter-belt (5) is arranged and moves without any interruption and with an essentially horizontal development between the outer surfaces of at least two cylinders (6), each presenting its longitudinal axis co-planar with and parallel to the longitudinal axis of corresponding cylinders, between the outer surfaces of which the conveyor belt (2) is arranged and moves, each cylinder (6) on which the conveyor belt (2) moves being essentially tangent with the corresponding cylinder (3) on which the counter-belt moves. A vacuum dryer according to claim 1, characterized in that a single intermediate belt (7) is placed between the conveyor belt (2) and its corresponding counter-belt (5). A vacuum dryer according to claim 1, characterized in that two intermediate belts (47,48), parallel with each other are placed between the conveyor belt (45) and its corresponding counter-belt (46). A vacuum dryer according to claim 1, characterized in that the evaporation chamber (10) obtained between the conveyor belt (2) and the counter-belt (5) is slanted in relation to the horizontal plane in the orthogonal direction in relation to the progress direction of the conveyor belt and of the counter-belt, the condensate collecting in the lower part of said evaporation chamber. A vacuum dryer according to claim 1, characterized in that the counter-belt (5) presents a plurality of single-acting valves (12) belonging to it, arranged in the direction of its longitudinal development and communicating with the evaporation chamber (10), wherein said valves, during the motion of the counter-belt (5), come into contact with corresponding openings for the suction of the steam and of the condensate. A vacuum dryer according to claim 1, characterized in that the spirals (8) forming each intermediate belt are joined together by means of connecting gudgeons (14) arranged transversally in relation to the longitudinal direction of progress of each of said intermediate belts. A vacuum dryer according to claim 1, characterized in that each intermediate belt (7,47,48) presents, in correspondence with each of its edges a listing (13) obtained by impregnation with an impermeable plastic material of the opposite ends of the spirals forming it. A vacuum dryer according to claim 8, characterized in that the evaporation chamber (10) is made tight toward the exterior by means of said intermediate belt (7,47,48) which creates the tightness along its edges because of the adherence of the listings (13) both to the conveyor belt (2) and to the counter belt (5) and at the beginning and at the end of the junction between the conveyor belt (2) and the counter-belt (5) through at least one of the gudgeons (14) connecting the spirals forming the intermediate belt (7,47,48), when one or more of said gudgeons is tangent, at the same time, both with the conveyor belt (2) and with the counter-belt (5), in correspondence with the winding cylinders.

Inventors: BAGGIO GIOCONDO, BAGGIO, GIOCONDO
Applicants: BAGGIO GIOCONDO, BAGGIO, GIOCONDO
Application year: 1992
Main classification: F26B13
IPC codes: F26B13, F26B5