A steam dryer, a heat exchanger assembly and a method for drying moist particulate material

Abstract

The invention relates to a steam dryer, a heat exchanger assembly and a method for drying moist particulate material. The steam dryer comprises a heat exchanger assembly having one heat exchanger being integrated into another heat exchanger.

Description

TECHNICAL FIELD

[0001] The present invention relates to the drying of moist particulate material, and in particular to the drying of moist particulate material such as sugar beet pulp.BACKGROUND OF THE INVENTION

[0002] Within the technical field of steam drying, and especially within the technical field of steam drying of particulate material, and in particular to the drying of particulate sugar beet pulp, several drying techniques are known, such as the ones disclosed in EP0537262A1, EP0955511A3, EP1044044A1, EP107023A1, EP1956326B1, US4,813,155A, US5,357,686A, US6,154,979A, US6,266,895B1, US6,438,863B1, US7,578,073B2 and EP3663687B1.

[0003] It if is further known from EP2801779B1 to arrange a steam dryer for drying moist particulate material where the efficiency of the drying of particulate material is stigmatically improved compared to the above-cited prior art, by the use of a two-part heat exchanger assembly within the steam dryer, where one part of the heat exchanger assembly functions as a preheater for the other part, by the use of hot water condensate from the other part of the heat exchanger.

[0004] However, there is a need for an even more energy efficient steam dryer, where the overall energy consumption of the dryer is reduced.

[0005] The energy consumption of the internal fan of the dryer, which fan establishes the fluid bed, and the flow of circulating steam is quite substantial in order to arrange a flow of 10 - 300 m 3< / sec of circulating steam within the dryer and through the internal heat exchanger. The two-part heat exchanger in EP2801779B1 has, compared to the other cited prior art, the advantage that a higher degree of the steam supplied to the dryer is utilized. However, the two-part heat exchanger in EP2801779B1 requires a greater performance from the impeller, compared to the other cited prior, as the pressure loss in the two-part heat exchanger is larger compared to a corresponding size heat exchanger disclosed in the first-mentioned prior art documents. The increased pressure loss is a result of the circulating steam which first enters the part of heat exchanger functioning as a pre-heater and then afterwards enters the part of the heat exchanger being heated by the supplied steam.

[0006] This "increased" pressure loss must be compensated for by an increased operation of the fan, which increased operation requires energy (electricity). In the above description, a corresponding size heat exchanger is to be understood as a heat exchanger capable of maintaining the same amount of flow of the circulating steam within the steam dryer.

[0007] Further, the known prior art heat exchanger assemblies are very large steel structures being up to 9 meters high, or more. Those large structures are most often not assembled on site and must be transported on trucks which, due to the size and load of the heat exchanger assembly, is an extremely cumbersome operation.

[0008] It is an object of the present invention to overcome the above issues and provide an energy efficient steam dryer and further a steam dryer having a heat exchanger assembly which is easier to handle and install into the steam dryer.

[0009] The above object and advantages, together with numerous other objects and advantages, which will be evident from the description of the present invention, are according to a first aspect of the present invention obtained by: A steam dryer for drying moist particulate material, such as sugar beet pulp, the steam dryer comprising: a closed container for maintaining an atmosphere comprising super-heated steam at an elevated pressure compared to ambient pressure, the closed container comprising a lower cylindrical part and an upper cylindrical part, a heat exchanger assembly located inside the closed container and comprising a main flow path for a main flow, for allowing the super-heated steam to be transported from inside the upper cylindrical part to inside the lower cylindrical part, a material inlet for feeding the moist particulate material into the lower part of the closed container and a material outlet for removing the dried particulate material from the closed container; an impeller for generating a flow of the super-heated steam going upwards on the outside of the heat exchanger assembly to the inside of the upper cylindrical part and downwards through the heat exchanger assembly, the heat exchanger assembly comprising a first heat exchanger and a second heat exchanger for heating the super-heated steam, the steam dryer further comprising: a first conduit for supplying a first flow of steam from a supplier of steam, such as a boiler, to the first heat exchanger for heating the first heat exchanger, the first heat exchanger being adapted for condensing the first flow of steam into a flow of condensed hot water, and comprising a hot water outlet for discharging the flow of condensed hot water from the first heat exchanger, the steam dryer comprises a second conduit for leading the flow of hot water from the first heat exchanger to the second heat exchanger in the form of hot water for heating the second heat exchanger, wherein the first heat exchanger comprising a first part of the main flow path and the second heat exchanger comprising a second part of the main flow path, the first and second parts of the main flow path being parallel connected.

[0010] In the steam dryer as disclosed in EP2801779B1, the heat exchanger assemble can be described as a two-step heat exchanger, where the circulated steam inside the chamber for contacting and drying the moist particulate material first enters the part of the heat exchanger assembly functioning as a preheater, then exits this part and enters the second part being heated by the steam from the supplier of steam, such as, but not limited to a boiler. During this operation process, the circulated steam encounters a pressure loss, due to the entering and exiting of the two parts of the heat exchanger assembly, which pressure loss puts a demand on the impeller, for creating the circulation motion of the circulating steam, which again requires energy.

[0011] By employing a heat exchanger assembly where a first heat exchanger comprising a first part of a main flow path and a second heat exchanger comprising a second part of the main flow path, and where the first and second parts of the main flow path being parallel connected, the heat exchanger assembly is capable of maintaining the same circulating flow of steam compared to a heat exchanger assembly comprising only a single heat exchanger, as disclosed in the introductory prior art, while at the same time utilizing a higher degree of the steam supplied to the dryer, due to the heat regenerating function from the first heat exchanger to the second heat exchanger. Thus, the steam dryer has the advantage of having an increased energy efficiency together with utilizing a higher degree of the supplied steam.

[0012] Examples of moist particulate material, normally non-homogenous materials suitable for being dried in accordance with the teachings of the present invention are: wood chip, wood pulp, bark chip, sugar beet pulp, sludge, wet distillers grain, bagasse, chopped or otherwise particulate material of alfalfa or other plants or vegetables, fish meal or the like or even combinations of the above materials with other ingredients or materials. Preferably, the particulate material is sugar beet pulp.

[0013] The steam dryer preferably comprises a plurality of guide plates positioned upright circumferentially around the heat exchanger assembly for guiding the moist particulate material along a path around the heat exchanger assembly. The term 'guide plate' is to be understood as a generic term including evidently technical solutions encompassed by the literal understanding of the term but also plates or walls serving to divide the closed container into several compartments and serving to control the transfer and transport of the humid particulate material within the cylindrical parts of the closed container and in particular to control the time of rest of the particulate material in the individual compartments and as described per se in several of the above-listed patent applications and patents.

[0014] The term 'upright' is to be understood as a generic term including evidently technical solutions encompassed by the literal understanding of the term but also orientations which are not strictly vertical, however, differing from a horizontal orientation and also including sloping orientation defined by the guide plate or guide plates.

[0015] The expression 'a plurality of guide plates positioned upright and circumferentially around the heat exchanger' as used in the present specification is to be understood not only encompassing the literal understanding of the expression but also technical solutions such as guide plates having any geometrical configuration including planer plates, curved or partially curved and planar plates or plates including one or more sections which are bent along a straight or curved line from the orientation of the remaining part of the plate, and in addition, the upright position of the plate is to encompass any overall orientation of the plate relative to the supporting horizontal plane e.g. defined by the geometrical center line of the geometrical structure or the plane defined by a part, in particular the major part, of the guide plate.

[0016] According to a further embodiment of the first aspect of the invention, the second heat exchanger being integrated into the first heat exchanger.

[0017] The first and second heat exchanger of the steam dryer assembly may in principle be parts arranged separately next to each other within the closed steam dryer or the second heat exchanger may be arranged outside the pressurized closed container and supplied with part of the circulating flow from the closed container and the hot water from the first heat exchanger via conduits, whereby it is possible to retrofit the second heat exchanger into an existing system having only one heat exchanger.

[0018] However, experience has shown, that by integrating the second heat exchanger into the first heat exchanger, it is possible to arrange a heat exchanger assembly which dimension and flow-wise corresponds to a single heat exchanger assembly such that the remaining parts of the steam dryer substantially fits the "two" heat exchanger assembly. Hereby, only minimal modification of the remaining parts of the steam dryer (compared to prior art) is required. It is important to note, that even slight variations to the internal layout of the components of the steam dryer may influence the internal flow dynamics of the circulating steam negatively, whereby minimal modification of the components are required.

[0019] According to a further embodiment of the first aspect of the invention, the second part of the flow path being less than 30% of the main flow path, preferably less than 20%, such as between 5 to 15% of the main flow.

[0020] In a preferred embodiment, the second part of the flow is substantially less compared to the first part of the flow, as defined above.

[0021] As the hot water flowing through the second heat exchanger has less energy compared to the steam flowing through the first heat exchanger, the second part of the flow path is less than 30% of the main flow path which is sufficient to achieve a sufficient overall heating of the circulating steam flowing through main flow path.

[0022] According to a further embodiment of the first aspect of the invention, the first heat exchanger comprising an outer circumferential wall and an inner circumferential wall defining a first space there between, the first space comprising the first part of the main flow path, the second heat exchanger comprising an outer circumferential wall defining a second space, the second space comprising the second part of the main flow path.

[0023] By the above-defined heat exchanger assembly, is it possible to arrange a steam dryer with a heat exchanger assembly where one heat exchanger of the assembly is integrated into to another heat exchanger of the assembly and where the two heat exchangers function separately and in parallel flow. The two heat exchangers may be separately interconnected or integrated as a single unit.

[0024] According to a further embodiment of the first aspect of the invention, the inner wall of the first heat exchanger encircling the outer wall of the second heat exchanger, the inner wall of the first heat exchanger and the outer wall of the second heat exchanger being arranged with a space therebetween.

[0025] Heat exchanger assemblies for steam dryers within the technical field, and as disclosed in the cited prior art, are very large circular structures of up to 9 meters or higher and weighing several tons. For that reason alone, such heat exchangers are assembled on site which means that several thousand long steel pipes, such as e.g. 5000 full length steel pipes, for the flow the circulating steam, are interconnected with the top and bottom plates of the heat exchanger, which is a cumbersome and time-consuming task.

[0026] By arranging the inner wall of the first heat exchanger encircling the second heat exchanger, and arranged with a distance to the outer wall of the second heat exchanger, provides the possibility of a heat exchanger assembly where a second heat exchanger is integrated into the first heat exchanger and arranged as a separate unit, such that the first and second heat exchangers are separate parts that may be individually assembled and afterward interconnected. Each heat exchanger is hereby easier to handle and may thus be assembled off-site and more easily transported to the building site.

[0027] Further, by arranging a space between the two heat exchangers, any unintentional heat transfer between the two heat exchangers is minimized.

[0028] According to a further embodiment of the first aspect of the invention, the outer wall of the second heat exchanger comprising a circumferential flange at one end thereof, the circumferential flange has a diameter being larger than a diameter of the inner wall of the first heat exchanger.

[0029] As the first and second heat exchangers are arranged as separate units where the second heat exchanger can be inserted into the first heat exchanger, arranging the outer wall of the second heat exchanger comprising a circumferential flange at one end thereof has proven to ensure an easy operation of interconnecting the two heat exchangers and to ensure a correct position and alignment between the two heat exchangers.

[0030] According to a further embodiment of the first aspect of the invention, the heat exchanger assembly comprising a flow regulation element, such as a valve, for regulating the second part of the main flow through the second heat exchanger, specifically.

[0031] Though the second heat exchanger may comprise less than 30% of the main flow there is a risk that the temperature of the circulating steam before exiting the second heat exchanger is too low. Thus, the amount of circulating steam flowing through the second heat exchanger may be too high. The desired exit temperature of the circulating steam leaving the heat exchanger assemble should be approximate 220 degrees Celsius. If the temperature of the circulating steam leaving the second heat exchanger is lower than the desired temperature, i.e. between 150-200 degrees Celsius, the flow through the second heat exchanger may too high.

[0032] Therefore a flow regulating element, such as a flow regulating element arranged at one end of the second heat exchanger, such as the top or bottom end, is positioned within the flow path for regulating the flow, such that if the temperature of the circulating steam flowing through the second heat exchanger is too low, the flow regulating element may partly or even in full close the flow of circulating steam flowing through the second heat exchanger.

[0033] According to a second aspect of the present invention, the above objects and advantages are obtained by: A heat exchanger assembly for a steam dryer according to any of the objects of the first aspect of the invention, the heat exchanger assembly having a main flow path and being arranged for heating a main flow of superheated steam within the steam dryer at a flow rate of 10 - 300 m 3< / sec, the heat exchanger assembly comprising a first heat exchanger and a second heat exchanger, the first heat exchanger comprising a first part of the main flow path and the second heat exchanger comprising a second part of the main flow path, the first and second parts of the main flow path being parallel connected.

[0034] By employing a heat exchanger assembly where a first heat exchanger comprising a first part of a main flow path and a second heat exchanger comprising a second part of the main flow path, and where the first and second parts of the main flow path being parallel connected, it is possible to provide a heat exchanger assembly capable of maintaining the same circulating flow of steam compared to a heat exchanger assembly comprising only a single heat exchanger, as disclosed in the introductory prior art, while at the same time, the heat regenerating function from the first heat exchanger of the steam dryer to the second heat exchanger of the steam dryer such that a higher degree of the steam supplied to the dryer is utilized. Thus, the steam dryer has the advantage of having an increase energy efficiency together with utilizing a higher degree of the supplied steam.

[0035] According to a further embodiment of the second aspect of the invention, the second heat exchanger being integrated into the first heat exchanger.

[0036] The first and second heat exchanger of the steam dryer assembly may in principle be parts arranged separately next to each other within the closed steam dryer or the second heat exchanger may be arranged outside the pressurized closed container and supplied with part of the circulating flow from the closed container and the hot water from the first heat exchanger via conduits, whereby it is possible to retrofit the second heat exchanger into an existing system having only one heat exchanger.

[0037] However, experience has shown, that by integrating the second heat exchanger into the first heat exchanger, it is possible to arrange a heat exchanger assembly which dimension and flow-wise corresponds to a single heat exchanger assembly such that the remaining parts of the steam dryer substantially fits the "two" heat exchanger assembly. Hereby, only minimal modification of the remaining parts of the steam dryer (compared to the prior art) is required. It is important to note, that even slight variations to the internal layout of the components of the steam dryer may influence the internal flow dynamics of the circulating steam negatively, whereby minimal modification of the components are required.

[0038] According to a further embodiment of the second aspect of the invention, the second part of the flow path being less than 30% of the main flow path, preferably less than 20%, such as between 5 to 15% of the main flow.

[0039] In a preferred embodiment, the second part of the flow is substantially less compared to the first part of the flow, as defined above.

[0040] As the hot water flowing through the second heat exchanger has less energy compared to the steam flowing through the first heat exchanger, the second part of the flow path is less than 30% of the main flow path which is sufficient to achieve a sufficient overall heating of the circulating steam flowing through the main flow path.

[0041] According to a further embodiment of the second aspect of the invention, the first heat exchanger comprising an outer circumferential wall and an inner circumferential wall defining a first space therebetween, the first space comprising the first part of the main flow path, the second heat exchanger comprising an outer circumferential wall defining a second space, the second space comprising the second part of the main flow path

[0042] According to a further embodiment of the second aspect of the invention, the inner wall of the first heat exchanger encircling the outer wall of the second heat exchanger, the inner wall of the first heat exchanger and the outer wall of the second heat exchanger being arranged with a space therebetween.

[0043] As mentioned earlier, heat exchanger assemblies for steam dryers within the technical field, and as disclosed in the cited prior art, are very large and heavy structures and therefore assembled on site which means that several thousand long steel pipes, such as e.g. 5000 full length steel pipes, for the flow of the circulating steam, are interconnected with the top and bottom plates of the heat exchanger, which is a cumbersome and time-consuming task.

[0044] By arranging the inner wall of the first heat exchanger encircling the second heat exchanger, and arranged with a distance to the outer wall of the second heat exchanger, provides the possibility of a heat exchanger assembly where a second heat exchanger is integrated into the first heat exchanger and arranged as a separate unit, such that the first and second heat exchangers are separate parts that may be individually assembled and afterward interconnected. Each heat exchanger is hereby easier to handle and may thus be assembled off-site and more easily transported to the building site.

[0045] According to a further embodiment of the second aspect of the invention, the outer wall of the second heat exchanger comprising a circumferential flange at one end thereof, the circumferential flange has a diameter being larger than a diameter of the inner wall of the first heat exchanger.

[0046] As the first and second heat exchangers are arranged as separate units where the second heat exchanger can be inserted into the first heat exchanger, arranging the outer wall of the second heat exchanger comprising a circumferential flange at one end thereof has proven to ensure an easy operation of interconnecting the two heat exchangers and to ensure a correct position and alignment between the two heat exchangers.

[0047] According to a further embodiment of the second aspect of the invention, the heat exchanger assembly comprising a flow regulation element, such as a valve, for regulating the second part of the main flow through the second heat exchanger, specifically.

[0048] Though the second heat exchanger may comprise less than 30% of the main flow there is a risk that the temperature of the circulating steam before exiting the second heat exchanger is too low. Thus, the amount of circulating steam flowing through the second heat exchanger may be too high.

[0049] Therefore a flow regulating element, such as a flow regulating element arranged at one end of the second heat exchanger, such as the top or bottom end, is positioned within the flow path for regulating the flow, such that if the temperature of the circulating steam flowing through the second heat exchanger is too low, the flow regulating element may partly or even in full close the flow of circulating steam flowing through the second heat exchanger.

[0050] According to a third aspect of the present invention, the above objects and advantages are obtained by: A method for drying moist particulate material, such as sugar beet pulp, by super-heated steam, the method comprising: providing a steam dryer according to any of the embodiments according to the first aspect of the invention, the method further comprising: supplying a flow of steam from a supplier, such as a boiler, to the first heat exchanger for heating the first heat exchanger and condensing the flow of steam within the heat exchanger into a flow of condensed hot water, discharging the flow of condensed hot water from the first heat exchanger, generating a flow of fluid from the flow of condensed hot water and leading the flow of fluid to the second heat exchanger in the form of hot water for heating the second heat exchanger, generating a flow of super-heated steam, by means of an impeller, going upwards on an outside of the heat exchanger assembly to the inside of the upper cylindrical part and downwards through the heat exchanger assembly, feeding the moist particulate material to the closed container and subjecting the moist particulate material to the flow of super-heated steam for converting the humid particulate material into dry particulate material.

[0051] The defined heat exchanger assembly where a first heat exchanger comprising a first part of a main flow path and a second heat exchanger comprising a second part of the main flow path, and where the first and second parts of the main flow path being parallel connected, the heat exchanger assembly is capable of maintaining the same circulating flow of steam compared to a heat exchanger assembly comprising only a single heat exchanger, as disclosed in the introductory prior art, while at the same time utilizing a higher degree of the steam supplied to the dryer, due to the heat regenerating function from the first heat exchanger to the second heat exchanger. Thus, the steam dryer has the advantage of having an increased energy efficiency together with utilizing a higher degree of the supplied steam. Fig. 1 shows an exploded perspective view of the heat exchanger assembly. Fig. 2 shows a perspective view of the heat exchanger assembly. Fig. 3 shows a cross-sectional view of the heat exchanger assembly including an impeller. Fig. 4 shows a cross-sectional view of a steam dryer. Fig. 5 shows perspective cross-sectional view of part of the first heat exchanger. Fig. 6 shows a perspective view of part of the second heat exchanger.

[0052] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will thus not be described in detail with respect to the description of each figure.

[0053] Fig. 1 shows an exploded perspective view of the heat exchanger assembly 10.

[0054] The figure shows the first heat exchanger 14 and the second heat exchanger 16.

[0055] The first heat exchanger 14 is arranged with an outer circumferential wall and an inner circumferential wall defining a circumferential first space between the two circumferential walls and an opening 28 though the second heat exchanger. A perforated plate, such as but not limited to a 1.4-inch-thick metal plate with openings is arranged at each end of the circumferential wall. Only the "upper" perforated plate is shown in the figure, but it must be understood that the opposite lower end likewise comprises a perforated plate. Between each opening 28 in the upper perforated plate and a corresponding opening in the lower perforated plate, there is arranged a tubing, such as a steel pipe. Hereby, a flow path for the circulating steam is arranged through the first heat exchanger, the flow path being arranged for circulating a first part of the main flow.

[0056] The second heat exchanger 16 also comprising an outer circumferential wall defining a second space, where the second space is arranged for the second part of the main flow. Like the first heat exchanger 14, the second heat exchanger 16 comprises a similar perforated plate having openings 26 at each end, and a plurality of tubing's connected with corresponding openings at the perforated plates at each end of the second heat exchanger 16. The second heat exchanger 16 hereby comprises a flow path arranged for the second part of the main flow.

[0057] As is apparent from the figure, the circumferential wall of the second heat exchanger 16 is arranged with a diameter being smaller than the diameter of the opening 28 in the first heat exchanger which is defined by the inner circumferential wall. Hereby, the second heat exchanger 16 may be inserted into the opening 28 of the first heat exchanger 14 such that the two heat exchangers 14, 16 are integrated.

[0058] The second heat exchanger 16 comprises at the "upper" end a circumferential flange 50 extending in a radial direction compared to a longitudinal centerline through the heat exchanger. The circumferential flange 50 may be arranged integrated with the "upper" perforated plate, i.e. the upper perforated plate being round and having a diameter larger than the diameter of the opening 28, such that the upper end of the second heat exchanger 16 abuts against the upper end of first heat exchanger 14. In an alternative embodiment, the second heat exchanger may comprise alternative elements, extending circumferential around the wall of the second heat exchanger, below the upper perforated plate. Such alternative elements may be a separate circumferential flange or spaced projections arranged for abutting against the upper end of the first heat exchanger 14.

[0059] It should be noted that the number of openings in the perforated plates illustrated in the drawings is very low for illustrative purposes.

[0060] When the second heat exchanger 16 is inserted into the first heat exchanger 14, there may preferably be arranged a space between the inner wall of the first heat exchanger 14 and the circumferential wall of the second heat exchanger. Hereby unintentional heat transfer between the two heat exchangers is minimized, and the operation of integrating the two heat exchangers is eased.

[0061] It should be noted that the terms "upper" and "lower" must be understood in relation to the orientation of the elements shown in the drawings which depict the actual vertical orientation of the elements when in use.

[0062] Fig. 2 shows a perspective view of the heat exchanger assembly 10. The figure illustrates the two heat exchangers, 14, 16 being interconnected into the heat exchanger assembly 12.

[0063] As shown in the drawing, the upper surface of the second heat exchanger 16, due to the thickness of the flange 50, is positioned slightly above the upper surface of the first heat exchanger 14. In an alternative embodiment, the flange 50 may be received in a recess in the upper surface of the first heat exchanger 14 such that the upper surface of the heat exchangers 14, 16 are substantially flush.

[0064] Fig. 3 shows a cross-sectional view of the heat exchanger assembly 10 including an impeller 32.

[0065] For clarification, it must be noted that the elements referred to by reference numbers, 18, 20 and 22 may also to be considered present in the embodiment shown in figures 1-2, though first mentioned in relation to figure 3.

[0066] The figure illustrates a heat exchanger assembly 12 having first and second heat exchangers 14, 16 as disclosed in relation to figure 1 and 2.

[0067] The heat exchanger assembly 12 is connected to a first conduit 8 for supplying a first flow of steam from a supplier of steam, such as a boiler, to the first heat exchanger 14. The steam which enters the first heat exchanger 14 may condense into hot water due to the function of the heat exchanger and the condensed hot water is lead to the second heat exchanger 16 via second conduit 20 by a pump 30. The hot water, by use of the pump, is lead out of the top of the second heat exchanger 16 and out of the steam dryer 10. Hereby is established a heat exchanger assembly utilizing a high amount of the supplied steam while the first and second heat exchangers function in parallel, as described above.

[0068] An impeller 32, such as a fan ensures that the circulation inside the steam dryer 10 is drawn down through the heat exchanger assembly 12 and up on the outer sides of the heat exchanger assembly.

[0069] The process of the supplied steam and the circulating steam will also be described in relation to figure 4.

[0070] The figure further illustrates a flow regulating element 34 arranged in connection with the lower end of the second heat exchanger 16.

[0071] The flow regulating element 34 is limited to a position at the lower end of the second heat exchanger 16 but may be arranged elsewhere such as at the upper end of the second heat exchanger 16 or integrated into the second heat exchanger 16.

[0072] The flow regulating element 34 is shown arranged as a plate shaped disc connected to a rod which may be rotated (shown with arrows) such that the plate shaped disc may close off the flow of circulating steam coming down through the second heat exchanger 16 and out through the bottom of the second heat exchanger 16 before entering the impeller 32.

[0073] The flow regulating element 34 has the function to regulate the flow of circulation steam through the second heat exchanger 16 in the event that the temperature of the steam flowing out through the second heat exchanger 16 is too low, and thus not heated enough. The desired exit temperature of the circulating steam leaving the heat exchanger assembly should be approximately 220 degrees Celsius. If the temperature of the steam leaving the second heat exchanger 16 is lower than the desired temperature, i.e. between 150-200 degrees Celsius, the flow through the second heat exchanger 16 is too high and may be closed off partly or in full by the flow regulating element 34.

[0074] Fig. 4 shows a cross-sectional view of a steam dryer 10. The figure shows the overall function of the steam dryer 10.

[0075] The steam dryer 10 comprises a closed container 10' for maintaining an atmosphere of steam at an elevated pressure. The closed container 10' comprising a lower cylindrical part and an upper cylindrical part, a heat exchanger assembly 12 as previous disclosed which is located inside the closed container and comprising a flow path through the heat exchanger assemble 12 for allowing the super-heated steam to be transported from inside the upper cylindrical part to inside the lower cylindrical part.

[0076] The heat exchanger assembly 12 having the first and second heat exchanger 14, 16 as previously disclosed.

[0077] The impeller generates a flow of the super-heated steam going upwards on the outside of the heat exchanger assembly 12 to the inside of the upper cylindrical part and downwards through the heat exchanger assembly 12.

[0078] The steam dryer comprises a material inlet 42, positioned above a gill / hole plate bottom, for feeding the moist particulate material into the lower part of the closed container 10', and the steam dryer 10 preferably comprises a plurality of guide plates positioned upright and circumferentially around the heat exchanger assembly for guiding the moist particulate material along a path around the heat exchanger assembly for subjecting the moist particulate material to the flow of the super-heated steam for drying the moist particulate material. The partition walls are not shown in the illustrated embodiments but are commonly known in the cited prior art e.g., EP3663687B1 which is referred to by reference.

[0079] The steam dryer 10 comprises a material 44 outlet for removing the dried particulate material from the closed container 10'.

[0080] The steam dryer 10 further comprises a first conduit 18 for supplying a first flow of steam from a supplier of steam, such as a boiler, to the first heat exchanger 14 for heating the first heat exchanger 14. The first heat exchanger 14 is adapted to condense the first flow of steam into a flow of condensed hot water, due to the exchange of heat between the supplied steam and a circulating steam inside the closed container 10'.

[0081] The first heat exchanger 14 comprises a hot water outlet at the bottom thereof, for discharging the flow of condensed hot water from the first heat exchanger and to the second heat exchanger 16.

[0082] For leading the flow of hot water from the first heat exchanger 14 to the second heat exchanger 16, by a pump 30 (or another flow generator), for heading the second heat exchanger 16, the steam dryer 10 comprises a second conduit 20 and the steam dryer 10 comprises a third conduit 22 for leading the hot water from the second heat exchanger 16 and out of the steam dryer 10.

[0083] Fig. 5 shows a perspective view of part of the first heat exchanger 14.

[0084] The figure shows the interior parts of the first heat exchanger. The second heat exchanger 16 is for illustrative purposes not shown in the figure.

[0085] It must be noted that the steel pipes extending between the ends of the second heat exchanger for the flow of the circulating steam are not shown.

[0086] It must further be noted that the figure only shows part of the length of the second heat exchanger 14.

[0087] The second heat exchanger 16 comprises an outer circumferential wall and an inner circumferential wall, where the inner wall is illustrated with dotted lines. Between these two walls there are several first partition plates 46 arranged in a parallel and staggered configuration. The partition plates 46 have several openings corresponding to the number of steel tubes, such that the tubes may extend through the first partition plates 46. The arrangement of the first partition plates 46 has the technical effect that the supplied steam entering the second heat exchanger 14 via the first conduit 18 is guided by the first partition plates 46 back and forth sideways (side to side) though the second heat exchanger, whereby it is ensured that a maximum amount of the supplied steam contacts all the pipes.

[0088] Each first partition plate 46 is shown arranged in a plane extending perpendicular to the longitudinal direction of the first heat exchanger 14.

[0089] The first partition plates 46 are shown covering more the 50% of the area between the circumferential walls. Preferably, the plates cover between 50-70% of the area between the walls which ensures that the supplied steam is forced from side to side through the first heat exchanger as explained above but limits the pressure loss from the arrangement of the first partition plates 46. The above range of 50-70% thus ensures optimal heat transfer between the supplied and the circulating steam while keeping the pressure loss at a minimum.

[0090] Fig. 6 shows a perspective view of part of the second heat exchanger 16.

[0091] The figure shows the internal parts of the second heat exchanger 16.

[0092] The circumferential outer wall of the second heat exchanger 16 and several pipes are not shown.

[0093] The basic principle of the interior of the second heat exchanger 16 basically corresponds to the principle of the first heat exchanger 14.

[0094] The second heat exchanger 16 comprises a circumferential outer wall (not shown) defining an interior space in which several throughgoing steel pipes and several second partition plates 48 are arranged. The throughgoing pipes are arranged similar to the throughgoing pipes of the first heat exchanger 14 and are arranged for accommodating the circulating steam within the steam dryer.

[0095] The second heat exchanger 16 comprises an inlet (not shown) at the bottom end and an outlet (not shown) at the top end such that the condensate (hot water) from the first heat exchanger 14 can enter and exit the second heat exchanger (see the description in relation to figure 4). Thus, the condensate may flow through the second heat exchanger 16 in the space between the throughgoing pipes in which the circulated steam flows.

[0096] The second heat exchanger 16 comprises several staggered and parallel arranged second partition plates 48 which function similar to the first partition plates 46 of the first heat exchanger 14. The second partition plates thus have a number of openings for accommodating a corresponding number of throughgoing pipes.

[0097] The second partition plates 48, thus guide the condensate from side to side through the second heat exchanger ensuring that a maximum amount of condensate flow past all pipes. Hereby an optimal heat transfer between the condensate and the circulating steam is achieved.

[0098] Each of the second partition plates 48 covers at least 50% of the cross-sectional area of the second heat exchanger 16 perpendicular to the longitudinal direction thereof, and preferably between 55-70% of the cross-sectional area.

[0099] It is hereby ensured that the condensate flows past all the tubes with a limited pressure-loss of the condensate from the second partition plates 48.

[0100] In the following is given a list of reference signs that are used in the detailed description of the invention and the drawings referred to in the detailed description of the invention. 10Steam dryer 10'Closed container 12Heat exchanger assembly 14First heat exchanger 16Second heat exchanger 18First conduit 20Second conduit 22Third conduit 24First heat exchanger flow channels 26Second heat exchanger flow channels 28First heat exchanger opening 30Pump 32Impeller 34Flow regulating means 36Main flow 38First part of main flow 40Second part of main flow 42Material inlet 44Material outlet 46First partition plate 48Second partition plate 50Circumferential flange 52Gill / hole plate bottom

Claims

1. A steam dryer for drying moist particulate material, such as sugar beet pulp, said steam dryer comprising: - a closed container for maintaining an atmosphere comprising super-heated steam at an elevated pressure compared to ambient pressure, said closed container comprising a lower cylindrical part and an upper cylindrical part, a heat exchanger assembly located inside said closed container and comprising a main flow path for a main flow, for allowing said super-heated steam to be transported from inside said upper cylindrical part to inside said lower cylindrical part, a material inlet for feeding said moist particulate material into said lower part of said closed container and a material outlet for removing said dry particulate material from said closed container; - an impeller for generating a flow of said super-heated steam going upwards on the outside of said heat exchanger assembly to the inside of said upper cylindrical part and downwards through said heat exchanger assembly, said heat exchanger assembly comprising a first heat exchanger and a second heat exchanger for heating said super-heated steam, said steam dryer further comprising: - a first conduit for supplying a first flow of steam from a supplier of steam, such as a boiler, to said first heat exchanger for heating said first heat exchanger, said first heat exchanger being adapted for condensing said first flow of steam into a flow of condensed hot water, and comprising a hot water outlet for discharging said flow of condensed hot water from said first heat exchanger, - said steam dryer comprises a second conduit for leading said flow of hot water from said first heat exchanger to said second heat exchanger in the form of hot water for heating said second heat exchanger, characterized in that said first heat exchanger comprising a first part of said main flow path and said second heat exchanger comprising a second part of said main flow path, said first and second parts of said main flow path being parallel connected.

2. A steam dryer according to claim 1, wherein said second heat exchanger being integrated into said first heat exchanger.

3. A steam dryer according to claim 2, said second part of said flow path being less than 30% of said main flow path, preferably less than 20%, such as between 5 to 15% of said main flow.

4. A steam dryer according to any of the previous claims, said first heat exchanger comprising an outer circumferential wall and an inner circumferential wall defining a first space there between, said first space comprising said first part of said main flow path, said second heat exchanger comprising an outer circumferential wall defining a second space, said second space comprising said second part of said main flow path.

5. A steam dryer according to claim 4, said inner wall of said first heat exchanger encircling said outer wall of said second heat exchanger, said inner wall of said first heat exchanger and said outer wall of said second heat exchanger being arranged with a space therebetween.

6. A steam dryer according to claims 4-5, said outer wall of said second heat exchanger comprising a circumferential flange at one end thereof, said circumferential flange has a diameter being larger than a diameter of said inner wall of said first heat exchanger.

7. A steam dryer according to any of the previous claims, said heat exchanger assembly comprising a flow regulation element, such as a valve, for regulating said second part of said main flow through said second heat exchanger, specifically.

8. A heat exchanger assembly for a steam dryer according to any of claims 1-7, said heat exchanger assembly having a main flow path and being arranged for heating a main flow of superheated steam within said steam dryer at a flow rate of 10 - 300 m3 / sec, said heat exchanger assembly comprising a first heat exchanger and a second heat exchanger, said first heat exchanger comprising a first part of said main flow path and said second heat exchanger comprising a second part of said main flow path, said first and second parts of said main flow path being parallel connected.

9. A heat exchanger assembly according to claim 8, said second heat exchanger being integrated into said first heat exchanger.

10. A heat exchanger assembly according to claim 9, wherein said second part of said flow path being less than 30% of said main flow path, preferably less than 20%, such as between 5 to 15% of said main flow.

11. A heat exchanger assembly according to any of claims 8-10, said first heat exchanger comprising an outer circumferential wall and an inner circumferential wall defining a first space therebetween, said first space comprising said first part of said main flow path, said second heat exchanger comprising an outer circumferential wall defining a second space, said second space comprising said second part of said main flow path.

12. A heat exchanger assembly according to claim 11, said inner wall of said first heat exchanger encircling said outer wall of said second heat exchanger, said inner wall of said first heat exchanger and said outer wall of said second heat exchanger being arranged with a space therebetween.

13. A heat exchanger assembly according to any of claims 8-12, said outer wall of said second heat exchanger comprising a circumferential flange at one end thereof, said circumferential flange has a diameter being larger than a diameter of said inner wall of said first heat exchanger.

14. A heat exchanger assembly according to any of claims 8-13, said heat exchanger assembly comprising a flow regulation element, such as a valve, for regulating said second part of said main flow through said second heat exchanger, specifically.

15. A method for drying moist particulate material, such as sugar beet pulp, by superheated steam, said method comprising: - providing a steam dryer according to any of claims 1-7, said method further comprising: - supplying a flow of steam from a supplier, such as a boiler, to said first heat exchanger for heating said first heat exchanger and condensing said flow of steam within said first heat exchanger into a flow of condensed hot water, - discharging said flow of condensed hot water from said first heat exchanger, - generating a flow of fluid from said flow of condensed hot water and leading said flow of fluid to said second heat exchanger in the form of hot water for heating said second heat exchanger, - generating a flow of said super-heated steam, by means of an impeller, going upwards on an outside of said heat exchanger assembly to the inside of said upper cylindrical part and downwards through said heat exchanger assembly, - feeding said moist particulate material to said closed container and subjecting said moist particulate material to said flow of super-heated steam for converting said humid particulate material into dry particulate material.

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