EP4314681A1

EP4314681A1 - Drying apparatus and method for drying boards

Info

Publication number: EP4314681A1
Application number: EP22718948.7A
Authority: EP
Inventors: Patrick VANLEUVEN; Michel Rigaudon
Original assignee: Prtc Nv; Etex Building Performance International SAS
Current assignee: Prtc Nv; Etex Building Performance International SAS
Priority date: 2021-03-30
Filing date: 2022-03-29
Publication date: 2024-02-07
Also published as: WO2022207632A1

Abstract

A drying apparatus for continuous drying of moist materials in superheated steam comprising: - an elongated drying enclosure having an input end and output end; - feed means for feeding intake air and/or intake superheated steam; - discharging means for discharging exhaust air and/or exhaust steam; - n horizontal drying decks arranged in superposed levels, each drying deck comprising means for carrying the moist materials from the input end through the drying enclosure, to be discharged at the output end of the drying apparatus as dried materials; - a feeding device for feeding the moist materials to any one of the n drying decks; - an unloading device for unloading the dried materials of any one of the n drying decks; - one or more drying modules creating one or more drying zones, each drying module comprising a plurality of opposing jet dryer boxes disposed above and below an upper surface of each of the n drying decks wherein each jet dryer box comprises a plurality of spaced orifices therein; wherein n is an integral of at least 1; wherein the drying apparatus further comprises means for generating superheated steam and means for coupling a source of heated air and/or superheated steam to the respective inlets of the jet dryer boxes; and wherein a mixture of air and superheated steam is provided through the plurality of orifices in the jet dryer boxes thereby impinging upon at least the upper and lower major surfaces of the moist materials.

Description

Drying apparatus and method for drying boards

FIELD OF THE INVENTION

The present invention relates to an improved apparatus for the uniform drying of boards, such as calcium silicate boards and a method for doing the same.

BACKGROUND OF THE INVENTION

The use of board dryers for drying calcium silicate hydrate “CSH” boards, such as xonotlite or tobermorite-based boards, or gypsum boards, or Calcium Aluminium Sulphate Hydrate boards such as ettringite, is well known in the art.

The drying of boards is carried out in most cases by means of a predominantly convective heat transfer in the form of cross-flowing of heated air. According to the prior art, drying plants are operated in most cases in circulating air mode. The drying air in this case is repeatedly directed onto the sheets and reheated after each contact. A characteristic feature of different types of driers is the way the air is guided over the boards: cross aeration, longitudinal aeration, or so-called impingement jet aeration. In the case of cross aeration, the drying air is guided over the material to be dried from the side, transversely to the feed direction of the sheet-like material. Since the drying air is increasingly cooled during its path over the material to be dried, different drying speeds are consequently created across the width. In the case of longitudinal aeration, the drying air travels over a long path along the longitudinal axis of the drier, flows over the boards in the process, dries this and cools down to a considerable extent as a result. In the case of impingement jet aeration, the drying air is blown perpendicularly onto the surface of the material to be dried. The advantages are that as a result of the construction consisting of a multiplicity of relatively short drying zones, which in each case can be individually controlled, the desired drying temperature and the climate over the length of the drier can be freely selected. Therefore, the drying conditions can be adapted to the requirements of the material to be dried. The drier, moreover, can be controlled in an excellent manner, e.g. during product changes. In general, boards are transported through a dryer vertically by use of fork conveyors or horizontally by use of roller conveyors or mesh belts.

US 4, 562, 650, discloses a typical fork type conveyor which carries the boards on forks in a vertical position into the dryer. These dryers have the disadvantage that the edges of the boards are more accessible than the large surfaces. Hence, the edges will dry faster and are easily overdried or even burned, resulting in discoloration and diminished quality of the boards.

US 6,655,041 discloses a typical horizontal roller belt drier which conveys the moist boards in a horizontal way. The larger surfaces of the boards are easily accessible, and the drying process is somewhat more uniform. However, due to the high production capacity of the upstream production plant and the necessarily long residence time, these drying units are relatively long.

Uniform drying across the width of the material to be dried without warping of the boards, remains an important challenge. Even more so in view of the increasingly stringent quality and technical requirements for the boards and the drive for a continuous cost reduction, time reduction and energy saving in the board manufacturing plants.

Therefore, there remains a need for a shorter, more economical and a more energy efficient drying apparatus which can dry boards efficiently and uniformly while maintaining excellent board qualities.

SUMMARY OF THE INVENTION

The inventors have now surprisingly found that it is possible to provide an improved drying apparatus and method fulfilling the above- mentioned needs.

The present invention provides a drying apparatus comprising: an elongated drying enclosure having an input end and output end; feed means for feeding intake air and/or intake superheated steam; discharging means for discharging exhaust air and/or exhaust steam; n horizontal drying decks arranged in superposed levels, each drying deck comprising means for carrying the moist materials from the input end through the drying enclosure, to be discharged at the output end of the drying apparatus as dried materials; a feeding device for feeding the moist materials to any one of the n drying decks; an unloading device for unloading the dried materials of any one of the n drying decks; one or more drying modules creating one or more drying zones, each drying module comprising a plurality of opposing jet dryer boxes disposed above and below an upper surface of each of the n drying decks wherein each jet dryer box comprises a plurality of spaced orifices therein; wherein n is an integral of at least 1; wherein the drying apparatus further comprises means for generating superheated steam and means for coupling a source of heated air and/or superheated steam to the respective inlets of the jet dryer boxes; and wherein a mixture of air and superheated steam is provided through the plurality of orifices in the jet dryer boxes thereby impinging upon at least the upper and lower major surfaces of the moist materials.

The present invention further provides for a method for drying moist materials such as boards including gypsum boards and calcium silicate hydrate boards.

DETAILED DESCRIPTION

The present invention will be described in the following with respect to particular embodiments but the invention is not limited thereto but only by the claims.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein. The term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Accordingly, the terms “comprising” and “including” encompass the more restrictive terms “consisting essentially of” and “consisting of”.

As used in the foregoing and hereinafter, the following definitions apply unless otherwise noted.

The term “at least one” is intended to denote one or more than one. Mixtures can also be used for the purpose of the invention.

The terms “materials” is intended to refer to all kinds of materials which may require to be dried, including raw materials, partly processed materials and finished materials, in particular to boards like “CSH” boards, such as ettringite, xonotlite or tobermorite-based boards, or gypsum boards.

The term “major surface” is intended to refer to the surface of the material that has the largest surface area.

The term “board” refers to a generally planar component suitable for attachment to an interior or exterior building surface, including but not limited to wall panels, fagade panels, roofing panels, lap siding, vertical siding, soffit panels, trim boards. It is understood that a board generally has two major surfaces, a top and a bottom surface, substantially parallel to each other and at least four edge or minor surfaces.

According to a first aspect, the invention relates to a drying apparatus for the uniform drying of moist materials, such as gypsum or calcium silicate boards. The moist materials to be dried are introduced into an elongated drying enclosure via an input end of said enclosure. A feeding device loads the moist materials onto a horizontal drying deck. The moist materials are carried horizontally from the input end, through the enclosure to the output end. The skilled person understands that by the time the materials arrive at the output end they are dried materials. An unloading device unloads the dried materials from the drying decks. The dried materials are then removed from the elongated drying enclosure via the output end of said enclosure. During the drying process, a mixture of air and superheated steam is provided through opposing jet dryer boxes which are disposed above and below an upper surface of the drying deck. This mixture impinges on the upper and lower major surfaces of the moist materials that are carried on the drying decks to evaporate the water inside the materials and to form dried materials.

The term “superheated steam” is intended to refer to steam at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured.

The inventors have surprisingly found that a horizontal position of the boards in the dryer in combination with impingement jet aeration with superheated steam, can provide a drying apparatus which is much more efficient in drying moist boards and which dries the boards uniformly. The inventors have found that this specific combination also creates a much more stable environment which is less susceptible to fluctuations in the drying temperature. The drying temperature remains rather constant with a temperature inside the moist materials of around 100 °C, which positively influences the final quality and performance of the dried materials.

The inventors believe that due to the superior heat transfer properties of superheated steam to hot air (higher thermal conductivity and heat capacity at the same temperature) and no resistance to diffusion of the evaporated moisture from the moist boards in its own vapor, in combination with the horizontal position of the boards and the jet aeration delivery of the air/steam mixture to the boards, higher drying rates can be achieved in the drying apparatus according to the present invention. Since the heat transfer is enhanced significantly, the drying process will be much more efficient and the residence time and/or the length of the oven can be shorter while also less energy is consumed.

Furthermore, the inventors have found that in the drying apparatus according to the present invention, the boards can be dried more homogenously without over drying or burning the edges and with a lower risk on explosion of the boards. The Applicants believe that the superheated steam is able to penetrate the moist boards which prevents the formation of a hard dried shell on the outside of the products. This in turn also prevents explosion of the boards. When boards are dried by direct heating in hot air or when they are heated up too fast, often a hard outside shell is formed because the moisture along the outside border of the board will evaporate first. When the moisture inside in the middle of the boards starts to evaporate, it cannot escape and the boards tend to explode. By using a less aggressive heating technique and by drying the materials in a more stable environment, these explosions are prevented.

The inventors have further found that the quality of the boards can be improved as well. The inventors believe that this is due to the much more stable drying temperatures that can be achieved in the drying apparatus according to the present invention. The less temperature fluctuations during the drying process, the better the quality of the dried boards. This is specifically true for ettringite-based boards where the crystal structure is easily destroyed in conventional dryers which would impact the quality dramatically. By using the drying apparatus according to the present invention, the intended crystal structure in the material is preserved instead of (partially) broken down which leads to better results in terms of quality and performance.

As said, the drying apparatus according to the present invention comprises means for generating superheated steam.

In a preferred embodiment of the apparatus according to the present invention, superheated steam is generated from the moisture in the moist material being dried by circulating an initial gas within the enclosure between a heat source and the moist material.

When the drying apparatus is started up, air or another gas is fed into the enclosure by feed means. The air or gas which initially fills the enclosure is recirculated between a heat source and the moist material so as to produce steam from the moisture in the moist material. In this initial start-up phase, the conveying of moist materials through the enclosure may be slowed- down or remain stationary. Alternatively or additionally, saturated steam generated by an external source, such as a steam raising boiler or other known steam generating means, can be injected into the enclosure through feed means.

The steam in the enclosure increasingly displaces the initial gas from the enclosure and continues to be heated by recirculation past a heat source until it becomes superheated. Once the steam inside the enclosure is superheated, the process can proceed continuously with the dried materials being unloaded at the output end and further moist materials being fed at the input end.

As said, the drying apparatus according to the present invention comprises n horizontal drying decks wherein n is an integral of at least.

In a preferred embodiment of the apparatus according to the present invention, n is an integral of at least 5, preferably at least 10. According to another preferred embodiment of the apparatus according to the present invention, n is an integral of maximum 20, preferably maximum 15..

Most preferably n is 5 or 10.

The skilled person will understand that the drying apparatus may comprise a plurality of drying decks which are arranged in superposed levels spaced along the height of the drying enclosure, hence, in a vertical fashion.

Each horizontal drying deck comprises carrying means for carrying the moist materials from the input end through the drying enclosure to be discharged at the output end of the drying apparatus as dried materials. It is understood that each horizontal drying deck carries at least one string of moist materials, however, it can be envisaged that one drying deck could also carry more than one string of moist materials. For example, two or more boards, each belonging to a different string, may be placed onto the deck next to each other thereby filling the usable width of the drying deck.

In one embodiment of the apparatus according to the present invention, the carrying means comprises one or more conveying means.

In one embodiment, the apparatus according to the present invention further comprises a plurality of endless drive chains or belts extending between the input and output ends along the drying decks and each coupled to at least one of the conveying means for advancing the same, respective means connected to the chains or belts at one of the ends for tensioning the chains or belts; a plurality of drive means each including an input drive motor connected to at least one of the chains or belts at the input end of the apparatus and an output drive motor connected to at least one of the chains or belts at the output end of the apparatus for advancing the chains or belts from the input end to the output end.

In a preferred embodiment of the apparatus according to the present invention, each chain or belt is coupled to two of the conveying means.

In a particular embodiment of the apparatus according to the present invention, the conveying means in itself is heated either directly, such as by internal electric heating elements, or indirectly by conduction and/or convection from another heat source in close proximity or in contact with the conveying means.

In a preferred embodiment of the apparatus according to the present invention, the conveying means are selected from a spaced roller conveyor or a meshed belt conveyor.

In a preferred embodiment, the apparatus according to the present invention comprises a plurality of drying modules, preferably between 2 and 20 drying modules, more preferably between 5 and 10 drying modules.

The skilled person will understand that the drying apparatus may comprise a plurality of drying modules which are located next to each other spaced along the length of the drying enclosure, hence, in a horizontal fashion. A further vertical division of the drying module is achieved when a plurality of drying decks is present.

In a preferred embodiment of the apparatus according to the present invention, the ratio air/steam can be controlled separately for each drying deck thereby controlling the drying conditions such as the drying temperature

By using jet dryer boxes and by incorporating a plurality of drying modules and/or drying decks the drying conditions in each, such as the temperature and the humidity, can be tuned very easily. By varying the ratio of air/steam that is supplied through the jet dryer boxes to each drying module and/or to each drying deck, a multitude of different moist materials requiring different drying conditions can still be dried in the same oven at the same time and in ideal conditions.

For example, boards having different dimensions, such as a different thickness, or even boards having a different composition can be dried simultaneously in the same oven. The only requirement being that one drying module and/or one drying deck in that module contains only one type of material and that the drying conditions are adjusted to that material.

It can also be envisaged that a changeover between products with different dimensions or different thicknesses can be realised much faster. For example, the drying conditions in the drying modules and/or drying decks closest to the input end, which are receiving the new product, can already be tuned to the needs of the new products being fed in while the drying conditions in the drying modules and/or drying decks near the output end, which still comprise the previous product, can maintain the ideal drying conditions for said product.

Furthermore, the drying conditions in each drying module and/or drying deck could also be adjusted to the drying stage of the materials. As a material passes through the drying apparatus, it partially dries and becomes lighter in weight. This would allow the air flow rate and the air temperature to be reduced in drying modules and/or drying decks which are closer to the output end. The ability of tuning the drying conditions in each drying module and/or drying deck allows for these reductions which also minimize the energy costs. For example, the air/steam ratio delivered to the drying modules and/or drying decks near the input end can be high, hence a high percentage of air and a lower percentage of superheated steam, while the air/steam ratio delivered to the drying modules and/or drying decks near the output end can be low, hence a high to 100% of superheated steam with little to no air being supplied at all. In this way, the most energy-efficient drying of the materials can easily be accomplished.

The control of the different jet-dryer boxes in the different drying modules and/or drying decks can easily be achieved by a control program that can include parameters based on characteristics of the particular product to be dried and/or that allows for independently controllable to operator- desired levels of parameters.

Jet dryer boxes supplying air are well known in the art. The jet dryer boxes according to the present invention comprise a plurality of spaced orifices through which air and/or superheated steam is delivered.

The person skilled in the art will understand that the optimum orifice configuration depends on the configuration of the drying apparatus, however, it should maximize the amount of heat transferred to the moist materials and ensure a flat moisture profile across the material width to ensure uniform drying.

In a preferred embodiment of the drying apparatus according to the present invention, each jet dryer box comprises an elongated duct having a closed end and an open end in communication with a source of air and/or superheated steam, at least one surface of the duct being a face defining a plane lying substantially parallel to and facing the moist material being dried, said surface having a pattern of rows of orifices, preferably at least five rows, directed toward the moist material and extending parallel to the long dimension of the duct, said orifices in any given row being spaced from each other, and said rows being spaced from each other with respect to a perpendicular line through a given row a distance that is the average spacing between orifices in a given row, wherein the orifices of one row are laterally displaced from those of its adjacent rows by a spacing determined by the formula LDF times the spacing between the corresponding adjacent orifices in any given row; where LDF = ((n ± 1)/2)/n when n is an odd integer, and LDF ((n ± 2)/2)/n when n is an even integer, and where LDF is the lateral displacement fraction and n is the number of rows in the pattern.

For example, when n is 5, the lateral displacement fraction LDF is 2/5 or 3/5. When n is 6, the lateral displacement fraction LDF is 1/3 or 2/3.

It is understood that the orifice size is determined to a large extent by practical considerations such as plugging by debris, ease of cleaning and the distance from the orifice to the most materials. The spacing between the orifices in a single row depends on the orifice size. In jet dryer boxes, the temperature of the air and/or steam delivered will lower with increasing distance from the entrance of the dryer box. More hot air and/or steam should be delivered where the temperature is lower, hence near the end of the box, in order to maintain a uniform delivery of energy across the width of the materials and hence to maintain a uniform drying. To accomplish a uniform moisture profile across the moist materials, the orifice area per square unit of face is increased from the inlet end of the jet dryer box to the closed end by varying the orifice size or orifice spacing. However, adjusting the orifice spacing is preferred because of ease of fabrication. By varying the spacing, the lateral distance between adjacent orifices in the same row is decreased from the inlet end to the closed end in order to provide a larger number of orifices per square unit of face area towards the closed end. Preferably, the total orifice area toward the closed end is in the order of 1.4 times the total orifice area toward the inlet end.

In a particular embodiment, the topmost and bottommost jet dryer box in the drying apparatus according to the invention comprise only one surface having a pattern of rows of orifices and facing the moist material being dried while the intermediate jet dryer boxes have at least two surfaces having a pattern of rows of orifices wherein a first surface faces the moist material above the jet dryer box and a second surface faces the moist material below the jet dryer box.

As said, the drying apparatus according to the present invention comprises at least one means for coupling a source of heated air and/or superheated steam to the respective inlets of the jet dryer boxes. The skilled person will understand that coupling means may comprise regular conduit means which are suitable for conducting air and/or steam into the inlets of the jet dryer boxes.

In practice, the air is fed into the enclosure by feed means. These feed means may form a source of heated air and/or superheated steam which is connected through a coupling means to the inlets of the jet dryer boxes. As said, the drying apparatus according to the present invention comprises at least one feed means for feeding intake air and/or intake superheated steam.

The skilled person will understand that feed means may comprise regular conduit means which are suitable for conducting air and/or steam into the enclosure. The feed means for air and steam can be combined or can be separate.

In an advantageous embodiment, the apparatus according to the present invention further comprises heating means to heat up the intake air. Preferably, the air is heated by indirect heating means such as indirect burners which pass the products of combustion through an air-to-air heat exchanger to prevent products of combustion from circulating in the air.

As said, the drying apparatus according to the present invention comprises at least one discharging means for discharging exhaust air and/or exhaust steam.

The skilled person will understand that discharging means may comprise regular conduit means which are suitable for conducting air and/or steam out of the enclosure.

In an advantageous embodiment of the apparatus according to the present invention, the discharging means are automatically controlled discharging means including a control device, such as a volumetric flow control valve, to control the amount of steam being discharged. Preferably, the amount of steam that is discharged is limited so that external air or other gas is not drawn into the enclosure through the input or output end to replace the discharged steam.

In a further advantageous embodiment, the apparatus further comprises means to recuperate the energy that is discharged from the drying apparatus. Preferably, this recuperated energy is used to pre-heat the intake air and/or the drying modules which have a lower temperature requirement.

In a preferred embodiment of the apparatus according to the present invention, the energy recuperating means comprises one or more heat exchangers and/or one or more heat pumps. In a more preferred embodiment of the apparatus according to the present invention, one or more heat pumps are used to generate the required energy and/or as energy recuperating means to recuperate energy that is discharged from the drying apparatus.

In a particular embodiment of the apparatus according to the present invention, the exhaust air and/or exhaust steam is reheated and recirculated into the enclosure as intake air and/or steam.

In one embodiment of the apparatus according to the present invention, the apparatus further comprises condensation means.

Preferably, a proportion of the superheated steam is discharged from the enclosure through discharging means and at least part of the thermal energy is recovered from the discharged steam by condensation in the condensation means optionally in combination with a heat exchanger.

By recuperating some of the thermal energy, the energy efficiency of the drying apparatus can be increased even more compared to conventional dryers. A conventional dryer that is directly fired on average uses around 0.9 kWh for every kilogram of water that needs to be evaporated while the drying apparatus according to the present invention only uses around 0.7 kWh or less per every kilogram of water, hence, an energy saving of up to 22% can be reached.

As said, the drying apparatus according to the present invention comprises an elongated drying enclosure having an input end and an output end. The enclosure is preferably made out of stainless steel to avoid potential oxidation, but galvanized steel is also possible. Preferably the enclosure is thermally insulated.

The skilled person will understand that the dimensions of the drying enclosure at least depend on the drying capacity that is needed and on the limitations of the factory.

For many commercial applications it is desirable for materials to be dried in a continuous process where the materials are continually passed through a drying location. In a batch method there is no difficulty in retaining the superheated steam in the enclosure while drying is taking place since the enclosure may simply be sealed in gas-tight manner after the batch of material has been introduced and opened again when it is necessary to remove the material after drying. In a continuous drying process, however, it is necessary to prevent the escape of the superheated steam from the enclosure, and the entry of air into the enclosure, during drying, while at the same time allowing the continual passage of materials into and out of the enclosure. Therefore, at least the elongated enclosure of the drying apparatus according to the present invention should be substantially air and steam tight except at its input and output ends where the boards enter and leave the drying apparatus.

In an advantageous embodiment of the apparatus according to the present invention, the drying deck in the lowest position, the bottom drying deck, is situated at a higher level relative to the input and output ends of the enclosure.

The inventors believe that by positioning the inlet and outlet ends at an appropriate level relative to the bottom drying deck, in use, steam tending to pass downwardly along said ends and external air tending to pass upwardly along the ends will meet to form a temperature and density differential stratification layer between said steam and air, which layer inhibits the escape of superheated steam from the enclosure through the inlet or outlet ends.

As said, the drying apparatus according to the present invention comprises at least one feeding device. The feeding device removes the moist materials from a supply. For example, the supply may be a conveying system that carries the moist materials from the forming area, in case of CSH boards this is typically a filter press machine to the drying apparatus.

In a preferred embodiment of the apparatus according to the present invention, the feeding device comprises a conveying means for picking up the moist materials from a supply and a lifting system for lifting the moist materials to a particular drying deck.

In a more preferred embodiment of the apparatus according to the present invention, the feeding device is at least partially enclosed within the enclosure of the dryer, more preferably substantially completely enclosed.

The inventors have found that it is important that the feeding device is at least partially enclosed within the enclosure of the drying apparatus. Otherwise the environment inside the enclosure cannot be guaranteed nor controlled; steam and hence humidity may be lost from within the enclosure.

However, the exact configuration of feeding device does not represent the essence of the invention and the skilled person will understand that any feeding device that is capable of picking up the moist materials from a supply and lifting the moist materials to a particular drying deck may be suitable for use in the drying apparatus according to the present invention. Suitable feeding devices may consist of a conveyor belt, chain and/or rollers used as the lifting platform which is lifted by means of chains, belts and/or cables to bring the moist materials up to the level where it is introduced onto the chosen drying deck.

As said, the drying apparatus according to the present invention comprises at least one unloading device. The unloading device removes the dried materials from the drying apparatus to a storage means. For example, the storage means may be a conveying system that carries the dried materials from the drying apparatus to a storage centre. Alternatively, the dried boards are stacked onto a pallet which functions as the storage means which is replaced by a new pallet once full.

In a preferred embodiment of the apparatus according to the present invention, the unloading device comprises a conveying means for picking up the dried materials from a particular drying deck and a lifting system for lifting the dried materials down after which they can be conveyed out of the drying apparatus to the storage means.

In a more preferred embodiment of the apparatus according to the present invention, the unloading device is at least partially enclosed within the enclosure of the dryer, more preferably substantially completely enclosed.

The inventors have found that it is important that the unloading device is at least partially enclosed within the enclosure of the drying apparatus. Otherwise the environment inside the enclosure cannot be guaranteed nor controlled; steam and hence humidity may be lost from within the enclosure.

However, the exact configuration of unloading device does not represent the essence of the invention and the skilled person will understand that any unloading device that is capable of picking up the dried materials from a particular drying deck and lifting the dried materials back down may be suitable for use in the drying apparatus according to the present invention. Suitable unloading devices may consist of a conveyor belt, chain and/or rollers used as the lifting platform which is lifted by means of chains, belts and/or cables to bring the dried materials back down.

According to a second aspect, the invention relates to a method for drying moist boards using the drying apparatus according to the invention.

In a preferred embodiment, the method for drying horizontal drying decks moist board according to the invention further comprises guiding the boards on through the drying apparatus which is split into drying modules, in which the boards are brought into contact with air and/or superheated steam wherein the superheated steam is generated by the evaporation of the moisture within the moist boards and by transforming the saturated steam to superheated steam though heating over heat exchangers in a main drying stage and a final drying stage by means of impingement jet aeration and wherein the impingement jet aeration is ensured by means of a plurality of opposed jet drying boxes, wherein the circulating air/steam mix is heated to 150 to 350°C, preferably to 150 to 250°C, the circulating air has a humidity of 150 g/kg to 850 g/kg, the air to superheated steam ratio in the main drying stage is between 1 to 10 % by volume and the air to superheated steam ratio in the final drying stage is between 1 to 20 % by volume.

Claims

1. A drying apparatus for continuous drying of moist materials in superheated steam comprising: an elongated drying enclosure having an input end and output end; feed means for feeding intake air and/or intake superheated steam; discharging means for discharging exhaust air and/or exhaust steam; n horizontal drying decks arranged in superposed levels, each drying deck comprising means for carrying the moist materials from the input end through the drying enclosure, to be discharged at the output end of the drying apparatus as dried materials; a feeding device for feeding the moist materials to any one of the n drying decks; an unloading device for unloading the dried materials of any one of the n drying decks; one or more drying modules creating one or more drying zones, each drying module comprising a plurality of opposing jet dryer boxes disposed above and below an upper surface of each of the n drying decks wherein each jet dryer box comprises a plurality of spaced orifices therein; wherein n is an integral of at least 1 ; wherein the drying apparatus further comprises means for generating superheated steam and means for coupling a source of heated air and/or superheated steam to the respective inlets of the jet dryer boxes; and wherein a mixture of air and superheated steam is provided through the plurality of orifices in the jet dryer boxes thereby impinging upon at least the upper and lower major surfaces of the moist materials.

2. The drying apparatus according to claim 1 , further comprising a heat source for heating air and/or superheated steam; wherein superheated steam is generated from the moisture in the moist material being dried by circulating an initial gas within the enclosure between the heat source and the moist material and/or by injecting superheated steam into the enclosure from an external source.

3. The drying apparatus according to claim 1 or 2, wherein the ratio air/steam can be controlled separately in each drying module, thereby controlling the drying conditions such as the drying temperature.

4. The drying apparatus according to any one of claims 1 to 3, wherein each drying deck comprises one or more vertically conveying means to convey the moist materials to be dried through the drying apparatus towards the output end.

5. The drying apparatus according to claim 4, further comprising a plurality of endless drive chains extending between the input and output ends along the drying decks and each coupled to at least one of the conveying means for advancing the same, respective means connected to the chains at one of the ends for tensioning the chains; a plurality of drive means each including an input drive motor connected to at least one of the chains at the input end of the apparatus and an output drive motor connected to at least one of the chains at the output end of the apparatus for advancing the chains from the input end to the output end.

6. The drying apparatus according to claim 4 or 5, wherein the conveying means are selected from spaced roller conveyors or meshed belts.

7. The drying apparatus according to any one of claims 1 to 6, further comprising condensation means and wherein a proportion of the superheated steam is discharged from the enclosure through discharging means and at least part of the thermal energy is recovered from the discharged steam by condensation in the condensation means.

8. The drying apparatus according to any one of claims 1 to 7, further comprising means for generating energy and/or re-using the energy coming out of the discharging means, to heat up the intake air and/or drying zones with a lower drying temperature, preferably by heat-exchangers and/or heat pumps.

9. The drying apparatus according to any one of claims 1 to 8 wherein n is an integer of at least 5, preferably at leastlO.

10. The drying apparatus according to any one of claims 1 to 9 wherein n is an integer of maximum 20, preferably maximum 15.

11. A method for drying moist boards using the drying apparatus according to any one of claims 1 to 10.

12. A method for drying moist boards according to claim 11 , further comprising: guiding the boards on horizontal drying decks through the drying apparatus which is split into drying modules, in which the boards are brought into contact with air and/or superheated steam in a main drying stage and a final drying stage by means of impingement jet aeration and wherein the impingement jet aeration is ensured by means of a plurality of opposed jet drying boxes, wherein - the circulating air is heated to 150 to 350°C, the circulating air has a humidity of 150 g/kg to 850 g/kg, the air to superheated steam ratio in the main drying stage is between 1 to

10 % by volume, the air to superheated steam ratio in the final drying stage is between 1 to 20 % by volume.