Method and dryer for drying panels

Abstract

1. A method for drying panels in a drying device comprising a first stage (A) and a second stage (B), each of the two stages (A, B) having a tier, the panels being placed on a surface formed in layers and passing through the drying device in each tier of the two stages (A, B), the panels being dried in contact with hot dry air in the first stage (A) and by cooler dry air in the second stage (B), the panels being supplied with hot air from a means for recovering heat in at least one of the two stages (A, B), the means being arranged outside the two stages and the hot air being supplied directly to at least one of the stages.

Description

[Technical Field]

[0001] The present invention relates to a method for drying panels and to a dryer used for this purpose.

[0002] When drying panels, particularly cement and gypsum-containing building panels, the panels are exposed to hot air as they are conveyed through the dryer.

[0003] Dry air can be supplied in the form of longitudinal ventilation, cross-flow ventilation, or cross-flow ventilation using a nozzle box with nozzles. In longitudinal ventilation, dry air is supplied at one end of the dryer, or at one end of a section if it is divided into sections, and discharged at the opposite end.

[0004] Cross-flow ventilation allows for more intensive drying in the dryer by supplying air at several points on one side of the dryer and discharging it on the other side. In particular, in turbulent flow, cross-flow ventilation through nozzles in nozzle dryers allows for more intensive drying.

[0005] In most cases, a recirculation process is used in which most of the drying air is recirculated, reheated after contact with the material to be dried, and then reused, and only a small portion of the drying air is discharged to the outside as exhaust air, with a corresponding amount of fresh air being supplied from the outside.

[0006] For example, fuel, i.e., primary energy, is required to heat the drying air using a combustor or heating coil, and electrical energy, i.e., secondary energy, is required to supply the air using a fan. In order to be able to manufacture such plates in a more energy-efficient manner, the use of both primary and secondary energy must be reduced.

[0007] DE 26 13 512 A1 describes a drying process that achieves low primary energy consumption by using the heat of condensation from exhaust air. This process is designed in two stages. Drying takes place at high temperature and humidity in the first drying stage and at low temperature and humidity in the second drying stage, where the drying capacity of the first stage is two to three times that of the second stage. The second drying stage is heated by the exhaust air from the first drying stage via a heat exchanger. In both stages, drying air is supplied in a recirculation process: in the first drying stage, longitudinal ventilation is used, and in the second drying stage, cross-flow ventilation is used with a high recirculation mass flow rate. However, the second stage requires a high recirculation mass flow rate, resulting in high secondary energy consumption.

[0008] When the heat of condensation of the exhaust air is also used to reduce primary energy consumption, a common problem arises: the waste heat of the exhaust air is only available at low temperatures. A lower dry air temperature can be compensated for with a higher air mass flow rate, but this results in an increase in secondary energy consumption.

[0009] WO 95 / 04908 A1 discloses a method for drying plates conveyed through a layered dryer, in which the plates are contacted with dry air in two stages A and B, and drying in stage A is carried out in a recirculating air process with hot dry air, at least moderate air humidity and a drying capacity 2 to 4 times higher than in stage B. In stage B, the exhaust air from stage A is passed through a heat exchanger arranged in the layer of the dryer; at the same time, dry air of low temperature and humidity is fed in countercurrent to the exhaust air from stage A.

[0010] According to WO2019 / 105888A1, at least two means for recovering heat are used, where the two means are arranged in series and the heat from the first means is passed to a combustor for further heating, and only then is it introduced into the first stage of the dryer. Summary of the Invention

[0011] It is an object of the present invention to further improve the method described in the preamble of claim 1.

[0012] According to the present invention, this object is achieved as specified in claim 1.

[0013] According to the invention, in at least one of the two stages (A, B), hot air is supplied to the panels using means for recovering heat, the means being located outside the two stages and the hot air being supplied directly to at least one of the stages.

[0014] This optimizes the use of both primary and secondary energy: in particular, the primary energy used is maintained by utilizing the waste heat of the exhaust air and the heat of condensation, without increasing the secondary energy requirements by circulating a large air mass.

[0015] Further advantageous configurations of the method emerge from the subclaims and the description, especially in conjunction with the drawings.

[0016] Preferably, the panels are heated in at least the first stage by a heat exchanger, by means of a heat pump, by means of a wet separator, directly by means of a combustor, indirectly electrically using hot steam or thermal oil, or by hot air generated using low-grade heat.

[0017] According to the invention, in the second stage, heat is at least supplied to the panels with only a single means for recovering the heat provided.

[0018] Preferably, according to the method of the invention, the panels to be dried are dried with minimal energy consumption by adapting the transport speed of the panels to the respective energy absorption and associated dehumidification of the panels.

[0019] The invention provides a method with low primary and secondary energy consumption. The primary energy used in the invention is kept low by utilizing the waste heat of the exhaust air and the heat of condensation. Compared to methods known from the prior art, the recirculated air mass flow rate is reduced, which reduces the mass flow rate of the drying air and therefore the secondary energy consumption.

[0020] The invention particularly makes use of waste heat from other processes taking place in the vicinity of the dryer, such as waste heat from a waste incineration plant or a combined heat and power plant; low-grade heat available as waste heat from other heat-generating processes can also be used in accordance with the invention.

[0021] Waste heat from a combined heat and power plant can also be used. This technology produces heat and electricity simultaneously. Using fuel to generate heat and electricity simultaneously in a single device is more efficient and cost-effective than generating heat and electricity separately in two different devices (COGEN technology or Combined Heat and Power Generation). Combined heat and power is up to 40% more efficient than generating heat and electricity separately. Preferably, combined heat and power is done with renewable fuels. The electricity produced in this way can also be used in the system to transport the panels and run the fans.

[0022] In one embodiment, the inventive dryer system is coupled to a solar thermal and / or photovoltaic system. Alternatively, the inventive dryer uses heat from a geothermal system.

[0023] Typically, the front stage A and the rear stage B have the same number of floors arranged one above the other to guide and transport the panels. The floors also typically have the same height and distance from each other, allowing the panels to move seamlessly from stage A to stage B. This applies in particular when the panels are transported at the same transport speed in both stages. However, this does not exclude the possibility of selecting different transport speeds in the two stages.

[0024] Therefore, according to one embodiment of the invention, different transport speeds can be used for the panels dried in the two stages. The speed selection for each stage, determined according to the desired drying process, results in a faster transport speed in stage A than in stage B, due to the higher drying temperature in stage A than in stage B. This means that a separate conveyor located between stages A and B reduces the speed of the panels leaving stage A to the speed level of stage B. At the same time, to avoid intermediate accumulation of panels in the conveyor area, the conveyor distributes the panels over a wider area in stage B according to the speed difference between stages A and B. This is done by dividing the panels into more levels or lanes in the area of ​​stage B than in stage A. A conveyor device, such as a discontinuous conveyor, is used for this purpose, preferably lifting the panels at a higher speed on the side facing stage A and sending them to more stages or tracks of stage B on the side facing stage B, where the conveyor operates at a slower speed. The conveyor has a turning point at the transition to stage B to distribute the panels to the level of stage B. If the number of trucks is higher, it is preferred that several panels are transported next to each other on one level, for example in 2 to 4 trucks.

[0025] This approach allows for a compact entry area into the dryer in stage A, allowing for high speed and temperature traversal, which is also suitable for ensuring the final activation of the coagulant, such as starch, contained in the panels. The intense heat in the previous stage A, combined with high humidity, accelerates the expansion of the coagulant contained in the building panels being dried, thus producing high-quality panels.

[0026] Implementing two stages with different panel conveyor speeds allows for maximum use of energy from stage A while adjusting the area and speed in stage B to achieve efficient drying of the panels.

[0027] To achieve rapid drying in stage A, the panels are preferably heated there by cross-flow of hot air without the use of a nozzle box. This allows the panels to be easily heated without the structural challenges associated with installing a nozzle box. The panels are preferably heated in the front section of the dryer in a recirculating air process. Due to the high air humidity of the panels, the dew point is preferably between 60 and 99°C, more preferably between 75 and 90°C.

[0028] The heat supplied to the panels in stage A is preferably at least partly generated by means for recovering heat, in particular a heat pump. At least part of the section of stage A is additionally equipped with a heater, in particular in the section preceding stage A. Alternatively, stage A can also be completely heated by a heater installed therein.

[0029] The extensive heating of the first stage results in high air humidity in this stage, which means that the dew point is increased to temperatures between 75° C. and 99° C., preferably between 75° C. and 90° C. The air humidity is between 200 and 600 g per kilogram of air.

[0030] To simplify the design of stage A, it is also possible to place the stage A area directly on the floor of the industrial building without a separate floor for the dryer. This allows stage A to be implemented with a lightweight structure; if stage A consists of a number of modules arranged one behind the other in the production direction, these modules can be easily installed and removed from the container, for example to repair or replace the area, especially the fans installed in the ceiling area.

[0031] Although nozzle boxes allow panels to be dried well in a short time, they require a high energy input to generate the necessary airflow. On the other hand, nozzle boxes also occupy a considerable amount of space in stage A, which usually has multiple sections, and nozzle boxes are arranged one above the other in each section depending on the number of floors in stage A.

[0032] According to the invention, Stage A longitudinal ventilation designs use nozzle boxes whenever possible.

[0033] Both stages are divided into fields or sections. In both stages A and B, for example, the drying device is provided with a conveyor system having several fields or sections extending back and forth in the conveying direction to transport the panels to be dried, which pass through several levels per field, the conveying device being arranged in layers and designed as a roller conveyor. Preferably, a separate conveyor chain is provided for the panels in each field; however, panels can also be driven in several fields by a conveyor chain common to these fields, so that the conveyor chain drives the panels over, for example, five fields.

[0034] Stage B requires a larger drying area due to the increased drying time of the panels being dried; it is longer than Stage A.

[0035] In both dryer stages A and B, dryers operating at low temperatures and drive systems adapted for a large number of panels, particularly gypsum board panels, are used, which are processed simultaneously in the low temperature dryer across multiple floors, for example 16 to 60 floors, particularly 20 to 50 floors, and particularly preferably 30 to 40 floors. The use of multiple floors in conjunction with the inventive drive systems allows for an increased residence time of panels, particularly gypsum board panels, in a low temperature dryer having the same dryer length as a high temperature dryer, while maintaining the same panel output as a high temperature dryer.

[0036] It can therefore be seen that the installation of the inventive dryer arrangement in an existing plant allows particularly large energy savings.

[0037] For optimal utilization of the waste heat, especially generated in stage A, the panels are dried in the first stage A and / or the second stage B by means of at least one external heat exchanger and / or alternatively an internal heat exchanger.

[0038] It is also advantageous if the panels in the first stage A are heated directly using recycled air through a combustor, indirectly electrically using hot steam or thermal oil, or using low-grade heat. In stage B, the panels are heated by low-grade heat either from stage A or from heat recovery from another process where heat is released at low temperature, for example a combined heat and power plant or a heat pump.

[0039] Preferably, the panels in the first stage A are dried with dry air at temperatures ranging between 90 and 160°C, in particular between 120 and 140°C. The choice of a low temperature allows the panels to dry slowly, so that no anhydrite is formed on the panels.

[0040] When the panels are dried in the first stage A in these temperature ranges, the hot air absorbs a large amount of moisture, and the temperature and circulation rate of the air are preferably selected so that the dew point of the hot air is between 60 and 99°C.

[0041] The drying section of the first stage A has either cross-flow ventilation or, alternatively or additionally, longitudinal ventilation. When high temperatures are reached, stage A is preferably indirectly and at least partially heated, for example by up to 50%, via a heat pump. Alternatively, the section of stage A is heated indirectly by a combustor or indirectly.

[0042] In a second stage B, the panel is dried with dry air at a temperature of from 20 to 90°C, preferably between 30 and 90°C.

[0043] The exhaust air from the drying process in the first stage A can be advantageously reused to preheat the drying air in the second stage B by passing it through a heat exchanger.

[0044] Even greater efficiency of the inventive drying process can be achieved if the panels are first dried in a pre-drying stage upstream of the first stage A, then in the first stage A, and finally in the second stage B.

[0045] Preferably, the panels are transported through stages A and B using separate conveyor systems for each stage A, B and / or each section. Alternatively, the conveyor systems may be driven by direct drive motors or may be at least partially connected to each other using gearboxes.

[0046] The invention also provides a dryer for drying panels in first and second stages A, B, each of which is equipped with a conveyor device for transporting the panels arranged in layers through the dryer, wherein the first stage (A) has at least one section, the first stage A has a feeding device, an exhaust device and a recirculation duct, as well as means for supplying supply air and means for exhausting air, with recirculation air conveying means and heating devices, and the second stage B receives the panels from the first stage A and is equipped with a dry air feeding device and a dry air exhaust device; alternatively, a heating device may also be provided in the second stage.

[0047] In particular, the dryer has low secondary energy consumption, since high conveying capacities for circulating air are avoided in the second stage.

[0048] Preferably the dryer housing is fitted with a door for each dryer stage A, B. Preferably the dryer, particularly stage B, does not have its own floor but is built on a screed in the factory hall.

[0049] An advantage of the dryer is that the first and second stages A, B each comprise at least one section or zone, and in particular the first stage A is equipped with means for flowing recirculating air transverse to the conveying direction of the panels.

[0050] For design reasons, the first stage A of the dryer is preferably divided into several sections, which are at least partially equipped with devices for cross-ventilation; fans are provided in the front section of stage A, especially in the ceiling area; however, fans can also be installed externally, especially above the top cover of the section, if the recirculated air generated by them is then directed into the section interior. For this purpose, air-guiding elements are preferably provided in the section. Cross-ventilation is achieved, for example, by hot air impinging on a surface.

[0051] Preferably, only longitudinal ventilation is provided in stage B, i.e. the subsequent stage, although this does not exclude providing additional and / or exclusive means for cross-flow ventilation in stage B. In stage B, the panels have a temperature between 30°C and 90°C; during drying of the panels in stage B, the air humidity there will be between 5 and 30 g / kg, i.e. between 5 and 30 g per kilogram of air.

[0052] It is advantageous to provide the second stage B of the dryer with means for flowing recirculating air against and / or in the direction of conveyance of the panels.

[0053] In a further advantageous embodiment of the dryer, the second stage B is provided with induction means or at least one exhaust fan for inducing the recirculating air in a spiral. Auxiliary induction means, for example in the form of guide panels, may be provided.

[0054] Preferably, a roller conveyor or a conveyor belt is provided as a conveying device for transporting the panels to be dried in the dryer.

[0055] The use of condensation heat, made possible by the low temperature of the dry air cooling the heat exchanger and the at least moderate humidity of the exhaust air from stage A, also results in intensive primary energy use.

[0056] When dry air is guided through the heat exchanger countercurrently to the exhaust air from stage A, the cooler dry air joins the already cooled exhaust air. This ensures that the water vapor contained in the exhaust air is condensed as much as possible, further improving the utilization of primary energy. This more intensive use of primary energy results in considerable primary energy savings.

[0057] Overall, Stage B dries a maximum of 60% of the drying capacity of Stage A.

[0058] Each stage A and B is equipped with a conveyor device for transporting the panels arranged in layers through the dryer, which can be designed as a roller conveyor dryer or a belt dryer, whereby the conveyor device has several roller conveyors or conveyor belts arranged one above the other.

[0059] The shelves are spaced apart by a distance between 100mm and 150mm, preferably 130mm.

[0060] Additional heating devices may be installed in Stage B for additional transient loads.

[0061] The invention is explained in more detail below with reference to embodiments: In a single figure, a dryer with two stages A and B and a heat pump is shown.

[0062] The dryer 1 (figure) comprises two stages A and B for drying panels which are fed to the dryer 1 via a conveyor device 100, such as a conveyor belt, in the direction of the arrow C. These panels are in particular building panels, such as plasterboard panels or gypsum wall panels.

[0063] Each of the two stages A and B is preferably divided into sections or zones 2. At least some of the zones 2 of stage A, particularly the front zone 2, are each equipped with a recirculation fan 16 for generating a flow transverse to the conveying direction of the panels.

[0064] Preferably, stage A has, on the inlet side, a sealed section 3, which is supplied with outside air heated by a heat pump 4 or heat exchanger 4 via a supply line 6 equipped with an openable flap 5; this supply of outside air not only serves to heat the panels but also to seal stage A against other air flows and against the ingress of outside air into stage A.

[0065] Fresh air heated by a fan 8 is distributed via a pipe 7 branching off from the supply pipe 6 to the individual pipes 9, 10, 11, 12, 13, and 14 via further pipes 80. From there, the fresh air reaches a heating device 15 installed in part of the zone 2, for example, in a ceiling box above the floor on which the panels are transported. The heating device 15 is preferably a direct heating device, such as a burner, or an indirect heating device, such as a steam or electric heater. At least one recirculation fan 16 is provided within the zone 2 or for several zones 2 together, generating a cross-flow of hot air as recirculation air in the zone 2. Alternatively, two recirculation fans 16 are provided for each zone 2. The moisture-laden air returns from the zone 2 to the heat exchanger 4 via an outlet 38, where the moisture condenses therefrom.

[0066] From stage A, the pre-dried panels are transported to stage B, which is designed as a vertical dryer.

[0067] Stage B is also supplied with heated ambient air from heat exchanger 4. Pipes 19 to 26 are used for this purpose.

[0068] Fans may also be installed in tubes 19 to 26. At the inlet of section 2 of stage B, the air flowing from tubes 19 to 26 into section 2 is heated by heaters 29 to 31. Heaters 29 to 31 are switched on when additional heating energy is needed; this is the case when the system is starting up, when the heat supplied from stage A is still insufficient and heat exchanger 4 has not yet received any warm exhaust air or enough warm exhaust air from stage A. Heaters are also needed when the system is shut down and there is insufficient hot air supplied from stage A to enter stage B. Heaters 29 to 31 may also be used when the panels to be dried contain higher moisture than expected, or when changing between different panel types, which may result in a lack of thermal energy in stage B. Therefore, heaters 29 to 31 are provided specifically for transient loads in stage B.

[0069] It will be appreciated that depending on the length of stage B, multiple lines for supplying air, in particular hot air from heat exchanger 4 or from another heat exchanger, may be provided to recover the enthalpy of evaporation of the water evaporated from the panels.

[0070] In principle, recirculation fans are not required in stage B; however, if such fans must be provided, they are constructed and positioned in the same manner as the recirculation fans in stage A. Both radial and axial fans can be used.

[0071] Exhaust fans 32 to 35, as well as recirculation fans, are distributed throughout the length of stage B. Moist air is drawn from stage B via these and chimneys 36 to 39.

[0072] Additional internal heat exchangers may be provided in both stage A and stage B, for example in the ceiling box above the nozzle box in stage A or above the conveyor in stage B, also in a ceiling box provided for this purpose.

[0073] Heat exchanger 4 is connected to section 2 of stage A via exhaust duct 38 and central exhaust duct 39. Warm, moisture-saturated air travels via exhaust fan 40 through exhaust ducts 38, 39 to heat exchanger 4 where it condenses and releases its moisture as water.

[0074] The heat exchanger 4 draws in fresh air via an fresh air fan 41. It exhausts stagnant air to the environment via a chimney 42. A condensate separator 43 is provided between the fresh air fan 41 and the heat exchanger 4.

[0075] Stage B is designed as a longitudinal drying area; depending on the strength of the air supply via lines 19 to 26 associated with the extraction of used air by fans 32 to 34, the air is directed countercurrently to the direction of panel transport, at least in the front area of ​​stage B.

[0076] Since it is preferable not to exceed a temperature of 160°C in the dryer as a whole, all kinds of building boards, in particular gypsum boards but also cement boards, can be dried very slowly in the inventive dryer with low energy consumption, thereby making it possible to produce high quality boards.

[0077] The relatively low number of floors also allows the panels to be dried very efficiently, since many panels can be dried simultaneously in a small space, and even if the panels are transported in several lanes next to each other, production efficiency is further increased. (Other possible items) (Item 1) 1. A method for drying panels in a drying apparatus comprising a first stage (A) and a second stage (B), wherein the first stage and the second stage (A, B) each have a tier, the panels are placed on a surface formed in tiers and pass through the drying apparatus in each tier of the first stage and the second stage (A, B), the panels are dried in contact with hot dry air in the first stage (A) and by cooler dry air in the second stage (B), the panels are supplied with hot air from a means for recovering heat in at least one of the first stage and the second stage (A, B), the means being arranged outside the first stage and the second stage, and the hot air is supplied directly to at least one of the stages. (Item 2) Item 1. The method according to item 1, wherein the panel is heated in at least the first stage by a heat exchanger, by using a heat pump, by using a wet separator, directly by a combustor, indirectly electrically by using hot steam or thermal oil, or by hot air generated using low-grade heat. (Item 3) 3. The method according to item 1 or 2, wherein the moisture contained in the air condenses in the first stage (A) at a dew point between 60°C and 99°C. (Item 4) 4. The method according to claim 3, wherein the moisture contained in the air condenses in the first stage (A) at a dew point between 75°C and 90°C. (Item 5) 5. The method according to any one of items 1 to 4, wherein the moisture content of the air in the second stage (B) is between 5 g / kg and 30 g / kg per kilogram of air. (Item 6) 6. The method according to any one of items 1 to 5, wherein the panel is dried by circulating air at least in the first stage (A). (Item 7) 7. The method according to any one of items 1 to 6, wherein the panel is dried in at least the first stage (A) by dry air at a temperature of 120°C to 160°C. (Item 8) 8. The method according to any one of items 1 to 7, wherein the panel is at least substantially dried in the area of ​​at least the first stage (A) by the use of a nozzle box. (Item 9) 9. The method according to any one of items 1 to 8, wherein the panel is dried in the second stage (B) by dry air at a temperature of from 20 to 90°C. (Item 10) 10. The method according to any one of items 1 to 9, wherein the exhaust air from the first stage (A) is fed into a heat exchanger (31) to preheat the dried air of the second stage (B). (Item 11) 11. The method according to any one of items 1 to 10, wherein the panel is first dried in a sealing stage (3) or pre-drying stage upstream of the first stage (A), then in the first stage (A) and finally in the second stage (B). (Item 12) A dryer for drying panels in a first stage (A) and a second stage (B), wherein at least one of the first stage and the second stage (A, B) can be supplied with hot air from a means for recovering heat, the means being arranged outside the first stage and the second stage, and the hot air can be supplied directly to the at least one stage. (Item 13) 13. The dryer according to item 12, wherein the means for recovering heat comprises a heat exchanger, a heat pump, a wet separator, and / or a combustor for direct or indirect heating of the hot air using hot steam or thermal oil, by indirect electrical heating, or using low-grade heat, and wherein a maximum of one means for recovering heat is provided. (Item 14) Item 14. The dryer according to item 12 or 13, wherein the panel is heatable in the first stage (A) by hot air flowing transversely to the direction of conveyance and in the second stage (B) by hot air flowing in the longitudinal direction. (Item 15) 15. The dryer according to any one of items 12 to 14, wherein the second stage (B) is equipped with means for flowing recirculating air against and / or in the direction of conveyance of the panels. (Item 16) 15. The dryer according to any one of items 11 to 14, wherein a condensate separator (43) is arranged upstream of the at least one heat exchanger (4). (Item 17) 17. The dryer according to any one of items 12 to 16, having at least 16 floors on which the panels are dried and transported. (Item 18) 18. The dryer of one of items 12 to 17, wherein the floors are spaced apart by a distance of 150 mm or less.

Claims

1. 1. A method for drying a panel in a drying apparatus having a first stage and a second stage, wherein the first stage and the second stage each have a tier, the panel is placed on a surface formed in layers and passes through the drying apparatus in the tiers of each of the first stage and the second stage, the panel is dried in contact with hot dry air in the first stage and is dried by cooler dry air in the second stage, and the panel is supplied with hot air from a means for recovering heat in at least one of the first stage and the second stage, the means being arranged outside the first stage and the second stage, and the hot air is supplied directly to at least one of the stages.

2. 10. The method of claim 1, wherein the panels are heated in at least the first stage by a heat exchanger, by a heat pump, by a wet separator, directly by a combustor, indirectly electrically by hot steam or thermal oil, or by hot air generated using low-grade heat.

3. 2. The method of claim 1, wherein moisture contained in the air condenses in the first stage at a dew point between 60°C and 99°C.

4. 4. The method of claim 3, wherein the moisture contained in the air condenses in the first stage at a dew point between 75°C and 90°C.

5. 2. The method of claim 1, wherein the moisture content of the air in the second stage is between 5 g / kg and 30 g / kg of air.

6. The method of claim 1 , wherein the panel is dried by circulating air in at least the first stage.

7. 10. The method of claim 1, wherein the panel is dried in at least the first stage with dry air at a temperature of 120°C to 160°C.

8. 10. The method of claim 1, wherein the panel is dried at least substantially in the area of ​​the first stage by use of a nozzle box.

9. 10. The method of claim 1, wherein the panel is dried in the second stage with dry air at a temperature of 20 to 90°C.

10. 10. The method of claim 1, wherein exhaust air from the first stage is fed into a heat exchanger to preheat the dried air of the second stage.

11. 11. The method of any one of claims 1 to 10, wherein the panel is dried first in a sealing or pre-drying stage upstream of the first stage, then in the first stage, and finally in the second stage.

12. A dryer for drying panels in a first stage and a second stage, wherein at least one of the first stage and the second stage can be supplied with hot air from a means for recovering heat, the means being arranged outside the first stage and the second stage, and the hot air can be supplied directly to the at least one stage.

13. 13. The dryer of claim 12, wherein the means for recovering heat comprises a heat exchanger, a heat pump, a wet separator and / or a combustor for direct or indirect heating of the hot air using hot steam or thermal oil, by indirect electrical heating or using low-grade heat, and wherein a maximum of one means for recovering heat is provided.

14. 13. The dryer of claim 12, wherein the panels are heatable in the first stage by hot air flowing transversely to the direction of conveyance and in the second stage by hot air flowing longitudinally.

15. 13. The dryer of claim 12, wherein the second stage is equipped with means for flowing recirculating air against and / or in the direction of conveyance of the panels.

16. The dryer of claim 12 , wherein a condensate separator is disposed upstream of the at least one heat exchanger.

17. 17. The dryer of any one of claims 12 to 16, having at least 16 floors on which the panels are dried and transported.

18. 18. The dryer of claim 17, wherein the floors are spaced apart a distance of 150 mm or less.

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