Method for drying panels, and dryer

Abstract

A method for drying panels in a drying device comprising a first stage (A) and a second stage (B), wherein the two stages (A, B) each have levels and the panels are in each case laid on surfaces formed level-by-level and, in the respective levels of the two stages (A, B), are guided through the drying device, wherein the panels in the first stage (A) are brought into contact with drying air of a high temperature and are dried, and in the second stage (B) are dried with drying air of a less high temperature, is characterised in that at least in one of the two stages (A, B) warm air is supplied to the panels from a means for recovering heat, wherein the means is arranged outside the two stages and wherein the warm air is supplied directly to the at least one stage.

Description

[0001] Process for drying plates and dryer

[0002] The present invention relates to a method for drying plates and to a dryer used therefor.

[0003] When drying boards, especially cement and gypsum-based building boards, the boards are conveyed through a dryer and brought into contact with heated air.

[0004] The drying air can be supplied by longitudinal ventilation, transverse ventilation, or cross ventilation using nozzle boxes equipped with nozzles. With longitudinal ventilation, the drying air is supplied at one end of the dryer or, if it is divided into several zones, at one end of a zone and exhausted at the opposite end.

[0005] With cross ventilation, the air is introduced at several points along the sides of the dryer and discharged at the opposite sides, enabling more intensive drying within the dryer. Particularly intensive drying is achieved with cross ventilation via nozzles through the jet dryers in impingement flow.

[0006] In most cases, a recirculation process is used, in which a large portion of the drying air is circulated. In this case, most of the drying air is reheated after contact with the material being dried and thus reused. Only a small portion of the drying air is discharged to the outside as exhaust air, and a portion equivalent to the exhaust air is supplied from the outside as supply air.

[0007] Fuel, i.e. primary energy, is required to heat the drying air, for example using burners or heating registers, and electrical energy, i.e. secondary energy, is required to supply the air using fans. The use of both primary and secondary energy must be reduced in order to enable more energy-efficient production of the above-mentioned panels. DE 26 13 512 A1 discloses a drying process in which low primary energy consumption is achieved by utilising the condensation heat of the exhaust air. This process has two stages. In the first drying stage, drying takes place at high temperature and high humidity, and in the second drying stage at low temperature and low humidity, with the drying capacity of the first stage being twice to three times that of the second stage and the second drying stage being heated from the exhaust air from the first drying stage via a heat exchanger.In both stages, the drying air is supplied using a recirculation process: longitudinal ventilation in the first drying stage and cross ventilation with a large recirculating air mass flow in the second drying stage. However, the second stage requires a large recirculating air mass flow and thus consumes a large amount of secondary energy.

[0008] When reducing primary energy consumption by also utilizing the condensation heat of the exhaust air, the general problem arises that the exhaust air's waste heat is only available at a low temperature level. A lower drying air temperature can be compensated for by larger air mass flows, but this leads to greater secondary energy consumption.

[0009] WO 95 / 04908 A1 discloses a method for drying boards conveyed through a dryer in tiers, in which the boards are brought into contact with drying air in two stages A and B, wherein in stage A drying is carried out using a recirculating air process with drying air of high temperature and at least medium humidity and with a drying performance two to four times higher than in stage B. In stage B, the exhaust air from stage A is passed through a heat exchanger arranged in the tiers of the dryer; at the same time, the drying air with low temperature and low humidity is passed in countercurrent to the exhaust air from stage A.

[0010] According to WO 2019 / 105888 A1, at least two means for recovering heat are used, wherein the two means are arranged in series and wherein the heat from the first means is passed through a burner for further heating and only then is it introduced into the first stage of the dryer.

[0011] It is the object of the invention to further improve the method according to the preamble of claim 1.

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

[0013] According to the invention, warm air is supplied to the plates in at least one of the two stages (A, B) by a means for recovering heat, the means being arranged outside the two stages and the warm air being supplied directly to the at least one stage.

[0014] This optimizes the use of both primary and secondary energy. In particular, the primary energy used is maintained by utilizing waste heat and the condensation heat of the exhaust air, without increasing the demand for secondary energy by recirculating large air mass flows.

[0015] Advantageous further developments of this method emerge from the subclaims and the description, in particular in conjunction with the drawings.

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

[0017] According to the invention, the heat is recovered at least , with at most a single means for recovering heat being provided which is supplied to the plates in the second stage.

[0018] Preferably, according to the method according to the invention, the transport speed of the panels to be dried is also adapted to the respective energy consumption and the associated dehumidification of the panels, so that they are dried with minimal energy expenditure.

[0019] The invention creates a process with low primary and secondary energy consumption. The primary energy used in the invention is kept low by utilizing the waste heat and condensation heat of the exhaust air. This reduces the mass flow of the drying air compared to prior art procedures, thereby lowering the consumption of secondary energy because the circulated air mass flows are reduced.

[0020] The invention utilizes, in particular, the waste heat from other processes taking place in the vicinity of the drying device, for example, the waste heat from a waste incineration plant or a combined heat and power plant; according to the invention, low-calorific heat available from other exothermic processes can also be utilized as waste heat.

[0021] The waste heat from a combined heat and power plant can also be used. This technology generates heat and electricity simultaneously. Using a single fuel to generate heat and electricity in a single unit is more efficient and cost-effective than generating heat and electricity separately in two different units (COGEN technology or combined heat and power generation). Combined heat and power is up to 40% more efficient than generating heat and electricity separately. Combined heat and power is preferably powered by a renewable fuel. The electricity generated in this way can also be used in the plant to transport the panels and operate fans.

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

[0023] Typically, the front stage A and the rear stage B have the same number of stacked levels for guiding and conveying the panels. The levels also typically have the same height and the same spacing, so that the panels are transferred seamlessly from stage A to stage B. This is especially true if the panels are conveyed at the same conveying speed in both stages. However, this does not preclude the possibility of selecting different conveying speeds for the two stages.

[0024] According to one embodiment of the invention, it is therefore possible to use different transport speeds for the boards to be dried in the two stages. The selection of the speed determined for each stage according to the desired drying progress results in a higher transport speed in stage A than in stage B for drying in stage A at a higher temperature than in stage B. This means that the speed of the boards emerging from stage A is reduced to the speed level of stage B in a separate conveyor arranged between stages A and B. At the same time, in order to avoid intermediate storage of boards in the area of ​​the conveyor, the conveyor distributes the boards over a larger area in stage B corresponding to the speed difference between stages A and B.This is done by dividing the panels into a larger number of levels or tracks in the area of ​​level B than in level A. A conveyor device, for example a discontinuous conveyor, is used here, which picks up panels at a higher speed on the side facing level A and delivers them to a larger number of levels or tracks of level B on the side facing level B, with the conveyor preferably having a lower speed on this side. The conveyor has a tipping point at the transition to level B in order to distribute the panels among the levels of level B. If there are a larger number of tracks, it is preferable to transport several panels next to one another on one level, for example in two to four tracks.

[0025] This measure creates a compact dryer entrance area with stage A, which can be passed through at high speed and high temperature. This is also suitable for ensuring the final activation of strengthening agents contained in the boards, such as starch. The intense heating in the front stage A, combined with the high humidity, promotes the swelling of the strengthening agents contained in the building boards being dried. This allows for the production of high-quality boards.

[0026] In the event that two stages with different conveying speeds of the panels are implemented, the area and speed within stage B can be adjusted to achieve sufficient drying of the panels while at the same time making the best possible use of the energy from stage A.

[0027] To achieve rapid drying in stage A, the boards are preferably heated by a crossflow of hot air without the use of nozzle boxes. This allows the boards to be heated easily without the construction effort required for installing nozzle boxes. Preferably, the boards are heated in the front part of the dryer using a recirculation process. Due to the high humidity of the boards, the dew point is preferably between 60 and 99 °C, most preferably between 75 and 90 °C.

[0028] The heat supplied to the panels in stage A is preferably generated at least partially by a heat recovery means, in particular by a heat pump. At least some of the zones of stage A are additionally equipped with heaters, in particular the front zones of stage A. Alternatively, stage A can also be heated entirely with the heaters installed within it.

[0029] Due to the extensive heating of the first stage, high humidity is generated; this means that the dew point rises to a temperature of 75°C to 99°C, with the dew point preferably being between 75°C and 90°C. The humidity is between 150 and 750 g per kilogram of air, preferably between 200 and 600 g per kilogram of air.

[0030] To simplify the construction of Stage A, it is also possible to mount the zones of Stage A directly on the floor of a production hall without a separate dryer floor. This allows Stage A to be constructed using a lightweight structure; if Stage A consists of a large number of modules arranged one behind the other in the direction of production, these modules can be easily installed and removed, for example, to repair or replace the fans installed in the zones, especially in their ceiling areas.

[0031] While the nozzle boxes enable good drying results for the boards in a short period of time, they require a high energy input to generate the required airflow. On the other hand, the nozzle boxes also take up considerable space within stage A, which typically comprises several sections, with the nozzle boxes in each section arranged one above the other, corresponding to the number of levels in stage A.

[0032] According to the invention, a design of stage A with longitudinal ventilation is possible, as is the use of nozzle boxes.

[0033] Each stage is divided into fields or sections. In both stages A and B, for example, a drying facility with a conveyor system comprising a plurality of fields or sections extending one behind the other in the conveying direction is provided for conveying continuous panels to be dried, in a plurality of levels per field, with conveyor devices arranged as roller conveyors in the levels. Each field preferably has its own conveyor chain for the panels; however, the panels can also be driven in a plurality of fields by a conveyor chain common to these fields, with one conveyor chain driving the panels across five fields, for example.

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

[0035] In both stages A and B of the dryer, a drive system is used that is adapted to a low-temperature dryer and to the large number of boards, in particular plasterboards, that are processed simultaneously in the low-temperature dryer over a large number of levels, for example, sixteen to sixty levels, in particular in twenty to fifty levels, particularly preferably in thirty to forty levels. By using a large number of levels in conjunction with the drive system according to the invention, a longer residence time of the boards, in particular the plasterboards, in a low-temperature dryer can be achieved with the same output of boards as in a high-temperature dryer, while maintaining the same dryer length as in a high-temperature dryer.

[0036] It is thus shown that particularly large energy savings are possible when the dryer arrangements according to the invention are installed in an existing plant.

[0037] In order to optimise the use of the waste heat generated in particular in stage A, the plates in the first stage A and / or in the second stage B are dried by at least one external heat exchanger and / or alternatively by an internal heat exchanger.

[0038] It is also advantageous if the panels are heated in the first stage A by circulating air from a burner directly, by hot steam or thermal oil, or indirectly electrically, or by low-calorific heat. In stage B, the panels are heated by low-calorific heat, which either comes from heat recovery from stage A or from another process that releases heat at low temperatures, for example, from a combined heat and power plant or a heat pump.

[0039] In the first stage (A), the boards are preferably dried in air with a temperature between 90 and 160°C, especially between 120 and 140°C. Selecting low temperatures allows for gentle drying of the boards. No gypsum anhydrite is formed in the boards.

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

[0041] The drying zones of the first stage A have either cross ventilation or, alternatively or additionally, longitudinal ventilation. If high temperatures are to be achieved, stage A is, in a preferred embodiment, heated at least partially indirectly via a heat pump, for example, to 50%. Alternatively, the zones of stage A are each heated by burners or indirectly.

[0042] In the second stage B, the panels are dried by drying air at a temperature of 20 to 90 °C, preferably between 30 and 90 °C.

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

[0044] An even higher efficiency of the drying process according to the invention can be achieved if the panels are first dried in a pre-drying stage prior to the first stage A, then in the first stage A and finally in the second stage B.

[0045] Preferably, the panels in stages A and B are conveyed through sections by means of separate conveyors for each stage A and B and / or each section. Alternatively, the conveyors are each driven by direct-drive motors, or they are at least partially interconnected using gears.

[0046] The invention also provides a dryer for drying boards in a first and a second stage A, B, each of which is equipped with a conveyor device for conveying the boards arranged in tiers through the dryer, wherein the first stage (A) has at least one zone, wherein the first stage A has a supply device, a discharge device and a recirculation duct with conveying means and a heating device for recirculation air, as well as means for supplying supply air and means for discharging exhaust air, and wherein the second stage B is equipped to receive the boards from the first stage A and is equipped with a supply device for drying air and a discharge device for drying air; alternatively, a heating device can also be provided in the second stage.

[0047] In particular, high flow rates for air circulation are avoided in the second stage, so that this dryer has only a low consumption of secondary energy.

[0048] Preferably, the dryer housing is equipped with a door for each drying stage A and B. Preferably, the dryer, especially in stage B, does not have its own floor, but is installed on the screed of a factory hall.

[0049] Advantageously, a dryer in which the first and second stages A, B each comprise at least one section or zone is equipped with means for the flow of circulating air transversely to the conveying direction of the plates, in particular in the first stage A.

[0050] For structural reasons, the first stage A of the dryer is preferably divided into several sections, which are at least partially equipped with cross-ventilation devices. In particular, fans are provided in the front zones of stage A, especially in the ceiling areas. However, the fans can also be mounted on the outside, particularly above the upper covers of the zones, if the recirculated air they generate is subsequently directed into the interior of the zones. Air guidance elements are preferably provided within the zones for this purpose. For example, cross-ventilation is achieved by means of an impingement flow of hot air.

[0051] Preferably, in stage B, i.e., in the rear stage, only longitudinal ventilation is provided, which does not preclude the provision of additional and / or exclusive means for cross ventilation in stage B. In stage B, the boards have a temperature of between 30°C and 90°C; during drying of the boards in stage B, the air humidity there is between 5 and 30 g / kg, i.e., per kilogram of air. Advantageously, the second stage B of the dryer is equipped with means for the flow of circulating air against and / or in the conveying direction of the boards.

[0052] In a further advantageous embodiment of the dryer, the second stage B is equipped with guide means for helically guiding the recirculating air or with at least one exhaust fan. Alternatively, guide means, for example in the form of baffles, are provided.

[0053] Roller conveyors or conveyor belts are preferably provided as conveying devices for transporting the panels to be dried in the dryer.

[0054] By also utilising the condensation heat, which is made possible by the low temperature of the drying air cooling the heat exchanger and the at least medium humidity of the exhaust air of stage A, the primary energy is used intensively.

[0055] When the drying air is directed countercurrently to the exhaust air passing through the heat exchanger in stage A, cooler drying air encounters already cooled exhaust air. This ensures the greatest possible condensation of the water vapor contained in the exhaust air and further improves the utilization of primary energy. The more intensive use of primary energy leads to significant savings in primary energy.

[0056] Overall, drying performance in stage B is at most 60% of the drying performance of stage A.

[0057] Each stage A, B is equipped with a conveyor device for conveying boards arranged in tiers through the dryer. The dryer can be designed as a roller conveyor dryer or a belt dryer, with the conveyor device comprising several roller conveyors or conveyor belts arranged one above the other. The tiers are spaced apart from each other at a distance of 100 mm to 150 mm, preferably 130 mm.

[0058] For additional transient loads, additional heating devices can be installed in stage B.

[0059] The invention is explained in more detail below using an exemplary embodiment. The single figure shows a dryer with two stages A and B and a heat pump.

[0060] A dryer 1 (figure) comprises two stages A and B for drying boards, which are fed to the dryer 1 via a conveyor device 100 such as a conveyor belt in the direction of an arrow C. These boards are in particular building material boards, for example plasterboard or gypsum wallboard.

[0061] Each of the two stages A and B is preferably divided into sections or zones 2. At least some zones 2, in particular the front zones 2, of stage A are each equipped with recirculation fans 15 for generating a flow transverse to the conveying direction of the plates.

[0062] Preferably, stage A has a sealing section 3 on the inlet side. The sealing section 3 is supplied by a heat pump 4 or a heat exchanger 4 with fresh air heated therein via a supply line 6 equipped with a closable flap 5. This fresh air supply serves not only to heat the panels but also to seal stage A against other air currents and the penetration of outside air into stage A.

[0063] Fresh air heated by a fan 8 is distributed via a line 7 branching off from the supply line 6 and distributed via a further line 80 to individual lines 9, 10, 11, 12, 13, 14. From these, the fresh air reaches heating devices 15 installed in some of the zones 2, which are arranged, for example, in a ceiling box above the levels in which the panels are conveyed. The heating devices 15 are preferably direct heating devices such as burners or indirect heating devices such as steam or electric heaters. Within the zones 2 or jointly for several zones 2, at least one recirculation fan 16 is provided to generate a cross-flow of the heated air in the zones 2 as recirculation air. Alternatively, two recirculation fans 16 are arranged for each zone 2.The moisture-enriched air from zones 2 is returned to the heat exchanger 4 via outlets 38, where the moisture condenses.

[0064] From stage A, the pre-dried panels are transported to stage B, which is designed as a longitudinal dryer.

[0065] Stage B is also supplied with heated fresh air from heat exchanger 4. Lines 19 to 26 serve this purpose.

[0066] Fans can also be arranged in lines 19 to 26. At the inlet of section 2 of stage B, the air flowing into section 2 from lines 19 to 26 is heated by heating devices 29 to 31. Heating devices 29 to 31 are switched on when additional heating energy is required; this is the case when the system is started up, when insufficient heat is yet available from stage A and heat exchanger 4 is not yet receiving any warm exhaust air or is not receiving enough warm exhaust air from stage A. The heating devices are also required when the system is shut down and insufficient warm air is no longer available from stage A to enter stage B.Heating devices 29 to 31 can also be used in the event that the panels to be dried have a higher moisture content than expected, as well as when switching between different panel formats, which can lead to a lack of thermal energy in stage B. Thus, heating devices 29 to 31 are provided especially for transient loads in stage B.

[0067] It is understood that, depending on the length of stage B, a plurality of ducts can be provided for supplying air, particularly warm air from heat exchanger 4 or from another heat exchanger, in order to recover the evaporation enthalpy of the water evaporated from the plates. As a rule, recirculation fans can be omitted in stage B; if such recirculation fans must nevertheless be provided, they are constructed and arranged like the recirculation fans in stage A. Both radial and axial fans are possible.

[0068] Just like the recirculation fans, exhaust fans 32 to 35 are also arranged along the entire length of stage B. Humid air from stage B is removed via these and chimneys 36 to 39.

[0069] In both stage A and stage B, additional internal heat exchangers can be provided, for example above the nozzle boxes in stage A in a ceiling box or above the conveying device in stage B, also in a ceiling box provided for this purpose.

[0070] The heat exchanger 4 is connected to zones 2 of stage A via exhaust air ducts 38 and a central exhaust air duct 39. Warm, moisture-saturated air is passed through the exhaust air ducts 38, 39 via an exhaust air fan 40 to the heat exchanger 4, where it condenses and releases its moisture as water.

[0071] The heat exchanger 4 draws in fresh air via a fresh air fan 41. It expels used 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.

[0072] Stage B is designed as a longitudinal drying zone; depending on the intensity of the air supply via lines 19 to 26 in relation to the extraction of the used air by fans 32 to 34, the air is directed countercurrently to the conveying direction of the panels, at least in the front area of ​​stage B.

[0073] Since temperatures of 160 °C are preferably not exceeded in the dryer, building boards of any kind, in particular gypsum boards, but also cement boards, are dried very gently and with low energy consumption in the dryer according to the invention, whereby boards of high quality can be produced.

[0074] The high number of tiers, which at the same time have a relatively low height, also enables very efficient drying of the boards, as a large number of boards are dried simultaneously in a small space. Furthermore, if the boards are conveyed side by side in several lanes, production efficiency is further increased.

Claims

Patent claims 1. A method for drying boards in a drying device comprising a first stage (A) and a second stage (B), wherein the two stages (A, B) each have tiers and the boards are each placed on surfaces formed in tiers and are guided through the drying device in the respective tiers of the two stages (A, B), wherein the boards are brought into contact with high-temperature drying air and dried in the first stage (A) and are dried with drying air of a lower temperature in the second stage (B), characterized in that warm air is supplied to the boards in at least one of the two stages (A, B) from a means for recovering heat, wherein the means is arranged outside the two stages and wherein the warm air is supplied directly to at least one stage.

2. A method according to claim 1, characterized in that the plates are heated at least in the first stage by warm air generated by a heat exchanger, by means of a heat pump, by means of a wet separator, by a burner directly or by means of hot steam or by means of thermal oil or electrically indirectly or by means of low-calorific heat.

3. Process according to claim 1 or 2, characterized in that the moisture contained in the air condenses in the first stage (A) at a dew point between 60 °C and 99 °C.

4. A method according to claim 3, characterized in that the moisture contained in the air condenses in the first stage (A) at a dew point between 75 °C and 90 °C.

5. Process according to one of claims 1 to 4, characterized in that the moisture contained in the air in the second stage (B) is between 5 g / kg and 30 g / kg per kilogram of air. Method according to one of claims 1 to 5, characterized in that the boards are dried by circulating air at least in the first stage (A). Method according to one of claims 1 to 6, characterized in that the boards are dried by drying air at a temperature of 120°C to 160°C at least in the first stage (A). Method according to one of claims 1 to 7, characterized in that the boards are dried at least in the region of the first stage (A) at least substantially by the use of nozzle boxes. Method according to one of claims 1 to 8, characterized in that the boards are dried in the second stage (B) by drying air at a temperature of 20 to 90°C. Method according to one of claims 1 to 9, characterized in that the exhaust air from the first stage (A) is passed into a heat exchanger (31) for preheating the drying air of the second stage (B).Method according to one of claims 1 to 10, characterized in that the boards are first dried in a sealing stage (3) or pre-drying stage preceding the first stage (A), then in the first stage (A) and finally in the second stage (B). Dryer for drying boards in a first (A) and a second stage (B), characterized in that at least one of the two stages (A, B) can be supplied with warm air from a means for recovering heat, wherein the means is arranged outside the two stages and wherein the warm air can be supplied directly to the at least one stage. Dryer according to claim 12, characterized in that the means for recovering heat comprises a heat exchanger, a heat pump, a wet separator and / or a burner for directly or indirectly heating the warm air by means of hot steam or by means of thermal oil or by. indirect electrical heating or by means of low-calorific heat, wherein at most a single means for heat recovery is provided. Dryer according to claim 12 or 13, characterized in that the plates can be heated in the first stage (A) by warm air flowing transversely to the conveying direction and in the second stage (B) by warm air flowing longitudinally. Dryer according to one of claims 12 to 14, characterized in that the second stage (B) is equipped with means for the flow of circulating air counter to and / or in the conveying direction of the plates. Dryer according to one of claims 11 to 14, characterized in that a condensate separator (43) is arranged upstream of the at least one heat exchanger (4). Dryer according to one of claims 12 to 16, characterized in that it has at least sixteen levels in which the plates are dried and transported.Dryer according to one of claims 12 to 17, characterized in that the levels have a distance of 150 mm or less from each other.

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