EA041314B1 - METHOD AND DEVICE FOR DRYING GYPSUM BOARD - Google Patents

Description

The present invention relates to a device for drying sheets, in particular gypsum building boards, especially gypsum boards, as well as a method for drying sheets, in particular gypsum building boards, especially gypsum boards.

Gypsum is capable of dehydrating to form a gypsum binder (calcium sulfate hemihydrate) which can subsequently be hydrated and cast, molded or otherwise formed into useful shapes such as slabs. Gypsum is typically prepared for use as a gypsum binder by grinding and calcining at relatively low temperatures of about 120 to 170° C., typically at atmospheric pressure.

In the production of gypsum boards, calcium sulfate hemihydrate (gypsum binder) is mixed with water and optionally additional additives to form a viscous slurry. Said slurry is then cast onto a facing layer, such as a layer of paper, in order to obtain layers of gypsum slurry of a certain thickness. The topmost layer of gypsum slurry is then covered with another cladding layer. A sandwich of two facing layers with layers of gypsum slurry in between then passes a pair of forming plates and/or rollers to form the board. Said structure extends along the conveyor line to allow the gypsum binder to rehydrate and harden. During curing, water reacts with the gypsum binder to produce calcium sulfate dihydrate, causing the gypsum to set by recrystallization. The cured structure is cut to obtain several slabs of the desired length and then transferred to a drying system to allow excess water to evaporate.

In order to be able to process a viscous gypsum slurry, sufficient water must be added to the gypsum binder to adjust the viscosity so that the slurry can be pumped and cast onto the facing layer. The slurry must be sufficiently fluid on the facing layer to be evenly distributed across the entire width of the facing layer. Therefore, the gypsum binder slurry must contain water in a hyperstoichiometric amount. The specified amount of water added to the gypsum binder is in excess of the stoichiometric amount of water required to set the gypsum. Excess water remains in the gypsum layer after setting and said water must be removed by evaporation.

The characteristics of a gypsum board are determined by the characteristics of the gypsum layers and the facing layers attached on opposite sides of the gypsum layer structure. While the gypsum layers impart stiffness and impact strength to said board, the facing layers impart bending strength to the gypsum board. Therefore, it is important to ensure sufficient adhesive strength between the gypsum layer and the facing layers.

Gypsum boards are usually produced in a continuous way on production lines. As described above, in the first step, a gypsum slurry is cast onto a facing layer, and then another facing layer is placed over the topmost wet layer of gypsum. After the gypsum layer has acquired sufficient stability during curing, the composite is cut into slabs of the desired size. The boards are then transported to a dryer to remove excess water. Removal of excess water is usually done by exposing the wet plate to hot air. Hot air flows along the surface of the plate to absorb water vapor. The gypsum board is dried and at the same time the air is cooled by the evaporation of water. Gypsum boards are also to be dried in dryers to evaporate excess water. Thus, in the following, the term gypsum board means gypsum boards and gypsum fiber boards or other gypsum building boards, unless otherwise indicated.

Since drying consumes a large amount of energy, the amount of excess water contained in the gypsum slurry is tried to be kept as low as possible. As an additional measure to reduce energy consumption, efforts are being made to use heat efficiently for drying, for example by recirculating the air used for drying. The air then repeatedly contacts the wet plates and heats up after each contact. The air is getting richer with moisture. Part of the moist drying air circulated is separated and vented to remove excess moisture from the system. The heat contained in the moist exhaust air can be recovered, for example, in a heat exchanger for heating the supply air.

The heat input along the longitudinal direction of the dryer is calculated according to the amount of excess water contained in the gypsum board. Gypsum boards have a thickness in the range of about 6-60 mm. Due to the heat generated during the setting process, the temperature of the gypsum board when it enters the dryer is about 25-45°C. The board is then uniformly heated to a temperature of about 80-100° C., preferably up to about 90-100° C. to accelerate drying. At the beginning of the drying process, the gypsum board contains a large amount of water. Therefore, a large amount of heat can be applied to the wet gypsum board to speed up the drying without the risk of calcining the outer layers of the gypsum board, while calcium sulfate dihydrate is slightly dehydrated to calcium sulfate hemihydrate. During the first stage of the drying process, the hot air typically has a temperature of over 200° C., so it can be used at high mass flow rates. Due to the evaporation of the water, the gypsum board is cooled and the temperature of the gypsum remains below about 100° C. even in the area bordering the facing layers enclosing the gypsum layers. Therefore, adhesion between the gypsum layers and the cladding layers attached to them is ensured.

The adhesion of the cladding layers to the gypsum layer is at least in part due to the growth of calcium sulfate dihydrate crystals in the cladding material. To prevent deterioration of the adhesion between the gypsum and the facing layer, heating of the gypsum layer to temperatures above about 100°C should be avoided. If the temperatures at the gypsum/cladding layer of gypsum boards rise above this threshold, the calcium sulfate dihydrate is converted to a hemihydrate with loss of crystal structure and hence also loss of adhesion between gypsum and cladding.

At a later stage of the drying process, the amount of water still contained in the gypsum layers becomes lower, and therefore the cooling effect due to water evaporation also decreases. In addition, the amount of excess water contained in the central part of the board is higher than in those parts adjacent to the facing layers located on the outside of the gypsum board.

At the end of the drying process, when the gypsum board contains only a small amount of excess water, the heat input must be reduced to avoid overheating the outer layers of the gypsum to form calcium sulfate hemihydrate.

At the end of the drying process, the temperature of the air used for drying is further reduced so that the dry gypsum boards can be removed from the dryer.

The exhaust air accumulated during the drying process still contains a large amount of energy, which is reused in said process to improve efficiency. The heat can be recovered, for example, in a heat exchanger in which the outlet air is directed countercurrently to the cold inlet air, so that the outlet air is cooled and the inlet air is heated. When the exhaust air cools below its dew point, the heat of condensation can also be used to preheat the supply air.

Alternatively, the exhaust air can be reused directly in the drying process. This can be done, for example, by mixing the exhaust air with the supply air and using the mixture as the intake air of the dryer section.

Since high temperature air is used in the early stage of the drying process, the exhaust air still has sufficient drying capacity to be used as a drying medium in a later stage of the drying process. The moisture contained in the drying air increases the heat capacity of the air, so moist air can be used very effectively to dry gypsum boards, as long as the air has sufficient capacity to further absorb moisture.

However, as already discussed, towards the end of the drying process, the temperature of the drying air must be lowered in order to avoid excessive heating of the gypsum with the conversion of calcium sulfate dihydrate to calcium sulfate hemihydrate.

When the temperature of the drying air drops, it must be ensured that it does not drop below the dew point. Condensation of water vapor below the dew point leads to the formation of liquid water. Condensed water can degrade the quality of gypsum boards and in particular cause corrosion of the dryer.

Consequently, in state of the art drying devices, only a part of the exhaust air accumulated in the early stages of the drying process is directly used in a later stage of the drying process, and the remainder, for example, is fed to a heat exchanger to heat the supply air.

The setting and drying of gypsum boards is carried out continuously. Thus, production lines for gypsum boards are quite long and often several hundred meters in length. Therefore, such production lines have rather high space requirements.

US 2012/0246966 A1 describes a method and apparatus for drying sheets. The sheets are guided through a dryer divided into drying chambers and brought into contact with the drying air by means of a falling jet aeration. When aerating with a falling jet, nozzle boxes with transverse aeration are used. The drying step is divided into a main drying step and a final drying step. The exhaust air from the individual drying chambers of the main stage is collected and introduced into the final drying stage. In practice, exhaust air from the main drying stage is introduced into the pressure chamber of one or more drying chambers in the first half of the final drying stage. Part of the recirculated exhaust air is used for drying in the drying chamber and the other part is introduced into the suction chamber of the respective subsequent drying chamber. Exhaust air from one or more drying chambers in the second half of said end

- 2 041314 drying stage is removed at a much lower temperature.

US 6,837,706 B2 describes a dryer for gypsum boards with a feeder comprising a plurality of feeder roller assemblies arranged at levels one above the other. The drying section is generally divided into several zones, in particular three longitudinally ventilated zones: two high temperature zones and a subsequent low temperature zone. Due to the high capacity of the upstream production plant and the long residence time, the dryers are very long. In the high temperature zones above and below the individual roller feed units, there are black plates that run the width of the roller feed units. Said black boards are heated solely by the flowing drying air and transfer additional heat to the drywall board by radiation. Due to the increased heat transfer coefficient, the length of the dryer can be reduced.

US 5,659,975 describes a board drying process and a dryer. To dry the boards, the boards are guided to the racks through a dryer and brought into contact with the drying air in two stages. In stage A with a higher drying capacity, the drying air is supplied at a higher temperature and at least medium humidity, while in another stage B it is supplied at medium temperature and low humidity.

The exhaust air from stage A is fed to stage B through a heat exchanger located in the dryer rack. The drying air in stage B is counter-current through a dryer with low humidity and temperature, so that the boards in stage B are dried by both condensing heat and radiant heat. Thus, only a reduced mass flow of drying air is required to transfer the heat of condensation. Consumption of primary and secondary energy is low.

US 2015/0308739 A1 describes a drying system for drying boards in which the board is transported through a dryer in the longitudinal direction. The rollers are designed to support the plate in a support plane containing the longitudinal direction of the dryer. Air supply means are provided to direct the air flow to the plate surfaces. The air inlet means comprises lower and upper channels extending across the longitudinal direction of the dryer. The lower and upper channels have a plurality of holes for directing the air flow towards the lower and upper sides of the plate, respectively. The air introduced through these openings is aligned with the longitudinal axis of the dryer. This increases the contact time between the air and the underlying boards and hence the drying speed. By having larger channels relative to the space available between adjacent rollers, the size of the gaps between the channels and the plates can be reduced and/or the size of the channel surface opposite the corresponding platen surface can be increased, thus helping to direct the air flow in the longitudinal direction of the dryer.

The annual production of gypsum boards can be several million square meters per production site. Therefore, the energy required to dry wet gypsum boards is quite high, and there is a continuing need to further reduce the energy requirements in the production of gypsum boards. In addition, the creation of a new production line requires significant financial investments.

Therefore, the object of the claimed invention is to provide a device for drying sheets, in particular gypsum boards, as well as a method for drying sheets, in particular gypsum boards, which has high drying efficiency with low energy consumption, and this device preferably has low investment costs and less requirements. to space than the production lines known from the state of the art.

These objectives are achieved by the sheet dryer according to claim 1 and the sheet drying method according to claim 6.

According to a first aspect of the present invention, there is provided a sheet dryer, comprising: a transport means for conveying sheets through the sheet dryer;

a first drying stage located towards the forward end of the sheet dryer and comprising at least one drying chamber;

first stage drying air supply means for introducing hot air into said at least one drying chamber of said first drying stage via at least one drying air inlet;

venting means for venting exhaust air from said at least one drying chamber of said first drying stage;

a second drying stage located behind the first drying stage and containing at least one drying chamber;

moving means for moving exhaust air discharged from said at least one first stage drying chamber to said at least one second drying stage drying chamber;

moist drying air supply means for introducing said exhaust air into

- 3 041314 said at least one drying chamber of said second drying stage, wherein said moist drying air supply means comprises at least one moist drying air inlet for introducing moist drying air into the upstream position of the second drying stage;

additional air supply means for introducing additional air into said second drying stage through at least one additional air inlet located downstream of said moist drying air inlet.

In the sheet dryer according to the present invention, the vent gases vented from the first drying stage and still at a rather high temperature are introduced into the second drying stage to be used for further drying of the sheets. The moisture contained in these exhaust gases is due to the evaporation of water from the sheets in the first drying stage. Higher humidity of the drying air results in higher drying efficiency.

In the sheet dryer according to the invention, additional air supply means are provided for introducing additional air into the second drying stage through at least one additional air inlet located downstream of the moist drying air inlet. The additional air mixes with the moist drying air containing exhaust air moved from the first drying stage and introduced into the second drying stage to the upstream position of the second drying stage and then advanced to the downstream position. In the forward part of the second drying stage, moist drying air takes additional moisture from the sheets, in particular gypsum boards, previously dried in the first drying stage and moves through the second drying stage. The absorption of moisture by the moist drying air simultaneously cools the moist drying air, which leads to an increase in its dew point. To avoid overheating of the sheets, the drying temperature, which is equivalent to the temperature of moist drying air, must be lowered towards the downstream end of the second drying stage while the sheets are advancing upstream in the second drying stage. The amount of moisture contained in the sheet is reduced, and therefore the cooling effect applied to the sheets by water evaporation is also reduced. Therefore, the temperature of the air and the temperature of the sheets moving through the second drying stage converge. To avoid calcination of calcium sulfate dihydrate to form calcium sulfate hemihydrate or anhydrite, the temperature of the sheets should be maintained below a threshold value. When drying gypsum boards, the temperature must be kept below about 100°C to avoid scorching. By adding additional air to the flow of moist drying air in the rearward position of the second drying stage, the dew point of the moist drying air can be lowered and, if necessary, the moist drying air can be cooled by appropriate selection of the temperature of the additional air. Thus, condensation of liquid water and excessive heating of the sheets is prevented.

Preferably, the additional air inlet is located at a rearward position of at least 30%, preferably at least 40%, of the total length of the second drying stage.

Since the temperature and dew point of the moist drying air can be controlled in the upstream position of the second drying stage by adding additional air, a high mass flow rate, preferably all of the exhaust air from the first drying stage, can be used. The exhaust air has a high energy content. Due to the high temperature and/or high moisture content, which can be reused without the risk of water condensing towards the end of the second drying stage or the risk of excessive heating of the gypsum board in the final phase of the drying process, which can initiate the conversion of calcium sulfate dihydrate to calcium sulfate hemihydrate . The temperature of the supplementary air can be adjusted appropriately to cool the humid drying air containing the exhaust air from the first stage to a level that avoids the calcium dihydrate becoming a hemihydrate and lowers the dew point of the drying medium (by diluting it with the dry supplementary air) enough to avoid water condensation.

By hot air is meant the air introduced into the drying chamber of the first drying stage for drying the sheets in the first drying stage.

It is assumed that the exhaust air exhausted from the first drying stage is at least part of the drying air accumulated during the drying of sheets, in particular gypsum sheets, in the first drying stage and exhausted from the first drying stage. The exhaust air exhausted from the first drying stage may be formed by exhaust air exhausted from one or more drying chambers forming the first drying stage or forming part of the first drying stage. In particular, in an embodiment in which the first drying stage comprises more than one drying chamber, the drying chambers may be operated at different temperatures. The exhaust air taken from the first drying stage can then, according to an embodiment, be formed by only a part of the exhaust gas taken from specific drying chambers,

- 4 041314 for example from drying chambers, where the sheets are dried at a higher temperature than in other drying chambers of the first stage of drying. According to another embodiment, the exhaust air exhausted from the first drying stage is formed by the combined exhaust air taken from all the drying chambers of the first drying stage.

Moist drying air is to be understood as the air used for drying sheets in the second drying stage, in particular gypsum boards. The moist drying air is at least partially formed by the exhaust gas vented from the first drying stage. The moist drying air contains the moisture collected in the first drying stage. The moist drying air may be formed entirely from exhaust gas from the first drying stage, or may additionally contain air from other sources. Upon entering the second drying stage, the moist drying air may be at a temperature provided by the exhaust air from the first drying stage, or may be at a higher temperature, for example by heating the exhaust air from the first drying stage in a heating device.

By additional air is meant (outside) air which contains less water than the moist drying air introduced into the second drying stage.

Sheets, in particular gypsum boards, are conveyed through the sheet dryer according to the invention. Sheets are transported in the longitudinal direction. Longitudinal direction refers to a direction corresponding to the direction of advancement of the sheets through the sheet dryer. The sheet dryer typically has the largest extension in the longitudinal direction, with sheets being fed into the sheet dryer at one end of the device and sheets exiting the device at the opposite end.

By transverse or cross direction is meant a direction orthogonal to the longitudinal direction and parallel to the plane of the sheets.

The vertical direction refers to a direction orthogonal to the longitudinal direction and orthogonal to the plane of the sheets.

Under the plane of the sheets understand the plane of the sheets, having the greatest elongation. The sheet thickness is the extension of the sheet in a direction orthogonal to the plane of the sheets. The sheets are evenly shaped with flat surfaces on opposite sides of the sheet.

The sheets enter the sheet dryer at one end of the sheet dryer and exit the sheet dryer at the opposite end. By upstream is meant the location in the sheet dryer closest to the end of the device where the sheets enter the sheet dryer. By backflow is meant the location in the sheet dryer closest to the end of the sheet dryer where the sheets leave the sheet dryer.

The sheet dryer comprises a transport device for transporting the sheets through the sheet dryer. Transport devices known from the state of the art can be used. Examples of conveying devices are belt conveyors or roller conveyors. The conveying device allows the sheets to be continuously conveyed through the sheet dryer.

The conveying device transports sheets, in particular gypsum boards, through the drying device in the longitudinal direction from the forward upstream end of the dryer to the downstream end of the dryer. At the upstream end of the dryer, the sheets are conveyed from the sheet making machine, in particular the gypsum board forming machine, into the dryer. In such a sheet making apparatus, sheets are formed and, according to an embodiment, cut to a certain size. The sheet making apparatus and the sheet drying apparatus may together form production units in a continuous production line for the production of, for example, gypsum boards.

The transport device can be designed to transport the sheets one by one through the dryer in one tier. According to an embodiment, the conveying device comprises several tiers arranged one above the other. The sheets can then be transported through the dryer in several tiers at the same time.

The productivity of the sheet making apparatus is generally quite high. Since the drying of the sheets takes time to reach the desired moisture level in the sheets, the use of a multi-deck dryer makes it possible to increase the capacity of the dryer when space is limited and to adjust the capacity of the dryer according to the capacity of the sheet making machine.

According to an embodiment, the number of tiers constituting the dryer is selected in the range from 4 to 16, according to a further embodiment, in the range from 8 to 12.

According to an embodiment, each tier is provided with a transport device for continuously transporting the sheets through the dryer.

The width of the conveying device is selected to allow the sheets to be transported through the first and second stage drying chambers. According to option

- 5 041314 width of the transport device is selected in the range from 2 to 6 m, according to a further embodiment in the range from 3 to 5 m. for drying, or the width of the roller used to transport the sheets. Then, several sheets to be transported through the dryer can be arranged side by side in the transverse direction of the conveyor.

The first stage of drying is located towards the forward end of the device for drying sheets and contains at least one drying chamber of the first stage.

The sheets enter the dryer by entering at least one first stage drying chamber at the upstream end of the first drying stage.

According to an embodiment, means for preheating the sheets may be provided upstream of the first drying stage. Sheets are received from a sheet maker and must be heated to start evaporating water. A separate device for preheating the sheets may be provided upstream of the first drying chamber of the first drying stage. Of course, the sheet preheater can also be integrated into the sheet dryer.

Sheets, in particular gypsum sheets, are transported in the longitudinal direction through the first drying stage by means of a transport means. In the first stage of drying, part of the water contained in the sheets is removed. Heat is applied to the sheets to remove water. According to an embodiment, heat is supplied by hot air flowing along the surface of the sheets, thereby absorbing moisture.

A first stage hot air supply means is provided for introducing hot air into at least one first drying stage drying chamber and an air exhaust means for exhausting exhaust air from said at least one first drying stage drying chamber. As the hot air inlet of the first stage, hot air introduction means known from the state of the art can be used. According to an embodiment, nozzles are provided to direct a stream of hot air onto the surface of the sheets.

According to an embodiment, hot air inlets are provided above and below the sheet conveyance plane in at least one first drying stage drying chamber. By blowing hot air on both surfaces of a sheet, in particular a gypsum board, it is possible to increase the drying speed and achieve a homogeneous drying progress of the sheet, thereby avoiding cracking or development of uneven surface in the sheet.

According to an embodiment, the hot air inlets for introducing hot air are located at the upstream end of the first drying stage of the sheet dryer, in particular at the upstream end of the first drying chamber of the first drying stage, adjacent to the inlet of the sheets into at least one drying chamber of the first stage drying device for drying sheets. The hot air then moves along the surface of the sheets in a longitudinal direction parallel to the sheet conveying direction through the sheet dryer.

Hot air can only be introduced at one point into the first drying stage through the first hot air inlets. However, according to an embodiment, it is also possible to arrange additional hot air inlets for introducing hot air into the first drying stage at locations downstream of the first hot air inlets.

According to an embodiment, the hot air inlets may be located at an intermediate position between both sides of the constriction means, or on one or both sides of the transport means. According to an embodiment, the air flow may be introduced in a direction parallel to the longitudinal direction. The air flow may be in the same or opposite direction relative to the sheet transport direction.

An air heater may be provided in the first stage drying air supply means for preparing hot air. Air heaters known from the state of the art can be used to heat the air. The air is directed into the air heater by means of an air supply. Typical air heating devices are heat exchangers, in which air moves in countercurrent to the heat carrier and is thereby heated. Such a heat carrier can be, for example, exhaust air having a high water vapor content. According to another embodiment, at least one burner is provided in the first stage drying air supply means, and the air is heated by burning fuel or natural gas. The hot air is then introduced into at least one drying chamber of the first drying stage at the drying air inlet.

Additionally, exhaust air venting means are provided for venting exhaust air from said at least one drying chamber of said first drying stage. After absorption of moisture from the sheets, hot air is removed from at least one drying chamber of the first drying stage as exhaust air. Since the vented exhaust air has a high temperature and may have an additional water absorption capacity, the vented exhaust air can be reused for (further) drying of the sheets.

According to an embodiment, a portion of the exhaust air expelled after advancing along the sheets and after absorbing moisture from the sheets is recirculated and reintroduced into the first drying stage, eg upstream, as recirculated air.

According to an embodiment, separating means are provided for dividing the exhaust air flow discharged from at least one first stage drying chamber into an exhaust part constituting exhaust air and a recirculated part to be re-introduced into the first drying stage in the form of recirculated air. .

According to an embodiment, the separating means are adjustable in order to regulate the discharged portion and the recirculated portion of the discharged air discharged from at least one drying chamber of the first drying stage. Preferably, all exhaust air is recirculated.

According to an embodiment, a recirculation pipe is provided. Said recirculation pipe is connected to a separation means for receiving a recirculated portion of the discharged air discharged from at least one drying chamber of the first drying stage. The recirculation pipe is additionally connected to at least one drying chamber of the first drying stage for re-introduction of recirculated air through the hot air inlet.

Heating means, such as a burner, may be provided in the recirculation pipe to preheat the recirculated air before being reintroduced into the first drying stage. In addition, a driving means, such as a fan or a compressor, may be provided in the recirculation pipe to drive the recirculated air through the recirculation pipe.

The exhaust air discharged from at least one drying chamber of the first drying stage is collected and at least partially moved to the second drying stage by means of a moving means for moving the exhaust air discharged from said at least one drying chamber of the first drying stage, at least one drying chamber of the second drying stage. The exhaust air can be mixed with other air, such as air recirculated in the second drying stage, and form moist drying air, which is used as the drying medium in the second drying stage. The moist drying air is then supplied to the second drying stage at the moist drying air inlet located in the upstream position of the second drying stage.

According to an embodiment, the moist drying air comprises exhaust air collected from the first drying stage, which is then directly introduced into the second drying stage. The transfer means can then be formed by a simple pipe moving the exhaust gas from the first drying stage to the second drying stage.

In accordance with a further embodiment, between the last drying chamber of the first stage, located at the rear end of the first drying stage, and the first drying chamber of the second stage, located at the forward end of the second drying stage, a bypass path can be, for example, an opening used to move sheets between drying chambers. The exhaust air of the first drying stage can then enter the second stage directly by passing from the first stage drying chamber to the second stage drying chamber.

Suction means may be provided to assist in moving exhaust air from the last drying chamber of the first stage, located at the downstream end of the first drying stage, to the second drying stage.

According to yet another embodiment, exhaust air collected from the first drying stage may be heated to a higher temperature before being introduced into the second drying stage. Accordingly, a heater, such as a burner, is provided in the conveying line, and the exhaust air exhausted from said at least one drying chamber of the first drying stage is heated before entering the second drying stage at the wet drying air inlet. The moist drying air inlet is located in the upstream position of the second drying stage.

The second drying stage, containing at least one drying chamber, is located behind the first drying stage.

The second drying stage may be similar in design to the first drying stage or may be of a different design than the first drying stage.

The first stage of drying of the drying device is formed by drying chambers in which exhaust air is collected. The second drying stage is formed by drying chambers in which the exhaust air collected in the first drying stage is used for drying the sheets previously dried in the first drying stage.

At the upstream end of the second drying stage, the sheets that have been partially dried in the first drying stage are introduced and transported longitudinally through the second drying stage, being further dried and exiting at the rear end of the second drying stage, opposite the upstream end. end.

Sheets, in particular gypsum boards, are transported through the second drying stage by means of transport means. The transport means are preferably designed in such a way that a continuous movement of the sheets from the first drying stage to the second drying stage is achieved.

In the second drying stage, the partially dried sheets, in particular gypsum boards, are further dried to remove residual moisture still contained in the sheets after the first drying stage. Since the amount of water still contained in the sheets becomes smaller, in particular towards the end of the drying process, care must be taken not to overheat the sheets. Towards the end of the second drying stage, the sheets may be cooled before exiting the second drying stage.

The second drying stage is provided with at least one moist drying air inlet for introducing moist drying air containing exhaust air vented in the first drying stage.

The moist drying air inlet is located in the forward position of at least one drying chamber of the second drying stage.

The second stage venting means is provided in at least one drying chamber of the second drying stage for removing moist drying air after passing through at least one drying chamber of the second drying stage.

According to an embodiment, the moist drying air inlet and the second stage vent are provided at or near opposite ends of the at least one drying chamber. According to embodiments, the moist drying air inlet and the second stage exhaust means may be located at the upstream and downstream ends of at least one drying chamber to provide longitudinal air flow for the moist drying air, or may be located transversely. away from the conveying direction, creating a transverse flow of moist drying air.

In accordance with the present invention, additional air supply means is provided for introducing additional air into said second drying stage at an additional air inlet located downstream of said moist drying air inlet.

As already explained above, towards the end of the drying process, the amount of free water contained in sheets, in particular gypsum boards, becomes low, and the cooling efficiency caused by water evaporation decreases. In addition, the amount of free water contained in the core of the sheet, in particular the gypsum board, is higher than on or close to the surfaces of the sheet. For evaporation, water must diffuse from the core of the leaf towards the surface of the leaf to then evaporate. In particular, in gypsum boards, water must also pass through the facing layer if it is provided on the outer surfaces of the outer gypsum layer.

By providing additional air supply means downstream of the moist drying air inlet, the temperature profile and humidity profile of the drying air in the longitudinal direction of the second drying stage can be controlled. High temperatures at relatively high humidity levels can be used in the earlier stages of the second drying stage, thereby increasing the efficiency of the drying process. By introducing additional air, the dew point of the moist drying air is lowered and the temperature of the moist drying air can be lowered without initiating liquid water condensation. The temperature and humidity of the additional air can be adjusted accordingly.

According to an embodiment, a heater is provided in the second drying stage for heating additional air. The heater may be, in accordance with an embodiment, a heat exchanger using heat generated in another process step, or may be a burner, in accordance with another embodiment. The burner can use fuel to prepare additional air.

A heater may be provided in the supply line to introduce additional air into the second drying stage. In the supply line, supply air or outside air can be fed into the heater to provide additional air. The additional air is then fed into the second drying stage.

Additional air can be supplied directly to the drying chamber of the second drying stage.

According to another embodiment, a second stage recirculation line is provided for recirculating moist drying air within the second drying stage, preferably to the last drying chambers. When passing through the drying chamber of the second stage, the container

- 8 041314 is not fully used for moisture adsorption by moist drying air. Therefore, at least part of the moist drying air can be recirculated and fed back into the second stage drying chambers.

According to an embodiment, a burner is provided in the second stage recirculation line. The burner heats the recirculated moist drying air before the moist drying air enters the second stage drying chamber again.

According to a further embodiment, means may be provided in the second stage recirculation line to accelerate the flow of moist drying air in the second stage recirculation line, such as a fan or compressor.

To remove moisture from the dryer, a second stage separator may be provided in the second stage recirculation line, and part of the moist drying air flowing in the second stage recirculation line is recirculated to the drying chamber, and the other part of the moist drying air is removed from the dryer.

In order to reuse the heat contained in the moist drying air removed from the dryer, a heat exchanger may be provided in a vent line to remove the moist drying air removed from the dryer.

According to an embodiment, an additional air inlet is provided in the second stage recirculation line. The additional air then mixes intensively with the moist drying air before re-entering the second stage drying chamber and contacting the sheets. In this way a homogeneous drying of the sheets is achieved.

In accordance with a further embodiment, an additional air inlet is provided in the recirculation line of the second stage, located downstream of the heating device for heating the recirculated moist drying air, in particular the burner.

According to an embodiment, the second drying stage comprises more than one drying chamber. The sheets are then sequentially passed through the drying chambers, which are arranged one after the other. Holes are provided in the walls of the drying chambers of the second stage, which make it possible to move sheets from one drying chamber to an adjacent drying chamber. Preferably, said openings are sized such that only limited amounts of humid drying air used in a particular drying chamber for drying are transferred to an adjacent drying chamber, and the exchange of moist drying air between adjacent drying chambers is kept to a minimum.

The direction of air flow in the drying chambers of the first and second drying stages can be chosen either longitudinal or transverse. In the drying chambers of the first and second drying stages, the direction of the air flow may be the same or may differ.

The longitudinal airflow refers to the airflow in the longitudinal direction of the sheet dryer. The air flow may be directed in the sheet conveying direction or may be opposite to the sheet conveying direction. For longitudinal air flow, the air supply means and the air exhaust means are located, according to an embodiment, at or near opposite ends of the drying chamber in the longitudinal direction. In an embodiment in which the first and/or second drying stage comprises more than one drying chamber, the direction of the longitudinal air flow can be selected individually for each drying chamber. Therefore, according to an embodiment, the longitudinal air flows in several drying chambers can have different orientations. While some of the dryers have a longitudinal airflow in the sheet transport direction, others may have a longitudinal airflow in the opposite direction to the sheet transport direction.

The location of the air supply means for introducing air and the air outlet means for removing air depends on the desired direction of air flow. If the direction of the longitudinal air flow is directed in the sheet transport direction, then the air supply means are arranged upstream of the air outlet means with respect to the sheet transport direction. If the direction of the longitudinal air flow is opposite to that of the sheet transport direction, then the air supply means are located downstream of the air outlet means relative to the sheet transport direction.

By transverse air flow is meant the air flow in a direction perpendicular to the direction of transport of the sheets, preferably parallel to the main surface of the sheets. Cross-flow air can be created by placing the air supply means and the air outlet means on opposite sides of the drying chambers at or near the side ends of the sheets. According to another embodiment, one of the air supply means and the air exhaust means is located at a location on or near the center line of the sheets extending in the longitudinal direction, and the other is located at or near one or both sides of the sheets.

Accordingly, in the first drying stage in the cross-flow embodiment

- 9 041314 the air supply means corresponds to the first stage drying air supply means for introducing hot air, and the air exhaust means corresponds to the first stage air exhaust means for venting the discharged air. In the second drying stage, the air supply means corresponds to the moist drying air supply, and the air exhaust means corresponds to the air exhaust means of the second stage.

According to an embodiment in which the first and/or second drying stage is formed with cross-ventilated drying chambers, the drying stage preferably comprises more than one drying chamber. The drying conditions in each of the drying chambers can be adjusted individually. The temperature profile can then be set in the longitudinal direction of the drying stage, so that the drying temperature can be adjusted according to the amount of water present in the sheets, in particular gypsum boards.

According to an embodiment, the first drying stage comprises a plurality of cross-ventilated drying chambers. Accordingly, at least one drying chamber of the first drying stage is a transversely ventilated drying chamber. According to an embodiment, all first stage drying chambers are cross-ventilated drying chambers. Cross-ventilated dryers are relatively short and are generally sized to take the width of the transport and the sheets transported on the transport.

The individual transversely ventilated drying chambers are arranged one behind the other in a transverse direction with respect to the longitudinal direction of transport of the sheets.

According to an embodiment, the drying chambers of the first drying stage can be individually heated and aerated.

In the first drying stage, the temperature profile can be set by individually adjusting the drying air temperature as well as the drying air flow rate for each drying chamber.

At the beginning of the drying process, sheets, in particular gypsum boards or gypsum boards, have a high water content, and efficient cooling of the sheets is achieved by evaporation of the water. High drying rates can be achieved using high temperature drying air and/or high flow rates.

According to an embodiment, the first drying stage comprises more than two drying chambers, according to an embodiment 1-4 or more than 5 drying chambers, and according to a further embodiment more than 6 drying chambers. According to a further embodiment, the first drying stage comprises less than 30 drying chambers, and according to a further embodiment, less than 20 drying chambers.

According to a further embodiment, a pre-drying zone is provided upstream of the first drying stage. In such a pre-drying zone, the freshly prepared gypsum board is heated to a temperature in the range of 50 to 100° C. before entering the first drying stage, where the high temperature air, i. above 100°C, used to evaporate water from damp gypsum board.

According to an embodiment, the second drying stage comprises at least one drying chamber which is ventilated in the longitudinal direction. Accordingly, at least one drying chamber of the second drying stage is a longitudinally ventilated drying chamber.

According to an embodiment, the second drying stage comprises more than 2 longitudinally ventilated drying chambers arranged one behind the other. According to an embodiment, the second drying stage comprises less than 5, according to a further embodiment, less than 4 longitudinally ventilated drying chambers.

In accordance with a further embodiment, all the drying chambers of the second drying stage are longitudinally ventilated drying chambers.

According to a further embodiment, the second drying stage comprises longitudinally as well as transversely ventilated drying chambers.

By aligning the air flow with the longitudinal axis of the drying chamber, the contact time between the air and the plates can be increased, thereby increasing the drying speed.

According to an embodiment, the longitudinally ventilated drying chambers of the second drying stage are individually heated and aerated. A longitudinal temperature gradient can be set.

According to an embodiment, control devices are provided for adjusting temperature and air flow.

According to an embodiment of the invention, an additional air inlet is provided in the second drying stage. Additional air can then be introduced into the second stage drying chamber to lower the dew point of the drying air and adjust the temperature of the air present in the drying chamber.

- 10 041314

An additional air inlet according to an embodiment is provided in at least one drying chamber of the second drying stage downstream of the moist drying air inlet, and the additional air can be introduced directly into the drying chamber. Nozzles may be provided in the drying chamber to blow additional air into the air stream circulating in the drying chamber. With regard to the longitudinal dimension of the second drying stage, the position of the additional air inlet corresponds to the embodiment selected in the second half of the second drying stage. In the forward half of the second drying stage, the sheets are further dried to remove most of the moisture still present in the sheets.

According to an embodiment, at least one longitudinally ventilated drying chamber of the second drying stage is provided with a second stage recirculation line, and an additional air inlet is provided in the second stage recirculation line. The second stage recirculation line is connected to the second stage drying chamber at the upstream and downstream ends. According to an embodiment, means are provided in the second stage recirculation line to accelerate the air flow, such as a fan or compressor, to pressurize the recirculated air into at least one second stage drying chamber. According to an embodiment, a heating device is provided in the second stage recirculation line, such as a direct fired burner system or indirectly heated systems, such as thermal oil or steam, preferably in a position downstream of the supplementary air inlet.

According to an embodiment, the second drying stage has at least two drying chambers and an additional air inlet is located in the last drying chamber in the downstream direction.

According to an embodiment, a cooling section is provided downstream of the last drying chamber of the second drying stage. In the cooling section, the dried sheets are cooled to about room temperature and then removed from the dryer.

In accordance with a further aspect of the invention, a method for drying sheets is described. Said method can be carried out with a dryer as described above.

In accordance with the present invention, there is provided a method for drying sheets, wherein a wet sheet is introduced into a sheet dryer (as described above), the wet sheet is conveyed through a first drying stage, and hot air is introduced into at least one drying chamber to contact the wet sheet, and for evaporating moisture from the wet sheet to obtain a partially dried sheet;

exhaust air is withdrawn from said at least one first-stage drying chamber, and said exhaust air is collected from said at least one first-stage drying chamber;

transporting the partially dried sheet through a second drying stage which includes at least one drying chamber;

include at least part of the discharged air collected from said at least one drying chamber of the first drying stage into the composition of moist drying air, and said moist drying air is introduced into at least one drying chamber of the second drying stage in the forward position of the second drying stage ;

introducing additional air into at least one drying chamber of the second drying stage at a rearward position from the forward forward position to introduce moist drying air to obtain a dried sheet; and removing the dried sheet from the sheet dryer.

According to the method of the invention, the wet sheet is partially dried in the first drying stage, thereby removing some of the (excess) water contained in the sheet after it has been made. Hot air is supplied to the sheet for drying, and after absorbing water from the wet sheet, the exhaust air is removed from the first drying stage. The exhaust air obtained in the first drying stage is collected and used, at least in part, in the second drying stage for drying the partially dried sheets obtained from the first drying stage.

The exhaust air collected in the first drying stage is still at a high temperature and, due to the humidity level, has a high heat capacity and therefore has a high drying capacity. It is therefore very advantageous if a high proportion of exhaust air contained in the moist drying air in the second drying stage is selected. According to the embodiment, the amount of exhaust air contained in the moist drying air at the place where the humid drying air is introduced into the second stage drying chamber is selected to be greater than 10% by volume, according to the embodiment, greater than 30% by volume is selected. According to an embodiment, the amount of vented gas contained in the moist drying air is less than 50% vol., according to a further embodiment, less than 40% vol., as determined, for example, by means of an anemometer or by means of measurements

- 11 041314 differential pressure. The amount of humid air introduced into the second stage drying chamber depends on the temperature and moisture content required in that drying stage. For example, a temperature of less than 160° C. allows a moisture content of less than 300 g/kg dry air, preferably less than 200 g/kg dry air.

In addition, it is advantageous to use humid, high temperature drying air to improve drying efficiency. The temperature of the moist drying air can be selected to be high if there is enough water in the sheets to allow effective cooling when the sheet material is temperature sensitive.

However, in particular when the longitudinal airflow is used in the second drying stage and there is a high water content in the airflow due to the use of exhaust air from the first drying stage, condensation of liquid water may occur towards the end of the drying process when the sheets must be cooled to removal from the dryer. In addition, since the amount of residual water present in the sheets is low towards the end of the drying process, the cooling efficiency due to water evaporation is also low. Therefore, overheating easily occurs, in particular, when a high level of humidity is used in the moist drying air.

In accordance with the present invention, additional air is added in the upstream position of the second drying stage, preferably in the last drying phase, to lower the dew point of the moist drying air stream used upstream of the second drying stage for drying partially dried sheets.

Therefore, the addition of additional air prevents liquid water from condensing when the temperature of the air used for drying in the second drying stage decreases in the second drying stage at the end of the drying process. In addition, by suitably adjusting the temperature of the additional air, a high temperature moist drying air flow can be used to dry partially dried sheets in the upstream second drying stage without the risk of excessive heating of the sheets in the downstream second drying stage. This is of particular importance when this process is used to dry gypsum boards, since the calcium sulfate dihydrate should be avoided when drying gypsum boards. The adhesion of the facing layers to the gypsum layers placed between them deteriorates if calcium sulfate dihydrate is converted to calcium sulfate hemihydrate or even anhydrite.

The wet sheet to be dried by the process of the present invention is obtained from a sheet production unit. The wet sheet is produced by known methods. According to an embodiment in which the sheet is a gypsum board, the gypsum board is formed by known methods by applying an aqueous slurry of gypsum binder to a facing layer and then covering the top of the formed layer of gypsum binder slurry with another facing layer. The facing layer is made of materials known from the state of the art. Typical materials for the facing layer are cardboard, synthetic materials in the form of foil or cloth, fiberglass. Other materials for the facing layer may also be used.

The thickness of the sheet, in particular the sandwich, consisting of facing layers with layers of gypsum located between them, is regulated by known methods. The boards may preferably be thicker than 6 mm and thinner than 60 mm, preferably thinner than 30 mm.

In general, sheets can be of virtually any size and thickness as long as the sheets can be processed in a dryer.

When using the method in accordance with this invention for drying gypsum boards, such boards can be of any size and thickness. The size and thickness of drywall boards are limited by the dimensions of the dryer. Gypsum boards that are dried using the method of this invention typically have a thickness of greater than 5 mm, in accordance with an embodiment greater than 6 mm, and in yet another further embodiment greater than 8 mm. According to a further embodiment, the gypsum boards are less than 60 mm thick, according to an embodiment less than 40 mm thick, and according to a further embodiment less than 30 mm thick. However, gypsum boards of greater or lesser thickness can also be dried by the process of the present invention. Typical dimensions of sheets or gypsum boards to be dried using the claimed invention are a width and/or length of more than 1 m, in accordance with an embodiment, more than 1.5 m, and in accordance with a further embodiment, of less than 9 m, preferably from 1.2 to 5 m.

Upon entering the first stage of the dryer, the sheet, in particular the gypsum board, has a free water content of more than 20% by weight, according to another embodiment, more than 30% by weight. According to an embodiment, the sheet, in particular the gypsum board, has a free water content of less than 40% by weight, according to an embodiment of less than 35% by weight.

Before entering the dryer, the sheet according to the embodiment has a temperature of more than 20°C, according to the embodiment, more than

25°C. According to an embodiment, the sheet has a temperature of less than

50°C, in accordance with the embodiment, less than 45°C.

The wet sheet is dried by passing through a first drying stage and a second drying stage. In the first stage of drying, most of the excess water contained in the leaf evaporates. In the second drying stage, the residual water still contained in the sheet after the first drying stage is removed and the sheet is cooled to room temperature, preferably before exiting the dryer.

By excess water or free water is meant water that is not chemically bound to the compound from which the sheet is made. It can be removed from the sheet by drying at about 100°C by evaporation. When gypsum boards are dried by the process of this invention, the excess water is understood to be water that is not bound in the form of calcium sulfate dihydrate.

In the first and second drying stages, the sheets are exposed to drying air. The drying air flows along the surface of the sheets and absorbs the water evaporated from the sheets. The air flow can be directed in a longitudinal direction with respect to the longitudinal direction of conveying the sheets. The longitudinal air flow can be directed in the sheet transport direction or opposite to the sheet transport direction.

According to another embodiment, the air flow of the drying air is in a transverse direction with respect to the longitudinal direction of transport of the sheets. A transverse airflow may be introduced into the drying chamber on one side of the sheet, then flow over the surface of the sheet and be vented on the opposite side, for example by drawing air laterally from the sheets. However, according to an embodiment, the crossflow of air can also be initiated by blowing drying air over the surface of the sheet at a position between the sides of the sheets, so that the drying air flows towards the sides of the sheets and is to be sucked from both sides of the sheets.

According to an embodiment, in the first and/or second drying stages, a first airflow is provided on the top side of the sheets and a second airflow is provided on the underside of the sheets to increase and match the drying efficiency.

Sheets are dried mainly at atmospheric pressure. In order to initiate air flow direction, a small pressure difference can be provided in the drying chambers so that the air flow is directed from a higher pressure level at the air inlet point to a lower pressure level at the air outlet point of the drying chamber. The pressure difference is provided by blowing in an air flow at one location in the drying chamber and/or extracting an air flow at a location remote from the blowing air flow.

The air flow rate in the drying chambers of the first and second drying stages is adjustable in accordance with the embodiment in the range from 2 to 40 m/s, in accordance with the embodiment in the range from 5 to 30 m/s, preferably from 8 to 22 m/s .

According to an embodiment, the sheets are continuously conveyed through the first and second drying stages. According to an embodiment, the conveying speed is selected in the range from 0.2 to 8.5 m/min, according to a further embodiment, in the range from 1.0 to 5.0 m/min.

The wet sheet is transported through the first drying stage and exposed to hot air. Hot air may be introduced into at least one drying chamber to contact the wet sheet and to evaporate moisture from the wet sheet to form a partially dried sheet.

According to an embodiment, cross-flow air is preferred for the first drying stage.

The average amount of moisture contained in the hot air used for drying in the first drying stage is adjusted according to the embodiment in the range from 200 to 800 g/kg, according to the embodiment in the range from 250 to 500 g/kg (g _water s/kg _{dry air} ).

According to a further embodiment, upon entering the drying chamber of the first drying stage, the temperature of the hot air used for drying in the first drying stage is controlled to be higher than 140°C, according to a further embodiment, to be higher than 170 °C

According to a further embodiment, upon entering the drying chamber, the temperature of the hot air used for drying in the first drying stage is controlled to be lower than 280°C; according to a further embodiment, it is adjusted to be lower than 250°C, and according to another further embodiment, it is adjusted to be lower than 200°C.

According to an embodiment, the first drying stage comprises several drying

- 13 041314 sections formed by drying chambers, and the sheets sequentially pass through the drying chambers/sections of the first stage. This makes it possible to individually adapt the conditions used for drying in each drying chamber/section. In particular, the temperature profile can be set in the longitudinal direction, i.e. in the direction of sheet transport.

According to an embodiment in which the hot air used for drying in the first drying stage passes the sheets in the transverse or longitudinal direction, the hot air in the first drying chamber is controlled to a temperature in the range of 120 to 300°C, according to a further embodiment implementation in the range from 160 to 270°C.

For example, the temperature of the hot air used for drying in the first drying stage increases stepwise from the drying chamber/section to the drying chamber/section in the sheet conveying direction until the maximum temperature is reached.

The maximum temperature according to the embodiment is chosen to be less than 300°C, according to a further embodiment is chosen to be less than 220°C. According to a further embodiment, the maximum temperature is chosen to be at least 110°C.

The temperature rise when moving from one drying chamber/section to the next drying chamber/section according to an embodiment is less than 30°C, according to a further embodiment is less than 20°C, and according to a further embodiment is less than 10°C. In accordance with a further embodiment, the temperature rise when moving from one drying chamber/section to the next drying chamber/section until the maximum temperature is reached is at least 2°C, in accordance with a further embodiment, at least 4°C.

According to a further embodiment, the temperature of the hot air used for drying in the first drying stage is increased by at least 10° C. from the first first stage drying chamber to the second first stage drying chamber.

According to a further embodiment, after the maximum temperature has been reached, the temperature of the hot air decreases in steps as it moves from one drying chamber/section to the next drying chamber/section. Towards the end of the first drying stage, the amount of residual water contained in the sheet becomes lower, and the cooling effect caused by water evaporation decreases. According to an embodiment, the temperature decreases by 1-20° C. when moving from one drying chamber/section to the next drying chamber/section.

The rise or fall in temperature as one moves from one dryer chamber/section to the next dryer chamber/section is controlled according to the amount of moisture present in the sheet and the cooling effect achieved by water evaporation. The temperature rise or fall is controlled so that the temperature of the sheet, in particular close to the surface of the sheet, in particular the temperature at the surface of the sheet, does not exceed a threshold value in order to avoid deterioration of the sheet during drying.

According to the embodiment, the temperature of the hot air used for drying in the first drying stage is controlled so that the temperature of the sheet is below 110°C, according to the embodiment below 105°C. The leaf temperature is measured at the surface of the leaf.

According to a further embodiment, the temperature of the hot air used for drying in the first drying stage is controlled so that the temperature of the core of the sheet is 100° C. or lower. In accordance with the embodiment, the temperature of the core of the sheet is at least 90°C, in accordance with the embodiment is at least 95°C. Efficient and uniform drying of the sheet is achieved within these limits.

In order to efficiently utilize the drying capacity of the drying air, according to an embodiment, at least a portion of the hot air used for drying in the first drying stage is recirculated after being drawn out of the first stage drying chamber. The vented exhaust air re-enters the drying chamber to absorb additional water.

According to a further embodiment, the recirculated drying air is heated before being re-entered into the drying chamber.

In order to remove moisture from the first drying stage, according to an embodiment, part of the hot air used for drying in the first drying stage is removed as exhaust air from the first drying stage after passing through the drying chamber.

The amount of hot air used for drying and discharged from the first stage of drying after passing through the drying chamber as exhaust air is adjusted depending on the capacity of the dryer, the amount of hot air passing through the first stage drying chamber, the temperature of the hot air, the amount of water contained in the sheets and hot air, and other process parameters. According to an embodiment, the amount of output

- 14 041314 air exhausted from the first stage of drying after passing hot air through at least one drying chamber, is selected so that it is at least 10% vol. less than 20% by volume of dryer exhaust air, and according to yet another further embodiment is less than 50vol% of dryer exhaust air as determined by, for example, anemometer or differential pressure measurements.

The exhaust air discharged from the first drying stage after passing through at least one first stage drying chamber, according to an embodiment, has a temperature in the range from 120 to 220°C, according to a further embodiment, has a temperature in the range from 140 to 180° C.

The exhaust air discharged from the first stage of drying contains moisture absorbed during the passage of hot air through at least one drying chamber of the first stage. According to an embodiment, the amount of water contained in the exhaust air removed from the first drying stage is controlled in the range from 200 to 800 g/kg, according to the embodiment, in the range from 250 to 500 g/kg (g _water /kg _{dry air} ).

In accordance with an embodiment, exhaust air exhausted from the dryers of the first drying stage is collected and combined with the combined exhaust air flow of the first stage. The combined exhaust air stream is then split into a portion that is recirculated to the first stage drying chamber(s) and a portion that is removed from the first drying stage as exhaust air and can be used in the second stage as moist drying air or sent to waste.

After passing through the first stage of drying, partially dried sheets are obtained. Sheets, in particular gypsum boards, according to an embodiment, contain water in an amount of 10 to 30% by weight, according to a further embodiment in an amount of 15 to 25% by weight, based on the wet weight of the board.

In accordance with the process of the present invention, the partially dried sheets, after passing through the first drying stage, enter the second drying stage for additional drying of the partially dried sheets and, optionally, for cooling.

To optimize energy efficiency and drying efficiency, at least part of the exhaust air collected from said at least one drying chamber of the first drying stage is included in the moist drying air introduced into at least one drying chamber of the second drying stage in the forward position of the second drying steps.

The exhaust air collected from the first drying stage is used in the moist drying air in the second drying stage. Humid drying air can be formed solely from exhaust air introduced from the first drying stage, or by mixing exhaust air from the first drying stage with drying air obtained from other sources. Such sources may be, for example, air recirculated to the second drying stage, or heated supply air, or air from other sources, such as air obtained from other processes.

The exhaust air obtained from the first drying stage and contained in the moist drying air enters the upstream position of the second drying stage. The forward position is understood to be the position closer to the place where the partially dried sheets are introduced into the second drying stage.

In an embodiment in which the second drying stage comprises more than one drying chamber, the exhaust air/humid air of the first drying stage may be introduced into the drying chamber of the upstream second drying stage. The forward position is understood to mean not the last drying chamber of the second drying stage, when viewed in the longitudinal direction of sheet transport. According to an embodiment, the forward position may be the first drying chamber of the at least two drying chambers of the second drying stage, as viewed in the longitudinal direction of sheet transport.

The exhaust air can be introduced into at least one drying chamber of the second drying stage directly, for example by providing an air flow between the last drying chamber of the first drying stage and the first drying chamber of the second drying stage, or by introducing the exhaust air of the first drying stage directly into the drying chamber, for example, through a nozzle. According to another embodiment, exhaust air from the first drying stage can be indirectly introduced into the second drying stage, for example, by adding exhaust air to the drying air recirculated in the second drying stage through the second stage recirculation line.

In an embodiment in which exhaust air from the first drying stage enters the second drying stage by mixing with recirculated air to obtain moist drying air, the moist drying air enters the second stage drying chamber according to the embodiment at one end of the drying chamber, and after passing through the drying chamber and absorbing water from the partially dried sheets, moist drying air is removed at the opposite end of the second stage drying chamber.

The temperature of the moist drying air at the entrance of at least one drying chamber of the second drying stage in the forward position according to an embodiment is controlled in the range from 120 to 180°C, according to another embodiment, in the range from 130 to 160°C .

The temperature of the moist drying air can be adjusted by suitable measures. In an embodiment in which the temperature is to be raised, the moist drying air is heated, for example, by a heater. Supply air can be added to lower the temperature or heat can be extracted by passing through a heat exchanger.

According to an embodiment, the moist drying air before entering the second stage drying chamber contains moisture in an amount in the range of 200 to 800 g/kg, in accordance with an embodiment in the range of 250 to 500 g/kg (g _water /kg _su x _th air).

In accordance with the method of the present invention, additional air is introduced into at least one drying chamber of the second drying stage at a rearward position from an upstream position to introduce moist drying air to control the dew point and temperature of the moist drying air.

In the process according to the invention, additional air is introduced into the rearward position of the second drying stage. By adding additional air, the dew point of the moist drying air is lowered. Therefore, it is possible to use in the upstream portion of the second drying stage moist drying air for drying having a high water content. Moist drying air having a high water content has a high heat capacity and can therefore be effectively used for drying sheets.

In addition, the moist drying air used to dry the sheets in the upstream section of the second drying stage may be at a high temperature to improve the drying efficiency.

According to an embodiment, the temperature of the supplementary air is chosen to be lower than the temperature of the moist drying air at the location of the supplementary air inlet, i. e. in a position where additional air is blown into the moist drying air. The temperature of the additional air is regulated in accordance with the embodiment in the range from 80 to 180°C, in accordance with the embodiment in the range from 100 to 140°C.

According to a further embodiment, the additional air is first mixed with the recirculated air and then heated to the desired temperature by passing through a heating device such as a burner. The air temperature after passing through the heating device is controlled according to an embodiment in the range from 80 to 180°C, according to a further embodiment in the range from 100 to 140°C.

The amount of moisture contained in the supplementary air is chosen to be lower than the amount of moisture contained in the moist drying air at the location of the supplementary air inlet, i. e. in a position where additional air is blown into the moist drying air. The amount of moisture contained in the additional air is selected in accordance with the embodiment in the range of 10 to 100 g _{of water} /kg _{of air} , in accordance with a further embodiment in the range of 20 to 80 g _{of water} /kg _{of air} .

After drying in the second drying stage, the sheets are optionally cooled and then leave the dryer.

The cooling section is provided according to the embodiment for cooling the sheets after drying. According to an embodiment, the temperature in the cooling section is controlled in the range of 10 to 80°C, in accordance with a further embodiment to a temperature of 20 to 60°C, to then be removed from the dryer. Cooling can be achieved by supplying cold air to the dried sheets in the cooling section.

The invention will be explained in more detail with reference to the accompanying drawings. The figures show:

fig. 1 is a diagram of a sheet dryer according to the present invention;

fig. 2 is a chart showing the temperature of the sheets as they pass through the sheet dryer according to the present invention.

In FIG. 1 shows a diagram of a drying apparatus 1 according to the present invention. Said drying device comprises a first drying stage 2 and a second drying stage 3. A preheating zone 4 is located upstream of the first drying stage 2, and a cooling zone 5 is located downstream of the second drying stage 3.

The first heat exchanger 6 is provided for heating the supply air. The first supply air pipe 7 is connected to the heating part of the first heat exchanger 6, and the supply air is sucked in by the first fan 8. After passing through the heat exchanger 6, the supply air becomes heated to obtain preheated air, the temperature of which is preferably controlled in the range of 80 to 150°C. Said preheated air is introduced into

- 16 041314 preheating zone 4 through pipe 9 for preheated air. After passing through the preheating zone 4, the preheated air is discharged through the preheated air exhaust pipe 10 provided with a second fan 11 for sucking the preheated air from the preheating zone 4.

In order to heat the supply air sucked in through the first supply air pipe 7, the heat exchanger 6 is connected by its cooling part to the outlet line of the first drying stage 2 .

In order to heat the supply air in the heat exchanger 6, the exhaust air obtained in the first drying stage, which has a high humidity as well as a high temperature, is introduced into the heat exchanger 6 through the humid hot exhaust air pipe 12. A third fan is provided in the humid hot exhaust air pipe 12 for blowing the humid hot exhaust air received from the first drying stage 2 into the heat exchanger 6. The humid hot exhaust air received from the first drying stage passes through the heat exchanger 6 in counterflow with the supply air introduced through pipe 7 for supply air. Due to the low temperature of the supply air as well as the preheated air, the humid hot exhaust air taken from the first drying stage is cooled below the dew point of the humid hot exhaust air, and therefore the heat of condensation can also be used to preheat the supply air, which will be be used as preheated air in preheating zone 4.

Sheets, in particular gypsum boards, are introduced into the heating device 1 via a preheating zone 4 . The sheets are heated to a core temperature of about 40-80° C. before entering the first drying stage 2.

The first drying stage 2 contains seven drying chambers 2a-2g, which are aerated in the transverse direction. Hot air inlets and a vent means for venting exhausted air (not shown) are disposed at opposite ends of the laterally aerated drying chambers 2a to 2g to create a lateral airflow.

Fresh air is sucked in by the fourth fan 14 from the environment and introduced into the second heat exchanger 15 through the second pipe 16 for fresh air.

After passing through the second heat exchanger 15, the supply air is heated to a temperature of preferably 80-140° C. and the heated supply air is introduced into the heated supply air pipe 17 .

Each drying chamber 2a-2g is equipped with a burner 18 connected to a pipe 17 for heated supply air. The heated supply air is mixed with fuel, such as natural gas, and the heated supply air is additionally heated during combustion. The hot air obtained from the burner 18 is then introduced into the drying chambers 2a-2g of the first drying stage 2 through hot air inlets (not shown).

After passing through the drying chambers 2a to 2g, the hot air absorbs water from the sheets conveyed through the first drying stage 2 of the drying apparatus 1 and is discharged as exhaust air through the first stage exhaust pipe 19 . Each first stage drying chamber 2a-2g is provided with a separate first stage outlet pipe 19a-19g which is connected to the combined outlet pipe 19.

In the separator 20 provided in the exhaust pipe 19, the exhaust air flow is divided into a part to be sent through the first exhaust pipe 12 to the first heat exchanger 6 to recover the heat contained in the exhaust air, and a part to be recirculated. The proportion of exhaust air directed to the first exhaust pipe 12 and the proportion of exhaust air to be recirculated can be adjusted. According to an embodiment, about 20-50% by volume of the exhaust air is directed to the first exhaust air pipe 12 and the rest of the exhaust air is recirculated.

The first stage recirculation pipe 21 is connected to the separator, and a fifth fan 22 is provided in the recirculation pipe 21 for blowing recirculated exhaust air through the first stage recirculation pipe 21. The first stage recirculation pipe 21 includes separate connecting pipes 21a-21g connected to the burners 18a-18g for introducing recirculated exhaust gas into the burners 18a-18g. The recirculated exhaust air mixes with the heated supply air introduced into the burner 18a-18g through the heated supply air pipe 17 . The recirculated exhaust gas is combusted along with the heated supply air and fuel to provide hot air to be introduced into the drying chambers 2a-2g of the first drying stage 2.

The air flow inside the first drying stage can be adjusted by valves 23.

The last drying chamber 2g located at the downstream end of the first drying stage 2 is connected to the port 24 of the first drying chamber 3a of the second drying stage 3. At port 24, the sheet, after passing through the first drying stage 2, is introduced into the second drying stage 3.

The second stage 3 drying contains three drying chambers 3a-3c of the second stage, sequentially located in the longitudinal direction. Drying chambers 3a-3c of the second stage are aerated in the longitudinal direction.

A sixth fan 25 is connected to port 24, and the fan draws air from port 24 through port 24.

- 17 041314 of the last rear drying chamber 2g of the first drying stage and from the first drying chamber 3a of the second drying stage. The discharged air from the last drying chamber 2g of the first drying stage and the moist drying air from the first drying chamber 3a of the second drying stage are mixed and, after passing through the sixth fan 25, are sent through the moist drying air recirculation pipe 26a. In the moist drying air recirculation pipe 26a, a second stage burner 27a is provided for heating the moist drying air by burning fuel introduced into the moist drying air flow.

The humid drying air splitter 28a is provided in the humid drying air recirculation pipe 26a upstream of the second stage burner 27a. The moist exhaust air is deviated from the humid drying air flow recirculated to the humid drying air recirculation pipe 26a in the humid drying air flow divider 28a, and is directed through the humid exhaust air pipe 29a to the second heat exchanger 15. Since the supply air introduced into the the second heat exchanger 15 through the second supply air pipe 16 has a low temperature below the dew point of the humid exhaust air, it is also possible to use the heat of condensation in transferring heat from the humid exhaust air to the supply air which flows countercurrent in the second heat exchanger 15. After passing through the second heat exchanger 15 moist drying air is cooled and can be discharged into the environment.

After passing the second stage burner 27a to heat the moist drying air, the hot moist drying air is introduced at the moist drying air inlet (not shown) provided in the upstream port 30. The hot moist drying air enters the first drying chamber 3a of the second stage in countercurrent sheet transport direction.

Downstream of the first drying chamber 3a of the second stage, an additional (second) drying chamber 3b of the second stage is provided, provided with an inlet port 31 and an outlet port 32. The second drying chamber 3b of the second stage is aerated in the longitudinal direction, while the flow direction of the moist drying air is the same direction as the sheet conveying direction.

Like the first drying chamber 3a of the second stage, the second drying chamber of the second stage is equipped with a seventh fan 33 and a recirculation pipe 26b for moist drying air. In the second air flow splitter 28b provided in the recirculating wet drying air pipe 26b, the recirculating moist drying air flow is divided into a part that is sent to the second heat exchanger 15 through the wet exhaust air pipe 29, and a recirculated part that is sent to the burner 27b for heating.

Humid drying air inlet means (not shown) are provided at the inlet port 31, and moist drying air heated in the burner 27b can be introduced at the inlet port 31 to then enter the second stage second drying chamber 3b.

A third drying chamber 3c of the second stage is provided downstream of the outlet port 32. The third drying chamber of the second stage is provided with an inlet port 34 and an outlet port 35, like the first and second drying chambers of the second stage.

The moist drying air recirculation pipe 26c is provided with an eighth fan 36 and a second stage burner 27c for heating the recirculated moist drying air. Humid drying air is exhausted from the third drying chamber 3c of the second stage at the outlet port 35, and then driven by the eighth fan 36 towards the burner 27c of the second stage, and then re-entered into the third drying chamber 3c of the second stage.

In accordance with the present invention, additional air is introduced into the last drying chamber 3c of the second stage to lower the dew point and temperature of the moist drying air used in the last drying chamber 3c of the second stage for drying the sheets.

Supplementary air is supplied to the heated supply air pipe 17, which is connected to the eighth fan 36. The heated supply air supplied to the heated supply air pipe 17 is mixed as supplementary air with the moist drying air sucked in from the outlet port 35. After mixing, the air is sent to the burner 27c for heating and then enters the second stage third drying chamber 3c at the inlet port 34.

In the following, the drying of the sheet will be described with reference to the drying of gypsum boards.

The wet gypsum board is transported through the drying device 1 by means of a transport means (not shown). The transport means transports the drywall sheets at a speed of, for example, 65 m/min.

The wet gypsum boards enter the dryer 1 by entering the pre-drying zone 4 at the upstream end. Typical wet gypsum boards used to explain the drying process have an edge length of 1200 mm and a thickness of 12.5 mm. The wet weight of the gypsum boards before entering the dryer is about 8-13 kg/m ² .

In the pre-drying zone 4, the wet gypsum board is heated to about 40-60° C. by contacting the wet gypsum board with preheated air supplied through the preheated air pipe 9. The preheated air has a temperature of about 100-140°C.

The preheated gypsum board then enters the first drying stage 2. The first drying stage 2 comprises seven first stage drying chambers 2a-2g which are aerated in a direction transverse to the transport direction of the gypsum boards.

The temperature and flow rate of the hot drying air in each of the first stage drying chambers can be adjusted individually by adjusting the amount of fuel burned in the burners 18 and adjusting the valves controlling the amount of hot drying air introduced into the first stage drying chambers 2a-2g.

The temperature of the hot drying air entering the drying chambers 2a-2g of the first stage, and the exhaust air leaving them, are summarized in table. 1.

Table 1

The temperature of the hot drying air used in the first stage dryers, e.g.

Sushil 2 2 2 2 2 2 2 camera A b With d e f g input 1 2 2 2 2 2 2 (°C) 76 08 18 25 40 49 42 Texit 1 1 1 1 1 1 2 (°C) 45 63 77 89 90 95 00

The temperature of the gypsum board as it moves through the first stage dryers is shown in FIG. 2. The temperature of the drywall sheet in the center of the sheet increases slowly and reaches a maximum of about 90°C. Due to the high rate of water evaporation from wet gypsum boards, the temperature of the gypsum board at its center and surface remains at a low level of about 90°C. The temperature in the center of the drywall board is about the same as on its surface. No excess heating is observed.

After passing through the first drying stage 2, the partially dried gypsum board has a moisture content of about 10-20% by weight.

The partially dried gypsum board then enters a second drying stage 2 comprising three second stage drying chambers 3a-3c which are aerated in the longitudinal direction.

The temperature of the moist drying air entering and leaving the drying chambers 3a-3c of the second stage is summarized in Table 1. 2.

table 2

An example of the temperature of moist drying air used in second stage drying chambers

Drying chamber Behind 3b Zs Tin (°C) 252 250 160 Tout (°C) 164 168 135

In the first drying chamber 3 a of the second drying stage, the moist drying air is controlled to a high temperature of 252° C. and enters the drying chamber 3 a at the downstream end to flow countercurrently to the transport direction of the gypsum boards. The exhaust air taken from the last drying chamber 2g of the first drying stage 2 and the moist drying air discharged from the first drying chamber 3a of the second stage are sucked in by the fan 25, mixed and heated in the burner 27a of the second stage to obtain moist drying air to be used for drying. drywall sheets. The exhaust air discharged from the drying chamber 2g has a temperature of 150-40°C and a water content of 200-800 g _{of water} /kg _{of air} . The exhaust air mixes with the moist drying air exhausted from the drying chamber 3a. After being heated in the burner 27a, the moist drying air introduced into the first stage drying chamber 3a has a temperature of 140-280°C and a water content of 150-600 gW/kgAd.

As can be seen from FIG. 2, the temperature of the core and the surface temperature of the gypsum boards during the passage of the first drying chamber 3 a of the second stage (zone I) remains at about the same level of about 90° C., increasing slightly. The temperature at the core of gypsum boards and at the surface of gypsum boards is approximately the same.

After passing through the first drying chamber 3 a of the second stage, the gypsum boards have a moisture content of about 10% by weight.

The drywall boards then enter the second drying chamber 3b of the second stage. Humid drying air enters the second drying chamber 3b of the second stage at port 31. Humid

Claims (8)

The dry drying air has a temperature of 140-200°C and a humidity of 150500 g _water /kg _air . As can be seen from FIG. 2, the temperature of the core and surface of the gypsum boards during the passage of the second drying chamber 3b of the second stage (zone II) remains at about 90°C. The temperature in the center of the plates is approximately the same as on the surface of the plates. After passing through the second drying chamber 3b of the second stage, the gypsum boards have a moisture content of about 10-20% by weight, preferably about 5-15% by weight. The boards then enter the second stage third drying chamber 3c at the port 34. Humid drying air is introduced into the port 34 to flow in the downstream direction of the transport direction of the boards. Humid drying air introduced into the second stage third drying chamber 3c at port 34 has a temperature of 90-170°C and a moisture content of 90-250 g _water /kg _air . Humid drying air is formed by mixing the exhaust air discharged from the third drying chamber 3c of the second stage at port 35, having a temperature of 90-150°C and a humidity of 90-200 g of _water /kg _{of air} , with heated supply air having temperature of 80-160°C and humidity of 10-80 g _water /kg _air . The mixed air is then heated in the burner 27c. As can be seen from FIG. 2 (Zone III), the temperature at the center of the drywall board is lower than at the outer surface of the board. However, the temperature remains below 120° C. on the outer surface of the board, and consequently there is no re-calcination of the calcium sulfate dihydrate on the outer portions of the gypsum board. List of positions. 1 - Device for drying; 2 - the first stage of drying; 3 - the second stage of drying; 4 - preheating zone; 5 - cooling zone; 6 - the first heat exchanger; 7 - the first pipe for supply air; 8 - the first fan; 9 - pipe for preheated air; 10 - pipe for the removal of preheated air; 11 - second fan; 12 - the first exhaust pipe; 13 - the third fan; 14 - fourth fan; 15 - second heat exchanger; 16 - second pipe for supply air; 17 - pipe for heated supply air; 18 - burner; 19 - exhaust pipe of the first stage; 20 - separator; 21 - recirculation pipe of the first stage; 22 - fifth fan; 23 - valve; 24 - port; 25 - sixth fan; 26 - recirculation pipe for moist drying air; 27 - burner of the second stage; 28 - air flow separator for moist drying air; 29 - pipe for wet exhaust air; 30 - rear port along the way; 31 - input port; 32 - output port; 33 - the seventh fan; 34 - input port; 35 - output port; 36 - the eighth fan. CLAIM 1. A device (1) for drying gypsum building boards, containing a transport device for transporting boards through a device (1) for drying gypsum building boards; a first drying stage (2) located towards the forward end of the device (1) for drying gypsum building boards and containing at least one drying chamber (2a-2g); first stage drying air supply means for introducing hot air into said at least one drying chamber (2a-2g) of said first drying stage (2) through at least one drying air inlet; venting means for venting exhausted air from said at least one drying chamber (2a-2g) of said first drying stage (2); a second drying stage (3) located behind the first drying stage (2) and containing at least one drying chamber (3a-3c); moving means for moving exhaust air discharged from said at least one drying chamber (2a-2g) of the first drying stage (2) to said at least one drying chamber (3a-3c) of the second drying stage (3); moist drying air supply means for introducing said exhaust air into said at least one drying chamber (3a-3c) of said second drying stage (3), said moist drying air supply means comprising at least one moist drying air inlet for introducing moist drying air into the upstream position of the second drying stage (2); supplementary air supply means for introducing supplementary air into said second drying stage (3) through at least one supplementary air inlet located downstream of said moist drying air inlet, and said supplemental air inlet is located in the position at the rear along the course constituting at least 30% of the total length of said second drying stage (3). 2. Device according to claim 1, characterized in that at least one drying chamber (2a-2g) of the first drying stage (2) is a transversely ventilated drying chamber. 3. Device according to claim 1 or 2, characterized in that at least one drying chamber (3a-3c) of the second drying stage (3) is a longitudinally ventilated drying chamber. 4. Device according to one of claims 1 to 3, characterized in that the second (3) drying stage comprises at least two drying chambers, and an additional air inlet is provided in the last drying chamber of the second drying stage at the downstream end of the second stages (3) drying. 5. A method for drying gypsum building boards, wherein the wet gypsum building board is introduced into the device (1) for drying gypsum building boards, preferably according to one of claims 1 to 4; transporting the wet gypsum building board through the first drying stage (2) and introducing hot air into at least one drying chamber (2a-2g) to contact the wet sheet and evaporate moisture from the wet sheet to obtain a partially dried gypsum building board; exhaust air is withdrawn from said at least one drying chamber of the first drying stage (2), and said exhaust air is collected from said at least one drying chamber (2a-2g) of the first drying stage (2); transporting the partially dried gypsum building board through a second drying stage (3) comprising at least one drying chamber (3a-3c); include at least a part of the discharged air collected from said at least one drying chamber (2a-2g) of the first drying stage (2) into the moist drying air, and said moist drying air is introduced into at least one drying chamber (3a- 3c) the second drying stage (3) into the forward position of the second drying stage (3); introducing additional air into at least one drying chamber (3a-3c) of the second drying stage (3) at a rearward position from the forward forward position to introduce moist drying air to obtain a dried gypsum building board; and removing the dried gypsum building board from the device (1) for drying gypsum building boards; moreover, the specified inlet for additional air is located in a position rearward along the course, constituting at least 30% of the total length of the specified second stage (3) drying. 6. Method according to claim 5, characterized in that the humidity of the additional air introduced into at least one drying chamber of the second drying stage (3) is lower than the humidity of the moist drying air introduced into at least one drying chamber (3a- 3c) of the second drying stage (3) into the forward position of the second drying stage (3). 7. Method according to one of claims 5, 6, characterized in that the second drying stage (3) comprises at least two drying chambers, and additional air is introduced into the last drying chamber at the downstream end of the second drying stage (3). 8. The method according to one of claims 5 to 7, characterized in that the hot air introduced at least -