Method and device for drying gypsum board

Abstract

A device for drying sheets includes: a conveying device for conveying sheets through the device for drying sheets, a first drying stage arranged towards an upstream end of the device for drying sheets 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 at a drying air inlet; air discharge means for discharging exhaust air from said at least one drying chamber of said first drying stage, a second drying stage arranged downstream of the first drying stage and comprising at least one drying chamber; transfer means for transferring exhaust air discharged from said at least one drying chamber of the first drying stage into said at least one drying chamber of the second drying stage; humid drying air supply means for introducing said exhaust air into said at least one drying chamber of said second drying stage, said humid drying air supply means comprising at an humid drying air inlet for introducing humid drying air arranged at an upstream position of the second drying stage; supplemental air supply means for introducing supplemental air into said second drying stage at an supplemental air inlet arranged downstream of said humid drying air inlet. A method for drying sheets is also disclosed.

Claims

1. A device for drying sheets, comprising: a conveying device for conveying sheets through the device for drying sheets, a first drying stage arranged towards an upstream end of the device for drying sheets and comprising at least one drying chamber, a first stage drying air supply for introducing hot air into said at least one drying chamber of said first drying stage with at least one drying air inlet; an air discharge for discharging exhaust air from said at least one drying chamber of said first drying stage, a second drying stage arranged downstream of the first drying stage and comprising a plurality of drying chambers; an air transfer device for transferring exhaust air discharged from said at least one drying chamber of the first drying stage into at least one drying chamber of the second drying stage; a humid drying air supply for introducing humid drying air comprising said exhaust air into said at least one drying chamber of said second drying stage; and a supplemental air supply for introducing supplemental air into at least the most downstream drying chamber of said second drying stage, wherein the supplemental air contains less water than the humid drying air, and only humid drying air is introduced into the most upstream drying chamber of the second drying stage.

2. The device according to claim 1, wherein the at least one drying chamber of the first drying stage is a crosswise ventilated drying chamber.

3. The device according to claim 1, wherein at least one drying chamber of the second drying stage is a longitudinally ventilated drying chamber.

4. The device according to claim 1, wherein said supplemental air inlet is arranged at a position downstream of at least 30% of the total length of said second drying stage.

5. The device according to claim 1, wherein the supplemental air introduced into at least the most downstream drying chamber mixes with humid drying air introduced into at least the most downstream drying chamber.

6. A method for drying sheets, wherein a wet sheet is introduced into a device for drying sheets, comprising the steps of; conveying the wet sheet through a first drying stage and introducing hot air is introduced into the at least one drying chamber to contact the wet sheet and to evaporate humidity from the wet sheet to obtain a partly dried sheet; discharging exhaust air from said at least one drying chamber of the first drying stage and collecting said exhaust air from said at least one drying chamber of the first drying stage; conveying the partly dried sheet through a second drying stage comprising a plurality of drying chambers; introducing humid drying air comprising at least part of the exhaust air collected from said at least one drying chamber of the first drying stage into at least one drying chamber of the second drying stage; introducing supplemental air into at least the most downstream drying chamber of the second drying stage to obtain a dried sheet; and removing the dried sheet from the device for drying sheets, wherein the supplemental air contains less water than the humid drying air, and only humid drying air is introduced into the most upstream drying chamber of the second drying stage.

7. The method according to claim 6, wherein the hot air introduced into the at least one drying chamber of the first drying stage passes the sheet in a direction transverse to a longitudinal transport direction of the sheet through said first drying stage.

8. The method according to claim 6, wherein the humid drying air introduced into the second drying stage passes the sheet in a direction parallel to the longitudinal transport direction of the sheet through said second drying stage, and wherein the direction is the same or opposite relative to the transport direction of the sheets.

9. The method according to claim 6, wherein the device for drying sheets comprises: a conveying device for conveying sheets through the device for drying sheets, a first drying stage arranged towards an upstream end of the device for drying sheets and comprising at least one drying chamber, a first stage drying air supply for introducing hot air into said at least one drying chamber of said first drying stage with at least one drying air inlet; an air discharge for discharging exhaust air from said at least one drying chamber of said first drying stage, a second drying stage arranged downstream of the first drying stage and comprising a plurality of drying chambers; an air transfer device for transferring exhaust air discharged from said at least one drying chamber of the first drying stage into at least one drying chamber of the second drying stage; a humid drying air supply for introducing humid drying air comprising said exhaust air into said at least one drying chamber of said second drying stage; and a supplemental air supply for introducing supplemental air into at least the most downstream drying chamber of said second drying stage via at least one supplemental air inlet.

10. The method of claim 6, wherein the humid drying air has a moisture amount in a range of 150 g.sub.water/kg.sub.air to 600 g.sub.water/kg.sub.air, and the supplemental air has a moisture amount in a range of 10 g.sub.water/kg.sub.air to 100 g.sub.water/kg.sub.air.

11. The method of claim 6, wherein the supplemental air introduced into at least the most downstream drying chamber mixes with humid drying air introduced into at least the most downstream drying chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1: a scheme of a device for drying sheets according to the invention

(3) FIG. 2: a diagram showing the temperature of sheets while passing the device for drying sheets according to the invention

DESCRIPTION OF THE INVENTION

(4) FIG. 1 displays a scheme of a drying device 1 according to the invention. The drying device comprises a first drying stage 2 and a second drying stage 3. Upstream of the first drying stage 2 is arranged a pre-heating zone 4 and downstream of the second drying stage 3 is arranged a cooling zone 5.

(5) A first heat-exchanger 6 is provided for heating fresh air. A first fresh-air pipe 7 is connected to the heating part of the first heat-exchanger 6 and fresh air is aspired by a first fan 8. After passage of heat exchanger 6 the fresh air has been heated to obtain pre-heating air having a temperature adjusted preferably within a range of 80 to 150° C. The pre-heating air is introduced into the pre-heating zone 4 via a pre-heating-air pipe 9. After passage of the pre-heating zone 4 the pre-heating air is discharged via pre-heating air discharge pipe 10 equipped with second fan 11 for aspiring pre-heating air form pre-heating zone 4.

(6) For heating of the fresh air aspired through first fresh-air pipe 7, heat ex-changer 6 is connected with its cooling part to the exhaust line of the first drying stage 2.

(7) For heating of the fresh air in heat exchanger 6, exhaust air produced in the first drying stage, having a high humidity content as well as a high temperature, is introduced into heat exchanger 6 via a humid hot exhaust air pipe 12. In humid hot exhaust air pipe 12 is provided a third fan for pressing humid hot exhaust air received from the first drying stage 2 into heat exchanger 6. The humid hot exhaust air received from the first drying stage passes heat exchanger 6 counter-currently to the fresh air introduced through fresh-air pipe 7. Due to the low temperature of the fresh air as well as of the pre-heating air, the humid hot exhaust air received from the first drying stage is cooled below the dew point of the humid hot exhaust air and therefore also heat of condensation can be used to warm up the fresh air to be used as pre-heating air in pre-heating zone 4.

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

(9) The first drying stage 2 comprises seven drying chambers 2a-2g which are transversally aerated. Hot air inlets and discharge means for discharging exhaust air (not shown) are arranged at opposite ends of the transversally aerated drying chambers 2a-2g to induce a transversal airflow.

(10) Fresh air is aspired by a forth fan 14 from the surroundings and is introduced into second heat exchanger 15 via second fresh-air pipe 16.

(11) After passage of second heat-exchanger 15 the fresh air is warmed up to a temperature of preferably 80 to 140° C. and the warmed fresh air is introduced into warmed fresh air pipe 17.

(12) Each drying chamber 2a-2g is equipped with a burner 18 connected to warmed fresh air pipe 17. The warmed fresh air is mixed with fuel, e.g. natural gas, and by burning the warmed fresh air is further heated. The hot air obtained from burner 18 then is introduced into drying chambers 2a to 2g of the first drying stage 2 via hot air inlets (not shown).

(13) After passage of the drying chambers 2a to 2g the hot air has absorbed water from sheets conveyed through the first drying stage 2 of the drying device 1 and is discharged as exhaust air through first stage exhaust pipe 19. Each first stage drying chamber 2a to 2g is equipped with an individual first stage exhaust pipe 19a to 19g which are joined to a joint exhaust pipe 19.

(14) At a splitter 20 provided in exhaust pipe 19, the flow of exhaust air is divided into a part guided through first exhaust pipe 12 to first heat exchanger 6 for recovery of heat comprised in the exhaust air, and a part to be recirculated. The share of the exhaust air guided into first exhaust pipe 12 and the share of the exhaust air to be recirculated can be adjusted. According to an embodiment, about 20 to 50 vol. % of the exhaust air is guided to first exhaust air pipe 12 and the remainder of the exhaust air is recirculated.

(15) A first stage recirculation pipe 21 is connected to the splitter and a fifth fan 22 is provided in recirculation pipe 21 for pressing recirculated exhaust air through first stage recirculation pipe 21. First stage recirculation pipe 21 comprises individual connecting pipes 21a to 21g connected to burners 18a to 18g for introducing the recirculated exhaust gas into burners 18a to 18g. The recirculated exhaust air is mixed with the warmed fresh air introduced into burner 18a to 18g via warmed fresh air pipe 17. The recirculated exhaust gas is burnt together with warmed fresh air and fuel to provide hot air for introduction into drying chambers 2a to 2g of the first drying stage 2.

(16) The airflow within the first drying stage can be adjusted by valves 23.

(17) The last drying chamber 2g arranged at a downstream end of the first drying stage 2 is connected to a port 24 of the first drying chamber 3a of the second drying stage 3. In port 24 a sheet, after having passed the first drying stage 2, is introduced into second drying stage 3.

(18) The second drying stage 3 comprises three second-stage drying chambers 3a to 3c consecutively arranged in a longitudinal direction. The second-stage drying chambers 3a to 3c are aerated longitudinally.

(19) A sixth fan 25 is connected to port 24, the fan aspirating air from the last downstream drying chamber 2g of the first drying stage and from the first drying chamber 3a of the second drying stage via port 24. Exhaust air from the last drying chamber 2g of the first drying stage and humid drying air from the first drying chamber 3a of the second drying stage are mixed and after passage of the sixth fan 25 are guided through humid drying air recirculation pipe 26a. In humid drying air recirculation pipe 26a is provided a second-stage burner 27a for heating the humid drying air by burning fuel introduced into the flow of humid drying air.

(20) A humid drying air airflow splitter 28a is provided in humid drying air recirculation pipe 26a upstream of second stage burner 27a. Humid exhaust air is deviated from the airflow of humid drying air recirculated in humid drying air recirculation pipe 26a at humid drying air airflow splitter 28a and is guided through humid exhaust air pipe 29a to second heat exchanger 15. Since the fresh air introduced into second heat exchanger 15 via second fresh-air pipe 16 has a low temperature below the dew point of the humid exhaust air also heat of condensation can be used when transferring heat from the humid exhaust air to the fresh air that flows counter-currently in the second heat exchanger 15. After passage of the second heat exchanger 15 the humid drying air is cooled and can be discharged to the surroundings.

(21) After passage of the second stage burner 27a for heating the humid drying air the hot humid drying air is introduced at a humid drying air inlet (not shown) provided at downstream port 30. The hot humid drying air enters the first second-stage drying chamber 3a counter-currently to the conveying direction of sheets.

(22) Downstream of the first second stage drying chamber 3a is arranged a further (second) second-stage drying chamber 3b provided with an entry port 31 and an exit port 32. The second second-stage drying chamber 3b is aerated longitudinally wherein the direction of flow of the humid drying air is in the same direction as the transport direction of the sheets.

(23) Similarly to the first second-stage drying chamber 3a, the second second-stage drying chamber is equipped with a seventh fan 33 and a humid drying air recirculation pipe 26b. At a second airflow splitter 28b provided in the humid drying air recirculation pipe 26b the flow of recirculated humid drying air is split in a part that is guided to second heat exchanger 15 via humid exhaust air pipe 29 and a recirculated part that is guided to burner 27b to be heated.

(24) Humid drying air inflow means (not shown) are provided in entry port 31 and humid drying air heated at burner 27b can be introduced at entry port 31 to then enter second second-stage drying chamber 3b.

(25) Downstream of exit port 32 is provided a third second-stage drying chamber 3c. The third second-stage drying chamber is provided with an entry port 34 and an exit port 35 similar to first and second second-stage drying chambers.

(26) A humid drying air recirculation pipe 26c is provided equipped with an eighth fan 36 and a second stage burner 27c for heating recirculated humid drying air. Humid drying air is discharged from the third second-stage drying chamber 3c at exit port 35 and then is driven by eighth fan 36 towards second-stage burner 27c and then reenters third second stage drying chamber 3c.

(27) According to the invention, supplemental air is introduced in the last second-stage drying chamber 3c to lower dew point and temperature of the humid drying air used in the last second-stage drying chamber 3c for drying the sheets.

(28) The supplemental air is supplied in warmed fresh air pipe 17 which is connected to eighth fan 36. Warmed fresh air provided in warmed fresh air pipe 17 is mixed as supplemental air with humid drying air aspired from exit port 35. After mixing the air is forwarded to burner 27c to be heated and then enters third second stage drying chamber 3c at entry port 34.

(29) In the following drying of a sheet will be described with reference to the drying of gypsum plasterboards.

(30) A wet gypsum plasterboard is conveyed through drying device 1 by conveying means (not shown). The conveying means convey the gypsum plaster sheets at a speed of for example 65 m/min.

(31) The wet gypsum plasterboards enter drying device 1 by entering pre-drying zone 4 at an upstream end. Exemplary wet gypsum plasterboards used for explaining the drying process have an edge length of 1200 mm and a thickness of 12.5 mm. The wet weight of the gypsum plasterboards before entering the drying device is about 8 to 13 kg/m.sup.2.

(32) In the pre-drying zone 4 the wet gypsum plasterboard is warmed up to about 40 to 60° C. by contacting the wet gypsum plasterboard with pre-heated air introduced through pre-heating-air pipe 9. The pre-heated air has a temperature of about 100 to 140° C.

(33) The pre-heated gypsum plasterboard then enters the first drying stage 2. The first drying stage 2 comprises seven first stage drying chambers 2a to 2g which are aerated in a direction transverse to the transport direction of the gypsum plasterboards.

(34) The temperature and the flow of the hot drying air in each of the first stage drying chambers can be adjusted individually by adjusting the amount of fuel burnt in burners 18 and adjustment of valves regulating the amount of hot drying air introduced into first-stage drying chambers 2a-2g.

(35) The temperature of the hot drying air entering the first stage drying chambers 2a-2g and of exhaust air leaving the same are summarized in table 1:

(36) TABLE-US-00001 TABLE 1 temperature of hot drying air used in first stage drying chambers, for example: Drying chamber 2a 2b 2c 2d 2e 2f 2g T.sub.in (° C.) 176 208 218 225 240 249 242 T.sub.out (° C.) 145 163 177 189 190 195 200

(37) The temperature of the gypsum plasterboard while travelling through the first stage drying chambers is displayed in FIG. 2. The temperature of the gypsum plaster sheet in the center of the sheet slowly increases to reach a maximum of about 90° C. Due to the high rate of water evaporation from the wet gypsum plaster sheets, the temperature of the gypsum plaster sheet at its center and its surface remains at a low level of about 90° C. The temperature at the center of the gypsum plasterboard is about the same as at its surface. No excessive heating can be observed.

(38) After passage of the first drying stage 2 the partly dried gypsum plasterboard has a moisture content of about 10 to 20 wt. %.

(39) The partly dried gypsum plasterboard then enters the second drying stage 2 comprising three second-stage drying chambers 3a to 3c that are longitudinally aerated.

(40) The temperature of the humid drying air introduced into and discharged from second stage drying chambers 3a to 3c is summarized in table 2:

(41) TABLE-US-00002 TABLE 2 Example of temperature of humid drying air used in second stage drying chambers Drying chamber 3a 3b 3c T.sub.in (° C.) 252 250 160 T.sub.out (° C.) 164 168 135

(42) In the first drying chamber 3a of the second drying stage the humid drying air is adjusted to a high temperature of 252° C. and enters drying chamber 3a at a downstream end to flow counter-currently to the conveying direction of the gypsum plaster sheets. Exhaust air received from the last drying chamber 2g of the first drying stage 2 and humid drying air discharged from first second-stage drying chamber 3a are aspired by fan 25, mixed and heated in second-stage burner 27a to obtain humid drying air to be is used for drying the gypsum plaster sheets. The exhaust air discharged from drying chamber 2g has a temperature of 150 to 240° C. and a water load of 200 to 800 g.sub.water/kg.sub.air. The exhaust air is mixed with humid drying air discharged from drying chamber 3a. After heating in burner 27a, the humid drying air introduced into the first second-stage drying chamber 3a has a temperature of 140 to 280° C. and a water load of 150 to 600 g.sub.water/kg.sub.air.

(43) As can be seen from FIG. 2, the core temperature and the surface temperature of the gypsum plaster sheets while passing the first second-stage drying chamber 3a (“Zone I”) remains on about the same level of about 90° C., slightly increasing. The temperature in the core of the gypsum plaster sheets and at the surface of the gypsum plasterboards is about the same.

(44) After passage of the first second-stage drying chamber 3a the gypsum plasterboards have a moisture content of about 10 wt. %.

(45) The gypsum plasterboards then enter second second-stage drying chamber 3b. Humid drying air enters the second second-stage drying chamber 3b at port 31. The humid drying air has a temperature of 140 to 200° C. and humidity of 150 to 500 g.sub.water/kg.sub.air. As can be seen from FIG. 2, the core and surface temperature of the gypsum plasterboards while passing the second second-stage drying chamber 3b (“Zone II”) remains at a level of about 90° C. The temperature at the center of the boards is about the same as at the surface of the boards.

(46) After passage of the second second-stage drying chamber 3b, the gypsum plasterboards have a moisture content of about 10 to 20 wt. %, preferably of about 5 to 15 wt. %.

(47) The boards then enter the third second-stage drying chamber 3c at port 34. Humid drying air is introduced at port 34 to flow in a downstream direction relative to the transport direction of the boards.

(48) The humid drying air introduced in third second-stage drying chamber 3c at port 34 has a temperature of 90 to 170° C. and a moisture content of 90 to 250 g.sub.water/kg.sub.air. The humid drying air is formed by mixing exhaust air discharged from third second-stage drying chamber 3c at port 35, having a temperature of 90 to 150° C. and a humidity of 90 to 200 g.sub.water/kg.sub.air with warmed fresh air having a temperature of 80 to 160° C. and a humidity of 10 to 80 g.sub.water/kg.sub.air. The mixed air is then heated in burner 27c.

(49) As can be seen from FIG. 2, “Zone 3”, The temperature in the center of the gypsum plasterboard is lower than at the outer surface of the board. However, also at the outer surface of the board, the temperature remains below 120° C. and, therefore, no re-calcination of the calcium sulfate dihydrate occurs at the outside regions of the gypsum plasterboard. 1 drying device 2 first drying stage 3 second drying stage 4 pre-heating zone 5 cooling zone 6 first heat-exchanger 7 first fresh-air pipe 8 first fan 9 pre-heating-air pipe 10 pre-heating air discharge pipe 11 second fan 12 first exhaust pipe 13 third fan 14 forth fan 15 second heat exchanger 16 second fresh-air pipe 17 warmed fresh air pipe 18 burner 19 first stage exhaust pipe 20 splitter 21 first stage recirculation pipe 22 fifth fan 23 valve 24 port 25 sixth fan 26 humid drying air recirculation pipe 27 second stage burner 28 humid drying air airflow splitter 29 humid exhaust air pipe 30 downstream port 31 entry port 32 exit port 33 seventh fan 34 entry port 35 exit port 36 eighth fan