METHOD AND DEVICE FOR DRYING A MOVING WEB MATERIAL

Abstract: The invention concerns a method and a device for drying a moving web material (P). The device (10) is fitted to direct infrared radiation at the moving web material (P) to be dried. The device (10) comprises at least one first infrared radiator (11) and at least one second infrared radiator (12), which are fitted at the vicinity of one another, and the wavelength ( lambda maximum)(T1) of the maximum intensity of the radiation of the first infrared radiator (11) is shorter than the wavelength ( lambda maximum) (T2) of the maximum intensity of the radiation of said second radiator (12). The first infrared radiator (11) is placed at one side of the web material (P) and the second infrared radiator (12) at the opposite side. The power density of the first infrared radiator (11) is 450...700 kW per sq m and the emitter temperature 2000...2800 DEG C, and the surface layer (15) of the second infrared radiator (12) is made of a metal, a metal alloy or a ceramic material whose emissivity is substantially higher than, or equal to, 0.6.

Description: Method and device for drying a moving web material The invention concerns a method for drying a moving web material, in which method infrared radiation is directed at the material to be dried and in which method the moving web material is passed through the radiation zone of an infrared radiator while the web material to be dried absorbs radiation into itself, in which method the radiation produced by at least one first infrared radiator and the radiation produced by at least one second infrared radiator are applied to the moving web material to be dried, said radiators being fitted in the vicinity of one another, and the wavelength of the maximum intensity of the radiation of said first infrared radiator being shorter than the wavelength of the maximum intensity of the radiation of said second infrared radiator, in which case, in the drying process, the spectrum of the overall radiation is optimal in view of the absorption spectrum of the material to be dried, and in which method the first infrared radiator is placed at one side of the web material and the second infrared radiator at the opposite side. The invention also concerns a device for drying a moving web material, which device is fitted to direct infrared radiation at the moving web material to be dried, and which device comprises at least one first infrared radiator and at least one second infrared radiator, which are fitted at the vicinity of one another, and the wavelength of the maximum intensity of the radiation of the first infrared radiator being shorter than the wavelength of the maximum intensity of the radiation of said second radiator, and in which device the first infrared radiator is placed at one side of the web material and the second infrared radiator at the opposite side. In paper and textile industries and also in other fields of industry, a moving web material is dried. In the production and finishing of paper, there are a number of stages at which drying has to be carried out by means of a method not contacting the web, for example by drying by means of radiation. The infrared radiator devices currently used for drying of a web material consist of high-temperature quartz-tube radiators or of gas-operated medium-wave radiators. The wavelength range of a high-temperature short-wave radiator is substantially 0.5...5.0 μm, while the peak is at about 1.2 μm. When a thin web is dried, the short-wave radiation penetrates through the web, because the absorption coefficient of the material is, as a rule, poor in the wavelength range between 0.5 μm and 2.0 μm, as the absorption peak is in a range substantially higher than 2 μm. Thus, the emission peak of the radiator and the absorption peak of the web material do not coincide. However, with a high-temperature short-wave radiator, a high power density per unit of area is achieved. The power density may be up to 450 kW per sq m, in which case the radiation energy absorbed into the web is higher than 130 kW per sq m. Power densities of said order are required in an attempt to obtain quick drying, which is again necessary, for example, in a process of coating of paper. The wavelength range of medium- wave infrared radiators is substantially 1.5 μm ... 6.0 μm. The wavelength corresponding to the maximum intensity is placed approxi¬ mately between 2.0 μm and 3.0 μm. One of the points of absorption maximum of the water to be evaporated is situated within said interval. At said interval, the absorptivity of cellulosic fibres is also good. Out of the reasons mentioned above, the radiation efficiency of the radiation of a medium- wave radiator is high, about 40- 60%, whereas the corresponding efficiency with short-wave infrared radiators, i.e. with a high-temperature radiator, is about 30—35 % when drying of thin web materials is concerned. When the thickness of the material is increased, the effi¬ ciency of absoφtion becomes higher especially for the short-wave radiators. The maximum power density attainable with medium-wave infrared radiators is 60...75 kW per sq m when a one-sided source of radiation is used, and 120...150 kW per sq m when a two-sided source of radiation is used. A dryer composed of an infrared radiator device, i.e. an IR-dryer, consists of a radiation face, which is placed as close to the face to be dried as possible. In the prior-art devices, the radiation face is enclosed in a box, and the box is fixed in a suitable location on the frame constructions of the process equipment either stationarily or as provided with a displacing mechanism. Further, in said dryers, the use of a backup reflector is known, which reflects the radiation that has passed through the material to be dried and thereby intensifies the process of drying . From the prior art, a number of different IR-dryers used for drying of a moving web or web material are known. The operation of these dryers is based on the ability of pieces to emit electromagnetic radiation, which is specific of the temperature of the piece. It is a second feature characteristic of radiation that, in stead of one wave- length, the radiator emits several wavelengths, whereby an emission spectrum specific of the radiator is formed. Further, in accordance with the laws of physics, it is characteristic of radiation that, when the temperature of the radiating piece rises, the transfer of radiation heat to the target material is increased in proportion to the difference between the fourth powers of the temperatures of the pieces. However, the temperature of the radiator does not alone determine how much radiation can be absorbed into the material to be dried. The temperature, moisture, thickness, material, surface roughness, and brightness of the piece to be dried determine an absorption coefficient, which indicates what a proportion of the radiation arriving on the face of the piece to be dried is absorbed into the material. However, as a rule, the absorption coefficient is a function of the wavelength, so that in a short-wave range the absoφtion coefficient of a thin material is inferior to that in a medium- wave or long- wave range. IR-radiation sources operating in the short-wave infrared range are considered radia¬ tors which emit a radiation whose wavelength of maximum intensity of radiation is in the wavelength range of 0.76...2.00 μm. IR-radiation sources operating in the medium-wave infrared range are considered radiators which emit a radiation whose wavelength of maximum intensity is in the wavelength range of 2.00...4.00 μm. The correspondence with temperature is obtained by means of Wien's displacement law from the formula λmaximum * T = 2.8978-10^ (mK) The temperature range of a short-wave radiator is obtained as 3540 °C ... 1176 °C, and that of a medium-wave radiator as 1176 °C ... 450 °C. The IR-dryers operating in the short-wave range are currently almost exclusively electrically operated. In them, usually a tungsten filament placed in a quartz tube is made to glow by means of electric current. The maximum emitter temperature of the glowing filament is usually about 2200 °C, in which case the wavelength correspon- ding to the maximum intensity of radiation is about 1.2 μm. In the prior-art short-wave infrared radiators, the lamps are, as a rule, arranged in heating modules of 3...12 lamps. The modules are attached side by side, and a drying zone extending across the web is obtained. The lamps are usually spaced so that the power density of the dryer per unit of area varies in a range of 100...450 kW per sq m. The dryers operating in the medium-wave IR range are either electrically operated or gas-operated. In electric devices, filaments are made to glow by means of electric current either in a quartz tube or behind a ceramic tile or a tile made of quartz. In the former case, the spiral filament operates directly as the emitter, whereas in the latter case the heat is transferred first into the tile, after which the tile operates as the emitter. The tile may also be partly penetrable by radiation. In gas-operated systems, a usually ceramic radiator is made to glow by means of a flame, which radiator starts glowing and thus operates as the emitter. Radiation is partly also emitted directly from the flame. As was stated above, the wavelength of maximum intensity of medium- wave infrared radiators is 2.00...3.00 μm, the corresponding temperature of the radiator being, as was stated above, in the range of 1176 °C ... 690 °C. With medium-wave infrared radiators, the maximum power density varies, depending on the method and the temperature, substantially in a range of 40...100 kW per sq m. Adverse aspects of short-wave infrared radiators include poor radiation efficiency in the shorter wavelength range of the radiator influencing the overall efficiency, expensive electric control system, high cost for electricity and ventilation systems. Adverse aspects of medium-wave infrared radiators include low power density per unit area when quick drying is aimed at, poor adjustability, slow heating and cooling, relatively high cost of electrical system and electricity in the case of electric infrared radiators. For gas operated systems the high cost for the gas feed system and the risk of explosion from handling of explosive gases can be mentioned. The difficulties to use the cooling exhaust air or the exhaust gases for an efficient improvement of the drying process is common for both gas- and electrical medium wave dryers. Thus, it can be considered that a major drawback of the prior art infrared heaters, ie. IR-dryers, consisting of short wave infrared radiators is poor efficiency because of the low absoφtion coefficient of the material to be dried in the shorter wave length range of the radiator. When the IR-dryer consists of medium wave infrared radiators, a particular draw¬ back can be considered to be the low power density and still the need for a relatively expensive electrical and ventilation system, poor controllability because of the slow heating and cooling of the medium-wave radiators and the difficulties to efficiently use the exhaust air or gases in the drying process. In the EP Patent 288,524, a method is described for drying a moving web material. In the method, infrared radiation is directed at the material to be dried, and the moving web material is passed through the radiation zone of the infrared radiator while the web material to be dried absorbs radiation into itself. In the method, the radiation produced by at least one first infrared radiator and the radiation produced by at least one second infrared radiator are directed at the moving web material to be dried, said radiators being fitted in the vicinity of one another. In this connection, the wavelength of the maximum intensity of the radiation of the first infrared radiator is shorter than the wavelength of the maximum intensity of the radiation of the second infrared radiator, in which case, in the drying process, the spectrum of the overall radiation is optimal in view of the absoφtion spectrum of the material to be dried. The maximum intensity of the radiation of the first infrared radiator occurs in the wavelength range of the radiation 0.76 μm < λ maximuιn < 2.00 μm, and the maximum intensity of the radiation of the second radiator is in the wavelength range 2.00 μm < λ ma χim u m < 4-00 M m - The radiators can be fitted at the same side of the moving web material, or they can be fitted so that the first radiator is placed at one side of the web material and the second radiator at the opposite side. By means of the method and the device in accordance with the EP Patent 288,524, a spectrum is obtained that is favourable in view of the drying. Then, an efficiency of radiation is achieved that is at least about 5 % better than with the prior-art solutions of equipment. From the prior art, it is known to provide the second radiator, placed at the opposite side of the web material to be dried, with a surface layer which in the short wave 0,5-2,0 μm spectra mainly reflects but partly also absorbs the radiation of the first infrared radiator that passes through the material web so that the temperature of the second infrared radiator rises to several hundreds of Celsius degrees. When a typical white ceramic material is used as the surface material, the temperature may rise to a value of an order of 500...700 °C for low grammage webs for example paper webs with grammages less than 110 g/m 2 . A temperature of 500...700 °C is not yet sufficient as the surface temperature of the second infrared radiator, while its power density is a function of its temperature level in Kelvin degree in fourth power, but additional electric energy can be fed into the surface layer of the second infrared radiator according to EP Patent 288,524, whereby the surface temperature can be raised further to a temperature of 800...1050 °C. Thus, the backup radiator described above is a device that receives the heat radiation passing through the web and uses this heat for heating the surface layer of the device. The backup radiator is a medium-wave radiator. The backup radiator is used together with a short-wave infrared radiator. Together, these two devices produce a good drying result and efficiency. i The object of the present invention is to provide an improvement over the method and the device described in the EP Patent 288,524 for drying a moving web material. A specific object of the present invention is to provide a method and a device wherein it is possible to avoid the supply of additional electric energy to the surface layer of the second radiator. The objectives of the invention are achieved by means of a method which is charac¬ terized in that (a) as the first infrared radiator, a radiator is used whose power density is 450...700 kW per sq m and whose emitter temperature is 2000...2800 °C, (b) as the web material to be dried, a web is used whose transmissivity is substan¬ tially higher than, or equal to, 0.18 for short wave infrared radiation 0.5— 2.0 μm, (c) as the second infrared radiator, a radiator is used whose surface layer is made of such a metal, metal alloy or ceramic material whose emissivity is substan¬ tially higher than, or equal to, 0.6, within the total wavelength range of 0.5—2.0 μm. in which case, of the power density of the first infrared radiator, such a percentage proportion passes through the web as is sufficient to be capable of heating the surface layer of the second infrared radiator to a temperature of substantially at least 800 °C. 0 On the other hand, the device in accordance with the invention is characterized in that the power density of the first infrared radiator is 450...700 kW per sq m and the temperature 2000...2800 °C, and the surface layer of the second infrared radiator is made of a metal, metal alloy or ceramic material whose emissivity is substantially higher than 0.6 within the total wavelength range of 0.5—2.0 μm. The device and the method in accordance with the present invention are particularly well suitable for thin web grades, which have a transmissivity τ equal or higher than 0.18 for short wave radiation for example corresponding to grammages equal or less than 110 g/m 2 for ordinary paper webs. As the first radiator, a radiator is used whose power density is 450...700 kW per sq m and whose temperature is 2000...2800 °C. As the second radiator, a radiator is used whose surface layer is made of a metal, metal alloy or ceramic material whose emissivity is substantially higher than 0.6 within the total wavelength range of 0.5—2.0 μm. In such a case, of the power density of the first radiator, such a percentage proportion of the energy passes through the web as is sufficient to heat the surface layer of the second radiator to a temperature of substantially at least 800 °C. In a preferred embodiment of the invention, the power density of the first radiator is chosen at a value of 530...650 kW per sq m, and the temperature with the maximum power density at the value 2100...2600 °C, and the emissivity of the surface layer of the second radiator is chosen at a value of 0,65-0,9 within the total wavelength range of 0.5-2.0 μm. In a preferred embodiment of the invention, the surface layer is formed of a metal alloy which contains 10...26 %-wt. (per cent by weight) of chromium, 0...84 %- wt. of iron, and 0...81 %-wt. of nickel and 0—25 %-wt. of aluminia. A metal alloy is particularly favourable which contains chromium, > 20 %-wt. of iron and alternatively nickel or aluminia or a metal alloy of chromium and nickel. In a preferred embodiment of the invention, ceramic material has been chosen from the group of carbides, nitrides and suicides. In an another preferred embodiment of the invention, ceramic material is a ceramic base, preferably an aluminium oxide, zirconium oxide, glass ceramic or quartz material, coated with a carbide, nitride, suicide, a metal or a metal alloy. The invention will be described in detail with reference to some preferred embodi¬ ments of the invention illustrated in the figures in the accompanying drawings, the invention being, however, not supposed to be confined to said embodiments alone. Figure 1 is a schematic side view illustrating a prior-art method for drying a web material. Figure 2 is a schematic side view illustrating the basic principle of the method in accordance with the present invention. Figure 3 is a perspective view of a first embodiment of a radiator tray which is a part of the second radiator in figure 2. Figure 4 is a planar view from above of the radiator tray shown in figure 3. Figure 5 is a view from above in figure 4. Figure 6 is a view from the left in figure 4. Figure 7 is a view corresponding to figure 6, but with the flanged sheet in the left edge dismounted. Figure 8 is a partially sectioned view according to the line VIII-VIII in figure 4. Figure 9 is an enlarged view of a part A of figure 8. Figure 10 is a perspective view of an alternative embodiment of a radiator tray which is a part of the second radiator in figure 2. Figure 11 is a planar view from above of the radiator tray shown in figure 10. Figure 12 is a view from above in figure 11. Figure 13 is a view from the left in figure 11. Figure 14 is a view corresponding to figure 13, but with the flanged sheet in the left edge dismounted. In the prior-art solution shown in figure 1, the web material to be dried is denoted with the letter P. The web material passes over the rolls 13 and 14, and the running direction of the web material P is denoted with the arrow A. The first infrared radiator 11 is placed at one side of the web P, and similarly the second infrared radiator 12 is placed at the opposite side of the web P. The infrared radiator 11 and the infrared radiator 12, respectively, may consist of one or several separate radiators. When solutions known from the prior art are used as the surface layer in the second radiator, the radiation of the first infrared radiator 11 that passes through the web P can heat the surface layer of the second radiator 12, at the maximum, to a temperature of about 500...700 °C. In figure 2, the surface layer in accordance with the present invention is denoted with the reference numeral 15. The power density of the first infrared radiator 11 is chosen as 450...700 kW per sq m, and the temperature is chosen as 2000...2800 °C. As the surface layer 15 of the second radiator 12, a metal, metal alloy or a ceramic material is used, whose emissivity is substantially higher than, or equal to, 0.6 within the total wavelength range of 0.5—2.0 μm. When a web material with equal or higher transmissivity T than 0.18 for short wave infrared radiation which for example with ordinary paper webs correspond to grammages substantially equal or less than 110 g/m 2 is used, such a percentage proportion of the intensity of the first radiator 11 passes through the web P as is sufficient to heat the surface layer of the second radiator 12 substantially at least to a temperature of 800 °C. Advantageously, the surface layer 15 contains 10...26 %-wt. of chromium, 0...84 %-wt. of iron, and 0...81 %-wt. of nickel, 0—25 %-wt of aluminium. In a preferred embodiment, the surface layer 15 contains a metal alloy with chromium, > 20 %- wt. of iron and alternatively nickel or aluminia or a metal alloy of nickel and chromium. The second radiator 12 in figure 2 have a frame on which box-shaped radiator trays according to figures 3—9 are mounted. The radiator tray according to figures 3—9 is as a whole marked with 20. It comprises a with heat insulation 22 of ceramic fibres filled radiator sheet box 23 together with radiator surface material 24, in one or several parts, building up the surface layer 15 in figure 2. A radiator surface material or part 24 according to the invention is shown from a side view in figure 8. As can be seen from figure 8 and figure 3 is this part bended showing longitudinal waves with tops 25 and grooves 26, in which row-vise are arranged holes 27 for mounting of bolts 28 with a head 29 and free ends 30 with lock pins 31. As can be seen from figure 9 is the outmost situtated longitudinal row of holes situated in an eccentric manner to press the outmost free wave effectively down. In this way the design will prevent the mentioned outmost waves from bending upwards forming an obstacle for the passing web or other parts. According to the design the bolts 28 can be surrounded by distance pipes 32 to secure a defined thickness of the total radiator tray 20. The radiator tray frame can be comprised by sections in which case two on the opposite side situated flanged sheets 33 are mounted to lay upon the radiator surface material parts and lock them up in the edges. An alternative embodiment of a radiator tray 20a according to the invention is shown in figures 10—14. The second radiator 12 in figure 2 have a frame on which box-shaped radiator trays according to figures 10—14 are mounted. The radiator tray according to figures 10—14 is as a whole marked with 20a. It comprises a with heat insulation 22 of ceramic fibres filled radiator sheet box 23 together with radiator surface matterial 24a in one or several parts building up the surface layer 15 in figure 2. The alternative embodiment can preferably be used if the radiator surface material 24a of ceramic material, metal or an metal alloy according to the invention have such a mechanical stability over 800 °C that the flanged sheets 33 on both sides are capable to keep the radiator surface material in a fixed position over its total surface. Above, just the solution of principle of the invention has been described, and it is obvious to a person skilled in the art that numerous modifications can be made to said solution within the scope of the inventive idea defined in the accompanying patent claims.

Claims: Claims 1. A method for drying a moving web material (P), in which method infrared radiation is directed at the material (P) to be dried and in which method the moving web material is passed through the radiation zone of an infrared radiator while the web material to be dried absorbs radiation into itself, in which method the radiation produced by at least one first infrared radiator (11) and the radiation produced by at least one second infrared radiator (12) are applied to the moving web material to be dried, said radiators being fitted in the vicinity of one another, and the wavelength of the maximum intensity of the radiation of said first infrared radiator (11) being shorter than the wavelength (^naximum)^) of the maximum intensity of the radiation of said second infrared radiator (12), in which case, in the drying process, the spectrum of the overall radiation is optimal in view of the absoφtion spectrum of the material to be dried, and in which method the first infrared radiator (11) is placed at one side of the web material (P) and the second infrared radiator (12) at the opposite side, characterized in that (a) as the first infrared radiator (11), a radiator is used whose power density is 450...700 kW per sq m and whose emitter temperature is 2000...2800 °C, (b) as the web material (P) to be dried, a web is used whose transmissivity (T) is substantially higher than, or equal to, 0.18 for short wave infrared radiation 0.5—2.0 μm, (c) as the second infrared radiator (12), a radiator is used whose surface layer (15) is made of such a metal, metal alloy or ceramic material whose emissivity is substantially higher than, or equal to, 0.6 within the total wavelength range of 0.5— 2.0 μm. in which case, of the power density of the first infrared radiator (11), such a percen¬ tage proportion passes through the web (P) as is sufficient to be capable of heating the surface layer (15) of the second infrared radiator (12) to a temperature of substantially at least 800 °C. 2. A method as claimed in claim 1, characterized in that as the web material (P) to be dried, a paper web is used whose grammage is substantially less than, or equal to, 110 grams per sq m. 3. A method as claimed in claim 1 or 2, characterized in that the power density of the first infrared radiator (11) is 530...650 kW per sq m and its emitter temperature is 2100...2600 °C. 4. A method as claimed in any of the claims 1 to 3, characterized in that the emissivity of the surface layer (15) of the second infrared radiator (12) is 0.65—0.9 within the total wavelength range of 0.5—2.0 μm. 5. A method as claimed in any of the claims 1 to 4, characterized in that the surface layer (15) is made of a metal alloy consisting of some of the metals chro¬ mium, aluminia, nickel and iron. 6. A method as claimed in claim 5, characterized in that the surface layer (15) contains 10...26 %-wt. of chromium, 0...84 %-wt. of iron, 0...81 %-wt. of nickel, and 0—25 %-wt. of aluminia. 7. A method as claimed in claim 5 or 6, characterized in that the surface layer (15) contains a metal alloy with chromium, iron > 20% and alternatively nickel or aluminia or a metal alloy of chromium and nickel. 8. A method as claimed in any of the claims 1 to 4, characterized in that the ceramic material has been chosen from the group of carbides, nitrides and suicides. 9. A method as claimed in any of the claims 1 to 4, characterized in that the ceramic material is a ceramic base, preferably an aluminium oxide, zirconium oxide, glass ceramic or quartz material, coated with a carbide, nitride, suicide, a metal or a metal alloy. 10. A device (10) for drying a moving web material (P), which device is fitted to direct infrared radiation at the moving web material (P) to be dried, and which device (10) comprises at least one first infrared radiator (11) and at least one second infrared radiator (12), which are fitted at the vicinity of one another, and the wavelength (λ max imumXTι) of the maximum intensity of the radiation of the first infrared radiator (11) being shorter than the wavelength of tne maximum intensity of the radiation of said second radiator (12), and in which device the first infrared radiator (11) is placed at one side of the web material (P) and the second infrared radiator (12) at the opposite side, characterized in that the power density of the first infrared radiator (11) is 450...700 kW per sq m and the emitter temperature 2000...2800 °C, and the surface layer (15) of the second infrared radiator (12) is made of a metal or metal alloy or ceramic material whose emissivity is substantially higher than, or equal to, 0.6 within the total wavelength range of 0.5—2.0 μm. 11. A device as claimed in claim 10, characterized in that the power density of the first infrared radiator (11) is 530...650 kW per sq m and the emitter temperature is 2100...2600 °C. 12. A device as claimed in claim 10 or 11 , characterized in that the emissivity of the surface layer (15) of the second infrared radiator (12) is 0,65-0,9 within the total wavelength of 0.5—2.0 μm. 13. A device as claimed in any of the claims 10 to 12, characterized in that the surface layer (15) is made of a metal alloy consisting of some of the metals chro¬ mium, aluminia, nickel and iron. 14. A device as claimed in claim 13, characterized in that the surface layer (15) contains 10...26 %-wt. of chromium, 0...84 %-wt. of iron, 0...81 %-wt. of nickel, and 0,25 %-wt. of aluminia. 15. A device as claimed in claim 13 or 14, characterized in that the surface layer (15) contains a metal alloy with chromium, iron > 20% and alternatively nickel or aluminia or a metal alloy containing chromium and nickel. 16. A device as claimed in any of the claims 10 to 12, characterized in that the ceramic material has been chosen from the group of carbides, nitrides and suicides. 17. A device as claimed in any of the claims 10 to 12, characterized in that the ceramic material is a ceramic base, preferably an aluminium oxide, zirconium oxide, glass ceramic or quartz material, coated with a carbide, nitride, silicide, a metal or a metal alloy. 18. A device as claimed in any of the claims 10 to 17, characterized in that the surface (15) is at least partly wavelike. 19. a device as claimed in any of the claims 10 to 18, characterized in that the outmost situated longitudinal row of holes (27) and bolts (28) are situated in an eccentric manner to put an extra press on the outmost free wave.

Inventors: ALEXANDERSSON BOERJE, JOHANSSON LEIF, ALEXANDERSSON, BOERJE, JOHANSSON, LEIF
Applicants: ALEXANDERSSON BOERJE, INFRAROEDTEKNIK AB, JOHANSSON LEIF, ALEXANDERSSON, BOERJE, JOHANSSON, LEIF
Application year: 1996
Main classification: F26B3
IPC codes: D21F5, F26B3, F26B13