Method for drying a printed paper web, and drying section

Abstract

The invention relates to a method for drying a printed paper web (P) by means of a drying section (2), wherein the drying section (2) has a pre-dryer (8) and a main dryer (10), the main dryer (10) is located downstream of the pre-dryer (8) with respect to the conveying direction (F) for the paper web (P), the paper web (P) is pre-dried by means of the pre-dryer (8), the main dryer (10) is a hot air dryer, by means of which the paper web (P) is hot-air-dried, and the pre-dryer (8) has a lower connected load or is shorter than the main dryer (10) or both. The invention also relates to a corresponding drying section (2).

Description

[0001] Description

[0002] Process for drying a printed paper web, drying section

[0003] The invention relates to a method for drying a printed paper web and a drying section therefor.

[0004] After printing a paper web with a print image, drying is required before further processing to protect the printed image from thermal and / or mechanical stress during further processing. Radiation dryers, especially IR dryers, and impingement jet dryers can be used to dry a printed paper web.

[0005] Reference is made to EP 3 363 635 A1 , US 2018 / 264 803 A1 , DE 10 2010 046 756 A1 , WO 2017 / 202 846 A1 , WO 2020 / 222 806 A1 , WO 2015 / 016 902 A1 , DE 10 2005 031 159 A1 , DE 10 2018 114 387 A1 , DE 10 2019 126 701 A1 , DE 10 2020 128 178 A1.

[0006] Against this background, one object of the invention is to improve the drying of a printed paper web. In particular, the drying process should be as energy-efficient as possible. To this end, an improved method for drying a printed paper web and an improved drying section are to be provided.

[0007] The object is achieved according to the invention by a method having the features according to claim 1 and by a drying section having the features according to claim 15. Advantageous embodiments, refinements, and variants are the subject of the dependent claims. The statements in connection with the method also apply mutatis mutandis to the drying section, and vice versa. If steps of the method are specified implicitly or explicitly below, advantageous embodiments for the drying section result from the fact that it is designed to carry out the method. For this purpose, the drying section has, in particular, a suitably designed control unit.

[0008] The method serves to dry a printed paper web, i.e., a paper web with a printed image, using a drying section. The drying section is arranged, in particular, downstream of a printer used to print the paper web. Downstream of the drying section, the printed paper web is further processed, in particular. Preferably, the printer and the drying section are each part of a corrugated board plant for producing corrugated board from multiple paper webs, including the printed paper web. Further processing then involves, for example, assembling the paper webs and / or semi-finished products thereof to form a corrugated board web.

[0009] Alternatively, the printer and the drying section are each part of a printing system, at the end of which the printed paper web is simply rolled up, for example. The printer is preferably an inkjet printer, also known as a digital printer.

[0010] The drying section has a pre-dryer and a main dryer. Accordingly, the drying section is designed in several stages, with the pre-dryer (also "first dryer unit") forming a first drying stage and the main dryer (also "second dryer unit") a second drying stage. The main dryer is arranged downstream of the pre-dryer with respect to the conveying direction of the paper web. The printed paper web therefore first passes through the pre-dryer and then the main dryer. In general terms, the terms "upstream" and "downstream" are understood here relative to the conveying direction of the paper web. The paper web continues to be conveyed at a web speed. This speed is, in particular, the same in the printer and along the drying section, but can fundamentally vary overall.

[0011] The pre-dryer pre-dries the paper web, i.e., before drying in the main dryer, the paper web is pre-dried in the pre-dryer. The pre-dryer is preferably a radiation dryer with a number of radiation elements with which the paper web is radiation-dried. This is assumed below without loss of generality. “A number of” is generally understood to mean “one or more,” but the radiation dryer preferably has not just one, but several radiation elements. These each extend, for example, in the conveying direction and can then expediently be switched off individually depending on a width of the paper web perpendicular to the conveying direction. Alternatively, the radiation elements each extend in a transverse direction perpendicular to the conveying direction and in particular across the entire width of the paper web. The radiation elements themselves are in particular parallel to one another.The preferred orientation in the transverse direction or in the conveying direction also applies in the case of only a single radiation element.

[0012] Preferably, the pre-dryer in the radiation dryer configuration is an IR dryer (infrared dryer), which will also be assumed below. The radiation elements are then each designed as an IR filament, e.g., a carbon filament. During operation, such a radiation element regularly has a high temperature of, e.g., 1200 to 1400°C. The main dryer, on the other hand, is an impingement jet dryer, also known as a "hot air dryer," with which the paper web is dried with hot air. Accordingly, a large amount of hot air is flowed onto the paper web to dry it. The hot air is heated, e.g., using a steam / air heat exchanger, gas burner, heating rod, or a comparable heating element.

[0013] Alternatively or in addition to a configuration as a radiation dryer, the pre-dryer has at least one heating element, preferably a plurality of heating elements, with which the paper web is dried. Each heating element is suitably designed as a heating plate, e.g., a heated metal plate. Preferably, the at least one heating element is used for radiation or contact drying, rather than impingement jet drying. However, such a drying process can be provided separately or additionally, as described further below. The heating element is spaced from the paper web for drying purposes or is in contact with it.

[0014] The pre-dryer suitably comprises both the described radiation elements and the described heating elements. Preferably, the radiation elements are arranged on a first side of the paper web, and the heating elements are arranged on a second side of the paper web, opposite the first. This optimally increases the heat flux density in the pre-dryer. Alternatively or additionally, it is also advantageous to use radiation elements and / or heating elements on both of the aforementioned sides of the paper web.

[0015] For the sake of simplicity, we will refer to the drying of the paper web. This specifically means that both the paper from which the original paper web is formed and the printed image, i.e., the printed ink, are dried.

[0016] In this case, the pre-dryer has a lower connected load than the main dryer, or is shorter (viewed in the conveying direction), or both. As a result, the drying and thus the heating of the paper web in the pre-dryer is limited, especially compared to the main dryer. The “connected load” is understood to mean in particular the maximum thermal power (heat per unit of time) that can be generated / is generated by the respective dryer for drying the paper web, particularly on its printed side. Both the pre-dryer and the main dryer each have a connected load. The power that is actually delivered by the respective dryer for drying at a given time is referred to as the “dryer power”; this can therefore correspond to a maximum of the connected load.In a suitable embodiment, the connected load of the pre-dryer corresponds to at least 40% and at most 80% of the connected load of the main dryer. The aforementioned value range for the connected load refers in particular only to drying by means of radiation elements of a radiation dryer and / or only on the printed side. Any additional heating elements, in particular for drying the side opposite the printed side (unprinted side), are preferably excluded and then, if present, result in an additional connected load. Nevertheless, an embodiment in which the connected load lies in the above-mentioned, preferred value range can also be combined with a embodiment in which the dryer has additional heating elements and / or in which the opposite side is also dried.

[0017] The connected load of the main dryer, in particular as defined above, is preferably in the range of 3 kW / (m / min) to 8 kW / (m / min) and is particularly dependent on the web speed, measured in m / min. “Shorter” is understood in particular to mean that the residence time for a given point on the paper web is shorter in the pre-dryer than in the main dryer, i.e. the pre-dryer is passed through in a shorter period of time than the main dryer. For a given web speed, the path along which the paper web is dried is therefore also spatially shorter in the pre-dryer than in the main dryer. The terms “shorter / short” and “longer / long” are therefore to be understood generally as referring to the path along which the paper web is dried in the respective dryer.In a suitable embodiment, the pre-dryer has a (first) path along which the paper web is dried, and this path is as short as possible, but the web is dried along this path with the highest possible drying performance. At a web speed of up to 300 m / min, this achieves, in particular, a surface temperature of the paper web of at least 40 °C. The main dryer, on the other hand, is preferably at least half an order of magnitude (i.e., a factor of 5) longer than the pre-dryer and accordingly has a (second) path along which the paper web is dried, which is correspondingly longer than the first path.

[0018] The pre-dryer serves, in particular, solely to preheat (also: precondition) the paper web for the most efficient drying in the downstream main dryer. Preferably, the environment of the paper web in the main dryer is characterized by a cooling limit temperature, and the pre-dryer specifically heats the paper web to the cooling limit temperature (e.g., with a tolerance of + / -20°C). More important than the precise dimensioning of the pre-dryer and main dryer is that a pre-dryer is used, followed by an impingement jet dryer as the main dryer. The relative dimensions of the two dryers are then coordinated with the aim of achieving the most energy- and process-optimized drying possible.

[0019] The pre-dryer is specifically the only dryer through which the paper web passes between the printer and the main dryer, or equivalently, all dryers between the printer and the main dryer constitute the pre-dryer. In other words, the pre-dryer is the first and only dryer through which the printed paper web passes before entering the main dryer. Preferably, no other drying, generally no temperature control, and / or no further processing of the paper web takes place between the pre-dryer and the main dryer.

[0020] The invention is based in particular on the observation that the evaporation rate (equivalently, the drying rate) as a function of the residence time in the respective dryer differs depending on the dryer type. While the evaporation rate in a radiation dryer is largely independent of the paper web's residence time within it, the evaporation rate in an impingement jet dryer is very high at the beginning and decreases sharply with increasing residence time. Accordingly, if the paper web is dried too intensively in the radiation dryer, it is already in an advanced drying phase by the time it reaches the impingement jet dryer, and the impingement jet dryer can no longer be operated at a high evaporation rate, thus becoming inefficient.

[0021] In this case, the pre-dryer uses as little energy as possible to first fix the print image on the paper web, initiate an initial drying process, and increase the temperature of the paper web and thus the partial pressure of volatile substances (e.g. solvents in the ink) by achieving the highest possible power density. However, the reduced dimensions of the pre-dryer compared to the main dryer prevent excessive heating of the paper web, thus making more efficient use of the downstream main dryer. Preferably, the paper web is preheated with the pre-dryer in such a way that an average evaporation rate of at least 4 g / (m 2 *s) up to 12 g / (m 2 *s) is achieved.

[0022] The principle according to which drying is carried out with the pre-dryer is initially of secondary importance; what is more important is that the web is pre-dried with the pre-dryer before impingement jet drying in the main dryer, so that the main dryer can then be operated as efficiently as possible.

[0023] Furthermore, it was observed that radiation dryers and impingement jet dryers each have specific disadvantages and different optimization potentials, both individually, specifically and in combination, as explained in more detail below.

[0024] Radiation dryers, for example, have the disadvantage of inhibited mass transport as a result of inadequate air management within the dryer. This results in the formation of a broad concentration boundary layer over the printed image, which limits drying. Although the paper web is heated considerably, the moisture contained in it (and in the ink) is only removed to a limited extent. This cannot be remedied by any additional air knives (“AirKnife”) that might

[0025] Concentration boundary layer can only be reduced locally at most. However, for a fresh print image, the inhibited mass transport is an advantage, as the risk of smearing of the still-fresh print image is correspondingly reduced. Conversely, an impingement jet dryer, due to the strong convective flow prevailing within it, has a higher risk of an insufficiently fixed print image compared to a radiation dryer, but at the same time it also has a higher drying potential. However, since an impingement jet dryer also has a higher thermal capacity and thus a higher thermal inertia than a radiation dryer, there is a risk of overdrying the paper web, which can only be partially compensated for by subsequent remoistening.In contrast, a radiation dryer can be activated and deactivated much more quickly, so that it can respond much better to changing drying needs.

[0026] The combination of a radiation dryer and an impingement jet dryer is therefore particularly advantageous in terms of achievable dynamic performance. The arrangement of the radiation dryer upstream of the impingement jet dryer is advantageous for fixing the print image and preconditioning the paper web. The relative dimensioning of both dryers offers optimization potential in terms of energy efficiency and required installation space.

[0027] Furthermore, drying generally requires a large amount of primary energy, which has a negative impact on the energy efficiency, operating costs, and CO2 emissions of the drying section in particular, and generally of the system into which the drying section is integrated. The unnecessary overdrying of the paper web offers corresponding optimization potential. Radiation dryers, in particular, have very high power consumption, which can amount to several hundred kilowatts in applications with a paper web width (in the cross direction, also known as the "working width") of 2.5 m or more and a web speed of 300 m / min or more—as in the corrugated board plant assumed here.

[0028] A core idea of ​​the invention presented here is, in particular, to optimize the drying of a printed paper web such that the paper web is dried with a heat flow that is as high as necessary, yet as low as possible. This is achieved in the present case by specifically coordinating the pre-dryer and the main dryer, i.e., by specifically dimensioning the two dryers relative to each other. This coordination of the heat flow is preferably further improved by the specific dryer type (radiation, impact jet) and their sequence in the conveying direction.

[0029] The pre-dryer is preferably activated and deactivated depending on the job, i.e., depending on the specific properties of the paper web. For example, with a paper web made of uncoated paper, the pre-dryer is operated at reduced power rather than full power, since fixation occurs automatically due to the capillary absorption of the paper and requires little or no drying. This saves energy and prevents overdrying of the paper web.

[0030] In an advantageous embodiment, a hot air function is integrated into the pre-dryer, i.e. in addition to radiation and / or contact drying, the paper web is also dried with hot air in the pre-dryer. Here, the printed paper web in the pre-dryer is additionally exposed to a stream of hot air (hot air stream). This advantageously increases the heat flux density on the paper web. In addition, the convective mass transport is correspondingly increased, thus preventing the formation of a concentration boundary layer and realizing increased mass transport. The hot air is suitably obtained from cooling air for the radiation elements. In an advantageous embodiment, a quantity of supply air, e.g. fresh air, is supplied to the pre-dryer as cooling air, with which the radiation elements are cooled by the supply air being directed over the radiation elements.The supply air is in particular in contact with the radiation elements and absorbs heat from them. The supply air is either unheated or already heated and is therefore already hot air itself. The supply air, now heated by the radiation elements, is then flowed onto the paper web as hot air, preferably at a flow velocity of at least 10 m / s. Regardless of how the hot air is generated in the pre-dryer, this hot air suitably has a flow velocity in the range of 10 m / s to 50 m / s. The flow velocity is expediently adjusted depending on the job in order to minimize the risk to the printed image, while at the same time always achieving the highest possible heat flux density and drying rate.In the case of the connected load of the pre-dryer already described, only the connected load of the radiation elements and / or the heating elements is taken into account; any additional heat sources such as heating elements that are only used optionally or pre-heated cooling air for the radiation elements are not taken into account.

[0031] Due to the intentionally limited pre-drying in the pre-dryer, a significantly higher evaporation rate is now achieved in the main dryer than with longer pre-drying, i.e. the main dryer is operated more efficiently. The less pre-drying is carried out with the pre-dryer, the higher the evaporation rate achieved in the main dryer. This improved efficiency of the main dryer also means that its dryer temperature, i.e. the temperature of the hot air flowing onto the paper web, can be reduced. This in turn advantageously results in a reduced primary energy requirement, e.g. in the form of natural gas, as well as a reduction in CO2 emissions. In a suitable embodiment, the main dryer is then operated at a dryer temperature of less than 180 °C, particularly preferably at a dryer temperature in the range of 100 °C to 150 °C.In principle, the lowest possible drying temperature is preferred, but the drying temperature can also be up to 200 °C.

[0032] In the main dryer, hot air is blown onto the paper web to dry it. The hot air is preferably jetted out onto the paper web using a plurality of air nozzles, preferably at a flow velocity (here also nozzle exit velocity) in the range of 30 m / s to 60 m / s. The air nozzles are designed, for example, as slot nozzles or perforated nozzles. The main dryer preferably has a nozzle array with a large number of corresponding air nozzles (in particular not air knives), where a large number is understood to mean in particular “at least 10” and preferably “at least 100”, although the exact number is not important; what matters is primarily an arrangement of the air nozzles over the largest possible area, e.g. in a matrix with several rows and columns.In a practical embodiment, the main dryer is operated in a recirculation mode, in which exhaust air from the main dryer is at least partially and optionally mixed with fresh air and fed back to the main dryer as supply air in order to dry the paper web.

[0033] In an advantageous embodiment, in addition to the pre-dryer and the main dryer, the drying section also has a post-dryer (third dryer unit, third stage), which is arranged downstream of the main dryer with respect to the conveying direction. The paper web thus passes through a third dryer, the post-dryer, downstream of the pre-dryer and the main dryer. The post-dryer is a radiation dryer; the explanations in connection with the pre-dryer apply analogously; in particular, the post-dryer is also preferably an IR dryer. However, the post-dryer is suitably at least as long as the pre-dryer. The post-dryer is expediently shorter than the main dryer. The post-dryer preferably has a connected load that corresponds to 30% to 40% of the connected load of the main dryer.In a suitable design, the pre-dryer and the post-dryer are dimensioned such that their combined connected loads correspond at most to the connected load of the main dryer. The main dryer thus bears the main load of the drying process. The pre-dryer primarily serves to precondition the paper web for subsequent drying in the main dryer, while the post-dryer primarily serves for on-demand, i.e., job-dependent, post-drying downstream of the main dryer. Preferably, no other drying process, generally no temperature control, and / or no further processing of the paper web takes place between the main dryer and the post-dryer.

[0034] The after-dryer thus enables further drying of the paper web downstream of the impact jet dryer. While the evaporation rate in the main dryer decreases with increasing residence time, the radiation dryer allows further drying to be achieved despite the advanced drying of the paper web. Furthermore, this arrangement increases the dynamics during job changes, as the radiation dryer can be activated and deactivated accordingly quickly. This means that the after-dryer can be used to react quickly and energy-efficiently to print jobs that require higher drying performance (e.g. a paper web made of coated paper). Furthermore, the after-dryer, which can be activated particularly quickly, may make it possible to dispense with additional heat-protection coating in some circumstances, which correspondingly reduces the installation space required by the corrugator (in general: system).

[0035] The after-dryer expediently has a drying output that is controlled depending on the paper moisture content of the paper web. For this purpose, the control unit of the drying section, for example, has a corresponding (first) controller. The drying output is thus automatically adjusted to the paper moisture content. The paper moisture is measured in particular downstream of the main dryer and expediently upstream of the after-dryer. The paper moisture is measured, for example, using a microwave sensor. The electrical power of the radiation elements of the after-dryer serves as a control variable for the controller. Depending on the electrical power, a corresponding temperature of the radiation elements (manipulated variable) results, which then influences the paper moisture (controlled variable). A certain target paper moisture level is specified as a reference variable, e.g. manually by an operator or automatically based on job data for a current print job.The target paper moisture content is determined, for example, by the further processing of the printed paper web downstream of the drying section.

[0036] The pre-dryer, the main dryer, and the optional post-dryer are fundamentally designed separately from one another, meaning they are not combined in a single unit with a common housing. Nevertheless, at least two of the three dryers are connected to one another via an air duct system. The air duct system serves primarily to remove exhaust air from and supply air to the individual dryers. As the paper web is passed through each dryer, the surface temperature of the paper web increases. This results in a partial pressure difference of volatile substances (solvents, especially water) between the surface of the paper web and the air in the dryer (dryer air). The partial pressure at the surface of the paper web is higher than in the dryer air. Typically, it can be assumed that the partial pressure at the surface corresponds to the saturation vapor pressure of the volatile substances.This partial pressure in turn depends on the temperature.

[0037] The partial pressure of water in the dryer air depends not only on the total system pressure (absolute pressure) but also primarily on the molar fraction of water in the dryer air. This means that dryer air containing water generally has a lower drying potential. However, this is negligible if the saturation vapor pressure at the end of the drying section is very high. This means that exhaust air from one or more dryers can be profitably used as supply air in one or more dryers further downstream and is advantageously used. For example, exhaust air from the pre-dryer is used as supply air in the main dryer and / or in the after-dryer and / or analogously, exhaust air from the main dryer is used as supply air in the after-dryer. In this way, advantageous energy recovery is realized because the drying potential still present in the exhaust air from one dryer is still used in another dryer.This further increases the energy efficiency of the process.

[0038] A particularly preferred embodiment is one in which exhaust air from the pre-dryer is used as supply air for the after-dryer. In this case, the exhaust air expediently also serves as cooling air for the typically much warmer radiation elements of the after-dryer. This realizes exhaust air recirculation from the first to the third drying stage. Alternatively or additionally, the exhaust air is flowed onto the paper web as hot air for additional hot air drying in the after-dryer, analogous to the above explanations in connection with the pre-dryer. The amount of exhaust air used as supply air is expediently adjusted by means of a (second) controller - e.g. as part of the control unit - in order to achieve optimum drying performance. The exhaust air has a humidity which is measured, for example, with a corresponding humidity sensor and which in any case serves as a controlled variable for the controller.Furthermore, a specific target humidity is specified as a reference variable for the controller. The humidity of the supply air is then adjusted by the controller by mixing the exhaust air with fresh air accordingly (amount of fresh air = manipulated variable). The fresh air has, in particular, been dehumidified beforehand or is at least as dry as possible. The amount of fresh air added is controlled, for example, via a flap angle (control variable) of a fresh air flap in the air duct system. The partial pressure of water can also be used for control purposes as an equivalent to humidity. The target partial pressure is then conveniently calculated as the difference between the saturation partial pressure and a predetermined partial pressure difference. The saturation partial pressure depends on the surface temperature of the paper web and is calculated, for example, using the Antoine equation.

[0039] In the following, exemplary embodiments of the invention are explained in more detail with reference to a drawing. In the drawings:

[0040] Fig. 1 a method,

[0041] Fig. 2 a drying section,

[0042] Fig. 3 a corrugated board plant with the drying section from Fig. 2,

[0043] Fig. 4 shows the drying section from Fig. 2 in a different representation, with a control unit.

[0044] Fig. 1 shows a schematic view of a method for drying a printed paper web P by means of a drying section 2. An exemplary embodiment of the drying section 2 is shown schematically in Fig. 2. Fig. 3 then shows an example of a corrugated board plant 4 into which the drying section 2 is integrated together with a printer 6. The drying section 2 is arranged downstream of the printer 6 with which the paper web P is printed. Downstream of the drying section 2, the printed paper web P is further processed. In Fig. 3, the further processing is, by way of example, an assembly of the paper web P with further paper webs P' and / or semi-finished products made therefrom to form a corrugated board web W. Alternatively, the printer 6 and the drying section 2 are each part of a printing plant (not shown), at the end of which the printed paper web P is, for example, simply rolled up. The printer 6 in the present case is an inkjet printer, also referred to as a digital printer.

[0045] The drying section 2 has a pre-dryer 8 and a main dryer 10 and is therefore designed in several stages. The main dryer 10 is arranged downstream of the pre-dryer 8 with respect to a conveying direction F for the paper web P. The printed paper web P therefore first passes through the pre-dryer 8 and then the main dryer 10. The pre-dryer 8 is also the only dryer through which the paper web P passes between the printer 6 and the main dryer 10. In general terms, the terms "upstream" and "downstream" are understood here relative to the conveying direction F of the paper web P. The paper web P continues to be conveyed at a web speed. This speed is the same in the printer 6 and along the drying section 2, but can fundamentally vary overall.

[0046] In the exemplary embodiment shown, the pre-dryer 8 is a radiation dryer with a number of radiation elements 12 with which the paper web P is radiation-dried in a first step S1. In addition to or instead of the radiation elements 12, other heating elements can be used. “A number of” is generally understood to mean “one or more,” but in the present case the radiation dryer has not just one, but several radiation elements 12. In Fig. 2, these each extend in a transverse direction Q perpendicular to the conveying direction F and across the entire width of the paper web P. The radiation elements 12 themselves are also parallel to one another here. Alternatively, the radiation elements 12 extend in the conveying direction F and can then be switched off individually, for example depending on a width of the paper web P perpendicular to the conveying direction F (not shown).The pre-dryer 8 shown here is an IR dryer (infrared dryer), and the radiation elements 12 are each designed as IR filaments, e.g., carbon filaments. The main dryer 10, on the other hand, is an impact jet dryer, also known as a "hot air dryer," with which the paper web P is hot-air dried in a second step S2. Accordingly, a quantity of hot air H1 is directed onto the paper web P to dry it. The hot air H1 is heated, e.g., using a steam-to-air heat exchanger, gas burner, heating rod, or a comparable heating element (not shown).

[0047] For the sake of simplicity, we will refer to the drying of the paper web P. This means that both the paper from which the original paper web P is formed and the printed image, i.e., the printed ink, are dried.

[0048] Compared to the main dryer 10, the pre-dryer 8 has a lower connected load or is shorter (viewed in the conveying direction F), or both. As a result, the drying and thus the heating of the paper web P in the pre-dryer 8 is limited. The "connected load" is understood to mean the maximum thermal power (heat per unit of time) that can be generated / is generated by the respective dryer 8, 10, 26 for drying the paper web P. Both the pre-dryer 8 and the main dryer 10, as well as the post-dryer 26 mentioned below, each have a connected load. The power that is actually delivered by the respective dryer 8, 10 for drying at a given time is referred to as the "dryer power"; this can therefore correspond to a maximum connected load.In the embodiment shown here, the connected load of the pre-dryer 8 corresponds to at least 40% and at most 80% of the connected load of the main dryer 10. The connected load of the main dryer 10 is, for example, in the range of 3 kW / (m / min) and.

[0049] 8 kW / (m / min) and is also dependent on the web speed, measured in m / min. By “shorter” is meant in particular that the residence time for a given point on the paper web P in the pre-dryer 8 is shorter than in the main dryer 10, i.e. the pre-dryer 8 is passed through in a shorter period of time than the main dryer 10. For a given web speed, a path W1, W2, W3 along which the paper web P is dried is therefore also spatially viewed shorter in the pre-dryer 8 than in the main dryer 10. This is immediately apparent in Fig. 2, where the path lengths W1, W2 are shown in a ratio of 1:6, and the main dryer 10 is correspondingly longer than the pre-dryer 8. The terms “shorter / short” and “longer / long” are to be understood generally in relation to the path W1, W2, W3 along which the paper web P is dried in the respective dryer 8, 10. In the embodiment of Fig.2, the pre-dryer 8 has a (first) path W1 along which the paper web P is dried. The main dryer 10, however, is in the present case approximately half an order of magnitude longer than the pre-dryer 8, ie a (second) path W2 of the main dryer 8 is correspondingly longer than the first path W1.

[0050] The pre-dryer 8 is used to preheat (also: precondition) the paper web P for the most efficient drying in the downstream main dryer 10. The environment of the paper web P in the main dryer 10 is characterized by a cooling limit temperature, and the pre-dryer 8 specifically heats the paper web P to the cooling limit temperature (e.g., with a tolerance of + / -20°C). More important than the exact dimensioning of the pre-dryer 8 and main dryer 10 is that a radiation dryer is used as the pre-dryer 8, followed by an impingement jet dryer as the main dryer 10, with the dimensioning of the two dryers 6, 10 being coordinated relative to one another with the aim of achieving the most energy- and process-optimized drying possible. A key idea in this case is to optimize the drying of the printed paper web P so that it is dried with a heat flow that is as high as necessary, but as low as possible.This is achieved by the special coordination of the pre-dryer 8 and the main dryer 10, i.e. by a special dimensioning of the two dryers 8, 10 relative to one another, as well as by the special type of dryer (radiation, impact jet) and their sequence in the conveying direction F. In this case, the pre-dryer 8 uses as little energy as possible to first fix the print image on the paper web P, to initiate an initial drying process and, through the high power density of the radiation, to increase the temperature of the paper web P and thus the partial pressure of volatile substances (e.g. solvents in the ink). However, the reduced dimensioning of the pre-dryer 8 compared to the main dryer 10 prevents the paper web P from heating up too much, in order to use the downstream main dryer 10 more efficiently.In this case, the paper web P is specifically preheated by the pre-dryer 8 in such a way that the highest possible evaporation rate is achieved in the main dryer 10.

[0051] In the embodiment shown here, a hot air function is integrated into the pre-dryer 8, i.e. in addition to radiation drying, the paper web P in the pre-dryer 8 is also hot air dried with hot air H2. Here, in the pre-dryer 8, the printed paper web P is additionally exposed to a stream of hot air H2 (hot air stream). The hot air H2 is obtained from cooling air for the radiation elements 12. In the embodiment according to Fig. 2, a quantity of supply air 24, e.g. fresh air 22, is supplied to the pre-dryer as cooling air, with which the radiation elements 12 are cooled by the supply air 24 being guided over the radiation elements 12. The supply air 24 is in contact with the radiation elements 12 and absorbs heat from them. The supply air 24 is either unheated or already heated.The supply air 24, now heated by the radiation elements 12, is now flowed as hot air H2 onto the paper web P, in this case with a flow velocity in the range of 10 m / s to 50 m / s.

[0052] Optionally, in the pre-dryer 8 shown here as an example, additional heat is supplied to the paper web P by means of one or more heating plates 14, which are arranged, for example, as shown in Fig. 2, on the opposite side of the paper web P with respect to the radiation elements 12. Alternatively or in addition to the heating plates 14, additional radiation elements 12 analogous to the radiation elements 12 already described are used, so that the paper web P is then radiation-dried on both sides (not shown). The heating plates 14 are generally heating elements, which, in an embodiment not shown, can also be used instead of the radiation elements 12 for the main pre-drying and must then also be taken into account in the connected load of the pre-dryer.

[0053] Due to the intentionally limited pre-drying in the pre-dryer 8, a significantly higher evaporation rate is achieved in the main dryer 10 than with a longer pre-drying period. The less pre-drying is performed in the pre-dryer 8, the higher the evaporation rate achieved in the main dryer 10. This also allows its dryer temperature, i.e., the temperature of the hot air H1 flowing onto the paper web P, to be reduced, e.g., to a dryer temperature in the range of 100°C to 150°C.

[0054] In the main dryer 10, hot air H1 is flowed onto the paper web P in order to dry it. In the exemplary embodiment in Fig. 2, the hot air H1 is jetted out by means of a plurality of air nozzles 16 and flowed onto the paper web P, e.g. at a flow velocity in the range of 30 m / s to 60 m / s. The air nozzles 16 are designed here, for example, as perforated nozzles, and the main dryer 10 has a nozzle array 18 with a plurality of corresponding air nozzles 16. In the embodiment shown here, the main dryer 10 is also operated in recirculation mode, in which exhaust air 20 from the main dryer 10 is at least partially and optionally mixed with fresh air 22 and fed back to the main dryer 10 as supply air 24 in order to dry the paper web P.

[0055] The drying section 2 shown here, in addition to the pre-dryer 8 and the main dryer 10, also has an optional post-dryer 26, which is arranged downstream of the main dryer 10 with respect to the conveying direction F. The paper web P thus passes through a third dryer, the post-dryer 26, downstream of the pre-dryer 8 and the main dryer 10 in a third step S3. The post-dryer 26 is a radiation dryer; the statements regarding the pre-dryer 8 apply analogously. The after-dryer 26 here is also an IR dryer, but the after-dryer 26 is at least as long as the pre-dryer 8 and at least 1 m and a maximum of 1.5 m long, in Fig. 2 1.5 m long and thus 1.5x as long as the pre-dryer 8. The after-dryer 26 is, however, shorter than the main dryer 10. The after-dryer 26 has a connected load which corresponds to 30% to 40% of the connected load of the main dryer 10.In one possible embodiment, the pre-dryer 8 and the post-dryer 26 are dimensioned such that their total connected loads correspond at most to the connected load of the main dryer 10. The main load of the drying process is thus borne by the main dryer 10. The pre-dryer 8 primarily serves to precondition the paper web P for subsequent drying in the main dryer 10, while the post-dryer 26 serves primarily for on-demand, i.e., job-dependent, post-drying downstream of the main dryer 10.

[0056] The drying section 2 has a control unit 28. An exemplary embodiment is shown schematically in Fig. 4.

[0057] In the present case, the after-dryer 26 has a drying output which is controlled as a function of the paper moisture content of the paper web P. For this purpose, the control unit 28 has a corresponding (first) controller 30. The drying output is thus automatically adjusted to the paper moisture content. The paper moisture content is measured downstream of the main dryer 10 and upstream of the after-dryer 26, e.g. with a microwave sensor S1. The electrical power of the radiation elements 12 of the after-dryer 26 serves as the control variable for the controller 30. Depending on the electrical power, a corresponding temperature of the radiation elements 12 results (manipulated variable), which then influences the paper moisture content (controlled variable). A specific target paper moisture content is specified as the reference variable, e.g. manually by an operator or automatically based on job data for a current print job.The target paper moisture content is determined, for example, by the further processing of the printed paper web P downstream of the drying section 2.

[0058] The pre-dryer 8, the main dryer 10, and the optional post-dryer 26 are essentially designed separately from one another, meaning they are not combined in a single unit with a common housing. Nevertheless, at least two of the three dryers 8, 10, and 26 are connected to one another via an air duct system 32. The air duct system 32 serves to remove exhaust air 20 from and supply supply air 24 to the individual dryers 8, 10, and 26.

[0059] In the present case, exhaust air 20 from one or more of the dryers 8, 10 is used as supply air 24 in one or more dryers 10, 26 located further downstream. Specifically, in Figs. 2 and 4, the exhaust air 20 from the pre-dryer 8 is used as supply air 24 for the after-dryer 26. The exhaust air 20 also serves as cooling air for the even significantly warmer radiation elements 12 of the after-dryer 26. This realizes exhaust air recirculation from the first to the third dryer stage. In addition, the exhaust air 20 is flowed onto the paper web P as hot air H3 for additional hot air drying in the after-dryer 26, analogous to the above explanations in connection with the pre-dryer 8. The amount of exhaust air 20 used as supply air 24 is adjusted by means of a (second) controller 34 as part of the control unit 28 in order to achieve optimum drying performance.The exhaust air 20 has a humidity that is measured with a corresponding humidity sensor S2 and which serves as a controlled variable for the controller 34. Furthermore, a specific target humidity is specified as a reference variable for the controller 34. The humidity of the supply air 24 is then adjusted by the controller 34 by mixing the exhaust air 20 accordingly with fresh air 22 (amount of fresh air 22 = manipulated variable). The fresh air 22 has been dehumidified beforehand or is at least as dry as possible. The amount of added fresh air 22 is controlled, for example, via a flap angle (control variable) of a fresh air flap 36 of the air duct system 32. The partial pressure of water can also be used for control purposes as an equivalent to humidity. List of reference symbols.

[0060] 2 drying sections

[0061] 4 Corrugated board line

[0062] 6 printers

[0063] 8 pre-dryers (radiation dryers)

[0064] 10 main dryers (impact jet dryers)

[0065] 12 Radiation element

[0066] 14 Heating plate

[0067] 16 hole nozzle

[0068] 18 nozzle field

[0069] 20 exhaust air

[0070] 22 Fresh air

[0071] 24 supply air

[0072] 26 post-dryers (radiation dryers)

[0073] 28 Control unit

[0074] 30 first controller

[0075] 32 Air duct system

[0076] 34 second controller

[0077] 36 Fresh air flap

[0078] F Conveying direction

[0079] H1 Hot air (in the main dryer)

[0080] H2 hot air (in the pre-dryer)

[0081] H3 Hot air (in the after-dryer)

[0082] P (printed) paper web

[0083] P' paper web

[0084] 51 first step

[0085] 52 second step

[0086] 53 third step

[0087] W Corrugated board web

[0088] W1 (first) route

[0089] W2 (second) route

[0090] W3 (third) route

Claims

Claims 1. Method for drying a printed paper web (P) by means of a drying section (2), a. wherein the drying section (2) has a pre-dryer (8) and a main dryer (10), b. wherein the main dryer (10) is arranged downstream of the pre-dryer (8) with respect to a conveying direction (F) for the paper web (P), c. wherein the paper web (P) is pre-dried with the pre-dryer (8), d. wherein the main dryer (10) is an impingement jet dryer with which the paper web (P) is hot-air dried, e. wherein the pre-dryer (8) has a lower connected load or is shorter or both than the main dryer (10).

2. Method according to claim 1, wherein the pre-dryer (8) is a radiation dryer, in particular an IR dryer, with a number of radiation elements (12) with which the paper web (P) is radiation-dried.

3. Method according to claim 1 or 2, wherein the pre-dryer has at least one heating plate (14) with which the paper web is dried.

4. Method according to claim 1 or 2, wherein the connected load of the pre-dryer (8) corresponds to at least 40% and at most 80% of a connected load of the main dryer (10).

5. Method according to one of claims 1 to 4, wherein the connected load of the main dryer (10) is in the range of 3 kW / (m / min) and 8 kW / (m / min).

6. Method according to one of claims 1 to 5, wherein an environment of the paper web (P) in the main dryer (10) is characterized by a cooling limit temperature, wherein the paper web (P) is heated to the cooling limit temperature by the pre-dryer (8).

7. Method according to one of claims 1 to 6, wherein the paper web (P) is preheated with the pre-dryer (8) in such a way that in the main dryer (10) an average evaporation rate of at least 4 g / (m 2 *s) up to 12 g / (m 2 *s) is achieved 8. Method according to one of claims 1 to 7, wherein a quantity of supply air (24) is supplied to the pre-dryer (8) as cooling air, with which the radiation elements (12) are cooled by guiding the supply air (24) over the radiation elements (12), wherein the supply air (24) absorbs heat from the radiation elements (12) and is then flowed as hot air (H2) onto the paper web (P), preferably with a flow velocity of at least 10 m / s.

9. Method according to one of claims 1 to 8, wherein in the main dryer (10) hot air (H1) is flowed onto the paper web (P) in order to dry it, wherein the hot air (H1) is jetted out by means of several air nozzles (16) and is flowed onto the paper web (P), preferably at a flow speed in the range of 30 m / s to 60 m / s.

10. Method according to one of claims 1 to 9, wherein the drying section (2) has a post-dryer (26) which is arranged downstream of the main dryer (10) with respect to the conveying direction (F), wherein the post-dryer (26) is a radiation dryer. 11 . Method according to claim 10, wherein the after-dryer (26) is at least as long as the pre-dryer (8), and / or wherein the after-dryer (26) is at least 1 m and at most 1.5 m long, and / or wherein the after-dryer (26) is shorter than the main dryer (10), and / or wherein the after-dryer (26) has a connected load which corresponds to 30% to 40% of the connected load of the main dryer (10), and / or wherein the pre-dryer (8) and the after-dryer (26) are dimensioned such that their connected loads in total correspond at most to the connected load of the main dryer (10).

12. Method according to claim 10 or 11, wherein the after-dryer (26) has a drying capacity which is controlled depending on a paper moisture content of the paper web (P).

13. Method according to one of claims 10 to 12, wherein exhaust air (20) of the pre-dryer (8) is used as supply air (24) for the after-dryer (26) and thereby serves as cooling air for radiation elements (12) of the after-dryer (26).

14. Method according to one of claims 1 to 13, wherein the amount of exhaust air (20) used as supply air (24) is adjusted by means of a controller (34), wherein the exhaust air (20) has a humidity which serves as a control variable for the controller (34).

15. Drying section (2) which is designed to carry out a method according to one of claims 1 to 14.

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