A paper web drying system comprising a combustion heater upstream of an electric heater

Abstract

The invention relates to a paper web drying system (1) for a paper manufacturing machine, the paper web drying system (1) comprising a drying assembly (10) arranged to direct heated air to a paper web (W), an air guide assembly (20) arranged to guide air to the drying assembly (10), an electric heater (30) arranged to heat air of the air guide assembly (20), and a combustion heater (40) arranged to heat air of the air guide assembly (20) The combustion heater (40) is upstream of the electric heater (30). The invention further relates to a method (100) for a paper web drying system.

Description

[0001] A PAPER WEB DRYING SYSTEM COMPRISING

[0002] A COMBUSTION HEATER UPSTREAM OF AN ELECTRIC HEATER

[0003] TECHNICAL FIELD

[0004] The present invention generally pertains to heating air in industrial applications, and in particular relates to paper web drying systems for paper manufacturing machines and methods for a paper web drying system.

[0005] BACKGROUND

[0006] Drying systems that employ heated air are essential in the production of paper products, including tissue paper. Common examples include through-air drying ("TAD") systems and Yankee hood drying systems. Many of these drying systems utilize process air recirculation in order to conserve energy. Combustion burners, often powered by fossil fuels, are frequently employed to heat or reheat air.

[0007] The drying of tissue paper is a particularly important application of TAD systems as described in WO2019212612A1.

[0008] Although existing drying systems generally fulfil their intended purposes satisfactorily, there remains room for improvement, for example in areas such as fossil fuel consumption, operational uptime, maintenance demands, and process control.

[0009] SUMMARY

[0010] A general object of the invention is to improve prior art systems and methods for heating air, particularly in drying applications within paper manufacturing, and to address the operational challenges identified above. Thereby, one particular object involves reducing maintenance demands to increase operational uptime in paper manufacturing machines.

[0011] In accordance with the present disclosure, the object is reached by a paper web drying system for a paper manufacturing machine. The paper web drying system comprises a drying assembly arranged to direct heated air to a paper web, an air guide assembly arranged to guide air to the drying assembly, an electric heater arranged to heat air of the air guide assembly,and a combustion heater arranged to heat air of the air guide assembly. The combustion heater is upstream of the electric heater.

[0012] Thereby, the combustion heater may not only serve to efficiently supply heat to the air, but also to oxidize or burn, thus reducing or eliminating, particles carried by the air. The combustion heater can be used not only to supply heat to the system, but also to at least reduce any build-up of dust / dirt at electrical coils or similar components of the electric heater. Even though there may remain some residues such as ashes downstream of the combustion heater, such residues are reduced in volume and typically exhibit physical properties that make them less likely to accumulate at the electric heater. As a result, an accumulation of particles at the downstream electric heater may be avoided or at least reduced. Thus, the need to stop the machine or its drying system for cleaning the electric heater is significantly reduced or eliminated, thereby contributing to improved operational uptime.

[0013] It is advantageous to reduce or avoid particles carried by the air from reaching the electric heater. Not only could such particles impair the function of the electric heater by blocking it and in effect thermally insulating it, such particles could also constitute a fire risk if they accumulate in the paper web drying system, e.g. at the electric heater. It is believed that with the combustion heater arranged upstream of the electric heater, any particles that could possibly accumulate, e.g. at the electric heater, are less likely to constitute a fire risk as they comprise little or no combustible substances. By “at” the electric heater is herein meant on, within, or in close proximity to the electric heater.

[0014] Moreover, the combination of an electric heater and a combustion heater increases flexibility with respect to energy sources for heating. For example, heating sources such as combustible fuels may be mixed with electricity. Additionally, the control of air heating may be facilitated. For example, one of the heaters may operate at a substantially constant and efficient power level, while the other may be load-dependently controlled.

[0015] The electric heater may be arranged to heat air guided by the air guide assembly. The combustion heater may be arranged to heat air guided by the air guide assembly. The air guide assembly may be arranged to guide air from the drying assembly to the combustion heater and from the combustion heater to the electric heater. The air guide assembly may be arranged to guide air from the electric heater to the drying assembly.Typically, the system is configured such that all the air that passes through the electric heater has already passed through the combustion heater.

[0016] The combustion heater may comprise a combustion chamber in which a flame or flames heat the air of the air guide assembly. Preferably, the combustion heater is configured such that a flame is in direct contact with the air to be heated. In other words, a flame of the combustion heater burns in the air to be heated. The combustion heater may alternatively be referred to as a burner. The combustion heater or its combustion chamber may be disposed close to the electric heater such that the combustion heater may bum off particles accumulated at the electric heater.

[0017] The electric heater and the combustion heater may each be of a capacity allowing one of them to provide sufficient heat to the paper manufacturing machine, which is beneficial for redundancy and for using only electricity or only combustible fuel. Alternatively, the electric heater and the combustion heater may be dimensioned such that both of them need to run simultaneously to provide sufficient heat to the paper manufacturing machine, which may be beneficial from an investment and / or operation cost perspective.

[0018] The paper web drying system may comprise an air-moving device, such as a blower or a fan. The air guide assembly may be arranged to guide air from the air-moving device to the combustion heater. In other words, the air-moving device may be upstream of the combustion heater and the electric heater, such that the air-moving device may operate at a comparably low temperature. Such a disposition of the air-moving device may be beneficial for its working point, and may enable the use of a comparably cost-efficient air-moving device.

[0019] The paper web drying system may be a Yankee drying system. The paper web drying system may be a through-air drying system.

[0020] In accordance with another aspect, there is provided a paper web drying system for a paper manufacturing machine. The drying system comprises a drying assembly arranged to direct process air to a paper web, an electric heater arranged to heat the process air, a combustion heater arranged to heat the process air, and an air guide assembly arranged to guide the process air in a recirculation path from the drying assembly to the heaters and back to the drying assembly. The drying system may further comprise a bypass arrangement arranged to bypass the drying assembly. Thereby, the system may operate safely under emergencyconditions, protecting the drying assembly and other components of the drying system from potential damage. Upon bypass, the process air may thus be circulated through the heaters. The drying system may further comprise a fresh-air inlet arrangement arranged to let in cold ambient air upstream of the electric heater and the combustion heater. Optionally, the paper web drying system may comprise only one or more electric heater(s) or heater modules, i.e. not combustion heater.

[0021] In accordance with another aspect, there is provided paper web drying system for a paper manufacturing machine. The paper web drying system comprises a drying assembly arranged to direct process air to a paper web, an electric heater arranged to heat the process air, and an air guide assembly arranged to guide the process air from the electric heater to the drying assembly. The paper web drying system further comprising a bypass arrangement configured to bypass the drying assembly and guide the process air back to the electric heater in case the air temperature downstream of the electric heater is above a temperature threshold value.

[0022] In accordance with another aspect, there is provided a method for a paper web drying system of a paper manufacturing machine. The drying system comprises a drying assembly arranged to direct heated air to a paper web, an air guide assembly arranged to guide air to the drying assembly, and first and second heaters arranged to heat air of the air guide assembly. The method comprises operating either the first or the second heater at substantially constant power and controlling the other one of the heaters at a load-dependent power. Thereby, the heating may be particularly efficient, and control thereof may be facilitated.

[0023] Typically, the heaters may be connected in series. The downstream one of the heaters may be controlled at a load-dependent power while the upstream one is operated at substantially constant power. Alternatively, the upstream one of the heaters may be controlled at a loaddependent power while the downstream one is operated at substantially constant power.

[0024] In accordance with another aspect, there is provided a method for a paper web drying system of a paper manufacturing machine. The drying system comprises a drying assembly arranged to direct heated air to a paper web, an air guide assembly arranged to guide air to the drying assembly, at least one electric heater, powered by electricity, arranged to heat the process air, and at least one combustion heater, powered by combustible fuel, arranged to heat the process air. The method comprises controlling the heaters to obtain a set ratio between the consumption of electricity and combustible fuel.The combination of an electric heater and a combustion heater increases flexibility with respect to energy sources for heating. For example, combustible fuels may be mixed with electricity. One of the heaters may operate at a substantially constant and efficient power level, while the other may be load-dependently controlled. Alternatively, a ratio may be set between electric heater and a combustion heater, or between a plurality of heaters powered by electricity and combustible fuel, respectively.

[0025] In accordance with another aspect, there is provided a paper web drying system for a paper manufacturing machine, comprising a first machine part and a second machine part, and an air guiding system arranged to guide air in a guiding path from the first machine part to a combustion heater, from the combustion heater to an electric heater, and from the electric heater to the second machine part. The first and second machine parts may be the same machine part. The paper web drying system may comprise a drying cylinder, and the machine part may be an enclosure arranged to direct heated air to a paper web on a surface of the drying cylinder.

[0026] In accordance with another aspect, the present disclosure provides a paper web drying system for a paper manufacturing machine, comprising a drying cylinder, an enclosure arranged to direct heated air to a paper web on a surface of the drying cylinder, and a recirculation system arranged to guide air in a recirculation path from the enclosure to a first heater, from the first heater to a second heater, and from the second heater back to the enclosure, wherein the drying system comprises arrangement providing a bypass path for the air so as to bypass the drying assembly. Thereby, the system may operate safely under emergency conditions, protecting the drying assemblyand other components of the drying system from potential damage.

[0027] In accordance with another aspect, the present disclosure provides a method for a paper web drying system of a paper manufacturing machine, the drying system comprising a drying cylinder, an enclosure arranged to direct heated air to a paper web on a surface of the drying cylinder, and a recirculation system arranged to guide air in a recirculation path from the enclosure to a first heater, from the first heater to a second heater, and from the second heater back to the enclosure, wherein the method comprises controlling one of the first and second heaters with a constant power load, and controlling the other of the first and second heaters so as to control the temperature of the air entering the enclosure on the recirculation path.In accordance with another aspect, the present disclosure provides a paper machine heater assembly comprising a combustion heater and an electric heater for heating a paper machine air flow, wherein the combustion heater and the electric heater are connected in series with the combustion heater arranged upstream of the electric heater. The air input to the paper machine heater assembly may e.g. be ambient air or process air from the paper machine.

[0028] In a general aspect, there is provided an air heating system comprising an electric heater arranged to heat air of the air heating system, and a combustion heater arranged to heat air of the air heating system, wherein the combustion heater is upstream of the electric heater.

[0029] Applicable to all aspects described herein, the air guide assembly may be arranged to guide the air from the heater or heaters to the drying assembly. The air guide assembly may be arranged to guide the air from the drying assembly to the heater or heaters. The air guide assembly may be arranged to guide the air from a first heater, e.g. a combustion heater, to a second heater, e.g. an electric heater. In the latter case, the air guide assembly may be arranged to guide the air from the second heater to the drying assembly. The air may be process air.

[0030] Also applicable to all aspects described herein, the bypass assembly may be arranged to guide air from one position in the air guide assembly to another position in the air guide assembly, so as for the air to bypass the drying assembly.

[0031] Further possible features, and associated advantages, are apparent from the other aspects described herein.

[0032] DESCRIPTION OF THE DRAWINGS

[0033] Below embodiments of the invention will be described with reference to the drawings, in which

[0034] figure 1 schematically illustrates a paper web drying system with a Yankee dryer of a paper manufacturing machine,

[0035] figure 2 generally illustrates a drying system with a bypass arrangement,

[0036] figure 3 schematically illustrates a paper web drying system with a through-air dryer of a paper manufacturing machine,

[0037] figure 4 illustrates an alternative to figure 1, and

[0038] figure 5 illustrates a method for a paper web drying system.DETAILED DESCRIPTION OF EMBODIMENTS

[0039] In paper manufacturing machines, such as tissue paper manufacturing machines, fuel burners are typically used as heat sources. Oftentimes, the fuel that powers the burners, or combustion heaters, may cause pollutant emissions, which lead to environmental issues and also incur costs in the form of carbon dioxide taxes. To address these challenges, some solutions incorporate electric heaters, often installed upstream of a circulation fan, to reduce reliance on fossil fuels and decrease emissions.

[0040] Referring to figures 1 to 4, each the presently exemplified paper web drying system 1 for a paper manufacturing machine comprises a drying assembly 10 arranged to direct heated air to a paper web W, such as a tissue paper web. In the examples of figure 1 and 4, the drying assembly 10 is a Yankee hood, more precisely the dry end (DE) of a Yankee hood. A portion of a Yankee cylinder 15 is indicated by a dashed line. The drying assembly could in addition or alternatively be connected to the wet end (WE) of the Yankee hood. In the example of figure 3, the drying assembly 10 is a hood of a through-air dryer. It is to be apprehended that the assembly 10 may in principle be any hood applicator or similar applicator that is arranged to direct air to a paper web, as is generally illustrated in figure 2.

[0041] The paper web drying system 1, or drying system, further comprises an air guide assembly 20 arranged to guide air to the drying assembly 10. The air guide assembly 20 may comprise ducts or pipes configured to guide air in a paper manufacturing machine. Furthermore, the air guide assembly 20 may typically comprise a number of undepicted baffles or deflectors for controlling the air flow of the drying system.

[0042] As denoted by a flash symbol in figures 1, 3 and 4, the drying system comprises an electric heater 30 arranged to heat air of the air guide assembly 20. A combustion heater 40 arranged to heat air of the air guide assembly 20 is denoted by a flame symbol.

[0043] A number of alternative arrangements of the electric heater 30 and the combustion heater 40 are feasible. In accordance with the present disclosure, the combustion heater 40 is upstream of the electric heater 30, as is shown in figures 1, 3 and 4. Figure 2 does not include any detailed representation as to the type of heater or heaters, but may for example include a combustion heater 40 upstream of an electric heater 30, or solely an electric heater.The configuration with the combustion heater 40 upstream of the electric heater 30 brings the advantage that particles encompassed in the air flow may be partly or completely burned by the combustion heater 40 before reaching the electric heater 30, such that the particles are less likely to reach the electric heater 30, and potentially accumulate at the electric heater 30. Especially in a paper making machine, the particles may comprise elongated cellulose fibre residues are that are prone to adhering to e.g. an electric heater 30. This problem may be particularly significant if process air is recirculated from a hood of a Yankee dryer or a through-air dryer (TAD).

[0044] By passing air to be heated in the electric heater 30 through the combustion heater 40 upstream of the electric heater, any particles entering the electric heater 30 may exhibit physical properties that make them less likely to accumulate at the electric heater 30. The drying system may be configured such that all the air, e.g. process air, that reaches the electric heater 30 has passed through the upstream combustion heater 40.

[0045] As is shown in figures 1 to 4, the drying system 1 may comprise an air-moving device 50, which may be referred to as a recirculation fan. The air guide assembly 20 may be arranged to guide air from the air-moving device 50 to the combustion heater 40, as is indicated by the arrows next to the air guide assembly 20.

[0046] The air guide assembly 20 may be arranged to guide air from the drying assembly 10 to the air-moving device 50. As is also exemplified in the figures, the guide assembly 20 may be arranged to guide air from the electric heater 30 to the drying assembly 10. Thus, the drying system 1 may be a process air recirculation system arranged to guide process air in a recirculation path from the drying assembly 10 to the heaters 30, 40 and back to the drying assembly 10.

[0047] The distance between the combustion heater 40 and the electric heater 30 may be selected such that the combustion heater 40 may bum off any particles that may possibly have accumulated at the electric heater 30. For example, it is plausible that particles may have accumulated at the electric heater 30 should the combustion heater 40 not have been active for a certain time of operation of the drying system 1. The combustion heater 40 may be adjacent to the electric heater 30. In some undepicted examples, the combustion heater 40 may be adjacent to the electric heater 30 such that there is no guide assembly 20 in the form of a pipe or duct extending between the combustion heater 40 and the electric heater 30. In suchexamples, the combustion heater 40 and the electric heater 30 may be arranged immediately next to one another.

[0048] Figure 3 shows two sets of electric heaters 30 and combustion heaters 40 in one drying system 1. The exemplified drying system comprises a through-air dryer with a drying cylinder 15 and a drying assembly 10 in the form of a hood, as shown to the left. As is indicated, process air may be recirculated in a recirculation path from the through-air dryer hood to the heaters 30, 40 and back to the drying assembly 10. As shown to the right, the through-air dryer may be connected to an air preheater system, or air preheater. Typically, the air preheater system may be connected to a plurality of through-air dryers. As exemplified, the air preheater system may recirculate process air in a recirculation path that includes the electric heater 30 and the combustion heater 40. The air preheater system may comprise an intake for intake of ambient air into the system, as is indicated by the arrow in the lower right comer.

[0049] Referring still to figure 3, the electric heater 30, here the electric heater of the air preheater system, may comprise at least two individually controlled electric heater modules 32, 34. Such a configuration may be applied generally to the electric heaters 30 of the present disclosure. Advantages may include improved redundancy and also lower electric power demand per electric feed line. In addition, the plural electric heater modules 32, 34 be operated by cost-effective power electric equipment. As indicated, the drying system 1 may comprise a first thyristor 32t arranged to modulate electric power supplied to the first electric heater module 32 and a second thyristor 34t arranged to modulate electric power supplied to the second electric heater module 34. In comparison to most alternatives, a thyristor is cost-effective in the present context. In an undepicted embodiment, one of the electric heater modules 32, 34 may be operated at constant power, and thereby not require any thyristor or similar, while the other one is load-dependently controlled, e.g. by a thyristor.

[0050] As is shown in figures 1, 3 and 4, the drying system 1 may comprise a drying cylinder 15. The drying assembly 10 may comprise an enclosure (e.g. a hood) arranged to direct heated air to the paper web W (schematically shown in figure 1 only) when arranged on a surface of the drying cylinder 15.

[0051] As has been described, the drying system 1 may be a process air recirculation system arranged to guide process air in a recirculation path from the drying assembly 10 to the heaters 30, 40 and back to the drying assembly 10. The system 1 may be configured such that selectively theelectric heater 30, the combustion heater 40, or both the electric heater 30 and the combustion heater 40 simultaneously, heat the air or the process air.

[0052] The drying system 1 may be configured such that either the electric heater 30 or the combustion heater 40 operates at substantially constant power, while the other one operates at a load-dependent power.

[0053] Referring to figure 2, a drying system 1 may comprise a bypass arrangement 25 configured to bypass the drying assembly 10. Such a bypass arrangement 25 may be present also in the examples of figures 1, 3 and 4. Thus, the drying system 1 for a paper manufacturing machine may comprise a drying assembly 10 arranged to direct process air to a paper web W, an electric heater 30 arranged to heat the process air, optionally a combustion heater 40 arranged to heat the process air, an air guide assembly 20 arranged to guide the process air in a recirculation path from the drying assembly 10 to the heaters 30, 40 and back to the drying assembly 10. The bypass arrangement 25 may be arranged to bypass the drying assembly 10. Thereby, as is illustrated, the bypass arrangement 25 may bypass the drying assembly 10 and guide the air back to the electric heater 30. Even not disclosed in detain, the bypass arrangement 25 may comprise a bypass inlet 25a connected to the air guide assembly 20 upstream of the drying assembly 10, and a bypass outlet 25b connected to the air guide assembly 20 downstream of the drying assembly 10.

[0054] Referring still to figure 2, the drying system 1 may comprise a fresh-air arrangement 26 configured to let comparably cold air (e.g. ambient air) into the drying system before, i.e. upstream of, the electric heater 30 and the combustion heater 40. Such a fresh-air arrangement 26 may be present also in the examples of figures 1, 3 and 4. As is illustrated, the fresh-air arrangement 26 may advantageously be configured to let cold air into the system upstream of the recirculation fan 50, thereby lowering the operation temperature of the recirculation fan 50 when air is introduced via the fresh-air arrangement 26.

[0055] The bypass arrangement 25 and / or the fresh-air arrangement 26 may preferably be operated in case of emergency conditions. Such emergency conditions may, for example, but not limited to, involve too high air temperature at the recirculation fan 50, i.e. an air temperature that is approaching or exceeding an equipment temperature limit. Another plausible emergency may be that the Yankee hood 10 is in a retracted position (i.e. positioned away from a paper webdrying position), or that there for other reasons exists no paper web W to dry, such that it would be of no use, and even dangerous, to provide hot air to the hood 10.

[0056] Figure 1 illustrates an exemplary combustion air shut-off arrangement 80 that may be comprised in the drying system 1. The combustion air shut-off arrangement 80 may be configured to shut a combustion air duct 84 that may be provided for supplying combustion air to the combustion heater 40, see top left corner of figure 1. When combustion air is not supplied via the combustion air duct 84, it may be beneficial to shut-off, i.e. close, the combustion air duct 84. The combustion air supplied via the combustion air duct 84 may be ambient air. The drying system 1 may be configured such that both process air from the airmoving device 50 and ambient air from the combustion air duct 84 is supplied to the combustion heater 40.

[0057] The combustion air shut-off arrangement 80 may be configured to shut and also pressurize the combustion air duct 84, when the combustion heater 40 is not in operation. As is illustrated, combustion air shut-off arrangement 80 may comprise first and second flow regulating devices 81 and 82 installed on the combustion air duct 84 and a second air-moving device 83, or second fan, to keep the combustion air duct 84 pressurized. As show, the second fan 83 may be connected to the combustion air duct 84 operatively between the first and second flow regulating devices 81 and 82. Such an air shut-off arrangement 80 is advantageous as it may eliminate the need to keep an undepicted main combustion air fan always on to avoid a backflow through the combustion air duct 84. The second fan 83 may be significantly less energy-consuming than the main combustion air fan. Such a shut-off arrangement 80 may be present also in the examples of figures 2 to 4, and it is to be noted that the shut-off arrangement 80 may be applied also to other drying systems than the ones described herein. For example, the shut-off arrangement 80 does not require there being an electric heater in the drying systems.

[0058] Referring to figure 1, the arrow to the left of the first flow regulating device 81 indicates ambient air being supplied from the main combustion air fan to the combustion heater 40 via the combustion air duct 84. The arrow below the second air-moving device 83 indicates ambient air supplied to pressurize the combustion air duct 84.

[0059] The drying system 1 may optionally comprise instruments for temperature reading, to monitor the temperature of the air or equipment of the drying system 1. Referring to figure 4, theremay be temperature sensors 91-95 arranged at various positions along the air guide assembly 20. Typically, temperature sensors 91-95 are positioned in between each component (fan, heater, etc). Similar sensors may be provided in the examples of figures 1 to 3.

[0060] For example, there may be a first temperature sensor 91 operatively positioned between the drying assembly 10 and the air-moving device 50, a second temperature sensor 92 operatively positioned between the air-moving device 50 and the combustion heater 40, a third temperature sensor 93 operatively positioned between the combustion heater 40 and the electric heater 30, and / or a fourth temperature sensor 94 operatively positioned between the electric heater 30 and the drying assembly 10.

[0061] If, as in the examples of figure 3 and 4, there are provided plural electric heaters or electric heater modules 32, 34, there may be a temperature sensor operatively positioned between each one of the electric heaters or electric heater modules. Thus, as is shown in the example of figure 4, the third temperature sensor 93 is operatively positioned between the combustion heater 40 and a first electric heater 32, the fourth temperature sensor 94 is operatively positioned between the first electric heater 32 and the second electric heater 34, and a fifth temperature sensor 95 is operatively positioned between the second electric heater 34, and the drying assembly 10.

[0062] During operation of the drying system, instruments for temperature reading, such as the temperature sensor 91-95, may be employed to control a heater so as to heat the air passing therethrough from a certain temperature at the heater inlet to a higher temperature at the heater outlet. For example, the power supply (e.g. fuel or electricity) to the heater may be controlled such that the air is heated (delta T) 100 degrees Celsius or 200 degrees Celsius.

[0063] Instruments for temperature reading, such as the fifth temperature sensor 95, may for example be positioned at the drying assembly 10 to ensure that the air temperature that the drying assembly 10 is exposed to does not exceed a certain threshold, which may be referred to as a first temperature threshold value. Should the drying system 1, e.g. by means of the first temperature sensor 95, detect that the air temperature that the drying assembly 10 is exposed to approaches or equals the first temperature threshold value, the drying system 1 may trigger the bypass arrangement 25. In this manner, the air temperature that the drying assembly 10 is exposed to may quickly be lowered. Especially when an electric heater is comprised in the drying system 1, it may be advantageous to provide for such a bypass function. One reasonbeing that electric heaters may respond relatively slow to power regulation. As a comparison, combustion heaters may respond faster to power regulation.

[0064] Instruments for temperature reading, such as the first temperature sensor 91, may for example be positioned at the recirculation fan 50 to ensure that the air temperature that the recirculation fan 50 is exposed to does not exceed a certain threshold, which may be referred to as a second temperature threshold value. Should the drying system 1, e.g. by means of the first temperature sensor 91, detect that the air temperature that the recirculation fan 50 is exposed to approaches or equals the second temperature threshold value, the drying system 1 may trigger the fresh-air arrangement 26. In this manner, the air temperature that the recirculation fan 50 is exposed to may quickly be lowered. Especially when an electric heater is comprised in the drying system 1, it may be advantageous to provide for such a fresh-air supply function, as discussed in the above paragraph.

[0065] Using e.g. the temperature sensors 91-95, the power level of the heater devices 30, 32, 34, 40 can be controlled in different ways. For example, but not limited to, every heater devices 30, 32, 34, 40 can use the temperature reading as a set-point for the is power regulation.

[0066] For example, a desired temperature of the air supplied to the drying assembly 10 may be in the range of 350-600 degrees Celsius or more typically 450-500 degrees Celsius. When the temperature of the air supplied to the drying assembly 10 is 450-500 degrees Celsius, the air exiting the drying assembly 10 may typically be approximately 300-400 degrees Celsius. Typically, the recirculation fan 50 arranged downstream of the drying assembly may have an upper operation temperature limit of 400 degrees Celsius.

[0067] In another example, one or more of the heater devices 30, 32, 34, 40 may be operated at a substantially constant power (which can be also zero, which means that the heater is turned off), while one or more of the other heater devices 30, 32, 34, 40 are controlled at a loaddependent power. The latter may involve that a respective temperature reading is input to a feedback system for controlling the heater devices 30, 32, 34, 40 load-dependently.

[0068] In yet another example, it may be considered that the amount of power provided to the respective heater devices 30, 32, 34, 40 is chosen according to the specific cost of the different energy sources of the drying system 1.With reference primarily to figure 4, a method 100 for a paper web drying system 1 of a paper manufacturing machine is next described. Figures 1 to 4 may facilitate embodiments of the method 100. The method comprises operating 110 either the first or the second heater 30, 40 at substantially constant power and controlling 140 the other one of the heaters 30 or 40 at a load-dependent power. The heaters may for example be the electric heater 30 and the combustion heater 40, or the electric heater modules 32, 34.

[0069] The method 100 may comprise comprise operating 110 either the first or the second heater 30, 40 at substantially constant power, obtaining 120 the temperature of the air guided to the drying assembly 10, and using the obtained temperature for controlling 140 the other one of the heaters 30, 40 at a load-dependent power.

[0070] In some detail, the method 100 may comprise operating 110 the combustion heater 40 at substantially constant power. This may comprise continuously supplying the combustion heater 40 with combustible fuel. Advantageously, the combustion heater 40 may be adapted or optimised to efficiently operate at said constant power. Simultaneously, the electric heater 30 may be controlled to ensure that the temperature of the air that is guided to the drying assembly 10 at least substantially equals a desired temperature, e.g. a preset desired temperature. The method may comprise obtaining the temperature of the air that is guided to the drying assembly 10, comparing the temperature to the desired temperature, and in case there is a difference, accordingly controlling the electric heater 30 to regulate the temperature of the air that is guided to the drying assembly 10. For example, the electric heater may be controlled by one or more thyristors, as described herein.

[0071] As has been discussed, it may be advantageous to instead operate the electric heater at substantially constant power, and to control the combustion heater to ensure that the temperature of the air that is guided to the drying assembly at least substantially equals a desired temperature. The reason being that combustion heaters may respond faster to power regulation as compared to electric heaters.

[0072] An alternative method for a paper web drying system 1 of a paper manufacturing machine is next described. The drying system comprises a drying assembly 10 arranged to direct heated air to a paper web W, an air guide assembly 20 arranged to guide air to the drying assembly 10, at least one electric heater 30, powered by electricity, arranged to heat the process air, and at least one combustion heater 40, powered by combustible fuel, arranged to heat the processair. The alternative method comprises controlling the heaters 30, 40 to obtain a set ratio between the consumption of electricity and combustible fuel. Thus, a share of the electric versus combustion heaters may be set.

[0073] As one example, it may be determined that a total heating power of 5.000 kW is required for the paper web drying. In view for example the availability and / or cost of electricity and combustible fuel, it may be desired to provide 1 / 5 of the total heating power by the use of electricity and 4 / 5 of the total heating power by the use of combustible fuel. Thus, the electric heater(s) 30 may be supplied with electricity to provide 1.000 kW heating power while the combustion heater(s) 40 may be supplied with combustible fuel to provide 4.000 kW heating power. In this connection, the efficiency of each heater may be taken into account when supplying the electricity and the combustible fuel. If the required total heating power decreases to 3.000 kW, the electric heater(s) 30 may be supplied with electricity to provide 600 kW heating power while the combustion heater(s) 40 may be supplied with combustible fuel to provide 2.400 kW heating power, assuming that the desired ratio between electricity and combustible fuel is maintained. The decrease in required heating power may result from reduced production, lighter paper grade, lowered Yankee dryer speed, etc. The method may comprise varying said ratio by the drying system or by an operator based e.g. on availability and / or cost of electricity and combustible fuel.

[0074] Referring to figure 4, the alternative method may comprise obtaining the temperature of the air upstream and downstream each heater 30, 40, using the temperature sensors 92-95. Based on the temperature upstream the first one of the heaters, in this case the temperature of sensor 92, and the desired temperature of the air to be supplied to the drying assembly 10, in this case the temperature of sensor 95, a total temperature increase to be provided by the heaters 30, 40 may be determined. The method may comprise determining a respective required air temperature increase to be provided by each heater 30, 40, and controlling each heater 30, 40 such that it provides its required air temperature increase.

[0075] It is to be understood that the invention is not limited to the particular embodiments and examples illustrated in the drawings and described herein; instead, it encompasses all modifications and variations within the scope of the appended claims.

Claims

CLAIMS1. A paper web drying system (1) for a paper manufacturing machine, the paper web drying system (1) comprisinga drying assembly (10) arranged to direct heated air to a paper web (W),an air guide assembly (20) arranged to guide air to the drying assembly (10), an electric heater (30) arranged to heat air of the air guide assembly (20), and a combustion heater (40) arranged to heat air of the air guide assembly (20), wherein the combustion heater (40) is upstream of the electric heater (30).

2. The paper web drying system of claim 1, comprising an air-moving device (50), wherein the air guide assembly (20) is arranged to guide air from the air-moving device (50) to the combustion heater (40).

3. The paper web drying system of claim 2, wherein the air guide assembly (20) is arranged to guide air from the drying assembly (10) to the air-moving device (50).

4. The paper web drying system of any preceding claim, wherein the distance between the combustion heater (40) and the electric heater (30) is selected such that the combustion heater (40) may burn off particles accumulated at the electric heater (30).

5. The paper web drying system of any preceding claim, wherein the electric heater (30) comprises at least two individually controlled electric heater modules (32, 34).

6. The paper web drying system of claim 5, wherein the drying system comprises a first thyristor (32t) arranged to modulate electric power supplied to a first one (32) of the electric heater modules and, optionally, a second thyristor (34t) arranged to modulate electric power supplied to a second one (34) of the electric heater modules.

7. The paper web drying system of any preceding claim, comprising a drying cylinder (15), wherein the drying assembly comprises an enclosure arranged to direct heated air to the paper web (W) when arranged on a surface of the drying cylinder (15).

8. The paper web drying system of any preceding claim, wherein the drying system (1) is a process air recirculation system arranged to guide process air in a recirculation path from the drying assembly (10) to the heaters (30, 40) and back to the drying assembly (10).

9. The paper web drying system of any preceding claim, configured such that selectively the electric heater (30), orthe combustion heater (40), orboth the electric heater (30) and the combustion heater (40) simultaneously,may heat the air.

10. The paper web drying system of claim 9, configured such the either the electric heater (30) or the combustion heater (40) is operated at substantially constant power, while the other one is operated at a load-dependent power.

11. The paper web drying system of any preceding claim, comprising a bypass arrangement (25) configured to bypass the drying assembly (10).

12. The paper web drying system of claim 11, configured to bypass the drying assembly (10) by means of the bypass arrangement (25) in case the air temperature downstream of the electric heater (30) is above a first temperature threshold value.

13. The paper web drying system of any preceding claim, comprising an air moving device (50) arranged downstream of the drying assembly (10) and a fresh air arrangement (26) configured to supply fresh air into the drying system (1) upstream of the air-moving device (50) and downstream of the drying assembly (10).

14. The paper web drying system of claim 13, configured to supply fresh air into the drying system (1) in case the air temperature upstream of the air-moving device (50) is above a second threshold value.

15. The paper web drying system of any preceding claim, comprising a combustion air shutoff arrangement (80) arranged to shut a combustion air duct (84) that is provided to supply combustion air to the combustion heater (40).1816. The paper web drying system of any preceding claim, wherein the drying assembly (10) is a hood of a Yankee dryer or of a through-air dryer (TAD).

17. A paper web drying system (1) for a paper manufacturing machine, the paper web drying system (1) comprisinga drying assembly (10) arranged to direct process air to a paper web (W),an electric heater (30) arranged to heat the process air,a combustion heater (40) arranged to heat the process air,an air guide assembly (20) arranged to guide the process air in a recirculation path from the drying assembly (10) to the heaters (30, 40) and back to the drying assembly (10), anda bypass arrangement (25) arranged to bypass the drying assembly (10).

18. The paper web drying system of claim 17, wherein the bypass arrangement (25) comprises a bypass inlet (25a) connected to the air guide assembly (20) upstream of the drying assembly (10) and a bypass outlet (25b) connected to the air guide assembly (20) downstream of the drying assembly (10).

19. A paper web drying system (1) for a paper manufacturing machine, the paper web drying system (1) comprisinga drying assembly (10) arranged to direct process air to a paper web (W),an electric heater (30) arranged to heat the process air,an air guide assembly (20) arranged to guide the process air from the electric heater (30) to the drying assembly (10), anda bypass arrangement (25) configured to bypass the drying assembly (10) and guide the process air back to the electric heater (30) in case the air temperature downstream of the electric heater (30) is above a first temperature threshold value.

20. The paper web drying system of claim 19, comprising an air moving device (50) arranged downstream of the drying assembly (10) and a fresh air arrangement (26) configured to supply fresh air into the drying system (1) upstream of the air-moving device (50) and downstream of the drying assembly (10).

21. A method (100) for a paper web drying system (1) of a paper manufacturing machine, the drying system (1) comprising19a drying assembly (10) arranged to direct heated air to a paper web (W),an air guide assembly (20) arranged to guide air to the drying assembly (10), and first and second heaters (30, 40) arranged to heat air of the air guide assembly (20), wherein the method (100) comprisesoperating (110) either the first or the second heater (30, 40) at substantially constant power andcontrolling (140) the other one of the heaters (30, 40) at a load-dependent power.

22. The method of claim 21, comprising operating (110) either the first or the second heater (30, 40) at substantially constant power, obtaining (120) the temperature of the air guided to the drying assembly (10), and using the obtained temperature for controlling (140) the other one of the heaters (30, 40) at a load-dependent power.

23. The method of claim 21 or 22, wherein the first heater is a combustion heater (40) and the second heater is an electric heater (30).

24. A method for a paper web drying system (1) of a paper manufacturing machine, the drying system (1) comprisinga drying assembly (10) arranged to direct heated air to a paper web (W),an air guide assembly (20) arranged to guide air to the drying assembly (10), at least one electric heater (30), powered by electricity, arranged to heat the process air, andat least one combustion heater (40), powered by combustible fuel, arranged to heat the process air,wherein the method comprises controlling the heaters (30, 40) to obtain a set ratio between the consumption of electricity and combustible fuel.

25. The method of claim 24, comprisingobtaining the temperature of the air upstream and downstream each heater (30, 40), determining a total temperature increase to be provided by the heaters (30, 40) to obtain a desired temperature of the air supplied to the drying assembly (10),determining a required air temperature increase to be provided by each heater (30, 40), andcontrolling each heater (30, 40) to provide its required air temperature increase.

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