The invention is applicable particularly in the drying section of a paper, paper board or pulp machine, where the drying section comprises one or more wire groups in which the web to be dried is guided, supported by a wire, over one or more drying cylinders in order to evaporate the moisture from the web. These wire groups may be provided with a single wire run or a twin wire run. Single wire run means a run where the web passes from one drying cylinder to the next supported by the same drying wire, also between the cylinders. In a twin wire run, which utilises top and bottom wires, the web has free, unsupported runs when it passes from one cylinder row to the next one.

The development of paper machines has resulted in machine constructions with higher and higher velocities. In order to retain the required drying capacity in these new high velocity machines it has been necessary to build correspondingly longer drying sections and machine rooms. This increases the construction costs considerably. Different solutions for increasing the evaporation capacity have been sought in order to keep the size of the drying sections at a reasonable level.

Wire through blowing, i. e. blowing air through the wire against the web which passes over the drying cylinder, has been presented as one possibility to increase the evaporation capacity of a multi-cylinder drying section. The increased evaporation provided by wire through blowing is based on two factors: -the ventilation effect, i. e. the fact that when the blow penetrates into the wire it improves the transfer of steam evaporated from the web into the air, and -the heating effect, i. e. the fact that heat is transferred by convection and conduction through the wire into the paper when hot air is blown against the wire.

In some applications of the wire through blowing the air is blown against the wire

from impingement blow hoods mounted above the drying cylinders. The object may then be to provide only a ventilation effect, i. e. to intensify the mass transfer, i. e. to intensify the transfer of steam through the wire.

Then the blown air does not have to be substantially hotter than the temperature of the air in the drying section. The temperature of the blown air can be, for instance, of the order of about 100 °C. However, regarding the ventilation effect the blown air should be as dry as possible. As blowing air it is therefore possible to use either supply air supplied to the drying section, or a mixture of this supply air and recycled air taken from the hood.

The impingement blow apparatus formed for the purpose of ventilation can have a relatively light and simple structure. However, this solution provides a relatively small increase in the drying effect.

In other applications the aim is also to transfer heat through the wire against the web to be dried. Then the blowing air must have a high temperature. It has, for instance, been proposed to mount a high capacity hood like a Yankee hood above a drying cylinder covered by a wire. In these high capacity hoods the blowing temperatures and blowing velocities can be the same as in Yankee hoods.

With these hoods a more efficient evaporation is obtained, but on the other hand, the extra evaporation brought by the hoods is only about half of that which could be achieved with impingement blow directly against the web. A great part of the heating effficiency is bound by the wire and released from the wire into the surrounding air in the return loop of the wire, i. e. when the wire returns from the last drying cylinder to the first one in the drying section. A large part of the heating power of the hoods is thus transferred into heating the air which surrounds the drying section and leaves this space together with the exhaust air. This is waste of energy.

The size of the air apparatus in the above described impingement blow hoods per

achieved extra evaporation is the double compared to the evaporation achieved by direct impingement blow. As structures they are large and as investments they are expensive, particularly because in them also all structures in the hood of the drying section must be dimensioned to withstand high temperatures.

In order to succeed, the impingement blow further requires very open wires, which presents a conflict regarding the requirements on runability and threading, at least in high velocity paper machines.

Therefore an object of the present invention is thus to provide a method and a device for drying a fibre web, in which method and device the above mentioned disadvantages are minimised.

The object is particularly to provide a method and a device which in the drying section of a paper machine or the like substantially can increase the efficiency of the evaporation of water from the web.

An object is also to provide a method and a device which is more efficient and simpler than previously, for the control and management of the moisture profile in the cross direction of the web.

A further object is to provide a method and a device which in a simple manner without great structural changes can be applied also in already operating paper machines or the like.

A further object is also to provide a method and a device which in use can shorten the length of a multi-cylinder drying section so that the evaporation efficiency will not suffer.

Thus an object is to provide a method and a device, thanks to which several drying cylinders can be omitted from the drying section, and which thus provide a drying section which is substantially shorter than previously.

In order to attain the above presented objects the method and the device according to the invention is characterised in that what is presented in the characterising clauses of the enclosed independent claims.

For a typical method according to the invention, in which the paper web is dried in at least one drying cylinder group known per se provided with a single wire run, where the web is supported by the wire when it is conveyed over the drying cylinders, it is then mainly characterising that the evaporation of the moisture from the web is intensified by increasing the temperature of the wire itself before the wire is conveyed over said at least one drying cylinder.

The invention is particularly well suited to be used in a drying cylinder group provided with a single wire run. When desired, the invention can also be applied in other connections, such as in a drying cylinder group provided with twin wire run.

In a solution according to the invention the temperature of the wire can be advantageously raised to a temperature of about 120 to 200 °C, typically to a temperature of about 150 °C. The temperature of the wire is advantageously raised so that the temperature of the heated wire is 0 to 100 °C higher than the temperature of the surface of the drying cylinder.

The temperature of the wire can be raised, for instance, by running the wire through a heating device where hot gas, advantageously hot air, is blown through the wire.

The invention is described below with reference mainly to applications, in which the wire is heated with hot air; however, the wire can also be heated with for instance hot steam or any other hot gas suitable for this heating. It is of course not necessary to actually run the wire through the device, but the blowing can be arranged to be effected against the wire from one side of it.

The temperature of the wire can be advantageously raised in a heating device using recirculated air, in which heating device the temperature of the recirculated air can be raised by e. g. gas heating to a temperature of about 200 to 300 °C, typically to

about 260 °C, by low pressure steam heating to a temperature of 140 to 150 °C, or by high pressure steam heating to a temperature of 160 to 200 °C. The blown air can be heated in the most advantageous manner available or in another manner which is the most suitable, for instance to a temperature of about 260 °C.

The heating device comprises a blow box or the like, from where hot air is blown on the first side of the wire. In addition, it is possible to arrange means on the other side of the wire, for instance a second separate box or a box integrated in the blow box, which from the other side of the wire recovers the air blown through the wire.

The air which is recovered in this way can be reheated and returned into the blow box. The air can be heated, as mentioned above, e. g. by gas heating or by directing it through a steam radiator. A gas burner is advantageous in that sense that with it the air temperature can be easily raised to a high temperature. With a steam radiator one will not easily reach the same temperature. However, steam heating, which typically means heating with low pressure steam (4 to 6 bar overpressure) or high pressure steam (10 to 16 bar overpressure), can be a useful alternative when suitable steam is available. It is also possible to use other devices and methods known per se for raising the air temperature. The air can e. g. be heated with oil radiators (thermal oil).

The air recirculation system including the heating can be arranged on the heat recovery level of the paper machine, or most advantageously on the roof of the hood in the drying section, whereby one will at least avoid long air channels and obtain an integrated system.

However, when hot air is used for heating the wire the air blown through the wire can also be allowed to spread into the space surrounding the drying cylinders and to discharge from this space mixed with other discharge air.

In order to prevent the hot gas blown through the wire from spreading freely into the air of the machine room, or even into the air of the drying section hood, the part

of the drying section provided with a hot wire according to the invention, or at least that part of the wire loop where the wire is heated, can be enclosed or provided with its own separate small-sized hood. The wire loop can be enclosed so that it is separated from the rest of the air space of the drying section hood. However, when required the enclosure can also extend as a uniform space through the entire drying section. If recirculated air is not used for heating the wire it is possible to take the required air from this enclosure or to recover it by using the recovery equipment described above.

Discharge air from the drying section can be removed from the above mentioned enclosure, from the drying section hood, or from both. The enclosure can be constructed so that bearings of the cylinders and rolls, the wire tensioning mechanisms etc., are left outside the enclosure, whereby it is not necessary to dimension these components for the high temperatures. Within the enclosure the air temperature can be relatively high, i. e. for instance 120 to 250 °C, typically about 150 °C. In practising the invention the air temperature in other places within the drying section hood can be the same as in conventional drying sections, i. e. about 70 to 90 °C. When the heated wire loop is enclosed it is possible to recover the heat of the hot air in a more economical way than letting the air first mix with the air in the hood or in the machine room.

Now it has been found that in wire through blowing the heat transfer occurs mainly by conduction through the wire, and that an effective heat transfer requires that -the wire has a high thermal conductivity, and -there is a high number of contact points and a large contact area of the wire against the web to be dried.

However, the permeability of the wire has not been found to have an obvious effect on the heat transfer. On the other hand, when the openness of the wire increases, its qualifications for an efficient heat transfer are reduced, because open wires are thicker and more bulky than less open wires, and because they generally have less

contact points and a smaller contact area against the web to be dried. A larger openness thus enables the blow to penetrate effectively into the wire, but the benefits from this are lost in the form of a poorer heat transfer in the wire itself, and from the wire to the web to be dried.

It has been found that in wire impingement blowing the ventilation effect depends on the permeability of the wire. The reason for this is supposed to be that the ventilation effect is based only on convection within the wire, and that thermal conductivity does not have any importance in this sense. With a relatively open wire, having a permeability of 3,700 in 3/M2/h, it is possible to achieve an almost threefold drying effect compared to a less open wire having a permeability of 1,500 m3/m2/h. But when one moves to very open wires, i. e. wires having a permeability of 7,300 m3/m2/h, one will not achieve any particularly high increase in the drying effect.

A heated wire according to the invention will continue to release heat to the web being dried when it passes through the cylinder group. With a heated wire it is possible to increase the drying capacity up to 30-50 %. The larger part of the drying section is provided with wire heating the greater is the effect of the solution according to the invention. It is most advantageous to heat the wire in the wire loop just before the cylinder group.

The manner according to the invention to increase the evaporation effect with the aid of a separately heated wire is substantially more advantageous than to blow hot air through the wire against the web, whereby a large part of the hot air and its heat will pass to the air space surrounding the drying cylinder. When required, it is of course possible to use impingement blowing in order to provide the ventilation effect mentioned earlier.

The method according to the invention is best suited to be used in a drying section provided with a single wire run having 3 to 12 drying cylinders and turn rolls,

where the web to be dried is guided over the drying cylinders between the wire and the drying cylinder. The invention may be applied for instance in the SymRun drying section of the applicant where the wire loop is located above the cylinder group. However, the invention can also be applied in drying sections provided with a twin wire run or in other drying sections where a wire or a similar supporting fabric is used to keep the web attached to the drying cylinder.

When the method according to the invention is used to dry a web with the aid of a heated wire from the side opposite the drying cylinders it will reduce the problems of one-sided web curling, i. e. drying effected only by the drying cylinder. Thus the method according to the invention can, when desired, be applied at the end of the drying section to control the curling at a point where the dry content of the web is > 70%.

With the method according to the invention it is further possible to effectively and rapidly control the drying, for instance in connection with a quality change. Then, for instance during start-up or after a web breakage, when the drying cylinders have not yet achieved their full drying capacity, the evaporation occurring on the wire can be momentarily kept at a higher level than during a normal run.

Correspondingly, when required, the evaporation can be kept at a lower level if it is desired to decrease the drying.

When desired, the heating of the wire can also be controlled in the cross direction of the web. Then the heating of the wire, the blowing of air/steam through the wire, is controlled in the cross direction of the web in order to obtain the desired temperature profile.

The method according to the invention will operate with most wires currently in use. However, the benefit provided by the invention will be greater if wires having a particularly good thermal conductivity and heat capacity, a high number of contact points and a large contact area were used in the solution according to the invention.

Thus, the wire can advantageously be dense, whereby the number of contact points between the wire and the web is high, and the wire has a large contact area with the web passing over the cylinder. The use of a dense wire is also advantageous regarding the runability.

When applying the invention, the wire typically has to withstand temperatures of only 150 to 200 °C. It is not necessary for the wire to withstand as large temperature differences as those wires which are used in applications with impingement blow, where the intention is to transfer heat, with the aid of hot air, through the wire against the web to be dried. The desired increase in the evaporation effect is obtained with the solution proposed in the invention, already at relatively low wire temperature increases, when the wire has a large contact area against the web and the wire has a good thermal conductivity and heat capacity. The thermal conductivity can be improved in wire materials known per se-plastics-by adding metal fibres or some other suitable material with a good thermal conductivity in the plastics material.

In the solution according to the invention the wire is advantageously heated to a temperature of about 150 °C (120 to 200 °C). Even a lower temperature can come into question if low pressure steam is the most advantageous heat source. The wire delivers heat to the web in all points where it touches the web. In solutions provided with a single wire run the contact of the wire to the web and its heat transfer is particularly effective on the cylinder. However, the heat transfer is very effective also on the turn roll between the drying cylinders, particularly on rolls provided with suction, such as the VAC rolls of the applicant, where the web is attached to the wire passing over the roll. The high negative pressure of VAC rolls improves the contact between the wire and the web.

The thermal mass of the wire is so large, that it does not substantially cool when it passes through the cylinder group. The grammage of the wire can be, for instance, 1,500 g/m2, when the corresponding grammage of the paper web is typically about

30 to 80 g/m2, even only 20 g/m2.

The following example shows that with the method according to the invention, which applies heating of the wire, it is possible with realistic process parameters to obtain a noticeable heat transfer from the wire to the web, i. e. a substantially more effective evaporation in the cylinder group of a paper machine, where the machine speed is 1,500 m/min (25 m/s), and where the cylinder group comprises -5 drying cylinders having a diameter of 1,830 mm, and -5 VAC rolls having a diameter of 1,500 mm.

The grammage of the wire in use is 1.5 kg/m2, and its characteristic heat capacity is 1.4 kJ/kg/°C. The characteristic heat consumption of the drying is 2,800 kJ/kgH20.

It is assumed that a wire heating increasing the Tappi square evaporation of the cylinder group by 15 kgH2O/m2/h is provided. The square evaporation in conventional drying groups is of the order 25 to 30 kgH2O/m2/h.

Then the increased evaporation per width metre in the cylinder group can be calculated as follows: 5 x n x 1.83 m x 15/3,600 kgH20/m2/s = 0.12 kgH20/m/s The heating power per width metre required for the increased evaporation is then 0.12 kgH2O/m/s x 2,800 kJ/kgH20 = 335 kW/m Because this energy is taken from the heat content of the wire, the temperature of the wire will decrease about 6 °C when it passes through the cylinder group, which can be calculated as follows: 335 kW/m/ (25 m/s x 1.5 kg/m2 x 1.4 kWs/kg/°C) = 6 °C When the wire passes over a drying cylinder there is formed a temperature gradient in the wire so that the greatest cooling occurs at the surface against the web. In order to keep the gradient at a reasonable level it is required that the wire has a good heat conductivity.

With the values in the above presented example the contact area of the wire against the web per width metre at the drying cylinders and the VAC rolls is 5 x n x (1.83 m + 1.5 m) x 240°/360° = 34.9 m2/m The required heat flow density from the wire to the web is thereby on the average 335 kW/m/34.9 m2lm = 9.6 kW/m2 When assuming that the average temperature of the wire is 150-6/2 = 147 °C and the temperature of the web is 75 °C, then the required heat transfer coefficient from the wire to the web is 9.6 kW/m2/ (147 °C-75 °C) = 0.133 kW/m2/°C The obtained value contains both the contact heat transfer coefficient from the wire to the web and the effect of the heat transfer occurring within the wire.

When the wire is heated by blowing hot air through it, it is advantageous that the wire has a large internal area and that the through flow causes an effective heat transfer. In the example case it is assumed that the heat transfer coefficient between the through blowing air and the wire is 1 kW/m2/°C, as calculated per the area of the wire. We further assume that the length of the through blowing hood (the blow box) of the wire is 1 metre for each drying cylinder, i. e. 5 metres in the example above.

Then, with the values of the example above, the required temperature difference between the through blowing air and the wire is D T = 335 kW/m (1 kW/m2/°C x 5 m2/m) = 67 °C The amount of the recirculated air is the lower the higher its temperature. Assuming that the initial temperature of the recirculated air is 260 °C and that it cools 86 °C when it flows through the wire, then the average D T between the air and the wire is 67 °C, as mentioned above. Then the mass flow of the recirculated air is m = 335 kW/m/1.40 kJ/kg dry air/°C x 86 °C) = 2.78 kg dry air/width metre It is assumed that the humidity of the recirculated air is 200 g H20/kg dry air. The

mass flow density of the recirculated air when blown through the wire is m/A = 2.78 kg dry air/s/m x 1.200/ (5 x 1 m) = 0.67 kg/m2/s which amount of air can be obtained at a reasonable pressure difference (a few hundred Pa) through the wire.

Thus the above mentioned calculation shows that the assumed added evaporation per area can be obtained by heating the wire.

The invention is described more closely in the following with reference to the enclosed drawings, in which Figure 1 shows a cross section of a part of the drying section taken in the travelling direction of the web, in which part a hot wire according to the invention is used, and Figure 2 shows a cross section of a drying section according to Figure 1.

The part of the drying section shown in Figures 1 and 2 comprises a cylinder group 10 provided with a single wire run where the wire 12 and the web 14 are arranged to pass over five drying cylinders 16 and five VAC rolls 18.

Means 22 are arranged in the wire return loop 20 above the drying cylinders 16 for heating the wire. The means 22 comprise a blow box 24 and a recovery box 26, where hot air is blown from the blow box through the wire to the recovery box.

The means 22 further comprise a recycled air heating device 28, such as a gas burner, which via the channel 30 is connected to the blow box 24 and via the channel 32 to the recovery box 26. A blower 34 is mounted in the channel 32, whereby the blower propels the air in the channel 32 from the recovery box 26 to the heating device 28 and further as reheated air in the channel 30 to the blow box 24.

The wire loop 12,20 of the cylinder group 10 is enclosed within the enclosure 36.

The other side of the enclosure 36 is shown with a shading in Figure 1. The enclosure 36 is as wide as the cylinders 16 and covers the actual cylinders, but leaves outside the enclosure the cylinder bearings and wire tensioning mechanisms which are not shown explicitly. The blow box 24 and the recovery box 26 are narrower than the enclosure and contained in the enclosure. On the other hand, the recirculated air heating device and the blower are mounted above and outside the enclosure 36.

The enclosure 36 is narrower than a conventional hood 38 in the drying section. On the other hand, the roof 40 of the enclosure 36 is formed by a part of the roof of the hood. It is, of course, also possible to construct a totally own roof for the enclosure.

The blower 34 and the heating device 28 have been arranged above the roof 40 of the enclosure.

The object of the enclosure is to prevent or minimise air exchange between the internal space within the enclosure and the other space within the hood of the drying section, so that it prevents the hot air from escaping from the enclosure, which would cause heat losses.

The wire heating device operates so that the recirculated air is heated in the heating device 28, for instance to a temperature of about 260 °C, and supplied via the channel 30 to the blow box 24 mounted above the return loop 20 of the wire, from where the air is blown through the wire. The hot air heats the wire to a temperature of about 150 °C. The wire travels from the blow box over the first VAC roll 18'to the first drying cylinder 16'. The VAC roll, which is a suction roll, sucks the web against the wire travelling over the VAC roll, whereby the web gets a good contact with the wire which heats the web. From the VAC roll the web and the wire travel together to the first drying cylinder 16', over which the web travels between the hot wire 12 and the cylinder. The hot cylinder and the hot wire evaporate effectively moisture from the web. The web is dried on both sides. The web and the wire pass through the whole cylinder group, passing alternately over a VAC roll and a drying

cylinder. The wire coming from the last drying cylinder has cooled to a temperature of about 144 °C, and guided by the rolls 42 it returns towards the integrated blowing/recovery box 24,26.

The air which is discharged from the recovery box 26 and has a temperature of e. g.

174 °C, is reheated in the heating device 28, which may be a gas burner, to a temperature of about 260 °C, and then it is again supplied as recirculated air to the blow box.

The invention can also be applied without a recovery box. Then the air blown through the wire is allowed to spread into the enclosure and to mix with the air in the enclosure. The air in the enclosure is supplied in any suitable manner known per se to be heated in a heating device, and then further to the blow box in order to heat the wire.

The invention is not intended to be limited to the above described embodiment of the invention, but on the contrary, the objective is that it can be widely applied within the scope of the inventive idea defined by the claims presented below.

Thus, in a drying group where the invention is applied, there may be, in addition to the drying cylinders or similar, other previously known dryers, such as impingement blow dryers, where the impingement blow is directed directly or through the wire against the web to be dried.