WO2004094723A1

WO2004094723A1 - Doctor for a paper or board machine

Info

Publication number: WO2004094723A1
Application number: PCT/FI2004/000253
Authority: WO
Inventors: Hannu Kasula; Kenneth Widell; Vesa Ahvenniemi
Original assignee: Metso Paper, Inc.
Priority date: 2003-04-24
Filing date: 2004-04-23
Publication date: 2004-11-04
Also published as: DE112004000672T5

Abstract

The invention relates to a doctor for a paper or baord machine, which doctor has been disposed in connection with a roll or a cylinder/drum (10) of the paper or board machine to keep the surface of the roll or the cylinder/drum (10) clean. The doctor (20) includes a doctor blade (21) of the doctor and an actuating means (22) connected therewith to produce a doctoring blow (P) to prevent a web (W) from wrapping around the roll or the cylinder/drum (10) during a web break and to assist the threading of the web (W). According to the invention, the doctoring blow (P) is a low pressure blow, so that the pressure of the blower air of the blow is 0.05 - 0.4 bar and the volumetric flow of the blow is 0.05 - 1 m3/second/width metre.

Description

Doctor for a paper or board machine

The present invention relates to paper and board machines. More specifically, the present invention relates to a doctor according to the preamble of claim 1.

A rotating roll, drying cylinder or reeling drum of the paper or board machine can be provided with a doctor. The purpose of the doctor is to prevent the web from wrapping around the roll or the cylinder/drum during a web break and to help tail threading. A further purpose of the doctor is to keep the roll or cylinder surface clean. There are two different types of doctors regarding their operating principle: a contacting doctor in which the doctor blade of the doctor extending across the entire width of the roll or the cylinder is placed against the outer surface of the roll or the cylinder, or a non-contacting doctor in which the doctor blade of the doctor is at a small distance, usually at a distance of a few millimeters, from the outer surface of the roll or the cylinder, so that air is blown into a gap between the tip of the doctor blade of the doctor and the outer surface of the roll from the trailing side of the doctor blade of the doctor to prevent the web from travelling with the roll or the cylinder. The present invention relates to a non-contacting doctor, which is called an air doctor in the following. As known in the prior art, the air doctors use pneumatic system air as the source of blowing air, in which connection the velocity of air blown from the doctor blade of the doctor is high, but the blow quickly stops in the stagnant air field in front of it and the doctoring force produced by means of it is small because of small momentum. The contacting doctors in turn wear, brake the rotating cylinder and load the drive of the cylinder. In addition, wearing doctors must be replaced regularly, which causes short breaks in production.

One problem with the air doctor applications known from the prior art is illustrated in Fig. 1. An air doctor 20' has been arranged in connection with a roll or a cylinder 10' for producing a doctoring blow P'. The problem situation shown in the figure where a web W runs away and is not guided into the pulper may occur in particular at lower speeds.

One problem with the arrangements known from the prior art is also that the noise nuisance caused by blowing is considerable when pneumatic system air is used.

An object of the invention is to provide a doctor in which the problems and drawbacks of the prior art arrangements are eliminated or at least minimized.

With a view to achieving the objects described above and those coming out later, the doctor of a paper or board machine according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

The doctor in accordance with the invention uses a large mass flow in blowing, i.e. a large volumetric flow is produced in blowing by means of an actuating means, most appropriately a blower. In addition, the nozzle of the blower is arranged such that the air that is blown comes at a lower pressure. When the volumetric flow is large, the blow extends farther before it stops although the pressure is lower than the pneumatic system pressure (7.5 bar max). The doctor in accordance with the invention is thus a so-called low pressure air doctor, which is intended to advantageously separate a full width web from the surface of the roll in two ways: by doctoring the web off the surface of the roll in a conventional manner and to separate the web from the surface of the roll before this meets the doctor, which facilitates the doctoring itself. Since the air volume, the volumetric flow, supplied by the low pressure doctor towards the roll is considerably larger than, even as many as about ten times that of the prior art air doctors operating by means of pneumatic system air, the velocity of air is higher at a small distance from the surface of the nozzle. In that connection, the static pressure of the air jet is lower, which facilitates the separation of the paper from the surface of the roll, even separates the web from the surface of the roll. The web separates from the surface of the roll at the latest when it comes closer to the tip of the air doctor, where the velocity of air is highest. In accordance with an advantageous feature of the invention, a slit nozzle is used as the blow nozzle of the doctor.

The air pressure used in the doctor in accordance with the invention is 0.005 - 0.4 bar (0.5 - 40 kPa, advantageously below 0.3 bar (30 kPa), most appropriately 0.01 - 0.2 bar (1 - 20 kPa). The nozzle slit is 0.5 - 8 mm, advantageously 1 - 6 mm and the air volume, the volumetric flow, supplied by the low pressure doctor towards the roll is 0.05 - 1 mVsecond/width metre, most appropriately 0.15 - 0.3 mVsecond/width metre. The pressure required in the nozzle chamber of the air doctor, as well as the angle between the air jet and the roll's tangent is 5 - 70°, most appropriately 15 - 40°.

In the invention, the web is advantageously brought under control and guided into a pulper by means of an air jet directed downwards from the doctor when an advantageous holding down blow of the entire width of the web in accordance with the invention is used. The holding down blow prevents the web from running away at lower speeds, typically below 1200 m/min.

In accordance with an advantageous application of the invention, in addition to the operation of the low pressure doctor, the doctor comprises a second operation, holding down blowing extending across the width of the web, which blowing also gets its air from the blower. The purpose of the holding down blow is to guide the web into the pulper. In the arrangement in accordance with the invention, holding down blowing and doctor blowing can use the same air.

In accordance with an advantageous additional feature, a slit nozzle is also used as the holding down blow nozzle. The blower air pressure in holding down blowing is about 0.05 - 0.4 bar, most appropriately 0.1 - 0.2 bar and the nozzle slit is 1 - 10 mm, most appropriately 1.5 - 5 mm. In accordance with another advantageous additional feature, a hole row nozzle is used as the holding down blow nozzle, the diameter of whose holes is 0.5 - 8 mm, most appropriately 1 - 4 mm, and the distance of whose holes from one another is 3 - 30 mm, most appropriately 5 -20 mm.

In accordance with one advantageous additional feature of the invention, the blowing force can be profiled in doctor blowing and in holding down blowing in a cross direction, i.e. in the width direction of the web.

An advantage of the invention with respect to the doctors known from the prior art is that the doctor in accordance with the invention does not brake the rotating cylinder, thus not loading the drive of the cylinder because the friction between the air flow and the roll is infinitesimal as compared with the prior art arrangement in the gap between the doctor blade of the doctor and the cylinder.

The low pressure air doctor in accordance with the invention provides a greater doctoring force as compared with those known from the prior art because the momentum of the mass flow discharging from the low pressure air doctor is considerably higher than that of the prior art air doctor operating with pneumatic system air. Thus, the air jet blown to the surface of the roll has more power.

When the low pressure doctor provided with a blower is used, the number of the paper mill's pneumatic compressors can be reduced, which in turn lowers investment costs. A compressor is considerably more expensive than a blower in respect of both its acquisition costs and its operating costs.

The device in accordance with the invention is advantageously attached to frame structures of the paper or board machine by means of articulated joints such that it can be opened into a cleaning position. During this pivoting function, the doctor blow can be kept switched on, so that the blow cleans the gap between the doctor and the cylinder at the same time. Advantageously, the control devices of the doctor and the blowing down device in accordance with the invention are connected to the break automatics of the paper or board machine, so that in case of a web break the blow function is automatically switched on and the timing of the doctor can be coupled, when desired, with the sequence of threading blows. The control device of the doctor and the control device of the blowing down device are connected to each other and there is a temporal delay between the control of the doctor and that of the blowing down device. Moreover, the devices can be automatically switched off immediately or after a possible delay after a web break has been dealt with.

In accordance with an advantageous application of the invention, using the holding down blow produced by the device placed below the doctor, the run of the web can be brought under control when the web is being passed down into the pulper. A pressure of less than 40 kPa (0.4 bar) is most appropriately used in the air doctor in accordance with the invention.

The air doctor in accordance with the invention is advantageously used in connection with a reeling drum, but it can also be applied on drying cylinders and at suitable locations of a press as well as at other locations where the web is run down from a roll/cylinder. The doctor can be used up to a speed of at least 1200 m/min and even up to 2500 m min, in particular when the basis weight of the web is low.

In the following, the invention will be described in greater detail with reference to the figures of the appended drawing, but the invention is by no means meant to be narrowly limited to the details of them.

Figure 1 shows one problem encountered in prior art applications in connection with the running away of the web when using a doctoring blow produced by means of an air doctor. Fig. 1 has been described in the general part of the description part in connection with the portion describing the prior art. Figure 2 is a schematic view of the operating principle of the air doctor in accordance with the invention when the web is separated from a cylinder/roll.

Figure 3 is a schematic view of the operating principle of the air doctor in the blowing down phase, by which blowing down the web is guided to a pulper when the air doctor blows air to between the cylinder and the web, facilitating the separation of the web from the cylinder.

Figure 4 is a schematic cross-sectional view of an application of the doctor in accordance with the invention.

Figure 5 a is a schematic view of another application of the doctor in accordance with the invention as viewed from the side.

Figure 5b is a schematic view of the operation of the application of Fig. 5a in a second stage.

Figure 5c is a schematic axonometric view of a part of the doctor in accordance with the invention.

In Figs. 2 and 3 as well as 4 and 5a-5c, the same reference numerals are used of the parts corresponding to one another, which reference numerals also correspond to the reference numerals used of the corresponding parts in Fig. 1 with the difference that a superscript comma is used in connection with the reference numerals in Fig. 1.

In Fig. 2, an air doctor 20 is arranged in connection with a cylinder 10 for producing a blow P, by which a web W is separated from the surface of the cylinder 10. A device 25 is arranged in connection with the doctor 20 for a holding down blow PA, by which the ran of the web W is guided towards a pulper. A guide plate 27 is most appropriately arranged between a doctor blade 21 of the doctor 20 and the blowing down device 25.

A blower of the doctor 20 is designated by the reference numeral 22 and it is attached to the doctor blade 21 of the doctor. The doctor, its blade structure 21, the blower 22 and the blowing down device 25 are most appropriately attached pivotably to frame structures (not shown) of a paper or board machine for the purpose of cleaning. In that connection, the blow P blown from the doctor blade

21 of the doctor in a pivoting situation is guided to clean a gap between the doctor blade of the doctor and the surface of the cylinder 10. The doctor 20 is most appropriately connected to the a break and threading control system 30 of the paper or board machine, so that it can be arranged to operate automatically in case of a break and/or taking account of the threading phases. In controls 31, 32 of the doctor 20 and the blowing down device 25, which controls are connected to each other, there is a temporal delay and both also possibly comprise a temporal duration.

In the air doctor 20, a slit nozzle is advantageously used which extends across the entire width of the web W and the nozzle slit of which is 0.5 - 8 mm, most appropriately 1 - 6 mm. The blowing down device 25 has a nozzle slit 26 that is also most appropriately a slit nozzle that extends across the entire width of the web, and the nozzle slit is 1 - 10 mm, most appropriately 1.5 - 5 mm.

In the situation shown in Fig. 3, the web W has been separated by means of the blow P of the air doctor 20 from the surface of the cylinder 10 and the web W is passed towards the pulper by means of the downward blow PA of the blowing down device 20. The blow PA produced by means of the blowing down device 25 is either a low pressure blow, in which case the air flow needed by it is produced by the same blower 22 as the doctor blow P, the air pressure used being 0.05 - 0.4 bar (5 - 40 kPa), most appropriately 0.1 - 0.2 bar (10 - 20 kPa). Fig. 4 schematically shows one application of the doctor 20, i.e. a doctor apparatus, in accordance with the invention, disposed in connection with a drying cylinder 10. The doctor 20 is intended to run the web down in a web forming machine, such as a paper or board machine. The web is usually run down from the roll or cylinder 10 that carries it. In Fig. 4, the drying cylinder 10, more simply a cylinder, rotates clockwise and it carries the web W with it. The doctor 20 is used for preventing the web W from wrapping around the cylinder, by which doctor the web W is separated and run down to broke handling. For this purpose, the doctor includes members, i.e. a doctor blade 21 to produce a separating force and to direct it at the web W.

In accordance with the invention, the web W is separated abruptly in a contact- free manner. For this purpose, the doctor blade 21 is connected to a machine unit arranged to produce an air flow, i.e. to an actuating means 22, for example, a blower. More specifically, the doctor blade 21 includes a system of ducts 114 for passing and guiding the air flow produced, i.e. a doctoring blow P, to the roll or cylinder 10. In addition, a slit nozzle 115 is connected to the system of ducts 114 to produce from the air flow an air jet that generates a separating force (Fig. 4). Thus, the web W situated on the surface of the roll or cylinder 10 can be separated readily and simply by means of the air jet in a contact-free manner. Further, in accordance with this application of the invention, the actuating means 22 is arranged such that the pressure level in the system of ducts is below 30 kPa.

By connecting the slit nozzle to a low pressure level, the doctor is readily provided with a large volumetric flow. In practice, an air jet produced by means of a large amount of air extends far and its separating effect is greater that that of an air jet produced by means of a high-pressure hole nozzle, which air jet has been used in connection with doctor blades. In addition, a low-pressure air jet is almost noiseless. Today, a pressure level of below 30 kPa is already achieved by a blower, which is considerably less expensive to purchase and use than a pneumatic compressor. In accordance with this application of the invention, the actuating means is thus formed by a blower that is arranged to form a part of the doctor. In that case, the building of long air ducts is avoided and the blower can be dimensioned according to the doctor. Furthermore, this kind of doctor is compact, and easy to place in connection with a roll or a cylinder. Further, the pressure level of the doctor is easy to regulate simply by adjusting the blower. Fig. 4 is an illustration of principle of the blower 116, but in practice it can be located very near the doctor, even in contact therewith.

Because of the slit nozzle and a low pressure level but a high volumetric flow, an advantageously low speed of air is sufficient. In spite of this, the separating force is sufficient. In accordance with this application of the invention, the blower and the slit nozzle are arranged such that the speed of air in the slit nozzle is below 200 m/s, preferably 70 - 120 m/s. In that connection, the speed of air is clearly below the speed of sound, which means that the noise level also remains low. On the other hand, the slit nozzle has a larger open surface area than that of a hole nozzle, thus simultaneously avoiding the whistle characteristic of hole nozzles.

In addition to the blower, the properties of the air jet, i.e. the blow P, are also affected by the properties of the slit nozzle and, above all, substantially by the open surface area of the slit nozzle of the width of the roll or cylinder.

To provide a general-purpose doctor, the slit is arranged to be adjustable in accordance with the invention. By this means, the air jet can be adjusted in a case- specific manner according to different positions and web grades. More specifically, the system of ducts 114 includes a distribution tube 117 extending substantially across the entire width of the roll or cylinder 10. In practice, the diameter of the distribution tube is 200 - 600 mm, preferably 300 - 500 mm, so that the air flow produced for the blower can be distributed evenly across the entire width of the machine. Further, a duct 118 arranged to become narrower is connected to the distribution tube 117 in which the slit nozzle 115 is disposed. Because of the duct, the air flow in the cross direction of the paper or board machine turns so that it flows in the direction of the duct across the entire width of the doctor. In practice, the length of the duct is at least 100 mm, more commonly about 200 mm, so that the air flow has time to even out in the duct.

To distribute the air flow into the duct 118, the distribution tube 117 has a suitable gap 119 or suitable openings at suitable intervals. In spite of the gap, the distribution tube can be used as a load-carrying part of the doctor, so that a separate frame structure is not needed. In other words, the doctor can be supported at the ends of the distribution tube on the paper or board machine. Correspondingly, the duct 118 is formed by plate parts 120 and 121, which are easy to shape for guiding the air flow. The plate parts are, however, so thick or they are supported so well that deformations arising from an overpressure are avoided. In practice, the slit nozzle 115 is formed by the edges of the plate parts 120 and 121, which edges thus define the size of the slit (Fig. 4). For the purpose of adjusting the slit nozzle 115, there is additionally an adjustment rib 122 at the edge of at least one plate part 121 on the side of the roll or cylinder 10. By changing the location and the position of the adjustment rib, the slit of the slit nozzle can be adjusted as desired. At the same time, it is possible to affect the shape and orientation of the air jet.

Advantageously, the slit nozzle is substantially of the same kind across the entire width of the doctor. Thus, the web separates in a controlled manner and at the same time at different points. However, there may be differences in the web and in its separation in the cross direction, which means that the web separates differently at different points. In practice, for example, the edges of the web tend to adhere to the surface of the roll or cylinder. To stabilize the separation process, the duct 118 comprises at least one partition 123 (Fig. 5c) that extends in the longitudinal direction of the paper or board machine. The duct 118 is thus divided in the cross direction by means of the partition 123 into more than one zone 124. Generally, there are two to four zones for each width metre of the doctor. In addition, control members 125 are arranged in connection with the partitions 123 for regulating the air flow by zones. In Fig. 5c, the partition 123 and control dampers arranged to serve as the control members 125 are illustrated with broken lines. The control members allow regulation according to the situation so that the web is separated evenly across the entire width of the doctor. In Fig. 5c, the intensity of the air flow, i.e. the blow, is indicated with the length of the arrows. Here there is thus a stronger air jet in the edge area of the web than elsewhere. In the other figures, the arrows depict the flow of air in general.

To assure the separation of the web, the air jet is directed at a relatively sharp angle towards the cylinder. The slit nozzle is generally disposed such that the angle α between the air jet and the roll's or cylinder's tangent is 30° - 80°, preferably 40° - 60°, the direction of the air jet being against the direction of movement of the web.

As shown in Figs. 4 and 5a, the web is separated in practice already before the doctor because of a strong blow that extends far. In any event, the web is always separated at the latest at the point where the blow, i.e. the air jet, impinges on the cylinder. Separation is certain because the slit nozzle is placed very close to the cylinder. Generally, the distance is a few millimetres. In spite of the small gap the operation of the doctor is contact-free, which means that the servicing operations comprise mainly cleaning of the doctor.

Fig. 4 shows an example of a modernized doctor, in which a previous doctor is made use of. Previously, in the position shown there was a prior art doctor provided with a doctor blade, and it was desirable to get rid of this doctor. In the new doctor arrangement there is as an old part a doctor beam 126 which has been pivotably mounted by means of bearings on the paper or board machine and which is shown by broken lines in Fig. 4. In that connection, to form a new doctor, a whole formed by the distribution tube 117 and the duct 118 was attached to the doctor beam 126. In doctors designed from the very beginning it is then possible to use different f eedoms, so that the doctor can be made small in size and easy to clean. Another structural special feature is constituted by the above- mentioned partitions, which make the duct stiffer.

To bring the web under control after separation, the doctor additionally includes a second blowing apparatus 25 provided with a similar slit nozzle. The blowing apparatus is used for producing a kind of holding blow, by means of which the web is guided down in a controlled manner to broke handling. Figs. 5a and 5b illustrate the operation of the doctor in successive stages. In Fig. 5a, the web W is just separating from the surface of the cylinder 10 clearly before the doctor. Very soon after this, the web is already below the doctor, where the air blow produced by means of the blowing apparatus 25 brings the web W under control. In that connection, the web W is tightened and separates earlier from the cylinder 10 than in the initial stage. The direction of the second blow, i.e. air jet, is arranged to be downwards according to the direction of movement of the web to be run down, so that it simultaneously pulls the web. Thus, by means of the two blows, i.e. air jets, the web can be separated reliably and guided down in a controlled manner. The same blower is advantageously used for producing both blows, i.e. air j ets.

The following Table 1 shows a summary of calculated values when high-pressure hole nozzles are compared with low-pressure slit nozzles.

Quantity Hole Slit Slit Unit

Area of the hole/slit 28 400 400 nm 2

Speed in the nozzle 313 94 168 m/s

Pressure in the chamber 6 0.055 0.19 bar

Mass flow 0.03768 0.3767 0.06939 m3/s

Force 57.71 17.28 57.71 N/m

Table 1

The first comparison examines the achievement of the same mass flow with different nozzles. The hole nozzles using a pressure of 6 bar and the slit nozzle using a pressure of 0.055 bar provide the same mass flow. Correspondingly, the speed of air from the slit nozzle is only 94 m/s when from the hole nozzle it is almost the speed of sound. On the other hand, by increasing the pressure in the slit nozzle to a level of 0.19 bar, the same force is achieved as that of the hole nozzle. In that case, the mass flow is, however, already many times higher, so that the force extends considerably farther than from the hole nozzles. The speed of air simultaneously remains advantageously low.

The doctor in accordance with the invention is inexpensive to manufacture and use. The contact-free doctor does not have components that wear and require servicing. At the same time, the surface of the roll or cylinder remains undamaged. In addition, force and mass flow can be readily regulated simply by adjusting the blower. Regulation can also be accomplished by changing the structure of the doctor or by controlling the air flow in the cross direction of the paper or board machine.

Above, the invention has been described only with reference to some of its advantageous exemplifying embodiments, but the invention is not by any means meant to be narrowly limited to the details of them.

Claims

1. A doctor for a paper or board machine, which doctor has been disposed in connection with a roll or a cylinder/drum (10) of the paper or board machine to keep the surface of the roll or the cylinder/drum (10) clean, which doctor (20) includes a doctor blade (21) of the doctor and an actuating means (22) connected therewith to produce a doctoring blow (P) to prevent a web (W) from wrapping around the roll or the cylinder/drum (10) during a web break and to assist the threading of the web (W), characterized in that the doctoring blow (P) is a low pressure blow, so that the pressure of the blower air of the blow is 0.05 - 0.4 bar and the volumetric flow of the blow is 0.05 - 1 m³/second/width metre.

2. A doctor as claimed in claim 1, characterized in that the doctoring blow (P) is substantially of the entire width of the web.

3. A doctor as claimed in claim 1 or 2, characterized in that in order to produce the doctoring blow (P), the blow nozzle of the doctor blade (21) of the doctor is a slit nozzle whose slit is 0.5 - 8 mm, most appropriately 1 - 6 mm.

4. A doctor as claimed in any one of claims 1 to 3, characterized in that a blowing down device (25) has been arranged in connection with the doctor (20) to produce a holding down blow (PA) for guiding the web (W) into a pulper.

5. A doctor as claimed in any one of claims 1 to 4, characterized in that the blowing down device (25) has been arranged to produce a holding down blow

(PA) substantially of the entire width of the web.

6. A doctor as claimed in claim 1 to 5, characterized in that the blowing down device (25) has been arranged to produce a holding down blow (PA) whose air pressure is 0.05 - 0.4 bar.

7. A doctor as claimed in any one of claims 1 to 6, characterized in that the blow nozzle of the blowing down device (25) is a slit nozzle whose nozzle slit is 1 - 10 mm, most appropriately 1.5 - 5 mm.

8. A doctor as claimed in any one of claims 1 to 6, characterized in that the blow nozzle of the blowing down device (25) is a hole row nozzle the diameter of whose holes is 0.5 - 8 mm, most appropriately 1 - 4 mm and the distance of whose holes from one another is 3 - 30 mm, most appropriately 5 - 20 mm.

9. A doctor as claimed in any one of claims 1 to 8, characterized in that a control device of the doctor (20) and a control device of the blowing down device (25) have been connected to a break and threading control system (30) of the paper or board machine to operate automatically in break situations and/or in web threading situations.

10. A doctor as claimed in any one of claims 1 to 9, characterized in that the control devices of the doctor (20) arid the blowing down device (25) have been connected to each other.

11. A doctor as claimed in claim 10, characterized in that in controls of the control devices (31, 32) there is a temporal delay between the doctor (20) and the blowing down device (25).

12. A doctor as claimed in claim 10 or 11, characterized in that in the controls of the control devices (31, 32) there is a temporal duration between the doctor (20) and the blowing down device (25).

13. A doctor as claimed in any one of the preceding claims, characterized in that the doctor (20) has been attached pivotably to frame structures of the paper or board machine, so that a gap between the doctor blade (21) of the doctor (20) and the cylinder surface can be cleaned by means of the doctoring blow (P).

14. A doctor as claimed in any one of the preceding claims, characterized in that a guide plate (27) has been placed between the doctor blade (21) of the doctor and the blowing down device (25).

15. A doctor as claimed in any one of the preceding claims, characterized in that the doctoring blow (P) can be profiled in a cross direction.

16. A doctor as claimed in any one of the preceding claims, characterized in that the holding down blow (P) can be profiled in a cross direction.

17. A doctor as claimed in claim 1, characterized in that the doctor comprises a doctor blade (21) which has been connected to an actuating means (22) arranged to produce a blow and which includes a system of ducts (114) for passing and guiding the blow produced to the roll or cylinder (10), and a slit nozzle (115) connected to the system of ducts (114) to produce a blow that generates a separating force, and that the actuating means (22) has been arranged such that the pressure level in the system of ducts (114) is below 30 kPa.

18. A doctor as claimed in claim 17, characterized in that the actuating means (22) has been arranged to form a part of the doctor and it is formed of a blower

(116).

19. A doctor as claimed in claim 17 or 18, characterized in that the actuating means (22) and the slit nozzle (115) have been arranged such that the speed of air in the slit nozzle (115) is below 200 m/s, preferably 70 - 120 m/s.

20. A doctor as claimed in any one of claims 17 to 19, characterized in that the slit of the slit nozzle (115) substantially of the width of the roll or cylinder (10) has been arranged to be adjustable.

21. A doctor as claimed in any one of claims 17 to 20, characterized in that the system of ducts (114) includes a distribution tube (117) extending substantially across the entire width of the roll or cylinder (10) and a duct (118) connected to the distribution tube and arranged to become narrower, the slit nozzle (115) being disposed in said duct.

22. A doctor as claimed in claim 21, characterized in that the duct (118) is formed by plate parts (120, 121), the edge of at least one of which plate parts on the side of the roll or cylinder (10) is provided with an adjustment rib (122) for adjusting the slit of the slit nozzle (115).

23. A doctor as claimed in claim 21 or 22, characterized in that in the duct (118) there is at least one partition (123) extending in the longitudinal direction of the paper or board machine to divide the duct (118) into more than one zone (124) in the cross direction.

24. A doctor as claimed in claim 23, characterized in that control members (125) have been arranged in connection with the zones (124) to regulate the blow by zones.

25. A doctor as claimed in any one of claims 17 to 24, characterized in that the slit nozzle (115) has been disposed such that the angle between the blow and the roll's or cylinder's tangent (10) is 30° - 80°, preferably 40° - 60°, the direction of the blow being against the direction of movement of the web (W).

26. A doctor as claimed in any one of claims 17 to 25, characterized in that the doctor includes a second blowing apparatus (25) provided with a similar slit nozzle for producing a holding blow when the web (W) is run down.

27. A doctor as claimed in claim 26, characterized in that the blowing apparatus (25) has been arranged in the direction of movement of the web (W) after the doctor and in it the direction of blowing has been arranged according to the direction of movement of the web (W) which is ran down.