The invention also relates to a method in connection with a suction roll of a paper or board machine according to claim 10.

Suction rolls are used in paper and board machines in the wire section i. e. in the web forming section as well as in the press section and in the drying section. The present invention is mainly related to applications used in the wire i. e. web forming section and in the press section, in which the suction roll serves to dewater the web being formed.

The suction rolls used in paper or board machines usually comprise a perforated roll shell attached to end flanges at the ends of the roll. The end flanges are in turn journalled rotatably on attachment flanges situated at the ends of the roll and attached to the frame of the machine. Inside the roll shell, there may be a static suction box attached to the attachment flanges enabling suction to be applied to a given sector of the suction roll. The interior of the roll may also be empty, in which case suction is applied to the entire circumference of the roll shell. The ends of the roll are provided with ducts by which an external source of negative pressure can be connected to the roll.

Published Finnish application no. 974480 and Finnish patent specification no.

103593 are known from prior art.

By means of a suction roll the web is dewatered and by means of negative pressure the water being removed is passed into holes in the suction roll shell.

Seals extending over the entire length of the roll and defining the suction sector/s in the circumferential direction are used on the circumference of the suction roll.

After the suction chambers defined by the seals the negative pressure is released and a dewatering area begins, in which water is removed from the perforation of the suction roll and passed to be discharged. In certain applications problems arise in respect of the entire operation process of the suction roll in the emptying of the perforation of the roll in the area where the negative pressure is removed and in which the water should be removed-the water is not removed as desired, e. g. rapidly enough, which results, for example, in mist formation.

The amount of air the roll has to process in order to form the desired negative pressure in the holes of the roll shell is essential for the operation of the suction roll. As the speeds of the paper or board machines used today continue to increase, the amount of air processed by the suction rolls also grows. In addition, the negative pressure used as well as the selected hole pattern have an effect in the operation of the suction roll. Since the air flows removed by means of suction rolls are large, a change in the negative pressure or in the hole pattern induces a great change in the situation. When the suction roll is changed so that the hole pattern, open area, level of negative pressure, length of the chamber or the like changes relative to the original roll, either the dry solids content after the wire section may diminish and/or the mist formation of the suction roll may increase, since the structure of the suction box of the roll, the arrangement of doctors, trough structure and the like do not enable rapid emptying of the holes after the seal rib.

It has been suggested that the above-mentioned problem be solved by emptying the holes with the help of positive pressure after the suction chamber. One such arrangement is presented in WO 02/3316S, which discloses a suction roll for a paper machine, which suction roll has an axis of rotation and a perforated jacket disposed to rotate around the axis of rotation and at least two stationary zones extending being arranged axially inside the jacket, of which the first zone is

connected to a source of negative pressure and the second zone is connected to a source of positive pressure. A serious disadvantage relating to this arrangement is its need for pressurized air, air fans, etc.

The above-mentioned problem arising in some prior art arrangements is also dealt with in Figure 6 of the accompanying drawing, in which figure a web passed via a suction zone C'of the suction roll 10'has the reference sign W'and a fabric the reference sign F'. A roll shell 12'comprises suction holes 18'. A suction chamber V'is arranged inside, on the upper side of the roll shell 12 of the suction roll 10', and the suction zone C'is defined by means of seals 15', 16', which seals are arranged in connection with the suction chamber V'axially to the suction roll 10'.

Pressure po is effective in the suction chamber V'. Pressure p2 is effective outside the roll shell 12'. The holes 12'of the suction zone C'are filled with water due to the effect of the negative pressure po. Reference sign S'shows the direction of rotation of the suction roll 10'. There is no negative pressure in area B'i. e. in the area after the outlet rib, but the pressure pi in the area B'is lower than the pressure p2 outside the shell 12', whereby the holes 18'become only partially empty due to centrifugal force, since the lower pressure in the area B'keeps the water in the holes 18'. The holes 18'are thus filled from outside and cause mist formation.

The holes 189 are emptied of water e. g. by means of doctors 22'and the pressure p3 inside the roll 10'corresponds to atmospheric pressure. The pressure p3 in the area A'is thus about the same as the pressure p2 outside the roll sell 12'.

An object of the invention is to eliminate or at least minimize the disadvantages related to the arrangements known from prior art.

An object of the invention is to provide an improvement and a change in the operation and structure of a suction roll.

To achieve the afore-mentioned objects and those that come out later the suction roll according to the invention is mainly characterized in what is presented in the characterizing part of claim 1.

The method according to the invention is in turn mainly characterized in what is presented in the characterizing part of claim 10.

It has been found in the invention that air can be introduced simply, efficiently and cost-effectively into the problematic area without separate positive pressure devices. In the invention, the roll is provided with means with the help of which the areas-having a negative pressure as a result of the rotational motion and partial emptying of the holes-after the suction chamber between the suction box and the shell can be filled with replacement air rapidly enough in order to empty the water from the hole. It is thus possible, in the invention, to rapidly after the suction chamber introduce the replacement air replacing the water and necessary for removing the negative pressure into the suction holes.

According to the invention replacement air is provided in the area after the suction chamber of the suction roll, which replacement air releases the negative pressure in order to allow water to discharge from the perforation of the suction roll as easily as possible without causing mist formation. Thus, by means of the suction roll according to the invention, a better dry solids content can be obtained, as the water removal capacity of the suction roll can be maximized and the rewetting of the web due to mist formation can be minimized.

According to the invention an amount of air sufficient to serve as replacement air is provided via the ends or sides of the roll and, according to an advantageous feature of the invention, the replacement air is introduced from where negative pressure is needed.

The roll according to the invention is a so-called breathing suction roll i. e. a suction roll provided with a breather, in which roll the suction air requirement of the suction roll can also be utilized elsewhere in the paper or board machine, e. g. in a cleaning device. According to the invention no separate pressurized blowing air feed is needed in order to improve the breathing of the roll, since the replacement air creates a sufficient amount of internal pumping energy, when a controlled air flow without great pressure losses is arranged to the roll via a roll end or through the shell or chamber from one side of the roll to the other. One end of an air duct used for improving the breathing of the suction roll can be coupled in connection with devices requiring a low negative pressure, such as a water- collecting trough external to the roll or to cleaning devices, whereby the amount of replacement air required by the suction roll creates a sufficient negative pressure.

Several different factors influence the operation of a suction roll. The capacity of the suction roll to remove water from the web is influenced by: the level of negative pressure and its duration of action, web properties (pulp, filler, graml11age), open area in the shell of the suction roll hole size/countersink, rewetting, rotation speed, centrifugal force and wire properties. The level of negative pressure of the suction roll affects the suction roll's air quantity and thus also the vacuum system. The centrifugal force of the suction roll has in turn an effect in the emptying of water contained in the holes of the roll. Also, the time factors have to be considered in the water removal, emptying of the roll/discharge of negative pressure, breathing, formation of negative pressure, and in the flow depth of water into the holes of the shell. It also has to be taken into account that the amount of air needed to provide the negative pressure in the holes increases as a function of the vacuums.

According to an application of the invention directing of air i. e. water removal is arranged in connection with the suction roll as directed only to a certain part of the roll, in order to ensure that the removal does not take place e. g. too close to the

web, but at a desired location. According to this application of the invention the internal air processing of the suction roll is utilized in the vacuum system, whereby the main flow can be decreased, since the air requirements of the roll are utilized internally.

According to an advantageous application of the invention replacement air holes (e. g. axial bores in a flange shaft) are added into the space outside the suction box, meaning that passing replacement air inside the roll creates a high-velocity air flow on the inner surface of the shell (blow). According to an advantageous characteristic of this application of the invention the blow generated by the replacement air can, if necessary, be directed at a certain point. For example, the replacement air is passed via a support of the suction box, inside the box, to an appropriate point and the point is defined by means of seals. This application provides, for example, the following advantages : the shell has an improved capability to stay clean, the dry solids increase after the wire, because the amount of water circulating with the shell decreases, thereby reducing the need to doctor the suction roll, since the water is more easily removed, the risk of roll-internal oil leaks diminishes as the shell-internal suction effect has been removed, mist formation of the shell decreases as the main flow of the replacement air in the suction hole is from the inside/to the outside.

According to an advantageous application of the invention, in connection with the invention the area of the suction zone of the suction roll is formed of a plurality of sectors meaning that suction can be gradually increased, for example such that in each section the minimum amount of suction sufficient to still remove water from the webs is used. This way it is possible to optimize the amount of air used by the suction roll by optimizing at the same the suction required and its duration of action.

By means of the replacement air used in connection with the invention it is possible to utilize e. g. the following items requiring low-pressure ventilation/a

vacuum, e. g. a suction deflector, wire cleaning devices, ventilation of the trough of the former roll and suction roll as well as other ventilation applications in the wire section, end-face ventilation/doctors and the like. The replacement air of the suction roll can be utilized in different ways. The replacement air is taken from a ventilation application to an area in the outlet side. The invention thus helps to gain vacuum/ventilation savings, as well as possibilities to make use of the energy of the suction roll instead of separate fans and pumps.

The invention will now be described in more detail with reference to the figures of the accompanying drawing Figures 1A and 1B schematically show the operating principle of the suction roll according to the invention.

Figures 2A and 2B schematically show an advantageous application of the invention.

Figures 3A and 3B schematically show another advantageous application of the invention.

Figure 4 schematically shows, as a function of chamber width and negative pressure, the amount of air needed for creating a negative pressure.

Figure 5 schematically shows an application of the invention in which gradual increase of suction is used.

Figure 6 schematically shows a problem underlying the invention relating to emptying of the holes of a suction roll in applications known from prior art.

The same reference numerals have been used in the figures for corresponding parts.

As shown in Figures 1A-1B, 2A-2B and 3A-3B the structure of a suction roll 10 consists of a shell 12 densely perforated with holes 18 and mounted rotatably on bearings and of a suction box 20 arranged fixedly inside the shell 12 in a closed space. In the area of a suction zone C water is sucked through the shell 12 and a fabric F conveying the paper web W from the paper web W by means of the suction box 20 defined by seals 15,16 and connected to a vacuum system V.

Arrow S indicates the direction of rotation of the roll 10.

In the application according to Figures 1A and 1B a discharge trough 11 is arranged around the suction roll 10, which trough surrounds the area in which water is removed from the perforation 18 of the roll 10, and doctors arranged in connection with the suction roll 10 have the reference number 22, the doctors typically being foil doctors, an example of which is disclosed e. g. in Finnish application no. 902910. According to the invention, in connection with the application in principle illustrated in Figures 1A-1B replacement air is passed from area A via a set of feed channels 17 to a distribution chamber B situated in the cross-direction of the roll 10. Negative pressure generated by means of a suction box 20 connected with a vacuum duct V is effective in a suction zone C.

Auxiliary ribs 14 are arranged in the cross-directional distribution chamber B in order to prevent return flows. Paper web running, supported by a fabric F, over the suction roll 10 has the reference sign W.

In the application of the invention illustrated in Figures 2A and 2B a suction zone C is produced on the shell surface 12 of the roll 10 by means of a vacuum system V of the suction roll 10. According to Figure 2B replacement air holes 32 to a space outside the suction roll 32, e. g. axial bores, preferably a hole/holes, arranged in a thrust shaft 36, are situated near a seal 15 of the discharge side of the suction chamber, where the need for replacement air is the greatest (area B), Figure 2A, and they make it possible to pass replacement air to the inside of the roll 10 by means of boxes/pipings 35, etc. The replacement air can be introduced

exactly to the desired point on the circumference of the nip, which generates a high-velocity air flow from the inner surface of the shell 12 to the outer surface, blow P. The blow P created by the replacement air can, if necessary, be directed at a certain point, as shown in Figure 2A. For example, the replacement air is passed via a support of the suction box 20 to an appropriate point inside the shell 12 to form a blow opening 38 and the point is defined with seals 33 and 34.

Figures 3A and 3B show another preferable exemplifying embodiment of the invention, in which area A has a separate (breather) chamber 41, into which the replacement air is passed via an end from outside the roll along a channel 42. The chamber 41 may extend over the entire axial length of the suction box or just part of it. The introduction of replacement air ensures that there is enough air in the area A to be pumped to area B. Breathing holes are indicates with the reference number 43. The suction roll 10 is provided with a breather tube i. e. a replacement air feed channel 17, by means of which the replacement air releasing the negative pressure from holes 18 is ensured such that after a suction zone C defined by seals 15,16 the water is removed from the holes 18 of a shell 12 without causing mist formation. In the blow area A both centrifugal force and an additional air feed affect the holes 18, resulting in the negative pressure to be removed more quickly, and thus the water is removed from the holes, meaning that when the wire touches the suction roll 10, there is no more water in the holes of the suction roll 10 or on its surface.

Figure 4 schematically shows, as a function of chamber width and negative pressure, the amount of air needed for creating a negative pressure, and as can be seen from the figure the air flow being removed and needed for creating a negative pressure per time unit grows significantly as a function of the desired negative pressure. The same thing happens as the rotation speed (running speed of the machine) grows. A great portion of the air amount in question has to be arranged inside the roll after the suction chamber as replacement air in order to empty the holes rapidly.

According to an exemplifying embodiment of the invention illustrated in Figure 5 a suction zone area C of a suction roll 10 is arranged as sections Cl... C8, such that each section is sealed by means of seals 5115, 5116 and each section Cl... Cs is provided with a separate control valve 52, by means of which the suction level of a single section is controlled, meaning that it is possible to increase the suction gradually such that in each situation it is possible to use e. g. the minimum amount of suction sufficient to still remove water from a web W. This way it is possible to optimize the air flow by optimizing at the same the suction required and its duration of action by controlling the suction level of each zone Cl... Cg.

Figure 6 schematically shows a problem underlying applications known from prior art. The same reference numbers have been used in this figure as in the previous figures but with the addition of superscript commas. In Figure 5 a web being passed via a suction zone C'of a suction roll 10'has the reference sign W' and a fabric the reference sign F'. A roll shell 12'comprises suction holes 18'. A suction chamber V is arranged inside, on the upper side of the roll shell 12 of the suction roll 10', and the suction zone C'is defined by means of seals 15', 16', which seals are arranged in connection with the suction chamber'in the axial direction of the suction roll 10'. Pressure po is effective in the suction chamber ß Pressure p2 is effective outside the roll shell 12'. The holes 12'of the suction zone C'are filled with water due to the effect of the negative pressure po. Reference sign S'indicates the direction of rotation of the suction roll 10'. There is no negative pressure in area B'i. e. in the area after a outlet rib, but the pressure pi in the area B'is lower than the pressure p2 outside the shell 12', whereby the holes 18'become only partially empty due to the effect of centrifugal force, since the lower pressure in the area B'holds water in the holes 18'. The holes 18'are thus filled from the outside and cause mist formation. The holes 18'are emptied from water e. g. by means of doctors 22'and the pressure p3 inside the roll 10' corresponds to atmospheric pressure. The pressure p3 in area A'is thus about the same as the pressure p2 outside the roll sell 12'.

The invention has been described above with reference to some of its advantageous exemplifying embodiments only, to the details of which the invention is, however, by no means intended to be narrowly confined.

The invention is especially suitable for use in suction rolls with a high level of negative pressure, e. g. at least 40 kPa. The former roll can be mentioned as a special application.