Patent Abstract:
A roll in a paper or board machine has a roll frame or center shaft ( 15 ) with bearings mounted at its ends and a groove-like surface structure ( 10 ). There is a contact between the surface structure and the center shaft so that an essentially closed structure is formed. The roll produces a vacuum that keeps the web attached to the outer surface of a fabric in the fabric wrap area of the roll circle by utilizing the boundary layer airflows of the surface structure flow and/or of the web. A dryer group has at least one contact dryer cylinder and at least one turning roll with a single fabric run arrangement.

Full Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority on Finnish Application No. 20031461, Filed Oct. 7, 2003, the disclosure of which is incorporated by reference herein. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention relates to rolls and dryer groups in a paper or board machine. 
     The use of double fabric run and/or single fabric run arrangements in the dryer groups of multi-cylinder dryer sections of paper or board machines is previously known. In the double fabric run arrangement the dryer cylinder groups have two fabrics pressing the web one from above and the other from below against heated cylinder surfaces. Between the dryer cylinder rows, generally horizontal rows, the web has, in the double fabric run arrangement, free and unsupported draws, which are liable to fluttering, which may cause web breaks, particularly at such drying stages, in which the web is still relatively moist and therefore weak for its strength. Due to this, during several past years there has been increasing use of the said single fabric run arrangement, in which each dryer cylinder group has only one dryer fabric, supported by which the web travels through the entire group, the dryer fabric pressing the web against the cylinder surfaces heated by the dryer cylinders and the web remaining outside the outer curve at the turning cylinders or turning rolls located between the dryer cylinders. Thus, in the single fabric run arrangement, the dryer cylinders are positioned outside the fabric loop and the turning cylinders or turning rolls are located inside the loop. In so called normal single fabric run groups the dryer cylinders are in the top row and the turning cylinders or turning rolls are in the bottom row, and correspondingly, in so called turned single fabric run groups, the dryer cylinders are in the bottom row and the turning cylinders or turning rolls are in the top row. 
     In known dryer groups adapting the single fabric run arrangement the dryer fabric and the paper web are transferred from the previous drying element, such as a contact dryer cylinder, to a turning or suction cylinder or similar in a common straight run, whereby a closing wedge space, also referred to below as closing nip, is formed between the dryer fabric and the last-mentioned turning cylinder or suction cylinder surface. The dryer fabric and the cylinder surfaces moving towards this nip tend to generate positive pressure in the said wedge space by means of the boundary layer flows conveyed by them. This again produces a pressure difference over the paper web supported by the dryer fabric, the pressure difference having a tendency to detach the paper web from the dryer fabric causing runnability problems, wrinkles, and even web breaks. On the other hand, for improving the efficiency of dryer sections, the need arises for using dryer sections with a more compact construction than heretofore, in which the contact dryer cylinders and the mentioned suction cylinders are as close as possible to each other. All these aspects together with rising web speeds increase the overpressure problems of the said closing nip. It is previously known that the transfer of the paper web in the single fabric run arrangement on the contrary from the turning suction cylinder to the contact dryer cylinder takes place after a so called opening nip, supported by the dryer fabric. In dryer sections suitable for the single fabric run arrangement, the term pocket space is used to refer to the pocket-like space, which is limited by two parallel dryer cylinders and the turning cylinder and dryer fabric between them. 
     In the solutions known in the prior art technique, attempts have also been made to remove the problems occurring in the area of the closing nip by means of roll suction, roll sector suctions and various types of vacuum-generating boxes as well as by using combinations of rolls and suction boxes, which, however, have not necessarily been able to completely eliminate the problems in this area in an energy-efficient manner. At high machine speeds the requirement of energy used for web stabilization also strongly increases. Typically the power requirement increases to the power of three in relation to the web speed. 
     A solution for removing the problems in this area is set forth in the FI patent No. 105573 (corresponding U.S. Pat. No. 5,996,244), which discloses a roll in a paper machine, particularly in a paper drying device, and a dryer group in a paper machine, in which the roll in the paper drying device comprises a shaft, supported by which the roll is adapted to rotate, and a surface structure, connected to the shaft with support pieces or similar, in which the openness of the surface structure of the roll is more than 10% and the surface structure of the roll is open in a slot-like manner so that during the roll rotation an effect is produced that aspirates air to inside the roll, whereby an air flow-through is created through the roll. In a dryer group of a paper machine, in which dryer group the single fabric run arrangement is adapted, at least one of the turning rolls of the dryer group is an open roll of the type described above. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to develop further the above-described corresponding technique in order to principally solve the problems related to the web transfer in the mentioned closing nip area as well as in the turning roll area covered by the web. 
     Particularly at high speeds, e.g. exceeding 1,400 m/min, an extremely critical area for runnability at the beginning of a dryer section equipped with a single fabric run arrangement is the above-described closing nip area of the turning cylinder, and the object of the invention is therefore to set forth more efficient and more energy-saving new constructions for removing the drawbacks in this area prone to runnability problems. 
     The object of the invention is to set forth a roll during the use of which runnability problems in the closing nip area of the turning roll are eliminated or at least minimized. 
     Another object of the invention is to set forth a dryer group of a paper machine that is better for runnability than the known solutions. 
     The invention is based on a roll having a large open surface area, with its key characteristic being the capability to create a vacuum in the fabric wrap area together with the dryer fabric or the dryer fabric and paper. The generation of vacuum is based on
         a) a change in the angular momentum of the in-going gas flow, such as airflow, taking place in the closing nip, which intensifies the inflow,   b) flow adjustment and pumping in the roll grooves and channels, which flows are influenced by the centrifugal forces created by the roll rotation, and   c) the vacuum effect of the opening nip, which aspirates gas, e.g. air, creating a vacuum, whose effect extends to the entire fabric wrap area.       

     Although the following description discusses airflows, air space, etc., this is not, however, intended to restrict the invention to such situations only, in which the flow is air. The flow can also be another gas (fluid). 
     The design of the roll according to the invention also allows increasing the vacuum-producing capacity of the roll along with a rising running speed, since the vacuum of the roll wrap area develops according to the formula
 
p=ζ f v b [b=1.3-2],
     where p=vacuum change,   ζ f =fluid density, and   v=running speed.   

     According to the invention the roll comprises a surface structure having a large, most preferably groove-like open surface area, such as plate disks, and a center shaft and/or alternatively roll frame with grooves machined in it. According to the invention, both the vacuum produced by the roll and its profile on the roll surface are optimized to the maximum so that a contact exists, if required, between the surface structure, such as the plates, and the center shaft, i.e. there is no free air space between the roll frame and the grooves. According to the invention, a roll construction preferable for efficiency is provided for example by attaching plates with different diameters to a solid frame roll/shaft, the plates forming the grooving when placed at suitable intervals. 
     In the roll according to the invention the surface structure is so formed that most preferably a grooving is formed, in which the depth of each groove is approximately 10-155 mm, most appropriately 18-85, and the groove width is 1-50 mm, most appropriately 6-10 mm. The proportion of the groove width to the land width is most appropriately 0.6-2.0. 
     The most common form for the roll surface grooving is the U shape or one very much similar to it. The grooving can be straight or spiral. The grooving can be made by disk cutting and/or turning the frame roll surface. It can also be produced by welding, gluing or mechanically locking separate plate disks, made of metal, polymer or combinations of several materials, to the frame roll surface, in which case the part in contact with the fabric or otherwise the most external part is highly wear-resistant. The grooving can also be produced using a so-called G strip technique, in which the strip is applied directly on top of the smooth frame roll or in special mounting grooves in the frame roll surface. As a special application, the G strip is applied to an old or new frame roll by means of a support strip. 
     When the roll functions in the application placed against the fabric, the edges in the land supporting the fabric can be rounded, e.g. with a rounding radius of R=1-3 mm, or the whole land can be made to a slight circular arch, say with a radius of curvature of R′=100-500 mm, or the form can be selected in some other way so that the surface pressure between the fabric and the roll is optimal and the fabric wear is reduced. 
     The roll according to the invention can have a perforation at the bottom of the grooves, or the frame roll can have a perforation independent of the grooving geometry. In addition, according to a special aspect of the invention, the perforation can also appear at the groove peak/land so that the perforation opens to the contact surface of the fabric. This construction is particularly useful in a tail threading situation, since the tail can be stabilized to the fabric surface more efficiently with a higher vacuum. 
     When using a roll according to the invention, a dryer fabric that is normal for permeability is intended, such as a dryer fabric, whose permeability is 500 m 3 /(m 2 h), preferably 1,000-35,000 m 3 /(m 2 h), most appropriately 1,000-5,000 m 3 /(m 2 h). 
     The advantages of the invention are its efficiency and simplicity. According to a preferable embodiment of the invention, the roll diameter is for example 1,500 mm. Separate plate disks have been attached for example by welding to a frame construction with a diameter of 1,300 mm, which does not need to be an actual frame roll, but a support construction similar to it. The height of the plate disks is 100 mm and their width is 6 mm. A 7-mm wide open groove remains between the plate disks. The peak of the plate disk is rounded with the radius of curvature R′=150 mm. A simulation model has provided results according to which the roll generates a good vacuum on its surface both in the closing and in the opening nip as well as a vacuum of almost the same level −500 to −900 Pa in the rest of the fabric-covered area on the roll surface. With the plate arrangement/grooving according to this preferable embodiment of the invention, the vacuum of the roll surface can be brought to a level of −500 to −900 Pa, based on the results achieved from the simulation, depending on the position of the wrap area. These pressure levels are for example of the same class as with the suction roll marketed with the trademark VacRoll™ of Metso Paper, Inc., in which the suction air volume is 400 m 3 /(hm) (2000 m/min). This vacuum in the fabric wrap area is achieved with the indicated power entirely without external aspiration or without runnability components in the pocket space. 
     The roll according to the invention, having a groove structure, such as lands of plate, preferably adapted around a solid center shaft or a frame roll, provides an inwardly air pumping phenomenon in the groove construction, which is generated when the air conveyed with the fabric hits against the roll surface and the groove walls, such as the plates. The airflow accelerates in the grooves and then exits from the opening nip. The inwardly pumping phenomenon in the roll according to the invention, providing a vacuum without special roll-external vacuum-providing equipment, is intensified with an increasing rotating speed. Thus, it automatically produces its own vacuum utilizing external boundary layer flows and/or blasting flows starting from the closing nip of the turning cylinder and continuing until to the opening nip, and this enables providing a preferable and efficient paper machine roll, which is particularly useful as a turning roll/cylinder of the dryer groups of a paper machine dryer section. 
     The roll creates the vacuum effect due to the fact that in the closing nip there occurs a change of angular momentum of the gas (fluid) flowing to the grooves. The gas flow directed to the roll grooves proceeds in the roll grooves to the area of the opening roll nip, whereby a vacuum effect is created, which extends over the entire fabric wrap area. 
     The roll surface structure is formed in such a manner that the vacuum effect and gas pumping are created by the friction between the structure and the fluid, the boundary layer, and accelerating movement of gas. 
     In the roll according to the invention, air hits against the surfaces of the groove walls, such as plates, placed around the center shaft, whereby the groove walls tend to pump the air forwardly and particularly in a closing nip, also inwardly towards the shaft. Air circulates around the roll frame until to the opening nip. In this way the vacuum effect is created by a combined effect of three factors, i.e. the impact, flow and opening nip. 
     In connection with the invention it is possible to use a blow box according to a preferable embodiment of the invention described below in more detail, which is used to intensify the effect of the opening nip for example with trailing side aspiration, or on the other hand, a blow directed to a closing nip is used to intensify the impact and flow effect on the side of the closing nip. In addition, it is preferable to separate these areas of influence of the opening and closing nip from one other with a sealing in the roll axial direction/a blow box wall construction. 
     According to a preferable further characteristic of the invention, in a drying geometry based on the single fabric run design, besides the roll according to the invention, a box constructed on the leading side is used, built up of a separate blow box including a flexible nozzle solution and a passive box space attached below it, which has aspiration/an ejection blow, if required, and is open at the bottom part. 
     In this embodiment of the invention, aspiration is directed to the suction zone of the blow box using the passive box section. When this box is realized according to the blow nozzle/flow divider principle, it is possible to achieve a vacuum of approximately −1000 Pa in the high-vacuum zone (2.2 mm nozzle, blow air volume 900 m 3 /(hm)). 
     This embodiment of the invention preferably also includes two flow divider/sealing elements in the box on the leading side, and by adjusting their distance to the fabric it is easy to adjust the vacuum of both the high-vacuum zone and the vacuum influencing in the open gap area, which keeps the web on the fabric surface before it comes to the influence area of the roll according to the invention. This allows efficiently preventing excessive bending of the fabric. 
     The total air volume requirement in the above-described system is 900 m 3 /hm per blow box, which is 50% of the present blow box/VacRoll™ total air volume. Consequently, the blower power requirement also decreases by 50%, which in practice can mean a decrease of approximately 1 MW in the power consumption in the dryer section of a large modern paper machine. It is remarkable that the roll according to the invention preferably replaces this underpressurized turning roll, in which case runnability components (e.g. blow boxes) that are almost like the present ones are used in the pocket space, designed to improve/intensify the performance of the proposed roll. 
     The roll according to the invention is also easy to keep clean, because the airflows automatically created by it simultaneously prevent dirtying of the roll surface structure and grooving. Therefore, an advantage of the roll according to the invention is also in its operating principle, the flow direction changes during the rotation cycle whereby each point in the roll grooves is subjected to inflow and outflow, and the roll is then kept clean for a longer time. 
     According to a preferable further aspect of the invention, it is possible, if required, to arrange, in connection with the roll according to the invention, an adjustment/suction possibility through a connection placed on the roll hub for adjusting the vacuum effect as desired. This kind of connection can also be used in a situation in which at least one part in the roll axial direction is realized according to the known technique. The area of the tail or both edges, for example, could be constructed for this kind of adjustment, because the intensification of the vacuum effect by dividing the blow box in the cross-machine direction is not necessarily as efficient or easy to implement. 
     According to a preferable further characteristic of the invention, in the roll axial direction there is at least one part in which the air is adapted flowing through the openings in the roll shell to the roll interior. 
     The invention is described below in more detail by making reference to the figures in the enclosed drawing, to the details of which the invention is not intended to be strictly limited in any way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1B  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1B.1  is an alternative partial enlargement A′ of the region A in  FIG. 1B , showing the radius of curvature at the corners of a land. 
         FIG. 1B.2  is another alternative partial enlargement A″ of the region A in  FIG. 1B , showing the radius of curvature formed large at a land. 
         FIG. 1C  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1D  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1E  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1F  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIG. 1G  is a schematic view of the surface structure of an embodiment of the roll of this invention. 
         FIGS. 2A and 2B  illustrate examples of the effect of closing the open space between the plates and the center area in the roll according to the invention. 
         FIG. 3  is a schematic view of the flow behavior taking place in connection with the roll according to the invention. 
         FIGS. 4-6  show a schematic view of embodiments according to the preferable further characteristics of the invention, in which there is a blow box arranged in connection with the roll. 
         FIGS. 7-8  show a schematic view of preferable embodiments of the invention in connection with dryer groups adopting the single fabric run arrangement. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1A  shows a surface structure  10  realized according to an embodiment of the roll according to the invention, in which the surface structure  10  is formed of turned or disk-cut grooves  11 , whereby lands  12  between the grooves  11  are simultaneously formed. In  FIG. 1B , on the contrary, the roll surface structure  10  is formed of separate plate disks  16 , attached with welds  13  or formed to the roll frame or center shaft  15  in another method known as such, which form the lands  12 , whereby the grooves  11  remain between them. In connection with  FIG. 1B , there are alternative partial enlargements A′, A″ of detail A indicated in  FIG. 1B.1  and  1 B. 2 , in which according to the partial enlargement A′, shown in  FIG. 1B.1 , a radius of curvature R can be used in the corners of the lands  12 , such as plate disks  16 , which R is approximately 0.1-7 mm, or according to the partial enlargement A″, shown in  FIG. 1B.2 , the surface  17  of the land  12 , such as a plate disk, can be formed large as to its radius of curvature R′, with R′ then being approximately 100-3000 mm. 
     According to the invention, regardless of the production method of the grooves, the edges  14 ,  17  of the land  12  supporting the clothing shown in the partial enlargements A′, A″ can be rounded or shaped so as to reduce wear of the clothing, such as a fabric. In this case the open surface area of the roll can be slightly increased, which improves the roll performance particularly in the closing nip area. When the open surface area increases, the groove depth can be slightly reduced, if required, without deteriorating the roll performance. 
       FIGS. 1C-1F  illustrate the formation of the surface structure  10  of the roll according to the invention using the G strip technique, either without a support strip,  FIG. 1C , or with the support strip,  FIGS. 1D  (grooved roll) and  1 E (smooth roll). According to  FIG. 1C , in the embodiment shown without a support strip, the G strip is applied slightly in a wedge form to the grooved roll shell  15 , and the bottom part of the G strip is slightly narrowing. According to  FIGS. 1D and 1E , a grooved support strip  18  is applied to the roll surface with the strip  12  forming the lands, and the dimensions of the actual groove strip  12  are e.g. 45×5 mm and the dimensions of the grooved support strip  18  are 8×12 mm. 
     To form an open surface structure  10  of the roll according to the invention of the various embodiments shown in  FIGS. 1A-1G  there are thus formed either grooves  11 , with the lands  12  remaining between them, or land sections  12  are attached to the roll center shaft or frame  15 , with the grooves  11  remaining between them. The depth S of each groove and thus the height of the land is approximately 10-155 mm, most appropriately 15-85 mm and the groove width L is 1-50 mm and the relation of the groove width L to the land width B is most appropriately 0.6-1.4. 
     According to a preferable embodiment shown in  FIG. 1B  the roll diameter is for example 1,500 mm. Separate plate disks have been welded to a frame construction having a diameter of 1,300 mm, which does not need to be an actual frame roll, but a support construction similar to it. The height of the plate disks is 100 mm and their thickness is 6 mm. A 7-mm wide open groove remains between the plate disks. A simulation model has given results according to which the roll generates a good vacuum on its surface both in the closing and opening nip as well as a vacuum of almost the same level −500 to −900 Pa in the rest of the fabric-covered area on the roll surface. With the plate arrangement/grooving according to this preferable embodiment the vacuum of the roll surface can be brought to a level of −500 to −900 Pa, based on the results achieved from the simulation, depending on the position of the wrap area and the running speed. 
     In  FIG. 1F , according to a preferable embodiment of the invention, the open surface structure  10  of the roll has been formed by such land sections  12 , which comprise a metal part  12 A forming the frame construction  12 A of the land, and by a filler section  12 B, which is made for example of plastic or metal based material forming the other side wall of the groove  11 . In the embodiment according to  FIG. 1F  the land sections  12  of the surface structure have been attached to the roll frame or center shaft with a support strip  18 . This structure according to FIG.  1 F has the advantage that the portions of metal and plastic can be optimized and the roll weight is also reduced and the renewal of the surface structure becomes easier. 
       FIG. 1G  shows a preferable embodiment of the invention in which separate plate units are attached to one another or to the center shaft using friction of form closing attachments. In the embodiment of  FIG. 1G  plate disks  12  and  19  of different size have been attached to the roll frame for example by a shrink fit, whereby the alternately positioned plate disks form a surface grooving between them. It is also possible to produce plate disks comprising the structure of the disks  12  and  19 , which are then attached to each other using for example a form closing attachment. The use of plate disks of a different size also enables providing at least one roll part with a grooving of different depth, allowing to optimize the groove depth at the roll edges, for example, based on the generation of the maximum vacuum output. 
       FIG. 2A  shows the effect of closing the open space between the plates and the center shaft of the roll used in the embodiment according to  FIG. 1A . In the figure, X-axis represents the roll circle and Y-axis the pressure in the roll grooves. The top curve  40  in the figure illustrates an arrangement in which there is an open space between the plates and the center shaft, i.e. an arrangement according to the prior art technique, the center curve  41  and the bottom curve  42  represent situations in which the space interval between the plates and the center roll is closed, i.e. embodiments of the invention. Number  43  indicates a closing nip and number  44  an opening nip. The figure also shows pressure peaks present in the rotational pressure at the opening nip and at the closing nip. As shown in the simulation figure, the roll according to the invention can provide the above-mentioned rotational pressure on the roll surface. In the simulation the roll diameter was kept at 1,500 mm and the groove depth was 250 in the cases ofthe curves  40  and  41 , and 125 mm in the graph of curve  42 . The curves show that a groove closed at the center space generates greater vacuum than an open solution, and for the groove depths, 250 mm produces less vacuum than 125 mm. 
       FIG. 2B  illustrates measurement results of the effect of the running speed on the vacuum level of the roll circle showing that a speed increase increases the vacuum. This is preferable, because with a rising speed it is preferable that the forces keeping the web attached to the fabric surface at the turning roll also increase. Curve  45  represents the influence of the running speed 1,200 m/min on the vacuum level of the roll circle, curve  46  represents the influence of the running speed 1,400 m/min on the vacuum level of the roll circle, and curve  47  represents the influence of the running speed 1,600 m/min on the vacuum level of the roll circle. A closing nip is marked with number  43  and an opening nip with number  44 . 
       FIG. 3  depicts the flow behavior in the roll according to the invention for example in an embodiment according to  FIG. 1A , in which arrows are used to indicate the airflow. According to the invention, in the area of the closing nip N 1  the boundary layer airflow and a possible blasting flow hit against the roll plates. The impact produces airflows in the radial direction, and the flow between the plates accelerates being essentially an airflow in the tangential direction of the periphery. The flow continues until to the opening nip N 2  essentially in the direction of the tangential flow of the periphery. 
     In the embodiments according to  FIGS. 4-6 , for preventing overpressurization of the pocket space T, a blow box  20 ;  20 A,  20 B,  20 C is used. The blow box  20  is located in the pocket space T formed by the dryer cylinders  21 ,  22  and the roll  23 , and it is provided with a flexible blow nozzle  25 , which is used to partly control the stream of the boundary layer flow conveyed with the fabric  24  to the closing nip N 1 . Further, the blow box is provided with projections  26 ,  27  having a distance adjustment possibility, if required. This can be used to prevent the fabric from conveying air with it and to seal the influence area of the vacuum zone restricted and thus more efficient. In boxes  20  producing a high vacuum it may also be preferable to use labyrinth sealing as shown in  FIG. 6  for the sealing  26 . In the embodiment according to  FIG. 4  the blow box  20  has a bottom surface  28  open towards the roll, from which the airflows P 1  are conveyed to inside the roll  23  according to the invention, intensifying in this way the in-pumping effect P 2  of the roll  23 . In the embodiments shown in  FIGS. 5 and 6 , the bottom plate is without perforations, and, if required, inward pumping can be controlled with the nozzle  32  or even by using a blow nozzle in position  26 . This allows leading the blast air also through the interior of the box with adjustment of the air volume led to the roll  23  by removing part of the blast air. This prevents the canal between the box  20  and the fabric  24  from becoming overpressurized, which would hinder the runnability. Arrow P 3  indicates the airflow led outwardly from the roll  23  according to the invention in the area of the opening nip N 2 , which further intensifies the runnability. Arrows S 1 , S 2  indicate the travel direction of the fabric  24 . In the embodiments shown in  FIGS. 4-6 , the sealing  29  separating the nip influence areas N 1  and N 2  is also shown in the boxes  20 . 
       FIG. 4  shows a so called half-pocket box  20 A, which fills the pocket space T on the side of the closing nip N 1  and leaving open the side of the opening nip N 2  of the pocket T, allowing the airflows of the trailing/up-going side to be freely removed form the pocket space without deteriorating the roll  23  performance. 
       FIG. 5  shows a so called box that fills the entire space, which fills the pocket space T and in which on the side of the opening nip N 2  there is created a blow that intensifies the airflow P 3  with the nozzle  30 , allowing thus to raise the vacuum production capability of the roll, while the aspiration  31  can still be used directly from the opening nip N 2 . 
       FIG. 6  shows a blow box  20 C, which in a sense represents an intermediate form between the blow boxes  20 A,  20 B illustrated in  FIGS. 4 and 5 , and it has further a blow nozzle  32 , which can be used to adjust/intensify the vacuum production capability of the roll  23 . 
     Via the embodiments of  FIGS. 4-8  it should be noted that the roll  23  functions as an active pump, in which case the blow boxes  20  and their use must be made in such a way that the flow-through air necessary for the roll operation exists. A preferable application principle is particularly the solution of  FIG. 5 , which can be used to efficiently control the roll  23  operation. 
     The solutions illustrated in  FIGS. 5 ,  6  and  8  are preferable for the vacuum control of the pocket space T and for the intensification and control of the vacuum generated by the roll  23 . Particularly at the beginning stage of drying, higher vacuum is required especially for the runnability component of the pocket space (blow box), and the control of vacuum generated by the roll is also more useful in this case. At the final stage of drying it may be preferable to use a box according to  FIG. 8  or even of  FIG. 4 , the paper being then drier and easier to control. 
       FIGS. 7 and 8  show schematic views of dryer groups R adapting single fabric run arrangements, with rolls  23  according to the invention located as turning rolls and pocket spaces T, equipped with a blow box  20 , being used in connection therewith. The top row dryer cylinders are indicated with reference number  37  and the dryer fabric and its lead and guide rolls are referred to with numbers  38  and  39  respectively.  FIG. 7  shows an embodiment, in which in the pocket spaces T of a dryer group R adapting a single fabric run arrangement, there is located a blow box  20 C, which corresponds primarily to the embodiment illustrated in  FIG. 6 , allowing to optimize the vacuum production of rolls  23  considering the position of the group in the dryer section. 
       FIG. 8  shows an embodiment, in which the blow box  20  located in the pocket spaces T corresponds primarily to the embodiment illustrated in  FIG. 4 , in which a plate without perforations  28  is applied and the suction at the opening nip of the cylinder is intensified with a separate suction connection  36 . Dryer groups R of the type illustrated in  FIGS. 7 and 8  are particularly suitable for use as dryer groups at the beginning of the dryer section. 
     The invention is described above by making reference only to some of its preferable embodiments to the details of which the invention is not, however, intended to be strictly limited in any way.

Technology Classification (CPC): 3