Abstract:
A device for continuous drying of a pulp web, particularly a tissue web, with a drying drum ( 1 ) and an air circulating system, where the drying drum ( 1 ) has a cylindrical shell ( 9 ) designed as a honeycombed body with an annular, flexible transition profile ( 13 ) at the edges of the shell and connected to the end covers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a device for continuous drying of a pulp web, particularly a tissue web, with a drying drum and an air circulating system, where the drying drum has a cylindrical shell designed as a honeycombed body.  
         [0002]     In conventional tissue plants, the drying process begins at an ingoing dryness of some 40 to 45%. In order to achieve higher paper volume, papermakers now dispense with preliminary mechanical dewatering, and the ingoing dryness of this newer type of device is around 20 to 25%. These plants operate with through-air drying. During the heating process, one or more consecutive through-air drying drums at ambient temperature are exposed abruptly to the supply air temperature of approximately 300° C. The drying drums currently in use have a thin-walled shell, for example a perforated or honeycombed body, that is joined to thick-walled end flanges. Due to the substantial differences in mass between drum shell and end flange, there is excessive stress at the transition points that leads to deformation and even structural damage. The same damage occurs if the drums are cooled down abruptly from operating to ambient temperature during an emergency shutdown, when they are sprayed with cold water in order to prevent the plastic wires enclosing the drums from being damaged.  
       SUMMARY OF THE INVENTION  
       [0003]     The invention now aims to eliminate this disadvantage and is characterized by the honeycombed cylinder shell of the drying drum having an annular, flexible transition profile at the edges. Thus, any changes occurring in diameter and any resulting thermal stress can be reduced.  
         [0004]     An advantageous further development of the invention is characterized by the transition profile being designed as a U-profile and preferably being butt-welded onto the honeycombed cylinder shell. With this design of transition piece, continuous heat transition is guaranteed during both the heating and the cooling process of the machine. The special type of joint leads to a reduction of the stresses in the welds to such extent that the welds suffer no deformation or structural damage at all.  
         [0005]     A favorable embodiment of the invention is characterized by the cross-section of the transition profile, preferably a U-profile, narrowing towards its center. As a result, the heat flow can be influenced particularly well. In addition, this design creates a flexible connection, which also guarantees that the cylinder shell is centered and thus, runs exactly true.  
         [0006]     It is an advantage if the honeycombed cylinder shell is wider than the paper web to be dried, thus allowing a defined variation of the paper web width.  
         [0007]     A favorable further development of the invention is characterized by an endless ring being shrunk on at each end and which extends beyond the transition profile and into the honeycombed cylinder shell. This prevents dust or fibers from entering the cavity of the U-profile.  
         [0008]     It has proved favorable to make the cylinder shell out of longitudinal ribs that are connected to upright, edged profiles. This achieves good stability in the cylinder shell.  
         [0009]     A favorable embodiment of the invention is characterized by the longitudinal ribs of the honeycombed cylinder shell being spaced at a distance of between 20 and 80 mm from one another, preferably between 30 and 40 mm. If the spacing is narrower, there is also less specific load and thus, reduced risk of marks on the paper web.  
         [0010]     An advantageous embodiment of the invention is characterized by the edged connecting profiles mounted in a honeycombed pattern protruding beyond the longitudinal ribs and supporting the paper web and the conveying wire. This results in a large supporting surface and a further reduction in the risk of marks on the paper web.  
         [0011]     It is particularly favorable it the honeycombed cylinder shell has an open area of at least 85%. The through-air drying process can thus be implemented particularly well.  
         [0012]     A particularly favorable further development of the invention is characterized by covers being provided on the face ends to stabilize the cylinder shell and by these covers being bolted to the cylinder shell, particularly to the transition pieces. This design guarantees improved stability of the drum shell; in particular, it prevents any sliding movement by the end cover and the drum shell if there is radial expansion caused by the temperature.  
         [0013]     An advantageous embodiment of the invention is characterized by the drying drum having a fully welded drum body. This design virtually excludes the risk of any areas where cracks could occur. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention will now be described in examples and referring to the drawings, where  
         [0015]      FIG. 1  shows a variant of a configuration of a through-air drying unit;  
         [0016]      FIG. 2  is a sectional view through  FIG. 1  along the line marked II-II;  
         [0017]      FIG. 3  shows a drying drum according to the invention;  
         [0018]      FIG. 4  shows detail IV in  FIG. 3 ;  
         [0019]      FIG. 5  shows detail V in  FIG. 3 ; and  
         [0020]      FIG. 6 a  sectional view along the line marked VI-VI in  FIG. 3 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]      FIG. 1  shows a possible configuration of a through-air drying process. The figure shows the drum  1  with its bearings  2  and  3 , and the drive  4 .  
         [0022]     Beneath the drum there is a two-part hood  5  and  6  (see  FIG. 2 ) from which the hot supply air flows through the paper web, through a conveying wire  8 , then through the drying drum  1  into the inside of the drum, and is removed from the drum on the drive side through an annular channel  10 . The hot supply air at a temperature of approximately 300° C. is cooled down to approximately 120° C. by the drying process. The exhaust air cooled in this way is then returned to its entry status in a processing system. At the outlet, the paper web  7  with the conveying wire  8  is carried over a deflection roll  11 . The cover device  12  is clearly visible here, covering the area of the drum  1  from the inside outwards in the sector that does not come into contact with the tissue web  7  and which also is not enclosed by the hood  5  and  6 . This prevents additional air from being drawn into the drying drum, which would greatly reduce the suction effect through the paper web. In principle, the air can also be conveyed from the inside of the drying drum  1  through the cylinder shell  9  to the outside.  
         [0023]      FIG. 3  show a sectional view through a drying drum  1 , comprising a perforated, preferably honeycombed cylinder shell  9  with a flexible ring  13  rolled into a horizontal U-profile, butt-welded onto the edges of the shell on both the operator and the drive side. Due to this U-shaped transition profile  13  between cylinder shell  9  and end covers  14 ,  15 , the maximum stresses in the connecting weld are reduced to approximately one third of those occurring in conventional designs, which guarantees damage-free operation of the drying drum over its entire service life.  
         [0024]      FIG. 4  shows a sectional view of the connection between the drum shell and the flexible ring  13 , as well as the weld joint itself and the bolted connection  17  to the drum cover  14 . As viewed in section, each flexible ring  13  is preferably a unitary (or two half-ring) member having a radially extending, relatively rigid inner rim portion that is butt welded to the outer edges of the longitudinal ribs  18  (see  FIG. 6 ), and a radially extending, relatively rigid outer rim portion that forms a flange for the bolted connection to a mating flange portion of the cover. A relatively thin, flexible, web portion extends axially between the inner and outer rim portions, forming the preferred “U” profile in section. As used herein, “flexible” should be understood in the context as semi-rigid with the capability to bend or flex under thermal or mechanical stresses, while retaining sufficient rigidity to transmit the rotational drive torque between one or both covers  14 ,  15  and the shell body  9 . A cavity or channel is formed by the flange portions and the web, and can be considered as having a center C that lies on an imaginary circle around the drum axis. Likewise, the web can be considered as having a center that lies in radial alignment with the center of the channel, and preferably has a varying width along the direction between the flange portions, which narrows toward the center.  
         [0025]     The external flanges of these flexible rings  13  are bolted to the drum covers  14  and  15 , which have journals to hold the two bearing assemblies  2  and  3  that are designed to take account of the changing length of the drying drum  1  in cross-machine direction, caused by the differences in temperature during heating up and cooling down. The temperature of the exhaust air is normally around 120° C., while the supply air entering the drying drum has a temperature of approximately 300° C. The two ends of the drum including flexible ring  13  are covered by an endless imperforate protective ring  16  from the outer edge of the outer flange portion inwardly beyond the inner flange portion to the edges P of the paper web. This arrangement prevents any dust or fibers from entering the cavity in the U-profile. This endless ring  16  is shrunk on in such a way that it cannot detach itself from the drum surface during the heating and cooling process, nor during drying operation.  
         [0026]     A view of the peripheral sector of the drum  1  is illustrated in  FIG. 5 . This drawing shows the covering ring  16 , which extends inwardly beyond the outer edges of the honeycombed cylinder shell  9  a distance D and marks the edges P of the paper web.  
         [0027]      FIG. 6  shows the supporting structure of the cylinder shell  9  with longitudinal ribs  18 , with advantageous spacing a of approximately 30 to 40 mm and the connecting profiles  19  protruding beyond the longitudinal ribs in radial direction to form the honeycomb and support the paper web  7  and the conveying wire  8 .