Abstract:
A pressurized box is positioned opposite a vacuum box. The pressurized box has a leading cross machine direction baffle with a ceramic shoe which engages a forming fabric, and a trailing cross machine direction baffle of similar construction. Two end deckles complete the pressurized box. The leading cross machine direction baffle presses against a resilient seal which causes a web contained between upper and lower forming fabrics to wrap about the baffle shoe a few degrees forming an effective end seal which does not allow air to bypass the baffle. The trailing cross machine direction baffle is positioned over a vacuum box which prevents air from leaking around the trailing baffle. A vacuum box draws air through the forming fabrics and the web, from the pressurized box increasing the total pressure gradient across the web to 20 to 30 psi or more. The pressure box and opposed vacuum box form an air press. The pressure box is positioned and held against the forming fabrics by opposing air tubes. An alternative embodiment utilizes a suction roll positioned beneath the pressure box and having a suction gland positioned beneath the portion of the roll which backs the pressure box.

Description:
This application is a continuation of U.S. Ser. No. 08/962,110 filed on Oct. 31, 1997, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatus for removing water from a paper web in general and to apparatus which draw air through a web in particular. 
     BACKGROUND OF THE INVENTION 
     Paper is formed from a stock containing less than one percent paper fibers by weight. The fibers contained in the stock are deposited on a forming fabric and a web is formed by draining water from the stock through the forming fabric. In many modem papermaking machines stock is injected between two forming fabrics in a so-called twin wire former. The web, however formed, is dewatered in three sections of the papermaking machine. The sections are referred to as the forming section, the pressing section, and the drying section. The paper web typically leaves the forming section with a fiber content of ten to twenty-five percent fiber by weight. The-paper web leaves the pressing section with a fiber content of between thirty-five and forty-five percent fiber by weight. Finally the paper web is dried to about ninety-five percent fiber dry weight in the drying section. 
     The direction of progress in the papermaking industry is to improve paper quality while reducing cost. Cost is reduced by increasing the speed at which paper is manufactured and by decreasing the amount of fiber required for a web with selected properties. Costs are also reduced by decreasing the amount of energy used in forming and drying the paper web. Quality is improved by better control over fiber supply, and the processes used in forming the finished paper. 
     Greater speed complicates the control of the processes by which the paper web is dried. Thus greater machine speeds drive a search for new and better processes. 
     Water removed in the drying section is the most costly water removed from a paper web. If a paper web is formed from stock containing one percent fiber, then approximately 99 pounds of water must be removed to form one pound of finished paper web. The last pound of water removed from a paper web which is being formed, which represents taking the web from fifty percent dry weight to ninety-five percent dry weight, is typically accomplished by evaporation in the drying section of a papermaking machine. This last pound of water costs as much to remove as the first ninety-eight pounds. 
     Thus methods of improving the dewatering processes in the forming section and pressing section are to be sought. In the pressing section the use of extended nip presses and high temperature pressing techniques has increased the amount of water which can be removed by a combination of pressure and temperature. In the forming section, where water is typically removed by drainage and vacuum, new methods of increasing water removal are needed. 
     One approach is to pass air through the web to draw or blow water from between the fibers which form the web. Air can be drawn by a vacuum, but vacuum has two limitations. First, the process takes place in Earth&#39;s atmosphere, and thus the maximum vacuum is limited to less than sea level pressure. Second, practical and cost considerations limit the cost-effective levels of vacuum obtainable in practice to considerably less than 14.7 psi. 
     The use of vacuum to draw water from a paper web is a fairly straightforward process. A box is placed on the side of a forming fabric opposite a paper web and air is drawn from the box. The low pressure pulls the forming fabric against the box forming a seal. The vacuum also controls the amount of pressure or force with which the forming fabric presses against the box and any fabric supports bridging the box. 
     If pressure is used, sealing the pressurized box to the web can be a problem. If the box is held against the forming fabric with insufficient force, air will leak around the box, causing a loss of air and possibly disrupting the web by blowing along the plane of the web. If too much force is used to hold the box against the forming fabric, excessive wear of the fabric results. The fabrics used to form the paper web are expensive and premature replacement of the forming fabrics results in additional costs caused by the lack of productivity while the machine is down. Unlike vacuum, which supplies its own clamping force, pressure requires a separate system to develop the sealing force. 
     Part of the answer is disclosed in U.S. Pat. No. 5,225,042 to Eaton et al. which discloses how a seal can be formed by pressing a sealing member against an unsupported portion of a forming fabric. Eaton et al. discloses a system useful for pressures to about ten psi. Further, Eaton et al. shows a gravity drain system opposite the pressure box. 
     Certain grades of paper, such as tissue paper or creped papers are typically formed by pressing the web onto a large diameter Yankee dryer, and creating a soft absorbent web by scraping the web off the dryer surface with a doctor blade. Alternative approaches hold out the possibility of increasing absorbency while overcoming the limitations of using a single large diameter Yankee dryer. If a web can be dried without pressing, an absorbent web can be formed without creping the web with a doctor blade. New approaches may lead to more cost-effective approaches to manufacturing these important and widely used grades of paper. 
     Critical to improving the manufacture of tissue paper without creping is an ability to reduce water content in the web as formed without compressing the web. The process of supplying high pressure air and vacuum simultaneously to the web in the forming section has the possibility of reducing web water content by three to five or more percent. This represents a significant reduction in cost compared with removing the same water by techniques which are solely dependent on evaporation for reducing the water content of the web. 
     What is needed is an apparatus which removes water from a paper web with high pressure air which does not disrupt the web and does not lead to excessive wear of the forming fabrics. 
     SUMMARY OF THE INVENTION 
     An air press for dewatering a web comprises a pressurized box which is positioned opposite a vacuum box. The pressurized box consists of a leading cross machine direction baffle with a ceramic shoe which engages a forming fabric, and a trailing cross machine direction baffle of similar construction. Two end deckles formed of high density polyethylene complete the pressurized box. The leading cross machine direction baffle presses against a resilient seal which causes a web contained between upper and lower forming fabrics to wrap about the baffle shoe a few degrees. This wrapping, in combination with a resilient foam backing or an applied vacuum, forms an effective seal which does not allow air to bypass the baffle. The trailing cross machine direction baffle is positioned over a vacuum box which prevent air from leaking around the trailing baffle. Between and opposite the leading and trailing baffles, a vacuum box draws air from the pressurized box and through the forming fabrics and the web, increasing the total pressure gradient across the web to 20 to 30 psi or more. The pressure box and opposed vacuum box form an air press. The pressure box is positioned and held against the forming fabrics by opposing air tubes. 
     An alternative embodiment air press utilizes a suction roll positioned beneath the pressure box and having a suction gland positioned beneath the portion of the roll which backs the pressure box. 
     The upper and lower forming fabrics diverge after the air press and the path of the web is controlled by a transfer slot into which the trailing baffle is positioned. 
     It is a feature of the present invention to provide a means for dewatering a paper web in the forming section of a papermaking machine. 
     It is a further feature of the present invention to provide a forming section which produces a paper web having a three to six percent greater fiber dry weight than conventional forming sections. 
     It is another feature of the present invention to provide an air press which will not cause excessive wear on forming fabrics passing through the air press. 
     It is also a feature of the present invention to provide an apparatus for sealing a pressurized box to a forming fabric without excessive wear of the forming fabric. 
     It is an additional feature of the present invention to provide a pressurized box loading system which is not prone to crashing into the forming fabrics. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an cutaway isometric view of the air press of this invention. 
     FIG. 2 is a cross-sectional view of an alternative embodiment of the air press of this invention, wherein a vacuum roll is opposed to a pressure box. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring more particularly to FIGS. 1-2, wherein like numbers refer to similar parts, an air press  20  is shown in FIG.  1 . The air press  20  is positioned about an upper forming fabric  22  and a lower forming fabric  24  and a web  26  positioned therebetween. The air press comprises a pressure box  28  positioned above the upper forming fabric  22  and a vacuum box  30  positioned below the lower forming fabric  24 . Air is supplied to the pressure box  28  and flows through the relatively permeable upper forming fabric  22 , through the web  26 , and finally through the relatively permeable lower forming fabric  24 . 
     The large volume of the pressure box  28  means that air flows with relatively low velocity and a relatively high pressure of about fifteen to thirty psi through the web  26  as it passes between the pressure box  28  and the vacuum box  30 . Pressure in the vacuum box  30  is typically about fifteen inches of Mercury below atmospheric or about seven psi vacuum. Thus the total pressure drop across the web  26  is approximately twenty to thirty-five psi and almost all the pressure drop occurs through the web  26  thickness. The web  26  is relatively thin, approximately a few hundredths of an inch or less, and thus the pressure drop across the web is one to a few thousand psi per inch. 
     The effect of the rapid expansion of air as it passes through the web  26  is to remove water from the web. A prior art air press device used on a tissue web having a solids content of about twenty-five percent and a thickness of about three hundredths of an inch was able to increase the solids content to approximately thirty-three percent. While these levels of water removal are desirable, the mechanical lever employed in the prior art device to urge the pressure box against the web can result in web breakage in the event of pressure loss. Although the precise mechanism is not clearly understood, the use of pressurized air is more effective than vacuum alone because of the higher pressures available and the greater air volume forced through the web. Prior art air presses have two basic problems: establishing an adequate seal between the pressure box and the forming fabrics without causing excessive wear of the forming fabrics; and controlling the loading of the pressure box against the fabric. A pressure of twenty psi with an area of six inches by 200 inches results in a force of 12 tons caused by the pressure in the box which must be counterbalanced. If a mechanical lever arm is used to allow movement of the pressure box, and if pressure in the box is suddenly released for any reason, such as a web break, which allows rapid venting through the fabrics, the pressure box can be driven into the forming fabrics with destructive results. 
     The pressure box  28  of this invention overcomes these problems by having a frame  32  which is rigidly mounted with respect to the forming fabrics,  22 ,  24 . An opened sided box  34  is mounted to the frame  32 . The top of the box  34  is open so that the box does not have to support the pressure directly away from the forming fabrics  22 ,  24 . The box  34  is constructed of a leading cross machine direction baffle  36  and a trailing cross machine direction baffle  38  and two end deckles  40  which extend in the machine direction. The baffles  36 ,  38  and deckles  40  are mounted for movement towards and away from the forming fabrics  22 ,  24 . The pressure within the box  34  acts only against the inwardly facing sides  42 ,  44  of the baffles  36 ,  38  and the inwardly facing sides  46  of the deckles  40 . The leading baffle  36  is structurally tied by cross beams  48  to the trailing baffle  38  so that the forces developed by the pressure in the box  34  on the baffles  36 ,  38  are statically equal and are carried by the cross beams  48 . 
     The width of the deckles  40  is only about 6 inches in a typical application and therefore the loads developed are small, a few hundred pounds, and can be resisted by using thick deckles which are supported in wide piston slots  50 . 
     The baffles  36 ,  38 , as shown in FIG. 1, have an L-shape, with the long legs  51 ,  52  of the L terminating in ceramic shoes  53 ,  54  which each ride in a pocket formed by the vacuum box  30  as discussed below. The short legs  55 ,  56  are mounted in cavities is  57 ,  58  in the frame  32 . The cavities  57 ,  58  are sealed from the interior of the box  34  by flexible flaps  60  which are attached to the frame  32  and extend downwardly over the interior surfaces  42 ,  46  allowing motion between the box  34  and the frame  32 . By preventing pressure from the interior of the pressure box  28  from acting on any horizontal surface, the pressure interior to the pressure box  28  does not load or move the baffles  36 ,  38  towards or away from the forming fabrics  22 ,  24 . 
     The loading of the ceramic shoes  53 ,  54  and the movement of the inner box  34  is controlled by pairs of opposed air tube. Upper air tubes  62 ,  64  move the baffles  36 ,  38  downwardly by expanding between upwardly facing surfaces  66 ,  68  of the short legs  55 ,  56 . Lower air tubes  65 ,  67  move the baffles  36 ,  38  upwardly away from the forming fabrics  22 ,  24  by expanding between portions  70 ,  72  of the frame  32  and downwardly facing surfaces  74 ,  76  of the short legs  55 ,  56  of the baffle  36 ,  38 . 
     The upper air tubes  62 ,  64  and lower air tubes  65 ,  67  are connected to a source of compressed air (not shown) and a controller (not shown) which controls the force with which the baffles  36 ,  38  are pressed against the upper forming fabric  22 . The L-shaped baffles  36 ,  38  have some cross machine direction flexibility which allows them to conform to the forming fabrics  22 ,  24 . 
     The end deckles  40  are supported by double acting air cylinders  78  which can raise and lower the deckles  40  into engagement with the forming fabrics  22 ,  24 . The deckles  40  and the structure  80  in which the piston slots  50  are formed can be mounted on machine screws  82  so that the end deckles  40  may be adjustably positioned for paper webs of different widths. Because of the high pressure contained in the pressure box  28 , sealing between the deckles  40  and the baffles  36 ,  38  is critical. As shown in FIG. 1 a cylindrical rubber seal member  84  in grooves  86  in the sides  88  of the deckle  40  forms a pressure seal with the sides of the baffles  36 ,  38 . To allow movement of the deckles  40  in the piston slots  50  an enlarged groove  90  in the piston and an enlarged groove  92  in the deckle support structure  80  allows movement of the deckles  40  while maintaining a seal with the baffles  36 ,  38 . 
     The pressure box  28  is positioned over the vacuum box  30 . The vacuum box  30  has a leading cross machine direction side  96  and a trailing cross machine direction side  98 , the sides have ceramic shoes  100  which engage the lower forming fabric  24 . A plurality of cross machine direction intermediate supports  102  are positioned between the leading and trailing vacuum box sides  96 ,  98 . The intermediate supports  102  have ceramic shoes  104  which engage and support the lower forming fabric  24 . Gaps  106  between the intermediate supports open into a vacuum duct  108 . 
     A pocket  110  formed between the leading side  96  and the first intermediate support  113  is shown filled by a resilient foam  112  which is supported by a solid support  114 . The shoe  53  of the leading edge baffle  36  is loaded by the upper air tube  62  which causes the shoe  53  to slightly depress the upper and lower forming fabrics  22 ,  24  and the web  26  sandwiched therebetween. The amount of deflection is adjusted depending on the fabric tensions and the type of web being formed (tissue, paper, linerboard) and the pressure in the pressure box  28 , so that a good seal is formed which prevents air leaks and air blowing through the web  26 . 
     A transfer slot  116  is formed between the trailing side  98  of the vacuum box and the last intermediate support  118 . The trailing baffle  38  is moved by the upper air tube  62  which causes the shoe  54  to slightly depress the upper and lower forming fabrics  22 ,  24  and the web  26  sandwiched therebetween. To effect a good seal between the fabrics and the trailing baffle  38 , a vacuum beneath the transfer slot  116  is necessary. In addition, for the slot  116  to effectively transfer the web to the lower forming fabric  24 , vacuum must be applied to the lower fabric as it is pulled away from the upper fabric  22 . The amount of vacuum beneath the transfer slot  116  should not be too high preferably about five inches of mercury below atmospheric as opposed to the fifteen inches of mercury in the vacuum box  30 . If the vacuum is to high in can pull the web  26  and the lower forming fabric  24  away from the upper fabric  22 . A separate source (not shown) of lower vacuum is therefore connected to the transfer slot  116 . 
     An alternative embodiment air press  120  is shown in FIG.  2 . As the machine direction width of an air press is increased, the amount of friction drag between the lower forming fabric and the vacuum box increases. The air press  120  can be used to form linerboard. As the web being dried increases in thickness, the time over which it can be usefully devolatized by remaining in the air press increases. The increased width can result in unacceptable drag forces between the lower forming fabric  122  and a vacuum box. The air press  120  employs a vacuum roll  124  which has a foraminous surface through which air can be drawn. A gland  126  is positioned within the roll  124  and remains stationary beneath the upper pressurized box  128 . The upper pressurized box  128  is similar to the pressurized box  28  and utilizes a design which provides gentle constant loading of ceramic shoes  130  mounted on cross machine direction leading and trailing baffles  136 ,  138 . 
     It should be understood that for some webs and machine conditions it may be desirable to dispense with the resilient foam  112  and the solid support  114  positioned in the pocket  110  as shown in FIG.  1  and simply use the vacuum to form the seal. If this is the case the amount of vacuum applied to the seal should be about five inches of mercury below atmospheric which is substantially less than the fifteen inches of mercury below atmospheric applied to the vacuum box  30 . 
     It should also be understood that the leading baffle and trailing baffle would not necessarily have to be tied together. If each baffle is allowed to move independently then the total height of the inside surfaces of the baffles should be minimized to minimize lateral loads which the outside plates  140  shown in FIG. 1 would be required to support. Lateral bearings made from low friction surfaces of ultra high density polyurethane or the like could be used to minimize the force necessary to move the baffles towards and away from the forming fabrics. 
     It should be understood that tension in the forming fabrics is typically in the range of from about 10 to about 60 pounds per linear inch and in particular, tensions in the range of around forty pounds per linear inch may be desirable. The forming fabrics can be any fabric permeable to air, for example Albany International 94M, Appleton Mills 2164B or the like. 
     The air which is supplied to the pressurized box may be heated to reduce its viscosity and thus increase its ability to flow through the paper web. Secondary benefits of heating the air may be some additional drying capability. Gases other than air should be understood to be included in the term “air.” Gases like combustion gases and other waste gases are most likely to be available. 
     It should be understood that key to making an air press function is an ability to seal the pressurized source of air to the forming fabrics without excessively loading the web so as to cause wear or permanent deformation of the fabrics. It is also important that loading of the fabrics does not increase significantly if a paper break or other malfunction causes air to be rapidly vented from the pressurized box. The position of a vacuum box or vacuum roll beneath the pressurized box is also important in effecting a good seal around the baffles making up the pressurized box. 
     It should be understood that if the endless loops of the forming fabrics have seams where the ends of the fabric are joined, the design of the air press must accommodate the passage of the thicker joints, as does the apparatus shown in FIG.  1 . 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.