Abstract:
A steam distributor includes a front screen equipped with steam perforations wherein the output area of at least some of the perforations can be adjusted to enable active control of the steam jet velocity. The steam velocity can be controlled independently of steam flow. The front screen is includes (i) a first plate that has a first set of apertures and (ii) a second plate that has a second set of apertures, wherein the second plate covers the first plate, and wherein the means fir varying the size of at least one of the perforations moves the first plate, the second plate, or both the first and the second plates in order to change the position of the first set of apertures relative to the second set of apertures.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a steam distributor for applying steam to a web such as a paper sheet that is moving along its side wherein steam is discharged through a plurality of perforations in a screen. By varying the output area of the perforations, optimal steam velocity can be attained to achieve the desired steam absorption into the web and/or achieve efficient moisture removal. 
       BACKGROUND OF THE INVENTION 
       [0002]    The steam heating of a paper sheet is widely practiced in papermaking. The increase in sheet temperature that results provides increased drainage rates for the water thus reducing the amount of water to be evaporated in the drier section. Water drainage is improved by the application of steam principally because heating of the sheet reduces the viscosity of the water, thus increasing the ability of the water to flow. Most of the heat transfer takes place when the steam condenses in the sheet. The condensation of the steam transforms the latent heat of the steam to sensible heat in the water contained by the sheet. 
         [0003]    A particular advantage of steam heating of the paper sheet is that the amount of steam applied may be varied across the width of the sheet along the cross machine direction so that the cross machine moisture profile of the sheet may be modified. This is usually carried out to ensure that the moisture profile at the reel is uniform. Moisture measurement devices are well known in the papermaking art that can sense the moisture profile of a sheet of paper. If such an apparatus is scanned over the paper sheet, downstream of a steam distributor, then after measuring the water profile in the sheet, steam can be applied in varying amounts on a selective basis across the sheet, thus achieving the required uniform moisture profile at the reel. 
         [0004]    A typical steam distributor is divided into compartments with laterally spaced-apart baffle plates that are covered with a partially perforated cover. Actuators supply steam to the compartments. By regulating the supply of steam into each compartment, it was possible to a limited extent to control the moisture profile of the sheet. Nevertheless, even with these improvements, the velocity of the steam passing through the perforated cover varies only with the actuator flow rates so ideal steam velocity cannot be achieved for different flow rates. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is based in part on the development of a steam distributor that includes a front screen that is equipped with steam perforations wherein the output area of at least some of the perforations can be adjusted to enable active control of the steam jet velocity. With respect to paper manufacturing, steam velocity affects penetration depth, boundary layer penetration, and response shape, especially the response width of the steam that is applied to the sheet. Excessive steam velocity causes sheet breakage whereas slowly delivered steam yields poor efficiency. With the present invention, the steam velocity can be controlled independently of steam flow to optimize efficiency and thereby avoid sheet upsets. 
         [0006]    Accordingly, in one aspect, the invention is directed to an apparatus to distribute steam that includes: 
         [0007]    a steam distribution header; 
         [0008]    a housing defining a steam discharge chamber that is in fluid communication with the steam distribution header; 
         [0009]    a front screen that covers the steam discharge chamber and which has a plurality of perforations through which steam exits; 
         [0010]    means for varying the size of at least one of the perforations through which steam exits; and 
         [0011]    means for regulating the flow of steam from the steam distribution header into the steam discharge chamber. 
         [0012]    In another aspect, the invention is directed to a method of distributing steam along a length of continuously moving sheet which includes the steps of: 
         [0013]    (a) positioning an apparatus having a leading edge and a trailing edge relative to the moving sheet, wherein the apparatus includes:
       (i) a steam distribution header;   (ii) a housing defining a steam discharge chamber that is in fluid communication with the steam distribution header; and   (iii) a front screen that covers the steam discharge chamber and which has a plurality of perforations through which jets of steam exit;       
 
         [0017]    (b) regulating the flow of steam from the steam distribution header into the steam discharge chamber to establish a predetermined, steam flow rate through the plurality of perforations; 
         [0018]    (c) adjusting the velocities of the Jets of steam to desired levels at the predetermined steam flow rate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a perspective view of a steam distribution apparatus; 
           [0020]      FIG. 2A  is a perspective view of the compartments in the steam distributor apparatus; 
           [0021]      FIG. 2B  is enlarged, partial view of the front screen panel; 
           [0022]      FIG. 2C  is a partial front view of the compartments formed by stationary baffles or dividers; 
           [0023]      FIG. 3  shows a pair of separated plates that form a front screen when they are combined; 
           [0024]      FIG. 4A  shows the cross sectional view of a screen consisting of two plates with apertures that are form perforations through which steam flows; 
           [0025]      FIGS. 4B and 4C  show the front views of the screen consisting of two plates with apertures wherein the apertures are fully and partially aligned, respectively; 
           [0026]      FIGS. 5A and 5B  show the front views of a screen consisting of two plates with apertures of different sizes at two different alignment positions: 
           [0027]      FIG. 6  is a cross sectional view of a compartment; 
           [0028]      FIG. 7A  is another perspective view of a compartment; and 
           [0029]      FIG. 7B  illustrates an actuator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  illustrates the overall assembly of a steam distribution apparatus or steam box  10  which includes an elongated housing  12  that is enclosed by end plates located at opposite ends. The length of the apparatus typically corresponds to the width of the sheet or web to which steam is to be applied. For papermaking operations the length can range, for instance, up to about 30 feet (9.1 meters). An external source of steam is connected to the steam distribution apparatus  10  and excess steam in the form of condensate is removed through a drain  16  which is located on the side of end plate  14 . The contour of the front screen panel or plate  18  preferably matches the external shape of the product to which steam is being supplied. The concave-shaped curvature of front screen panel  18  is particularly suited for apply steam to a roll of material. The front screen panel can also have a planar configuration to match the straight run of a moving sheet. 
         [0031]    As further described herein, front screen panel  18  has steam outlets or perforations (not shown) that are formed thereon. The perforations are arranged so that exiting steam expands and impacts the surface of adjacent moving sheet to form a desired pattern (or response shape) of condensate. In one embodiment, the response shape is uniform along the width (or cross direction) of the moving sheet. With the present invention, the steam velocity can be optimized independent of the steam how rate. 
         [0032]    The steam distributor apparatus  10  is preferably separated into a plurality of steam discharge chambers or compartments along its length. By regulating the amount of steam that passes through each compartment, it is possible to control the level of condensate that is applied along the cross direction of the moving sheet. For example, the amount of steam that enters into the individual chambers can be controlled in response to variations in measured properties of the sheet along its cross direction. Furthermore, the perimeter(s) of one or more of the compartments that define that steam profiling zone for the steam application can also be modified. This permits control of the steam profile along the cross direction as well. The invention is illustrated in an apparatus with multiple steam discharge chambers or compartments. The partitions or baffle panels that are laterally spaced apart create corresponding profiling zones that are covered by a perforated screen plate through which steam passes. It is understood however that the invention can be implemented with a steam distributor having a single discharge chamber. 
         [0033]      FIG. 2A  shows a partially disassembled exposed portion of the housing  30  of the steam distributor apparatus. The housing  30  encloses a steam distribution header  36  which is connected to at least one source of steam (not shown). Header  36  runs the length of the steam distribution apparatus. The header  36  is flanked by an interior wall  60  and an exterior wall  62 . The inner enclosure  34  shields the pneumatic actuators  32  with a removable cover that is secured by the hand tightened screws  64 . A plurality of baffles or partition panels  40 , that are laterally spaced apart, are secured to the exterior wall  62  thereby creating a number of steam discharge chambers or compartments once the front screen panel segment  31  is secured to the forward part of the housing. As further described herein, screen panel segment  31  comprises an interior plate  130  that is coupled to exterior plate  120  that faces a moving web. 
         [0034]    In this embodiment, the middle of front screen segment  31  of front screen panel  18  ( FIG. 1 ) is fully populated with outlets  20 , which as shown in  FIG. 2B . Outlets  20  are preferably circular but it is understood that the individual outlets can have non-circular configurations. The number and size of the outlets are designed to achieve the desired steam flow rate and velocity. The size of the outlets  20  should be sufficiently small to minimize the amount of fibers and other debris from the sheet of material being heated that enters into the discharge chambers. Nevertheless, in operation, as steam is applied through the perforations  20  onto a moving sheet of paper, for instance, the middle of front screen segment  31  can come into contact with the sheet. In this regard, it is may be preferred to avoid excessive blank areas on the middle of front screen segment since there may be a tendency for debris to accumulate in areas on the panel that are not populated with outlets. As is apparent, the number of front screen panel segments  31  required to cover a steam distribution apparatus will depend on the total cross directional length of the steam distribution apparatus and the cross directional length of each panel segment  31 . 
         [0035]    Each pneumatic actuator  32  is operatively connected to a pipe  42  which has an inlet end located within the header  36  and an outlet end that is located in a discharge chamber. In this embodiment, the inlet end of the pipe  42  is partially covered by a sleeve  44 . A piston is attached to the actuator  32  by a connecting rod to regulate the inlet into pipe  42  and thus control the steam flow between the header  36  and the control chamber. 
         [0036]    As shown in  FIG. 2C , a plurality of oblique-oriented baffles  40 , which are not aligned with the machine direction of movement of the traveling sheet (not shown), form a plurality of steam discharge compartments  66  along the cross direction or width of the steam distribution, apparatus  10  ( FIG. 1 ). While baffles  40  are illustrated as being planar, it understood that they can be curved or other non-planar configuration. The perimeter(s) of discharge compartments  66  define a series of trapezoidal-shaped profiling zones  22  through which steam from outlets  68  passes as it travels toward the steam perforations  20  ( FIG. 2B ). In this arrangement, adjacent trapezoidal-shaped profiling zones are inverted with respect to each other. The profiling zones  22  can exhibit other shapes depending on the configuration of partition panels  40 . Where adjacent panels  40  are vertical and parallel, the profiling zones are rectangular. 
         [0037]      FIG. 3  illustrates the front screen  31  ( FIG. 1 ) when dissembled into an exterior plate  120  and an interior plate  130 . (Only a few of the perforations in front screen  31  are represented.) The exterior plate  120  has a plurality of apertures  124 ,  126 ,  128  that form a pattern of apertures as shown on the front surface  122 . Similarly, the interior plate  130  has a plurality of corresponding apertures  134 ,  136 ,  138  that form a pattern of apertures as shown on the front surface  132 . The dimensions and curvature of exterior plate  120  match that of interior plate  130  so that when the two plates are slidably fitted together, they form front screen  31  ( FIG. 1 ). In this embodiment, the size of the circular apertures in both plates  120  and  130  are the same; moreover, the pattern of the apertures in plate  120  is also aligned with the pattern of the apertures in plate  130 . Thus, for example, apertures  124 ,  126  and  128  of exterior plate  120  are directly above apertures  134 ,  136  and  138 , respectively, when exterior plate  120  and interior plate  130  are assembled to form the front screen  31 . The configurations and positions of the apertures in the plates can be varied as desired in order to achieve optimum steam velocities. For example, while the cross sectional area of the apertures is preferably circular the area can be rectangular or other polygonal shape. In the case where the cross sectional area is circular, its diameter typically ranges from 0.0625 to 0.25 inches (1.59 to 6.35 mm) and preferably from 0.0625 to 0.125 in. (1.59 to 3.18. mm). Regardless of the geometry, the cross sectional area of each aperture typically ranges from 0.003 to 0.05 sq. in. (1.94 to 32.3 sq. mm) and preferably from 0.003 to 0.012 sq. in. (1.94 to 7.74 sq. mm.) The thickness of the exterior plate  120  is preferably the same as that of interior plate  130 ; the thickness of each plate typically range from about 0.0313 to 0.125 in. (0.795 to 3.175 mm) and preferably from about 0.0625 to 0.125 in. (0.795 mm to 3.175 mm). 
         [0038]      FIGS. 4A and 4B  depict a partial cross sectional and front view of screen  31  ( FIG. 1 ) that is formed by pressing exterior plate  120  against interior plate  130 . The apertures in exterior plate  120  are fully aligned to those of interior plate  130  and, as an illustration, apertures  152 ,  154 ,  156  and  158  are located along one side of exterior plate  120  and are aligned with corresponding apertures  162 ,  164 ,  166 , and  168 , respectively on one side of interior plate  130 . In this configuration, apertures  152  and  162  form perforation  142 , apertures  154  and  164  form perforation  144 , apertures  156  and  166  form perforation  146 , and apertures  158  and  168  form perforation  148  on screen plate  31 . In the fully aligned arrangement shown in  FIG. 4B , the output area of the perforation is the highest which means that for a given steam flow rate into a discharge chanter, the steam jet velocity is at the lowest. 
         [0039]    Lateral movement of exterior plate  120  relative to interior plate  130  shifts the positions of the apertures in exterior plate  120  relative to those in exterior plate  130  so as to reduce the size of the perforations in the screen plate as shown in  FIG. 4C . For example, aperture  152  partially covers aperture  162  so that the area of perforation  142 A is smaller than that of perforation  144  ( FIG. 4B ) when aperture  152  is fully aligned with corresponding aperture  162 . In this fashion, the cross sectional area of each perforation (such as perforations  142 A,  144 A,  146 A and  148 A) in the screen plate  31  is adjusted to the same degree. Lateral movement of exterior plate  120  relative to interior plate  130  can be accomplished by moving one or both plates. As shown in  FIGS. 48 and 4C , in this embodiment, the exterior plate  120  is connected to a precision manual or motorized displacement device  108 . Suitable manual devices include screw mechanisms and suitable motorized devices include linear actuators. As further described herein, the effect of partially reducing the output area is to increase the steam jet velocity for a given steam volumetric flow into the discharge chamber. 
         [0040]      FIGS. 5A and 5B  illustrate an embodiment of a screen plate  190  that includes exterior plate  180  and a lower interior plate  170  where the sizes of the apertures vary. Only three rows of perforations on the screen plate  190  are illustrated. In this construction, the circular apertures in the interior plate  170  all have the same diameter whereas the circular apertures in the first and third rows of exterior plate  180  are three times larger while the remaining apertures in the exterior have the same diameter as the apertures in the interior plate  170 . As shown in  FIG. 5A , when the apertures in the two plates are fully aligned, the output area of each screen plate perforation is restricted only by the size of the smaller interior apertures. In the top and third row of perforations, the larger apertures of the exterior plate  180  (such as aperture  182 ) are aligned with the smaller apertures in the interior plate  170  such as aperture  172 ). Thus, the output area for steam flow for perforation  182  is the same as the area of aperture  172 . In the second row, because the size of the apertures in both plates is the same, the output area of the perforation  194  will also be the same. For this design, shifting of the exterior plate  180  relative to interior plate  170  effects the output areas of the first and second row perforations different than the output areas of the middle row perforations. For example, as shown in  FIG. 5B , when the exterior plate  180  is moved a distance equal to the diameter of the smaller interior plate  170 , the top row perforations retain the same output areas because the larger aperture  172  is sufficiently large to still fully expose the underlying smaller aperture  162 . In contrast, the output areas of middle row perforations is effectively eliminated since the aperture  182  in the exterior plate and the aperture  172  in the interior plate are not align at all. The effect is to increase the jet velocity through the top and lower row perforations if the steam volumetric flow is the same, but no steam flows through the middle perforations as they are closed. As is apparently, if the exterior plate  180  is moved a distance of less than the diameter of the smaller interior plate  170 , the output areas of the middle low perforations would be reduced thereby increasing the jet velocities in all of the perforations. 
         [0041]    As is apparent, the shape, dimensions and arrangement of the apertures in the movable exterior and interior plates can be selected to create the desired steam output areas for the perforations in a screen that is formed. Indeed, while the screen plate is usually formed with two plates with apertures, additional plates can be used to provide additional features to the screen plate. Once the exterior and interior plates are slidably engaged, lateral movement of one or both plates changes the output area so as to modify the steam jet velocities of the steam exiting the perforations of the screen and impinging on the moving web. 
         [0042]    In operation of the steam box as shown in  FIGS. 6 and 7A , high pressure steam that is supplied to the header  36  is drawn into the pipe  42  through the annular opening between the pipe  42  and the sleeve  44 . The amount of steam drawn is controlled by the actuator  32  which is connected to a pneumatic supply  35  which tunes or regulates the actuator by pressurizing a diaphragm that is on top of a piston that is located inside the actuator  32 . The piston is connected to a measuring plug that moves inside the sleeve  44  to control the amount of steam that goes into each discharge chamber. Steam from the pipe  42  initially enters into a discharge chamber  66  through the pipe outlet  68 . The high velocity steam is dispersed within the discharge chamber  66  before exiting through the perforations of the from panel screen segment  31  and contacting a continuous moving sheet  33  located in front of the perforations. Preferably, a target plate  92  is positioned to disperse the high velocity steam uniformly throughout the discharge chamber  66  before the steam permeates through the perforations in the screen plate  31 . In this fashion, there is uniform steam distribution from the leading edge  104  to the trailing edge  106  of the steam distribution apparatus as the sheet of material moves across the screen plate  31  in the machine direction. The speed at which moving sheet  33  determines the boundary layer velocity (or cross flow velocity), which is the velocity of the gaseous fluid flowing adjacent the moving sheet. Condensate that forms on the bottom of the discharge chamber  66  seeps through a drain hole and out through a condensate drain  38 . 
         [0043]    With respect to paper manufacturing, the desired or ideal steam velocity depends on, among other things, furnish (or paper pulp) composition, machine speed, and machine configuration. Steam velocities that are too low or excessively high degrade steam shower performance which result in reduced production, wasted steam and fiber build up in the steambox that in turn leads to sheet breaks, steam cloud, dripping and other problems. With the present invention, the steam jet velocity can be optimized to accommodate different paper production rates, paper grades and other criteria. Referring to  FIG. 6 , optimizing the jet velocity can takes into account various factors including, for example: (i) the distance (H) between screen  31  (equipped with the perforations) and moving sheet  33 , (ii) output area (B) of the perforations; and (iii) boundary layer velocity. Typically, the H is between 0.125 to 0.5 in. (3.18 to 12.7 mm) and the boundary layer velocity is 300 to 7000 ft./sec. (91.4 to 2,134 m/sec.) For a particular paper production rate and grade, once H and the boundary layer velocity are established, the steam jet velocity can be optimized by adjusting the output area of the perforations. The jet velocity should range from 50 to 150 ft./sec. (15 to 45.7 m/sec.). 
         [0044]    By monitoring and controlling the steam flow into each of the discharge chambers, the steam profile that is injected onto the sheet along its cross direction can be continuously and independently regulated. The steam profile as measured along the length of the steam distribution apparatus can be uniform or non-uniform so that the sheet or web of material can be exposed to a steam curtain having different steam velocities in the cross direction. Adjustment to the output areas of the perforations can be made in response to cross direction sensors, such as moisture profile sensors, located upstream and/or downstream of the steam distributor. 
         [0045]    As shown in  FIG. 2A , the front screen panel segment  31  has a concaved exterior contour. A backing bar  98  is secured to the lower end of the laterally spaced baffles  40 . The front screen panel segment  31  can be welded onto a portion of the backing bar  98  as well as onto the baffles  40 . In this fashion, the front screen panel segment  31  forms the front perforated wall of the steam discharge chambers. The front of the backing bar  98  also defines a series of dowel pins  84  that helps align the cleanout bar  48  as it is secured with screws  50  to the body of the steam distribution apparatus. When it is necessary to clean the steam discharge chambers between the baffles  40 , it is only necessary to remove the cleaning bar  48  to gain access to the discharge chambers through access slots that are located at the lower end of each discharge chamber. 
         [0046]    The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing, from the scope of the present invention as defined by the following claims.