Abstract:
A proactive steam and mist removal apparatus is employed along a point of application of a rewet or steam shower system to remove excess steam and water droplets to prevent damage to machinery and products. The apparatus includes an internal chamber equipped with cooled baffles that promote condensation and a condensate collection mechanism. A vacuum source draws a stream of ambient steam and water droplets from the environment along a path into the chamber and a projected gas stream directs the stream into the chamber wherein at least a portion of the ambient steam condenses into condensate. The remaining portion of ambient steam is separated into water and cold air that is recycled as the projected gas. Steam and moisture that escape capture into the internal chamber are heated by a hot air blower that raises their relatively humidity so that the dried air dissipates harmlessly into the atmosphere.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to techniques for removing excess steam and moisture and particularly to devices for removing water vapor and mist that are generated by steaming and rewet systems employed in papermaking machines. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric as water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is, in essence, a water removal, system. 
         [0003]    Conventional papermaking machinery includes equipment to set the sheet properties of the paper as it is being manufactured. One of the more basic operations on a papermaking machine is control of the cross-direction (CD) moisture profile by rewetting or remoisturizing with water sprays administered by spray nozzles. By applying water to the drier areas of a sheet, a uniform CD moisture profile can be created. This is usually carried out to ensure that the moisture profile at the reel is uniform. Similarly, steam heating of a paper sheet is widely practiced in papermaking. The increase in sheet temperature that results provide increased drainage rates for the water thus reducing the amount of water to be evaporated in the dryer section. Water drainage is improved by the application of steam principally because the 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. As in the case of rewetting, in steam heating the amount of steam applied may be varied across the width of the sheet along the cross machine direction so that the CD moisture profile of the sheet may be modified. 
         [0004]    Unfortunately, in both rewet and steam shower processes, water tends to accumulate in the vicinity adjacent the points of application. Large water droplets develop on surrounding structures that can result in the formation of rust and related problems. In addition, the rotating rolls and airflow of the machinery can propel droplets onto the paper sheet. Conventional vacuum boxes, which are not energy efficient, demonstrate only limited success in removing the excess steam and moisture; as a result, the excess water is detrimental to the production of high quality paper and the operations of the machinery. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is based in part on the development of a proactive steam and mist removal apparatus that can be employed in any environment where excess steam and water droplets are generated. The apparatus is particularly suited for use in an enclosed manufacturing environment where the steam and water droplets if not removed would cause damage to the machinery and products being made. 
         [0006]    In one aspect, the invention is directed to an apparatus for removing ambient steam and water droplets from the environment that includes: 
         [0007]    a. a housing defining a chamber comprising contact surfaces that promote condensation; 
         [0008]    b. a vacuum source that draws ambient steam and water droplets from the environment along a path into the chamber; and 
         [0009]    c. a source of gas that projects a stream of gas along the path of ambient steam and water droplets to direct the ambient steam and water droplets into the chamber wherein at least a portion of the ambient steam condenses into condensate and wherein the remaining portion of the ambient steam is directed into a separation means for separating the remaining portion of ambient steam into a liquid component comprising water and a gaseous component. 
         [0010]    The inventive apparatus is particularly suited for used in a papermaking machine to capture excess steam and water droplets in the form of misty air that is generated in rewet and steam shower applications. In a preferred embodiment, the configuration of the housing facing the misty air takes advantage of the natural motion of the cloud of misty air that develops by defining an entry point into the chamber that initially accelerates the velocity of the misty air. Moreover, the internal structures within the chamber near the entry point are preferably designed with an abrupt turn away from the direction of flow so that centrifugal forces facilitate the separation of free droplets from the steam of the misty air. The free droplets are readily collected and drained away from the papermaking machine. 
         [0011]    In another aspect, the invention is directed to a method of removing ambient steam and water droplets that includes the steps of: 
         [0012]    a. providing a housing defining a chamber comprising contact surfaces that promote condensation; 
         [0013]    b. drawing the ambient steam and water droplets from the environment along a path into the chamber; 
         [0014]    c. projecting a stream of gas along the path of ambient steam and water droplets to direct the ambient steam and water droplets into the chamber wherein at least a portion of the ambient steam condenses into condensate and wherein the remaining portion of the ambient steam is separated into a liquid component comprising water and a gaseous component. 
         [0015]    In a further aspect, the invention is directed to an improved papermaking machine equipped with a mist removal system that includes: 
         [0016]    a source of mist that is generated at an application point along the machine direction of the papermaking machine; and 
         [0017]    a vacuum-assisted device positioned adjacent the application point that includes:
       i. a housing defining a chamber comprising contact surfaces that promote condensation;   ii a vacuum source that draws ambient steam and water droplets from the environment along a path into the chamber; and   iii. a source of gas that projects a stream of gas along the path of ambient steam and water droplets to direct the ambient steam and water droplets into the chamber wherein at least a portion of the ambient steam condenses into condensate and wherein the remaining portion of the ambient steam is directed into a separation means for separating the remaining portion of ambient steam into a liquid component comprising water and a gaseous component.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a cross sectional schematic view of a steam and mist removal apparatus; 
           [0022]      FIGS. 2 and 3  are front and rear views, respectively, of a steam and mist removal apparatus; and 
           [0023]      FIG. 4  depicts an example of a papermaking machine employing steam and mist removal apparatuses. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    The steam and mist removal device of the present invention can be employed in any environment such as an industrial setting where excess steam and/or mist are continuously generated. The device is particularly suited for employment in an enclosed area where steam and water droplets do not readily dissipate into the atmosphere. 
         [0025]    As illustrated in  FIG. 1 , the steam and mist removal device  2  includes a housing  4  that defines a chamber  8  with an opening that is partially shielded by front panel  6 . Upper flange  10  of housing  4  and the upper end of panel  6  define an upper entrance  50  into chamber  8 . Lower flange  12  of housing  4  and the lower end of panel  6  define a lower entrance  52 . A rear inner panel  14  having a surface that preferably matches that of housing  4  forms a rear channel  18  where condensate collects. Similarly, a lower front inner panel  16  having a surface that preferably matches that of housing  4  forms a front channel  20  where condensate collects. Channels  18 ,  20  collect condensate and free water droplets into a steam trap  22 , which has a drain through which water is disposed. Located at the lower end of housing  4  is a vacuum duct  46  through which a suction force is applied into chamber  8 . A plurality of baffles  54 ,  56  is situated within chamber  8  between upper entrance  50  and vacuum duct entrance  58 . 
         [0026]    Vacuum duct  46  is connected to the inlet of a gas-liquid separator  26 , which for instance can comprise a condensing cyclone separator having a drain  24  at the lower conical section and an upper gas outlet tube  28  that is connected to a fan blower  30 . Air from fan blower  30  is diverted into an upper air header  34  and a lower air header  32 . Valves  62  and  64  controls the flow rates into upper and lower air headers  34  and  32 , respectively. Relief valve  66  diverts excess airflow through channel  36  and into the atmosphere. Preferably about 20% by volume of the air from fan blow  30  is diverted into upper air header  34  and projected out of upper air discharge outlet  44  while preferably about 20% by volume of the air from fan blow  30  is diverted into lower air header  32  and projected out of lower air discharge outlet  42 . The remaining 60% of the air flows into the atmosphere through channel  36 . Under these operating conditions, it is expected that (i) approximately 95% by volume of the misty air that enters the steam and mist removal device  2  will flow into chamber  8  and into contact with baffles  54 ,  56  (with a portion thereof eventually circulating into channel  18 ) and (ii) the remaining approximately 5% of the misty air will enter into channel  20 . 
         [0027]    To facilitate condensation, selected surfaces of housing  4  and baffles  54 ,  56  as well as condensing cyclone  26  are preferably cooled by heat pump  38  which deliveries cold water to cooling coils (not shown) that are in thermal contact with surfaces on these structures. A heat pump  38  can be employed whereby warm water is recycled from each of these units. Heat pumps are described for example in U.S. Pat. No. 7,665,225 to Goldberg et al., which is incorporated herein by reference. Finally, one or more hot air apparatuses  40 , such as a hot air blower that is supplied by heat pump  38 , can be employed to dry escaping steam or mist that is not drawn into the vacuum of steam and mist removal device  2 . 
         [0028]    As depicted in  FIG. 1 , device  2  is strategically positioned adjacent moving web of paper  1  and to a source of the steam and mist so that excess steam and mist, which consists of free water droplets, which are collectively labeled as “misty air,” can be efficiently removed. A continuously moving cloud of misty air is generated adjacent paper  1  and flows naturally toward the entry point of device  2  that faces the incoming cloud. The vacuum-generating device  2  is energy efficient as it is configured and positioned to along the natural path of the misty air. In operation, a vacuum in chamber  8  is established when fan  30  is activated. The misty air flows toward the direction of lowest pressure that, in this embodiment, is into upper entrance  50  and lower entrance  52 . Upper entrance  50  defines a narrow gap where the velocity of the misty increases. As the fast moving misty air suddenly turns toward rear channel  18 , the centrifugal force drives the heavier free water droplets against the inner surface of flange  10 . The free water droplets are collected by and drained down through rear channel  18 . Furthermore, as the misty air then is pulled through chamber  8 , some of the water vapor comes into contact with cooled baffles  54 ,  56  and condense into water. Condensate along with water droplets in the misty air are collected by gravity at the bottom of chamber  8  where airflow induced by the vacuum force is at the lowest level in order to minimize the impact of air to flow of condensate. Chamber  8  is preferably configured with a relatively narrow top portion and an expansive bottom portion that causes the velocity of the air to be lower at the bottom. The result is that gravity drains the water away faster with minimum obstruction and less misty air is blown toward vacuum entrance  58 . Steam trap  22  is preferably configured to operate by gravitational pull so as to avoid air leakage through chamber  8 , rear channel  18  and front channel  20 . 
         [0029]    Misty air, which is not condensed within chamber  8 , along with entrained water droplets are drawn through vacuum duct  46  whose entrance  58  within chamber  8  is preferably located above steam trap  22 . Within condensing cyclone  26 , a high-speed airflow is established within the upper cylindrical and lower conical interior by centrifugal force. Air flows in a spiral pattern, beginning at the top (wide end) of condensing cyclone  26  and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) water droplets in the rotating stream collide with the conical wall and fall to bottom of cyclone  26  and are removed via drain  24 . 
         [0030]    The substantially water-free air from condensing cyclone  26  is directed into lower air header or duct  32  and upper air header or duct  34  where it is projected through upper air discharge outlet  44  and lower air discharge outlet  42 , respectively. Most of the misty air along with residual free water droplets is recycled through valves  60 ,  62 , which controls the relative volumes of air flowing into each duct. 
         [0031]    Upper air discharge outlet  44  is configured so that high velocity airflow is projected preferably perpendicular to the path of the approaching high humidity misty air. The force of the projected jet of air redirects the misty air towards front panel  6  whereupon high velocity airflow projected from lower air discharge outlet  42  pushes most of the air along with the air from outlets  42  and  44  into upper entrance  50 . Misty air that is not captured into steam and mist removal device  2  is heated by hot air from hot air apparatus  40 . The temperature of the hot air is sufficient to change the relative humidity of the misty air to cause it to dry out and dissipate harmlessly into the atmosphere. 
         [0032]      FIGS. 2 and 3  illustrate the front and rear views, respectively, of a steam and mist removal device  102  that is particularly suited for capturing misty air that emanates from a source that generates a layer of misty air expanding over an extended length. As shown, this embodiment includes an upper discharge outlet  70  and a lower discharge outlet  72 , with each outlet comprising an elongated structure through which a jet of high velocity air is projected onto approaching misty air. A front panel  78  shields the internal chamber that houses baffles or condensing ribs  80 ,  82 . Given the length of the internal chamber, two vacuum ducts  74 ,  76  are employed in order to maintain a more uniform vacuum within the chamber. The longer vacuum duct  74  extends farther into the internal chamber. The lower surface  88  of the internal chamber has a pyramid-like contour with an apex at the center and smooth sloping surfaces on each side of the apex as viewed along the length of the steam and mist removal device  102 . This configuration facilitates drainage of water into steam traps  90 ,  92 . The external periphery of the steam and mist removal device  102  is equipped with a pipe or header  104  with holes (not shown) through which hot air is projected into contact with the escaping steam and mist. 
         [0033]    In operation, a vacuum is generated within the internal chamber as steam and mist are drawn through vacuum ducts  74 ,  76  and into a condensing cyclone  26  ( FIG. 1 ). Recycled cold air from a condensing cyclone  26  is projected through upper and lower air discharge outlets  70 ,  72  into the misty air. A source of cold water  84  delivers cold water to baffles  80 ,  82  and condensing cyclone  26  ( FIG. 1 ) to facilitate condensation. A source of hot air  86  is connected to header  104  to dry escaping mist and steam along the periphery of steam and mist removal device  102 . 
         [0034]      FIG. 4  illustrates use of the steam and mist removal device  102  ( FIGS. 2 and 3 ) at strategic stages in a papermaking machine  108 . In particular, an inventive device can be positioned adjacent any source of steam and mist at a point of application along the machine direction (MD) of the papermaking machine. As shown in  FIG. 4 , a web or sheet of paper  110  from a forming section and press section (not shown) of papermaking machine  108  is conveyed through a series of drying cylinders  112  where additional water is removed and the web or sheet structure consolidated. A gauge/controller assembly  114  measures one or more properties such as the moisture level of web of paper  110  along its cross direction. In this example, papermaking machine  108  is equipped with three rewet systems for web surface re-moisturizing in order to improve surface properties and simultaneously apply cross directional moisture control to sheet of paper  116 . 
         [0035]    The first rewet (or remoisturizing) system includes a rewet spray device  118  with an associated steam and mist removal device  124  that are both positioned on the topside of sheet  116 . The second rewet system includes a rewet spray device  120  with an associated steam and mist removal device  126  that both are positioned on the bottom side of sheet  116 . Finally, farther downstream is a third rewet system includes a rewet spray device  122  with an associated steam and mist removal device  128  that are both positioned on the topside of sheet  116 . In each rewet system, water is sprayed onto sheet  116  and the associated steam and mist removal device is positioned so that the suspension of misty air that is generated is on the front side of the device. Rewet systems and associated actuators and controls are described for example in U.S. Pat. Nos. 6,334,579 to Zarbi and 7,513,975 to Burma, which are incorporated herein by reference. Instead of rewet systems, steam shower distribution systems can be employed with the inventive steam and mist removal device. Steam application devices are described for example in U.S. Pat. Nos. 4,543,737 to Boissevain, 5,798,026 to Wacinski et al., and 7,459,061 to Passiniemi, which are incorporated herein by reference. 
         [0036]    Each remoisturizing spray device provides a fine water spray to selected areas on sheet  116  along the cross direction for CD moisture profile control. The spray devices can be controlled by signals from gauge/controller assembly  114  and/or scanner  144 . They provide on-line measurements of sheet properties, such as thickness, moisture, gloss or smoothness that are made by scanning sensors that travel back and forth across the width of the sheet of paper in the cross-machine direction. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. As water is sprayed onto the surface of sheet  116 , clouds of misty air  154 ,  156  and  158 , each comprising an aggregation of minute globules of water suspended in the atmosphere, are continuously generated. Steam and mist removal devices  124 ,  126  and  128  are positioned to effectively remove the misty air in order to prevent water droplets from being indiscriminately deposited onto sheet  116  and water damage to the machinery. 
         [0037]    Following the re-moisturizing process, the paper contacts a series of oil-heated rolls  132 ,  134 ,  136  and a finishing metal belt in the finishing section  130  where improved surface characteristics are imparted on the paper sheet. Sheet  138  is calendered through a pair of rolls  140  to allow for thickness control and to further smooth the surface. A heating device  142  selectively heats a calender roll surface to control the sheet caliper in the cross direction. Scanner  144  measures moisture content and/or other properties of the dried, calendered sheet before it is accumulated by winding onto a reel (not shown). 
         [0038]    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.