Method for conditioning paper and paperboard webs

Abstract

The specification discloses embodiments of a process and related apparatus for conditioning a fibrous web in order to improve the efficiency of drying and calendering thereof. In the process, a moving fibrous web is conditioned after the drier unit of a papermaking machine by applying a flow of moistened gas through one or more arrays of radial jet reattachment nozzles placed in close proximity to the web surface prior to a calendering unit or prior to a steaming unit placed between the nozzles and the calender unit to cool the web and/or increase its moisture content. Webs treated according to the invention exhibit improved properties including less moisture streaking, enhanced smoothness and avoidance of optical property loss.

What is claimed is:  
     
         1 . A method for treating an elongate moving web which comprises conditioning the web by applying a flow of moistened gas to a surface of the moving web across its width and along at least a portion of its length from a plurality of substantially overlapping flow zones wherein the flow in each zone is sufficient to create a combination of suction and pressure forces on the surface of the web to promote convective heat transfer and thereby decrease its surface temperature.  
     
     
         2 . The method of    claim 1    wherein the moving web is a fibrous paper or paperboard web proceeding from the dryer unit of a papermaking machine and has a moisture content below about 8% by weight and a temperature of at least about 80° C.  
     
     
         3 . The method of    claim 1    wherein the temperature of the moistened gas is in the range of from about 10° to about 65° C.  
     
     
         4 . The method of    claim 1    further comprising steaming the web after the conditioning step to increase its moisture content by at least about 0.3 wt. %.  
     
     
         5 . The method of    claim 4    wherein the steamed web has a moisture content ranging from about 3 to about 8% by weight.  
     
     
         6 . The method of    claim 1    wherein the flow velocity of moistened gas applied to the web is in the range of from about 100 to about 300 feet per second.  
     
     
         7 . The method of    claim 6    wherein the flow of moistened gas is applied to the surface of the web from a plurality of overlapping flow zones by flowing the moistened gas through a plurality of spaced-apart radial jet reattachment nozzles placed in close proximity to the surface of the web.  
     
     
         8 . The method of    claim 7    wherein the nozzles are spaced from about 0.5 to about 2 inches from the surface of the web.  
     
     
         9 . The method of    claim 1    wherein the moistened gas is air containing a relatively fine mist of water droplets which is applied to the surface of the web at a rate of from about 0.05 to about 1.0 pounds per minute per foot width.  
     
     
         10 . The method of    claim 9    wherein the air has an absolute humidity of at least about 0.01.  
     
     
         11 . The method of    claim 1    wherein the surface temperature of web is decreased to at least about 80° C. or lower.  
     
     
         12 . A web treated by the process of    claim 1   .  
     
     
         13 . The method of    claim 1    further comprising calendering the web after the condition step.  
     
     
         14 . The method of    claim 1    wherein the web is moving at a velocity of at least about 120 meters per minute.  
     
     
         15 . A method for treating a fibrous web proceeding from a dryer unit of a papermaking machine, the web having a temperature of at least about 80° C., which comprises conditioning the web by applying a flow of moistened gas to a surface of the moving web across its width and along at least a portion of its length from a plurality of substantially overlapping flow zones wherein the flow is sufficient to create a combination of suction and pressure forces on the surface of the web to enhance convective heat transfer and thereby decrease its surface temperature, and thereafter calendering the web.  
     
     
         16 . The method of    claim 15    wherein the web proceeding from the dryer unit has a moisture content below about 8% by weight.  
     
     
         17 . The method of    claim 15    wherein the temperature of the moistened gas is in the range of from about 10° to about 65° C.  
     
     
         18 . The method of    claim 15    further comprising steaming the web to increase its moisture content by at least about 0.3 wt. % after conditioning the web and prior to calendering.  
     
     
         19 . The method of    claim 18    wherein the steamed web has a moisture content ranging from about 3 to about 8% by weight.  
     
     
         20 . The method of    claim 15    wherein the flow velocity of moistened gas applied to the web is in the range of from about 100 to about 300 feet per second.  
     
     
         21 . The method of    claim 15    wherein the flow of moistened gas is applied to the surface of the web from a plurality of flow zones by flowing the moistened gas through a plurality of space-apart radial jet reattachment nozzles placed in close proximity to the surface of the web.  
     
     
         22 . The method of    claim 21    wherein the nozzles are spaced from about 0.5 to about 2 inches from the surface of the web.  
     
     
         23 . The method of    claim 15    wherein the moistened gas is air containing a relatively fine mist of water droplets which is applied to the surface of the web at a rate of from about 0.05 to about 1.0 pounds per minute per foot width.  
     
     
         24 . The method of    claim 23    wherein the air has an absolute humidity of at least about 0.01.  
     
     
         25 . The method of    claim 15    wherein the surface temperature of web is decreased to at least about 80° C. or lower.  
     
     
         26 . A paper or paperboard product treated by the process of    claim 15   .  
     
     
         27 . A method for treating a moving cellulosic web proceeding from a dryer unit of a papermaking machine, the web having a moisture content below about 8 wt. % and a temperature of above about 80° C., which comprises conditioning the web by applying a flow of moistened gas having a temperature in the range of from about 10° to about 65° C. to a surface of the moving web across its width and along at least a portion of its length from a plurality of spaced apart radial jet reattachment nozzles to thereby increase the moisture content of the web by at least about 0.2% and decrease its temperature, and thereafter calendering the web.  
     
     
         28 . The method of    claim 27    further comprising steaming the web to increase its moisture content by at least about 0.3 wt. % after conditioning the web and prior to calendering.  
     
     
         29 . The method of    claim 28    wherein the steamed web has a moisture content ranging from about 3 to about 8% by weight.  
     
     
         30 . The method of    claim 27    wherein the flow velocity of moistened gas applied to the web is in the range of from about 100 to about 300 feet per second.  
     
     
         31 . The method of    claim 27    wherein the nozzles are spaced from about 0.5 to about 2 inches from the surface of the web.  
     
     
         32 . The method of    claim 27    wherein the moistened gas is air containing a relatively fine mist of water droplets which is applied to the surface of the web at a rate of from about 0.05 to about 1.0 pounds per minute per foot width.  
     
     
         33 . The method of    claim 32    wherein the air has an absolute humidity of at least about 0.01.  
     
     
         34 . A paper or paperboard product treated by the process of    claim 27   .  
     
     
         35 . A papermaking process which comprises depositing an aqueous slurry of cellulosic and/or synthetic fibers at a consistency of from about 0.2 to about 1.5% by weight on a moving web former screen thereby forming a layer of slurry on the screen, dewatering the slurry on the moving screen to form a fibrous web, pressing the thus formed fibrous web with one or more wet press nips to provide a pressed web having a solids content in the range of from about 32 to about 45% by weight, drying the pressed web to provide a dried web having a moisture content of from about 0.2 to about 6% by weight, conditioning the dried web by applying a flow of moistened gas to the web from a plurality of radial jet reattachment nozzles placed in close proximity to the web to provide a conditioned web having a moisture content which is substantially uniformly increased across its width by at least about 0.2% relative to the moisture content prior to conditioning, and calendering the conditioned web in a calendering unit.  
     
     
         36 . The process of    claim 35    further comprising rewetting the conditioned web to provide a moistened web and thereafter calendering the moistened web.  
     
     
         37 . The process of    claim 35    wherein the dried web has a temperature above about 120° C.  
     
     
         38 . The process of    claim 35    wherein the conditioning cools the web to a temperature below about 120° C.  
     
     
         39 . The process of    claim 35    wherein from about 20 to about 80 cubic feet per minute of moist gas per nozzle is applied to the web to condition the web.  
     
     
         40 . The process of    claim 39    wherein the moistened gas has an absolute humidity of at least about 0.01.  
     
     
         41 . A paper or paperboard product made by the process of    claim 35   .  
     
     
         42 . An apparatus for conditioning an elongate moving web which comprises a means for producing a moisturized gas and moisturized gas application means for applying a flow of the moisturized gas onto a surface of the moving web such that the web is urged toward the gas application means while providing a cushion of air between the gas application means and the web to thereby limit contact between the application means and the web and cause moisture in the gas to contact the web.  
     
     
         43 . The apparatus of    claim 42    wherein the application means comprises an array of radial jet reattachment nozzles arranged across the width and along at least a portion of the length of the web, each nozzle having an elongate cylindrical sleeve with a gas inlet and a circular open end adjacent the web opposite the inlet, a flow director at lease partially disposed within the cylindrical sleeve comprising a head spaced outwardly from the open end of the sleeve toward the web and a rod axially aligned within the elongate sleeve so as to provide an annular flow space between the sleeve and the rod, the head being connected to one end of the rod, wherein moistured gas flows through said annular space out said open end of said sleeve between said open end and said head and then onto the surface of the web.  
     
     
         44 . The apparatus of    claim 43    wherein the rod is substantially solid.  
     
     
         45 . The apparatus of    claim 43    wherein said rod includes an interior opening along at least a portion of its length in flow communication with a source of liquid and includes means for emitting liquid from said interior opening out into said annular flow space.  
     
     
         46 . The apparatus of    claim 45    wherein said means for emitting comprises slots or perforations in said tube providing flow communication between said interior opening and said annular flow space.  
     
     
         47 . The apparatus of    claim 43    wherein said head comprises a circular disc having a dome-shaped surface adjacent said open end of said sleeve which surface is spaced from said open end of said sleeve from about 2 to about 7 millimeters.  
     
     
         48 . The apparatus of    claim 43    wherein said rod contains an interior opening in flow communication with a source of moisturized gas and a distal open end adjacent said head in flow communication with said interior opening, and the flow director further comprises a deflector supported adjacent said head so as to face said distal open end of said rod in spaced relation with said head, said distal open end comprising an orifice configured to emit moisturized gas therefrom in the direction of said deflector.  
     
     
         49 . The apparatus of    claim 48   , wherein at least a portion of said deflector is provided by a cone or pyramidal-shaped protrusion whose apex is adjacent said orifice so that moistened gas emitted from said orifice is caused to flow radially outwardly relative to the axis of said rod by reason of contact of the gas with said protrusion.  
     
     
         50 . The apparatus of    claim 42    wherein said gas application means comprises an elongate cylindrical sleeve having an elongate opening therethrough providing flow communication with a distal open end thereof, and a flow director including a head disposed outside of said sleeve in spaced apart adjacency with said distal open end of said sleeve and means for supporting said head in said spaced-apart adjacency with said distal open end of said sleeve, said head having a dome, conical, pyramidal or trumpet-like configuration adjacent said distal open end of said sleeve so that a flow of moistened gas directed through said elongate opening of said sleeve and out said distal open end thereof flows against said head and is caused to flow radially outwardly relative to the length axis of said sleeve.  
     
     
         51 . The apparatus of    claim 43    wherein said rod contains an interior opening in flow communication with a source of liquid and a distal open end adjacent said head in flow communication with said interior opening, and the flow director further comprises a deflector supported adjacent said head so as to face said distal open end of said rod in spaced relation with said head, said distal open end comprising an orifice configured to emit a liquid stream therefrom in the direction of said deflector.  
     
     
         52 . The apparatus of    claim 51   , wherein at least a portion of said deflector is provided by a cone or pyramidal-shaped protrusion whose apex is adjacent said orifice so that liquid emitted from said orifice is caused to flow radially outwardly relative to the axis of said rod by reason of contact of the liquid stream with said protrusion.