Abstract:
Apparatus and method for the non-contact drying of a web of material. The apparatus includes air flotation nozzles for floating the web, and direct air impingement nozzles for enhanced drying of the web. The nozzle arrangement is particularly well-suited to float and dry light weight webs under moderate to high tension. Increased cushion pressure is created to support the web preferably with the same horsepower as conventional arrangements. The increased cushion pressure pad of the nozzle arrangement allows for good flotation with reduced velocities below about 11,500 FPM. Machine direction wrinkles are removed and the result is positive flotation with no marking on the web or ink build up on the air bars. The nozzle arrangement includes pairs of flotation nozzles directly opposing pairs of direct impingement nozzles. A perforated member can be positioned between flotation nozzles within a pair of flotation nozzles to control return air.

Description:
This application claims priority of U.S. Provisional Application Ser. No. 61/190,283 filed on Aug. 27, 2008, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     In drying a moving web of material, such as paper, film or other sheet material, it is often desirable to contactlessly support the web during the drying operation in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example. 
     When floating light weight webs under medium to high tensions, machine direction corrugations often form in the web which can not be removed with prior art dryer nozzle arrangements. In addition, these hard-to-float light weight webs tend to have ink build up and marking problems when conventional nozzle arrangements are employed. 
     It therefore would be desirable to provide a flotation dryer having a nozzle arrangement that provides excellent web flotation, excellent drying and heat transfer performance, minimal or no web flutter, and minimal or no corrugation formation even in light weight webs. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art have been solved by the embodiments disclosed herein, which provide an apparatus and process for the non-contact drying of a web of material. The apparatus includes air flotation nozzles for floating the web, and direct air impingement nozzles for enhanced drying of the web. Specifically, a plurality of air flotation nozzles or air bars are mounted in one or more sections of a dryer enclosure in air-receiving communication with headers, preferably both above and below the web for the contactless convection drying of the web. In conjunction with these air flotation nozzles, one or more sections of the dryer also includes direct impingement nozzles such as hole-array bars or slot bars. The drying surface of the web is thus heated by both air issuing from the air flotation nozzles and from the direct impingement nozzles. As a result, the dryer has a high rate of drying in a small, enclosed space while maintaining a comfortable working environment. The nozzle arrangement includes pairs of flotation nozzles directly opposing pairs of direct impingement nozzles. A perforated member can be positioned between flotation nozzles within a pair of flotation nozzles to control return air. 
     The paired nozzle arrangement is particularly well-suited to float and dry light weight webs under moderate to high tension. Increased cushion pressure is created to support the web preferably with the same horsepower as conventional arrangements. The increased cushion pressure pad of the nozzle arrangement allows for good flotation with reduced velocities below about 11,500 FPM. Machine direction wrinkles are removed and the result is positive flotation with no marking on the web or ink build up on the air bars. 
     In its method aspects, embodiments include providing a web dryer enclosure having a web inlet slot and a web outlet slot, floatingly guiding a running web in the dryer enclosure with first and second opposed arrays of nozzles for floatingly supporting and drying the web, each array comprising at least a pair of air flotation nozzles and at least a pair of direct impingement nozzles, the pair of direct impingement nozzles opposing the pair of flotation nozzles, and wherein between each flotation nozzle with the pair of flotation nozzles, providing a member having a plurality of apertures for directing return air from said flotation nozzles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic view of a flotation nozzle/direct impingement nozzle arrangement in accordance with certain embodiments; 
         FIG. 2  is a cross-sectional view of a direct impingement nozzle in accordance with certain embodiments; 
         FIG. 3  is a side view of the bar of  FIG. 2 ; 
         FIG. 4  is a top view of a direct impingement nozzle in accordance with certain embodiments; 
         FIG. 5  is a top view of a return air member in accordance with certain embodiments; 
         FIG. 6  is a schematic view of nozzle arrangement in a dryer entry section in accordance with certain embodiments; 
         FIG. 7  is a perspective view of a paired nozzle arrangement with a return air member; 
         FIG. 8  is a perspective view of a dryer-gear side arrangement of a paired nozzle and return air member; 
         FIG. 9  is a perspective view of an operator-side arrangement of a paired nozzle and return air member; 
         FIG. 10  is a perspective view of the apertured portion of a return air member in accordance with certain embodiments; and 
         FIG. 11  is a perspective view of a return air member in accordance with certain embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the present invention is not limited to any particular flotation nozzle design, it is preferred that flotation nozzles which exhibit the Coanda effect such as the HI-FLOAT® air bar commercially available from Megtec, Inc. can be used, in view of their high heat transfer and excellent flotation characteristics. Standard 1× HI-FLOAT® air bars are characterized by a spacing between slots of 2.5 inches; a slot width of 0.070 to 0.075 inches, usually 0.0725 inches; an installed pitch of 10 inches; and a web-to-air bar clearance of ⅛ inch. Air bar size can be larger or smaller. For example, air bars ½, 1.5, 2 and 4 times the standard size can be used. Air bars 2 times the standard size are characterized by a slot distance of 5 inches and slot widths of 0.140 to 0.145 inches (available commercially as “2× air bars” from Megtec, Inc.). In general, the greater distance between the slots results in a larger air pressure pad between the air bar and the web, which allows for increasing the air bar spacing. Another suitable flotation nozzle that can be used in the present invention is the Tri-Flotation air bar disclosed in U.S. Pat. No. 4,901,449, the disclosure of which is hereby incorporated by reference. 
     Means for creating direct air impingement on the web, such as a direct impingement nozzle having a plurality of apertures, such as a hole-array bar or slot bar, provides a higher heat transfer coefficient for a given air volume and nozzle velocity than a flotation nozzle. As between the hole-array bar and the slot bar, the former provides a higher heat transfer coefficient for a given air volume at equal nozzle velocities. Although maximum heat transfer is obviously a goal of any dryer system, other considerations such as air volume, nozzle velocity, air horsepower, proper web flotation, dryer size, web line speed, etc., influence the extent to which optimum heat transfer can be achieved, and thus the appropriate design of the direct impingement nozzle. 
     Turning now to  FIG. 1 , there is shown schematically a preferred flotation nozzle/direct impingement nozzle arrangement, such as for use in the first section (following the entry section) of a flotation dryer, with flotation nozzles or air bars denoted “AB” and direct impingement nozzles or hole bars denoted “HB”. Horizontal web W is shown floatingly supported between upper and lower flotation nozzle/direct impingement nozzle arrays. Two flotation nozzles AB are preferably positioned on 7″ centers with two direct impingement hole bars opposing them. Between the flotation nozzles is a member  42 , preferably made of metal, with return air holes in it. This member creates an additional pressure pad area and when in combination with the two Coanda air bar pressure pads doubles the support force on the web and gives good flotation particularly to hard-to-float webs. In certain embodiments, in the nozzle array above the web, the pair of flotation nozzles AB, AB is followed by, in the web direction, a pair of direct impingement nozzles HB, HB, which in turn is followed by another pair of flotation nozzles, and so on. A similar arrangement is provided in the nozzle array below the web, provided that each pair of flotation nozzles above the web is opposed by a pair of direct impingement nozzles below the web. Preferably the opposing nozzles are directly opposed, as shown, rather than positioned in a staggered relationship. 
     The member  42  between each flotation nozzle in a given pair of flotation nozzles opposed by a pair of direct impingement nozzles functions to restrict and control the amount of return air creating an additional pressure pad area which in turn enhances the pressure pad of the two flotation nozzles AB, AB. With the increased pressure pad area, good flotation of previously hard-to-float webs is achievable. The increased pressure pad area eliminates the ink build up and web marking found in conventional arrangements when floating light weight, wet webs under higher tensions. The controlled return air allows for improved flotation without adding air horsepower requirements from larger supply fans. 
     The member  42  is mounted between the air bars to control the amount of air that leaves the two flotation nozzles AB, AB and becomes trapped between the web W, the member  42  and the face of each air bar AB. As can be seen in  FIGS. 5 ,  10  and  11 , the member  42  between the two flotation airs bars AB is preferably an elongated, flat plate, and has a calculated hole pattern in the center third of the plate  42  to meter the correct amount of air leaving the pressure pad area above the plate. The pressure pad area over the member  42  and the two adjacent Hifloat pressure pads in this arrangement create a cushion pressure to float a web that is over twice the strength of the cushion pressure pad of the flotation nozzles in certain conventional nozzle arrangements. The opposing grouped direct impingement bars HB, HB on 7″ centers allow for increased heat transfer and form return air openings between the direct impingement bars to remove the solvent air via the dryer exhaust. The increased cushion pressure pad area developed from this arrangement eliminates the web marking and ink build up on air bars when floating high tensioned hard-to-float webs. The increased cushion pressure pad area of this arrangement allows for good flotation with reduced velocities below about 11,500 FPM required for prior art arrangements. Those skilled in the art will appreciate that the term “plate” as used herein is not to be construed as limiting the return air member  42  to any particular thickness or shape; although a flat planar sheet is preferred, other designs are within the scope of the present disclosure. 
       FIG. 5  shows an embodiment of member  42  suitable for use with the flotation bars AB. The member  42  is preferably a flat sheet having a length that corresponds to the length of the flotation bars AB. A portion of the member  42  includes a plurality of spaced apertures  43 , which are preferably located in the center third of the plate in the cross-machine direction, aligned linearly and centrally in the member  42  in the machine direction. In the embodiment shown, there are twelve such apertures  43 , although the number and size of the apertures is not particularly limited, and depends upon the desired return air characteristics to be achieved (note the embodiment of  FIG. 10  includes 14 circular apertures). One suitable configuration has 1″ diameter circular apertures, at 2.5″ centers. Other shaped apertures could be used. An open area of about 10% is preferred.  FIG. 7  shows the member  42  mounted between each flotation nozzle AB in a paired flotation nozzle assembly. The arrows illustrate one possible direction of return air flow through the apertures in the member  42 . 
       FIGS. 8 and 9  illustrate a suitable dryer gear-side and operator-side attachment of the member  42  to the paired air flotation bars  50 . The gear-side hold down tabs of member slide under the air bar  50  bodies to secure that end of member  42 . The operator-side hold down  70  includes a flat plate for affixing it to the air bar bodies, and an apertured right angle flange that positions under the member for inclusion of an adjustment bolt to adjust the open to flow area in the member  42  and balance the return air while on the run. The member is thus a damper in that the 1″ holes can be made to cover or overlap with the adjustment of the sliding bottom plate  62 , also shown in  FIG. 11 . The bottom plate  62  and the plate  42  slide relative to one another, with the bottom plate  62  functioning to regulate the extend to which, if it all, any of the apertures  43  in the top plate are blocked. 
     A suitable dryer entry section arrangement of nozzles is shown in  FIG. 6 . The dryer entry section is the first zone of the dryer, and is followed by the dryer section containing the air bar/hole bar arrangement discussed above. In the embodiment shown, respective air knives AK above and below the web W are the first upper and lower element of the dryer. Those skilled in the art are familiar with air knives; they are used to provide an air seal at the dryer to prevent a “belching” effect out of the dryer and a minimization of cold air into the dryer through the web slot. The design of the air knives that are used is not particularly limited. The dryer entry section also has an array of nozzles above the web W that includes a first dampered flotation nozzle  50  that is spaced from the upper air knife by a “blank off”, which may include a return air plate or baffle with appropriate apertures. These help to balance the pressure in these open areas to allow for stable web flotation through this section prior to the first air bars. The first dampered flotation nozzle  50  is unopposed by a nozzle below the web (as indicated by “blank off”), and is followed by a first direct impingement nozzle  60 A spaced from the dampered nozzle  50  by at least the distance necessary to accommodate another flotation nozzle (e.g., the center of the first direct impingement nozzle  60 A is 14 inches from the center of dampered nozzle  50 ) and directly opposed by a flotation nozzle  50 A, which in turn is followed by a second direct impingement nozzle  60 B in the web direction that is also directly opposed by a flotation nozzle SOB below the web W. Preferably the air flotation nozzles  50 A,  50 B and the direct impingement nozzles  60 A,  60 B are positioned on 7-inch centers. Preferably the flotation nozzles  50 A and  50 B are paired nozzles with return air member  42  as shown. The dampered flotation nozzle  50  has independent air pressure control via a damper in the inlet to the nozzle. It always runs at less pressure than the remaining air bars to center the web in the web slot opening on the dryer. The dampered nozzle does not have an opposing hole bar so the pressure must be reduced to keep the web from floating too high off the dampered nozzle. 
     Turning now to  FIGS. 2 and 3 , a preferred embodiment of a direct impingement nozzle hole bar HB is shown. Hole bar HB is installed in air-receiving communication with a header  11  having a port  13 . Header  11  feeds air into hole bar compartment  12 . The air emits from the hole bar HB via a plurality of apertures, in this case spaced circular holes in the top surface  14  of the hole bar HB. Preferably the top surface  14  of hole bar is crown shaped and approaches a central apex  15  at about a 5 degree angle. This design encourages the return air to flow over the edges of the hole bar after impingement on the web W. A flatter top surface  14  tends to result in return air traveling down the face of the hole bar in the cross-web direction, which is undesirable. The angle of the crown can vary from about 0 degrees to about 10 degrees. In general, the closer the hole bar is to the web W, the larger the angle of the crown. Hole bars at a large distance from the web could be flat. 
     The particular pattern and configuration of apertures in the top surface  14  of the hole bar HB is not critical, as long as relatively uniform coverage of the web is provided, and the impingement of air is not directly over the center of the pressure pad generated by an opposing air bar. The percent open area of a hole bar or an air bar is defined by the following equation: 
               [       ∑     i   =   1     i     ⁢           ⁢       A   sperfi     ⁢     n   i         ]     ⁢     lA   top     ×   100         
Where:
         j=number of perforation types   A sperf =cross-sectional area of a perforation type   n=number of copies of a perforation type   A top =exterior surface area of hole or air bar top   where perforations are located
 
The percent open area of the hole bar HB is from 1.8 to about 7.5% of the total area of the hole bar, preferably about 2.4% of the total area of the hole bar. The total dryer effective open area is defined by the following equation:
       

               [       ∑     i   =   1     i     ⁢           ⁢       (     A   openi     )     ⁢     (     n   i     )     ⁢     (     C   di     )         ]     ⁢     lA     surface   ⁢           ⁢   web   ⁢           ⁢   based       ×   100         
Where:
         A open =% open area/100×A top  of bar type   n=number of duplicates of a bar type   j=number of bar types in dryer   C d =discharge coefficient of bar type   A surface  web heated=total surface area of web being heated
 
The dryer effective open area can be based on measured or calculated discharge coefficients, and is preferably in the range of 1.4 to 4%, most preferably 1.5% of the total web surface area being heated in the dryer enclosure. In the embodiment shown in  FIG. 4 , the hole bar open area is accomplished with 8 horizontal rows  25   a - 25   h  of circular holes  18 , each horizontal row of holes  18  consisting of 31 holes spaced at 1.83 inch intervals. It should be understood by those skilled in the art that the number of rows of holes and the number of holes per row can vary, depending in part upon the size of the hole bar for the application. In the embodiment shown, the top row  25   a  commences 0.488 inches from the side edge  20  of the hole bar, and 0.421 inches from the top and bottom edges  21   a  and  21   b . Each subsequent horizontal row  25   b - 25   h  is spaced an additional 0.229 inches from the side edge  20 . Each horizontal row  25   a - 25   h  is vertically spaced 0.454 inches from its neighboring row, except the rows nearest the center of the bar. In order to reduce web disturbance at close spacing to the web, it is preferred that the center of the hole bar be devoid of holes. Preferably the dimensions of this central portion devoid of holes is such that two symmetrical rows of holes could be accommodated therein if such holes were present.
       

     Where the apertures of the hole bar are of a different configuration, such as diamonds, square or rectangular slots, preferably they have an equivalent diameter of from about 0.06 to 0.5 inches. Also, the slots can be continuous along the length of the bar. 
     Although an end feed hole bar is shown in  FIG. 4 , a center feed design can also be used, depending upon the application. 
     Depending upon the size of the holes  18 , “whistling” and web fluting or wrinkling problems, particularly in the machine-direction, can arise. These problems should be minimized without compromising good flotation and heat transfer characteristics. Hole diameters of 0.164, 0.172 and 0.1875 inches result in minimal web fluting and whistling in graphic arts applications, with hole diameters of 0.1875 inches being especially preferred. The optional use of a hole bar diffuser plate (not shown) coupled to flanges  9  ( FIG. 2 ) between the header  11  and the compartment  12  may also be used in reducing whistle. A flow straightener  30  may also be positioned in chamber  12  of hole bar HB to improve the air flow characteristics. 
     Also of importance in optimizing flotation and heat transfer characteristics is the height of the hole bars HB from the web W. If the hole bars are too close to the web centerline, web instability and web touch-down on the air bar top can occur. However, moving the hole bars too far away from the web centerline can cause an undesirable loss in heat transfer. Accordingly, preferably the hole bar should be from about 2 to about 10 equivalent aperture diameters (or slot widths) away from the web. Actual hole bar clearances ranging from about ⅛ to 1¾ inches from the web are preferred. 
     Suitable nozzle velocity is in the range of 1000 to 12000 feet per minute, with a nozzle velocity of from about 8000 to 10000 fpm being preferred. 
     The flotation nozzles and direct impingement nozzles need not be fed by the same header systems; separate headers can be used, especially if different operating velocities and/or air temperatures in the direct impingement nozzles and flotation nozzles are desired. Independent control of velocities may be important where heat transfer and flotation requirements are at odds, such as where low web tensions require reduced flotation velocity, yet the heat transfer required remains the same. 
     Similarly, the air bars and hole bars can be separately dampered such that they operate at different nozzle velocities.