Tri-flotation air bar

Abstract: Air flotation bar for use in floating and drying continuous webs of material in a dryer using dimensionally enlarged air bars. Air flows through three Coanda air slots (34, 36, 38; 204, 206, 208) to provide an air flow distributed over a large area of a passing web to maintain heat transfer capabilities and flotation capabilities. A central Coanda slot (36, 208) air impinges against a web for enhanced heat transfer while outer Coanda slots (34, 38;204, 206) create a flotation pressure pad.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention - The present invention relates to an air flotation bar for use in positioning, drying or curing of a continuous planar flexible material such as a printed web, news print, film material or sheet plastic. The present invention more particularly pertains to an air flotation bar which includes two individual air bars in the upper region of the air flotation bar, each having an air discharge slot about its longitudinal outer edge and a third air discharge slot between the two air bars. The outer slots provide for web flotation and heat transfer, and the third slot between the outer air bars provides for additional heat transfer air flow and flotation by air impinging upon the web. 2. Description of the Prior Art - Prior art air flotation bars have been up-scaled in physical size to provide an air bar twice the original size for higher flotation clearance and better web control. The detrimental effect of up-sizing is the degradation of the heat transfer coefficient. The present invention overcomes the disadvantages of the prior art by providing an air flotation bar where the same flotation capability is maintained, as well as enhanced heat transfer. Three small air slots instead of two larger air slots provide for an equal air flow orifice area in addition to a substantially equal distributed air flow. GB-A-1302091 discloses an air bar structure having two Coanda slots along the edges of the air bar, directing airflows in towards the centre of the air bar, and the embodiment of Figures 8 to 15 disclosed therein includes third air discharge means in the form of slots or holes which extend along the top plate of the air bar and are described as being intended to peel the Coanda airstream from the Coanda surface along the top of the air bar. The air leaving the additional air discharge means is illustrated in Figures 8 to 10 as moving perpendicular to the Coanda surface at the top of the air bar. Accordingly, the present invention provides an air flotation bar comprising: an air bar header; first and second substantially parallel, longitudinal air discharge Coanda slots positioned on the margins of a top surface of said air bar header; a third longitudinal air discharge slot between and parallel to said first and second longitudinal air discharge Coanda slots and on said top surface; chamber means in said air bar header for passing air to each of said air discharge slots; and air supply means operative to supply air to said chamber means; characterised in that in use of said air flotation bar the air flow from said third slot exhibits the Coanda effect; and in that there is at least one central longitudinal suction hole between said third longitudinal discharge slot and each of said first and second longitudinal discharge slots. In one embodiment of the present invention, there is provided an air flotation bar with longitudinal parallel mounted air bars mounted about the upper regions of an air bar header. The first and second air discharge slots, which use the Coanda effect, are formed along the outer longitudinal edges of each air bar and the third air discharge slot is formed between the inner longitudinal edges of the air bars. A support channel member is placed longitudinally across the greater portion in the upper region of the air flotation bar to support the inner ends of the air bars. Individual chambers with perforated elements direct pressurized air from the intermediate regions of the air flotation bar to each of the air discharge slots. Another larger chamber with perforations in the intermediate region and beneath each of the individual chambers uniformly channels pressurized air to each of the smaller individual chambers. Another chamber in the lower region in turn delivers air to the chamber in the intermediate region. A significant feature of the present invention is the ability to increase the size of the air flotation bar and maintain the same flotation capability without loss of the heat transfer coefficient. A further significant feature of the present invention is the use of three smaller sized air slots instead of two normal sized slots, providing for a more widely distributed uniform drying air flow with enhanced heat transfer. BRIEF DESCRIPTION OF THE DRAWINGS Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein: FIG. 1 illustrates a perspective view of an air flotation bar, according to the present invention; FIG. 2 illustrates a partial cutaway view of the air bar header with the header end plate removed; FIG. 3 illustrates a cross-sectional view of the air flotation bar taken along line 3-3 of FIG. 2; FIG. 4 illustrates a partial front view and a partial cutaway view taken along line 4-4 of FIG. 3; FIG. 5 illustrates a view of FIG. 3 including the air flow in and about the air flotation bar; and FIG. 6 illustrates an alternative embodiment including air flow in and about an air flotation bar with negative pressure in the interior air bar channel members. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a perspective view of an air flotation bar 10 for use in a web dryer. Externally visible members in the figure illustrate the air flotation bar 10 including a channel like air bar header 12 with opposing canted sides 14 and 15, and a bottom 16. Opposing and parallel vertically aligned air bar header end plates 18 and 20 affix between the sides 14 and 15 with each end plate having an air bar alignment tab 22 and 24, as also illustrated in FIG. 4. Holes, slots or other various openings can be fabricated in the air bar alignment tabs 22 and 24 for securing, mounting or positioning of the air flotation bar 10 in a dryer. V channels 26 and 28 are formed in and aligned horizontally in sides 15 and 14, respectively, to accommodate air bar mounting flanges, as later described in detail. Air bars 30 and 32 align longitudinally in a precise manner between the upper regions of sides 14 and 15 longitudinally to form aligned Coanda slots 34, 36, and 38 as illustrated. Two outer air discharge slots, in this case Coanda slots, 34 and 38 position as illustrated with an inner third slot 36, also using the Coanda effect, between air bars 30 and 32. An oval shaped air inlet 40 positions on the bottom 16 to accept dryer system air flow for the air flotation bar 10. FIG. 2 illustrates a partial cutaway view of the air flotation bar 10 with the air bar header end plate removed for purposes of the illustration. All numerals correspond to those elements previously described. Reference to this FIG. and FIG. 3 also facilitates an understanding of the following disclosed subject matter. Air bars 30 and 32 are mirror images of each other, and position between the upper regions of sides 14 and 15. Air bar 32 includes an upper air bar channel member 42 and a lower air bar channel member 44 tightly secured and affixed within the upper air bar channel member 42 forming an air bar chamber. The upper air bar channel member 42, with several integral and planar members, includes a horizontal planar surface 46, which intersects an inner vertical surface 48 to form a uniform defined radius Coanda curve 50, and also intersects an outer vertical surface 52 to form a uniform defined radius Coanda curve 54. The outer vertical surface 52 is bent at a right angle to form a horizontally aligned flange member 56 which in turn is accommodated by the V channel 26. The flange member 56 includes a plurality of holes 58a-58n where hole 58a and other like holes in the series are illustrated in the figure. A lip 60a of a sidewall 60 extends a finite distance inwardly at a right angle from the upper region of side 15 and on a plane lower than that of the horizontal planar surface 46 of the air bar 32 to form a Coanda slot 38 of a finite distance between the lip 60a and Coanda curve 54. An outer chamber 62 is also formed by the flange member 56, the upper portion of side 15, the outer vertical surface 52 and lip 60a. Air bar 30 is constructed in a like and similar manner to that of air bar 32, and includes a horizontal planar surface 66, an inner vertical surface 68, a Coanda curve 70, an outer vertical surface 72, a Coanda curve 74, a flange member 76, holes 78a-78n where only hole 78a and the other like holes in the series are illustrated, a lip 80a of sidewall 80 and an outer chamber 82. A support channel member 90 positions between the outer vertical surface 52 and outer vertical surface 72, and includes a plurality of orifices 92a-92n where only orifice 92a is illustrated. Vertically oriented struts 94 and 96 are positioned perpendicular on the support channel member 90 to support the inner ends of air bars 30 and 32, thus stabilizing the geometrical configuration of the inner Coanda slot 36 and forming outer support chambers 62 and 82. A central support chamber 91 is formed by struts 94 and 96, the support channel member 90, and the lower portions of the air bar channel members 44 and 64. A diffuser plate 100, including a plurality of holes 102a-102n secured between sides 14 and 15, and below the support channel member 90, provides for even flow of drying air from the oval shaped air inlet 40 of FIGS. 1 and 3. The diffuser plate 100, sides 14 and 15, air bar header end plates 18 and 20 of FIG. 1, and the bottom 16 define a first lower air flow chamber 104. The portions of the sides 14 and 15 just below the V channels 26 and 28, air bar header end plates 18 and 20, the support channel member 90 and the flange members 56 and 76 define a second upper diffused air flow chamber 106. An angled oval member 108 secures to the bottom 16 and adjacent to and about the oval shaped air inlet 40 to form a gasket chamber 111 about the oval shaped air inlet 40 as illustrated in FIG. 3. FIG. 3 illustrates a cross-sectional view of the present invention taken along line 3-3 of FIG. 2 where all numerals correspond to those elements previously described. FIG. 4 illustrates a partial front view and partial cutaway view taken along line 4-4 of FIG. 3 of the air flotation bar where all numerals correspond to those elements previously described. MODE OF OPERATION FIG. 5 illustrates a view of FIG. 3 with air flow in and about the air flotation bar 10 where all numerals correspond to those elements previously described. Dryer system air flows first through the oval shaped air inlet 40 and out of the Coanda slots 34, 36 and 38 as previously described. Air passing through the Coanda slots 34, 36, and 38, forms a broad air flow area to support a web. Air passing through the Coanda slot 36 projects and moves upwardly to, in effect, widen the distance between the flow of air flowing along towards the web and to provide a wider upper flow area beneath the web. The drying air flow has a wider foot print to provide a larger more effective drying area with heat transfer on the web. Dryer system air flow passes first through the oval shaped air inlet 40 of FIG. 3, through the first lower air flow chamber 104, through the diffuser plate 100 where the air flow is distributed evenly and diffused through the second upper diffused air flow chamber 106, and simultaneously through a plurality of holes 58a-58n, 78a-78n, and 92a-92n into chambers 62, 82, and 91, respectively. The diffuser plate straightens the air flow. Any other like structure which creates a pressure drop would act as a flow straightener. The air flow then continues from chambers 62, 82, and 91, and through Coanda slots 38, 34 and 36. The width of each slot is about 0.035-0.2″ by way of example and for purposes of illustration and not to be construed as limiting of the present invention, and in a range of preferably about 1.3-1.9% open area of the plane. The open area of the slots is in a range of 1-5% of the open area of the plane. FIG. 6 illustrates an alternative embodiment of the air flotation bar where all numerals correspond to those elements previously described. Negative pressure is applied to the interior chambers 110 and 112 of air bars 30 and 32 to create an area of low pressure in the areas of longitudinal holes 114 and 116, thus affecting air flow from the outer and inner Coanda slots 34 and 38, and 36, respectively, in the manner as illustrated by the air flow arrows. It should be noted that the air flow arrows in Figures 5 and 6 are purely schematic, and that particularly in Figure 6 the intention is to depict the turbulent nature of the flow pattern which leads to a high heat transfer coefficient. The air flotation bar can be used for drying of printed webs, coated webs, or any other suitable air flotation applications.

Claims: An air flotation bar comprising: an air bar header (12); first and second substantially parallel, longitudinal air discharge Coanda slots (34, 38) positioned on the margins of a top surface of said air bar header; a third longitudinal air discharge slot (36) between and parallel to said first and second longitudinal air discharge Coanda slots and on said top surface; chamber means (104, 106) in said air bar header for passing air to each of said air discharge slots; and air supply means operative to supply air to said chamber means; characterised in that in use of said air flotation bar the air flow from said third slot exhibits the Coanda effect; and in that there is at least one central longitudinal suction hole (114, 116) between said third longitudinal discharge slot and each of said first and second longitudinal discharge slots. An air flotation bar according to claim 1, wherein said chamber means comprise:- a main chamber (104, 106); a central support chamber (91) and two lateral support chambers (62, 82) to each side of said central support chamber, said central support chamber being connected to said main chamber (104, 106); upper air bar channel chambers (44, 64) forming therebetween said third air discharge slot above said central support chamber; and side walls (60, 80) with lips (60a, 80a) positioned about said support chambers and outer sides of said upper air bar channel chambers spaced therefrom forming said first and second air discharge Coanda slots (34, 38). An air flotation bar according to claim 2, including a diffuser plate (100) with holes (102a ... 102n) therein, secured mid-way up said chamber (104, 106). An air flotation bar according to claim 2, including flow straightening means (100; 102a ... 102n) in said chamber. An air flotation bar according to claim 4, wherein said chamber means further comprise:- a lower air flow chamber (104) and an upper air flow chamber (106), and said flow straightening means are positioned therebetween. An air flotation bar according to claim 2, including means (100; 102a ... 102n) for creating a pressure drop in said main chamber. An air flotation bar according to any one of claims 2 to 6, wherein said chamber means further comprise:- a bottom member (16) with an inlet hole (40) therein, air bar header end plates (18, 20) affixed to said bottom member (16), two sides (14, 15) extending upwardly from said bottom member; opposing right angled flanged members (56, 76) secured to said side members, each said flanged member including holes (58a ... 58n) in a base thereof, a top edge (54) of each said flanged member and a top edge (60a, 80a) of the adjacent said side member (15, 14) forming a respective one of said first and second Coanda slots (38, 34) about each side member; a support member (90) with centre orifices (92a ... 92n) extending between said flanged members; opposing vertical struts (94, 96) secured about each side of said orifices; opposing lower and upper air bar channels secured between said flanged member and said struts and forming said third Coanda slot therebetween. An air flotation bar according to any one of claims 2 to 7, wherein the corners of said upper air bar channel members are curved. An air flotation bar according to any one of claims 1 to 8, wherein all of said air discharge slots (34, 36, 38) (204, 206, 208) are substantially equal in width. An air flotation bar according to any one of claims 1 to 9, wherein each of said air discharge slots (34, 36, 38) (204, 206, 208) is 2.54 mm (0.1″) wide. An air flotation bar according to any one of claims 1 to 10, wherein the open area of said air discharge slots is 1 to 5% of the area of said air bar (10) (150). An air flotation bar according to claim 11, wherein said open area of each of said slots is 1.3 to 1.9% of the area of said air bar. The use of an air flotation bar according to any one of claims 1 to 12 for drying a printed web. The use of an air flotation bar according to any one of claims 1 to 12 for drying a coated web.

Inventors: WIMBERGER RICHARD J, WIMBERGER, RICHARD J.
Applicants: GRACE W R & CO, W.R. GRACE & CO.-CONN.
Application year: 1989
Main classification: F26B13
IPC codes: F26B13, B65H23, B41F23