GB2146303A - Device for supporting web on a bed of air - Google Patents

Abstract

An air cylinder for supporting a travelling web has a plurality of nozzle units disposed in a part- circular array. Each nozzle unit comprises a pair of first, Coanda-type nozzle orifices (35a,35b) arranged back-to-back whereby the air which passes over their respective transversely extending Coanda surfaces (32a,32b) flows in opposite circumferential directions towards respective radially inwardly directed air outlets (40). Each nozzle unit also comprises a pair of second nozzle orifices (36a,36b), each disposed in a respective one of said transversely extending Coanda surfaces (32a,32b) at a position spaced downstream from the adjacent Coanda nozzle orifice. Each second nozzle orifice (36a,36b) is adapted to establish an air flow which is normal, or substantially normal, to the flow of air over the transversely extending surface (32a,32b) from the respective Coanda nozzle orifice, such that the two air flows from the first and second nozzle orifices interact in the region of each second nozzle orifice (36a,36b) to provide additional support for the travelling web (14). <IMAGE>

Description

SPECIFICATION Float treatment apparatus The present invention relates to float treat ment apparatus for treating a floating web of material and is concerned particularly with such apparatus for use in drying continuously formed material webs, such as paper.

Our existing EuropeanPatent (U.K.) No.

3414 (to which reference is hereby directed) discloses a float treatment apparatus which comprises a plurality of separate but adjacent arrays of nozzles of the Coanda type, each nozzle array comprising a plurality of nozzle pairs disposed in an arcuate, generally Ushaped arrangement that has a curved length corresponding to more than a semi-circle, adjacent nozzle arrays being U-shaped in opposite directions and being positioned with respect to each other so that a web passing thereover moves in a serpentine path that takes it around a greater than 180 circumferential portion of at least one of the arrays.

Each nozzle pair of each array is arranged with the respective transversely extending surfaces of the two nozzles in each pair extending in opposite directions whereby the gaseous medium flowing over these two extended surfaces flows in opposite circumferential directions to respective radially inwardly directed gaseous medium outlets.

The aforegoing apparatus makes use of socalled Coanda nozzles, the basic principle of which has been known in the art for some time. A Coanda nozzle is one in which the discharged gaseous medium, normally air, flows over a laterally extended curved lip surface of the nozzle between that extended surface and the adjacent surface of the web being treated, the discharged gaseous medium being caused to cling to such extended surface by the so-called Coanda effect. Provided that dimensions and pressures are suitably chosen, the web of material will float stably on the nozzle at a small distance from the extended surface.

The use of Coanda nozzles in this manner has the result that, rather than being merely loosely supported in the radial direction as in the case of previously known air cylinders, the moving web is positively held to the contour of the cylinder by virtue of the air flow pattern achieved. As a result, very wide variations in tension in the web can be accepted from substantially zero to approaching web breakage whereby the mechanical running problems encountered with the known arrangements are eliminated.

Furthermore this arrangement enables the amount of floor space occupied to be reduced considerably compared with the traditional solid roller arrangements and also compared with the known curved path arrangements using conventional nozzles such as shown in G.B. 11 66729 which only permit gently undulating paths which have little effect on the floor space occupied.

The present invention has as its principal object to improve further the ability of the apparatus of European Patent (U.K.) No.

3414 to provide stable support for the web during its passage through it.

In accordance with the present invention in its broadest aspect, a second nozzle is provided downstream of each Coanda nozzle, considered in the direction of air flow through the latter nozzle, the second nozzle being of a non-Coanda type which is arranged to provide an air flow normal, or substantially normal, to the air flow from the Coanda nozzle such that the two air flows interact in the region of the second nozzle to provide additional support for the travelling web.

In accordance with a second aspect of the present invention, there is provided an air cylinder for supporting a travelling web, the air cylinder being formed by a plurality of nozzle units disposed in a part-circular array, each nozzle unit comprising a pair of first, Coanda-type nozzle orifices arranged back-toback whereby the air which passes over their respective transversely extending Coanda surfaces flows in opposite circumferential directions towards respective radially inwardly directed air outlets, and a pair of second nozzle orifices, each disposed in a respective one of said transversely extending Coanda surfaces at a position spaced from the first nozzle orifice, each said second nozzle orifice being adapted to establish an air flow normal, or substantially normal, to the flow of air over the transversely extending surface from the respective Coanda nozzle orifice, such that the two air flows interact in the region of the second orifice to provide additional support for the travelling web.

Advantageously, the second orifice is defined by an extended lip surface which is disposed normal to the transversely extending surface leading from the adjacent Coanda nozzle orifice and which terminates in a sharp edged corner or an edge of small radius so that on leaving that edge the air flow from the second orifice creates an area of low pressure in its immediate vicinity into which the air flow from the Coanda nozzle is drawn.

When the aforegoing apparatus is in operation there can exist a tendency for the air flow over the transversely extending Coanda surfaces to move progressively outwards from the centre of the air cylinder to the edges of the web being processed. This can result in an undesirable loss of air flow control due to excessive edge spillage to atmosphere, with a corresponding loss of float height in the vicinity of the radially inwardly directed air outlets.

It is a further object of the present invention to substantially eliminate this problem.

This can be achieved by providing a plural ity of boundary layer fences, whose radially inner sides are shaped to the profile of the air cylinder and whose radially outer sides are shaped to the path of the web, said boundary layer fences being positioned over each of said first pair of nozzle orifices and extend respectively between each of said second pair of nozzle orifices, whereby lateral movement of the air flow over the transversely extending Coanda surfaces is substantially prevented.

The boundary layer fences may be disposed just at or adjacent the lateral edges of the air cylinder or across its whole length.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical section through part of a float treatment apparatus embodying the present invention; Figure 2 is a diagrammatic illustration of a plurality of part cylindrical arrays embodying the present invention and arranged for transporting a material web; Figure 3 is a diagrammatic section through one of a plurality of nozzle units which together make up part of the cylinder of Fig. 1; Figure 4 is a diagrammatic sectional view of the embodiment of Fig. 3 but modified to include end fences; Figure 5 is a partial plan view of the apparatus of Fig. 4; Figure 6 is a diagrammatic sectional view of part of another embodiment of the present invention;; Figure 7 is a plan view of the apparatus of Fig. 6, and Figure 8 is a diagrammatic front view of part of a float treatment apparatus of the present invention.

In order to understand the invention properly it is necessary, first, to describe in detail the apparatus to which it is applied.

Fig. 1 shows a cylinder 10 which comprises a plurality, seven in this instance, of individual nozzle units 1 2 disposed in a part circular array that has a curved length corresponding to more than a semi-circle, the nozzle units 1 2 each being constructed as shown in more detail in Fig. 3. The radially inner ends of the nozzle units 1 2 communicate with a cylindrical central chamber (not shown) connected via pipework to a heated pressure medium supply (not shown) which would normally be hot air, the air exiting from the radially outer ends of the nozzle units 1 2 being such as to be capable of supporting and guiding around the cylinder a travelling web 14, as shown in Fig.

1.

With reference to Fig. 3, each nozzle unit 1 2 comprises a pair of nozzle boxes 1 6 which communicate at their radially inner ends with a chamber 1 8 to which hot air is supplied.

The nozzle boxes 1 6 are formed by a pair of longitudinal side walls 22, 24 formed from sheet metal, a base wall 26 having a plurality of oval openings 28 leading to the chamber 18, a common intermediate wall 30 which separates the two boxes 16, and a top wall defined principally by a pair of profiled sheet metal members 32a, 32b, referred to herein after as air bars. The adjacent ends of the two air bars 32a, 32b are smoothly curved (as indicated at 33a, 33b) and spaced by a respective uniform gap from an edge "a" defined by an overhanging sheet metal projection 34a, 34b carried by the common central wall 30.

The smoothly curved parts 33 of the air bars 32, together with the sharp edges a define so-called Coanda nozzles 35a, 35b, whereby the air flow leaving the boxes 1 6 through these nozzles is caused to cling to and follow the curve of the parts 33 and thence the air bars 32 as indicated by the arrows, whereby to form a cushion of air supporting the moving web 1 4 at a substantially constant height above the nozzle unit.

In contrast to the known construction shown in our earlier European Patent (U.K.) No. 3414, a further orifice 36 is provided in each box 1 6 whose exiting air is caused to interfere with the air exiting from the Coanda nozzles, in a manner described further hereinafter, in order to assist in providing a stable support for the web. Each orifice 36a, 36b is formed by an edge "b" of the relevant side wall 22, 24 uniformly spaced from an extended air bar lip 38 which extends in a direction normal to the air flow arriving from the adjacent Coanda nozzle, the lip 38 terminating in a sharp edge "c" or in a very small radius.

The arrangement is such that in passing through the orifice 36, the air flow is made to attach to the extended lip 38 so that on leaving the sharp edged corner "c" thereof it generates a depression (low pressure area) in the immediate vicinity of the latter edge. The air then travelling over the surface of the air bar 32 from the Coanda nozzle is induced into this depressed area, the result being that the air flow forms a rolling vortex 42, as shown by the arrows, the effect of which is to provide a stable flotation cushion for the web 14 at the downstream end of each air bar 32.

Downstream of the orifices 36, the air has to be collected again and recycled to the nozzle units. This is achieved in the spaces 40 between each pair of adjacent nozzle units.

Because the return air spaces 40 must be at a relatively low pressure, typically around or below atmospheric, their presence is one which tends to oppose flotation at these regions. If too dominant, these regions would therefore pull the web against the air return aperture between the nozzle boxes and air circulation would cease. The arrangement is therefore made to be such that the depression at the point of air re-entry is kept to a minimum by making the return velocity of the air at this point very low, e.g. 2%, in compari son with the air velocity at the outlet of the Coanda nozzle whereby the supporting effect of the air flow at the cylinder surface maintains a clear path for air return.In addition, a suitable restrictor device (not illustrated) is included in the return channel between the nozzle units at a position well below the web line, in order to ensure that an even depres sion occurs all around the cylinder at every return position.

It is advantageous to include within the nozzle boxes 1 6 some means of "straighten ing", i.e. rendering more uniform, the flow to the various nozzles. In the simplest form, this flow straightener cancomprise a perforated plate 44 designed and positioned to have a diffusing effect on the total head of air coming from the feed pipes or other air entry orifices.

Without such a device, the possibility exists of pressure profile variations across the face of the cylinder.

The aforegoing apparatus is found to operate more satisfactorily than that of our earlier European Patent (U.K.) No. 3414 and results in a particularly stable air flotation transport of the web, with the added benefit of increased clearance over the cylinder.

Due to the arrangement of the nozzle units to form a partial cylinder shown in Fig. 1, the web 14 is continually supported on a cushion of pressure medium in passing over the cylinder so that it is maintained at a substantially constant distance from the cylinder. The latter distance will vary in response to tension and/or air exit velocity changes, although the changes will be marginal-typically in the range 6-20mm. Furthermore, when hot air is used at the pressure medium applied to the underide of the web, this serves to dry the web whereby the air cylinder acts as a float drier. If desired, drying can be assisted by the provision of an outer convected air dryer of conventional construction which can be mounted around the cylinder opposite the outer side of the web.Such a drier could be in the form of an air cap or accelerator hood whereby to increase the evaporation rate from the web.

As explained in our earlier European Patent (U.K.) No. 3414, a cylinder of this type can be used in place of conventional rotating cast iron cylinders used, for example, in paper machines and for drying textiles and indeed for heating or cooling any web material.

When used in place of known flat float drying apparatus, it will be appreciated that considerable floor space can be saved by the cylindrical nature of the present arrangement. Fig. 2 illustrates how the cylinders are arranged to maximise the length of web which can be treated while minimising floor space utilisation. The web 14 is passed around a plurality of cylinders, alternate ones of which are inverted whereby the web extends substantially tangentially between adjacent cylinders.

It will be noted that in this arrangement in passing around the nozzle arrays the web 1 4 moves in a serpentine path that takes it around a greater than 1806 circumferential portion of at least the intermediate arrays.

The intermediate wall 30 of the nozzle units need not be common to both nozzle boxes and could comprise separate members for each box 16.

It may be that the device will also be fitted with a simple canopy enclosure on the outer side to handle vapour generated there. The latter canopy enclosure can be connected into the air recirculation circuit.

A problem which can arise with the abovedescribed embodiment is that there is a tendency for the air flow over the air bars 32 to move progressively outwards from the centre of the cylinder 10 to the edges of the web being processed. This can result in an undesirable loss of air flow control due to excessive edge spillage to atmosphere, with a corresponding loss of float height in the vicinity of the return air spaces 40.

Figs. 4 to 8 show some ways in which this problem can be overcome.

Fig. 4 shows a boundary layer fence 46 which is attached to the existing airbar units 32. The boundary layer fence 46 ensures that the flow of air exiting from the Coanda nozzles and flowing over the air bars 32, takes the shortest route to the return air spaces 40 situated between the nozzle units 1 2.

In Fig. 4 the boundary layer fence 46 is shown as a single element. It could, however, be made up of two or more in-line pieces. The fence 46 is shaped to the profile of the cylinder and web path and extends betweenorifices 36a, 36b, of each nozzle unit. It is also formed aerodynamically to minimise the air flow interruption where it passes over the Coanda outlets. Fig. 5 shows a plan view of the fence 46 when in position on the nozzle unit. As can be seen, it is a thin, elongate element. In use, the web is arranged to be supported well clear of the radially outer edges of the fences.

Fig. 8 shows a multiplicity of such fences 46 applied to the face of a cylinder. The pitch and number of these fences may vary with each application, but typically the pitch would be of the order of 25 mm from one fence to the next. The number needed depends for the most part on the variation of the web width.

Generally the fences 46 are positioned around the edge of the web, but it is usually ensured that 2 or 3 fences are positioned under the web to take into account all variations in the positions of the edges of the web. Of corse, it is also possible to place these fences 46 across the whole length of the cylinder.

Fig. 6 shows a variation on the boundary layer fence of Fig. 4. This fence is made up of 3 overlapping pieces 48a, b, c, in a stagger pattern as shown in plan in Fig. 7. The advantage of this fence is that the masking-off effect of the impingement air is greatly reduced, though at the expense of a more complicated fence.

Claims (7)

1. An air cylinder for supporting a travel ling web, the air cylinder being formed by a plurality of nozzle units disposed in a partcircular array, each nozzle unit comprising a pair of first, Coanda-type nozzle orifices arranged back-to-back whereby the air which passes over their respective transversely extending Coanda surfaces flows in opposite circumferential directions towards respective radially inwardly directed air outlets, and a pair of second nozzle orifices, each disposed in a respective one of said transversely extending Coanda surfaces at a position spaced downstream from the first Coanda nozzle orifice, each said second nozzle orifice being adapted to establish an air flow which is normal, or substantially normal, to the flow of air over the transversely extending surface from the respective Coanda nozzle orifice, such that the two air flows from the first and second nozzle orifices interact in the region of each second nozzle orifice, to provide additional support for the travelling web.

2. An air cylinder according to claim 1, wherein each said second orifice is defined by an extended lip surface which is disposed normal to the transversely extending surface leading from the adjacent Coanda nozzle orifice and which terminates in a sharp edged corner or an edge of small radius, so that onleaving that edge the air flow from the second orifice creates an area of low pressure in its immediate vicinity into which the air flow from the Coanda nozzle orifice is drawn.

3. An air cylinder as claimed in claim 1 or 2, including means for diffusing the total head of air upstream of the first and second nozzle orifices whereby to eliminate pressure profile variations across the working face of the cylinder.

4. An air cylinder as claimed in claim 3, including a plurality of boundary layer fences whose radially inner sides are shaped to the profile of the air cylinder and whose radially outer sides are shaped to the path of the web, said boundary layer fences being positioned over each of said first pair of nozzle orifices and extending respectively between each of said second pair of nozzle orifices, whereby lateral movement of the air flow over the transversely extending Coanda surfaces is substantially prevented.

5. An air cylinder as claimed in claim 4, wherein the boundary layer fences are located only at or adjacent the lateral edges of the air cylinder.

6. An air cylinder as claimed in claim 4, wherein the boundary layer fences are located at spaced intervals across the whole length of the cylinder.

7. An air cylinder substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.