GB2236584A - Air-float web treatment apparatus - Google Patents

Abstract

An air bar for air float treatment apparatus has infra-red heaters (38) disposed transversely thereof behind an infra-red transmissive window (32) which forms a pad plate (18). Heated air from Coanda nozzle (22) flows over rounded edges (20) and over the pad plate (18) to form a web-supporting air cushion. The infra-red heat enhances the heating of the web, e.g., for drying purposes. The ends of the heaters (38) are supported in side walls (16). <IMAGE>

Description

DESCRIPTION AIR-FLOAT TREATMENT APPARATUS The invention relates to air-float treatment apparatus, such as float dryers, and more particularly to air bars for such apparatus.

An air bar for float treatment apparatus is described in British Patent Specification No. 1,443,679.

Such an air bar comprises an elongate pad plate whose opposite longitudinal edges are rounded and air discharge slots are provided alongside said rounded edges such that the discharged air flows over the rounded edges and on to the flat surface of the pad plate due to the Coanda effect, whereby to maintain a web-supporting air cushion on said pad plate. If the web is to be heated, e.g. to dry a surface coating thereon, the air to be discharged is heated.

The drying rate can be increased by increasing the air velocity but the latter is limited if the Coanda effect is not be destroyed and often the air impingement velocity which the web can withstand is limited.

It is an object of the invention to provide for radiant heating of the web in an air float treatment apparatus. More particularly, it is an object of the invention to provide an air bar. with a radiant heater.

According to the present invention, an air bar comprises a pad plate which is at least partly transmissive to radiant heat and a plurality of elongate radiant heating elements disposed in the air bar transversely thereof and behind the transmissive part of the pad plate.

The radiant heating elements, which are preferably infra-red heaters, can be arranged perpendicular to the air bar i.e. parallel to the direction of travel of the web. Alternatively, they can be at an angle to the direction of web travel.

If there is a tendency for the web to dry nonuniformly transversely thereof, e.g., if it tends to dry streaky, this can be compensated by adjusting the heating elements individually or in banks of several of the heaters.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional detail view of an air bar according to the invention; Fig. 2 is a longitudinal sectional detail view of the air bar of Fig. 1; and Fig. 3 is a detail plan view, partly sectioned, of another embodiment of air bar.

Figs. 1 and 2 show the nozzle side of an air bar according to the. invention for an air float dryer.

The approximate path of the web to be treated and its direction of travel are denoted by the arrow 10 in Fig.

1. In the drawings it is assumed that the web travels horizontally but it can travel at any other inclination to the horizontal. Usually there will be two banks of nozzle bars arranged on opposite sides of the path of travel of the web to be treated. The nozzle bars may be opposed or may be staggered. The arrangement of the air bars is described more fully in British Patent Specification No. 1,443,679.

Each air bar comprises an elongated box or pressure chamber which extends transversely of the direction of travel over the full width of the web to be treated. The air under pressure is supplied to the pressure chamber at one or both ends of the air bar or alternatively medially of the air bar at its side (not shown) remote from the nozzles. Fig. 1 shows the front wall 12 of the air bar to which is attached a nozzle assembly 14. The nozzle assembly has side walls 16 which support a pad plate 18 which faces the web to be treated. The free edges 20 of the side walls 16 are rounded so as to blend with the pad plate 18. Coada nozzles 22 are formed adjacent the rounded edges 20 but set back slightly therefrom so that the discharged air follows the curvature of the rounded edges 20 due to the Coanda effect and is fed to the outer surface of the pad plate 18.A web supporting cushion of air is thereby maintained between the pad plate 18 and the web. The Coada nozzles 22 are formed between the side plates 16 and'side panels 24 which are mounted on the front wall 12. The side panels 24 and the side plates 16 form ducts 26 for the supply of air to the Coanda nozzles 22.

These ducts receive air under pressure from the pressure chamber 28 via apertures 30 in the side plates 16.

In a conventional air float dryer utilising air bars of the kind illustrated in British Patent Specification No. 1,443,679 the air supplied to the pressure chamber is heated for the purpose of drying the web. Usually the purpose of heating is to dry a coating on the web although it could be used for drying the web itself provided that the web has sufficient strength in its wet state. Another use is to dry printed matter on the web. The advantage of air float dryers is that it is possible to dry the web without touching the web even when the web is wet on both sides and the rate of drying can be enhanced.

Because the Coanda effect will not work if the air velocities at the nozzles are too high and because some webs are not sufficiently strong to be able to withstand air impingement at high velocities the drying effected by means of air float dryers having air bars is still restricted but could be enhanced if an auxiliary means is provided for imparting further heat to the web.

The simplest auxiliary means are radiant heaters such as infra-red heaters.

The purpose of the present invention is to enable infr-red and similar radiant heaters to be built into the air bars themselves whilst retaining an efficient compact structure. To this end the pad plate 18 comprises a plate of quartz glass or other heat transmissive material which is hereinafter called a lens 32. Thus, as can be seen in Figs. 1 and 2 the lens 32 forms substantially the whole of the pad plate 18. The lens 32 may be secured to the side walls 16 by spring clips (not shown). A series of infra-red heaters 34 is arranged inside the nozzle assembly 14 and the heaters are supported at their ends in the side walls 16. The ends of the heating elements in the infra-red heaters are connected to bus bars 36 insulatedly supported inside the ducts 26 by suitable lead wires.The drawings show the case wherein there is one bus bar in each duct 26 and the lead wires extend from the opposite ends of the infra-red heaters. If both lead wires extend fron one end of the heaters both bus bars would be arranged in the same duct 26. Reflectors 38 are built into one side of the infra-red heaters which are so arranged as to direct the radiant heat through the lens 32 and to the web. The heaters 34 are exposed to the air in the pressure chamber 28 and are thereby prevented from overheating as are their ends which extend into the ducts 26.

Usually, the air fed to the pressure chamber 28 is pre-heated but it need not be, the total heat input then being provided by the infra-red heaters 34.

As can be seen from Fig. 1 the individual infr-red heaters 34 extend transversely of the air bar, more particularly parallel to the direction 10 of travel of the web. Whilst it is possible to so space the infra-red heaters and so arrange the reflectors 38 that the heat is directed substantially uniformly on to the web transversely thereof, uniformly can be further enhanced by inclining the infra-red heaters 34 to the direction 10 of web travel as shown in Fig. 3. Thus, in Fig. 3 the heaters 34 are so arranged as to overlap one another by an amount equal to the pitch between the heaters.

Another advantage of the arrangement of the heaters 34 transversely of the air bar is that the drying rate transversely of the web can be controlled individually. In that event, separate terminals would be provided for the infra-red heaters so that the heating current to each can be controlled independently.

Another advantage of the illustrated air bar is that the length of each individual infra-red heater 34 is independent of the length of the air bar.

Accordingly, special infra-red heaters are not required for long air bars e.g. of a length of two to three metres or even up to nine metres.

For maintenance purposes the air bars themselves are usually removable from the apparatus.

To enable infra-red heaters to be changed all that it is necessary to do is to remove the relevant air bar and then remove the side panels 24. In the case of apparatus using a large number of air bars it is feasible to have a standby air bar so that should a heater fail in one of the air bars in operation the whole air bar can be replaced quickly and the faulty heater can be replaced at leisure.

Claims (13)

1. An air bar comprising a pad plate which is at least partly transmissive to radiant heat and a plurality of elongate radiant heating elements disposed in the air bar transversely thereof and behind the transmissive part of the pad plate.

2. An air bar as claimed in claim 1, in which the radiant heating elements are infra-red heaters.

3. An air bar as claimed in claim 1 or 2, in which the heating elements are arranged perpendicular to the air bar i.e. parallel to the direction of travel of the web.

4. An air bar as claimed in.claim 1 or 2, in which the heating elements are at an angle to the direction of web travel.

5. An air bar as claimed in any preceding claim, in which the heating elements are connected so that they can be adjusted individually or in banks of several of the heaters.

6. An air bar as claimed in any preceding claim, in which the pad plate is substantially wholly transmissive of radiant heat.

7. An air bar as claimed in any preceding claim, which comprises a box having a front wall and a nozzle assembly having side walls, which are attached to said front wall and whose edges remote from said front wall are rounded and blend with the pad plate, the nozzle assembly also having side panels which are also attached to said front wall so that Coanda nozzles are formed between the side panels and the side walls and adjacent said rounded edges of the latter but set back slightly therefrom, the side panels and the side walls forming ducts therebetween for the supply of gaseous medium to the Coanda nozzles.

8. An air bar as claimed in claim 7, in which the heating elements are supported by their ends through holes in said side walls.

9. An air bar as claimed in claim 8, in which a bus bar for supplying power to the heating elements is supported in one of said ducts.

10. An air bar as claimed in claim 9, in which another bus bar is supported in the other duct.

11. An air bar'as claimed in any preceding claim, in which the pad plate comprises a heat transmissive lens with which said rounded edges blend.

12. An air bar as claimed in any preceding claim, wherein each radiant heating element has a built-in reflector.

13. An air bar constructed substantially as herein described with reference to and as illustrated in the drawings.