ARRANGEMENT FOR DRYING A RUNNING WEB BACKGROUND OF THE INVENTION
The invention relates to an arrangement for drying a running web or the like by heat treatment, which is carried out by means of heat emitting sources, preferably by electrical heat means comprising at least one separate heat unit for the emittance of wavelengths in infrared light spectra. The heat unit is provided by light reflecting elements.
PRIOR ART
The drying of the running web is rather important if the web is coated with a coating substance, which includes water as the fluency element. During the application of coating substance onto the web and the levelling of the coating layer in a levelling member, say a nip member, water maintains the fluency property of the substance and, hence, faciliates an incomplicated and easy working phase. A commonly used substance concentration includes about 40% of water, which must be removed from the levelled coating layer and the running web, in which a portion of the water amount has penetrated. A typical method to dry the web is to blow air streams either against or parallel to the web so, that the web is maintained contactless from air stream supply boxes, and preferably at a predetermined fixed distance from them. This method is based on the application of unsaturated gas substance, which carriers away the vaporized water from the web and the coating layer. And as widely known, the vaporization is speeded up if the temperature of the wet coating or of the air stream is increased, the contact area between liquid and gas phases is enlarged, the saturation degree of the gas phase is decreased, the flow rate in the gas phase is increased or the penetration of the water vapour phase from the web/coating layer is speeded up.
A typical drying process is based on heated air which raises the temperature of the web and the wet coating substance. The heated air has an increased ability to receive watervapour, that is, the relative saturation of the air stream is initially lowered by heating. It is important to realize an economical system and at the same time an effective vapour carrier media. A portion of the heated air stream is guided out of the heating process and replaced by an amount of insaturated and heated air. Hereby the total humidity amount carried by the air stream will not result in to a value too near the saturation point. The indirect heating of the wet web by means of the heated air is slow and rather ineffective, since the major part of the heated air hardly comes into contact with the web.
Another method is based on a direct heating of the wet web by means of infrared heating. If a suitable light spectra range is used, a quick and an even vapourization is possible. The direction of the infrared rays is effected by reflecting elements placed in co-operation with light emitting lamps. In this method, however, the air in the vicinity of the heat treated web does not move fast enough, and hence, the air is very near to the saturation point and the vapourization is hampered.
OBJECT AND SHORT DESCRIPTION OF THE INVENTION
The object of the invention is to create a drying arrangement, which includes the favourable properties of the devices of the prior art, and at the same time avoids the drawbacks of the prior art.
The object of the invention is realized by means of the arrangement presented in the main claim. The invented arrangement includes members for a direct heating of the wet web by infrared light and members for keeping the vapor saturation value at low level in the vicinity of the running web. The low level is maintained by a partial recirculation of the heated air streams which receive the heat from light emitter lamps. The air streams are governed to create an air cushion, Coanda or the like effect, so that the web is out of contact with the box-like body member extending over the web transverse length. The infrared light penetrates through a barrier element located between the web and the lamp.
A portion of the air flow in the air cushion can be directed through recirculation elements back into the body member to be reheated. The heat economy is hereby positively effected. The air outtake members being properly arranged, the running web can be held at a predetermined distance from the barrier element.
A typical barrier element is a glass plate with low infrared light absoprion index. The best heat transfer effect, and the cooling of the hot infrared lamp, is possible by guiding the air circulation around the lamp, which can be placed parallel to web running direction or web transverse length. Another possibility Is to guide streams around a conversion member of the lamp, said conversion member being a typical eletricity-light emittance conversion element producing heat energy with high rate.
The outtake members can be elongated and parellely placed slots. A favourable set includes at least two separate heat units, placed for instance, so that each web side is treated at least by one unit.
The air streams from two neighbouring units can be either parallely or oppositely directed. The oppositely directed streams can either approach or move away from each other. If a number of heat units are placed in a row one after another in the web running direction, at least one unit directs air streams in a direction opposite to other streams.
BRIEF DESCRIPTION OF DRAWING
The invention is described more in detail with reference to the attached drawing in which fig 1 discloses an embodiment of the invention in a general view and partly sectioned, - fig 2 shows a sectional side view of the device of fig 1, - fig 3-6 show some arrangements of a number of devices set in co-operation with a running web.
The heat unit is marked with reference numeral 8. An infrared emitter lamp 1 is provided by a reflector mirror 2 for directing and concentrating infrared light on a running web 3. Air stream 4 is guided through a slot 5 of a box-like body 18 into a space between web 3 and a glass plate 6, which forms the top (or bottom) cover of body 18. Body 18 is provided by an air tube 7 for the air intake into body 18. A favourable arrangement is such that air intake is carried out around the electric-light convension member 9 of lamp 1. If several units 8 are placed in a row, air flows from two neighbouring units 8 can be directed in several alternative ways 10-13 relative to each other or the web running direction 26. Unit 8 can also be in co-operation with a reflector 14 so that web 3 is running between unit 8 and reflector 14. In certain cases it is favourable to place ordinary air stream boxes 15,16 in co-operation with unit 8, whereby air streams from boxes 15,16 maintain web 3 at a fixed distance from unit 8. At the same time, web 3 is pre- and posttreated with a measure, which accelerates the vapour motion away from the vicinity of the web, which in turn decreases the saturation level near the web.
Body 18 can be further provided by air tube 17 for the recirculation of air stream indicated by numerals 4,10-13. The lamp cooling stream is marked with numeral 19, air circulation around lamp 1 by 20,21 and the space between glass 6 and web 3 by 22. The interior 23 of body 18 is not shown in detail. The front plate 24 of body 18 is shaped to guide air stream 4,10-13 in a suitable manner. A portion of air stream 4,10-13 is recirculated in body 18, this portion 25 is mixed in interior 23 with fresh stream 19. Reference numeral 27 refer to the recovery or collector elements of the air recirculation system or to the outflow of air streams from heat unit 8 to the surrounding.
The invention is not limited to embodiment shown, but several modifications thereof are feasible within the scope of the attached claims.