Drying and gas or vapor contact with solids – Apparatus – Sheet – web – or strand
Patent
1996-08-05
1998-05-12
Bennett, Henry A.
Drying and gas or vapor contact with solids
Apparatus
Sheet, web, or strand
34643, E26B 900
Patent
active
057491640
DESCRIPTION:
BRIEF SUMMARY
This application is a national stage application, according to Chapter II of the Patent Cooperation Treaty. This application claims the priority date of Nov. 19, 1993 for Great Britain Patent Application No. 9323954.9.
FIELD OF THE INVENTION
The invention described here relates to improvements in drying webs and is applicable with particular advantage although not limited to a process carried out in a paper making machine, that of the application of "size" or "coating".
Such coatings are normally aqueous based and need to be dried by evaporation of the aqueous content. One particular problem associated with the application of "on machine" size/coating is that unless it is sufficiently dried before contact with the drying cylinders a phenomenum known as "picking" takes place. "Picking" results in areas of size/coating building up on the after-drying cylinders causing cylinder contamination, and leading to product quality problems. Hence there is a requirement for a device that will adequately dry the size or coating once applied to the paper web which can be positioned within the area of the paper making machine under consideration. Furthermore the area in question is very often limited in terms of space and thus the required device must be extremely compact and of high intensity in terms of its evaporation potential.
In recent times the approach to this problem has been to position a non contact "air turn" device between the size/coating application and the first "after-drying" cylinder, thereby setting up the correct sheet geometry through the size press/coater and creating a long uninterrupted length of web path into which an infra red radiation heating unit could be positioned. The purpose of the infra red unit is to achieve the required evaporation of water from the coated paper web.
There are problems however which can be attributed to the application of infra red radiation devices, namely: contamination, poor quality and risk of sheet creasing. temperatures and consequent heat loss, with attendant inefficiencies. i.e. slow response on both start-up and shut-down. radiation heat transfer with low air movement.
Thus, it is desirable to identify an improved method of web drying. Typically webs coated with `wet` size or coating are handled by air flotation systems. Such systems allow a web to be supported in a cushion of gas, usually air, without contact, and therefore damage to the coated surface.
A typical air flotation system comprises two air bar assemblies, between which a web travels. Each air bar assembly includes a plurality of parallel and spaced apart air bars, each air bar being elongate and arranged such that its longitudinal axis is transverse to the direction of travel of the web. Each air bar has a web facing surface through which gas, typically air, flows from the air bar.
Although the gas used in air flotation systems is often air, at times different gases need to be used to support the web. In the specification the term `air bar` and `air flotation system` will be used to encompass systems which could also operate with gases other than air.
In one form of air bar, each air bar includes an air inlet and on the web facing surface, a pair of parallel linear nozzles through which air passes to impinge on the web to support it. The nozzles are arranged such that they are aligned with the longitudinal axis of the air bar so that the nozzle lies transverse to the direction of travel. Many such air bars operate using the Coanda effect which causes the air flow to converge. This is arranged by providing a central top plate lying between the nozzles where each edge of the top plate forms one edge of a respective nozzle which is radiused such that air flowing from the nozzle flows over the radius and across towards the centre of the top plate. A typical air bar operating using these principles is described in UK patent specifications 1 302 091 and 1 302 092.
An alternative form of air bar also operates using the Coanda effect, and is known as an `air foil`. Here the air bar is elongate but is a
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patent: 5105562 (1992-04-01), Hella et al.
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Bennett Henry A.
Doster Dimmatia
Spooner Industries Limited
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