Patent Application
20050223593
The present invention relates to devices for contactlessly drying and guiding traveling webs, and more particularly, an improved web air flotation device that minimizes, eliminates or removes web wrinkles.
In web coating, printing and drying operations, it is often desirable that the web have contactless support, in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web during drying includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web as it travels through the dryer.
Important characteristics of any flotation system are the amount of cushioning provided by the flotation device, and the stability of the web as it passes over the device. Adequate support removes web wrinkles that typically are caused by higher tensions in lightweight webs. Airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the dryer, resulting in coating disturbance or web damage. Web flutter can be manifested in a multitude of forms, ranging from a violent flapping of the web to a high frequency drumming.
Single slot air bars are known in the art as air foils. They differ from double and triple slot air bars in that they have both a positive and negative pressure on the face of the air bar, whereas the double and triple slotted bars have only positive pressure. As a result, double and triple slotted air bars can be operated over a wider range of pressures and clearances; typical flotation clearances of air foils being about 2.3 mm compared to 6.3 mm for double and triple air bars. Air foils also have a dramatic decrease in both heat transfer and flotation stability as clearance is increased, whereas the heat transfers for double and triple air bars are relatively stable up to a clearance of 25 mm (single size bar). A typical application for single slot air bars is where flotation must be accomplished with air on only one side of the web.
Conventional air foils discharge air at about 45° to the web, which pushes the web up and relies on the flatness of the web to trap the air and force it to follow the air foil face. This creates a negative pressure to pull the web back down and hold it in place over the air foil. When floating lightweight webs under medium to high tensions, machine direction corrugations will form in the web. These corrugations allow the discharged air from the 45° slot to escape and not trap the air between the air foil face and the web, thereby reducing or eliminating the velocity created to draw the web down to the air foil face. This can result in poor flotation and can render the air foil ineffective.
It is therefore an object of the present invention to provide an air foil for floating a web that provides excellent web support for a wide range of web weights, and provides excellent web stability.
The problems of the prior art have been overcome by the present invention, which provides a step air foil particularly for one-sided flotation of a running web, and a web dryer having one or more such air foils. The air foil design includes two discharge slots which allow for increased draw down force, which flattens machine direction wrinkles in a floating web. The design does not rely on a flat web to help create a cross-face velocity to draw the web to the face for proper flotation as in conventional designs. Air discharged from the primary slot is gathered into the air stream of the secondary slot and creates an increased air cushion to provide greater support to the moving web and thereby remove machine direction web wrinkles caused by higher tension in light weight webs. The two air discharge slots blow gas (air) parallel to the web over a longer area than conventional designs thereby increasing the draw down force against the web.
The air foil includes a primary discharge slot and a second discharge slot spaced from and stepped down from the primary discharge slot, a first web support surface between the primary discharge slot and the secondary discharge slot, and a second web support surface downstream of the secondary discharge slot in the direction of web travel. The air foil is in communication with an air supply which provides a supply of air that is uniformly distributed to the primary and secondary slots.
The air foil can be primarily used for one sided flotation, but also can be used with two sided arrangements for enhancement of drying.
The air foil of the present invention is a unique design that incorporates a secondary slot that discharges air parallel to the web in order to maintain a constant pull down force that is independent of a flat web. It is particularly useful for one-sided flotation applications.
Turning now to
The flange portions 12a, 12b and bent portion 13 of header 1, together with the trailing top plate 2 and trailing bottom plate 3, define the primary and secondary slots of the air foil. Specifically,
The plate 3 also includes a relatively long portion 33 that extends from the short portion 31 at an angle therefrom. The relatively long portion 33 forms the wing of the air foil, as best seen in
The distance between the primary slot P and the secondary slot S is important for proper air flow and web flotation. If the distance is too small, the air issuing from the primary slot P will not flow parallel to the web. If the distance is too great, the primary slot airflow will lose its velocity. Preferably the distance between the slots is from about 2.5 inches to about 6.5 inches, with 3.25 inches particularly preferred.
Turning now to
In order to adequately support the wing extension of the trailing bottom plate, a plurality of gussets 60 (
As shown in
Thus, in operation, air flow is discharged through primary and secondary slots or orifices. The design allows for the reclamation of the discharged air from the primary slot to be gathered into the air stream of the secondary slot and create an increased air cushion to give greater support to the moving web, which in turn removes the machine direction web wrinkles caused by higher tensions in lighter weight webs. A higher flotation height (e.g., a positive 0.125 inch flotation height off the air foil face regardless of line speed) is possible for higher tensioned webs. Because air is discharged below and parallel to the web, there is always a velocity across the air foil face to draw the corrugated web down to the face and hold it in place for controlled transport. The increased cushion pressure of the secondary slot stretches the web, removing any machine direction wrinkles that may have formed in the web, thereby creating a glass-like appearance to the web. By incorporating two discharge slots on two different face locations and thus providing two large flat face areas, the draw down force is doubled, which is a necessity when flattening machine direction corrugation wrinkles.
The range of web weights and tension conditions with which the present invention exhibits excellent flotation characteristics is more than twice that of conventional designs.
The performance of the air foil allows for a larger spacing between air foils when installed in a dryer and allows for a greater window of operation with different web weights and web tensions. It can be used as a web-stabilizing device because of the strong web capturing characteristics of the design.
