Radiant energy – Irradiation of objects or material – Ion or electron beam irradiation
Reexamination Certificate
1998-01-13
2001-09-04
Colilla, Daniel J. (Department: 2854)
Radiant energy
Irradiation of objects or material
Ion or electron beam irradiation
C250S324000, C250S325000
Reexamination Certificate
active
06285032
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device for removing the gaseous laminar boundary layer from at least one of the two sides of a material web, paper for example, said web being moved in the transport direction, preferably at high speed.
Gaseous laminar boundary layers on material webs moved in air are known of themselves to be troublesome. Thus, for example when material webs are wound on a roll to form paper or film bales, including the laminar boundary layer in the winding produces a larger bale diameter for the actual length of material web to be wound than when the boundary layer is not included. In addition, for example during drying in a press, an attempt is made to expel the solvent in the printing ink(s) from the material web by drying. So-called boundary layer doctors are known for this purpose, with air jets mounted at right angles or transversely to the surface, said jets blowing air at high energy and speed against the material web to convert the laminar boundary layer in the microscopic range into a turbulent flow whose vortices have increasingly larger diameters than the thickness of the boundary layer, so that firstly they allow the solvent from the printing ink to pass through better and secondly they can be influenced by conventional blowing and/or suction nozzles for macroscopic elimination.
Such systems for drying are known especially for example in gravure, rotary-offset, and flexographic printing. In all of these printing methods, the ink dries as a result of the expulsion of the solvent or solvents, which are hydrocarbons or mixtures of alcohol and water. Because of the high speed at which the material web is transported, pronounced laminar boundary layers result that hinder both the transport of heat into the material web and the transport of solvent material out of it. Both physical principles are significant for drying.
Heat transport in a dryer based on hot air systems is responsible for the heating and therefore for the temperature increase of the material web. As a result of heat transport, the energy required to expel the solvent is provided. On the other hand, the material transport corresponds to its solvents driven out of the material web. Since drying is performed as a rule with material web temperatures above 100° C., there is still a small amount of water present that evaporates from the paper.
It is clear that the quality of a drying system depends upon the highest possible transfer of heat and material with a simultaneous small temperature differential between the ambient air and the material web. A small temperature differential necessarily means that there is a lower energy requirement provided the rest of the dryer system remains the same.
It is known from flow technology that in particular laminar boundary layers with relatively low Reynolds numbers, in conjunction with the high kinematic viscosity of hot air, exhibit low thermal and material transfer.
Since heat and material transfer in turbulent flow is a multiple of the value for laminar flow, an attempt is made in known drying systems to convert the laminar boundary layer both pneumatically and mechanically to change it to a turbulent boundary layer using the above-mentioned boundary layer doctor. As a rule, specially designed blowing nozzles are used, directed against at least one side of the material web. The results are not satisfactory despite the application of high energy. The reason is not known exactly despite intensive research. Possibly the reason for the unsatisfactory results lies in the fact that with surface roughnesses of the material web of approximately 2 to 4 &mgr;m in the case of paper, despite the conversion from a laminar to a turbulent boundary layer, a thin laminar so-called residual boundary layer is embedded so to speak in the unevennesses in the surfaces of the material web produced by the roughness, so that heat and material transport are both hindered.
This so-called boundary layer doctor can be used not only in dryers in printing presses, but theoretically in all other applications as well. Nevertheless, the efficiency remains poor.
SUMMARY OF THE INVENTION
The goal of the invention is to design a device according to the species in such fashion that boundary layers can be removed more simply and with much better efficiency.
This goal is achieved in a device for removing the gaseous laminar boundary layer from at least one of the two sides of a material web moved in transport direction preferably at high speed, for example said web being made of paper, characterized by at least one corona-charging electrode provided with at least one elongate tip, and connectable to a positive (+U) or negative high-voltage source, and characterized by at least one counterelectrode associated with the latter and connectable to a negative (−U) or positive high voltage or ground, with corona-charging electrode on the side of material web having the boundary layer to be removed, and the associated counterelectrode being located on the other side.
In the solution according to the invention, therefore, a corona-charging electrode is used in which, in the electrical field from the counterelectrode to the corona-charging electrode, a plasma channel is formed from the material web to the corona-charging electrode in the sense of a hard corona charge with direct current flowing through the latter, through which channel the charge, namely electrons, are conducted from the surface of the material web to the corona electrode, which has at least one and preferably however a plurality of points directed against one side in the direction of the surface of the material web. This produces collision ionization of the electrons in the plasma channel with gas molecules in the surrounding atmosphere, so that this molecule is ionized. According to one unconfirmed model concept, both as a result of the collision momentum of the electron on the gas molecule in the direction away from the surface of the material web on the one hand and the electrostatic force acting on the ionized gas molecule in the electrostatic field on the other hand, material transport takes place in the direction of the corona-charging electrode. The direction of movement, as already mentioned, runs transversely with respect to the flow direction of the boundary layer of the material web. As a result of this so-called ionic wind, the change from laminar to turbulent flow of the boundary layer is effected even below the critical Reynolds number. It is known that above a Reynolds number of 3×10
6
, a partially turbulent boundary layer is created spontaneously. However, this turbulent flow of the boundary layer has a greater thickness than that of the laminar flow and therefore interacts more readily with macroscopic influences, for example other pronounced or applied air flows, for example those from the boundary layer doctor.
In addition, in the turbulent flow, vorticized areas are created with directions of movement and speeds that oppose those of the transport direction of the material web and are approximately of the same magnitude as far as their contributions are concerned, so that in this quasi-backward moving vortex, there is no relative velocity, or only a slight one, with respect to the material web, which considerably facilitates the escape of solvents and/or water.
Surprisingly, however, it has also been found that the so-called ionic wind described above, even with reversed polarity, is capable of changing the laminar flow of the boundary layer into a turbulent flow. In conjunction with the tips of the electrodes of the corona-charging electrode spaced
5
mm apart, it was observed that with negative charging of the surface of one side of the material web, in other words with electron transport from the corona-charging electrode to the surface of one side of the material web, the change took place with a much higher charge transport that corresponds to the current flowing through the corona-charging electrode than at a corona-charging electrode connected to a positive high vo
Hahne Ernst August
Knopf Franz
Antonelli Terry Stout & Kraus LLP
Colilla Daniel J.
Eltexelektrostatik GmbH
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