1. Field of the Invention
The invention relates to paper machines and in particular to a wet forming section of a paper machine and the operation thereof.
2. Description of Related Technology
A paper machine wet forming section having a head box with at least one nozzle for discharging pulp in a jet in a width approximately equal to the width of the machine is known in the art. Such a machine may include a forming cylinder disposed downstream of the head box with respect to the direction of conveyance of the paper pulp through the machine and two continuous-loop forming wires, each wire wrapping about a selected peripheral portion of the forming cylinder, the wires forming a wedge-shaped inlet gap for receiving a jet stream of pulp from the head box nozzle. Such a machine may further include a plurality of guide rolls, one of which being an inlet guide roll around which one of the forming wires is wrapped. It is known to include a displacement device adapted to change the position of the inlet guide roll in order to change the location or length of the peripheral portion of the forming cylinder wrapped by both of the forming wires.
A paper machine twin-wire web forming system disclosed in Parker et al., U.S. Pat. No. 3,726,758 (Apr. 10, 1973) includes a wedge-shaped inlet gap or nip defined by two continuous-loop forming wires. A pulp jet stream is discharged from a nozzle of a head box into the wedge-shaped gap. FIG. 2 of the Parker et al. patent shows such a device in which the extent that one of the forming wires wraps about a forming cylinder can be altered, as well as the angle the pulp jet is directed to the inlet gap. When the nozzle of the head box is directed appropriately, the pulp jet can be injected either exactly in the middle of the inlet gap, or it can deviate so that the jet stream selectively impinges one or the other wire. As a result, a preliminary dewatering of the pulp occurs on the impinged wire before dewatering is performed between the two wires. The swiveling of the nozzle of the head box serves to continuously change the angle of the pulp jet stream.
Wolf, U.S. Pat. No. 3,944,465 (Mar. 16, 1976) discloses a head box for a paper machine forming section having only a single continuous-loop forming wire or screen. With reference to a side view thereof, the head box swivels in such a way that an opening of a nozzle of the head box moves along a circular arc when the head box is swiveled. The circular arc has a curvature opposite to that of a surface of a breast roll around which the wire is wrapped. The angle of a pulp jet discharged from the nozzle opening can therefore be continuously changed so that the pulp jet stream leaving the nozzle will be discharged either parallel to the wire or impinge the wire at an acute angle. If necessary, the stream is directed upwardly at an acute angle so that the jet is a parabolic trajectory.
Both the Parker et al. and Wolf patents disclose ways of influencing dewatering in the first phase of sheet formation. However, such devices have not proved satisfactory as they have been found to also alter the quality of the paper web formed.