| abstract |
An improved vacuum box is described for holding a paper web onto a supporting fabric wherever velocity stresses would otherwise separate the web from its supporting fabric and expose the web to those stresses. Specially shaped vacuum boxes are fitted into the dryer "pockets" formed between the rows of a double row of adjacent drying cylinders and the web traveling a serpentine path between them. Pressure differential zones of the box hold the web onto its supporting fabric as the web and fabric travel between heated drying cylinders. A first zone leads the departure of the web and fabric from the web-wrapped cylinder to ensure that the web is positively held to the fabric as it leaves the cylinder. A second arcuate suction zone adjacent a fabric-wrapped cylinder evacuates grooves in the adjacent cylinder. This system holds the web onto its fabric, overcoming centrifugal stresses on the web, as the web-fabric combination passes about a cylinder with the fabric in direct contact with the cylinder and the web in indirect contact. A third pressure differential zone ensures the web is held to its supporting fabric as it travels from the fabric-wrapped cylinder to the next web-wrapped cylinder in the drying sequence. The vacuum box is provided with seals between box surfaces and the fabric that reduce air leakage into the box yet accommodate waste paper passing between the drying cylinder surfaces and box surfaces without damage to the box or fabric. Box end seals perform a similar function. The end seal is provided with a spring and pivot arrangement which cooperate to accommodate waste. The top portion of the vacuum box is a curved surface designed to deflect at least a portion of the stray currents that typically flow in the dryer pockets and tend to lift the web from its supporting fabric. While the suction surface zones are generally open surfaces, a roller wearing surface may be provided which reduces the wear of the felt in passing through the vacuum box suction zones. The vacuum box may be compartmentalized across the width of the machine to permit reducing vacuum demand during start-up of the paper web through the machine. |