A study in how rewetting can be reduced in the paper machine with focus on the forming section

Abstract: This master thesis provides an overview of the paper machine with focus on the forming section. The forming section is the first part in the paper machine where the paper pulp is injected through a head box. The paper pulp contains about 99.5% of water and 0.5% fiber. The water as content is reduced by vacuum and gravity. The problem to be studied in this project is related to external rewetting. This is water going back to the paper web from the forming fabric after the dewatering zone. The dewatering is based on vacuum slots under the forming fabric. The vacuum slots absorb water from the soaked paper pulp through the forming fabric. External rewetting causes problem, hence the paper will have a higher dry content when leaving the forming section. The paper should have as low dry content as possible in the end of the forming section. Three different forming fabrics from Albany International were chosen for the project. The structures of the forming fabrics were two different double layers and one plain weave. The performance of the fabrics was studied by 4 different methods. The methods used were 2 different wicking tests, a vacuum dewatering trial and one foulard test. Also micro tomography was done to understand the structure of each design. The main test was a foulard test where the aim was to see in what way the rewetting got affected by different pores sizes. The results showed higher water content for the paper that was on top of the forming fabric with the larger pores.