2.2 Principle of the headbox The water fiber suspension flows in the headbox from the headers to the nozzle. From the nozzle a jet with uniformed cross machine velocity is delivered, with a thickness between 420mm and a width between 3-8meters depending on headbox model. To achieve a good formation and even water fiber suspension distribution in the paper, the turbulence levels in the headbox needs to be sufficient. The turbulence generator inside the headbox is designed to give a good fiber distribution and sufficient turbulence intensity. After the turbulence generator, the flow enters the nozzle which is designed to give a short free jet length. To adjust the paper sheet properties the geometry of the jet can be adjusted with a number of process parameters. The quality of the paper sheet is depending on both the headbox and the former roll. When the jet has left the headbox, the former roll only preserves the formation that the headbox has generated therefore the jet geometry is important for the paper sheet quality. To achieve the best mechanical properties and formation of the paper, a short free jet length and optimized jet geometry are essential. Depending on the type of tissue paper which will be produced, the setting is optimized regarding to the jet geometry. To validate the adjustments, a measurement of base weight and the formation is conducted on dry paper with a scanner about 2 seconds after it has exited the headbox. One important factor to be able to produce a paper with high quality is to minimize the build-up of fibers and chemicals inside the headbox. This means that the hydrodynamic design in the headbox is important to avoid sticking of deposits to increase the quality of the paper. (Valmet, 2014)
2