Three mechanisms have been previously identified that determine the dry content of a web in suction box dewatering, i.e. web compression, displacement of water by air and rewetting. In the present work, the relative importance of the three mechanisms was investigated through direct measurement of the web deformation and the dry content changes during and after the suction pulse. Suction pressure, suction time and rewetting time were varied. The experiments were done with both chemical and mechanical pulp webs of various grammages. It was found that a large web deformation took place during the suction pulse, particularly at its beginning. Compression dewatering was found to be the most dominant dewatering mechanism. Displacement dewatering started after most of the web compression had occurred. Its contribution to the increase in dry content was most pronounced for higher suction pressures, longer suction times and for chemical pulp webs. A surprisingly large expansion of the web was observed immediately after the suction pulse. This expansion was the effect of rewetting. This rewetting strongly reduced the dry content of the web if the web had not been immediately separated from the forming fabric at the end of the suction pulse. Rewetting for mechanical pulp webs was more pronounced and occured faster than for chemical pulps. Under the conditions studied, the decrease in dry content amounted to the order of 3 to 6 %. Rewetting was smaller for longer suction times and higher suction pressures. A considerable air flow through the web occurred under these conditions. This air flow apparently moved water from the forming fabric into the suction box, thus making less water available for rewetting.
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