Internal Heat Generation in Press Roll on High Speed Paper Machine

Abstract

With ever increasing paper machine speed, it has become desirable to minimize the sheet moisture content at the exit of press in order to maintain higher production efficiency and higher drying efficiency.For this purpose, elastoma cover materials for press rolls are to be kept as hard as possible and also nip pressure as high as possible, which is recent fundamental designing theme of high speed paper machine press part. This, however, constitutes a very severe condition for elastoma covered rolls and cover damages have taken place on press rolls in high speed paper machines. The reasons of these damages are mechanical problems and elastoma cover internal heat generation by hysteresis, both of which are caused by the above mentioned condition (i.e, high nip pressure at high speed). The hysteresis problem cam not be neglected as the machine speeds become higher. This paper shows results of analysis of this problem and calculates how much power is absorbed for internal heat generation in relatoin to total press section power consumption. These calculations were made on the basis of actual operating conditions on two machines. The calculations of actual amount of internal heat generation was also made by knowing the measurable factors for power consumption. Another investigation was performed by using experimental apparatus and a covered roll as a model at which point the generated internal heat damages the cover. It was found that the damage occurs when heat generating amount per rubber cover unit weight reaches up to 0.0840.087 kcal/kg/sec. In case of the two machines mentioned, nip pressure were 80 kg/cm, and machine speed was 600 m/min for one and 720 for the other. The generated heat was measured to be 0.0210.024 kcal/kg/sec. So, in these two cases, thermal safety factor is 34. In a machine of this size, however, if machine speed goes up to 1, 000 m/min, heat generation radically increased to 0.044 kcal/kg/sec (safety factor : 2), which will probably causes operation and maintenance trouble for elastoma covered rolls. Temperature distribution in the elastoma cover by internal heat generation appears higher from the surface to the inside. I would like, however, to leave this problem as a theme to be investigated in the future. The cover material tested in this report was polyurethane type synthetic material (hardness P & J 5).