On the mechanism of steam forming during impulse pressing of wet paper webs

Steam forming processes in wet paper webs undergoing impulse pressing are characterized here in two different, yet complementary studies. In the first study, we measured the transient temperature profiles of wet paper webs undergoing compression at a constant rate in a heated platen press. Eight micro-thermocouples were embedded into 500 g/m(2) SBK wet paper webs at different elevations. The experimental conditions were such that the length of the pressure pulse was varied from 100 ms to 15 s, the initial temperature of the platen press was set from 150 C to 300degreesC, and the initial dryness of the paper webs were held constant at approximately 30%; approximately 600 experiments were conducted. In the second study, we visualized the flashing event using a novel experimental device. In this part of the work, a 10 cm bed of SBK fibres, with an initial dryness of 40%, was pressurized to approximately 5 bar and heated to 170degreesC. The hydraulic pressure was then released rapidly causing the water to flash inside the fibre network. The flashing event was recorded by high-speed video and the temperature profile was measured dynamically using fast response thermocouples. The results indicate that steam was formed during the compression phase of the pressure pulse only with long compression times, i.e. t > 1 s. In this regime, we found evidence of a heat pipe. Indeed. we found no evidence of a heat pipe with shorter compression times, i.e. t < 500 ms. We also observed a sudden increase in temperature in the middle of the paper sample at the end of the pressure pulse and attributed this to flashing. We speculated that the expanding steam displaced the unbound water and allowed heat to be convectively transferred from the top of the sheet to its lower portions. This mechanism was confirmed in the subsequent study in the flashing apparatus.