Experiments in a pilot-scale impulse unit - energy efficiencies

Experiments were conducted to study the energy efficiency of the impulse un it in the pilot-scale research paper machine at the Swedish Pulp and Paper Research Institute (STFI) In Stockholm, Sweden. The paper produced was made of kraft softwood pulp with basis weights of 60-120 g/m(2). The roll surfa ce temperature ranged from ambient temperature to 350 degreesC and the pres s loads were between 400-800 kN/m. The paper was produced at machine speeds : of 300-800 m/min. Two different impulse drying efficiencies were defined, the total specific energy use (TSEU) and the impulse drying specific energy use (ISEU). The TS EU was defined as the electrical power input divided by the total water rem oval. The TSEU should be used as an overall efficiency parameter and should be compared with existing dryer section efficiencies. The TSEU was mainly dependent on the roll surface temperature, the ingoing moisture ratio of th e paper web, the press load and the basis weight. In general, the impulse u nit had higher energy efficiency than a modern dryer section. The ISEU was defined as the power transferred to the paper web divided by t he water removal due to the elevated temperature. This value should be comp ared with the latent heat of vaporisation and it provides information about the water removal mechanism in impulse drying. The ISEU was found to depen d on the roll surface temperature and the ingoing moisture ratio of the pap er web. The ISEU was close to the latent heat of vaporisation indicating th at the water removal mechanism in impulse drying was hot pressing combined with flashing of superheated water.