Experiments in a pilot-scale impulse unit – energy efficiencies

Abstract

Experiments were conducted to study the energy efficiency of the impulse unit 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 surface temperature ranged from ambient temperature to 350 degreesC and the press 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 TSEU was defined as the electrical power input divided by the total water removal. 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 the paper web, the press load and the basis weight. In general, the impulse unit had higher energy efficiency than a modern dryer section. The ISEU was defined as the power transferred to the paper web divided by the water removal due to the elevated temperature. This value should be compared with the latent heat of vaporisation and it provides information about the water removal mechanism in impulse drying. The ISEU was found to depend on the roll surface temperature and the ingoing moisture ratio of the paper web. The ISEU was close to the latent heat of vaporisation indicating that the water removal mechanism in impulse drying was hot pressing combined with flashing of superheated water. (Less)