Infrared assisted through-air drying of lowgrammage sheets

Abstract: Through air drying (TAD) enables production of premium tissue products with increased softness, absorbency and bulk. On the other hand, the energy consumption of the TAD process is considerably higher than for conventional tissue drying alternatives. Previous studies on the TAD process have indicated that the drying rate for low grammage sheets is independent of the flow of air through the sheets.   The objective of this work has been to investigate and quantify how drying times and drying rates for low grammage sheets are affected by the addition of external web heating in a TAD process. Moist Eucalyptus and softwood sheets with grammages ranging from 15 to 60 g/m2 were dried in a laboratory process by an air flow through them and an IR-dryer with a variable power output. During drying, pressure drop and air flow were measured and an IR-camera recorded surface temperatures which enabled calculation of drying times and drying rates.   Using the IR-dryer to dry sheets shortened the drying time with at least 20 % and up to 60 % compared to sheets dried without IR-heating. Both pulp types and all grammages showed a linear relationship between drying times and the amount of evaporated water. Mass specific drying rates however, were very high for low grammage sheets and decreased rapidly with increasing grammage. Especially for low grammage sheets the drying rate had a very strong dependency on the IR-power and increased significantly with every increasing IR-power level. This finding implies that heat transfer could be a limiting factor when drying low grammage sheets in the TAD process. Another interesting phenomenon was observed for all grammages of the Eucalyptus sheets. Through these sheets the air flow rate increased with increasing IR-power, something that was not seen at all for the softwood sheets.   To summarize, adding external web heating to a TAD process resulted in a positive effect on drying times and drying rates, especially for low grammage sheets typical for the TAD process.