Through air drying : Thermographic studies of drying rates, drying non-uniformity and infrared assisted drying

The removal of water is an integral part of papermaking. The drying process is responsible for a great share of the energy used in the paper machine. Premium grade tissue products are dried by through air drying. Large volumes of natural gas are burned to heat the air drawn through the paper web to achieve the drying. The low grammages for which this technique is used are believed to have material properties differing from the bulk properties achieved at higher grammages. If through air drying could be performed more efficiently, premium products could be produced with less environmental impact and at a lower cost. The objective of this thesis was to investigate the non-uniformity and the rate of through air drying. The aspects considered were grammage, pulp type, formation, web-fabric interaction and infrared radiation. A method was developed, where the time-dependent change in surface temperature of a drying sample was recorded. The experimental equipment allowed paper samples to be dried by drawing air through them, with the option of additional energy from infrared radiation. An infrared camera captured the spatial variations in surface temperature during drying.The objective of this thesis was to investigate parameters concerning the drying rate and the non-uniformity in the through air drying process. Parameters considered were grammage, pulp type, formation, web-fabric interaction and infrared radiation. A piece of equipment was therefore developed which allowed the paper samples to be dried by air being drawn through them, with the option to supply additional drying energy through infrared radiation. The time-dependent local surface temperature of a drying sample was recorded using an infrared camera. In addition, the air flow through the samples was measured. Samples with grammages ranging from 15 to 60 g/m² were made on a laboratory sheet former from a range of different commercial chemical pulps. The pulps comprised both hardwoods and softwoods. Samples with both good and bad formation were made. The measurements showed that the air flow through the sample varied with grammage and pulp type. The air permeability, i.e. the specific air flow, was constant at higher grammages, as could be expected. In contrast to that, at lower grammages, the air permeability was higher, and also a function of grammage. The permeability was also highly influenced by the fibre morphology, with softwood samples being much more permeable. The non-uniformity in drying increased with bad formation and was influenced by the web-fabric interaction. Local drying time maps quantified the spatial non-uniformity of drying. Formation had little or no impact on the total drying time, but the non-uniformity of drying increased with worse formation. For commercial through air drying fabrics, the effect of web-fabric interaction was observed for hardwood samples, where the web areas in contact with the knuckles of the fabric weave had longer drying times. However, virtually no non-uniformity could be measured for the softwood samples. The average drying rate was mainly influenced by pulp type, and was increased by the addition of infrared radiation. Interestingly, even though the permeability differed significantly between pulps, the drying rate was independent of the varying permeability at lower grammages. Thus, a higher air flow did not increase the drying rate. Adding additional drying energy by using an infrared radiator allowed for an increase in the drying rate. However, as the radiator power was increased, the corresponding increase in the drying rate was less than proportional. Energy utilisation was evaluated for both the case with only air flow, and the combination of air flow and infrared radiation. It was found that the energy supplied by air correlated well with the theoretical energy required to remove the water in the fibre wall. When the infrared radiation was added, the efficiency in using the supplied energy appeared to diminish as the radiator power increased.Artikel 5 ingick i avhandlingen som manuskript, nu publicerat.