Laboratory trials were carried out in order to clarify the interaction between felt and web structure and its impact on dryness variability. Oriented laboratory sheets of 80gsm and consisting of 100% never dried unbleached softwood kraft were prepared using a dynamic sheet former. The sheets were pressed with two different orientations; aligned with the felt aligned in the machine direction or with web aligned in the cross direction. The wet web samples were immediately transferred after pressing to a hot plate and dried under restraint. A bench-scale method was developed using near infrared (NIR) imaging in order to characterise the interaction between felt and web structure during pressing and drying. It was found that the average press dryness had a low dependence on interactions between web and felt structure. Moisture variability in the interaction layer was found to be highly dependent on the web fibre orientation relative to the press felt. High levels of moisture variability on the web surface were observed with large differences in fibre alignment. It was observed that moisture variations in the web surface after pressing continued through the drying process and further increased the total drying time of the paper web.

The use of infrared radiation for heating the web in the through air drying process was investigated in lab scale. The hypothesis was that infrared radiation should be a more efficient method to transfer drying energy to the wet web compared to hot air, but that a certain air flow is still required as a transport medium for the evaporated water. A trial program comprising handsheets made of two types of bleached chemical pulps, five grammages (15, 22, 30 and 60 g/m2), and dried with five radiator power levels was performed on a lab scale through air drying equipment. Drying times of the samples were determined from temperature data recorded with an infrared camera. The use of infrared radiation shortened drying times, especially for low grammage samples. The shortening of the drying time ranged between 10 and 45 %. The most substantial shortenings were obtained for the lowest grammages and the highest radiator power level. However, the increase of power did not linearly shorten drying time. After an initial shortening at the lowest power level, the positive effect of the IR heating decreased as the power was further increased.

A method has been developed in order to determine the in-plane non-uniformity of drying of wet paper samples dried by an air flow. The surface temperature of the samples, recorded by an infrared camera, is used to determine a mean drying time and the local drying time of each pixel. Based on the initial dryness and the mean drying time, a mean drying rate can be obtained, and furthermore, the drying time of each pixel can be presented as a 2-dimensional map. Apart from conventional statistical information on the variation in drying time, the map also gives information regarding the size and shape of the drying non-uniformity. The pressure drop over the sample and the air flow rate through the sample were used to calculate a flow resistance as a function of grammage. Laboratory sheets with grammages between 15 and 45 g/m2, made from an unrefined bleached chemical hardwood pulp, were analysed. A considerable variation in local drying time was observed, despite their anticipated uniform formation. The mean drying time increased linearly with increasing grammages, thus the mean drying rate was not dependent on grammage. The flow resistance of the sheets increased with increasing grammage. The air flow rate through the sheet appeared not to be critical for the drying rate at the given experimental conditions.

The influence of grammage and pulp type on through air drying was studied. The temperature of a sample was measured during the drying process and the observed temperature changes were used to evaluate the drying process. Laboratory sheets with grammages 15-60 g/m2, from two softwood and two hardwood bleached chemical pulps were used. All samples were analysed with respect to formation, flow resistance, modified permeability, mean drying time, non-uniformity of drying time, and area- and mass-specific drying rate. The pulps had different modified permeabilities but showed similar behaviour when analysed as a function of grammage. A constant value was found for higher grammages, while an increase in modified permeability was found at decreasingly low grammages. Almost all pulp and grammage combinations had similar areaspecific drying rates, but the mass-specific drying rates decreased with grammage. However, the samples with lower grammages had mass specific drying rates independent of modified permeability, where samples with increasing grammage showed an increasing dependency. This implies that the drying efficiency at low grammages was not controlled by the volume flow of the drying air. A good correlation was found between energy needed to evaporate water and energy supplied by the drying air as estimated from the surface temperature and air flow measurement. The surface temperature can therefore be used to quantify the drying process.