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6.6.2.2 Air Impingement Drying
This drying principle is mainly used in tissue production or in coating machines but also for enhancing the drying capacity of drying cylinders in multi-cylinder dryer sections. Hot air is blown through a nozzle plate at high velocity onto the paper. The impinging air transfers heat to the web and takes up the evaporated water. The air is then sucked back into the hood. Heat transfer in impingement drying increases with increased air temperature and velocity and by reducing the spacing between the nozzle plate and the paper surface.
6.6.2.3 Through Air Drying
This method is used in the drying of tissue and nonwovens. Hot air is sucked or blown through the air permeable paper web supported by a heat resistant wire. Heat is transferred directly into the fiber network and the evaporated water is carried off. Through air drying results in the highest drying rates.
6.6.2.4 Infrared Drying
This heat transfer method is mainly used to enhance the drying capacity in coaters when the web is wet. Infrared heaters are usually gas fired. The gas heats a mesh to a temperature of about 900 to 1100 °C. The low thermal inertia of the mesh allows fast control of the mesh temperature and the heating rate as well as preventing fires in the case of sheet breaks. In some cases electrical heaters are in use with temperatures up to about 700 °C, exhibiting a fast cool down of the emitter plates. Infrared drying units need sufficient air flow in order to carry off the evaporated water and to prevent coat quality problems.
6.6.2.5 Press Drying
This method is a combination of pressing and drying. First the web is dewatered mechanically in a press nip and brought into tight contact with the hot surface on one side. At the opposite side the web is covered by a permeable belt such as a felt or a wire which continues to press the web to the hot surface over a longer dis¬tance. The vapour escapes through the permeable cover or is stored therein. There are only a few installations of this dryer type worldwide.
6.6.2.6 Impulse Drying
This method is also a combination of pressing and drying. The process takes place in a press nip (for instance with a shoe press) where one surface, which is in direct contact with the web, is heated. The other web side is in contact with a felt. The wet web is compressed and thus mechanically dewatered. The vapor generated at the hot surface pushes the water through the compressed capillaries towards the felt and finally the generated vapor can flow freely through these channels. This kind of process is still in development.
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