Coating Machine Driers
After its application, the wet coat must be dried. Three different drying principles are used: radiation (infrared), convection (hot air), and contact (cylinders).
Infrared drying yields a very good energy transport, whereas mass transfer is inferior. Therefore, infrared is especially suitable for heating the paper or board web to temperatures where considerable drying occurs. Heating of the infrared radiators can be by electricity or gas. Gas-heated infrared dryers reach higher tem¬peratures and are usually cheaper. Electrical infrared dryers are easier to control and are, therefore, often used for moisture profiling in the cross machine direc¬tion. It depends very much on the local conditions at the mill site, such as availabil¬ity and prices, which heating principle is used for infrared drying.
Air flotation dryers are widely used for the drying of coat. In an air-flotation dryer, hot air is blown onto the paper surface. Air temperature can be as high as 350 °C, sometimes even higher. The impingement velocities of the air can exceed 40ms–1. The impingement effect gives a very effective heat and mass transfer between air and paper surface. The blowing nozzle sections in the dryer are alter¬nately placed on one side of the paper and on the other side. This gives a sinusoidal web run through the dryer with a web-stabilizing and wrinkle-avoiding effect.
Cylinder drying of coat is possible, as soon as the freshly coated surface is dry enough that the coating color does not stick on the cylinder surface. Therefore, drying cylinders are only found after a certain amount of contact-free drying, i. e. towards the end of the dryer section after a coater station. In modern coating machines, cylinder drying contributes only a small amount to the total drying capacity, sometimes less than 20 %. Contact drying has usually the lowest specific costs. Furthermore, the cylinder group is an excellent fix point for the web run. When no cylinders are used, a separate fix point between the coater stations has to be included, such as an S-wrap or a pull stack.
In some machines, the last cylinders are used to cool down the web rather than for drying. After drying, the web can reach temperatures well above 70 °C and does not cool sufficiently before the next coater station. Too high a web temperature causes excessive penetration of the coating color into the web so cooling provides a higher product quality.