Dryer fabrics, often called dryer “felts”, are used to hold paper against the outer surfaces of steam-heated drying cylinders. The pressure from the dryer fabrics increases the rate of heat transfer from the dryer to the paper. The pressure that is applied to the paper depends on the fabric tension and the diameter of the dryer. The pressure can be calculated, as a first approximation, by the equation:
P = 2 T / D
In this equation, P is the contact pressure (in pounds per square inch), T is the fabric tension (in pounds per inch of width), and D is the outside diameter of the dryer (in inches).
Increasing the fabric tension increases the pressure on the paper and that in turn increases the drying rate. There is, however, a point of diminishing return. The dryer fabric tension is normally in the range of 12 to 18 pli, with the higher values used for larger diameter dryers on machines that are producing lightweight, low-permeability grades of paper at high speed. Higher tension is generally better, although excessive tension can damage the fabrics and the papermaking machinery with no practical gain in drying capacity. Dryer performance evaluations can help you get the most out of your dryer section and provide you with the recommended pressure range and fabric tension for your dryer section.