Suction Press Roll

6.2.2.3 Suction Press Roll
This type of roll is an open press roll with vacuum application at part of the circumference. The vacuum removes the air between the web and the felt ahead of the press nip and holds the web on the felt. It enables the water squeezed out in the press nip to flow from the web and felt to the roll void volume where it is stored and released after leaving the vacuum zone.

These rolls are very sensitive with regard to their stressing and material strength. Their main load is the linear force (up to about 120 N mm–1) in the press nip for dewatering. Additional loading occurs from the forces due to vacuum, felt tension and dead weight. These forces result in a dynamic stressing of the shell. Stress concentration due to the drilling pattern and open area as well as the shell thickness has an impact on the actual maximum stress which may be either more “beam bending” (rolls with small diameter, high wall thickness) or more “shell deflection” (rolls with large roll diameter, low wall thickness).

Suction press rolls run in a corrosive ambience, so corrosion fatigue strength is the important material property. Corrosion fatigue strength decreases with the number of cycles and time. As a rule of thumb it can be stated that 10 % more stress (or less corrosion fatigue strength) leads to a factor of about ten in lifetime reduction, which can be translated into a lifetime of either 10 years or 1 year. Suc¬tion press rolls are designed for a lifetime of at least 109 cycles. The material applied is bronze or special alloys exhibiting good corrosion fatigue character¬istics.

Shell thickness and open area define the amount of air to be removed constantly during vacuum build up which affects the amount of energy consumption. Hole diameters (about 4 mm, open area about 15 to 30 %) in the metal shell are larger than those in the cover (about 2.5 to 4 mm, open area about 10 to 20 %). The holes in the cover are drilled after the coating has been applied. The coating holes have to match the pattern in the shell which is easier with smaller hole diameters in the coating.