Perforated rolls are used in the wire section as well as in the press and dryer sections. Their open area (2 to 85 %, total area of the openings/holes related to the overall roll surface) and their design vary considerably. Their tasks are
. • to dewater the paper web
. • to store the water and to release it in a controlled way
. • to remove the air between web and felt ahead of the press nip
• to hold the web to the roll surface for a certain circumference.
So perforated rolls are used under various operating conditions:
. • Without vacuum application, just storing water when it is pressed into the voids of the roll and releasing it afterwards. These rolls are used at lower machine speeds. A special application of such a roll type is the Dandy roll for formation improvement.
. • With vacuum application in the wire section and dryer sections.
. • With vacuum application and line load as suction press rolls in press nips.
Most rolls with vacuum application emit a loud noise due to the siren effect when the holes under vacuum are suddenly refilled with ambient air. The noise level may be so high as to make ear protection measures necessary. The actual noise level depends on the roll drilling pattern, the vacuum level, machine speed and the design of the sealing of the vacuum chamber inside the roll. Some typical perfo¬rated rolls are described in more detail below.