VacRoll suction holes and the grooves of all rolls should be cleaned regularly. Paper dust is released from the web on the dryer section and drawn into the VacRoll suction holes and grooves by the negative pressure inside the roll. These suction holes and grooves may get completely clogged due to web moisture, particularly at the start of the dryer section. Regular cleaning ensures efficient operation of the rolls. VacRolls should be cleaned at least once every three months in the first dryer groups, and once every six months in other groups.
For on-the-run dryer vacroll cleaning, consider the Valmet Dryer Vac Roll Edge Cleaner.
Rolls with perforated shells must also be cleaned internally with a vacuum cleaner. In such cases a longer shutdown is required as the roll must be opened prior to cleaning.
In addition to shell cleaning, the cleanliness of the bearing housing make-up air holes must be monitored. The make-up air holes may get clogged particularly in the first few groups of the dryer section. Monitor the cleanliness of the make-up air holes to find out how quickly they get clogged. Based on this monitoring, the optimal cleaning frequency of the holes can be defined. The rate of clogging varies notably between different machines. It is especially affected by the stock type used.
When cleaning the make-up air holes, make sure the dirt comes out instead of e.g. blowing it inside the housing with air. The purpose is to get the hardened stock off mechanically and out of the housing e.g. with a vacuum cleaner. This permits cleaning the make-up air holes without removing the flanges on both sides of the cord packing. This way the cord packing does not have to be replaced.
If the holes are clogged and the cord packing cannot fully keep the negative pressure in the air duct, the negative pressure may leak to the bearing housing. As a result, lubrication oil leaks past the cord packing to the air system.
If high-pressure water is used to clean the roll shell, the following considerations should be kept in mind:
Larger temperature differences may deform the shell permanently.
Rolls can be lifted by their journals or shell. To lift by the shell, use lifting straps that are at least 150 mm wide. The distance of a strap from the end of the shell must not exceed 0.2 x shell length.
Dryers and VacRolls must not be lifted at the center of the shell or at one lifting point. They must also not be lifted by a connected bearing housing or drive gear case.
If you lift the drive gear case with a roll, make sure to keep the case from rotating.
Lifting straps are not made for temperatures exceeding 80°C.
Typically, the hood ceiling is equipped with rails for the lifting of lead rolls, guide rolls, and stretcher rolls. Never use these rails for lifting dryer cylinders or VacRolls.
For more information on the care of VacRolls or any other rolls in your mill, contact your Valmet representative.