 |
|
| |
| |
 |
Double DoctorsSystem advantages:
- Reduced re-wetting of paper web
- Improved efficiency at suction rolls
- Increased dry content
- Fewer paper breakages in press section
- Reduced maintenance costs
- Prolonged roll life
- Improved paper quality
Double doctors have two blades which fulfill different functions. Depending on the application, the primary doctor blade is provided for either roll cleaning (fibres, fillers and surface water) and/or paper removal. This closely corresponds to the function of a normal single doctor. In critical applications such as on hard press rolls or on suction rolls, a significant improvement in the performance of the paper machine is achieved by using a secondary doctor blade.
Double doctors on suction rolls:
After the outgoing nip of the first doctor blade, water and fibres are evacuated out of the holes in the roll shell by centrifugal forces due to the surface tension of the water being broken by the first doctor blade.
The special construction of the double doctor with the positioning of the secondary doctor blade prevents re-wetting of the forming fabric or felt. The dry content of the paper web is consequently increased.
Our experience has shown that a 1.0 – 2.5% increase in dry content is common and as a result this capital investment can be repaid in a very short time. Another advantage is the considerably increased time between required roll maintenance since the holes in the suction rolls no longer plug up as a result of using a double doctor. An optimally conditioned suction roll also ensures an improved CMD moisture profile in the paper web and thus improved paper quality.
Double doctors on hard press rolls:
Similar to doctors on suction rolls, a low vacuum is produced (foil effect) at the back of the primary doctor blade. This causes an accumulation of fillers and fines on the back of the blade. This accumulation will drop on to the sheet and pass through the nip, leading to paper defects and sheet breaks.
In order to ensure that the double doctor works optimally, an internal shower is installed which sprays onto the back of the primary doctor blade. This prevents fillers and fines from accumulating at the back of the blade and equalizes the moisture profile on the roll surface.
Advantages for the user:
The primary doctor blade removes coarse contaminants such as fibres from the roll surface. This allows the secondary doctor blade to efficiently clean fines from the roll.
The secondary blade is attached close in front of the nip in order to ensure that the roll is as clean as possible in the press nip. Using a double doctor on hard press rolls reduces paper breaks, improves the moisture profile and enhances the overall performance of the press section and the paper machine.
|
|
|
|
|
