Cleaning Flashcards
(6 cards)
Draw a basic sketch of centrifugal cleaning! How is the separation process controlled? Name and explain the influencing factors of both control means.
Stock is fed into the cleaner which has a cylindrical shape. This creates rotational energy, creating a vortex. The diameter of the cone decreases downward which increases velocity – the leads to centrifugal forces. Lightweight material is drawn toward the centre of the cone and then upwards to the accept outlet. Whereas heavy material is sent to the outer wall, the vortex then pulls it out through the reject outlet. Forward cleaners remove heavy material such as sand, metal pieces and shives. Reverse cleaners can also be used to remove light contaminants such as foams and other plastic material.
Controlled by density differential, therefore the contaminants must have a different density from water.
Draw sketches of heavy cleaners, light cleaners and combi cleaners! Which contaminants do you remove with the different systems (2 examples)
Heavy cleaners remove sand, metal, shives
Light cleaners remove plastic, and foams
Name and explain 4 parameters/properties, which will influence the removal efficiency of a hydro cyclone.
High pressure differential between inlet and accepts – this creates high tangential velocity and high centrifugal acceleration. But this needs to be balanced with energy demand for cost reasons.
Low stock consistency allows free particle motion and therefor better separation.
Low flake content – flakes resist particle motion
High temperature – Reduces water viscosity and therefore reduces flow resistance to particle motion
Stock air content – air reduces efficiency
What is a vortex rotation in centrifugal cleaning and why is it so important? What is the problem, if there is not sufficient enough kinetic energy in the system? What rotation mode does occur and what are the consequences?
Vortex rotation is when the tangential velocity is proportional to 1/r. It means the velocity close to the centre of the cone is higher than the outside. This causes internal shearing action and fluidisation of fibre flocks because there is continuous distortion of the suspension. It is important for maintaining sedimentation of contaminants towards the wall. But high kinetic energy needed to overcome internal shear forces. If this is not reached solid body rotation can occur which would mean no distortion of elementary volume, no internal shearing and therefor formation of fibre flocks.
- Explain in a few words the main tasks and the function principle of thick stock cleaning!
Thick stock cleaning happens at 2-5%. Used pre-cleaning to take out coarse and heavy particles. Usually placed after pulper and before screens. To remove particles larger than 1mm with a higher density. Things like staples and paper clips. These rejects are then removed batchwise.
Explain in a few words the main tasks and the function principle of thin stock cleaning!
Stock is 05-1% consistency. Energy consumption is high because they are small but separation efficiency is high. Can be used to separated light and heavy rejects. Light weight contaminants such as Styrofoam, waxes, stickies and air. Heavy contaminants such as sand metal etc.
