The working principle of a FENRA hydrocyclone



The separation of heavy contaminants from pulp suspensions and other liquids is caused by the centrifugal force inside a hydrocyclone.


At the tangential inlet to the hydrocyclone, pressure is converted to velocity through a suitable nozzle and the velocity is creating the active force. To reduce the power needed of the velocity increase, the FENRA hydrocyclone is equipped with a special inlet design.


Contaminants with a higher density than the liquid are concentrated at the periphery of the cyclone and these impurities are transported downwards into the conical section.


To improve the removal of heavy contaminants the FENRA hydrocyclone can be equipped with an upper HELIX cone and a lower spiral threaded cone. The design of these cones is force feeding the contaminants towards the so called reject orifice where the impurities are leaving the cyclone.


The pulp suspension, free of heavy impurities, is collected in the centre part of the cyclone and leaving through a concentric pipe at the upper part of the cyclone. The clean pulp suspension is called accepts and a part of the kinetic energy is converted to pressure at the accept outlet.


Impurities with a lower density than the pulp suspension are affected by another force – the centripetal force. This force is pulling the light impurities towards the very centre of the cyclone and they can be removed as a separate part of the accept flow. Free air in the pulp suspension is also concentrated in that section of the cyclone and can be efficiently removed by using an external vacuum source.


The reject flow with heavy impurities still contains a lot of useful fibres and these fibres must be recovered. Therefore the reject flow is treated in a number of steps and in each step the concentration of impurities is increasing and the concentration of fibres is decreasing.


In almost all cases the design of the system is a classic cascade and the basic principle is to mix flows of the same cleanliness. As an example the reject flow of the first step is mixed with the accept flow of the third step and the mixture, diluted with water, is feeding the second step. Depending on the capacity a hydrocyclone plant can consist of 2 – 5 steps. In some applications only a single step is used.