Working Principle of the Rotational Particle Separator.
Core of the rotational particle separator is the filter element. It consists of a multitude of axial channels which rotate as a whole around a common rotation axis. The channels can have many cross-sectional forms which have as common feature that each channel is enclosed by a single continuous wall. Each channel is bounded by a wall in radial as well as tangential direction. In case of channels formed by concentric cylinders each channel is partitioned by at least one radially extending wall.
Solid and/or liquid particles present in the gas flowing through the channels are centrifugated towards the outer collecting walls: see fig. 1. As a result of the action of centrifugal forces, van der Waals' forces and/or forces due to surface tension, particles remain at the wall. The purified gas leaves the channels while the particles material collected at the walls can be removed periodically, for example by spouting gas or liquid at high pressure and velocity through the channels.
The radial height of the channels is small, up to a few millimeters, so that the radial distance to be travelled by a particle is small. At limited circumferential speed of the filter element (some tens of meters per second) and relatively large axial gas velocities (a few meters per second) particles of (sub)micron size can still reach the outer collecting wall for sufficient axial length of the channels (up to about one meter). The diameter of the filterelement can amount to one meter allowing gas flows of 10.000 m3/hr. Length and height of the channels are dimensioned such that the pressure loss is limited: up to some hundreds of Pascal.
Fig.1 Centrifugally inducd particle separation
Practical Designs of the Rotational Particle Separator
To meet the demands of the application various designs of the rotational particle separator have been accomplished. This includes the axial version and tangential version where axial and tangential refer to the direction of the gas flow at the inlet.
In case of the axial version shown in Fi. 3, gas containing particles material is directed axially into an impeller. The impeller is fitted upstream of the filter element. In the impeller the contaminated gas is distributed over the channels of the filter element. Downstream of the filter element, another impeller is fitted. In this impeller, purified gas leaving the filter element is directed radially towards a volute. In this volute the gas is directed towards the outlet analogous to the functioning of a radial fan. Upper and lower impeller and filter element are connected to each other and are driven by a motor positioned in the center.
An advantage of the axial version of the rotational particle separator is its compactness. The filter element, however, is not easily accessible for removing particles material collected in the channels. The axial version is therefore suited for applications, where the concentration of particles material in the gas is limited. The axial version is also suited for cases where the gas is contaminated with liquid particles material. These particles form a stable liquid film along the outer collecting walls of the channels, which flows downwards towards the lower impeller. Leaving the channels, the liquid is centrifugated towards the outer radial wall of the impeller.
Small holes in the outer wall of the impeller and further provisions allow for the continuous transport of the separated liquid particles material out of the rotational particle separator.
The tangential version of the rotational particle separator is shown in Fig. 4. The contaminated gas is directed tangentially into a cyclone-type inlet-housing. Here the course fraction of the particles material is centrifugated towards the stationary walls to leave the apparatus via the particles outlet at the bottom. The gas is subsequently guided towards the rotating filter element within which the fine particles material is separated. Downstream of the filter element, a radial impeller is mounted which serves to bring the gas at the desired pressure. The gas leaves the apparatus via the volute enclosing the impeller.
The blades of the impeller extend over a radius which is larger than the outer radius of the filter element. In this way the possibility is created to blow gas or liquid into the channels of the filter element from a provision mounted on top of the rotational separator. Particles material collected in the channels can thus be removed, either under filter operation or during a period of shut-off or shut-down of the apparatus.
Impeller and filter element are connected to the driving shaft of a motor fitted on top and flexibly (e.g. by rubber dampers) mounted on the housing. The impeller is designed such that an overpressure exists in the volute with respect to the cyclone. This prevents the occurrence of unwanted leakage flows of non-filtered gas in the cyclone to filtered gas in the volute. Large tolerances are allowed between housing and rotating element. Easy assembly and disassembly is accomplished by lifting the rotating element out ot the housing via a flange at the top.
Fig.3 The radial version of the RPS