Filtering materials should, above all, excel in the following:
- Low pressure loss
- Good separating capability
- Homogeneous, stable pressure build-up
- Favourable retention capacity.
In certain cases, however, other qualities may play a determining role, such as chemical and thermal resistance. For this reason, the selection of filters in relation to optimum requirements is likely to cause a compromise. When selecting the filtering material, overcoming the secondary effects (see Section 4.4.2) caused by the dynamic phase represent a task of primary importance. This ability, therefore, determines the underlying concept of the filter. Given the characteristic values of the oil particles to be separated, thought must be given to filter structure and fibre surface. Here, fibres from different materials are available. With the selection of suitable filtering material, the question of stability of the package arises. The solution lies as much in the construction of the filter as in the robustness of the fibrous layer. The latter can be significantly influenced when determining optimum filter parameters and inlet flow velocities. In fibrous layers which are subjected to high velocity, compression of the filter material becomes noticeable from a certain load onwards, and this can falsify the appearance of the filter behaviour but can be compensated for through appropriately high packing densities.
Diagram 4.9.1.1
For good separation performance, adequate layer thicknesses of more than 1 mm are necessary. To prevent re-entrainment, fitting a foam sleeve around the supporting grid offers a useful solution. At the usual inlet velocities, this highly porous layer prevents bubble formation and leads to an improved drainage of the emerging liquid. The foam material exerts no influence concerning secondary generation of particles smaller than 6 microns. In this range, fitting a fibrous material complementing the main drainage layer immediately downstream has proved to be advantageous. Diagram 4.9.1.2 shows the effectiveness of the foam material given an inlet velocity of 0.1 m/s.
Diagram 4.9.1.2
Graded density filter combinations, the individual filter layers of which display diminishing fibre diameter in the direction of flow, offer a higher degree of separation at relatively low pressure loss in comparison to a single layer. Such variations offer improvements in the stationary phase, i.e. when all parameters are constant.
