The overwhelming number of known publications on the subject of particle separation in fibre filters assumes that the particles, separated in the fibre layers, do not cause any structural changes. This means the filters are envisaged on the basis of their unloaded state.
However, knowledge of this so-called stationary separation phase is, in practice, not sufficient in order to carry out an effective assessment and optimum design of the filters. To achieve this, the dynamic phase has to be taken into account. This includes all changes in the course of time because of permanent filter loading which, at the end of the elapsed time period, causes pressure loss and decrease of separating efficiency.
For these changes, questions of filter structure are important but the present state of the art offers no satisfactory answers. The following questions, therefore, remain open:
- Amount and intensity of the increase in pressure loss accompanied by simultaneous decrease of separating efficiency
- Assumed mutual influence of pressure loss and separation efficiency
- Structural changes, saturation behaviour and regeneration of the filter
- Degree of probability of particle adhesion
- Particle separation at the fibre is, in the stationary phase, determined by two basic sub-phases:
- The particles must reach the sphere of influence of the fibre surface
- Adhere to this fibre surface
- The dynamic separation phase required study of filter systems and leads to the following further occurrences:
- Agglomeration (accumulation) of the adhering particles at the fibre surface because of coalescing.
- Bridges and films of liquid between the fibres.
- By displacement of the liquid from the saturated fibre layer, the fibre is again available for particle/aerosol retention.
