Functional principles of filtration scrubbers

The principle implemented in the separation of particles (aerosols) in filtration scrubbers involves passing dust particles dispersed in untreated air into a liquid, in which they are bound. This binding process increases the mass of the particles to be separated. In order to enable this technique, it is first necessary to create phase limits between gas and liquid – and then to produce relative movements between these two phases.

There are two possibilities to initiate this separation mechanism:

  1. Dispersion of one phase into the other
    A gas dispersion is created in a liquid (more or less fine gas bubbles are present in the liquid). Or, a liquid dispersion is created in the gas (liquid is present in droplets in the gas, or as a more or less distributed stream). In both cases, these methods produce a mixture of gas and liquid.
  2. Use of solid surfaces wetted with liquid
    With these mixtures, the only process taking place is transport of the aerosols contained in the untreated gas to the liquid surface.

Basic mechanisms involving the following forces take place here:

  • Forces of inertia (predominating)
  • Forces of flow
  • Forces of pressure
  • Field forces
  • Diffusion forces.

Mode of operation of a cyclone scrubber:
The cyclone scrubber intakes the untreated gas containing aerosols in such a manner that the gas impinges on the surface of the scrubbing liquid. A curved constriction begins to form at the interfaces of the two phases.

The following processes then take place:

  1. An aerosol mixture forms as a result of the change in direction (owing to forces of inertia)
  2. The energy of the gas leads to an entrainment of wall liquid, with simultaneous liquid dispersion.

Basic conditions that must be observed:
Cyclone scrubbers must be operated with air volume flow that is practically constant. If the flow is not constant, the following will occur:

Reduction in the flow of untreated air produces unsatisfactory dispersion of the scrubbing fluid. If the volumetric flow of air increases, this will produce a reduction in the level of the scrubbing fluid on the upstream side (untreated-gas side), which in turn means that the cyclone zone will not be sufficiently flooded.

Droplet collectors are installed in the cyclone scrubber to enhance the separation of the droplets formed in this process. A cyclone zone with a spiral form will enhance the phase-transition process. The pressure difference among cyclone scrubbers is around 1500 … 3000 Pa: i.e., greater than in scrubbers with or without inserts. The separation efficiency, however, is greater.

Additional advantages:
In addition to the separation of aerosols, the gas flowing through is moistened (conditioned) and cooled: whereby gaseous aerosols (e.g., oil vapour) condense out and can be separated.

An additional advantage of scrubbers is their ease of maintenance. There is no filter to replace, and the only work necessary is removal of the sludge from the bottom of the scrubbers. The use of sludge removers makes this work fully automatic.

Medium-pressure washers can be ideally combined with electrostatic and duct air filters, in order to achieve even more satisfactory results.