Wet type dust collectors are liquid (usually, but not necessarily, water) to capture and separate particulate matter (dust, mist, and fumes) from a gas stream. Some scrubbers operate by spraying the scrubbing liquid into the contaminated air. Others bubble air through the scrubbing liquid. In addition, many gybrid wet scrubbing designs exist.

Particle sizes, which can be controlled by a wet scrubber, range from 0.3 to 50 microns or a bit larger. Wet collectors may be used for the collection of most particulates from industrial process gas streams where economics allow for collection of the material in a wet state.

Advantage of wet collectors over dry type dust collectors include:

* Constant operating pressure;
* No secondary dust sources;
* Little spare parts requirements, low maintenance in operation;
* Ability to collect both gases and solid / liquid particulates;
* Ability to handle high temperature and high humidity gas streams, as well as to reduce the possibility of fire or explosion;
* Reasonably small space requirements for this type air scrubbers;
* Possibility to continuously collect sticky and hygroscopic solids without becoming fouled.

The mechanisma involved in the capture and removal of paricularsin scrubbers are inertial impact, Brownian diffusion, and condensation:

1) Inertial Impaction occurs when a dust particle and a liquid droplet collide, resulting in the capture of the particle. The resulting liquid/gas particle is relatively large and may be easily removed from the carrying gas stream by gravitational force or impingement on separators.
2) Brownian Diffusion happens when the dust particles are extremely small and have motion independent of the carrier gas stream. These small particles collide with one another, making larger particles, or collide with a liquid droplet and are captured.
3) Condensation occurs when the gas or air is cooled below its dew point. When moisture is condensed from the gas stream, fogging occurs, and the dust particles serve as condensation nuclei. The dust particles become larger as a result of the condensed liquid, and the pobability of their removal by impaction is increased.

Wet scrubbers perform two individual operations. The first one happens in the contact zone, where the dirty gas comes in contact with liquid; the second is the separation zone, where the liquid that has captured the particulates is removed from the gas stream.

All Canadian Air Systems air scrubbers use the following principles:

- high liquid-to-gas ration;
- intimate contact between the liquid and dust particles, which may be accomplished by formation of large numbers of small liquid droplets or by breaking up the gas flow into many small bubbles that are driven through a bath of scrubbing liquid, to increase the chances that contaminants will be wetted and collected;
- abrupt transition from dry to wet zones to avoid particle buildup where the dry gas enters the collector.

For additional information please refer to http://nis-co.com/scrubber/Index.html.

Oleg Tchetchel
Air Systems Engineer and Designer
Canadian Air Systems
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