Fundamentals of wet dust filter unit separation

A wet filter unit typically separates solid particulate from a gas stream by passing it through an atomized liquid whereby particles get encapsulated by droplets and subsequently submerged into a tank where sedimentation occurs. As the method of separation is provided by a liquid which is recovered and re-used within the unit, there is no requirement for replaceable filter elements such as those in a dry unit.

Scrubbing Stages

  • Primary inertial separation onto water surface of sump
  • Spray generation zone – fine droplets/mist
  • Irrigated baffle surfaces
  • Final water curtains
  • Collected dust settles in bottom of tank

Wet dust collectors, or wet dust scrubbers, are mass separators. The efficiency of wet dust collection is determined by the mass of the dust particle present in the extracted dust cloud relative to the size of the water droplets generated.

Wet collectors are more suited to particles with heavier mass and they function by generating very fine droplets or aerosols in the 'spray generation zone'. These droplets will impinge on, and encapsulate dust particles of similar momentum.

Wet units usually require dusts of relatively high specific gravity (>2500kg/m3) and with particle sizes >10 microns to achieve acceptable filtration efficiencies of >99%. Wet collectors are recommended when machining reactive white metals such as aluminium, titanium and magnesium etc. due to their highly volatile nature and are also ideal for handling sticky particulates or dusts extracted with liquid moisture.

Limitations of wet dust filter units

Unlike a dry dust extraction system where filtered dust is discharged from the unit, then collected and disposed of, wet dust extraction systems effectively create a liquid waste stream as the means of filtration and is also the medium in which the particulate accumulates. This requires regular manual cleaning and therefore some limitation of operation during maintenance down-time.

Treatment is also required to be added to the water in a wet unit to minimise corrosion, and this requires regular checks to maintain correct levels. In addition, the scrubbing liquor will collect water soluble salts, liquids and gases and will therefore require periodic drainage. Hydrophobic (water repellent) dust particulate will not be contacted by the droplets and therefore not filtered out in the unit.

A proportion of water is consumed and given off as a vapour and a visible discharge plume from the unit so a permanent water supply is required to be connected to it to allow operating levels to be maintained. Consideration must be given to the dust extractor discharge position because of the vapour plume and it is also worth noting that the plume can sometimes be misinterpreted as excessive carry-over and require validation by the supplier to the end user.

Due to the mechanics of atomized liquid filtration, the efficiency of wet units is linked to the specific gravity of the extracted dust as well as dust particle size distribution. A SG of 2,500kg/m3 and typical particle sizes >10 micron are usually required to achieve acceptable efficiencies as stated above, and this can rule out wet collector application in some industries.

Filtration efficiency based solely on particle size is limited when compared to dry dust units. Typically, the mean particle sizes in a wet dust extractor need to be >25 microns to achieve the same levels of efficiency as dry dust extractors handling dusts at 5 microns and less. Therefore careful consideration of required filtration efficiency and dust particle size distribution should be applied before selecting a wet dust extractor. The main applications for wet filtration are based around mechanically generated dusts such as polishing, linishing, grinding, crushing and fettling - typically in the metals and stone masonry sectors as the particle sizes and specific gravity of dusts generated from these processes generally fall within the capabilities of the equipment.