A venturi scrubber passes dirty flue gas through a converging nozzle into a narrow throat at high velocity (150 to 350 ft/s / 45 to 105 m/s). Water injected at the throat is atomized by extreme aerodynamic shear into millions of microscopic droplets (30 to 80 μm). Because gas decelerates around droplets while high-inertia dust particles travel straight, the particles impact into the water droplets and are captured via inertial impaction.

Higher pressure drop yields greater relative gas-liquid velocity and smaller atomized water droplets, which dramatically increases inertial impaction efficiency for fine particles (PM2.5 and sub-micron dust). However, fan electrical power increases linearly with pressure drop. Sizing balances required EPA emission limits against operating electricity costs.

Developed by Seymour Calvert in EPA-sponsored research, the cut-diameter (d50) is the aerodynamic particle diameter collected at exactly 50% efficiency. Particles larger than d50 are captured at 90% to 99%+ efficiency, while particles smaller than d50 penetrate the scrubber unless throat velocity or liquid-to-gas ratio is increased.

In boilers, incinerators, and furnaces, gas flow rates vary with production output. In a fixed throat, a 20% drop in airflow reduces throat velocity and drops pressure drop by ~36% (ΔP ∝ v²), causing collection efficiency to collapse. An adjustable throat damper modulates throat cross-sectional area to maintain constant gas velocity and pressure drop regardless of gas volume swings.

The venturi throat merely impacts dust particles into liquid droplets; it does not remove the droplets from the airstream. The downstream cyclonic separator imparts centrifugal force to spin the heavy water-dust slurry droplets against the outer casing wall, where they drain out the bottom while clean, droplet-free exhaust gas exits out the top.