Venturi scrubbers are used for the removal of fine particulate. Gas is accelerated at a high speed through a Venturi throat. Water is injected perpendicular to the gas flow. The large water drops injected into the gas stream collide with the fine particulate through a process called impaction.
The efficiency of this process is dependent primarily on the size and velocity of the particulate. Superfine, sub-micron particulate are able to follow a stream line around the water drops and are not collected. Micron-size and larger particulate are not able to slip around the water drops fast enough due to inertial effects. The exact "cut" of the Venturi depends on the velocity; smaller particles are captured at higher gas velocities. Venturi scrubbers are excellent particulate control devices for particulate at or above a micron in size.
The performance of a Venturi though is dependent on maintaining the gas velocity at design conditions. The above video details a Venturi scrubber with an adjustable throat. In most industrial applications, the gas flow rate varies and with a fixed opening, the velocity of the gas through the Venturi would vary as well, impacting performance. An adjustable throat offers one method for ensuring the Venturi works over a wide range of operating conditions.
The adjustable throat is a damper blade controlled by a positioner. The blade is positioned to maintain a constant pressure drop across the Venturi. The pressure drop across the Venturi is directly related to the gas velocity. Essentially, the damper blade maintains the gas velocity in the Venturi even at much lower gas flow rates. As stated above, gas velocity is critical to particulate removal in a Venturi. An adjustable throat ensures that the gas velocity remains constant, so that particulate removal is unaffected by operation in a "real" environment.