Tableware: Venturi scrubber

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Figure 1 - Venturi scrubber
A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively referred to as wet scrubbers.
Venturi devices have also been used for over 100 years to measure fluid flow (Venturi tubes derived their name from Giovanni Battista Venturi, an Italian physicist).
About 35 years ago, Johnstone (1949) and other researchers found that they could effectively use the venturi configuration to remove particles from gas streams. Figure 1 illustrates the classic venturi configuration.[1]
A venturi scrubber consists of three sections: a converging section, a throat section, and a diverging section. The inlet gas stream enters the converging section and, as the area decreases, gas velocity increases (in accordance with the Bernoulli equation). Liquid is introduced either at the throat or at the entrance to the converging section.
The inlet gas, forced to move at extremely high velocities in the small throat section, shears the liquid from its walls, producing an enormous number of very tiny droplets.
Particle and gas removal occur in the throat section as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section, where it is forced to slow down.
Venturis can be used to collect both particulate and gaseous pollutants, but they are more effective in removing particles than gaseous pollutants.

Figure 2 - Wetted throat venturi scrubber
Liquid can be injected at the converging section or at the throat. Figure 2 shows liquid injected at the converging section.[1] Thus, the liquid coats the venturi throat making it very effective for handling hot, dry inlet gas that contains dust. Otherwise, the dust would have a tendency to cake on or abrade a dry throat. These venturis are sometimes referred to as having a wetted approach.
Figure 3 shows liquid injected at the venturi throat.[1] Since it is sprayed at or just before the throat, it does not actually coat the throat surface. These throats are susceptible to solids buildup when the throat is dry. They are also susceptible to abrasion by dust particles. These venturis are best used when the inlet stream is cool and moist. These venturis are referred to as having a non-wetted approach.
Venturis with round throats (Figures 2 and 3) can handle inlet flows as large as 88,000 m3/h (40,000 cfm) (Brady and Legatski 1977). At inlet flow rates greater than this, achieving uniform liquid distribution is difficult, unless additional weirs or baffles are used. To handle large inlet flows, scrubbers designed with long, narrow, rectangular throats (Figure 4) have been used.[1]

Figure 3 - Non-wetted throat venturi scrubber
Simple venturis have fixed throat areas and cannot be used over a wide range of gas flow rates. Manufacturers have developed other modifications to the basic venturi design to maintain scrubber efficiency by changing the throat area for varying inlet gas rates.
Certain types of orifices (throat areas) that create more turbulence than a true venturi were found to be equally efficient for a given unit of energy consumed (McIlvaine Company 1974).
Results of these findings led to the development of the annular-orifice, or adjustable-throat, venturi scrubber (Figure 5).[1] The size of the throat area is varied by moving a plunger, or adjustable disk, up or down in the throat, thereby decreasing or increasing the annular opening. Gas flows through the annular opening and atomizes liquid that is sprayed onto the plunger or swirled in from the top.
Another adjustable-throat venturi is shown in Figure 6.[1] In this scrubber, the throat area is varied by using a movable plate. A water-wash spray is used to continually wash collected material from the plate.
Another modification can be seen in the venturi-rod or rod deck scrubber. By placing a number of pipes parallel to each other, a series of longitudinal venturi openings can be created as shown in Figure 7.[1] The area between adjacent rods is a small venturi throat.

Figure 4 - Rectangular throat venturi scrubber
Water sprays help prevent solids buildup. The principal atomization of the liquid occurs at the rods, where the high-velocity gas moving through spacings creates the small droplets necessary for fine particle collection. These rods must be made of abrasion-resistant material due to the high velocities present.
All venturi scrubbers require an entrainment separator because the high velocity of gas through the scrubber will have a tendency to entrain the droplets with the outlet clean gas stream.
Cyclonic, mesh-pad, and blade separators are all used to remove liquid droplets from the flue gas and return the...(and so on)