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216 Lawrence K. Wang et al.

As soon as the water is atomized, it collects particles by impaction of the particles on
the water droplets. This impaction process is possible as the result of the difference in
velocities between gas (high) and the water droplets (slow). As the gas–water droplet
mixture passes out of the throat, the velocities of both gas and water droplets decelerate.
At this point, further impaction causes the water droplets to reform into the bulk liquid
phase. Particles that were captured by water droplets in the throat will remain in the bulk
liquid phase. The water is then sent to a separator, where it is separated from the clean
gas stream (18–21).
2.5.2. Venturi Scrubber Design Variables
The temperature of the gas stream entering a Venturi scrubber needs to be held to
50–100°F above the dew point of the gas stream. The determination of the dew point of
a gaseous flow is explained elsewhere in this handbook. Cooling or heating is called for
if the gaseous stream needing treatment is not within the temperature range stated. If
such temperature adjustment of the gas stream is needed, then the physical properties of
the stream will be altered. A Venturi scrubber’s primary design parameters are the sat-
urated gas flow rate, Q , which is a direct function of the temperature of the gas
e,s
stream, the flow rate at actual conditions (Q ), particle size distribution in the pollut-
e,a
ed gas stream, and throat size (18, 22). If the temperature of the gas stream is changed
in a pre-heating or precooling step, the actual flow rate of the gas will also be changed.
This new actual flow rate will, in turn, affect the saturated flow rate. Another pretreat-
ment step that may be required is mechanical dust collection. If large particulate matter
is present in the gas stream being cleansed, this pretreatment may be appropriate.
When designing a Venturi scrubber, three choices become apparent:

1. Rely on a previous design for a similar or identical application.
2. Do a pilot test on the air to be cleansed.
3. Collect empirical data about the air stream to be treated (particle size distribution, flow rate,
temperature) to allow for use of published performance curves for a given Venturi system.

The first choice, although simple and direct, runs the risk of missing recent advances
within the industry. The second option is time-consuming but will provide for a result
that is based on timely data. The third option has the advantage of using industry data
to reach an advanced design in a timely fashion. Therefore option 3 is discussed here.
The primary consideration in this design will be pressure drop in the Venturi scrubber;
a secondary consideration will be construction materials (9).
2.5.3. Venturi Scrubber Sizing
AVenturi scrubber must be sized after the decision is made that this technology is
the best fit for the air pollution problem at hand (see Fig. 1c). A Venturi scrubber may
be sized using the airflow at inlet conditions (Q ) or the saturated airflow rate (Q )
e,a e,s
may be used for sizing calculations. Venturi scrubber original equipment manufacturers
(OEMs) use either method, based on their own preferences. Cost data are generally
based on emission stream flow rate at inlet conditions, Q . The saturated emission
e,a
stream flow rate, Q , can be found as shown below (9,20). Psychometric charts (Fig.
e,s
4) are available to determine saturated air temperature (T ).
e,a
Q is then determined:
e,s