Annual Costs

The total annual cost of a wet scrubber consists of both direct and indirect costs. Direct annual
costs are those associated with the operation and maintenance of the scrubber. These include labor
(operating, supervisory, coordinating, and maintenance), maintenance materials, operating materials, electricity, sludge disposal, waste water treatment, and conditioning agents. Heating and cooling may
be required in some climates to prevent freezing or excessive vaporation loss of the scrubbing liquid.

Indirect annual costs include taxes, insurance, administrative costs, overhead, and capital
recovery. All of these costs except overhead are dependent on the TCI. Table 1 lists the
parameters that impact wet scrubber annual costs with typical values provided for each parameter.
Table 2 provides the annual cost factors for scrubbers. Annual costs for scrubbers are difficult to
generalize because these costs are very site-specific.

Table 1.Annual Cost Parameters for Particulate Scrubbers

Table 2. Annual Cost Factors for Particulate Scrubbers

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Capital cost for Packed-bed Scrubbers

The costs for packed-bed scrubbers depend on the inlet gas velocity/column diameter, orientation of the column (vertical vs. horizontal), height of packing material, and the presence of any auxiliary equipment. Figures 1 and 2 present costs curves for two types of packed-bed scrubbers. Figure 1 presents a cost curve for a small vertical column packed-bed scrubber. The costs for this unit vary with the column diameter, which can range from 1 to 2.5 feet. Gas flowrates range from 200 to 1200 ACFM. 

For Figure 1, the scrubber is assumed to be constructed of FRP with 6 feet of polypropylene packing. Costs also include the costs for a spray nozzle, liquid distributor, and mist eliminator. Figure 2 provides a cost curve for a large packed-bed scrubber with horizontal gas flow from 800 to 80,000 ACFM. Costs for this unit are based on the use of PVC or FRP construction materials and a design that includes a spray section, a 1-foot packed bed, and a mist eliminator

Figure 1. Vertical Packed-bed Scrubber Capital Costs

Figure 2. Horizontal Packed-bed Scrubber Capital Costs

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Capital cost for Impingement Plate Scrubbers

Impingement plate scrubber costs are dependent on the number
of plates and the total gas flowrate. The costs for impingement scrubbers are based on data that
corresponds to a total gas flowrate between 900 and 77,000 ACFM or above. For total gas flowrates
above 77,000 ACFM, multiple scrubbers are required. Figure 1 presents cost curves for
impingement plate scrubbers with total gas flowrates between 900 and 77,000 ACFM. Cost curves
for scrubbers with total flowrates above 77,000 ACFM are shown in Figure 2 and require the
use of 2, 3, or 4 identical scrubber units. All the cost correlations shown here are for sieve plate
scrubbers with three plates. Impingement plate scrubbers are usually constructed with carbon steel.
Some applications may require more expensive materials, such as coated carbon steel, FRP, or
polyvinyl chloride (PVC)

Figure 1. Impingement Scrubber Capital Costs, Inlet Flowrate <77,000 ACFM

Figure 2. Impingement Scrubber Capital Costs, Inlet Flowrate >77,000 ACFM

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Capital cost for Venturi Scrubbers

Venturi scrubber costs are based on data for two ranges of gas flowrates. Cost curves for scrubbers treating less than 19,000 ACFM are provided in Figure 1. Cost curves for venturi scrubbers capable of handling greater than 19,000 ACFM but less than 59,000 ACFM are shown in Figure 2. For total flowrates greater than 59,000 ACFM, the gas stream should be divided evenly and treated by two or more identical scrubbers (with inlet flowrates of <59,000 ACFM) operating in parallel.

The most common construction material for venturi scrubbers is carbon steel. Special applications may require other materials, such as rubber-lined steel, epoxy-coated steel, fiberreinforced plastic (FRP), that will increase the cost of the unit. Separate cost curves for carbon steel and other specialized materials are included in Figures 1 and 2.

Figure 1. Venturi Scrubber Capital Costs, Inlet Flowrate < 19,000 ACFM

Figure 2. Venturi Scrubber Capital Costs, Inlet Flowrate >19,000 ACFM, <59,000 ACFM

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Capital Costs

The total capital investment (TCI) for scrubbers includes all of the initial capital costs, both
direct and indirect. Direct capital costs are the purchased equipment costs (PEC), and the costs of
installation (foundations, electrical, piping, etc.). Indirect costs are related to the installation and include engineering, construction, contractors, start-up, testing, and contingencies. The PEC is calculated based on the scrubber specifications. The direct and indirect installation costs are calculated as factors of the PEC. Table 1 provides the TCI factors for a typical scrubber.

Table 1, Capital Cost Factors for a Typical Scrubber

Wet scrubber costs are dependent upon the type of scrubber selected, the required size of the
scrubber, and the materials of construction. Scrubber sizing incorporates several design parameters,
including gas velocity, liquid-to-gas ratio, and pressure drop. Gas velocity is the primary sizing factor. Increasing the gas velocity will decrease the required size and cost of a scrubber. However, pressure drop will increase with increasing gas velocity. This will also result in increased electricity consumption and, therefore, higher operating costs. Determining the optimum gas velocity involves balancing the capital and annual costs. In most cases, scrubbers are designed to operate within recommended ranges of gas velocity, liquid-to-gas ratio, and pressure drop. These ranges are provided in Table 2.

Table 2. Recommended Gas Velocities, Liquid/Gas Ratios,

and Pressure Drops for Particulate Wet Scrubbers

Another important scrubber parameter that affects costs is the temperature of the gas stream at
saturation once it has been cooled by the scrubber liquid. This temperature affects the volumetric
flowrate of the outlet gas and, consequently, the size of the scrubber. In addition, the saturation
temperature impacts the scrubbing liquid makeup and the wastewater flowrate. The saturation
temperature is a complex function of essentially three variables: the temperature of the inlet gas stream, the absolute humidity of the inlet gas stream, and the absolute humidity at saturation. Typically, the saturation temperature is determined graphically from a psychometric chart once these three variables are known. For this document, the sizing and costing of wet scrubbers were aided by the use of the CO$T-AIR Control Cost Spreadsheets, that employ an iterative procedure for estimating the saturation temperature.

Once a scrubber has been properly designed and sized, the costs can generally be expressed as
a function of the inlet or total gas flowrate. Cost curves are shown below for the following types of
scrubbers: venturi, impingement plate, and packed tower.

All the estimates for scrubber capital costs have been escalated to third quarter 1995 dollars.
However, the capital costs presented in this section can be escalated further to reflect more current
values through the use of the Vatavuk Air Pollution Cost Control Indexes (VAPCCI), which are
updated quarterly, available on the OAQPS Technology Transfer Network (TTN), and published
monthly in Chemical Engineering magazine. The VAPCCI updates the PEC and, since capital costs
are based only on the PEC, capital costs can be easily adjusted using the VAPCCI. To escalate
capital costs from one year (Costold) to another more recent year (Costnew), a simple proportion can be used, as follows

The VAPCCI for wet scrubbers for third quarter 1995 was 114.7.

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Costs of PM Wet Scrubbers

The costs of installing and operating a scrubber include both capital and annual costs. Capital
costs are all of the initial costs related to scrubber equipment and installation. Annual costs are the
direct yearly costs of operating the scrubber, plus indirect costs such as overhead, capital recovery,
taxes, insurance, and administrative charges. The following sections discuss capital and annual costs forscrubbers, referenced to the third quarter of 1995 unless otherwise noted.

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Applicability

Wet scrubbers have numerous industrial applications and few limitations. They are capable of collecting basically any type of dust, including flammable, explosive, moist, or sticky dusts. In addition, they can collect suspended liquids (i.e. mists) or gases alone or with PM simultaneously. However, while scrubbers have many potential applications, there are some characteristics that limit their use. The most significant consideration is the relatively low collection efficiency for fine PM, especially those less than 1.0 :m in diameter. Therefore, conventional scrubbers may not be suitable for processes which emit many submicrometer particles. As discussed above venturi, condensation, and charged scrubbers are capable of collecting submicrometer particles at higher efficiencies than other scrubbers and, therefore, can be used effectively in applications where there are a large percentage of fine PM in the gas stream.

Gas stream composition may also be a limiting factor in scrubber application for a specific industry, since wet scrubbers are very susceptible to corrosion. The use of wet scrubbers also may not be desirable when collecting valuable dust which can be recycled or sold. Since scrubbers discharge collected dust in the form of a wet slurry, reclaiming clean dry dust from this slurry is often inconvenient and expensive. Because of design constraints, particulate scrubbers are generally not used in very large installations, such as utilities where gas flowrates exceed 250,000 ACFM, since multiple scrubbers are needed once flowrates exceed 60,000-75,000 ACFM.

Table 1 lists current applications of wet scrubbers.It should be noted that the level of PM control supplied by each of the scrubber types listed in Table 1  will vary according to the level of control currently required by each industry and/or facility. The driving

force for PM control in many industries and/or facilities is the Federal, State, and local air pollution

regulations. As more stringent PM regulations are put into place, a shift toward the use of higher

efficiency scrubbers is likely to occur. Table 2 rates the various scrubber types according to their

potential for controlling fine particles

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Collection Efficiency

Collection efficiencies for wet scrubbers are highly variable. Most conventional scrubbers can achieve high collection efficiencies for particles greater than 1.0 :m in diameter, however they are generally ineffective collection devices for submicrometer (<1 μm) particles. Some unconventional scrubbers, such as condensation and charged, are capable of high collection efficiencies, even for submicrometer particles. Collection efficiencies for conventional scrubbers depend on operating factors such as particle size distribution, inlet dust loading, and energy input. Figure 1 provides scrubber efficiency curves for coal and oil combustion, wood combustion, and coke production. Table 1 presents the PM-10 and PM-2.5 collection efficiencies.

Conventional scrubbers rely almost exclusively on inertial impaction for PM collection. As discussed above, scrubber efficiency that relies on inertial impaction collection mechanisms will increase as particle size increases. Therefore, collection efficiency for small particles (<1 μm) are expected to be low for these scrubbers. The efficiency of scrubbers that rely on inertial impaction can be improved, however, by increasing the relative velocity between the PM and the liquid droplets. Increasing velocity will result in more momentum for all PM, enabling smaller particles to be collected by impaction. This can be accomplished in most scrubbers by increasing the gas stream velocity. Unfortunately, increasing the gas velocity will also increase the pressure drop, energy demand, and operating costs for the scrubber.

Another factor which contributes to low scrubber efficiency for small particles is short residence times. Typically, a particle is in the contact zone of a scrubber for only a few seconds. This is sufficient time to collect large particles that are affected by impaction mechanisms. However, since submicrometer particles are most effectively collected by diffusion mechanisms that depend on the random motion of the particles, sufficient time in the contact zone is needed for this mechanism to be effective. Consequently, increasing the gas residence time should also increase the particle/liquid contact time and the collection efficiency for small particles.

An important relationship between inlet dust concentration (loading) and collection efficiency for fine PM in scrubbers has been recently found. Collection efficiency for scrubbers has been found to be directly proportional to the inlet dust concentration. That is, efficiency will increase with increasing dust loading. This suggests that scrubber removal efficiency is not constant for a given scrubber design unless it is referenced to a specific inlet dust loading. In contrast, it has been shown that scrubber outlet dust concentration is a constant, independent of inlet concentration.

Cumulative collection efficiency for PM wet scrubber at coal, oil, wood, and bark combustion source and coke production operation

 Table 1. PM-10 and PM-2.5 Cumulative Collection Efficiencies for Wet Scrubbers at Coal, Oil, Wood, and Bark Combustors; and Coke Production Units

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Fiber-Bed Scrubbers

In a fiber-bed scrubbers, the moisture-laden gas stream passes through mats of packing fibers,

such as spun glass, fiberglass, and steel. The fiber mats are often also spray wetted with the scrubbing

liquid. Depending on the scrubber requirements, there may be several fiber mats and an impingement

device for PM removal included in the design. The final fiber mat is typically dry for the removal of any droplets that are still entrained in the stream. Fiber-bed scrubbers are best suited for the collection of soluble PM, i.e. PM that dissolves in the scrubber liquid, since large amounts of insoluble PM will clog the fiber mats with time. For this reason, fiber-bed scrubbers are more often used as mist eliminators, i.e., for the collection of liquids, rather than for PM control.

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Charged Scrubbers

Charged, or electrically-augmented, wet scrubbers utilize electrostatic effects to improve
collection efficiencies for fine PM with wet scrubbing. Since conventional wet scrubbers rely on the
inertial impaction between PM and liquid droplets for PM collection, they are generally ineffective for particles with diameters less than 1 :m. Pre-charging of the PM in the gas stream can significantly
increase scrubber collection efficiency for these submicrometer particles. When both the particles and
droplets are charged, collection efficiencies for submicrometer particles are highest, approaching that of an ESP.
There are several types of charged wet scrubbers. Particulate matter can be charged negatively
or positively, with the droplets given the opposite charge. The droplets may also be bipolar (a mixture
of positive and negative). In this case, the PM can be either bipolar or unipolar. Figure  is a
schematic of a charged wet scrubber.

Schematic diagram of a charged wet scrubber

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Condensation Scrubbers

Condensation scrubbing is a relatively recent development in wet scrubber technology. Most
conventional scrubbers rely on the mechanisms of impaction and diffusion to achieve contact between the PM and liquid droplets. In a condensation scrubber, the PM act as condensation nuclei for the formation of droplets. Generally, condensation scrubbing depends on first establishing saturation conditions in the gas stream. Once saturation is achieved, steam is injected into the gas stream. The steam creates a condition of supersaturation and leads to condensation of water on the fine PM in the gas stream. The large condensed droplets can be removed by several conventional devices. Typically, a high efficiency mist eliminator is also used.

A high-efficiency condensation "growth" PM scrubber has been developed that is suitable for
both new and retrofit installations, and is designed specifically to capture fine PM that escapes primary PM control devices. This type of scrubber utilizes a multistage process, including pretreatment and growth chambers, that provide an environment that encourages the fine PM to coagulate and form larger particles. A schematic diagram of this scrubber is provided in Figure

Schematic diagram of condensation growth scrubbers 

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Orifice Scrubbers

Orifice scrubbers, also known as entrainment or self-induced spray scrubbers, force the
particle-laden gas stream to pass over the surface of a pool of scrubbing liquid as it enters an orifice.
With the high gas velocities typical of this type of scrubber, the liquid from the pool becomes entrained in the gas stream as droplets. As the gas velocity and turbulence increases with the passing of the gas through the narrow orifice, the interaction between the PM and liquid droplets also increases.
Particulate matter and droplets are then removed from the gas stream by impingement on a series of
baffles that the gas encounters after the orifice. The collected liquid and PM drain from the baffles back into the liquid pool below the orifice. Orifice scrubbers can effectively collect particles larger than 2 :m in diameter. Some orifice scrubbers are designed with adjustable orifices to control the velocity of the gas stream. A typical orifice scrubber is shown in Figure

Diagram of an orifice scrubber

Orifice scrubbers usually have low liquid demands, since they use the same scrubbing liquid for
extended periods of time. Because orifice scrubbers are relatively simple in design and usually have
few moving parts, the major maintenance concern is the removal of the sludge which collects at the
bottom of the scrubber. Orifice scrubbers rarely drain continually from the bottom because a static
pool of scrubbing liquid is needed at all times. Therefore, the sludge is usually removed with a sludge
ejector that operates like a conveyor belt. As the sludge settles to the bottom of the scrubber, it lands
on the ejector and is conveyed up and out of the scrubber. Figure  shows a typical sludge ejector

Diagram of a sludge ejector in an orifice scrubber

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Venturi Scrubbers

A venturi, or gas-atomized spray, scrubber accelerates the gas stream to atomize the scrubbing
liquid and to improve gas-liquid contact. In a venturi scrubber, a "throat" section is built into the duct
that forces the gas stream to accelerate as the duct narrows and then expands. As the gas enters the
venturi throat, both gas velocity and turbulence increase. The scrubbing liquid is sprayed into the gas
stream before the gas encounters the venturi throat. The scrubbing liquid is then atomized into small
droplets by the turbulence in the throat and droplet-particle interaction is increased. After the throat
section in a venturi scrubber, the wetted PM and excess liquid droplets are separated from the gas
stream by cyclonic motion and/or a mist eliminator. Venturi scrubbers have the advantage of being
simple in design, easy to install, and with low-maintenance requirements. An example of a venturi
scrubber is provided in Figure

Schematic diagram of venturi scrubber with cyclonic separation  

The performance of a venturi scrubber is dependent to some extent on the velocity of the gas
through the throat. Several venturi scrubbers have been designed to allow velocity control by varying
the width of the venturi throat. Because of the high interaction between the PM and droplets, venturi
scrubbers are capable of high collection efficiencies for small PM. Unfortunately, increasing the venturi scrubber efficiency requires increasing the pressure drop which, in turn, increases the energy
consumption.

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Mechanically-aided Scrubbers

Mechanically-aided scrubbers (MAS) employ a motor driven fan or impeller to enhance gasliquid
contact. Generally in MAS, the scrubbing liquid is sprayed onto the fan or impeller blades. Fans
and impellers are capable of producing very fine liquid droplets with high velocities. These droplets are effective in contacting fine PM. Once PM has impacted on the droplets, it is normally removed by
cyclonic motion. Mechanically aided scrubbers are capable of high collection efficiencies, but only with a commensurate high energy consumption. An example of a mechanically aided scrubber is provided in Figure

Because many moving parts are exposed to the gas and scrubbing liquid in a MAS, these
scrubbers have high maintenance requirements. Mechanical parts are susceptible to corrosion, PM
buildup, and wear. Consequently, mechanical scrubbers have limited applications for PM control.

Diagram of a Mechanically-aided Scrubbers

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Impingement Plate Scrubbers

An impingement plate scrubber is a vertical chamber with plates mounted horizontally inside a
hollow shell. Impingement plate scrubbers operate as countercurrent PM collection devices. The
scrubbing liquid flows down the tower while the gas stream flows upward. Contact between the liquid and the particle-laden gas occurs on the plates. The plates are equipped with openings that allow the gas to pass through. Some plates are perforated or slotted, while more complex plates have valve-like openings. Figure 1 shows common plate designs used in impingement plate scrubbers.

Common plate design for Impingement Plate Scrubbers

The simplest impingement plate is the sieve plate, which has round perforations. In this type of
scrubber, the scrubbing liquid flows over the plates and the gas flows up through the holes. The gas
velocity prevents the liquid from flowing down through the perforations. Gas-liquid-particle contact is achieved within the froth generated by the gas passing through the liquid layer. Complex plates, such as
bubble cap or baffle plates, introduce an additional means of collecting PM. The bubble caps and
baffles placed above the plate perforations force the gas to turn before escaping the layer of liquid.
While the gas turns to avoid the obstacles, most PM cannot and is collected by impaction on the caps
or baffles. Bubble caps and the like also prevent liquid from flowing down the perforations if the gas
flow is reduced.
In all types of impingement plate scrubbers, the scrubbing liquid flows across each plate and
down the inside of the tower onto the plate below. After the bottom plate, the liquid and collected PM
flow out of the bottom of the tower. A typical impingement plate scrubber is shown in Figure 2
Impingement plate scrubbers are usually designed to provide operator access to each tray, making
them relatively easy to clean and maintain. Consequently, impingement plate scrubbers are more
suitable for PM collection than packed-bed scrubbers. Particles greater than 1 :m in diameter can be
collected effectively by impingement plate scrubbers, but many particles <1 μm will penetrate these
devices.

Schematic diagram of a plate tower scrubber

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Packed-Bed Scrubbers

Packed-bed scrubbers consist of a chamber containing layers of variously-shaped packing
material, such as raschig rings, spiral rings, and berl saddles, that provide a large surface area for liquidparticle contact. These and other types of packings are illustrated in Figure 1 The packing is
held in place by wire mesh retainers and supported by a plate near the bottom of the scrubber.
Scrubbing liquid is evenly introduced above the packing and flows down through the bed. The liquid
coats the packing and establishes a thin film. In vertical designs, the gas stream flows up the chamber
(countercurrent to the liquid). Some packed beds are designed horizontally for gas flow across the
packing (crosscurrent).

Typical packing materials for packed bed scrubbers

In packed-bed scrubbers, the gas stream is forced to follow a circuitous path through the
packing, on which much of the PM impacts. The liquid on the packing collects the PM and flows down the chamber towards the drain at the bottom of the tower. A mist eliminator (also called a "de-mister") is typically positioned above/after the packing and scrubbing liquid supply. Any scrubbing liquid and wetted PM entrained in the exiting gas stream will be removed by the mist eliminator and returned to drain through the packed bed. A typical packed-bed scrubber is illustrated in Figure 2

Schematic Diagram for packed tower scrubber

In a packed-bed scrubber, high PM concentrations can clog the bed, hence, the limitation of
these devices to streams with relatively low dust loadings. Plugging is a serious problem for packedbed scrubbers because the packing is more difficult to access and clean than other scrubber designs. Mobile-bed scrubbers are available that are packed with low-density plastic spheres that are free to move within the packed bed. These scrubbers are less susceptible to plugging because of the
increased movement of the packing material. In general, packed-bed scrubbers are more suitable for
gas scrubbing than particulate scrubbing because of the high maintenance requirements for control of
PM

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Spray Chambers

Spray chambers are very simple, low-energy wet scrubbers. In these scrubbers, the
particulate-laden gas stream is introduced into a chamber where it comes into contact with liquid
droplets generated by spray nozzles. These scrubbers are also known as pre-formed spray scrubbers,
since the liquid is formed into droplets prior to contact with the gas stream. The size of the droplets
generated by the spray nozzles is controlled to maximize liquid-particle contact and, consequently,
scrubber collection efficiency.
The common types of spray chambers are spray towers and cyclonic chambers. Spray towers
are cylindrical or rectangular chambers that can be installed vertically or horizontally. In vertical spray
towers, the gas stream flows up through the chamber and encounters several sets of spray nozzles
producing liquid droplets. A de-mister at the top of the spray tower removes liquid droplets and
wetted PM from the exiting gas stream. Scrubbing liquid and wetted PM also drain from the bottom of
the tower in the form of a slurry. Horizontal spray chambers operate in the same manner, except for
the fact that the gas flows horizontally through the device. A typical spray tower is shown in
Figure 1

Schematic diagram of  spray tower scrubber

A cyclonic spray chamber is similar to a spray tower with one major difference. The gas
stream is introduced to produce cyclonic motion inside the chamber. This motion contributes to higher
gas velocities, more effective particle and droplet separation, and higher collection efficiency.1
Tangential inlet or turning vanes are common means of inducing cyclonic motion. Figure 2
provides an example of a cyclonic spray chamber.

Schematic Diagram of Cyclic spray chamber scrubber

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Types of Wet Scrubbers

There are a great variety of wet scrubbers that are either commercially available or can be
custom designed. While all wet scrubbers are similar to some extent, there are several distinct methods of using the scrubbing liquid to achieve particle collection. Wet scrubbers are usually classified according to the method that is used to contact the gas and the liquid.

The most common scrubber design is the introduction of liquid droplets into a spray chamber,
where the liquid is mixed with the gas stream to promote contact with the PM. In a packed-bed
scrubber, layers of liquid are used to coat various shapes of packing material that become impaction
surfaces for the particle-laden gas. Scrubber collection can also be achieved by forcing the gas at high
velocities though a liquid to form jet streams. Liquids are also used to supersaturate the gas stream,
leading to particle scrubbing by condensation.

1.Spray Chambers
2.Packed-Bed Scrubber
3.Impingement Plate Scrubbers
4.Mechanically- Aided Scrubbers
5.Venturi Scrubbers
6.Orifice Scrubbers
7.Condensation Scrubbers
8.Charged Scrubbers
9.Fiber Bed Scrubbers

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Particle Collection and Penetration Mechanisms

The dominant means of PM capture in most industrial wet scrubbers is inertial impaction of the
PM onto liquid droplets. Brownian diffusion also leads to particle collection, but its effects are only
significant for particles approximately 0.1 micrometer (:m) in diameter or less. Direct interception is
another scrubber collection mechanism. Less important scrubber collection mechanisms utilize
gravitation, electrostatics, and condensation.
Inertial impaction in wet scrubbers occurs as a result of a change in velocity between PM
suspended in a gas, and the gas itself. As the gas approaches an obstacle, such as a liquid droplet, the
gas changes direction and flows around the droplet. The particles in the gas will also accelerate and
attempt to change direction to pass around the droplet. Inertial forces will attempt to maintain the
forward motion of the particle towards the object, but the fluid force will attempt to drag the particle
around the droplet with the gas. The resultant particle motion is a combination of these forces of fluid
drag and inertia. This results in impaction for the particles where inertia dominates, and by-pass for
those particles overwhelmed by fluid drag. Large particles, particles i.e. greater than 10 μm are more
easily collected by inertial impaction because these particles have more inertial momentum to resist
changes in the flow of the gas and, therefore, impact the droplet. Small particles (i.e. particles <1 μm)
are more difficult to collect by inertial impaction because they remain in the flow lines of the gas due to the predominance of the fluid drag force.
Collection by diffusion occurs as a result of both fluid motion and the Brownian (random)
motion of particles. This particle motion in the scrubber chamber results in direct particle-liquid
contact. Since this contact is irreversible, collection of the PM by the liquid occurs. Diffusional collection effects are most significant for particles less than 0.1 μm in diameter. Direct interception
occurs when the path of a particle comes within one radius of the collection medium, which in a
scrubber is a liquid droplet. The path can be the result of inertia, diffusion, or fluid motion.
Gravitational collection as a result of falling droplets colliding with particles is closely related to
impaction and interception, and is a minor mechanism in some scrubbers. Gravitational settling of
particles is usually not a factor because of high gas velocities and short residence times. Generally,
electrostatic attraction is not an important mechanism except in cases where the particles, liquid, or
both, are being deliberately charged, or where the scrubber follows an electrostatic precipitator.
Some scrubbers are designed to enhance particle capture through condensation. In such cases, the
dust-laden stream is supersaturated with liquid (usually water). The particles then act as condensation
nuclei, growing in size as more liquid condenses around them and becoming easier to collect by inertial impaction.
The collection mechanisms of wet scrubbers are highly dependent on particle size. Inertial
impaction is the major collection mechanism for particles greater than approximately 0.1 :m in
diameter. The effectiveness of inertial impaction increases with increasing particle size. Diffusion is
generally effective only for particles less than 0.1 :m in diameter, with collection efficiency increasing with decreasing particle size. The combination of these two major scrubber collection mechanisms contributes to a minimum collection efficiency for PM approximately 0.1 :m in diameter.
 The exact minimum efficiency for a specific scrubber will depend on the type of scrubber, operating conditions, and the particle size distribution in the gas stream.

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WET SCRUBBERS

Wet scrubbers are PM control devices that rely on direct and irreversible contact of a liquid
(droplets, foam, or bubbles) with the PM. The liquid with the collected PM is then easily collected.
Scrubbers can be very specialized and designed in many different configurations. Wet scrubbers are
generally classified by the method that is used to induce contact between the liquid and the PM, e.g.
spray, packed-bed, plate. Scrubbers are also often described as low-, medium-, or high-energy,
where energy is often expressed as the pressure drop across the scrubber. This section addresses the
basic operating principles, designs, collection efficiency, applicability, and costs of wet scrubbers.
Wet scrubbers have important advantages when compared to other PM collection devices.
They can collect flammable and explosive dusts safely, absorb gaseous pollutants, and collect mists.
Scrubbers can also cool hot gas streams. There are also some disadvantages associated with wet
scrubbers. For example, scrubbers have the potential for corrosion and freezing. Additionally, the use
of wet scrubbers can lead to water and solid waste pollution problems. These disadvantages can be
minimized or avoided with good scrubber design.

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