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Mechanically Aided Scrubbers

Device Type

Mechanically aided scrubbers are defined as devices that use a moving mechanism in the gas stream to achieve the desired particulate or contaminant gas removal.

Mechanically aided scrubbers are generally used for dust control involving particulate larger than approximately 10 pm diameter and at low loadings (below 5 grains per dry standard cubic foot). They have been used to control dusts from loading and unloading facilities, fugitive dusts from storage facilities, wet coating operations, and numerous other applications. They were particularly popular in the 1970s in the mining industry for dust control where a wet product was required and are still used for that purpose. They are noted for their low cost, compact size, and reliability. Ever-tightening codes have shifted the focus to low- to medium-energy Venturi scrubbers on many applications that had been dominated by mechanically aided designs.

Typical Applications and Uses

These designs are used to control high-dust loadings of relatively large dust where it is desirable to recover the dust wet. Controlling dust from conveyors, blenders, mullers, mills, and other high-dust-loading sources are areas where this type of scrubber has been used. In these applications, the dust is usually returned to the process as wet slurry or is separated in a pond or clarifier.

They are effective on particles 10 pm or larger. They are not used as much where the particulate is hygroscopic and may buildup on the scrubber inlet. Similarly, they are not used for gas absorption, although they may be used ahead of a gas absorption device (such as a packed tower).

These scrubbers have been applied to the control of emissions from dissolving tank (smelt tank) vents in the paper industry. Some have been enhanced through the addition of a Venturi reverse spray scrubber ahead of the mechanically aided scrubber. Some recovery boiler suppliers routinely supply mechanically aided scrubbers for use on the dissolving tank vents. In recent years, packed towers have been added after the mechanically aided scrubber for additional gas absorption and odor control.

Their compact size and low cost make them attractive, where codes allow, for general dust-control applications.

Operating Principles

As was mentioned previously, it is believed that a given amount of energy input into a gas cleaning device is required to achieve a given amount of pollutant removal (the equivalent energy theory). This energy may be introduced using the gas velocity (produced by a fan or other prime mover), through a pump or other means or pressurizing liquid, or by a moving mechanical device.

The most common moving mechanism used in mechanically aided scrubbers is a rotating fan wheel or modified fan wheel. The wheel is usually sprayed with scrubbing solution and the liquid is shattered into the desired droplets. Locally, high relative velocities exist between the gas and the liquid so that impaction is enhanced. These designs basically include the fan with the scrubber, so no additional fan is needed.

Two popular mechanically aided scrubbers are the American Air Filter (AAF) W RotoClone series and the Ducon UW-4 arrangement. These use sprayed wheel-type contacting stages but approach the problem differently.

Figure 11.1 shows the AAF RotoClone unit. The Roto comes from the specially designed rotating element, and the Clone comes from the cyclone-type separation that was used. In this design, the gas stream usually is ingested into the rotating wheel where high local velocities and centrifugal force were applied, along with scrubbing liquid, to impact the particulate into the droplets. The scroll-shaped housing served to separate droplets from the gas stream. The type N RotoClone is used for higher efficiency dust control.

Please note the drag chain conveyor (extending off to the left) that is an integral part of the sump. This drag chain allows the continuous or periodic removal of settleable solids from the scrubber. This makes for a very compact arrangement.

Another variant is the type W RotoClone. Shown in Figure 11.2, the type W provides for direct injection into the specially designed fan wheel. In this model, the motor (seen at the right) is providing power to the wheel via a V-belt drive.

The Ducon UW-4 type scrubber uses a primary cyclonic separation stage followed by a wetted fan, followed by another cyclonic separation stage. The primary cyclonic zone is used to centrifugally separate large particulate and droplets and thereby reduce the loading to the wetted fan. The wetted fan

FIGURE 11.1

AAF RotoClone type N (American Air Filter).

FIGURE 11.2

Type W RotoClone (American Air Filter).

was sometimes preceded by a spray duct to add to the droplet loading and liquid/gas surface area. The wetted fan could be equipped with sprays both at the fan eye and in the housing to help keep the wheel clean. The spray regimen leaves the wetted fan discharge and is separated in a cyclonic separator. The captured liquid is returned through a trap to the primary separation stage or is diverted out of the vessel to a separate drain.

There are modified versions of the UW-4 type scrubber supplied by a variety of vendors. There were literally thousands of these scrubbers sold; some are running to this day.

An interesting mechanically aided design is the T-Thermal Hydrop® scrubber. Shown diagrammatically in Figure 11.3, the gas stream enters through a wetted approach section (at the top) and proceeds to a special inlet duct that discharges into the rotating wheel section of the device, where the stream is subjected to centrifugal and shear forces, causing the particulate to be combined with the injected liquid. A cyclonic separator is used to separate the droplets from the liquid stream.

Other mechanically aided designs have been the subject of experimentation including those using sonic pulses, microwaves, vibrating elements, or combinations thereof. In each case, some external force other than simply fan velocity or pump pressure supplements the total energy input.

FIGURE 11.3

Hydrop scrubber (T-Thermal Company).

FIGURE 11.4

Rotary scrubber (TREMA Verfahrenstechnik GmbH).

The mechanically aided rotary scrubber in Figure 11.4 supplied by TREMA uses tangential gas inlets to provide prescrubbing. They can be seen near the base of the unit.

Primary Mechanisms Used

The primary particulate capture mechanism in these devices is thought to be impaction, given the high relative velocities between the gas and liquid. Interception could also play an important role in mechanically aided scrubbers.

Because the agitation is sustained only in the confined zone at or near the moving element, diffusion is not a likely capture mechanism. Diffusion is seen more often in designs that create a spray and sustain that high surface area spray for an extended time. Likewise, many of these devices use centrifugal force applied close to the moving element to separate the gas and liquid; therefore, gas absorption is somewhat limited as the liquid surface area per unit volume is decreased during this separation.

Design Basics

Most mechanically aided scrubbers are proprietary designs that have been refined over the past few decades.

In general, the gas inlets and outlets are sized based on the conveying velocities that these devices need to successfully control dust at high loadings. Gas inlet velocities of 45-60 ft/s are not uncommon. The ducting to the moving device must be carefully designed to load the scrubber uniformly. Obviously, imbalance can be a problem with any moving device; therefore, the designers take care to allow uniform loading and to clean surfaces upon which dust may build.

The total horsepower input of these devices typically includes the energy required to move the gas, liquid, and dust. Also, a pump or source of pressurized liquid is needed.

Operating Suggestions

The vendors of these devices have accumulated a wealth of experience in a wide variety of applications. It is best to contact them regarding specific requirements.

Because many of these designs use spray nozzles at some point, the use of a solids separation device (strainer) is often required but not a standard part of the scope of equipment supply. The drag chain shown in Figure 11.1 helps remove large solids before they reach an injurious concentration level. In addition, external strainers or liquid cyclones are sometimes suggested to remove the captured particulate.

Given their simple designs, mechanically aided scrubbers often use a simple wire type overflow for level control. Open impeller pumps are often seen on these designs, given their use on generally high-solids applications. Spare pump wetted parts such as impellers, seals, shafts, and shaft sleeves are recommended.

Because the moving element is often sprayed, vibration detection devices and amperage meters can be used to monitor that important element. Increases in fan amperage can offer a warning of wheel solids buildup. A differential pressure gauge is often used to monitor the scrubber pressure drop. In keeping with the simplicity of the design, little if any additional instrumentation is used.

When the scrubbers are shut down for inspection or service, particular attention should be paid to corrosion or wear on the rotating element. Firms that use multiple mechanically aided scrubbers (such as foundries for sand dust separation) often have spare rotating elements on hand.

Packed Towers

Device Type

Packed towers are gas absorption devices that utilize internal media of a variety of types to enhance the mass transfer of gases into an absorbing liquid. Please also see filament/mesh scrubbers (Chapter 9) which share many of the same design and use characteristics of the packed tower.

 
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