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Filtration and Vacuum Specialists since 1976

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A Scientific Review of Dust Collection - Part 2

Airwashers (Scrubbers)

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next few months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Most air scrubber designs were developed as attempts to improve the performance of inertial collections. The limitations on inertial separators were that the dust particles as they reached the collecting surface did not agglomerate sufficiently. The finer dust particles did not stay on the collection surfaces and were swept back into the air stream.

Modification of Cyclone Collectors

The firs modification came when the standard design cyclones were modified. Water was sprayed on the interior wall of the cyclone. This improved the collection efficiency, but the difficulties came with keeping the surfaces coated and getting the water distribution on the interior of the barrel and the cone. Any surface that was not kept wet would form mud and sludge, which resulted in frequently cleaning the collector interior. The next evolution of the design was to spray water into the inlet of the wet cyclone. The slurry that was formed had a long distance to travel inside the collector. Aso the inner vortex was frequently a problem that interfered with the water dropping into the expansion chamber. These slurry droplets were typically swept upward into the outlet.


The collection efficiencies of these modified cyclones were much higher than the dry units. Two applications that compare efficiencies between clay and wet cyclones are listed below:

Application
Cyclone Efficiency
Wet Cyclone Efficiency

Material Handline (Rock)
80 - 85%
90 - 93%

Dryer
75 - 80%
92 - 96%

In order to have an efficient scrubber, the gas velocities in the scrubber had to be sufficient for the dust to be driven through the surface tension of the water coated surfaces and / or water droplets. For a good design, the scrubbing or washing action also produces a secondary generation of water droplets and incudes a mist collection sections. See Firgure 2-1


The Dynamic Wet Precipitator

consists of adding water sprays to a cnetrifugal type dry collector which is shown in Figure 2-2. The blade design of the centrifugal collector is modified to handle dust and a flow of water. A spray is centered in the inlet and the blades are coated with water. As the air hits the water surfaces at a moderate velocity, the slurry is thrown into the outer walls and into the drain. The liquids water enters the centrifugal separator and the mist enters the drain. This design is limited in the load it carries because the wear on the blades is high due to the solids content.



Figure 2-2
Orifice Scrubbers

These scrubbers are sometimes called orifice scrubbers as illustrated in Figure 2-3. it is essentially and inertial trap / inertial separator except that the air impinges against a water surface. Spray nozzles, however , offera agreater degree of spray dispersion. All of these scrubbers produce coarse water droplets and separate the droplets from the air by changing the flow directions at least once or twice which results in a pressure drop range of 3-6" w.c. These units are generally shorter than other types of wet collectors and they can be installed inside the plant.



Figure 2-3
Collection Efficiency Options for Low Pressure Scrubber Designs

In order to increase the collection efficiency while maintaininga a low to moderate power load, there are several design approaches that may be taken by scrubber suppliers:

  1. The velocity of the blades is increased so that the dust impacts the water surface at a faster velocity rate.

  2. The gas streams are separated into small individual jets so that the dust stays in contact with the water surfaces for a longer time. Some collectors are designed with orifice plates. These orifices range from 1/10 to 1/4 inch in diameter. Also, there are other orifices that are designed with smooth spheres on a course grid. In this case, the air bubbles would travel upward to the water surface while accomplishing a very effective scrubbing action.

  3. The velocity of the water sprays are increased in an effort to collect finer particles.


Figure 2-4
Basic Limitations of Scrubbing Action

In all of these designs, the collection of the finest dust and powder fractions are limited by one main factor which is the deflection of the fine particles away formt he water surface due to the water surface tension. To increase the penetration and collection efficiency of the fine dust, the venturi scrubber (Figure 2-4) is introduced


Venturi (High Pressure Scrubbers)

By increasing the air velocities to between 15,ooo and 20,000 feet per minute at the venturi throuat and by adding 4 to 6 gallons of water per 1,000 CFM of cleaned air at the venturi throuat, very fine water droplets are formed. The impact of these very fine water droplets at the hgih air velocity allows for the efficient collection of the fine particles. The pressure drop ranges from 15 to 60 inches water column. After the dust is entrappped in the liquid slurry, a mist eliminator is needed to separate the mist from the air stream. Mist eliminator designs are similar inertial separator designs where the mist from air separation is either accomplished by the change ina ir flow direction or by the spin in the air stream which created the centrifugal forces.

Also, as the slurry impinges against the collecting surface, the slurry is directed to the scrubber outlet. This invariably requires tha the water mucs flow through a hydraulic trap. Typically, the leakage around these traps cause the dirty droplets to exit the scrubber outlet.

Humidification is required int ehscrubbing process. If the air stream is not close to the saturatio point, the entrapped dust may again be liberated as the slurry evaporates. In most applications, the exhaust air is seldom returned to the work environment.

Exhause Plumes

When a warm humid air stream is mixed with colder air, a white plume will usually be formed due to condensed water vapor. Even though the air may be buoyant, the droplets may increase until the density of the air cause it to descend towards the ground. In some cases, the plume may reach ground level miles away as the plume becomes visible.

Application of Scrubbers

Scrubbers are most often applied to separate from process air streams the solids that are explosive. They are also applied whre the slurry is used in oter parts of the process or where the mixture si sold ina slurry form. Some scrubbers are applied so that chemical reactions will be generated within the scrubbing actions. In other applications theyare even applied as air absorbers.

We would like to aknowledge the contribution of Scientific Dust Collectors for their contributions to this newsletter. Exerpts taken from " A Scientific Review of Dust Collection" by Scientific Dust Collectors.

Look for "Airwashers (Scrubbers) next month.



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