Principle of Operation
The D.S.I. system agglomerates (attaches) the airborne dust particle to a water droplet so that the particle becomes heavy enough to be returned to the product stream by the force of gravity. It is similar to the technology utilized in wet scrubbers, where the dust-laden air is scrubbed or cleaned by water droplets in a controlled environment. However, unlike a wet scrubber, the D.S.I. system uses very little water and energy and a complex system of ductwork is not required. In essence, wet scrubber technology is applied at each point of dust generation. The following picture and quotation illustrates why the D.S.I. system works so efficiently.
"Consider a water droplet about to impinge on a dust particle, or what is aerodynamically equivalent, a dust particle about to impinge on a water droplet, as shown in the drawing. If the droplet diameter is much greater than the dust particle, the dust particle simply follows the air stream lines around the droplet, and little or no contact occurs. In fact, it is difficult to impact micron-size particles on anything, which is why inertial separators do not work well at these sizes. If, on the other hand, the water droplet is of a size that is comparable to that of the dust particle, contact occurs as the dust particle tries to follow the stream lines. Thus the probability of impaction increases as the size of the water spray droplets decreases. This explains why water "sprays" are not very effective on respirable dust; typical droplet sizes are 200-600 microns, much larger tan the respirable dust, which is less than 5 microns. Thus, water sprays can be improved by designing nozzles which produce smaller droplets." Coal Age Magazine, April 1976.
To achieve agglomeration at the dust source point, two conditions need to exist: (1.) Enough water droplets of the same size as the dust particles have to be generated and sustained and (2.) Both dust particles and water droplets have to be contained in an enclosed area so that agglomeration can occur. generated and sustained and (2.) Both dust particles and water droplets have to be contained in an enclosed area so that agglomeration can occur.
How Droplet Size Affects Agglomeration
The design of the D.S.I. system is based on a unique device which can produce a very dense fog of 1–10 micron size water droplets which literally blanket the dust source and keep the dust particles from becoming airborne. It does this at a low cost, both from a capital and a maintenance standpoint.
The D.S.I. fogger is an air driven acoustic oscillator for fogging liquids by passing them through a field of high frequency sound waves. This is accomplished by compressing air upstream of a specially designed converge section of the fogger. The result is an air stream that will accelerate past the speed of sound in the converge section. When it passes the speed of sound, a primary acoustical shock wave is generated at the mouth of the fogger. To further enhance the fogging capabilities, a resonating chamber in the path of the air stream reflects the air stream back at itself to amplify and compliment the primary shock wave.
Once this standing shock wave is generated, water is delivered through annular orifices where it is first sheared into relatively small droplets. These small droplets are then carried by the primary air stream into the intense shock wave where the sound energy is converted into work by exploding the droplets into thousands of micron size droplets. After having done its work, the air then escapes around the resonating chamber and carries the droplets downstream in a soft, low velocity fog.
The design of this fogger has obvious benefits regarding the control of respirable dust. However, its inherent design features also make it extremely reliable from a maintenance standpoint. Since the D.S.I. fogger does not rely on high pressure in the water circuit to achieve maximum atomization, wear problems are virtually eliminated as is the need for high pressure water pumps. The D.S.I. fogger cleans itself while operating with high frequency sound waves much the same way that a sonic laboratory cleaner works. The nozzle has no moving parts and is constructed of 100% stainless steel as compared to some competitors who use brass parts. This eliminates wear and corrosion and insures years of maintenance free service even with poor water quality as many material handling plants have. Undissolved sediment in the water that is
larger than the liquid orifices is easily filtered out by
An actual high speed photograph of droplets generated from an D.S.I. fogger. The droplets were captured on a greased microscope slide in a chamber that was at 100%
R.H. to eliminate evaporation of the droplets. The small squares are 2 microns across. Any fugitive dust particles encapsulated by such densely packed fog droplets has little or no chance of escaping.