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754 CHAPTER 34 1953 Guide the fan inlet, and elimination is effected largely on the fan blades and also on the sur faces beyond. A special two-element fan is used;-the air with its dust content and water spray enters one side of the wheel and is discharged from the inlet of the other wheel. As the air passes through the cyclonic chamber, a high degree of scrubbing action takes place. Relatively high pressure losses are encountered, with resulting high horsepower requirements. Centrifugal Dust Collectors , Centrifugal collector design can be divided into four groups: 1. Cyclone Collector. This type is commonly applied for the removal of coarse I dusts from an air stream, as a precleaner to more efficient dry or wet dust collectors, or as a separator in product conveying systems using an ai r stream to transport j material. Principal advantages are low cost and low pressure drop (% to in. .. water), but this type cannot be used for high efficiency collection of fine particles. 2. High Efficiency Centrifugal Collectors. These have been developed, to obtain higher centrifugal force action on dust particles in a gas stream. Centrifugal force I is a function of peripheral velocities and angular acceleration, and improvement in j dust separation efficiency has been obtained by (a) increasing velocities through a' cyclone shaped collector; (b) utilizing a skimmer or other design feature; (c) using ! a number of small diameter cyclones in parallel; and (d) in some unusual applications, i by placing units in series. While such collectors do not generally reach an efficiency on small particles equal i to that of the electrostatic, fabric, or some wet type units, their effective collection i range is extended appreciably beyond that of the conventional cyclone. Pressure ! losses of collectors in this group range from 3 to 8 in. water column. ; 3. Dry Type Dynamic Precipitator. In this type, dust is precipitated by centrif| ugal force upon specially shaped blades of an exhauster wheel, and then conveyed through a dust circuit in the fan casing to the dust storage hopper. 4. Cinder Catching Fan. This is a special type of collector in which removal of j solids is effected by a slotted scroll. It is a fan unit which is often used for the dual j purpose of dust removal and induced draft service. - ! | Settling Chambers |. While it would be possible in theory to remove dust by settling in a large i chamber when conveying velocities are, reduced to the point where the | particles would no longer be held in suspension, such devices have little | practical application in dust collecting equipment. Extreme space re- ! quirements and the presence of eddy currents may nullify the effective ; velocity. The settling chamber type of collector can therefore be used ! only for removal of extremely coarse particles. Pressure drop should be j M t /4 iQ- water column, plus that necessary to accelerate the air motion i to the required conveying velocity at discharge of the settling chamber. i Sonic Agglomerators ` ; Sonic agglomerators, now in an early stage of development, may prove i to be valuable accessories in the dust collecting field. High-intensity sound j can be utilized to coagulate low-micron and sub-micron sized aerosols such as smoke, fumes, or very fine dusts into resultant particles which are large j enough to be collected efficiently with several of the types of dust collecting I equipment previously described: ! This high-intensity sound can be created by various means such as j piezo-electric crystal generators, magneto-striction units, or high-powered ! sirens which are practical for units with greater air flow capacities.8 Air Cleaning 755 TESTING METHODS Methods of determining the performance of industrial air cleaning de-' vices will depend upon the nature of the air contaminant, its quantity, the required accuracy of the test, and the type of air cleaning device. The technics used in collecting the samples are the same as those utilized in the field of industrial hygiene. The tests may be facilitated by feeding, uniformly, a known amount of the material to be removed. The performance efficiency maybe calculated from the amount of material introduced, the quantities of air involved, the amount of material intercepted, and the quantity that escapes. When it is necessary to test the device under actual use, the material entering and leaving must be -sampled simultaneously over a sufficiently long period of time to collect an adequate amount for analysis. If feasible, the quantity of material removed by the cleaning device during the test period should also be determined. Unusual care must be exercised in collecting the samples to insure that the sampling areas selected are actually representative of the material entering and leaving the cleaning device. With gases, vapors and fresh fumes, this problem is not great. On the other hand, mists, dusts and aged fumes may present considerable trouble. When the material is confined in a duct system, it is common practice to collect the sample along the center fine. The sampling tube is located parallel with, and facing, the flow. The sampling velocity should be approximately the same as the air velocity in the duct. With large ducts it may be desirable to make traverse tests to locate an optimum sampling position. REFERENCES 1 Bronchial Asthma and Allied Allergic Disorders, by S. S. Leopold and C. S. Leopold (Journal of the American Medical Association, March 7,1925, Vol. 84, p. 731734). 3 Dirt Patterns on Walls, by R. A. Nielsen (A.S.H.V.E. Transactions, Vol. 46, 1941, p. 247). 3 Industrial Dust, by Philip Drinker and Theodore Hatch (McGraw-Hill Co., New York, N. Y.). 4 A.S.H.V.E. Standard Code for Testing and Rating Air Cleaning Devices Used in General Ventilation Work (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 225). 8 A Test Method for Air Filters, by Richard S. Dill (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 379). 6 Ventilation Problems in Safe Handling of Radioactive Materials, by W. W. Mc Intosh. Paper, presented at ASME Spring Meeting, March 24--26, 1952 7 Filtration of Radioactive Aerosols by Glass Fibers, Document HW-20847, Han ford Works (General Electric Co.) 8 Industrial Sonic Agglomeration and Collection Systems, by H. W. Danser and E. P. Neuman (Industrial and Engineering Chemistry, November, 1949). Agglomera tion of Smoke, Fog or Dust Particles by Sound, by H. W. St. Clair (Industrial and Engineering Chemistry, November, 1949). BIBLIOGRAPHY Design and Application of Oil-Coated Air Filters, by H. C. Murphy (A.S.H.V.E. Transactions, Vol. 33, 1927, p. 73). Operation and Maintenance of Air Filters, by W. G. Frank (Healing, Piping and Air Conditioning, May, 1931, p. 378). Size and Characteristics of Air-Borne Impurities, by W. G. Frank (Heating, Piping and Air Conditioning, January, 1932, p. 35). Fundamental Principles in the Design of Dry Air Filters, by Otto Wechsberg (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, April, 1933, P- 217).