Document Yr0eQ7M91Rv4mB6oygznYgG2K

HEATING VENTILATING AIR CONDITIONING GUIDE 1940 SMOKE STACKS Gases discharged from smoke stacks may contain relatively large particles of cinders and carbon as well as the fine smoke which remains in relatively permanent suspension. Large particles settle quickly and are likely to constitute a serious nuisance in the immediate neighborhood. Smoke tends to spread over a large area, but in cities the large number of stacks may pollute the atmosphere over the entire city. A variety of cinder catchers are available for catching the large cinders. Devices of the cyclone type remove much finer particles, but for anything approaching complete cleaning, electrostatic precipitators are usually required. For this purpose they usually consist of wires (at potential of 30,000 to 100,000 volts negative) suspended between vertical plates or hanging in the center of vertical cylinders. The dust collects on the plates or cylinders which are periodically vibrated for rapped to cause the dust to fall down into hoppers. REFERENCES 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 (Heating, 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, Healing, Piping and Air Conditioning, April, 1933, p. 217). Operation, Maintenance of Cloth-Screened Dust Collectors, by W. F. Terry (Heating, Piping and Air Conditioning, May, p. 259, June, p. 304, 1933). Testing and Rating of Air Cleaning Devices Used in General Ventilation Work, by Samuel R. Lewis (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 277). Air Filter Performance as Affected by Kind of Dust, Rate of Dust Feed, and Air Velocity Through Filter, by F. B. Rowley and R. C. Jordan (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 415). ~- The Dust-Free Space Surrounding Hot Bodies, by H. H. Watson (Transactions of the Faraday Society, Vol. 32, 1936, Pt. 2, p. 1073). A New Electrostatic Precipitator, by G. W. Penney (Electrical Engineering, January, 1937, p. 159). 514 Chapter 28 FANS Classification, Performance, Fan Efficiency, Characteristic Curves, System. Characteristics, Selection of Fans, Volume Control, Fan Designations, Motive Power IN heating and ventilating practice, fans are used to produce air flow except where positive displacement is required, in which case com pressors or rotary blowers are used. Fans are classified according to the direction of air flow as (1) axialflow or propeller type if the flow is parallel with the axis, and (2) radial flow or centrifugal type if the flow is parallel with the radius of rotation. Axial flow fans are made with various numbers of blades of a variety of forms. The blades may be of uniform thickness (sheet metal), either flat or cambered, or may be of varying thickness of so-called aerofoil section (airplane propeller type). Where an axial flow fan is intended for operation at comparatively high pressures the hub sometimes is enlarged in the form of a disc and the fan is known as a disc fan. Radialflow or centrifugalfans include steel plate fans, pressure blowers, cone fans, and the so-called multiblade fans. All the foregoing types have variations which may be obtained by modification of the proportions or change, in the curvature and angularity of the blades. The angularity of the blades determines the operating characteristics of a fan; a forward curved blade is found in a fan having slow speed operating characteristics, while a backward curved blade is found in a fan having high speed operating characteristics. A wide variation exists in the demands which have to be met by fan installations. A fan may be required to move large quantities of air against little or no resistance or it may be required to move small quanti ties against high resistances. Between these two extremes innumerable specific requirements must be met. In general, fans of all types in each general class can be made to perform the same duty, although mechanical difficulties, noise or lack of efficiency may limit the .use to one or another type. The most common field of service for fans of the propeller type is in moving air against moderate resistances, especially where no long ducts or heavy friction must be overcome and where noise is not objectionable, whereas centrifugal fans are commonly employed for operation at the comparatively higher pressures and where extreme quietness is necessary. FAN PERFORMANCE Fans of all types follow certain laws of performance which are useful in determining the effect of changes in the conditions of operation. These 515