Document Lxoe5QJpGoG6Zqm7D9a6L0NQ

HEATING VENTILATING AIR CONDITIONING CUIDE 1940 market, such as fabric filters, dust traps, settling chambers, centrifugal separators, electrical precipitators, and gas scrubbers, described in Chapter 27. The cinder particles are usually larger in size than the dust particles; they are gray or black in color, and are abrasive. Being of a larger size, the range within which they may annoy is limited. The dust particles are usually extremely fine; they are light gray of yellow in color, and are not as abrasive as cinder particles. Being ex tremely fine, they are readily distributed over a large area by air currents: The nuisance created by the solid particles in the air is dependent on the size and physical characteristics of 'the individual particles. The difficulty of catching the dust and cinder particles is principally a function of the size and specific gravity of the particles. Lower rates of combustion per square foot of grate area will reduce the quantity of solid matter discharged from the chimney with the gases of combustion. The burning of coke, coking coal, and sized coal from which the extremely fine coal hasbeen removed will not as a general rule produce as much dust and cinders as will result from the burning of non-coking coals and slack coals when they are burned on a grate. Modern boiler installations are usually designed for high capacity per square foot of ground area because such designs give the lowest cost of construction per unit of capacity. Designs of this type discharge a large quantity of dust and cinders with the gases of combustion, and if pollution of the atmosphere is to be prevented, some type of catcher must. be installed. NATURE'S DUST CATCHER Nature has provided means for catching solid particles in the air and depositing them upon the earth. A dust particle forms the nucleus for each rain drop and the rain picks up dust as it falls from the clouds to the earth. However, it was found in recent studies6-that rain was not a good air cleaner of the material below about 0.7 micron. REFERENCES Bulletin, Air Hygiene Foundation, Inc., Pittsburgh, Pa. Determination and Control of Industrial Dust, by J. J. Bloomfield and J. M. Dalla Valle (U. S. Public Health Bulletin, No. 217, 1935). Journal of Industrial Hygiene and Toxicology, Harvard School of Public Health, Boston, Mass. Reports of the National Silicosis Conference: Washington, D. C.. To be published by the U. S. Department of Labor. Saranac Symposium on Silicosis, 1937, Saranac Laboratories, Saranac, N. Y. Industrial Dust, by Philip Drinker and Theodore Hatch, McGraw Hill Co., N. Y. Noxious Gases, by Y. Henderson and H. Haggard, Chemical Catalog Co., N. Y. Occupation and Health, International Labour Office. Preventive Medicine and Hygiene, by Milton J. Roseriau, D. Appleton-Century Co., N. Y. ' Atmospheric Pollution of American Cities ior the years 1931-1933, by 1. E. lyes et al (V. S. Public Health Bulletin No. 224. March, 1936). 84 Chapter 5 HEAT TRANSMISSION COEFFICIENTS AND TABLES Methods of Heat Transfer, Coefficients, Conductivity of Homogeneous Materials, Surface Conductance Coefficients, Air Space Conductance, Practical Coefficients, Table of Con ductivities and Conductances, Tables of Over-all Coefficients of Heat Transferfor Typical Building Construction, Combined Coefficients of Transmission IN order to maintain comfortable living temperatures within a building it is necessary to supply heat at the same rate that it is lost from the building. The loss of heat occurs in two ways, by direct transmission through the various parts of the structure and by air leakage or filtration between the inside and outside of the building. The purpose of this chapter is to show methods of calculation and to give practical trans mission coefficients which may be applied to various structures to deter mine the heat loss by direct transmission. The amount lost by air filtration is determined by different methods, as outlined in. Chapter 6, and must be added to that lost by direct transmission to obtain the total heating plant requirements. METHODS OF HEAT TRANSFER Heat transmission between the air on the two sides of a structure takes place by three methods, namely, radiation, convection and conduction. In a simple wall built up of two layers of homogeneous materials separated to give an air space between them, heat will be received from the high temperature surface by radiation, convection and conduction. It will then be conducted through the homogeneous- interior section by con duction and carried across to the opposite surface of the air space by radiation, conduction and convection. From here it will be. carried by conduction through to the outer surface and leave the outer surface by radiation, convection and conduction. The process of heat transfer through a built-up wall section is complicated in theory, but in practice it is simplified by dividing a wall into its component parts and considering the transmission through each part separately. Thus the average wall may be divided into external surfaces, homogeneous materials and interior air spaces. Practical heat transmission coefficients may be derived which will give the total heat transferred by radiation, conduction and convec tion through any of these component parts and if the selection-and method of applying these individual coefficients is thoroughly understood it is usually a comparatively simple. matter to calculate the over-all heat transmission coefficient for any combination of materials. 85