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168 CHAPTER 8 1950 Guide Analytical Chemistry of Industrial Poisons, Hazards and Solvents, by M. B. Jacobs (Interscience Publishers, New York, 1941). Bibliography of Industrial Hygiene, 1900-1943 (U. S. Public Health Service Bul letin No. 289, 1945). Clouds and Smokes, by W. E. Gibbs (P. Blakiston's Son & Co., Philadelphia, Pa., 1924). Determination and Control of Industrial Dust, by Bloomfield and DallaValle (XJ. S. Public Health Service Bulletin No. 217,1935). Dust, by S. C. Blacktin (The Sherwood Press, Cleveland, 1934). (The) Environment and Its Effect upon Man (Harvard School of Public Health, Boston, 1937). Industrial Dust, by Drinker and Hatch (MeGraw Hill Book Co., New York, 1936). Industrial Health Engineering by A. D. Brandt (John Wiley & Sons, Inc., New York, 1947). Industrial Hygiene and Toxicology, edited by F. A. Patty (Interscience Publishers, Inc., New York, Vol. 1,1948 and Vol. II, 1949). Journal of Industrial Hygiene and Toxicology (monthly) (Harvard School of Public Health, Boston, Mass.). See cumulative abstract and subject indexes. Manual of Industrial Hygiene, by W. M. Gafafer, et al (U. S. Public Health Service, W. B. Saunders Co., Philadelphia, 1943). Noxious Gases and the Principles of Respiration InfluencingTheir Action, by Hen derson and Haggard (American Chemical Society Monograph Series No. 35, Reinhold, New York, 1943). Occupation and Health, two volumes (.International Labor Office, Washington, D. C.). Toxicology and Hygiene of Industrial Solvents, by Lehmann and Flury, trans lated by Eleanor King and H. F. Smyth, Jr. (Williams and Wilkins, Baltimore, 1943). CHAPTER 9 HEAT TRANSMISSION COEFFICIENTS OF BUILDING MATERIALS Heat Transfer Symbols; Calculating Overall Coefficients; Conductivity of Homo geneous Materials; Surface Conductance; Air Space Conductance; Practical Coefficients and Their Use; Computed Heat Transmission Coefficients; Roof Coefficients; Combined Ceiling and Roof Coefficients; Basement Floor, Basement Wall, and Concrete Slab Floor Coefficients; Cal culating Surface Temperatures; Water Vapor and Condensation; Vapor Transmission; Condensation Control THE design of air conditioning or heating systems for buildings requires a knowledge of the thermal properties of the walls enclosing the space. The rate of heat flow through the walls under steady-state conditions at design temperatures is usually the basis for calculating the heat required. For a given wall under standard conditions the rate is a specific value desig nated as Uy the overall coefficient of heat transmission. It may be deter mined by test in a guarded hot box apparatus, or it may be computed from known values of the thermal conductance of the various components. Because testing of all combinations of building materials is impracticable, the procedure and necessary data for calculation of the value of U are given in this chapter, together with tables of computed values for the more common constructions. HEAT TRANSFER SYMBOLS U = overall coefficient of heat transmission (air to air); the time rate of heat flow expressed in Btu per (hour) (square foot) (Fahrenheit degree temperature difference between air on the inside and air on the outside of a wall, floor, roof or ceiling). The term is applied to the usual combinations of materials in construction and also to single materials, such as window glass, and includes the surface conductance on both sides. k = thermal conductivity; the time rate of heat flow through a homogeneous mate-. rial under steady conditions through unit area per unit temperature gradient in the' direction perpendicular to the area. Its value is expressed in Btu per (hour) (square foot) (Fahrenheit degree per inch of thickness). Materials are considered homogene ous when the value of & is not affected by variation in thickness or size of sample within the range normally used in construction. C = thermal conductance; the time rate of heat flow through a material from one of its surfaces to the other per unit temperature difference between the two surfaces. Its value is expressed in Btu per (hour) (square foot) (Fahrenheit degree). The term is applied to specific materials as used, which may be either homogeneous or hetero geneous. / = film or surface conductance; the time rate of heat flow between a surface and the surrounding air. Its value is expressed in Btu per (hour) (square foot of surface) (Fahrenheit degree temperature difference). Subscripts i and o are used to differenti ate between inside and outside'surface conductances, respectively. a = thermal conductance of an air space; the time rate of heat flow through an air space per unit temperature difference between the boundary surfaces. Its value is ex pressed in Btu per (hour) (square foot of area) (Fahrenheit degree). The conduct ance of an air space is dependent on the temperature difference, the height, the depth, the position and the character of the boundary surfaces. The relationships are not linear, and accurate values must be obtained by test and not by computation. R = thermal resistance. Its value is expressed in Fahrenheit degrees per (Btu) (hour) (square foot). It may represent any of the following and must therefore be 169