Document 3QMR24MKngbJNpMVBOOLpNbxn

168 CHAPTER 8 1957 Guide 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 Influencing Their 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). i1f: tr CHAPTER 9 HEAT TRANSMISSION COEFFICIENTS OF BUILDING MATERIALS Heat Transfer Symbols; Calculating Overall Coefficients; Conductivity of Homo geneous Materials; Soil Conductivity and Specific Heat; Surface Conductance; ' Air Space Conductance; Practical Coefficients and Their Use; Computed Coefficients of Walls, Roofs, Ceilings and Floors; Effect of Insulation; Combined Ceiling, Roof, and Floor Coefficients; Glass Coefficients; Calculating Surface Temperatures THE design of air conditioning or heating systems for buildings requires a knowledge of the thermal properties of the walls enclosing the space. (The term walls in this case, includes windows, doors, ceilings, floors, roofs and skylights). 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 designated as U, the overall coefficient of heat transmission or thermal transmittance. It may be determined 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 it is impracticable to test all combinations of building materials, the procedure and necessary data for calculation of the value of U are given in this chapter, together with tables of computed values for a large number of the more common constructions. HEAT TRANSFER SYMBOLS , ^overall coefficient of heat transmission or thermal transmittance (air to air); the time rate of heat flow expressed in Btu per (hour) (square foot) (Fahrenheit de gree temperature difference between air on the inside and air on the outside of a wall, Uor> f00* or .ceding). The term is applied to the usual combinations of materials, and also to single materials, such as window glass, and includes the surface con ductance on both sides. This term is frequently called the V value. k = thermal conductivity; the time rate of heat flow through a homogeneous ma terial under steady conditions per unit temperature gradient through unit area per pendicular to the temperature gradient. Its value is expressed in Btu per (hour) (.square foot) (Fahrenheit degree per inch of thickness). Materials are considered Homogeneous when the value of k 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 unit area of a ma terial from one of its surfaces to the other per unit temperature difference between the nso surfaces. Its value is expressed in Btu per (hour) (square foot) (Fahrenheit degree). The term is applied to specific materials as used, either homogeneous or Heterogeneous, for the thickness or construction stated, not per inch of thickness. / = film or surface conductance; the time rate of heat flow between a unit area of a surface and the surrounding air. Its value is expressed in Btu per (hour) (square us if 8\F^ace) (Fahrenheit degree temperature difference). Subscripts i and o are used to differentiate between inside and outside surface conductances, respectively. ' 0 " thermal conductance of an air space; the time rate of heat flow through a unit IN i an- a*r space per unit temperature difference between the boundary surfaces. s value is expressed in Btu per (hour) (square foot of area) (Fahrenheit degree). ]Ae,cont*uctance of an air space is dependent on the temperature difference, the fa'"*' Q-*e depth, the position, character, and temperature of the boundary sur- f ?8- Since the relationships are not linear, accurate values must be obtained by iest and not by computation. . . ' 169