Document a1ZOzmeObBNEz51E8kjpex9vN

American Society of Heating and Ventilating Engineers Guide, 1935 Heat is lost from a building by transmission through all of those sur. faces which separate heated spaces from the outside air or from unheated colder spaces within the building. In general, five kinds of surfaces are involved: (1) outside walls; (2) outside glass; (3) inside walls or parti, tions next to unheated spaces; (4) ceilings of upper floors, either below a cold attic space or as the underside of a roof slab; and (5) floors of heated rooms above an unheated space. The net inside wall surface is usually determined by reference to the scale p|ans and elevations of the building concerned. In some cases, of course, the actual building may have to be measured. The total area of all outside openings which are occupied by windows and doors is accurately measured and listed as glass. The glass area is then deducted from the total outside wall area for each room and the difference is the net wall area. If there are no partitions, measure from the inside face of one wall to the inside face of the next wall. The areas of walls, ceilings and floors next to cold or unheated spaces are found, of course, by taking the inside dimensions of such areas, measured on the heated side. COEFFICIENTS OF TRANSMISSION The coefficients of transmission may be determined by means of the guarded hot box or the Nicholls heat meter described in Chapter 40, or they may be calculated from fundamental constants. Because of the unlimited number of combinations of building materials, it would be impractical to attempt to determine by test the heat transmission co efficient of every type of construction in use; consequently, in most cases it is advisable to calculate these coefficients. Symbols The following symbols are used in the heat transmission formulae in this chapter: XJ -- thermal transmittance or over-all coefficient of heat transmission; the amount of heat expressed in Btu transmitted in one hour per square foot of the wall, floor, roof or ceiling for a difference in temperature of 1 deg F between the air on the inside and that on the outside of the wall, floor, roof of ceiling. ' k = thermal conductivity; the amount of heat expressed in Btu transmitted in one hour through 1 sq ft of a homogeneous material 1 in. thick for a difference in temperature of 1 deg F between the two surfaces of the material. The conductivity of any material depends on the structure of the material and its density.- Heavy or dense materials, the weight of.which per cubic foot is high, usually transmit more heat than light or less dense materials, the weight of which per cubic foot is low. Ca = thermal conductance per unit area; the amount of heat expressed in Btu trans mitted in one hour through 1 sq ft of a non-homogeneous material for the thickness or type under consideration for a difference in temperature of 1 deg F between the two surfaces of the material. Conductance is usually used to designate the heat transmitted through such heterogeneous materials as plaster board and hollow clay tile. f -- film or surface conductance; the amount of heat expressed1 in Btu transmitted by radiation, conduction and convection from a surface to the air surrounding it, or vice versa, in one hour per square foot of the surface for a difference in temperature of 1 deg F between the surface and the surrounding.air. To differentiate between inside and outside wail (or floor, roof or ceiling) surfaces, /i is used to designate the inside film or surface conductance and/o the outside film or surface conductance.. a -- thermal conductance of an air space: the amount of heat expressed injBtu trans mitted by radiation, conduction and convection in one hour through an area of 1 sq ft of 92 Chapter 5--Heat Transmission Coefficients and Tables an air space for a temperature difference of 1 deg F The conductance of an air space depends on the mean absolute temperature, the width, the position and the character of the materials enclosing it. , ./ or cRon=d- urecstiivstiatyn,cie.eo.:r resistivity which is the reciprocal of transmission, conductance' ~tT = over-all or air-to-air resistance --- = internal resistivity. k -- = internal resistance. 1 Ca - -j- = film or surface resistance. -i- -- air-space resistance. a Fundamental Formulae The formula of the over-all coefficient for a simple wall * inches thick is; U (2) and for a compound wall of several materials having thicknesses in inches of *1, x,, etc., the coefficient is: 1 U= T+ X+?TT + 2 + fo +etc' (3) In the case of air-space construction, an air-space coefficient for each air space must be inserted in either Equation 2 or 3. Thus for a simple wall with one air space, 1 V (4) and for a simple wall of several air spaces having conductances of a,, a,, a,, etc., the coefficient is: U = +ir+-j-+J++f- + -r-+ir+T-+et`- J1 i. r% a> ki ./o (6) With certain special forms of materials which have irregular air spaces (such as hollow tile) or are otherwise non-homogeneous, it is necessary to use the conductance (Ca) for the unit construction, in which case t- is replaced by ~ . * Ca As in the case of the simple wall, fi and /<> are always the inside and outside surface coefficients for the two materials in contact with air. If 93