Document zdG5pqyg3pDwRG0652J5Ve0Yg

1 American Society of Heating and Ventilating Engineers Guide, 1937 HEAT TRANSFER COEFFICIENTS The symbols representing the various coefficients of heat transmission and their definitions are as follows: U = thermal transmittance or over-all coefficient of heat transmission; the amount o| 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 or 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. C = thermal conductance; the amount of heat expressed in Btu transmitted 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. / = film or surface conductance; the amount of heat expressed 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 degF between the surface and the surrounding air. To differentiate between inside and outside wall (or floor, roof or ceiling) surfaces, f\ is used to designate the inside film.or surface conductance and f0 the outside film or surface conductance. a = thermal conductance of an air space; the amount of heat expressed in Btu trans mitted by radiation, conduction and convection in one hour through an area of'l sq ft rf an air space for a temperature difference of 1 deg F. The conductance of an air spaa depends on the mean absolute temperature, the width, the position and the character of the materials enclosing it. R = resistance or resistivity which is the reciprocal of transmission, conductance, or conductivity, -i- = over-all or air-to-air resistance. -r- = internal resistivity. R X. = internal resistance. ' --j-- = film or surface resistance. --a1-- = ai.r-space resistance.. As an example in the application of these coefficients assume a wall with over-all coefficient U. Then, H -- AU (l -- t0) ' (1) where H = Btu per hour transmitted through the material of the wall, glass, roof or floor. A = area in square feet of wall, glass, roof, floor, or material, taken from building plans or actually measured. (Use the net inside or heated surface dimensions in all cases.) I -- to = temperature difference between inside and outside air, in which t must always be taken at the proper level. Note that t may not be the prealhing-litR temperature in all cases. 100 --------------Chapter 5--Heat Transmission Coefficients and Tables If the heat transfer between the air and the inside surface of the wall is being considered, then, H = A fi - /,) (2) where J- - inside surface conductance. t and ti -- the temperatures of the inside air and the inside surface of the wall re spectively. In practice it is usually the over-all heat transmission coefficient that is required. This may be determined by a test of the complete wall, or it may be obtained from the individual coefficients by calculation. The simplest method of combining the coefficients for the individual parts of the wall is to use the reciprocals of the coefficients and treat them as resistance units. The total over-all resistance of a wall is equal numeri cally to the sum of the resistances of the''various parts, and the reciprocal of the over-all resistance is likewise the over-all heat transmission coef ficient of the wall. For a wall built up of a single homogeneous, material of conductivity k and x inches thick the over-all resistance, U (3) If the coefficients/i,/0 and k, together with the thickness of the material x are known, the over-all coefficient [/ may be readily calculated as the reciprocal of the total heat resistance. For a compound wall built up of three homogeneous materials having conductivities ki, ki and k3 and thicknesses xu x and x3 respectively, and laid together without air spaces, the total resistance, For a wall with air space construction consisting of two homogeneous materials of thicknesses Xi and x2 and conductivities ki and ki, respectively, separated to form an air space of conductance a, the over-all resistance, U 1 fi xt k, + - + -- + Rz (5) Likewise any combination of homogeneous materials and air spaces can be put into the wall and the over-all resistance of the combination may be calculated by adding the resistances of the individual sections of the wall. In certain special forms of construction such as tile with irregular air spaces it is necessary to consider the conductance C of the unit as built instead of the unit conductivity k, and the resistance of the section is equal to The method of calculating the over-all heat transmission coefficient for a given wall is comparatively simple, but the selection of the proper coefficients is often complicated. In some eases the construc tion of the wall is such that the substituting of coefficients in the accepted 101