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HEATING VENTILATING AIR CONDITIONING GUIDE 1943 PER DEG FAHR PER INCH THICKNESS) Fig. 7. Heat Loss Coefficients for Insulated Ducts3 For round ducts less than.30 in. diameter, increase heat transmission values by the following percent* ages: Thickness of Inslilation (Inches) 1 m2 . -1% SI12 tos21 in. Duct Diameter_____________________________ 3% 2% 5% 3% 7% 790 CHAPTER 43. PIPE AND DUCT HEAT LOSSES HEAT LOSSES FROM DUCTS The. thermal transmission coefficient U for an uninsulated metal "duct, can be obtained from the equation: U = J_+i fi ^ Jo (1) where U = thermal transmittance, Btu per square foot per hour per degree Fahrenheit difference in temperature between the average temperature inside the duct and the air outside the duct. , fi = film conductance inside the duct, Btu per hour per square foot per degree Fahrenheit. fo = film conductance outside the duct, Btu per hour per square foot per degree Fahrenheit. Film conductance fi for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable inside coefficient can be calculated from Schultz's modified equation: 0.32 Fo08 L (2) where V0 = velocity of air in duct, feet per second. D = diameter of duct, feet. Film' conductance /0 depends on a number of variables including tem perature, diameter, and emissivity of the outer surface and can readily be calculated from data in Chapter 3. From this explanation, it is seen that it is unwise to recommend a given value of U for all uninsulated metal ducts. .' The heat loss from a given length of duct can be expressed by: ' Q = UPL [ J: (3) The heat given up by the air in the duct is: Q = 0.24 M (f, -- k) = 14.4 A Vd {h - k) (4) Equating 3 and 4 enables the determination of the temperature drop in the duct: h + k - 2f, 28.8 A Vd h-k UPL . . , , . , . . .T ,, Let x = 2--8.U,877P7AL7V--d cfo. r rectangular ducts,. = 7.2 DVdr ---- `or round ducts, solving for <i and h: h (* + 1) - 2/, (f-D = h (x - 1) + 2t, (x + 1) 791 i!