Document LJK12GNvZq6JNn0EM505gO1ad

' 478 Chapter25. 1945 Guide pressure drop of flowing fluids, by assuming that molecules of a fluid lose :their momentum upon contact with a solid. The fact that a coil starts to condense moisture when the surface tem perature drops below the dew-point temperature of the entering air makes it possible, to measure the surface temperature of a coil, ari other wise practically impossible task. After the surface temperature has been determined, it is possible to analyze completely the surface film coefficient of both the air side and refrigerant side of a coil. The air side coefficient, ha, of a dry coil of particular dimensions is an exponential function of the mass velocity of the air: where ... Aa = ZG ' (5) Aa = film coefficient of heat transfer, Btu per (hour) (square foot external surface) (degree Fahrenheit mean temperature difference between air and average surface temperature). G = air mass velocity, pounds per (hour) (square foot of coil face area). Z and n = constants which depend upon both air turbulence and surface arrange ment. The difficulty of obtaining sufficient tests to evaluate the-constants Z and n for all conditions of coil design and operation makes it desirable to use Equation 6 for determining the air side coefficient: Aa = 0.245 X.-j- X 2.3 X login (6) where 0.245 = specific heat of humid air, Btu per (pound) (degree Fahrenheit). 2.3 = the constant which converts logarithms from base e to base lD. A -- external surface area, square feet per square foot of coil face area. ' i)c = coil efficiency, a decimal less than 1.0. * This formula gives values of Aa after tests have been made to evaluate the coil efficiency. Equation 6 can be derived1 by combining the basic equations of heat transfer, mean temperature difference and coil efficiency: 2s " Aa X A X AfrZJa co 2 33MTDa--------------_ (8) ^ _ h.------h (by definition) n -- *S (9) where 2s . 0.245 X to X (<i - h) (10) 2s = sensible heat transferred, Btu per hour per square foot of coil face area. <i = temperature of air entering coil, degrees Fahrenheit. h = temperature of air leaving coil, degrees Fahrenheit. Is = average temperature of coil external surface, degrees Fahrenheit. MTDz = logarithmic mean temperature difference between air and coil surface. Coil Efficiency ' One method of expressing air-coil contact efficiency is the ratio between the weight of air that actually contacts the coil surface and the total weight of air passing through the coil. Due to the fact that the specific heat of air is fairly constant over a wide range-of temperature, coil. 4Loc. Cit. Note 2. Heat Transfer Surface Coils' 479 efficiency* can be'expressed as equal to the number of degrees that the entire amount of air is cooled, divided by the degrees difference between the entering air temperature and the coil surface temperature. . For a particular heat transfer surface, coil efficiency is only a-function of the mass velocity of the air, which may be observed By equating Formulae 5 and 6 and combining all constants into K and u: , , '^(i-*) =#r (u) This equation can be used in graphical form by plotting coil efficiency Fig. 12. Relation of Coil Efficiency to Mass Velocity -' against mass velocity as shown in Fig. 12. The significance of .coil efficiency can be visualized in Fig. 13, where the length of the line CD, divided by the length of line C-E, measures the coil efficiency. - The relation between coil capacity and coil efficiency is given by: - where 2 = DcG (A, - Aa) (12) hi = specific enthalpy of air entering coil, Btu per-pound. Aa = specific enthalpy of saturated air at surface temperature, Btu per pound. When no latent heat is being removed.from air, the. change in enthalpy *When coil efficiency is used herein it is intended to express air-coil contact efficiency and does not express total performance efficiency.