Document Z8JpwdV5mpGRjJKngG9V2d5k0

768 CHAPTER 35 1951 Guide AZ,, = logarithmic mean temperature difference between air and coil surface = t, -- Zi Zi -Z. log, Zj-t. If Equation 5 is combined with another equation expressing sensible heat transfer in terms of mass velocity and temperature difference, the variables may be arranged in the following form (which is useful for the solution of dehumidification problems and for the determination of /, from test data): f,,AN (Zi - Z.) = 0.243G(Zi - Z,) Zi-Z. log. it -- z. or, foAN _ Zi Z. 0.243G " E* Z, - Z. (6) where 4 0.243 = specific heat of humid air, Btu per (pound) (Fahrenheit degree). , G = air mass velocity, pounds per (hour) (square foot of coil face area). An examination of Fig. 12 will reveal that when t, is at the dew-point of the entering air: Zi -- Z. __ Zi -- Zdpt Zi Z. Z. Zspi and when Z,, is below the dew-point: Zi -- Z. _ Zi --Zdpi Zi -- Z. Zi -- Zdpi Therefore, Equation 6 may be written in its most useful form as h.AN 0.243G = log. Zi Zs " Zdpi Zdpi = log. fi-1. Zi-Z. where (7) Z. = minimum dry-bulb possible without dehumidification, Fahrenheit degrees. Zdpi = dew-point of air entering coil, Fahrenheit degrees. Zdpi = dew-point of air leaving coil, Fahrenheit degrees. This equation may be used to establish a line, as AT2-3, for a given coil if fa is known for the coil, or it may be used to determine f,, from test data for the purpose of rating coils. The use of this equation for coil selection is illustrated in Example 1 at the end of the chapter.- Equation 7 is also important as a means of determining the external film coefficient. External Film Coefficient While formulas have been developed expressing the film coefficient f0 for air passing parallel to a plane surface, they cannot be used directly for fins on tubes because of air turbulence, and because of the temperature gradient Air Heating and Cooling Coils -769 prevalent from the edge of a fin to its center. It is therefore necessary to make tests to evaluate the combined term ij/,,. The term, ij/,,, will be written merely fa in this discussion, as there is no necessity for separately evaluating v, and because values of /,, are usually applied only to the partic ular coils for which tests are made. The air side coefficient, fot of a coil of particular dimensions is an expon ential function of the mass velocity of the air: /. = ZG" (8) where fQ <= film coefficient of heat transfer, Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference between air and average surface temperature). G a* 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. Evaluation of constants Z and n may be accomplished through the use of test data in Equation 7 which gives values of A. directly from the results of any wet coil test. If /,,, calculated in this manner, is plotted against values of G which prevailed during the tests, a straight line should result on logarithmic coordinates. The slope of this line is the value of n. The value of Z may then be determined by direct substitution in Equation 8. For finned coils of different designs, values of Z and n are extremely vari able, depending on the particular design and arrangement of the coil surface. Therefore, it is desirable that these constants be determined directly from test data for each type of coil surface. Internal Film Coefficient The internal film coefficient,/! which appears in Equation 3, is evaluated in various ways, depending upon the nature of the fluid, and whether the fluid is changing state. When evaporating refrigerants are used in tubes, the temperature of the fluid is fairly constant, being affected principally by pressure drop through the tubes, by superheat of the evaporated refrigerant, and by the presence of oil in solution. To obtain maximum coil capacity it is necessary to keep the pressure drop through the tubes at a minimum, to keep the superheat as low as possible without carrying liquid back to the compressor, and to arrange for good separation and return of oil to the compressor. Another important factor is the removal of gas to keep the tube surface flooded with liquids as much as possible. The internal film coefficient is markedly in creased by heavy heat loads, because the increased turbulence and gas velocity cause good contact of the liquid with the tubes. Values of f, usually lie between 150 and 450. For rating of dehumidifying coils, satis factory results are obtainable by first determining the average'external sur face temperature from Equation 7, and then using the difference between the external film temperature and the refrigerant for evaluating f, in Equation 9. ff* f; = AN R (Z. - (,) (9)