Document yb7KrZyxyBEeQQnpm4GDwN1Q3

784 CHAPTER 35 1952 Guide Ata = logaritfimic mean temperature- difference between. airarid::c6il surface = t\ - i., h ^ t. log. fa -- f . 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): foANiU-t,) = 0.243G, - ti) t1 - t, log. t,-t. or. foAN _ t, -- l, 0.243G~ ge(, - i. (6) where 0.243 = specific heat of humid air, Btu per (pound) (Fahrenheit degree). O = air mass velocity, pounds'per (hour) (square foot of coil face area). An examination of Fig. 12 will reveal that ..when fe-is at the dew-point of the entering air: fl (. tl tdpl ti -- 1, 1. Idpl and when U is below the dew-point . . .A ~ t1 . Idpl., ti ti fdpl Therefore, Equation 6 may be written in its most useful form as , h&AN ti -- fdpi . fi f 0.2430 6 ft -- <dpi fi - t. .. where - , .. ; ; . ... ti = minimum dry-bulb possible without dehumidification, Fahrenheit'degrees, fdpi = .dew-point of air entering coil/Fahrenheit degrees. fdpi; = -dewTpoint.of air leaving coil, Fahrenheit degrees. This equation may be used to establish a line, as A-2-3, for a given coil if /o is known for the coil, or it may be used to determine f0from test data for the purpose of rating coils: Theuse 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 /<> 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 785 prevalent from the edge of a fin to its center. It is therefore necessary to make tests to evaluate the combined term i;/,,. The term, yf9, will be written merely/0 in this discussion, as there is no necessity for separately evaluating q, and because values of /0 are usually applied only to the partic ular coils for which tests are made. - The air side coefficient, fa, of a coil of particular dimensions, is anexponential function of the mass velocity of the air: : /. = Z 0" ' (8) ' where . fo = film coefficient of heat transfer, Btu per (hour) (square foot external surface) (Fahrenheit degree 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. Evaluation of constants Z and n may be accomplished through the use of test data in Equation 7 which gives values of K directly from the results of any wet coil test. If calculated in this inanner, 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. lie 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. h = AN 3t R Cl. - t.) (9)