Document ZBXbqNORdq8eq1vyJ4DG7KzMd

?S2 CHAPTER 35 1950-Guide Afa = logarithmic mean temperature difference between air ana coil surface 1 = : ti-l,- fa fa log. tt fa 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): " f0AN(ti fa) = 0.243G(fa - fa) h -fa fa-fa or, foAN = fi - i, 0.243G = S'fa -fa (6) iwhere 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: (l -- ( t (dpi fa t ( ~" (dpi and when t, is below theldew-point -rfa --fa fa (dpi fa -- fa fa -- (dps . . , : Therefore, Equation: 6 may be written in'its most useful form as :; h.AN . (l* (dpi fa-- (. 0.243G ' ;log. fa -- (dpS log, fa-fa W where; fa = minimum dry-bulb possible without dehumidification, Fahrenheit degrees ; (dpi = dew-point of air entering coil, Fahrenheit degrees.' ' _'[ '* (dps = dew-pbint of air leaving coil, Fahrenheit degrees:. This equation'may be used` ttf establish a line; as A-2-3, foragivencoil if /0 is known for the coil, or it may be used to-determine /:from test'data for the purpose of rating coils. The use of this equation for coil selection is illustrated in Example I 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 ' /. 5f^53 prevalent from the edge of a fin to its center. It is therefore necessary-;to make tests to evaluate the combined term rjf0. The term, rjf0, will be written merely jf0'in this ;discUssiomas'there is ho necessity for^separately evaluating 17, and because values of /,, are usually applied only to the partic ular coils for which,tests are made. T i The air side coefficient,/,, of a coil of particular dimensions is an expon ential function of the mass velocity of the air: where fo -- Z G" (8) ; = film coefficient of heat transfer, Btu per (hour)'(Square foot external . surface) (Fahrenheit degree mean temperature difference between air and average surfaco 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 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 mayThen 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 particulardesign 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. vs Internal.Film Coefficient The internal film coefficient,/ whichappears 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, toikeep 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- i.creased by heavy heatdoads, because.the increased turbulence- and - gas velocity cause goodicontact of the liquid with the tubes. Values of / usually lie between 160 and 450. For, rating of, dehumidifying coils, satis factory results are obtainable by first determining the average external sur- l/ace temperature from Equation 7, and1 then using .the difference between ..the extemal . film temperature, and the refrigerant ;for evaluating / in Equation 9. . .. ' ' \ . . h = AN' R (tm :Q (9)