Document 0qam6Zappm7Y5Xznp454dd7Yx

156 CHAPTER 7 ' 1946 Guide If this equation 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 sqlution of dehumidification problems and for the determination of Aa from test data): Aa A N (h - tt) t\ -- (g = 0.243 G (<, ~ f,) log. or, where 0.243 G Aa A N 0.243G -- ,loge' /i h -- -- ts ts : specific heat of humid air, .Btu per (pound) (Fahrenheit degree), air mass velocity, pounds per (hour) (square foot of coil face area). (6) An examination of Fig. 12 will reveal that when ts is at thedew-point of the entering air: ti -- ts __ /i --/dpi /a " /s /a -- /dpi and when ts is below the dew-point: t\ -- ta _ t\ -- /dpi h -- /s /j -- /dp2 Therefore, Equation 6 may be written in its most useful form as: where /a /dpi /dpa h*AN 0.243G = log. tj h -- -- /dp! /dps = loge (7) minimum dry-bulb possible-without dehumidification, Fahrenheit degrees. : dew-point of air entering coil, Fahrenheit degrees. ; dew-point of air leaving coil, Fahrenheit degrees. j..This.equation may be used to establish a line as A-2-3 for a given coil if Aa is known for the coil, or it may be used to determine.Aa 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 formulae-have been developed expressing the film coefficient Aa 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 prevalent from the edge of a fin to its center. It is therefore necessary to make tests to evaluate the combined term tyAa. The term,. tyAa, will be written merely Aa in this discussi'on as there is no necessity for separately evaluating rj and because values of Aa are-usually applied .only to the particular.coils for which tests are made. The air side coefficient, Aa, of a coil of particular dimensions is an exponential function of the mass velocity of the air: where ' Aa Aa = ZG* (8) film coefficient of heat transfer, Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference between air and average surface temperature). t Transfer Surface Coils 157 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 Aa directly from the results of any wet coil test. If Aa, 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 fine is the value of n. The value of Z may then be determined by direct substitution in Equa tion 8. Internal Film Coefficient The internal film coefficient, Ar which appears in Equation 3, is evalu ated 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 (M lb Per square inch), 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 liquid as much as possible. The internal film coefficient is markedly increased by heavy heat loads, because. the increased turbulence and gas velocity cause good contact of the liquid with the tubes. Values of Ar usually lie between 150 and 450. For rating of dehumidifying coils, satisfactory results are obtainable by first determining the average external surface temperature from. Equation 7, and then using the difference between, the external film temperature and the refrigerant for evaluating Ar in Equation 9. where . hr <=> internal film coefficient of heat transfer, Btu per (hour) (Square foot of internal tube surface) (Fahrenheit degree). tr = average refrigerant temperature, Fahrenheit degrees. The term (ts -- tr) is commonly written Af. . To evaluate' Ar by this method the same tests that were required to determine Aa may be used. When water is the cooling medium in tubes, the rate of heat transfer is a function of its velocity, which influences the number of contacts of the water molecules with the tube surface, per unit of time.- Increased water velocity and reduced tube diameter cause increased heat transfer. Heat transfer is also greater at higher temperatures of the water. The basic formula for the film coefficient of heat transfer for flow of water is as follows: Ar= 1.5 (t + 100)'-^- (io) where . V =. water, velocity, feet per second. D. -- internal diameter of tube, inches. / a* average water temperature, Fahrenheit degrees. ' .'