Document NEvLG49ynQwDB24M33oxq2N9R

154 -CHAPTER 7 ,, ' 1949 Giiide line at a constant horizontal distance from'the saturation curve-. It should be understood that the line l-A-2-3 is not intended to represent the path of the condition of the air as it passes through the coil from row to row. It is simply the path traced by the exit air conditions as the surface temper ature is gradually reduced with other, conditions remaining constant2. In the process of dehumidification, since heat is being transferred to the coil surface by two different mechanisms, (convection and condensation), it is evident that an over-all coefficient of heat transfer cannot be deter mined by,the same method used for heating and for dry cooling coils. However, if it; is assumed that the sensible heat transfer of a dehumidifying coil is unaffected by the presence of moisture on its surface, Equation 5 may be obtained to express this part of the heat transfer in terms of the external film coefficient and the surface temperature. where q. = K Y. A xi'X (MTD.) (5) q. = sensible heat transferred, Btu per (hour) (square foot of coil face area). <i = dry-bulb temperature of air entering coil, Fahrenheit degrees. <i = dry-bulb temperature of air leaving coil, Fahrenheit degrees. t. = average temperature of coil external surface, Fahrenheit degrees. MTD = logarithmic mean temperature difference between air and coil Burface = fi-f. U -t. log. U-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 h, from test data): h.AN{U -- u) U-t. log. U-t. 0.243G(t, - (,) or, KAN _ t, - t, 0.243G V g* t, -- U (6) where 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. 1 will reveal that when l, is at the dew-point'of the entering air: tl ta 11 flip! ta t. t. tdpi and when 4 is below the dew-point: : U-t. tv -- tdpl _ h -- ta ti -- ta x ) Performance; of Air Heating and Cooling Coils 155 Therefore, Equation 6 may be written in its most useful form as h.AN , ti -- tdpi U-t. ------ = log.---------- = log, 0.243G ti - Up, U-t, (7) where = minimum dry-bulb possible without dehumidification, Fahrenheit degrees. ; , = dew-point of air entering coil, Fahrenheit degrees. /jp) = dew-point of air leaving coil, Fahrenheit degrees. This equation may be used to establish a line as A-2-3 for a given coil if A is known for the coil, or it may .be used to. determine h. 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 h, 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 Vti- The term, yh,, will be written merely h, in this discussion as there is no necessity for separately evaluating q and because values of K are usually applied only to the partic ular coils for which tests are made. The air side coefficient, K, of a coil of particular dimensions is an expon ential function of the mass velocity of the air: K = ZG- i , . (8). where K = 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 K, 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, h, 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 y