Document zoKkxvG3YpyB8LmJVaKZ0bamm

7 HEATINC VENTILATING AIR CONDITIONING CUIDE 1941 A numerical integration (Table 4) is employed to determine the Number of Transfer Units (NTU) required. Temperature increments of 2 F are used between successive determinations of the quantity The energy balance indicates that the enthalpy increments corresponding to these temperature increments are: AA = ^ X 2 = 2.4 Btu per pound. The result of the integration is that the Number of Transfer Units required (NTU) = 0.757. Unit gas mass velocity, = 830 lb per hour per square foot. From Fig. 6, Ka = 138 lb per hour per cubic foot. The tower volume required is: V = ~Aa (NTU) = loo X 0.757 = 217 X 0.757 = 164 cu ft. Height of the packed section is: 164 6X6 4.6 ft. The graphical solution for the Number of Transfer Units required for the desired performance is plotted in Fig. 7. A" -- Aa is plotte_ d as a function of A, and the are-a under the curve from the initial enthalpy of the air, 25 Btu per pound, to the final enthalpy of the air, 77.8 Btu per pound is 0.757, the Number of Transfer Units required. The effect of the rapid decrease in potential due to the cooling of the water is indicated by comparison of area A,, A,, and A, of Fig. 7. Each represents the Number of Transfer Units required to achieve a water temperature reduction of about 15 F. 4=0 132 NTU (144 to 130 F) 4 =- o0 l2i5o1 NNTTUU (130 (115 to to 115 100 F) F^ t 100 102 104 106 108 110 112 114 116 128 120 122 124 126 128 130 132 134 136 138 140 142 144 Table 4. Integration of Number of Transfer Units Aa 25.0 27.4 29.8 32.2 34.6 37.0 39.4 41:8 44.2 46.6 49.0 51.4 53.8 56.2 58.6 61.0 63.4 65.8 68.2 70.6 73.0 75.4 77.8 A 71.4 75.0 78.9 83.0 87.3 91.8 96.7 101.7 107.1 112.8 118.9 125.2 132.1 133.2 146.8 154.9 163.7 172.9 182.6 193.1 204.3 216.3 229.0 a - ha 46.4 47.6 49.1 60.8 52.7 57.3 59.9 62.9 66.2 69.9 73.8 78.3 83.0 88.2 93.9 100.3 107.1 114.4 122.5 131.3 140.9 151.2 Aa A* - ha 0.052 0.050 0.049 0.047 0.045 0.044 0.042 0.040 0.038 0.036 0.034 0.032 0.030 0.029 0.027 0.025 0.023 0.022 0.021 0.020 0.018 0.017 0.016 0.757 520 CHAPTER 26. SPRAY EQUIPMENT When the relationship between the enthalpy of the saturated air and the temperature is linear over the ran^e of water temperatures involved, it can be shown7 that.the logarith- cnlc mean of the terminal potentials, Ahim, is the correct driving force. This is true to a good approximation when the water cooling does riot exceed 15 F. The approximation to linearity may be determined by inspection of Table 6, Chapter 1, or the temperature enthalpy diagram of Fig. 5. When the logarithmic mean is a valid potential, Equation 6 mayy be written: hiAM--mh* " KaGV {7) and the need for tKfe numerical integration for the determination of the tower volume is The application of the foregoing design method to atmospheric towers js difficult because rate coefficients and flow conditions are not yet well Fig. 6. Unit Conductances for Various Types of Packing Construction defined for such equipment. If these are known, application of Equation 7 to sections of the tower small enough to justify use of the logarithmic mean potential will yield the tower volume required for each section. The sections must be taken perpendicular to the path of water flow. A correction to adjust the logarithmic mean potential, evaluated as for counter-flow, to the reduced effectiveness of cross-flow, has been derived for heat transfer and may be applied to this case8. .-- Atmospheric towers operate with natural draft, produced in a vertical direction by the stack action of the tower structure, at zero velocity of the *Loc. Cit. Note 3, p. 79. Heat Transmission, by W. H. McAdams (McGraw-Hill Co., New York City. 1933. p. 157). 521 >I