Document 2qXV3reN4O09eoQ54wnYD35b5

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 of Ka which were obtained for complete wetting. Within the cooling tower operating range the overall rate coefficient for energy transfer is nearly the same numerically as the overall rate coefficient for mass trans fer. The conditions of test corresponding to the data presented in Fig. 6 are not well enough known in most cases to warrant recomputation of Ka. Therefore the magnitudes of the overall rate coefficient for mass transfer presented in Fig. 6 may be used directly in Equations 5 and 6, the units of Ka in these equations being Btu per (hour) (cubic foot) (Btu per pound). A typical design procedure is illustrated in the example following; Example 1. The rate of air flow, arbitrarily assumed in the data given, is related to the tower volume by economic considerations. A balance between air rate and tower volume rests on consideration of the costs of producing air flow and of the tower con struction*. A counter-flow forced draft cooling tower is to cool 36,000 lb of water per hour from an initial temperature of 110 F to a final temperature of 80 F. Air having an initial condition of 65 F dry.-bulb and 58 F wet-bulb temperature will be forced through the tower counter to the direction of water flow at the rate of 30,000 lb of dry air per hour. The cross-section of the tower is to be 8 ft x 8 ft and the packing is to be of the type producing a rate coefficient as indicated in curve No. 2 of Fig. 6. For this type of packing the average cross-sectional area for air flow will be 36 sq ft. Solution: Initial air enthalpy = 25.1 Btu per pound. Final air enthalpy: (hi - hi) = (I, - ,) (hi - 25.1) 36,000 X 30,000 1 (110 - 80) hi = 61.1 Btu per pound dry air. Table 4. Water Temperature Interval Dec F Numerical Integration for the Number of Transfer Units F 1 Mean Water Temperature Dec F Mean Air Enthalpy. Aa Btu per Lb Dry Air Saturated Air Enthalpy, A" Btu per Lb Dry Air Enthalpy Potential A" - ha Ah A" - ha 80-82 82-84 84-86 86-88 88-90 81 83 85 87 89 26.3 44.6 18.3 0.131 28.7 46.9 18.2 0.132 31.1 49.2 18.1 0.133 33.5 51.7 18.2 0.132 35.9 54.4 18.5 0.130 90-92 92-94 94-96 96-98 98-100 91 93 95 97 99 38.3 57.1 18.8 , 0.128 40.7 60.0 19.3 0.124 43.1 63.0 19.9 0.121 45.5 66.2 20.7 0.116 47.9 69.6 21.7 0.111 100-102 102-104 104-106 106-108 108-110 101 103 105 107 109 50.3 73.2 22.9 0.105 52.7 77.0 24.3 0.099 55.1 80.9 25.8 0.093 57.5 85.1 27.6 0.087 59.9 89.5 29.6 0.081 1.723 Loc. Git. Note 2, p. 142. CHAPTER 27. SPRAY EQUIPMENT 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 -- A/ = X 2 = 2.4 Btu per pound. The result of the integration is that the Number of Transfer Units required (NTU) = 1.72. Unit gas mass velocity, G 30^000 = 830 lb per (hour) (square foot). Fig. 6. Unit Conductances for Various Types of Packing Construction From Fig. 6, Ka -- 138 Btu per (hour) (cubic foot) (Btu per pound). The tower volume required is: v = wa {NTU) = nsr x 172 = 217 x 172 = 374 cu ft- Height of the packed section is: 374 8X8 5.9 ft. The graphical solution for the Number of Transfer Units required for the desired performance is plotted in Fig. 7. ,, --, is plotted as a function of h, and the area under the curve from the initial enthalpy of the air, 25.1 Btu per pound, to the final enthalpy of the air, 61.1 Btu per pound is 1.72, the Number of Transfer Units required. 1 he effect of the rapid decrease in potential due to the cooling of the water is indicated 557