Document JvxYJo7QmMYV6V3av7zvMpXYO

510 Chapter 26___________________________________________ /~ - 1945 Guide - since a reduction of the water.rate may reduce the wetted area within the exchanger10. The results included in Fig. 6 probably represent magnitudes of Kd which were obtained for complete wetting. Within the cooling tower operating range the over-all rate coefficient for energy transfer is1' nearly the same numerically as the over-all 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 over-all 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 per pound of dry. air). A typical design procedure is outlined in an illustrative example: Example 1. The rate of air ffow, 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 struction11. 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. "Loc. at. Note S. uLoc. at. Note 4, p. 142. Spray Equipment 511 Solution: Initial air enthalpy = 25.1 Btu per pound of dry air. Final air enthalpy: (hi -- hi) Lc (<r- h) - ,,(hi -- 25.1) = 36,30o0y0o(Xj(j 1 (11 - 80) fti = 61.1 Btu per pound dry air. 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 iWorminatinns of the Quantity .. - ^ . The energy balance indicates that the enthalpy Table 4. Numerical Integration for the Number of Transfer Units Water Temperature Interval FDec - 80-82 82-84 84-86 86-88 88-90 90-92 92-94 94-96 96-98 98-100 100-102 102-104 104-106 106-108 108-110 Mean Water Temperature Dec F * 81 83 < 85 87 89 91 93 95 97 99 101 103 105 107 109 Mean Air Enthalpy, Aa L .Btu per b Dry Air 26.3 28.7 31.1 33.5 35.9 38.3 40.7 43.1 45.5 47.9 50.3 52.7 55.1 57.5 59.9 Saturated Aib Enthalpy. A" Btu per Lb DrV Air 44.6 46.9 49.2 51.7 54.4 . 57.1 60.0 63.0 66.2 69.6 73.2 77.0 80.9 85il 89.5 Enthalpy Potential A" - A* 18.3 18.2 18.1 18.2 18.5 18.8 19.3 19.9 20.7 21.7 22.9 24.3 25.8 27.6 29.6 AA A" - Aa 0.131 - 0.132 0.133 0.132 ,0.130 0.128 0.124 : 0.121 0.116 0.111 0.105 0.099 0.093 0.087 0.081 1.723 Increments corresponding to these temperature increments are: AS = ~ At = X 2 = 2.4 Btu per pound of dry air. The result of the integration is that the'Number of Transfer Units required (NTU) ~ 1.72. Unit gas mass velocity, '-Gj- = 3--0g0g0--0 = 830 lb per (hour) (square . foot average cross-sectional air flow area). From Curve 2, Fig. 6, Ka = 138 Btu. The tower volume required is: V = Ya (-NTU) = X 1-72 = 217 X 1.72 = 374 cu ft! Height of the packed section is: OTi 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