Document BQaBoa9bwOrqMdQKBwVx48jj

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 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 Fig. 6. Unit Conductances for Various Types of Packing-Construction producing a rate coefficient as indicated in curve No. 2 of Fig. 6. the average cross-sectional area for air flow will be 36 sq ft. For this type of packing Solution: < Initial air enthalpy = 25.1 Btu per pound of dry air. Final air enthalpy: (hi - h.) = ^ (f. - t,) hi = 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 determinations of the quantity The energy balance indicates that the enthalpy uLoc. Cit- Note 4, p. 142. 540 CHAPTER 27. SPRAY EQUIPMENT Table 4. Numerical Integration for the Number of Transfer Units Water Temperature Interval Deg F 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 Deg F 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 Mean Air haEnthalpy, Btu per Lb 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 Air h"Enthalpy, Btu per Lb Dry 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 85.1 89.5 Enthalpy Potential A" - ha . 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 Ah h" - ha 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: Ah = At -- gQ-'j^Q 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. tU,nit gas mass ve,loc.ity, -Gj- = 3--0g,0g0--0 = 830 lb per hour per square foo,,t average cross-sectional air flow area. From, Curve 2, Fig. 6, Ka = 138 Btu. The tower volume required is: V = ^ (NTU) 30,000 X 1.72 = 217 X 1.72 = 374 cu ft. 138 Height of the packed section is: 374 = 5.9 ft. 8X8 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. The effect of the rapid decrease in potential due to the cooling of the water is indicated by comparison of area A i, A,, and A, of Fig. 7. Each represents the Number of Transfer Units required to achieve a water temperature reduction of about 10 F. A i = 0.471 NTU (110 to 100 F) A i = 0.595 NTU (100 to 90 F) A, = 0.657 NTU ( 90 to 80 F) The use of the logarithmic mean driving potential is illustrated by applying Equation 7 to example 1: _ (Ka) V iVi V -- n ki-h AJn_ 541