Document x2YOrbd5K0rJoob7jryo8Z8Q

HEATING VENTILATING AIR CONDITIONING CUIDE 1943 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 outlined in an illustrative example: 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 struction10. A counter-flow forced draft cooling tower is to cool 36,000 lb of water per hour from Bn 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* 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 - h,) - ^ (h - h) hi = 61:1 Btu per pound dry air. Table 4: Numerical Integration for the Number of Transfer Units Water Temperature Interval. . Dbg 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 Enthalpy, Aa Btu per Lb Dry Air 26.3 28.7 31.1 33.5 35.9 Saturated Air Enthalpy. Btu per Lb Dry Air 44.6 46.9 49.2 51.7 ^54.4 38.3 57.1 40.7 60.0 . 43.1 63.0 45.5 66.2 47.9 69.6 50.3 .73.2 52.7 77.0 55.1 80.9 57.5 85.1 59.9 89.5 Enthalpy Potential h" - 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 A" - 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 "Loc. Cit. Note 3, p. 142. 550 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 = ^ At = 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, --Gj- = 3--0^0--00 = 830 lb per (hour) (square foot). .; S/ DATA ON PACKING CONSTRUCTION a/ 4 1. Raschtg rings 15 mm dam by 15 mm high. Counterflow; forced draft 2- Wood streamlined sections 2f in. long. Counterflow: forced draft ' 3. Slat packed tower. 4. Badger type spray tower. 5. Slat packed tower. Slats 45 deg with horizontal. Natural draft chimney tower, counterflo*. 6. Tower No.5 with packing removed. Water distribution by spray noales placed in basin. 6 7. Tower No.3 operating with natural draft 8. Atmospheric cooling towmr. Slats 3 x i in. Spacing 8 in. between decks. Cross or counterflow, depending upon wind velocity. i i i .- 0 250 500 750 1000 1250 1500 1750 2000 2250 -lB DRY AH) R SQ FI PtR HR 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 = (NTU) = X 1-72 = 217 X 1.72 = 374 cu ft. . AB iOO 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 filial 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 551