EXA MPLE 19.3. A coonterflow induced-draft cooling tower operates with inlet and 7,99,6(6) exit water temperatares of 105 and 852F when the air has dry-bulb and wet-bulb temperatures, respectively, of 90 and 76F. The tower has 4ft of stacked plastic fill. and the flow rates are Gy=2,000lbh - ft2 and GA=2,200lohh-ftr. (a) Determine the number of transfer units, the beight of a transfer unit based on the overall gas-phase driviag forec, and the temperature approach. (b) If the oooling load remains the same bat the air temperature drops to 70 F with a wet-bulb temperature of 60F, predict the water temperuture and the temperiture approach. 638 shetion tv: Mass Transfer and les Applications. Solution (a) From the psychrometric chart (Fie. 19.2) the inlet humidity and bumid heat are determined. K2=0.017IbwaterfibairKA=0.0310.017100=55%ci=0.248Bm/bbF52.64.7,H.=0.248(9032)+1.075(,017)=32.7Bra/b2,200(1.0)(10585)=2,000(H232.7)H=54.7Btu/lb The points Tna=105,Ha=54.7 ) and Tnb=85,Hb=32.7 ure plotted in Fig. 19.10 as the endpoints of the operating line. The number of transfer units is obtained by NT/W determining the driving rorce at the mitdle of the column and using the log mean H Cf. 472=Hoy=1.824=2.2ft=ky2f The temperature approach is 8576=9F. (b) For Ty=70F and Tv=60F Kb=0.009?ci=0.244Hb=0.244(7032)+1.075(0.009)=18.9Bta/b For the same cooling load and constant water and airflows 2,200(1.0)(20)Hu=2,000(He18.9)=18.9+22=40.9 The operating line is located by trial to give the same number of transfer units. For Tm=95F,Tm=75F,V=1.78, which is close enough to 1.82 . The operating line is shown as a dasthed lifte in Fig. 19.9. The approsch to the wet-bolb temperature is TibTw=7560=15F 19.4. For the cooling tower described in Example 19.3, predict the effect of increasing the air rate by 20 percent with the same liquid rate and a wet-bulb temperafure of 76F. 26