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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 3 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 664 An introduction to mass transfer 11.9 Figure 11.20 The wet bulb of a sling psychrometer. perature is directly related to the amount of water in the surrounding air.12 The highest ambient air temperatures we normally encounter are fairly low so the rate of mass transfer should be small. We can test this suggestion by computing an upper bound on BmH2O under conditions that should maximize the evaporation rate using the highest likely air temperature and the lowest humidity. Let us set those values say at 120 F 49 C and zero humidity mH20 e 0 . We know that the vapor pressure on the wet bulb will be less than the saturation pressure at 120 F since evaporation will keep the bulb at a lower temperature -V O s s Psat 120 F Patm 11 671 Pa 101 325 Pa 0.115 12The wet-bulb temperature for air-water systems is very nearly the adiabatic saturation temperature of the air-water mixture the temperature reached by a mixture if it is brought to saturation with water by adding water vapor without adding heat. It is a thermodynamic property of an air-water combination. 11.9 Simultaneous heat and mass transfer 665 so with eqn. 11.67 mH2O s 0.0750 Thus our criterion for low-rate mass transfer eqn. 11.74 is met ll O s - mH2O e Bm H2O Ĩ_ ắ 0.0811 2 1 - mH2O s Alternatively in terms of the blowing factor eqn. 11.104 ln l m f 0.962 Bm H2O This means that under the worst normal circumstances the low-rate theory should deviate by only 4 percent from the actual rate of evaporation. We may form an energy balance on the wick by considering the u s and e surfaces shown in Fig. 11.20. At the steady temperature no heat is conducted past the u-surface into the wet bulb but liquid water flows through it to the surface of the wick where it evaporates. An energy balance on the region between the u and s surfaces gives nH2O shH2O s - Qs nH2O uhH2O u .--------------------- __ enthalpy of water heat converted enthalpy of liquid vapor leaving to the wet bulb water arriving Since mass is
