3. Water is to be cooled in a packed tower from a temperature of 45°C to 32°C using a counter current stream of air. The liquid flow rate is 6500 kg/h m2, and the air enters at 30°C and 80% relative humidity. Previous experience with similar towers suggests: ha = -1.338 and k a = 0.7 G' k,a where the units of kya are kg/h m3 G are kg/h m2 S ha are W/m2K (C) Determine the minimum gas flow rate. (d) Determine the tower height for a gas flow rate equal to 2.5 times the minimum. DATA The heat capacity of liquid water = 4.19 kJ/kg K The average humid heat of the air = 1.069 kJ/kg K The enthalpy of saturated air is given by the following table: 10 29 20 58 30 99 40 170 50 276 60 462 Temperature (°C) Enthalpy of Saturated Air (kJ/kg dry air) Enthalpy of Air - vapor mixture (kJ/kg dry air) 8 150 200 ΟΟΕ 350 8 8 Liquid Temperature (°C) 120 125 130 135 45 35 40 140 50 YA us 110 00:56 145 55 0.033 0.032 0.031 0.030 0.029 0.028 0.027 0.026 -0.025 -0.024 0.023 0.022 0.021 -0.020 0.019 0.018 0.017 70% 80 -50% Relative Humidity Enthalpy At Saturation, kJ/kg Dry Air 60 65 70 75 40% 55 20 OS 30% 45 15 -0.05 50.95 0.016 -0.015 0.014 0.013 0.012 0.011 0.010 0.009 0.008 0.007 0.006 °C 40 20% 10 Moisture Content, kg/kg Dry Air Figure 1: Psychrometric chart [Ref: Felder and Rousseau, 2005] 5 10% 0 Wet Bulb or Saturation Temperature, WATU 2012 -0.005 0.004 0.003 0.002 2 S0.4 40.6 +0.8 0.001 +0.00 55 35 40 45 50 -10 110 15 20 25 30 0.75 0.80 Dry Bulb Temperature, °C 0.85 Below 0°C Properties and Enthalpy Deviation Lines Are For Ice Humid Volume, m/kg Dry Air 0.90