Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
Air-water vapor mixture at 30°C DBT, 40% RH is heated to 65°C DBT, 30°C WBT and used as drying medium in the constant-rate period for spinach leaves. Given saturation vapor pressures and latent heats at 30/45/65°C, find the absolute humidity (kg water vapor/kg dry air) for the dryer-exit air at 45°C DBT (same wet-bulb line). (Rounded to 3 decimal places)
Official Answer Key:
0.017 to 0.020
Step-by-Step Solution & Explanation
During the constant-rate drying period, exit air follows the constant wet-bulb-temperature (adiabatic saturation) line originating from 30°C WBT. Saturation humidity at 30°C: Ws = 0.622×Pws/(Patm−Pws) = 0.622×4.16/(101.325−4.16) ≈ 0.0266 kg/kg. Along the adiabatic saturation line: Ws − W = cp,humid-air×(T−Tw)/hfg,w. Using cp≈1.005 kJ/kg·K and hfg at 30°C=2430.7 kJ/kg: W ≈ 0.0266 − 1.005×(45−30)/2430.7 ≈ 0.020 kg/kg dry air.
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