GATE 2020 AG – Question 61
Air at30°C DBT and40% RH is heated to65°C DBT,30°C WBT and used for constant-rate drying. Dry-air and vapour heat capacities are1.005 and1.88 kJ/(kg °C). Find exit humidity in kg/kg dry air at45°C DBT (three decimals). At30,45,65°C, saturation pressures are4.16,9.48,24.92 kPa and latent heats2430.7,2394.9,2346.3 kJ/kg.
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Correct answer: 0.017 to 0.020
Explanation
Using standard atmospheric pressure101.325 kPa, initial vapour pressure=0.4×4.16=1.664 kPa and humidity w₁=0.622p/(P−p)=0.01039. Heating does not change w. For nearly adiabatic constant-rate drying, conserve moist-air enthalpy h=1.005T+w(2500+1.88T). Equating inlet at65°C to outlet45°C yields w₂≈0.0183, or0.018. The source’s DBT/WBT data are approximate; the humidity lies in its retained range.