Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
In a tray drying under adiabatic process, hot air enters the dryer at 50°C dry bulb temperature (DBT) and 10% relative humidity (RH) with enthalpy of 70.2 kJ per kg dry air. The air leaving the dryer gains 20% moisture. Considering saturated water vapour pressure as 12.349 kPa at 50°C DBT, the enthalpy of air leaving the dryer in kJ per kg dry air is __________.
Official Answer Key:
70.0 to 70.3
Step-by-Step Solution & Explanation
Adiabatic tray drying is, to a very good approximation, a constant-enthalpy (adiabatic saturation) process on the psychrometric chart — the sensible heat given up by the drying air is converted into the latent heat of the moisture it picks up, so the total enthalpy of the air-water-vapour mixture is essentially unchanged from inlet to outlet, regardless of the amount of moisture picked up. Hence enthalpy leaving ≈ enthalpy entering = 70.2 kJ/kg dry air.
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