GATE 2020 ME (ME2) – Question 63

Thermal Engineering Applications · Psychrometry and air-conditioning · 2 marks · Numerical answer

Moist air at 105 kPa, 30°C and 80% RH passes a cooling coil in an insulated air-conditioning duct. Saturated air exits at 100 kPa and 15°C. Water saturation pressures at 30°C and 15°C are 4.24 and 1.7 kPa. Molecular weights are 18 g/mol for water and 28.94 g/mol for air. Find condensate (g/kg dry air), nearest integer.

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Correct answer: 10 to 10

Explanation

Humidity ratio is $w=(18/28.94)p_v/(p-p_v)$. Initially pv=0.8×4.24=3.392 kPa,finally pv=1.7 kPa. Compute1000[(18/28.94)(3.392/101.608−1.7/98.3)]≈**10 g/kg dry air**. Dry-air mass provides the conserved basis.