Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
A heater is placed in front of a continuous countercurrent dryer. Air at 40 °C and 70% RH is fed into the heater from which the air exits at 65 °C. If saturation vapour pressure at 40 °C and 65 °C are 0.074 bar and 0.250 bar respectively, then relative humidity of the air coming out of the heater and entering the dryer is
Options:
(A) 21%
(B) 27%
(C) 32%
(D) 38%
Official Answer Key:
A
Step-by-Step Solution & Explanation
Heating at constant pressure does not change the actual (partial) vapour pressure of moisture in the air — only the RH changes because P_sat rises. Actual vapour pressure entering heater: p_v = RH₁ × P_sat(40°C) = 0.70 × 0.074 = 0.0518 bar (unchanged after heating). RH₂ = p_v / P_sat(65°C) = 0.0518/0.250 = ≈21%. (D) mistakenly re-applies RH to the new saturation pressure directly rather than tracking the fixed vapour pressure.
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