In a drying experiment, the constant rate of drying is found to be 3.6 kg water m⁻² h⁻¹. Dry bulb and wet bulb temperatures of the drying air are 75°C and 37°C, respectively. Latent heats of vapourization at the dry bulb and the wet bulb temperatures are 2321 and 2414 kJ kg⁻¹, respectively. Convective heat transfer coefficient in W m⁻² K⁻¹ for the drying operation is ______
Official Answer Key:
63 to 64
Step-by-Step Solution & Explanation
In the constant-rate drying period, all heat supplied by convection is used for evaporation at the wet-bulb temperature: h_c×(T_db−T_wb) = R_c×λ_wb, where R_c is the constant drying rate and λ_wb is the latent heat evaluated at the wet-bulb temperature (since evaporation occurs at the wetted surface, at T_wb). h_c = R_c×λ_wb / (T_db−T_wb) = (3.6 kg/m²h × 2414 kJ/kg) / (75−37)°C = 8690.4/38 = 228.7 kJ/m²·h·K. Converting to W/m²K: 228.7×1000/3600 = 63.5 W/m²K, within the accepted band 63–64.
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