Thermodynamics: An Engineering Approach 8th Edition

Published by McGraw-Hill Education
ISBN 10: 0-07339-817-9
ISBN 13: 978-0-07339-817-4

Chapter 14 - Gas-Vapor Mixtures and Air-Conditioning - Problems - Page 751: 14-65

Answer

$q_{out}=14.2\text{ kJ/kg dry air}$

Work Step by Step

The amount of moisture in the air remains constant ($\omega_1 =\omega_2$) as it flows through the cooling section since the process involves no humidification or dehumidification. The inlet and exit states of the air are completely specified, and the total pressure is $1$ atm. The properties of the air at the inlet state are determined from the psychrometric chart to be $$ \begin{aligned} h_1 & =76.1 \mathrm{~kJ} / \mathrm{kg} \text { dry air } \\ \omega_1 & =0.0159 \mathrm{~kg} \mathrm{H}_2 \mathrm{O} / \mathrm{kg} \text { dry air }\left(=\omega_2\right) \\ T_{\mathrm{dp}, \mathrm{I}} & =21.3^{\circ} \mathrm{C} \end{aligned} $$ The exit state enthalpy is $$ \left.\begin{array}{rl} P & =1 \mathrm{~atm} \\ T_2 & =T_{\mathrm{dp}, 1}=21.3^{\circ} \mathrm{C} \\ \phi_2 & =1 \end{array}\right\} h_2=61.9 \mathrm{~kJ} / \mathrm{kg} \text { dry air } $$ From the energy balance on air in the cooling section, $q_{\text {out }}=h_1-h_2=76.1-61.9=14.2 \mathrm{~kJ} / \mathbf{kg~dry~air}$
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