A 1 lbm / s flow of saturated moist air (relative humidity...

A $1 \mathrm{lbm} / \mathrm{s}$ flow of saturated moist air (relative humidity $100 \%$ ) at 14.7 psia and 50 F goes through a heat exchanger and comes out at 80 F. What is the exit relative humidity, and how much power is needed?

Key Concepts

Saturated Moist Air

Saturated moist air is air that contains water vapor at its maximum capacity for a given temperature and pressure, which corresponds to a relative humidity of 100%. This condition is pivotal in processes that involve phase changes such as condensation or evaporation, and it provides a baseline for analyzing changes in humidity upon heating or cooling.

Sensible Heating

Sensible heating refers to the process of adding energy to a substance (in this case, air) without changing its phase, thus changing its temperature. When moist air is heated sensibly, its capacity to hold moisture increases, leading to a reduction in relative humidity if the moisture content does not change.

Psychrometrics

Psychrometrics is the study of the thermodynamic properties of moist air, including temperature, humidity, and enthalpy. It involves using tools such as the psychrometric chart to understand and predict how these properties interact and change during processes like heating, cooling, humidification, and dehumidification.

Relative Humidity

Relative humidity is a measure of the current amount of water vapor in the air compared to the maximum amount the air can hold at the same temperature, expressed as a percentage. As the temperature of air increases or decreases, the air’s capacity to hold water vapor changes, thereby affecting the relative humidity even if the moisture content remains constant.

Heat Exchangers and Energy Balance

Heat exchangers are devices used to transfer thermal energy between two or more fluids without mixing them. In thermodynamic processes involving air, applying an energy balance allows one to calculate the power or energy required to achieve the desired temperature change by accounting for the mass flow rate and the specific heat capacity of the air.

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