When moist air is heated up,

Question

BlackTom AI · Accepted Answer

Consider the question: When moist air is heated up, what happens to various properties?
Option a: 'The wet bulb temperature does not change.' In reality, wet bulb temperature tends to rise when air is heated, especially if humidity changes are involved; this option overlooks the thermodynamic response of moist air to heating, so it is unlikely to be correct.
Option b: 'the enthalpy of moist air per mass of dry air does not change.' Heating moist air adds energy to the system, so the enthalpy per unit mass of dry air increases rather than staying constant; this makes this statement false.
Option c: 'the absolute humidity does not change.' Absolute humidity is the mass of water vapor per unit volume of air. When air is heated at constant pressure, its capacity to hold water vapor increases, but the actual amount of water vapor (and thus absolute humidity) remains the same unless moisture is added or removed. Therefore, absolute humidity staying unchanged while heating is consistent with a closed system for water content.
Option d: 'The relative humidity of the air does not change.' Relative humidity depends on both actual vapor pressure and saturation vapor pressure, which increases with temperature; as a result, relative humidity typically changes with heating unless moisture content adjusts. Hence this option is incorrect.
In summary, the only statement that correctly reflects the behavior of moist air upon heating—assuming no moisture added or removed—is that the absolute humidity does not change, while the other properties generally undergo change due to the temperature dependence of saturation and energy content.

BH20A0720 Engineering Thermodynamics - Blended teaching 2.9.2024-15.12.2024 2nd full exam 11.03.2025 at 16:30-19:30

When moist air is heated up,

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