Understanding Sensible Heating of Moist Air

Sensible heating of moist air is a thermodynamic process where the temperature of the air is increased without adding or removing moisture. This process typically occurs at a constant pressure. We will analyze how different properties of the moist air change during this process.

Analysis of Moist Air Properties During Sensible Heating

• Relative Humidity (RH): Relative humidity is the ratio of the partial pressure of water vapor in the air to the saturation pressure of water vapor at the dry-bulb temperature. When moist air is heated sensibly, the dry-bulb temperature increases. The saturation pressure of water vapor increases significantly with increasing temperature. Since the amount of moisture (and thus the partial pressure of water vapor) remains constant during sensible heating, the relative humidity decreases because the denominator (saturation pressure) increases while the numerator (partial pressure) stays the same.
• Humidity Ratio (Specific Humidity): Humidity ratio is defined as the mass of water vapor per unit mass of dry air. In sensible heating, no water vapor is added to or removed from the air, and the mass of dry air also remains constant. Therefore, the humidity ratio remains constant during this process.
• Wet Bulb Temperature (WBT): Wet bulb temperature is the temperature measured by a thermometer covered in a wet wick and exposed to air flow. It is related to the enthalpy of the moist air. Sensible heating increases the dry-bulb temperature and thus increases the enthalpy of the air-vapor mixture. While the humidity ratio remains constant, the increased dry-bulb temperature and enthalpy lead to an increase in the wet bulb temperature, although the increase in WBT is less than the increase in dry-bulb temperature.
• Specific Volume: Specific volume is the volume occupied by a unit mass of the moist air mixture (mass of dry air plus mass of water vapor). Assuming the moist air behaves like an ideal gas mixture and the process occurs at constant pressure, the specific volume is directly proportional to the absolute temperature. Since sensible heating increases the dry-bulb temperature (absolute temperature), the specific volume of the moist air increases. This can be seen from the ideal gas law $PV = mRT$, or $\frac{V}{m} = \frac{RT}{P}$, where $\frac{V}{m}$ is specific volume, $R$ is the specific gas constant, $T$ is the absolute temperature, and $P$ is the pressure. With $P$ constant and $T$ increasing, specific volume increases.

Summary of Changes

Based on the analysis:

• Relative Humidity decreases.
• Humidity Ratio remains constant.
• Wet bulb temperature increases.
• Specific volume increases.

The question asks which of the listed properties increase. From our analysis, Wet bulb temperature and Specific volume increase during sensible heating of moist air.

Therefore, the correct answer involves Wet bulb temperature (3) and Specific volume (4).