Question

Unsaturated air (with dry bulb temperature and dew point being \( 35^\circ\text{C} \) and \( 18^\circ\text{C} \) respectively) is passed through a water spray chamber maintained at \( 15^\circ\text{C} \). The air will be:

• A. Cooled and humidified
• B. Cooled and dehumidified with increase in wet bulb temperature
• C. Cooled at the same relative humidity
• D. Cooled and dehumidified with decrease in wet bulb temperature

Hint:

When air contacts a water spray maintained at temperature \( T_s \): - If \( T_s > \text{DPT} \): Evaporation occurs \( \rightarrow \) Humidification. - If \( T_s < \text{DPT} \): Condensation occurs \( \rightarrow \) Dehumidification. Since \( 15^\circ\text{C} < 18^\circ\text{C} \), the process must be dehumidification. Because the air is cooled, its total enthalpy drops, meaning the wet bulb temperature must decrease.

Answer 1

The Correct Option is D

Concept: Psychrometric process tracking depends directly on comparing the initial state parameters of moist air against the physical temperature of the water spray matrix (the apparatus dew point or spray temperature, denoted as \( T_s \)). Key temperature parameters defined for unsaturated air are:
• Dry Bulb Temperature (\( \text{DBT} \)): The actual thermodynamic temperature of the air sample.
• Dew Point Temperature (\( \text{DPT} \)): The precise temperature at which the water vapor present in the air starts to condense out as liquid droplets.

Step 1: Identifying the operating regime by comparing temperatures.
From the problem text, we extract the initial state parameters of the entering unsaturated air stream:
• Entering Dry Bulb Temperature, \( \text{DBT}_{in} = 35^\circ\text{C} \)
• Entering Dew Point Temperature, \( \text{DPT}_{in} = 18^\circ\text{C} \) The temperature of the water spray chamber is explicitly given as: \[ T_{\text{spray}} = 15^\circ\text{C} \] Let us perform a critical comparison between the water spray operating temperature and the initial air dew point temperature: \[ T_{\text{spray}} = 15^\circ\text{C} < \text{DPT}_{in} = 18^\circ\text{C} \]

Step 2: Determining the physical mechanism taking place.
Because the water spray temperature is maintained lower than the dew point temperature of the incoming air stream, the air cools below its dew point as it contacts the cold water droplets. As a result, water vapor in the air must condense out of the gas stream onto the cold water droplets. This leads to a reduction in the absolute humidity (moisture content) of the air stream, a process known as dehumidification. Additionally, because the spray temperature (\( 15^\circ\text{C} \)) is significantly lower than the initial dry bulb temperature (\( 35^\circ\text{C} \)), sensible heat transfer occurs from the air to the water, which lowers the dry bulb temperature of the air stream (cooling). Combining these effects, the process is a Cooling and Dehumidification process.

Step 3: Determining the trend for the Wet Bulb Temperature.
During a standard cooling and dehumidification process, sensible heat is removed and latent heat is lost as water vapor condenses. Both total enthalpy and moisture content drop continuously during this operation. Since the wet bulb temperature (\( \text{WBT} \)) is a direct thermodynamic monotonic function of the total enthalpy of the moist air mixture, a continuous loss of total enthalpy ensures that the leaving wet bulb temperature drops below its initial value: \[ \text{WBT}_{out} < \text{WBT}_{in} \] Thus, the process is classified as cooling and dehumidification accompanied by a definitive decrease in the wet bulb temperature.