Question

When the relative humidity of air reaches 100%, the 'Wet bulb temperature' becomes

• A. Higher than dry bulb temperature
• B. Lower than dry bulb temperature
• C. Equal to dry bulb temperature
• D. Equal to humid heat

Hint:

Psychrometric Equilibrium: At 100% Relative Humidity (Saturated Air) $\implies \mathbf{\text{Dry Bulb Temp (DBT)} = \text{Wet Bulb Temp (WBT)} = \text{Dew Point Temp (DPT)}}$.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Psychrometrics of saturated air: at 100% relative humidity ($ ext{RH} = 1.0$), air is fully saturated with water vapor, evaporative cooling from the wet bulb wick ceases, and Dry Bulb Temperature (DBT), Wet Bulb Temperature (WBT), and Dew Point Temperature (DPT) become identical.
Key Formula or Approach:
\[ \text{At } \mathbf{100\% \text{ Relative Humidity (Saturated Air)}}: \quad \mathbf{DBT = WBT = DPT} \quad (\text{Wet Bulb Depression } = 0) \]

Step 2: Detailed Explanation:
In psychrometry and air conditioning engineering for spray dryers and cold stores:
1. Dry Bulb Temperature (DBT): The actual ambient thermodynamic temperature of air measured by a standard shielded thermometer.
2. Wet Bulb Temperature (WBT): The dynamic equilibrium temperature reached by a wet wick thermometer due to evaporative cooling of water into unsaturated air.
3. At 100% Relative Humidity (RH = 100%) (C):
- The ambient air is completely saturated with water vapor and can hold no additional moisture ($p_v = p_{vs}$).
- Net evaporative mass transfer from the wet wick stops entirely; therefore, there is zero evaporative cooling depression.
- Consequently, the Wet Bulb Temperature becomes exactly Equal to the Dry Bulb Temperature (and equal to the Dew Point Temperature).

Step 3: Final Answer:
Hence, when relative humidity reaches 100%, Wet bulb temperature becomes Equal to dry bulb temperature, matching option (C).