Question

The evaporative heat transfer coefficient is typically

• A. Dependent on the latent heat of vaporization
• B. Independent of air velocity
• C. Higher for still air compared to moving air
• D. Not affected by the liquid surface area

Hint:

In evaporative processes, always consider latent heat as a primary factor — it defines the energy needed for phase change and directly influences heat transfer.

Answer 1

The Correct Option is A

Evaporative heat transfer involves the removal of heat through the phase change of a liquid to vapor. The heat transfer coefficient in this process depends on several physical factors, the most fundamental being the latent heat of vaporization.
Latent heat of vaporization ($h_{fg}$) is the amount of heat required to convert unit mass of a liquid into vapor at constant temperature and pressure.
The rate of heat transfer during evaporation is given by:
\[ Q = h_{evap} \cdot A \cdot (T_{liquid} - T_{air}) \]
Here, $h_{evap}$ — the evaporative heat transfer coefficient — is directly influenced by the latent heat. A higher latent heat implies more energy is needed to vaporize the same amount of liquid, thus directly impacting the heat transfer rate.
Incorrect option analysis:
Option 2: Evaporation is strongly influenced by air velocity — more airflow enhances mass and heat transfer.
Option 3: Moving air increases evaporation, not still air — incorrect.
Option 4: Surface area of liquid affects the rate of evaporation — more area enhances heat transfer.
Therefore, the correct and most fundamental dependency of the evaporative heat transfer coefficient is on the latent heat of vaporization.