Step 1: Understanding the Problem:
The question asks to identify why the condensation of steam on the utility (heating) side of a food evaporator is highly advantageous for the evaporation process.
Step 2: Key Formula or Approach:
The rate of heat transfer ($q$) in an evaporator is defined by:
\[ q = U A \Delta T \]
where:
$U$ = overall heat transfer coefficient ($\text{W/m}^2\cdot\text{K}$),
$A$ = heat transfer surface area ($\text{m}^2$),
$\Delta T$ = temperature difference between the steam and the boiling food liquid ($\text{K}$).
The overall heat transfer coefficient ($U$) is heavily dependent on the individual convective film heat transfer coefficients of both the steam side ($h_s$) and the product side ($h_p$).
Step 3: Detailed Explanation:
• Phase Change Heat Transfer: Condensation is a phase-change process. When steam condenses into liquid water on the metal tubes of an evaporator, it releases its latent heat of vaporization ($h_{fg}$).
• High Heat Transfer Coefficient: Convective heat transfer with phase change (condensation or boiling) is exceptionally efficient. The convective film heat transfer coefficient for condensing steam ($h_s$) is extremely high (often in the range of $5000 - 15000\text{ W/m}^2\cdot\text{K}$), which is significantly larger than that of sensible heating of gases or liquid water.
• Impact on Evaporation Rate: This high steam-side heat transfer coefficient directly raises the overall heat transfer coefficient ($U$), allowing rapid thermal energy transfer to the boiling food liquid, maximizing water removal rates.
• Why Other Options are Incorrect:
• Condensation utilizes latent heat, so it does not reduce latent heat transfer.
• Steam condensation occurs on the utility side, meaning it has no physical contact with the product side. Therefore, it cannot prevent product-side fouling or mineral scaling (which are caused by product thermal degradation and concentration).
Step 4: Final Answer:
Steam condensation is critical because phase-change heat transfer provides an exceptionally high heat transfer coefficient, making option (B) the correct choice.