Question

Which of the following alternatives could be employed to increase the capacity of evaporator?

• A. Decrease the concentrations of solids in the feed
• B. Use the copper tube in the fabrication of calandria
• C. Heat the feed beyond its boiling temperature corresponding to pressure in the evaporator
• D. Cool the feed below its boiling temperature corresponding to pressure in evaporation

Hint:

Evaporator Capacity Optimization: PREHEATING FEED ABOVE BOILING POINT induces instant flash evaporation and maximizes net steam evaporation capacity.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Feed enthalpy optimization and flash evaporation: preheating feed milk above the boiling temperature of the first effect ($T_{ ext{feed}} > T_{ ext{boil}}$) causes instant adiabatic flash evaporation upon entry into the vacuum chamber, dedicating 100% of calandria steam heat to evaporation rather than sensible heating.
Key Formula or Approach:
\[ q = \dot{m}_{\text{feed}} C_p (T_{\text{boil}} - T_{\text{feed}}) + \dot{m}_{\text{evap}} \lambda \quad \xrightarrow{T_{\text{feed}} > T_{\text{boil}}} \quad \mathbf{\dot{m}_{\text{evap}} \uparrow \text{ (Flash Evaporation Boost)}} \]

Step 2: Detailed Explanation:
In thermal engineering and multiple effect evaporator capacity optimization:
- Total evaporation capacity ($E = \text{kg water evaporated/hour}$) is governed by the heat available for phase change:
1. If feed enters colder than the boiling temperature ($T_{\text{feed}} < T_{\text{boil}}$), a large fraction of the steam heat in the calandria is wasted simply warming the liquid (sensible heating), reducing net water evaporation.
2. Heating Feed beyond its Boiling Temperature (C):
- When feed milk is preheated in external plate preheaters above the first-effect saturation temperature ($T_{\text{feed}} > T_{\text{boil}}$), it carries excess enthalpy.
- Upon entering the vacuum distribution head of the calandria, it undergoes instantaneous Adiabatic Flash Evaporation, flashing off vapor instantly before touching the tube walls.
- 100% of the calandria heat flux is then utilized purely for latent heat of vaporization, substantially increasing total evaporator capacity.

Step 3: Final Answer:
Therefore, capacity is increased by Heating the feed beyond its boiling temperature in the evaporator, corresponding to option (C).