Dairy & Food Engineering-II

• Heat Transfer Mechanisms in Vacuum Dryers:
• Under deep vacuum ($\lt 10\text{ kPa}$), gas density is extremely low, suppressing convective air currents.
• Heat transfers to drying product trays primarily by conduction from heated shelves and thermal radiation from upper shelf plates.
• Conclusion: Conduction and radiation (Option 2).

Correct option: (2) Conduction and radiation

• Evaporator Selection for Viscous Tomato Puree:
• Rheological Considerations:
• Tomato puree is a highly viscous, non-Newtonian, heat-sensitive suspension with high particulate solids that foul tubular heat transfer walls.
• For small-to-medium batch concentration, steam-jacketed open kettles equipped with mechanical scraping agitators prevent burn-on fouling while processing thick slurries effectively.
• (Tubular falling or climbing film evaporators are unsuitable due to immediate tube plugging).
• Conclusion: Jacketed open kettle evaporator (Option 1).

Correct option: (1) Jacketed open kettle evaporator

• Chronological Sequence of Evaporator Operations:
• In dairy and food engineering, the operational procedure for a falling film or forced circulation evaporator follows a rigorous thermodynamic sequence:

• Step 1: Preheating the feed solution: Raw liquid feed is preheated close to the boiling temperature using regenerate heat exchangers to conserve energy.
• Step 5: Heating the feed solution to its boiling point: The liquid is brought to its saturation boiling point within the evaporation chamber/calandria tubes.
• Step 2: Vaporization of the solvent from the feed solution: Latent heat of vaporization causes rapid water evaporation, concentrating the non-volatile milk solutes.
• Step 3: Condensation of the vaporized solvent to produce distillate: Separated water vapor passes to a condenser (or mechanical vapor recompressor, MVR) to produce condensate.
• Step 4: Removal of concentrated solution or residue: The concentrated product (condensed milk/dairy syrup) is continuously discharged through the bottom extraction pump.
  • Conclusion: The correct chronological sequence is 1 - 5 - 2 - 3 - 4 (Option 1).