Step 1: Understanding the Concept:
Evaporation is a energy-intensive unit operation used to concentrate liquid foods by boiling off water.
Multiple-effect evaporators are designed to maximize energy efficiency by reusing steam.
Detailed Explanation:
In a multiple-effect evaporator, several evaporator vessels (effects) are connected in series:
- Fresh, high-pressure steam is supplied from a boiler *only* to the first effect to heat the product.
- The water evaporated from the product in this first effect becomes steam (vapor).
- This produced steam is then directed into the heating chest of the second effect, serving as its heating medium.
- To allow this to work, the second effect is operated at a lower pressure (and therefore a lower boiling point) than the first.
- This process continues through subsequent effects, with each effect utilizing the vapor produced by the previous one.
This design significantly reduces steam consumption compared to using fresh steam for every stage.
Step 2: Final Answer:
In a multiple-effect evaporator, the steam produced in the first effect is used to heat the next effect.