Questions and Answers
What is the main purpose of evaporation in food process engineering?
To remove water from dilute liquid foods to obtain concentrated liquid products
How does evaporation differ from distillation in food process engineering?
Evaporation results in a liquid final product, while distillation does not
What is the role of a non-contact heat exchanger in the evaporation process?
To provide a means of transferring heat from low-pressure steam to the product without mixing them
Why does evaporation involve reducing the temperature for boiling in food processing?
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What is the main benefit of using a multiple-effect evaporator over a single-effect evaporator?
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Which type of evaporator is best suited for highly heat-sensitive products like orange juice?
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How can fouling of heat-exchange surfaces impact evaporation efficiency?
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How many effects are feasible for a rising-film evaporator given the conditions: steam at 110°C, boiling temperature of 50°C, and a required temperature differential of 14°C?
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In a forced-circulation evaporator, what prevents the liquid food from boiling inside the heat exchanger tubes?
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Which statement is true about agitated thin-film evaporators?
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What is a key advantage of plate evaporators compared to tubular evaporators?
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Which type of pump is generally used in forced-circulation evaporators to maintain high circulation rates?
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What is the primary advantage of using vapors from previous effects as the heating medium in subsequent effects of a triple-effect evaporator?
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Which type of evaporator is best suited for processing heat-sensitive food products?
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What is the primary function of the shell-and-tube heat exchanger mentioned in the context of natural circulation evaporators?
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What is the primary disadvantage of a batch-type pan evaporator compared to other evaporator types?
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In a triple-effect evaporator, what is the primary difference between the forward feed system and the backward feed system?
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How does the concentration of the liquid food product affect the performance of the evaporation process?
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Study Notes
Evaporation in Food Processing
- Evaporation is an important unit operation used to remove water from dilute liquid foods to obtain concentrated liquid products, providing microbiological stability and reducing transportation and storage costs.
Types of Evaporators
- Rising-Film Evaporator: requires 14°C temperature differential, only 4 effects are feasible with a 60°C total available temperature differential.
- Falling-Film Evaporator: can handle more viscous liquids, best suited for highly heat-sensitive products, typical residence time is 20-30 seconds.
- Rising/Falling-Film Evaporator: combines rising-film and falling-film sections, attains final concentration in two stages.
- Forced-Circulation Evaporator: involves a non-contact heat exchanger, eliminates boiling, and has low capital and operating costs.
- Agitated Thin-Film Evaporator: suitable for very viscous fluid foods, uses high-pressure steam and wiper blades for high agitation and heat transfer rates.
Characteristics of Evaporation
- Boiling Point Elevation: increase in boiling point over that of pure water, at a given pressure, can be estimated using Duhring's rule.
- Heat Transfer: reduced heat transfer rates occur due to boiling point elevation, fouling, and heat sensitivity of food products.
Evaporation Systems
- Single-Effect Evaporator: vapors produced are discarded, and the steam condenses inside the heat exchanger.
- Multiple-Effect Evaporator: vapors are reused as the heating medium in another evaporator chamber, increasing energy-use efficiency.
- Forward Feed System: partially concentrated product from the first effect is introduced as feed into the second effect.
Other Types of Evaporators
- Batch-Type Pan Evaporator: one of the simplest and oldest types of evaporators, used for heat-sensitive products, with a small heat transfer area and long residence time.
- Natural Circulation Evaporators: uses short vertical tubes and natural convection, resulting in smaller convective heat transfer coefficients and reduced processing capacities.
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