ANS:C - Agitated film evaporator

A forced circulation evaporator is specifically designed to handle liquids with challenging characteristics, such as viscous, salting, and scale-forming liquors. Among the options provided, the Agitated film evaporator is an example of a forced circulation evaporator. Explanation:

1. Long vertical evaporator: Long vertical evaporators typically operate on natural circulation principles, relying on the density difference between the heated and unheated sections to drive circulation. They are suitable for relatively simple liquids but may not be optimal for handling viscous or scale-forming liquors.
2. Horizontal tube evaporator: Horizontal tube evaporators can operate on both natural and forced circulation principles. While they can handle a variety of liquids, their suitability for viscous, salting, and scale-forming liquors depends on specific design features and operational considerations.
3. Agitated film evaporator: Agitated film evaporators are a type of forced circulation evaporator. They feature mechanisms such as agitators or rotating blades to maintain turbulent flow and prevent fouling on the heat transfer surfaces. This agitation facilitates efficient heat transfer and helps prevent the buildup of scale or deposits, making them suitable for processing challenging liquors.
4. Calandria vertical tube evaporator: Calandria vertical tube evaporators typically operate on natural circulation principles and may not be specifically designed to handle viscous, salting, or scale-forming liquors as effectively as forced circulation evaporators.