ANS:A - long tube

A long tube evaporator is a type of heat exchanger used for the concentration of liquids through evaporation. In a long tube evaporator, the liquid to be concentrated flows through long tubes, typically arranged in a vertical or horizontal configuration, while steam or another heating medium is passed through the surrounding jacket or shell to provide the necessary heat for evaporation. Here's how a long tube evaporator works:

1. Liquid Feed: The liquid to be concentrated, often called the feed or liquor, is introduced into the long tubes of the evaporator.
2. Heating Medium: Steam or another heating medium is circulated through the shell or jacket surrounding the tubes. The heating medium transfers heat to the liquid inside the tubes, causing it to evaporate.
3. Evaporation: As the liquid flows through the long tubes, it is heated by the surrounding steam or heating medium. This heat causes the liquid to boil and evaporate, leaving behind the concentrated product.
4. Concentration: The evaporated vapor is typically collected and condensed back into liquid form, yielding a concentrated product. The concentrated product may be collected separately from the remaining liquid or continuously withdrawn from the evaporator.

Long tube evaporators are characterized by their relatively long tubes, which provide a large surface area for heat transfer and evaporation. This design allows for efficient heat transfer and evaporation, making long tube evaporators suitable for the concentration of various liquids, including juices, dairy products, pharmaceuticals, and chemicals. One advantage of long tube evaporators is their ability to handle viscous or fouling liquids, as the long tubes help to reduce the likelihood of fouling and allow for easy cleaning and maintenance. Additionally, long tube evaporators can be operated under vacuum conditions to reduce the boiling point of the liquid and minimize thermal degradation of heat-sensitive products. Overall, long tube evaporators are versatile and widely used in industries where liquid concentration is required, offering efficient and reliable operation for a range of applications.