In this type of evaporator, boiling takes place inside the tubes, due to heating made (usually by steam) outside the same. The creation of water vapor bubbles inside the tube creates an ascensional flow enhancing the heat transfer coefficient. This type of evaporator is therefore quite efficient, 

the disadvantage being to be prone to quick scaling of the internal surface of the tubes. This design is then usually applied to clear, non-salting solutions. Tubes are usually quite long, typically 4+ meters. Sometimes a small recycle is provided.