Evaporator is usually used to remove water from most liquid streams. Typically, heat is added to the pumping circuit and a large vapor / liquid chamber is used to separate the two fluids. A vacuum can be used to help this flash, but it is not required. By connecting the units in series so that steam from one stage can be used in subsequent (lower pressure) stages (multi-effect) or by compressor (mechanical recompression) or steam injection (thermal recompression) is used to increase steam Stress and allow reuse in the same phase. Generally, multi-effect evaporators appear as flash systems and are controlled by VLE.
Dryers are commonly used to remove moisture from most solid materials in order to reduce their water content. This is typically done by providing heat, and is usually enhanced by blowing a lot of hot air through the bed and providing mechanical agitation. Where large amounts of air are involved, dryers are generally less thermally efficient and it is difficult to reuse evaporated water. VLE certainly affects its performance, but liquid absorption isotherms and mass transfer are the main mechanisms.
The evaporator produces a concentrated liquid product (sometimes very sticky), while the dryer produces a solid product. The stages of feed are not a complete distinction, as there are some dryers used with liquid feeds, such as spray dryers, vacuum dryers and film dryers. The latter can be used as a dryer or evaporator (also known as a scraped film or scraped surface).
