1.Read the text, prepare its essay. Evaporators

Evaporator Construction. Principles of Unit Operation. Evaporation is the operation of concentrating a solution by boiling away the solvent. The concentration is normally stopped before the solute begins to precipitate from solution. Basically, then, an evaporator must consist of a heat exchanger capable of boiling the solution and a device to separate the vapor

phase from the boiling liquid. In its most simple form it might be a pan of

liquid sitting on a hot plate. The surface of the hot plate is a simple heat

exchanger, and vapor disengaging is obtained by the large area for vapor flow and its consequent slow rate of flow. In industrial operation the equipment is usually arranged for con­tinuous operation; the heat-exchange surface is vastly increased; boiling is much more violent, and vapor evolution is rapid. Such problems as foaming, scaling, heat sensitivity, corrosion, and space limitations are met. These problems have resulted in variations and refinements in evaporator design to meet different combinations of solution properties and economic conditions.

Evaporator-body construc­tion falls into a few general categories. The horizontal-tube evaporator is one of the classic construction types and has been widely used for many years. The solution to be evaporated boils outside of horizontal tubes within which steamcondenses. Solution boils inside vertical tubes with the heating medium, usually condensing steam, held in a chest through which the tubes pass. In the basket evaporator, the steam chest forms a basket hung in the center of the evaporator. Boiling or heating of the liquid in the tubes causes flow upward through the tubes, and unevaporated liquid flows downward through the annulus around the basket. The tubes are inserted into tube sheets by rolling or welding.

Vocabulary

unit прибор, блок,

precipitate,

sediment осад

solute розчин

solvent розчинник

solubility розчинність

supersaturation перенасиченість

rate of flow швидкість потоку

foam піна

scale рівень, шкала

sensitivity чутливість

refinement очищення

evolution, discharge вихід, випуск

basket evaporator камерний випарник

2. Complete the sentences.

1. Evaporation is the operation of ____2. Basically, an evaporator must consist of ____. 3. The surface of the hot plate is ____. 4. In industrial operation the equipment is usually ____. 5. Such problems as ____. 6. These problems have resulted in ____. 7. Evaporator-body construc­tion ____. 8.The horizontal-tube evaporator is ____. 9. The solution to be evaporated boils outside of horizontal tubes within which steam condenses.10. In the basket evaporator, the steam chest forms a basket hung in the center of the evaporator.