13. Centrifuges

Centrifuges are machines used for solid-liquid separation. Process requirements for a solid-liquid separation are maximum clarification of liquid and minimum cake moisture.

Sedimentation centrifuges. These remove solids from liquids by causing solid particles to "settle" through the liquid radially toward or away from the centre of rotation. If the solids are less dense than the liquid they move inward to the liquid surface. If they are more dense than the liquid they "sink" outward and deposit on the inner surface of the bowl, from which they are removed manually, or mechanically by a conveyer or a knife. In some centrifuges heavy solids are not allowed to deposit on the bowl wall but excape continuously through nozzles as a thick slurry. Sedimentation centrifuges are usually comparatively small in diameter and may operate at extremely high speeds.

Most sedimentation centrifuges operate continuously, or nearly so, with respect to the feed and liquid discharge. In some applications it is necessary to interrupt the flow periodically to permit removal of accumulated solids. This interruption may be as long as 30 min or as a few seconds. In many designs clarified liquid and concentrated solids are both discharged continuously, so that the machine may operate for many hours or days without interruption.

The sedimentation centrifuges having many important characteristics resulted in their being widely used in chemical process industries. There are many types of cedimentatiom centrifuges, a solid-bowl centrifuge being one of them.

Solid-bowl centrifuges. This is a continuous-sedimentation unit, commonly called a decanter. It has no filter medium; separation is effected by density difference.

These devices are available in both horizontal and vertical types, their mode of operation being similar. They consist essentially of two concentric rotating elements surrounded by a stationary housing. The outer element is a solid-bowl tapered at one end; the inner element is a screw conveyer shaped to closely fit the bowl contour.

Slurry entering the cylindrical portion of the bowl is acted upon by centrifugal force, which separates the solids from the liquor and drives them against the bowl wall. The rotating conveyer - operating at a slightly different speed om

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that of the bowl - conveys the solids up through the conical portion of the vessel and discharges them as cake product. Clarified liquor continuously discharges over adjustable weirs from the other end of the bowl. Various types of bowl configurations are available to achieve good solids dryness and clarity of liquid.

Limited to solids that can be readily conveyed, these machines cannot handle some soft, slimy products, and viscous slurries reduce capacity. The units can, however, process a wide range of particle sizes - from 5 to 2,000 microns, depending on density and available G forces.

to discharge - разгружать

10. removal - удаление, устранение

11. housing - корпус, коробка

12. screw conveyer - шнековый конвейер

13. portion - зд.: часть /цилиндра/

14. adjustable -^ регулируемый

15. weir - водослив

16. slimy - илистый

17. unit - агрегат

Conmentary

1. ... shaped to closely fit the bowl contour - ... форма которого /конвейера/ соответствует контуру цилиндра

2. with respect to - в отношении, что касается

3. farces = gravity forces - силы тяжести

Words and word combinations

cake - осадок /после центрифугирования/

sedimentation - оседание, осаждение

to settle - оседать, осаждаться

bowl - чан, резервуар, цилиндр solid-bowl centrifuge -сплошной цилиндрический декантер

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5. to deposit - осаждаться, давать осадок

5. nozzle - выпускное отверстие slurry - суспензия, пульпа feed - подача, питание discharge - выход, сток

Answer the following questions:

1. What are centrifuges?

2. What are process requirements for a solid-liquid separation?

3. How do sedimentation centrifuges remove solids from liquids?

4. How are solids removed from the inner surface of the bowl?

5. How do sedimentation centrifuges operate?

6. What is a decanter?

7. What does a solid-bowl centrifuge consist of?

8. How does a solid-bowl centrifuge operate?

9. What products cannot be handled by solid bowl centrifuges?

Exercises I. Translate the following sentences paying attention to the meaning of the words "cause" and "both" :

1. Capitalist system is the cause of unemployment.

2. Heat causes many chemical changes in substances.

3. The increase in motion causes the molecules to collide with each other.

4. Change in air pressure is the chief cause of winds.

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5. What causes a cyclon, and what does it gauge in the open sea?

6. Both Lomonosov and his friend Rihman worked at atmospheric electricity. Both of them made experiments on that subject.

7. Both above and below 4 °С the density of water decreases.

8. Both substances were examined before solving.

9. We wrote to them both.

II. Translate the following sentences paying attention to:

1. the Absolute Participle Constructions and

2. Gerundial Constructions :

1. The equipment having been received, we began the experiments from the very beginning.

2. An architect designs the building, a civil engineer providing for its construction.

3. The temperature rising, the bodies expand, their volume increasing.

4. There are many good conductors of electricity, silver and copper being the best of all.

5. The elements having been arranged in the Periodic Table, it became easier to predict new elements.

6. Germanium was predicted by Mendeleyev the German scientist Winkler discovering it seventeen years later.

7. Your reading English articles on your speciality will help you, to mastering English.

8. We know of S.V.Lebedev's having made the first synthetic rubber in the world.

9. S.V.Lebedev's having obtained synthetic rubber was very important for our industry.

10. Plants are useful sources of energy thanks to their storing the sun's radiation in chemical form.

11. We were informed by the radio of a new spaceship having been launched into orbit.

12. Most metals may readily be recognized by their metallic lustre, when polished, and by their being good conductors . of heat and electricity.

13. The workers succeeded in completing the construction of the plant in time because of their having introduced new methods of work.

14. Before the Revolution few people knew of Tsiolkovsky's working at the problem of space flights.

III. Ask questions on all the parts of the following sentences!

1. A new experiment was made in our laboratory last week.

2. Peter'e brother has been working at the car factory for five years.

IV. Translate the following text using a dictionary:

AUTOMATION

"Automation" is a new word for a new purpose. Ordinarily automation is any Improvement in the control of some activity or process by non-human, i.e., automatic; means, but sometimes the term is defined more narrowly. Recent steps in automation have followed each other with unusual speed.. Many people are surprised to learn that in industries like chemical and oil-refining entire processes have become very nearly automatic

Electronic computers are becoming very good , at routine clerical work in offices and factories.

Automation has many sides. It includes, for example, mechanical equipment for handling and assembly. Machines of this kind are automatic In that they do the actual work on their owe; the operators only watch them and correct them whenever they go wrong - when, for instance, tools wear out.

But automation can also - mean automatic control of processes and machinery, and this' is a very different thing from mechanization, though the two go together. Control is necessary in a vast number of processes in order to maintain the quality of a product when the operating conditions, such as tem-

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perature and pressure, change from time to time.