Tube Vertical Evaporator

The tube proportions in the basket evaporator are about the same as in the standard vertieal. One important feature of the basket-type evaporator is the ease with which a deflector may be added in order to reduce entrainment from spouting. The boiling in the vertical-tube evaporator is quite violent, and this tends to cause entrainment losses. This condition is accentuated if the liquor level in the evaporator is low. A baffle such as shown in Figure 9 largely prevents these losses and is much more easily added to the basket type than to the standard type. Other differences between the standard and the basket type are largely details of construction. 

Figure 11. Vertical tube evaporator (A) Tube sheets (B) Downtake (C) Condensate outlet (D) Non-condensed gas outlet (E) Liquor inlet (F) Thick liquor outlet.

Tube evaporators may be designed as vertical tube evaporators (VTE) or horizontal tube evaporators (HTE). In the VTE. vapor produced in one effect is condensed in the next effect. The effect is an evaporator chamber receiving heat from an external source or from a higher effect and producing vapor and brine which may serve as a heat source for ihc next elfecl. To obtain high efficiency in the use of heal energy, the process is repeated in several evaporator effects arranged in series. Sec also Evaporation. 

Figure 8. Quadruple effect. natural circulation vertical tube evaporator.

Several types of evaporators exist.34 The older, more traditional, evaporators are the Roberts and the Kestner, both rising film, tubular evaporators. The Roberts, first introduced in the 1800s, is known as a short tube, natural circulation, vertical tube evaporator. The tubes, inside which the evaporation takes place, are in the range of 1.5 to 3 meters in length. The Kestner evaporator consists of numerous long vertical tubes, 6 to 7.5 meters long, inside a cylindrical shell. In both, the juice to be concentrated is fed to the bottom of the tubes and heated, causing the juice to... 

It has been shown that the actual amount of heat transmitted will depend on a great many factors and will vary considerably with the type of apparatus and the kind of liquor. Outside of water distillation, the extreme limits in actual practice are probably 8 lb. per hour per square foot for electrolytic caustic liquor from 25 to 48° in a vertical-tube evaporator and 62 lb. evaporation per hour per square foot for malt extract from 5 to 30°Bd. in a rapid circulation film type evaporator with steam at 5 lb. and a vacuum of 28 in. Practical results of various liquids are given in later paragraphs, p. 375 et seq. 

Vertical Type.— Figure 12 The first vertical tube evaporator was built by Robert, and has the one great advantage over the horizontal type, that it can easily be cleaned with any ordinary flue cleaner. Figure 13 The same construction as the Robert evaporator with an addition of a very large downtake in the center to improve the circulation of the liquor. This type was first constructed by Claassen, and is now used to a large extent in the cane-sugar and malt-extract industries under the name of Standard Evaporator. ... 

Figure 14 shows an evaporator where the central downtake has been replaced by a large annular downtake. The steam chest does not form an integral part of the evaporator, but is placed inside the shell on separate supports. This construction was first built by Kauffmann, and introduced in the United States by Mantius. Figure 15 A vertical-tube evaporator with the steam inside and the liquor outside the tubes. The top of the tubes is closed, and the steam either enters through the main tubes (Sanborn), or through a small concentric tube (Pecqueur and Mantius).

Distilled Water.—Water will boil with a small temperature difference, and the apparatus is simple and cheap. It is therefore economical to combine a num ber of effects to one unit, and quadruple- and decituple-effect evaporators of the vertical-tube and film type are common practice. They are operated under pressure and vacuum, and the capacity will vary from 3 to 6 gal. per square foot according to the total temperature difference. Evaporator shells are made of cast iron or steel, and tubes of steel, brass or copper. There is no foaming, but generally all raw water contains considerable amounts of scale forming material and tubes have to be cleaned frequently. Sea water has to be used to a considerable extent for the manufacture of distilled water, and usually the Reilly coil type is used aboard ships, as it is claimed by the inventor that scale will not deposit on the tubes. Sometimes the 20°B6. liquor coming from the sea water stills is concentrated further for the recovery of sea salt in standard vertical-tube evaporators with salt filters. 

Ammonium Chloride.—The weak liquor will contain from 6 per cent to 10 per cent of salt, and is usually concentrated to 40 per cent in a horizontal-tube or vertical-tube evaporator. Capacity is about 2 gal. per square foot with a... 

Ammonium Nitrate.— The weak liquors are usually concentrated from 25 to 80 per cent in single effects of the horizontal-tube, or special vertical-tube evaporators. The steam pressure is from 5 to 20 lb. and the vacuum from 18 to 26 in. Evaporators have been built with cast-iron shells and cast-iron tubes, and also with aluminum tubes and enameled bodies. 

Potassium Bichromate.—Concentrated from 25 to 50 per cent in a vertical-tube evaporator with a steam pressure of 5 lb. and a vacuum of 26 in., at a rate of 1 gal. per square foot. Sodium chloride is separated in salt filters. Evaporators must be built of cast-iron shells and charcoal-iron or steel tubes. 

Potassium Sulphate.—The weak liquors containing from 10 to 20 per cent of sulphate crystals are concentrated in vertical-tube evaporators with a steam pressure of about 5 lb. and a vacuum of 27 to 28 in., at a capacity of 1 to IH gal. per square foot. The sulphate crystals are recovered in salt filters attached to the evaporators, which are built of either cast-iron or steel with steel or charcoal-iron tubes. 

The short-tube vertical evaporator. Fig. %(d), also known as the calandria or Robert evaporator, was the first evaporator to be widely used. Tubes 4 and 8 long, often 2" to 3" in diameter, are located vertically inside a steam chest enclosed by a cylindrical shell. The early vertical tube evaporators were built without a downcomer but did not perform satisfactorily, so the central downcomer appeared very early. There are many alternatives to the center downcomer different cross sections, eccentrically located downcomers, a number of downcomers scattered over the tube layout, downcomers external to the evaporator body. 

A solution of organic colloids is to be concentrated from 15 to 50 percent solids in a vertical-tube evaporator. The solution has a negligible elevation in boiling point. 

ATF Agitated Thin Film Evaporator VTE Vertical Tube Evaporator MSF Multi Stage Flash... 

A guide to the overall performance of a wide variety of typical vertical evaporator tubes with condensing outside and vaporization inside is given in Fig. 11.25. This survey by Alexander and Hoffman [164] was specifically directed at vertical-tube evaporators for desalination systems. It is seen that the best surfaces yield increases in overall coefficient up to 200 percent. 

FIGURE 11.25 Heat transfer enhancement in vertical tube evaporator (tubes 48, 70, 94 axial flutes others spiral flutes) [164]. 

L. G. Alexander and H. W. Hoffman, Performance Characteristics of Corrugated Tubes for Vertical Tube Evaporators, ASME Paper 7I-HT-30, ASME, New York, 1971. 

H. H. Sephton, Interface Enhancement for Vertical Tube Evaporator A Novel Way of Substantially Augmenting Heat and Mass Transfer, ASME Paper 71-HT-38, ASME, New York, 1971. 

H. H. Sephton, Upflow Vertical Tube Evaporation of Sea Water With Interface Enhancement Process Development by Pilot Plant Testing, Desalination (16) 1-13,1975. 

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