Natural Circulation Evaporator

Figure 5. Natural circulation, long tube film evaporator.

Natural circulation Forced circulation Short-tube evaporators 200-600 400-2,000 hio = (16.1/dJ [Bik(cjx/k)f ... 

Piret, E. L. and H. S. Isbin, H.S., Two-Phase Heat Transfer in Natural Circulation Evaporator, presented at AlChE Heat Transfer Sym., St. Louis, MO., Dec. 13, (1953). 

Figure 14.17. Natural circulation evaporator with horizontal tubes...

Brooks, C. H. and Badger, W. L. Trans. Am. Inst. Chem. Eng. 33 (1937) 392. Heat transfer coefficients in the boiling section of a long-tube, natural circulation evaporator. 

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

Fig. 1. Natural circulation evaporators where C = condensate, E = entrainment return, F = feed, N = noncondensibles vent, P = product or concentrate, S = steam, V = vapor, and M = knitmesh separator (a) horizontal-tube, (b) short-tube vertical, (c) propeller calandria, and (d) long-tube recirculating.

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... 

The type of evaporator to be used and the materials of construction are generally selected on the basis of past experience with the material to be concentrated. The method of feeding can usually be decided on the basis of known feed temperature and the properties of feed and product. However, few of the listed variables are completely independent. For instance, if a large number of effects is to be used, with a consequent low temperature drop per effect, it is impractical to use a natural-circulation evaporator. If expensive materials of construction are desirable, it may be found that the forced-circulation evaporator is the cheapest and that only a few effects are justifiable. 

Two general types of evaporators are used, and their names refer to the type of circulation used to transfer heat to the liquor for evaporating the water. Natural circulation evaporators rely on a thermosiphon to circulate liquors while forced circulation units use a pump to achieve the required circulation. The heating tubes may be inside or outside the evaporator body, but most designs, especially the older calandria style evaporators, use internal tubes for heating (Figure 2). 

Natural circulation evaporators have overall coefficients of the order of 1.1-3.4 kW/(m °K) = 200-600 Btu/(h ft °F). Adding forced circulation may raise this to the order of llkW/(m °K) = 2000Btu/(hft °F). In agitated-film units, for Newtonian liquids with viscosity of the order of water, coefficients of the order of 2.3kW/(m °K) = 400Btu/(hft °F) may be obtained. As the viscosity increases tolO Newton sec/m = 10,000 centiPoise, the coefficient will drop to the order of 0.7kW/(m °K) = 120Btu/(hft °F). More extensive listings of overall coefficients for evaporators may be found in the literature. ... 

---