Natural circulation evaporators

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.

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. ... 

It is convenient to classify evaporators into natural circulation evaporators, forced-circulation evaporators, and film evaporators. 

Evaporators in which circulation is maintained, independent of the evaporation rate or heating temperature, through the heating element are known as forced circulation evaporators. Forced circulation systems are illustrated in Figs. i(a) and S(b). Forced circulation systems are more expensive than comparable natural circulation evaporators and are, therefore, used only when necessary. 

The basic t5q)es of evaporators are pot evaporators and circulation—either natural or forced—evaporators. Figure 11.20 shows a natural-circulation evaporator. To improve the economy of the process, vapor compression may be employed. Vapor-compression evaporators make the latent heat of condensation available at a higher temperature to use the energy potentials of vapors by compressing it and combining it with fresh steam input. 

Figure 11.20 Natural circulation evaporator with external vertical-tube heat exchanger. From Cooley and Clark fC4J.)...

FORCED-CIRCULATION EVAPORATORS. In a natural-circulation evaporator the liquid enters the tubes at 0.3 to 1.2 m/s (1 to 4 ft/s). The linear velocity increases greatly as vapor is formed in the tubes, so that in general the rates of heat transfer are satisfactory. With viscous liquids, however, the overall coefficient in a natural-circulation unit may be uneconomically low. Higher coefficients are obtained in forced-circulation evaporators, an example of which is shown in Fig. 16.2. Here a centrifugal pump forces liquid through the tubes at an entering velocity of 2 to... 

Liquid-side coefficients. The liquid-side coefficient depends to a large extent on the velocity of the liquid over the heated surface. In most evaporators, and especially those handling viscous materials, the resistance of the liquid side controls the overall rate of heat transfer to the boiling liquid. In natural-circulation evaporators the liquid-side coefficient for dilute aqueous solutions is between 1500 and 3000 W/m -°C (300 and 600 Btu/ft -h- F). The heat flux may be conservatively estimated for nonfouling solutions from Fig. 15.13. 

To summarize the experience gained, it was found in the case of the natural circulation evaporator that a small leak did not cause the system to plug when... 

---