Evaporators horizontal-tube natural circulation

For short-tube vertical evaporators the heat-transfer coefficients can be estimated by using the same methods as for the long-tube vertical natural circulation evaporators. Horizontal-tube evaporators have heat-transfer coefficients of the same order of magnitude as the short-tube vertical evaporators. 

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

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.

For solution evaporation, circulation evaporators are often used in the form of a tube bundle apparatus with vertical, horizontal or slanting tubes, operated with natural or forced solution circulation. TVibes may be mounted inside or, with respect to better cleaning, outside (Fig. 7-20) of the evaporator. The solution evaporates inside the tubes and is lifted by the vapor bubbles generated according to the principle of an air lift pump. Natural circulation of the solution is mainly intensified by adding circulation pumps (forced circulation). 

Vertical-type natural circulation evaporator. In this type of evaporator, vertical rather than horizontal tubes are used, and the liquid is inside the tubes and the steam condenses outside the tubes. Because of boiling and decreases in density, the liquid rises in the tubes by natural circulation as shown in Fig. 8.2-lb and flows downward through a large central open space or downcomer. This natural circulation increases the heat-transfer coefficient. It is not used with viscous liquids. This type is often called the short-tube evaporator. A variation of this is the basket type, where vertical tubes are used, but the heating element is held suspended in the body so there is an annular open space as the downcomer. The basket type differs from the vertical natural circulation evaporator, which has a central instead of annular open space as the downcomer. This type is widely used in the sugar, salt, and caustic soda industries. 

The heat transition which in the natural circulation evaporator is rather low in the liquid phase, is improved by means of a circulation pump. The pump presses up the product through the heating grid with tube speeds of up to 3 ms . This pump can be arranged vertically, or as shown in Figure 11.8, horizontally. 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

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