Long-tube evaporator

Screw-pressed oil is aUowed to stand to settle out suspended soUds, filtered through plate filter presses, and then pumped to storage. The oil-rich solvent (miscella) from the solvent-extraction process is filtered or clarified, and most of the solvent is removed in a long tube evaporator. FinaUy, the concentrated oil passes through a stripping column where sparging steam is injected to remove the residual solvent. A metric ton of cottonseed yields ca 91... 

Figure 10.32. Long-tube evaporators (a) Rising film (b) Falling film...

Over-all heat transfer coefficients54 Long tube evaporators... 

Although equipment such as long-tube evaporators, thermosiphon reboilers, and tubular steam generators has been used for many years, rela-... 

Anderson el al. (A4), 1961 Theoretical and experimental study of heat transfer to liquid films in vertical long-tube evaporators. Theory based on use of universal velocity profile. 

Costs for forced circulation evaporators, long tube evaporators, and agitated falling-film evaporators are presented in Figs. 15-26 and 15-27. Information on purchased cost of specialized heaters is given in Figs. 15-28 and 15-29, while Figs. 15-30 and 15-31 present costs for process furnaces and direct-fired heaters. 

In long-tube evaporators, the liquid flows as a thin film on the walls of a long, vertical, heated tube (see Figure 10.32). Both falling film and rising film types are used. They are high-capacity units, suitable for low-viscosity solutions. 

Short-tube evaporators are being replaced today by long-tube evaporators to achieve a higher heat transfer coefficient (1). 

LONG-TUBE EVAPORATORS WITH UPWARD FLOW. A typical long-tube vertical evaporator with upward flow of the liquid is shown in Fig. 16.1. The essential parts are (1) a tubular exchanger with steam in the shell and liquid to be concentrated in the tubes, (2) a separator or vapor space for removing entrained liquid from the vapor, and (3) when operated as a circulation unit, a return leg for the liquid from the separator to the bottom of the exchanger. Inlets are provided for feed liquid and steam, and outlets are provided for vapor, thick liquor, steam condensate, and noncondensable gases from the steam. 

Circulation in the standard short tube evaporator depends upon boiling. Should boiling stop any solids present in the liquid will settle out. The earliest type of evaporator that perhaps could be called a forced-circulation system is the propeller calandria illustrated in Figure 11-10. Basically a standard evaporator with a propeller added in the downcomer, the propeller calandria often achieves higher heat transfer rates. The propeller is usually placed as low as possible to avoid cavitation and is placed in an extension of the downcomer. The propeller can be driven from above or below. Improvements in propeller design have permitted longer tubes to be incorporated in the evaporator. Propeller evaporators are sometimes used in Europe when forced circulation or long tube evaporators would be used in the United States. 

More evaporator systems employ this type than any other because it is versatile and often the cheapest per unit capacity. Long tube evaporators normally are designed with tubes 1 to 2 inches in diameter and from 12 to 30 feet in length. Long tube evaporators are illustrated in Figure 11-11. 

The long tube evaporator described above is often called a rising or climbing... 

The falling film version of the long tube evaporator (Figure 11-12) eliminates the problems associated with hydrostatic head. Liquid is fed at the top of long tubes and allowed to fall down the walls as a film. Evaporation occurs on the surface of the highly turbulent film and not on the tube surface. This requires that temperature differences be relatively low. 

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