Evaporation design

Evolving Evaporator Design to Improve Heat Integration... 

Figure 15.3 Evaporator design with the help of the grand composite curve. (From Smith and Linnhoff, Trans. ICkemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Also like distillation, the thermal profile of evaporators can be manipulated by changing the pressure. However, the degrees of freedom in evaporator design open up more options. 

Optimization The primary purpose of evaporator design is to enable production of the necessaiy amount of satisfactoiy product at the lowest total cost. This requires economic-balance calculations that may include a great number of variables. Among the possible variables are the following ... 

The most common evaporator design is based on the use of the same heating surface in each effec t. This is by no means essential since few evaporators are standard or involve the use of the same patterns. In fac t, there is no reason why all effects in an evaporator must be of the same type. For instance, the cheapest salt evaporator might use propeller calandrias for the early effects and lorced-circiilation effects at the low-temperature end, where their higher cost per unit area is more than offset by higher heat-transfer coefficients. 

Major evaporator designs include forced-circulation, long-tube vertical (both rising and falling film), and calandria-type evaporators. The economics of a particular process will dictate the evaporator style and model best suited to a particular application. Forced-circulation and calandria evaporators are required for processes where crystals are formed. These evaporators are designed to keep crystals suspended in solution to prevent scaling of the equipment. Long-tube vertical evaporators are used to concentrate a liquid that does not have solids present. 

A variety of forced-circulation evaporator designs are available, with and without elutriating legs or cyclones. Different inlets and outlets are used on the vapor body, tailored for the particular application. For some applications, such as super-... 

A great variety of evaporator designs have been developed for specialised applications in particular industries. The designs can be grouped into the following basic types. 

Figure 10.22 The principal degrees-of-freedom in evaporator design.

EVOLVING EVAPORATOR DESIGN TO IMPROVE HEAT INTEGRATION... 

When a process requires an evaporation step, the problem of evaporator design needs serious examination. Although the subject of evaporation and the equipment to carry out evaporation have been studied and analyzed for many years, each application has to receive individual attention. No evaporation configuration and its equipment can be picked from a stock list and be expected to produce trouble-free operation. 

There are two basic evaporator designs that are typically used atmospheric and vacuum evaporation (Metals Handbook 1987). Atmospheric evaporation principles are similar to those of a heated open tank, with the exception that the heated liquid is sprayed over plastic packing in order to increase its surface area and accelerate evaporation. Atmospheric evaporators on chrome plating lines have sometimes been used simultaneously as evaporators and as plating bath fume scrubbers. Atmospheric evaporators are considerably less expensive than vacuum evaporators. Typical atmospheric evaporator capital costs range from 2500 to 4000, while vacuum evaporator costs can be an order of magnitude or more higher. In atmospheric evaporator systems, however, vaporized water is not recovered, as it can be in vacuum systems. 

Evaporation as a process operation involves the concentration of one more or solutes by transfer of the solvent from the liquid into the vapor phase. Evaporation also may be simply formation of vapor from a liquid. The heating medium, usually steam, is introduced in the steam chest connected to a set of tubes inside the evaporator body. The steam condenses causing some of the liquid outside of the tubes to vaporize. As a matter of economy, often a multiple series of evaporators are connected so that the vapor from one evaporator is introduced (at a lower pressure) into the steam chest of the next evaporator where it condenses, and so on. Dissolved solids can be deposited on the exterior of the heating tubes (scaling) so that different interior evaporator designs are used to reduce scaling. 

Chen, I.Y. KocamustafaoguUari, G. An experimental study and practical correlations for overall heat transfer performance of horizontal tube evaporator design. In Heat Transfer Equipment Fundamentals, Design, Applications, and Operating Problems Shah, R.K., Ed. The American Society of Mechanical Engineers New York, 1989 108, 23-32. 

Evaporators Design Examples Nomenclature Reeerences Appendix... 

If scale forms inside and/or outside the tube walls, additional resistance terms (Fig. 4) should be added to Eq. (2). Some typical values of overall heat-transfer coefficients are given in Table 3 for various evaporator designs. 

A series of articles, "Studies in Evaporator Design, by W. L. Badger, has also been published in 1920 and 1921 in Chemical and Metallurgical Engineering, Badger gives experimental data with special reference to the horizontal-tube evaporator. 

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