Evaporators Types

Heat rejected to the environment by most industries is of Httle consequence. Cooling flows of air or water are deployed over equipment or through heat exchangers and the relatively small quantities of heat are dissipated to the surrounding air. Small cooling towers, often of the evaporative type, have become ubiquitous in an industrial faciUty. 

Evaporator may refer either to the type of constmction utilized or to the entire assemblage of equipment in a single installation. Thus a single multiple-effect evaporator may contain a number of effects of either the same or different evaporator types. An effect is a section of the evaporator heated by steam at one pressure and releasing vapor (water) at a lower pressure to another section. The term steam generally indicates the heat supply, whereas vapor means the material evaporated. Thus vapor from one effect becomes steam at the next effect. The term prime steam identifies the steam suppHed from an outside source to operate the evaporator (see also Steam). An effect may consist of several bodies, all operating at the same steam and vapor pressures. The purpose of more than one body in an effect may be to handle Hquor at different concentrations, or the result of size limitations or of additions to increase the capacity of an existing evaporator. 

FIG. 11-122 Evaporator types, a) Forced circulation, (h) Siibmerged-tiihe forced circulation, (c) Oslo-type crystallizer, (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (ij) Horizontal-tube evaporators. G = condensate F = feed G = vent P = product S = steam V = vapor ENT T = separated entrainment outlet. 

Bonilla [Trans. Am. Inst. Chem. Eng., 41, 529 (1945)] developed a simplified method for distributing the heating surface in a multiple-effect evaporator to achieve minimum cost. If the cost of the evaporator per unit area of heating surface is constant throughout, then minimum cost and area will be achieved if the ratio of area to temperature difference A/AT is the same for all effects. If the cost per unit area z varies, as when different tube materials or evaporator types are used, then zA/AT should be the same for all effec ts. 

If the crystallizer is not of the evaporative type but rehes only on adiabatic evaporative cooling to achieve the yield, the heating element is omitted. The feed is admitted into the circulating hue after withdrawal of the slurry, at a point sufficiently below the free-hquid surface to prevent flashing during the mixing process. 

Ambient dry bulb temperature for air-cooled condensers Available water temperature for water-cooled condensers Ambient wet bulb temperature for evaporative types... 

The selection of the most suitable evaporator type for a particular application will depend on the following factors ... 

Evaporator type Very viscous > 1000 Medium viscosity < 1000 max Low viscosity < 100 Foaming Scaling or fouling Crystals produced Solids in suspension ... 

Figure 10.36. Circulating magma crystalliser (evaporative type)...

In contrast, the noncharring liquid evaporative-type material reaches a steady peak burning rate. For both types of materials, finite thickness effects and their backface boundary condition will affect the shape of these curves late in time. This is seen by the increased burning rate of the 2.5 cm thick polypropylene at 70 W/m2 in Figure 9.3(c), but not for the 5 cm thick red oak material at 75 kW/m2 in Figure 9.4(a). 

Operational category Evaporator type Feed condition3 Suitable for heat-sensitive products Retention timeb (s) Holding volume3 (m3)... 

The submitters used a still of the evaporative type, such as a Hickman still. The checkers used a similar still modified to include a magnetic stirring bar. This modification greatly decreases the time required for removal of the last traces of solvent, minimizes the danger of bumping just before evaporative distillation occurs, and increases the rate of distillation of the product. 

Cost Analysis of Large Evaporative Type Cooling Towers Chatfield, D. L. Streeton, D. F. 

A method is given for predicting the costs of large evaporative type natural and mechanical draft cooling towers as functions of the main design parameters. The costs and parameter factors are expressed analytically for use in power plant optimization programs. 3 refs, cited. 

Since most evaporators are purchased from outside suppliers either prefabricated or on-site-fabiicated, such suppliers can be an excellent source of information on selection of evaporator type. Suppliers and addresses can be found in the literature. ... 

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