Evaporator forced-circulation

The application of natural circulation, climbing-film and falling-film evaporators will be limited if the product is very viscous or if it strongly tends to incrustations. In this case, a forced-circulation evaporator can be used. In this evaporator type. 

The high speeds achieve similar good k values as in the falling-film evaporator. Coatings are rinsed off from the tube wall at the high-flow speeds. 

With forced-circulation evaporators, products can be concentrated until they can be pumped. It has to be made sure that the supply of the product to the circulation pump is good. 

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Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

Fig. 2. Forced circulation evaporators (a) submerged-tube, shown as circulating magma crystallizer (b) submerged-tube, shown as suspension type...

Forced-Circulation Evaporators In evaporators of this type in which hydrostatic head prevents boiling at the neating surface, heat-... 

Frequent difficulties with forced-circulation evaporators ... 

The type of evaporator to be used and the materials of construc tion 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 produc t. 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-circiilation evaporator. It expensive materials of construction are desirable, it may be found that the forced-circulation evaporator is the cheapest and that only a few effec ts are justifiable. 

FK . 18-64 Forced -circulation (evaporative) crystallizer, (Swenson Process Equipment, Inc.)... 

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-... 

Figure 9.18 Continuous crystallizers, a) draft-tube and baffle (DTB), (b) single effect forced-circulation evaporative, (c) Oslo or Krystal type after Rohani, 2001)...

In forced circulation evaporators the liquid is pumped through the tubes. They are suitable for use with materials which tend to foul the heat transfer surfaces, and where crystallisation can occur in the evaporator. 

Forages, as ruminant feed, 10 863 Foraminifera, 17 690 Forastero cocoa beans, 6 353 Forced air cooling, of food, 21 560 Forced circulation crystallizer, 8 136 Forced-circulation evaporators, in sodium chloride solution mining, 22 803, 804 Forced convection, 13 245 Forced drainage experimental procedure, 12 12... 

Figure 14.21. Forced circulation evaporator with an external pump...

For the handling of corrosive fluids, forced-circulation evaporators have been constructed in a variety of inert materials, and particularly in graphite where the unique combination of chemical inertness coupled with excellent thermal conductivity gives the... 

For some products it has been found advantageous to pressurise the plate heat exchanger, with an orifice or valve preventing boiling until the liquor enters the separator, in what is known as the APV Paraflash system. This is a special case of the forced circulation evaporator described earlier. 

Figure 8.16. Some types of evaporators, (a) Horizontal tube, (b) Calandria type, (c) Thermocompressor evaporator, (d) Long tube vertical, (e) Falling film, (f) Forced circulation evaporator-crystallizer, (g) Three types of Oslo/Krystal circulating liquid evaporator-crystallizers.

Because the feed to one effect in this type of evaporator is the partially concentrated sea water discharged from another effect, it was necessary to have a source of partially concentrated sea water for the pilot plant. Rather than trying to store previously concentrated sea water, with the possible result that some of the potential scale might deposit on storage, sea water is concentrated continuously in one or two forced-circulation evaporators which can be operated single or double effect, as necessary, and can provide sea water feed to the LTV at any desired concentration. 

In the second test, the acid free rate was adjusted to maintain an LTV blowdown pH of 7.5. The acid was fed to the forced-circulation evaporator preceding the LTV, so that the additional time available permitted evening out fluctuations in acid feed rate. Under these conditions, some of the total alkalinity was lost and the LTV operated under the following scaling environment ... 

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