uncommon to find seven to ten effect evaporators, for example, in the production of pulp and paper, an industry with high energy costs and one that must evaporate enormous quantities of water. Figure 18 is a photograph of a large, outdoor installation of an eight effect, long tube, vertical evaporator system.
In addition to the reduction in steam usage, there is also a reduction in cooling water required to operate the last effect condenser. Approximately 30 pounds of cooling water must be provided for each pound of steam supplied to the first effect. The increased energy economy of a multiple effect evaporator is gained only as a result of increased capital investment, which tends to increase at about the same rate as the required area increases. A five-effect evaporator will usually require more than five times the area of a single effect because of the staging of the driving force, AT, which is less than a single effect evaporator. The only accurate method to predict changes in energy economy and heat transfer surface requirements as a function of the number of effects, is to use detailed heat and material balances together with an analysis of the influence of changes in operating conditions or rates of heat transfer. This, of course, requires a copious amount of engineering effort and computational work, a task performed best by sophisticated computer programs.
The distribution in each effect of the available temperature difference between condensing steam and process liquid can be allocated by the evaporator designer. Once the evaporator is put into operation, the system establishes its own equilibrium. This operating point depends upon the amount of fouling and the actual rates of heat transfer. Usually it is best not to interfere with this operation by attempting to control temperatures of different effects of an evaporator. Such attempts result in a loss of capacity since control usually can be accomplished by throttling a vapor imposing an additional resistance. The pressure loss results in a loss of driving force and a reduction in capacity.
The designer has a number of options to achieve the greatest energy economy with a given number of effects.[21' These are usually associated with the location of the feed in respect to the introduction of the steam. Figure 17 illustrates several methods of operation which are: forward feed, backward feed, mixed feed, and parallel feed.
Usually, heat transfer rates decrease as temperature decreases, so that the last effects have the lowest rates of heat transfer. By leaving the resistance of these effects higher, the designer can increase the temperature difference across them, thus increasing temperature and heat transfer rates in all the earlier effects. It has been shown that the lowest total area is required when the ratio of temperature difference to area is the same for all effects. When the materials of construction or evaporator type vary among effects, lowest total cost is achieved when the ratio of temperature difference to cost is the same for each effect. However, in most cases where evaporator type and materials of construction are the same for all effects, equal heat transfer surfaces are supplied for all effects.
Often in multiple-effect evaporators the concentration of the liquid being evaporated changes drastically from effect to effect, especially in the latter effects. In such cases, this phenomenon can be used to advantage by staging one or more of the latter effects. Staging is the operation of an effect by maintaining two or more sections in which liquids at different concentrations are all being evaporated at the same pressure. The liquid from one stage is fed to the next stage. The heating medium is the same for all stages in a single effect, usually the vapor from the previous effect. Staging can substantially reduce the cost of an evaporator system. The cost is reduced because the wide steps in concentrations from effect to effect permit the stages to operate at intermediate concentrations, which result in both better heat transfer rates and higher temperature differences.
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