Multiple-effect evaporation steam economy

Summary For the same capacity, steam requirements are significantly less for multiple-effect evaporators, than for single-effect evaporators generally, the economy increases with the number of effects. The price that is paid for this increased steam economy is greater equipment costs generally the cost of the equipment is proportional to the number of effects. 

There are several ways to increase the steam economy, or to get more evaporation with less steam input, for certain types of evaporation applications. The use of multiple-effect configurations or compression evaporation can be considered for large flow rates of relatively dilute aqueous solutions. Both multiple-effect and compression evaporation systems require a sizable incremental capital investment over single-effect evaporators, and these systems are larger and more complex than the simpler one-stage evaporators. Like the multiple-effect evaporators described above, compression evaporation systems can only be justified by a reduced level of steam consumption. 

Energy economy and evaporative capacity are the major measures of evaporator performance. When evaporating water with steam, the economy is nearly always less than 1.0 for single-effect evaporators, but in multiple-effect evaporators it is considerably greater. Other performance variables to be considered include product quality, product losses, and decrease in performance as scaling, salting, or fouling occurs. 

The principal measures of the performance of a steam-heated tubular evaporator are the capacity and the economy. Capacity is defined as the number of kilograms of water vaporized per hour. Economy is the number of kilograms vaporized per kilogram of steam fed to the unit. In a single-effect evaporator the economy is nearly always less than 1, but in multiple-effect equipment it may be considerably greater. The steam consumption, in kilograms per hour, is also important. It equals the capacity divided by the economy. 

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