In considering the heat balance for a single-effect evaporator, the heat content (enthalpy) of the evaporated vapor is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1 lb/hr of vapor will be produced by 1 lb/hr of live steam, since the values for the specific heat of evaporation on the heating and product side are about the same.
The steam consumption of evaporation plants can be considerably reduced by using the enthalpy (heat of condensation) of the vapor to heat a second effect. The vapor produced in this effect can be further used for heating of a third effect and so on.
As shown in the figure, 3 lb/hr of water can be evaporated by 1 lb/hr of live steam in a 3-effect plant. This corresponds to a specific steam consumption of 33%.
The total temperature difference is that between the maximum heating temperature in effect 1 and the lowest boiling temperature in the last effect. This is distributed between the individual effects and therefore the larger the number of effects, the smaller the temperature difference for each effect. This in turn increases the heating surface required to achieve a given evaporation rate. Increasing the number of effects increases the complexity of the plant arrangement and renders the operation and control more difficult. The product residence time will also increase.
Source: http://www.niroinc.com/html/evaporator/eenergy.html 3/2007;