If we consider the heat balance of a single-effect evaporator we find that the heat content (enthalpy) of the evaporated vapour is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1 kg/hr of vapour will be produced by 1 kg/hr of live steam, as the specific evaporation heat values on the heating and product sides are about the same. If the amount of vapour produced by primary energy is used as heating steam in a second effect, the energy consumption of the overall system is reduced by about 50 %.
This principle can be continued over further effects to save even more energy. The maximum allowable heating temperature of the first effect and the lowest boiling temperature of the final effect form an overall temperature difference which can be divided among the individual effects.Consequently, the temperature difference per effect decreases with an increasing number of effects. For this reason, the heating surfaces of the individual effects must be dimensioned accordingly larger to achieve the required evaporation rate, but with a lower temperature difference. A first approximation shows that the total heating surface of all effects increases proportionally to the number of effects. Consequently, the investment costs rise considerably whereas the amount of energy saved becomes increasingly lower.
|Live steam||Vapour||Specif. steam consumption|
|1-effect-plant||1 kg/h||1 kg/h||100 %|
|3-effect-plant||1 kg/h||3 kg/h||33 %|