In case of multiple-effect evaporation plants, the exhaust vapors from the product are used to heat the downstream-arranged evaporation effect so that the steam consumption will be reduced accordingly.
Considering the heat balance of a single-effect evaporator the heat content (enthalpy) of the evaporated vapor is approximately equal to the heat input on the heating side.
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 (delta T). 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.