A heat exchanger device used to concentrate a solution by boiling. The solution is heated until vapour is released and removed, leaving behind a more concentrated solution. A drying process may then follow evaporation and low pressures may be used for heat-sensitive materials such as milk and other thermally labile foods. Several evaporators or ‘effects’ can be connected in series as a cascade process to obtain a desired evaporation with the benefit of good energy utilization. They may be operated as feed-forward or feed-backward. Cold feeds use a backward process because the boiling temperature in the last effect is lower and consequently the required temperature for evaporation is less than that required in the first evaporator. See multiple effect.