- Working Principle

Main information on the technical data and the process
MEDESA® is a MVC/MED plant that works on the principle of a combination of mechanical vapour compression with so called multi-effect distillation.

Figure 1 gives a diagrammatic view of the general function principle. One of the main components of a MEDESA® plant is a novel plate heat exchanger. On one side a falling film of seawater turns partially into vapour leaving the salt behind. At the same time the salinity of the remaining seawater increases and finally it is sluiced out as brine. The vapour is forced to the other side by a compressor. There it condenses and delivers its condensation energy through the heat exchanger plates to the evaporating falling film, serving as the source of evaporation energy. The produced distillate is very pure sweet water with salinity lower than 50 ppm.

The above described process establishes a perfect re-use of heat, avoiding totally extern heat-demand. Due to the driving force temperature difference in the heat-exchanger and the boiling point elevation of seawater the condensation side needs a pressure higher than the evaporation side. The compressor has to overcome this obstacle, hence it needs a powerful driving force, in this case delivered by an electric motor. This is the sole energy demand of MEDESA® main part and is very much lower than evaporation heat.

In order to cut the unit product costs it is useful to arrange two ore more heat exchangers as a cascade, connecting the evaporation side of one heat exchanger together with the condenser side of another. At this case the capacity increases without enlargement of the compressor. This arrangement is a so-called multi-effect plant consisting of one compressor combined with 1 to 4 heat exchangers. The latter will be called “effects”.

Figure 2 shows a four effect process, related to the heat-exchange processes, with one compressor. The rated water output may be adjusted to customer’s demands with the variation of the number of effects. To avoid sediments, minimize the expenditure for additive chemicals in the process, avoid bio-toxic effects, use cost-effective construction materials, guarantee low wear, low maintenance costs and high reliability the MEDESA® project is especially designed for a low-temperature range process (process temperatures lower than 60 °C).

The challenge of this process is the design and realisation of a novel water vapour compressor and the required heat exchangers. Therefore, and due to the demand for low specific water costs a highly efficient compressor with a high flow rate is definitely neede