Or how the efficiency of the plant is improved by addition of vapour thermocompressor


The MED-TVC evaporator is basically an MED evaporator fitted with a thermocompressor. The purpose of the thermocompression of the vapour is to take advantage of the pressure of the available steam, when this pressure is sufficient (i.e. above 2 bar abs), to enhance the units' performance.

The incoming steam, called motive steam, is fed into the thermocompressor through a sonic nozzle. Its expansion will allow low pressure steam from a cell of the evaporator to be sucked out. Both steams will be mixed in the thermocompressor body. The mixture is then compressed to the pressure of the first bundle through a shock wave. The latent heat of the sucked vapour is thus recycled in the evaporator and is again available for desalination, leading to energy savings.

The performance of a thermocompressor is expressed by the mass of sucked steam (in kg) per kg of motive steam. This ratio is usually called w. The higher the motive steam pressure, the greater the w value (for a given temperature difference between the extraction of the sucked steam and the condensation temperature of first cell). On the other hand, for a given motive steam pressure, the higher the temperature difference, the lower the value of w.

Very high gain output ratios (GOR) can be obtained with MED -TVC units. For example, Sidem has designed units with twelve cells and a motive steam pressure of 30 bar, having a GOR of 17 (i.e. 17 kg of distillate water produced per kg of steam fed into the thermocompressor). They have been in operation since 1989 in the French West Indies. MED -TVC is the only process capable of such performance.