Evaporators, also called cooling towers, are used in many chemical processes.  In the desalination process, the evaporator is used to separate clean water from the salt water.  This is essentially an extraction process.

Salt water enters the system at the top of the unit at a temperature of 90°C.  This water comes from the solar collector and passes through a heat exchanger.  It is gently sprayed from above, in tiny droplets.  As the tiny drops of salt water fall downward, warm, dry air flows from the bottom of the unit to the top.  Some of the water in the droplets will vaporize; the salt will not vaporize, and will continue with the rest of the drop down the column.  With optimal sizing of the water droplets (by choosing the appropriate type of sprayer), the most water will change from the salt water phase to the humid air phase.  The proper droplet is found by maximizing the ratio of volume to surface area.  The droplet must be large enough (have enough mass) to fall at a sufficient velocity down the column, but the more surface area available, the more water mass transfer from the droplet to water vapor can occur. 

When the water moves from the salt water phase to the humid air phase, only water and other contaminants will vaporize, not the salt.  This will result in exiting water which has a higher concentration of salt than the entering water.  The humid air leaving the evaporator at the top, which is fed to the condenser, contains virtually no salt.  The brine  (the salty water exiting the evaporator) at the bottom will be recycled and combined with the feed solution at the entry point. 

The determination of exact flow rates, temperatures, and amounts of water removed from depends on many factors.  A complete design analysis was performed, using heats of vaporization and mass balances, among other tools.  The final design was found by combining this analysis with cost estimation.  The ideal evaporator was found to have the dimensions described in Table 8.

Table 8: Design parameters for evaporator