Design of a new spray-type seawater evaporator

The objective of this work is to design a low-cost evaporator. The new type of evaporator suggested here is of the spray-type, i.e., spaying the seawater into fine droplets to evaporate the water. The cost of this type of evaporator is comparatively low because no heat exchangers are required. The evaporator is designed based on the theory of cooling towers. The system is modeled and optimized using heat and mass transfer relations. With proper design the spray-mode of evaporation is superior to both pool boiling and thin-film evaporation. It is shown that the rate of evaporation is mainly influenced from the droplet size and temperature, i.e., the evaporation is enhanced by having small-diameter and hightemperature water droplets. A limitation of the suggested system is that either good filtration equipment needs to be used or the droplet size could not be low enough. Also it is desirable not to operate the systems with temperatures higher than 70°C, a temperature that can easily be obtained with comparatively cheap flat-plate solar collectors. Typical expected results are presented which prove the viability of the proposed system. The complete system consisting of the evaporator, a solar collector 1 m2 in area and pumps is modeled with TRNSYS. Such a system gives 11.2 m3/y of fresh water.

The objective of this work is to design a lowcost spray-type evaporator. The design of the unit was based on the theory of cooling towers. The complete unit was modeled with the TRNSYS program. The annual production of a unit with a collector area of 1 m2 is 11.2 m3, which is a α cpm — — Gt — Ka — Kw — K′ — M m — — ma mw n Pws qL — — — — — qs — qw — r M = 3.4×10 *T −0.0002527*T + 0.0055 −6 — ∆T — V Wa W″ — — — Area of interface, m2/m3 Humid specific heat of moist air on a dry air basis, J/kg K Total solar radiation falling on the collector surface, W/m2 Unit conductance sensible heat transfer from interface to main air stream, W/m2K Unit conductance heat transfer from bulk water to interface, W/m2K Unit conductance mass transfer from interface to main air stream, kg/s m2 Quantity of water evaporated, kg/s Mass transfer rate from interface to air stream, kg/s Air mass flow rate, kg/s Inlet water mass flow rate, kg/s Collector thermal efficiency Saturation pressure, Pa Rate of latent heat transfer from interface to airstream, W Rate of sensible heat transfer from interface to air steam, W Rate of total heat transfer from bulk water to interface, W Latent heat of evaporation (constant), kJ/kg Temperature difference, °C (i.e., inlet temperature to collector minus ambient temperature) Cooling volume, m3 Humidity ratio of air, kg/kg Humidity ration of interface (film), kg/kg