Evaporators for Open-Cycle Systems

Open-cycle flash-evaporators include those with open-channel flow, falling films, and falling jets. These conventional evaporators typically perform to within 70% to 80% of the maximum thermodynamic performance at acceptable hydraulic losses. Research at the Solar Energy Research Institute (SERI), now the National Renewable Energy Laboratory (NREL), led to the development of a vertical-spout evaporator that can perform to within 90% of the thermodynamic limit. In this evaporator, water is drawn upward through a vertical pipe (a spout) and violently sprayed outward by escaping steam (Bharathan and Penney 1984). To enhance performance, the spray may fall on screens that further break up the droplets and increase the evaporation rate. To avoid pressure loss, the evaporator has simple intake and exit systems that separate the steam from the discharge. Steam continues through the system, and the remaining seawater is discharged from the bottom of the evaporator. This design was tested using both freshwater and seawater, and it was found to be equally effective under both circumstances (Rogers and Trenka 1989).

Violent flashing in a spout evaporator causes seawater droplets to be entrained by the steam. If they are not removed, these droplets can cause erosion and stress-corrosion cracking in turbine blades and contaminate the desalinated water discharge as well. Passing the steam through the commercially available mist eliminators used in the process industry removes a sufficient quantity of these seawater droplets (Bharathan and Penney 1983b).