An even more cost-effective idea would be to apply "the DualDuplex" concept and limit the use of 2205 to the lower part only (where conditions are hostile) and employ LDX 2101® or 2304 for the upper part of the evaporators.

	Melittah, the first MSF plant with solid Duplex stainless steel evaporator vessels, has a capacity of 15,840 m3/day. (Courtesy of Reggiane)

Once-through
For once-through plants, the internal environment plays a decisive role in materials selection due to the presence of oxygen and residual chlorine. The use of solid austenitic 254 SMO for the shell has proven to be successful in the first most hostile stages – though Super duplex SAF 2507 would be equally effective. Downstream stages should certainly utilize 2205.

Internal components
Orifice plates should preferably be made of the same grade as the shells: SAF 2507 or 2205. The selection of material for other internal components should be based on environmental conditions as described in the section on corrosion resistance.

Vent systems Even though stainless steel 1.4404 is commonly used for vent piping, this grade has suffered severe pitting and stress corrosion cracking in several plants.

Fresh water handling Use either 1.4307 or 1.4404 depending on the chloride content of the water and the desired safety margins, see material selection chart. Duplex LDX 2101®and 2304 are cost-effective alternatives to both these grades if the design calls for strength, as is the case for large water storage tanks. The external environment must also be considered and may occasionally play an important role in materials selection. Two Duplex 2205 absorber towers on their way to one of the largest MSF-plants in the UAE. (Courtesy of Ferrofab)