How to select materials for multi-stage evaporators

Material selection for multi-stage evaporators (especially in seawater desalination, high-salinity wastewater treatment, and fine chemical industries) must simultaneously consider corrosion resistance, mechanical strength, thermal conductivity, processability, and economy. Different components and operating conditions require different material combinations. Based on comprehensive industrial practice and recent research, the following tiered selection principles can be followed:

I. Selecting the main material based on the corrosive medium.

II. Material selection by temperature-pressure zone

III. Material Selection by Component

IV. Economic Efficiency and Risk Control Strategies

1. Phased Investment: When water quality is uncertain, 316L or carbon steel heat exchangers can be used initially (pH adjusted to neutral). After stable operation, depending on corrosion levels, these can be partially replaced with titanium or duplex steel heat exchange elements, reducing initial investment risk.

2. Composite/Lined Structure: Titanium-steel composite plates and stainless steel-carbon steel composite pipes ensure corrosion resistance while saving on precious metal usage.

3. Surface Treatment: Polishing, passivation, or ceramic coating on 316L/2205 further improves pitting corrosion resistance and wear resistance.

4. Electrochemical Protection: Low-carbon steel sacrificial anodes are used at the titanium-dissimilar metal connections and replaced regularly to prevent galvanic corrosion.

5. Operation and Maintenance: High flow rate design (titanium pipe ≥3m/s) inhibits scaling, reducing cleaning frequency and downtime losses.

V. Quick Selection Reference Process

1. Measure the concentrations of Cl⁻, SO₄²⁻, F⁻, and NH₄⁺ in the influent, as well as pH, temperature, and solids content.

2. Use a table to initially determine the main material based on the "medium-temperature-pressure" triaxial analysis.

3. Compare the life cycle cost (LCC) of high-corrosion sections (first stage, heat exchange tubes, tube sheet): Titanium > 2205 > 316L > Carbon Steel.

4. Based on the initial investment budget, decide whether to adopt a composite plate, lining, or phased upgrade solution.

5. During the design phase, clarify the welding process (titanium requires argon arc welding + back shielding), anode specifications and replacement cycle, flow rate control, and monitoring point layout.

Conclusion

1. There is no "one-size-fits-all" material for multi-stage evaporators; selection must be tailored to the specific water conditions, temperature, and components.

2. High chlorine, high temperature → Titanium tubes + 2205 shell/tube sheet;

3. Medium chlorine, medium temperature → 2205/2507 entire section;

4. Low chlorine, low temperature → 316L or FRP;

5. Strong acid/fluorine-containing → Glass enamel/PTFE lining or Hastelloy alloy.

By combining strategies such as thin-walled titanium tubes to reduce thermal resistance, duplex steel to reduce wall thickness, composite plates to reduce precious metal usage, and sacrificial anodes to control galvanic corrosion, the total life-cycle cost of the equipment can be minimized while ensuring a 10-20 year lifespan.