PTQ Q4 2025 Issue

Chloride concentrations as a function of temperature for different materials

Chloride content

Maximum temperature

20°C

30°C

60°C

80°C

120°C SS 304 SS 316 SS 316 SS 316

130°C SS 316 SS 316

= 10ppm =25 ppm = 50 ppm = 80 ppm

SS 304 SS 304 SS 304 SS 316 SS 316 SS 316 SS 316 SS 316

SS 304 SS 304 SS 304 SS 316 SS 316 SS 316 SS 316

SS 304 SS 304 SS 304 SS 316 SS 316 SS 316

SS 304 SS 304 SS 316 SS 316 SS 316

Ti Ti Ti Ti

= 200 ppm = 300 ppm = 700 ppm = 1,000 ppm > 1,000 ppm

Ti Ti

Ti Ti Ti Ti

Ti Ti Ti

– – –

Ti Ti

Table 2 Chloride concentrations as a function of temperature for different materials ( Outokumpus Corrosion Handbook, 11th edition, 2015 )

seawater input must be closely controlled, or else the back- wash filtration systems installed in the upstream become ineffective, and the heat exchangers function as filters. High amounts of dissolved oxygen and chloride pro- mote fouling and corrosion, especially in materials that are not intended for use in sea-cool applications. Oxygen also accelerates the corrosion of metallic surfaces. For example, titanium surfaces can oxidise and result in the formation of TiO₂ layers with lower thermal conductivity (~11.8 W/mK) compared to pure titanium (~22 W/mK), reducing heat trans - fer efficiency. The asymptotic behaviour is observed when the fouling deposition rate is in equilibrium with the fouling removal rate. The fouling layer reaches a comparatively constant thickness steadily, and the removal and deposition processes are bal- anced. Specific operating conditions and cleaning techniques used may affect the actual thickness of the fouling layer. Optimising the frequency and intensity of cleaning is necessary to manage the asymptotic behaviour of fouling in sea-cool systems and maintain an appropriate degree of fouling without causing excessive energy consumption or equipment damage. The effective operation of sea-cool systems and the reduction of fouling’s influence on heat transfer efficiency depend on routine monitoring of fouling

levels, performance metrics, and periodic maintenance and cleaning, as seen in Figure 1 and Figure 2 . More frequent cleaning can also allow the design of the heat exchangers to be optimised, further increasing the positive effect. Material selection Appropriate material selection is essential to prevent corro- sion, and most of the stainless-steel and carbon steel grades struggle in saline environments, whereas titanium is consid- ered a favoured material due to its greater corrosion resist- ance. Furthermore, titanium is recommended for offshore applications due to its natural low density. When weight savings are sought, titanium’s comparatively low density (4,500 kg/m3) compared to steels (7,200-8,000 kg/m³) and copper alloys (8,600-8,900 kg/m3) is advantageous. The chloride tolerance of several metals at different temper- atures, emphasising titanium’s durability, is outlined in Table 2 (sourced from Outokumpu’s Corrosion Handbook ). The performance of the heat exchanger does not solely depend on the selection of material but also necessitates advance- ments in flow distribution and turbulence optimisation. Next-generation heat exchangers The key features that outperform traditional and conventional

100%

Average eciency level

75%

50%

Average eciency level

25%

Time

Figure 1 Performance deterioration in traditional plate heat exchangers over time

Figure 2 Optimal performance of plate heat exchangers with periodic maintenance and optimised plate design

PTQ Q4 2025