Seawater-Cooled Shell and Tube Heat Exchangers: Material Selection & Design Guide

Seawater is abundant and free, which is why it is the default cooling medium for ships, offshore platforms, coastal power stations, desalination plants and LNG terminals. It is also one of the most aggressive fluids a heat exchanger will ever see: ~3.5% chlorides drive pitting and crevice corrosion, dissolved oxygen accelerates general corrosion, biofouling from marine organisms restricts flow and creates deposit-cell corrosion, and entrained sand causes erosion at tube inlets. A shell and tube heat exchanger cooled with seawater must be engineered around these mechanisms — not just for its thermal duty.

Standard 304/316L stainless steel is not acceptable on the seawater side; chloride pitting will perforate the tubes within months. The proven options are: (1) Titanium Grade 2 — effectively immune to seawater corrosion at any temperature and velocity, the default choice for plate exchangers and increasingly for tube bundles; (2) Cu-Ni 90/10 — the traditional marine tube material, good biofouling resistance and moderate cost, limited to ~3.5 m/s velocity and clean seawater; (3) Cu-Ni 70/30 — higher strength, higher velocity limit (~4.5 m/s) and better resistance to polluted or sulphide-bearing seawater; (4) super-duplex stainless (UNS S32750 / S32760) — excellent chloride pitting resistance with high strength, common on FPSOs and offshore utilities; (5) nickel alloys (Alloy 625, Alloy C-276) — reserved for hot, contaminated or sour seawater services where duplex is not enough.

The shell side often runs clean fresh water, glycol or lube oil, so carbon steel or 316L stainless is usually acceptable there. The critical interface is the tubesheet, which sees seawater on the tube-side face: use a solid Titanium, Cu-Ni or super-duplex tubesheet, or clad a carbon steel tubesheet with the tube material by explosion bonding or weld overlay. Water boxes and channel covers should match the tube material or be internally coated (rubber lining, epoxy, or metallic cladding) — a mismatch between water-box and tubesheet material sets up galvanic attack.

Keep tube-side velocity between 1.5 and 3.5 m/s for Cu-Ni (up to 4.5 m/s for 70/30 and titanium) — too low invites deposits and biofouling, too high causes impingement and erosion-corrosion at the tube inlet. Fit inlet ferrules or plastic inserts in the first 100–150 mm of each tube to protect against inlet erosion. Design for periodic mechanical cleaning: removable channel covers, straight tubes, and 25 mm minimum tube ID to accept brush cleaning or bullet cleaning systems. Provide sacrificial anodes (typically zinc or aluminium) or an impressed-current cathodic protection system on Cu-Ni water boxes. Avoid dead legs where seawater can stagnate and support anaerobic bacteria.

Marine and offshore seawater coolers are typically built to ASME VIII Div. 1 (or PED 2014/68/EU) with class-society approval — ABS, DNV, LR, BV, CCS or KR depending on the vessel. Testing typically includes hydro on shell and tube sides, helium leak or eddy-current tube testing, and PMI (positive material identification) on every tube and tubesheet — a single mis-supplied 316L tube in a Cu-Ni bundle will fail in service. Full traceability with mill certificates (EN 10204 3.1 or 3.2) is standard.

We fabricate seawater-service shell and tube heat exchangers in-house in Titanium, Cu-Ni 90/10 and 70/30, super-duplex and nickel alloys, with PMI on every pressure part, tube-to-tubesheet welding qualified to ASME IX, and class-society-witnessed hydro and NDT. Long tube lengths, double tubesheets for critical services, and removable water boxes for high-fouling seawater are supplied as standard options. Send your seawater analysis, flow rates and temperatures and we will return a material recommendation, thermal selection and quotation — contact Evan at jxmike@shheatex.com or WhatsApp +86 173 1725 8304.