Evaporation leaves the salts behind

Makeup water arrives with whatever the local supply carries: 40 to 90 mg/L of chloride in most inland cities, several hundred at a coastal site. Inside the tower, water leaves as vapour and everything dissolved stays in the basin. At four cycles of concentration a 60 mg/L supply becomes 240 mg/L, held at 35 to 45 C and broken into droplets in a moving air stream.

Chloride, temperature and oxygen are the classic pitting combination, and silt on the basin floor turns a pit into a crevice no oxygen can reach. A grade that behaves on a municipal water line can fail here inside four years. The problem is settled on the water analysis, not on the tonnage.

What is actually steel in the tower

Casing and fill are plastic, the basin usually concrete, the fan blades plastic or aluminium. Steel is in the fan deck frame, the access platforms, the headers and risers, the outlet strainers, the flumes and weirs, and the tube bundle of a closed-circuit unit.

That split creates two purchasing packages handled by different people. The structural package is a galvanized section order measured in hundreds of tonnes: hot-dip galvanized H-beam for the deck, angle and channel for bracing, Z275 coil for casing sheet where the tower is not plastic. The wetted package is stainless, priced per metre of pipe.

The structure lives on zinc, until it does not

Hot-dip galvanizing works because the zinc-iron alloy layer formed in the melt is both a barrier and a sacrificial anode, and it works only while the surface can breathe. On a deck hosed down weekly, that is a coating measured in decades. Under a permanently wet, silted bottom course it is not: zinc cannot rebuild its oxide film without oxygen, and white rust becomes a hole. Drainage decides between those outcomes, not coating weight.

Within a few hundred metres of the sea, chloride deposition eats the zinc faster, and surface preparation before the coating is the step most often cut on price. Galvanized processing - cut, formed, welded, then dipped - keeps the coating continuous at the joints; galvanizing first and welding afterwards leaves bare metal at every seam.

The basin line is where stainless starts

Sumps, outlet strainers, flumes and weirs sit at the water line, the hardest place in the plant: alternately wet and dry, with a band of silt and biological growth at the boundary. 304 sheet at 240 mg/L chloride and 40 C will pit under that deposit. 316L, with 2 to 3 per cent molybdenum, slows the attack without changing the mechanism, and 317L adds a little more molybdenum for a little more money and much the same outcome.

The step that changes the mechanism is a bigger addition. 317LMN is specified as superior to 316L and 317L in chloride service, nitrogen doing part of the work. 254 SMO goes further, with a molybdenum level of 6.0 to 6.5 per cent and nitrogen of 0.18 to 0.20 per cent, and its application list opens with seawater cooling and cooling water pipework. Its yield strength is nearly twice that of the 300 series, so a strainer basket made from thinner sheet keeps its stiffness.

Between the two sits 2205 duplex, a 22Cr-3Mo alloy with 5 to 6 per cent nickel: pitting and crevice resistance above 316L and 317L, roughly twice the yield strength, and a service range of -50 to +600 F. For a pump body, fan hub or shaft that sees chloride and vibration together, it is usually the cheapest answer that lasts. For a small circulating pump, a Monel 400 shaft is the other answer in the yard. Above both, S32760 covers a PREN 40 duty where 2205 already has a history.

Biocide changes the question

Continuous chlorination at 1 to 2 mg/L free chlorine is a duty 316L handles in most towers. Shock dosing at 10 to 20 mg/L, or a hypochlorite room with drips, fumes and a bund, is a different duty. C-276 is one of the few alloys whose data sheet covers wet chlorine gas, hypochlorite and chlorine dioxide. 904L, whose nickel runs 24 to 26 per cent and molybdenum 4 to 5 per cent, covers the middle ground where concentration is known and temperature moderate; 825 is the fallback where the stream alternates between oxidising and reducing. The dosing room is small tonnage and high consequence, and specifications there are often copied from the last project without asking whether the biocide changed.

Closed circuit: titanium is not the expensive option

A closed-circuit tower cools process water through a bundle: outside, circulating tower water; inside, the process stream. Where the outside is seawater or a high-chloride supply, titanium is the standard answer: 4.51 g/cm3, a corrosion rate below 0.005 mm per year in seawater, one twentieth that of 316L, with a wall thin enough to hold the heat transfer coefficient.

The trade is stiffness. Grade 2 titanium runs at 345 MPa tensile, so tube supports and baffles need closer spacing than a stainless bundle, and fan-induced vibration can set the life before corrosion does. Where the water is brackish rather than full seawater, 254 SMO is the substitute its own page recommends against high-nickel and titanium alloys, and it welds in an ordinary shop.

Headers and risers are the other wetted purchase. Large-diameter stainless pipe runs to 2,500 mm outside diameter with wall from 1 to 150 mm, which covers most distribution mains. Where the water is brackish, 2205 seamless pipe does the same job at higher strength and lower nickel. A 304/304L pipe remains right for a treated, monitored, low-chloride loop.

Fabrication is where the specification is lost

A tower is assembled from plate, tube and section, and the shop decides whether the alloy performs. A fan deck frame in Q235 plate, cut on a plate cutting line, bent and welded, then galvanized, is plain work; the steel structure behind it is standard fabrication. A stainless sump is not: welds must be pickled and passivated, cut edges dressed, carbon steel tooling kept off the surface.

Handrails, ladders and walkway posts are usually left in carbon steel long after the basin has gone stainless, and the first thing maintenance replaces. Square stainless tube from 10x10 to 300x300 mm, wall 0.5 to 75 mm, is the usual fix, and it takes the repaint off the schedule. H-section is the frame for the same walkway. A bolted joint between that tube and galvanized steel needs an isolating washer, or the zinc around the bolt is consumed as the anode.

The six numbers to send with the enquiry

Chloride in the makeup water, cycles of concentration, basin temperature, biocide type and peak dose, seawater yes or no, and design life. Add the structural finish as a zinc mass per square metre or a paint system, the basin dimensions, and whether the headers are flanged or welded in the shop.

With those, alloy and coating stop being opinions. A cooling tower is a water treatment vessel that happens to be tall. The steel is chosen on the water; the structure is chosen on the weather.