Stainless steel for heat exchangers
The grade for a heat exchanger is chosen by medium and temperature: water, steam and food products — 304 or 316L, chlorides, sea water and acids — 316L or duplex, heating above 600 °C — 321 and 310S.
Corrugated stainless pipe for a sauna heat exchanger usually comes in a diameter of about 20 mm in 304 or 316L; the exchange catalogue offers round pipe 16–108 mm, from which coils, registers and shell-and-tube bundles are welded.
Media and temperatures
A heat exchanger works with two flows: the heating one and the heated one. The grade is chosen by the more aggressive of them, not by the average.
| Medium | Temperature | Grade | Comment |
|---|---|---|---|
| Water, steam | up to 400–600 °C | 304, 316L | The basic option for heating and DHW |
| Drinking water, food products | up to 150 °C | 304, 316L | Smooth surface, passivation of seams |
| Sea water, brines | up to 100 °C | 316L, duplex 2205 | Chlorides leave no choice |
| Acids and alkalis | depending on the medium | 316L, 904L | Selected for the specific medium |
| Flue gases, combustion products | 600–1000 °C | 321, 310S | Heat resistance is needed |
| Salt heat carrier, antifreeze | up to 150 °C | 316L | Depends on the composition of the solution |
Temperature matters not by itself but together with the load. A coil pipe at 500 °C and internal pressure works under different conditions than a casing sheet without pressure. Where there is heating under load, a grade with a margin is taken: 321 with titanium or 310S with a high content of chromium and nickel.
Choice of grade
304 covers water, steam, heating, DHW and food circuits without salt. It is the most common grade for registers, boilers and coils.
316L adds molybdenum and low carbon. Molybdenum gives resistance to chlorides, low carbon gives resistance of the weld seam without subsequent annealing. For sea water, brines, chemicals and food lines with aggressive washing, this is the main grade.
321 contains titanium, which binds carbon and prevents chromium from going into carbides during heating. It is taken for assemblies operating at 600–800 °C, where ordinary 304 loses resistance along grain boundaries over time.
310S withstands up to 1000 °C and works in gas ducts, furnace heat exchangers and assemblies with an oxidising atmosphere. It is a heat-resistant (scale-resistant) grade, not a heat-strength one: under a heavy load at high temperature its use is limited.
Duplex 2205 is chosen for sea water and chloride media, where strength and resistance to stress corrosion cracking are added to resistance. It is harder to weld than austenitic grades.
If there are chlorides in the flow, 304 is not suitable, even if the temperature is moderate. Pitting starts in stagnant zones and under deposits, and a heat exchanger consists precisely of narrow channels and crevices.
Pipes and sheets
A heat exchanger is assembled from two main types of rolled product.
- Round pipe — for coils, registers, shell-and-tube bundles and sauna heat exchangers. The catalogue offers diameters from 16 to 108 mm.
- Sheet — for plates, casings, bottoms and partitions. Thicknesses from 0.8 to 10 mm, formats 1250×2500 and 1500×3000 mm.
- Profile pipe — for frames, supports and housings where heat transfer through the wall is not needed.
Corrugated pipe for a sauna heat exchanger is a separate case. The corrugation gives a large contact area at a small diameter and withstands thermal expansion well, so it is taken for registers in stoves and tanks. The diameter is about 20 mm, grade 304 for water and 316L if the water contains chlorides or there is an aggressive medium in the circuit. Smooth pipe from the catalogue is used where a predictable flow and simple welding are needed: a coil, a register, a shell-and-tube unit.
The wall thickness is selected by pressure and flow velocity. A thin wall transfers heat better, but withstands pressure and erosion from a flow with mechanical particles worse.
Corrosion risks
- Chlorides in water. Pitting and crevice corrosion in the gaps between the pipe and the tube sheet, under gaskets and deposits.
- Stress corrosion cracking. Residual stresses after welding and bending plus chlorides and temperature produce cracks that are not visible from outside.
- Intergranular corrosion of the seam. Carbon in ordinary 304 and 316 binds into carbides during welding. The solution is low-carbon 304L and 316L.
- Scale and a depleted layer. A seam without cleaning and passivation becomes a centre of corrosion, especially in a water circuit.
- Stagnation and deposits. Scale and dirt hold moisture and chlorine at the surface, so places with a low flow rate corrode first.
- Galvanic couple. A carbon steel casing paired with a stainless bundle without insulation accelerates corrosion of the less resistant element.
- Erosion. A high flow velocity with abrasive particles removes the passive film faster than it recovers.
The practical minimum for a unit: a grade suited to the medium, a low-carbon variant for welding, cleaning and passivation of seams, flushing of the circuit before start-up and monitoring of deposits in operation.
Frequently asked questions
Which stainless steel is needed for a heat exchanger
For water and steam — 304, for chlorides, brines and sea water — 316L, for heating above 600 °C — 321 or 310S.
Is 304 suitable for a heat exchanger
Yes, if the medium is water, steam or a food product without salt. In chloride water, brine and sea water 304 is not used.
Which pipe is needed for a sauna heat exchanger
Usually corrugated pipe about 20 mm in diameter in 304, and in 316L if the water is aggressive. Smooth round pipe is taken for coils and registers.
Which grade is suitable for sea water
316L or duplex 2205. Ordinary 304 in sea water quickly becomes covered with pitting.
Is passivation of heat exchanger seams needed
Yes. After welding the seam is cleaned and passivated, otherwise corrosion will start along the heat-affected zone.
How does 316 differ from 316L for a unit
In carbon content. 316L does not lose resistance in the weld seam zone and does not require annealing after welding.
What to do about scale on the heat exchange surface
Remove it mechanically or chemically without abrasives that scratch the surface. After cleaning it is useful to repeat the passivation.
Can a stainless bundle be placed in a steel casing
It can, but insulation from the galvanic couple is needed: gaskets and bushes made of a dielectric, otherwise the casing will corrode.
What to look at next
- AISI 304 or 316: how they differ and what to choose — a breakdown of the main pair of grades.
- Heat-resistant and heat-strength grades: 310S, 20Х23Н18 — operation at high temperatures.
- Stainless round pipe — diameters 16–108 mm for coils and bundles.
- Stainless sheet — thicknesses 0.8–10 mm for plates and casings.
Place an order
Choose the grade, diameter and surface in the catalogue of stainless steel products and place an order with the required shipment week. The Incoterms basis and the mode of transport are set in the order before matching.