Selecting a Cryogenic Heat Exchanger for LNG Service
Regasification, boil-off gas handling and cold-box duty all come down to the same problem: transferring a large amount of heat across a very small temperature difference, at metal temperatures where ordinary carbon steel becomes brittle. A cryogenic heat exchanger for LNG is not a scaled-down process exchanger — it is a different design problem, with different materials, different failure modes and a different set of questions on the datasheet.
This guide covers the vaporizer and exchanger types used in LNG service, the material rules below −100 °C, thermal shock and cooldown control, the sizing numbers that matter, and the specification checklist to send to a fabricator.
The Main Cryogenic Exchanger Types and Where Each Fits
Four families cover almost all LNG heat transfer duty. They are not interchangeable; each is dominant in a particular part of the plant.
| Type | Heat source | Typical duty range | Strength | Weakness |
|---|---|---|---|---|
| Open rack vaporizer (ORV) | Seawater falling over panels | Large base load, 100–300 t/h per unit | Very low operating cost, simple, no fuel | Needs clean seawater; discharge temperature limits; marine growth and corrosion |
| Submerged combustion vaporizer (SCV) | Burner firing into a water bath | Base load and peak shaving | Fast start, compact, climate-independent | Fuel and CO₂ cost, visible plume, burner management complexity |
| Ambient air vaporizer (AAV) | Atmospheric air over finned tubes | Small and modular, 10–50 t/h per unit | Zero operating cost, no utility, passive | Large footprint, capacity collapses in cold weather, icing |
| Intermediate fluid vaporizer (IFV) | Seawater or waste heat via propane or glycol loop | Mid to large | Separates LNG from the heat source, good control | Two heat transfer steps, higher capex, more equipment |
Alongside these, shell-and-tube and brazed plate units appear as interstage coolers, condensers, boil-off gas recondensers and heater circuits inside the cold box. The choice between plate-fin, brazed plate and shell and tube follows the usual rules for pressure, fouling and cleanability — with the cryogenic constraints layered on top.
Why Cryogenic Duty Is Not Simply Cold Process Duty
Three effects separate LNG service from ordinary refrigeration work.
Brittle fracture is a material property, not a stress calculation
Carbon steel and most ferritic steels transition from ductile to brittle behaviour well above LNG temperatures. At −162 °C a material that has plenty of strength margin will still shatter at a stress far below yield if it is in the wrong metallurgical condition. Material selection therefore follows impact-test rules, not allowable-stress rules — this is the core of the stainless shell and tube design work used on low-temperature duties.
Thermal shock limits how fast the unit may be cooled
A wall that sees −160 °C on one face and +20 °C on the other carries a thermal gradient across a few millimetres of thickness. With a coefficient of thermal expansion around 16 × 10⁻⁶/K for austenitic stainless steel, a 180 K drop across a wall produces stresses measured in hundreds of MPa. The unit is usually fine at steady state and fails during a fast cooldown, or at a badly designed nozzle or tubesheet joint. Cooldown rate limits, warm-up rate limits and maximum permissible differential temperature between shell and tubes are all contractual numbers, not suggestions.
Heat leaks in as parasitic load
Every metre of uninsulated piping, every support and every instrument connection adds heat that must be removed again. On small boil-off gas or satellite duty, parasitic heat can be a double-digit percentage of the design duty. Insulation specification, vapour barriers and cold-bridge detailing belong in the exchanger scope, because the question “how much of my duty is heat leak?” is normally answered by the fabricator.
Material Selection Below −100 °C
The practical shortlist is narrow and well established.
| Material | Typical minimum service temperature | Notes for LNG service |
|---|---|---|
| Austenitic stainless 304 / 304L | −196 °C and below | The default for tubes, shells and internals; face-centred structure stays ductile |
| Austenitic stainless 316 / 316L | −196 °C | Used where chloride from seawater or an IFV loop is present |
| Aluminium 5083 / 6061 and finned extrusions | −196 °C | Standard for AAV finned tube panels and plate-fin cores; excellent conductivity, poor weldability in thin sections |
| 9% nickel steel | −196 °C | Pressure vessels and storage tank shells where strength and cost per tonne matter |
| Duplex 2205 / super duplex 2507 | −50 °C, with impact testing | Seawater-side components in ORV and IFV loops, not for the coldest LNG side |
| Copper alloys, brass, carbon steel | Not suitable | Brittle or unacceptable at LNG temperature — excluded on the LNG-wetted side |
Two documentation points matter as much as the material grade. First, impact testing must be specified with a test temperature below the lowest expected metal temperature, not the design fluid temperature, because a depressurisation or a cold blowdown can drive the wall colder than the steady-state condition. Second, all welds on the cold side need procedure qualification for the low-temperature regime, plus a documented leak and pressure test — covered on the pressure-retaining side by D1/D2 pressure vessel certification and ISO 9001/14001 quality systems when the exchanger is built as a vessel.
Heat Transfer Design: What Changes at Low Temperature
LNG-side film coefficients stay high
Liquid methane has a high density and a low viscosity, so the tube-side coefficient in a pumped LNG stream is large — commonly in the range of 2,000–6,000 W/m²K at realistic velocities. The controlling resistance is nearly always on the warm side: condensing steam, a water bath, seawater, or air over fins.
The warm side sets the surface area
Because the driving force is small relative to the absolute temperatures, and because the warm-side coefficient is low, the surface area is driven by the warm-side limitation. This is why SCVs and steam-heated vaporizers use plenty of tube surface even though the LNG side is thermally easy, and why AAVs need an extremely large fin area for a modest duty.
Two-phase and frosting behaviour govern control
Vaporising LNG passes through a wide quality range, so the wall temperature distribution is non-uniform along the tube. Frost, ice or hydrate formation on the cold face of an AAV progressively insulates the surface and cuts capacity, which is why AAV banks are arranged with hot-gas bypass, alternate-bank operation and timed defrost cycles rather than as a single passive block.
The Icing Trap in Ambient Air Vaporizers
An ambient air vaporizer is the cheapest to operate and the easiest to get wrong. Capacity falls with air temperature, humidity and fin blockage simultaneously, and the three arrive together in the worst month of the year.
- Capacity is rated at a design ambient, not an average. Ask for a performance curve across the expected seasonal range, not a single point.
- Humidity drives icing more than temperature alone. A humid 5 °C day can be harder on the unit than a dry −10 °C day.
- Standby banks are part of the design, not a spare. Most AAV installations divide the array into banks that alternate, allowing one to defrost while the others carry the load.
- Spacing cannot be compressed. AAV arrays need large air-side clearances, and blocking the inlet face is the fastest way to lose a third of the rated duty.
For sites with a reliable steam header, a steam-heated shell and tube vaporizer is the more compact answer, and for sites with neither air space nor steam, a submerged combustion unit is the usual fallback.
Specification Checklist for a Cryogenic Heat Exchanger
- Duty in kW or send-out rate in t/h, at the design and minimum turndown points
- LNG supply pressure and required outlet pressure and temperature
- Composition of the gas, including nitrogen and heavy ends — this sets the temperature profile
- Warm-side medium, inlet and outlet temperature, and available flow
- Design and minimum metal temperature, and the code basis for impact testing
- Permitted cooldown and warm-up rates, and the maximum differential temperature across the tube wall
- Material specification for tubes, shell, tubesheet, baffles, fins and all cold-side welds
- Corrosion allowance or the decision to use it, per side
- Heat-leak allowance, insulation type and vapour barrier scope
- Hydrostatic and leak test pressure, plus any helium or vacuum test requirement
- Third-party inspection, certification scope and the documentation package
- Footprint, weight and lifting points for transport and installation
- Defrost or hot-gas bypass arrangement, if ambient air is the heat source
- Instrumentation: temperature, pressure and any frost or flow monitoring
The Bottom Line
Cryogenic exchangers are selected on material toughness, thermal gradient control and warm-side surface area — not on a simple heat-balance calculation. Get the impact-test regime and the cooldown limits into the specification before the first drawing is issued, and the rest of the project is ordinary mechanical design.
Zhenggong Cooling and Heating Equipment manufactures shell and tube, plate and air-cooled heat exchangers together with low-temperature refrigeration packages from a 35,000 m² facility in Shandong, China, holding D1/D2 pressure vessel certification and ISO 9001 plus ISO 14001 systems, and supplies industrial customers worldwide. For a cryogenic or low-temperature duty, send the process conditions to the engineering team for a thermal and mechanical assessment. Our wider heat exchanger and refrigeration product range is summarised on the product pages, and the same material logic is examined for a different environment in our guide to titanium and cupronickel exchangers for marine cooling.