Heat exchangers in LNG systems

Heat exchangers in LNG systems enable the cooling/liquefaction of natural gas to -162°C(using refrigerants like propane/methane) or vaporize LNG back to gas (using seawater/air). They work via heat transfer across barriers—such as in Plate-Fin or Coil-Wound units—to manage high pressure and cryogenic temperatures, maximizing thermal efficiency during production or regasification.

Key Working Principles & Types

• Liquefaction (Cryogenic Heat Exchangers): In the liquefaction process, the main cryogenic heat exchanger (MCHE), typically a large coil-wound unit, reduces the temperature of pre-cooled natural gas to approximately -146°C  (-231°C), turning it into a liquid. This is often achieved through a closed-loop system using mixed refrigerants.
• Regasification (Vaporizers): These exchangers convert LNG back into gaseous form for pipelines.

• Submerged Combustion Vaporizers (SCVs): Use a burner in a water bath to create hot water, which warms the LNG.
• Open Rack Vaporizers (ORVs): Use seawater flowing over external panels containing LNG.
• Ambient Air Vaporizers (AAVs): Use fans to blow ambient air across heat exchanger tubes.

• Reliquefaction: On LNG carriers, heat exchangers (often plate-fin) are used to re-cool boil-off gas (BOG) for storage.
• Key Components: The process uses sophisticated equipment to withstand extreme cryogenic temperatures and high pressures. The primary heat exchanger types in LNG processing are:

• Plate-Fin Heat Exchangers (excellent for multi-stream services).
• Coil-Wound Heat Exchangers (used for high-capacity, heavy-duty applications).

Operational Principles: Heat transfer typically uses counter-current flow, where the LNG and the heating/cooling medium travel in opposite directions to achieve maximum temperature difference, according to Slideshare.

How the Heat Transfer Happens

• Spiral Wound Heat Exchangers (SWHE): Imagine thousands of tiny aluminum tubes wrapped around a central core. The natural gas flows inside the tubes, while a refrigerant (like a mix of nitrogen, methane, and ethane) flows over the outside of the tubes. The refrigerant absorbs the heat from the gas, causing the gas to liquefy.
• Printed Circuit Heat Exchangers (PCHE): These are compact blocks with chemically etched channels. They are used when space is tight (like on a ship) because they can handle extremely high pressures and provide massive surface area in a small footprint.

1. Pre-cooling: The "warm" natural gas enters the exchanger.
2. Thermal Exchange: A cold medium (refrigerant or seawater) flows adjacent to the gas. Because of the temperature difference, heat naturally moves from the warmer gas to the colder medium.
3. Condensation: As the gas loses energy, its molecules slow down and bond together, turning the gas into a liquid (LNG). This reduces its volume by about 600 times, making it easy to transport.