LNG Regasification Systems: How Vaporizers Convert LNG to Pipeline Gas
600 Cubic Meters from One Cubic Meter
The fundamental challenge of LNG regasification is one of phase change and expansion. One cubic meter of LNG at -162°C, when vaporized and warmed to 0°C, produces approximately 600 cubic meters of natural gas at standard conditions. This is the physical basis of the entire LNG industry — natural gas is compressed to 1/600th of its gas volume by cooling it to a liquid, making it practical to transport in ships, trucks, and pipelines. But it also means that the regasification system must handle a massive volumetric expansion while simultaneously heating the gas from -162°C to above 0°C and boosting its pressure to pipeline specification.
The vaporizer is the heart of this process. It is a heat exchanger — a device that transfers heat from a warm source to the cold LNG flowing inside tubes. The engineering challenge is to do this efficiently, reliably, in all weather conditions, and at the required flow rate. The answer to "which vaporizer type?" depends on three things: climate, required flow rate, and budget.
Three Vaporizer Types: Comparison
The LNG Send-Out Train: From Tank to Pipeline
The regasification system is not just the vaporizer — it is a complete send-out train that includes several functional stages between the LNG storage tank and the pipeline connection.
Cryogenic pump: A centrifugal or reciprocating pump inside the LNG tank (or in a pump well adjacent to the tank) draws LNG and boosts its pressure from tank pressure (0.8–1.6 MPa) to a higher pressure that will allow the gas to reach pipeline pressure after vaporization. For a pipeline requiring 1.0 MPa, the pump might boost to 1.5–2.0 MPa. Multiple pumps are installed in parallel for redundancy and to allow turndown.
Pre-heating section: In some vaporizer designs, a small portion of the vaporized gas is diverted back through a heat exchanger to pre-heat the incoming LNG, improving overall efficiency.
Vaporizer: The main heat exchange step — LNG absorbs heat and vaporizes. The output is natural gas at approximately 0°C and pump discharge pressure.
Superheating section: After vaporization, the gas passes through a superheater that raises its temperature to 5–15°C above ambient, ensuring no liquid droplets or ice forms in downstream piping.
Gas metering: The flow rate of regasified gas is measured by a custody transfer flow meter (typically ultrasonic or turbine type) for commercial transactions.
Pipeline injection: Gas enters the pipeline network at the required pressure, typically controlled by a pressure control valve.
Climate Considerations and Ice Formation
Ambient air vaporizers work well in temperate climates — coastal China, southern Europe, most of North America. But in climates where winter temperatures regularly fall below -15°C, ambient air vaporizers develop a serious problem: ice formation.
When the tube surface temperature of the vaporizer falls below 0°C (which happens at high LNG flow rates or low ambient temperatures), moisture from the ambient air freezes on the tube surfaces. The ice layer progressively blocks airflow between the tubes, reducing heat transfer efficiency, which causes more ice to form in a self-reinforcing cycle. Eventually the vaporizer becomes completely blocked and cannot vaporize LNG.
Sizing a Regasification System
| Application | Typical Send-Out Rate | Recommended Vaporizer Type | Vaporizer Sizing Note |
|---|---|---|---|
| Small LNG fueling station | 500–5,000 kg/hr | Ambient air vaporizer (temperate) or water bath (cold climate) | 1 or 2 vaporizers; one duty, one standby |
| Industrial gas distribution plant | 5,000–50,000 kg/hr | Water bath vaporizer | 2–3 vaporizers; N+1 redundancy |
| City gate station / peak shaving | 50,000–200,000 kg/hr | Water bath or SCV | Multiple units; N+1 or N+2 redundancy |
| Large LNG terminal | 200,000–1,000,000+ kg/hr | SCV (primary) + water bath (backup) | Large SCV batteries; full redundancy required |
Frequently Asked Questions
An LNG vaporizer is a heat exchanger that transfers heat from an external source (ambient air, warm water, or combustion gases) to LNG flowing through tubes. As LNG absorbs heat, it vaporizes and warms from -162°C to above 0°C. Cryogenic pumps inside the tank boost liquid pressure before vaporization so that after vaporization and heating, the gas is at the required pipeline pressure. For higher pipeline pressures, additional gas compression is needed downstream.
An ambient air vaporizer (AAV) uses forced or natural convection of ambient air over aluminum heat exchange tubes to vaporize LNG. AAVs are the most economical option in temperate climates where winter temperatures rarely fall below -15°C. Below this temperature, ice builds up on tube surfaces and progressively blocks airflow, eventually making the vaporizer inoperable. AAVs are not suitable for cold climates without supplementary anti-icing measures.
A water bath vaporizer passes LNG through tubes submerged in a warm water bath (typically 40–80°C, heated by gas burners or steam). Because the tube surface stays above 0°C, no ice forms — making water bath vaporizers reliable in all climates including extreme cold. They are more expensive to operate than AAVs (fuel cost for heating water) but are the standard choice in cold climates and anywhere reliable all-weather vaporization is required.
An SCV fires combustion gases directly into a water bath, bubbling hot gases through the water to heat it. The water bath in turn heats LNG through submerged tubes. SCVs offer the highest thermal efficiency and are immune to cold weather. They are used in large LNG terminals and peak shaving stations where very high vaporization capacity is needed. They are more complex and expensive but offer superior performance in demanding conditions.
When LNG vaporizes from liquid to gas, volume increases approximately 600×. This vaporization happens at roughly the same pressure as the liquid (0.8–1.6 MPa). Cryogenic pumps inside the LNG tank boost liquid pressure before vaporization to compensate for the slight pressure drop during heating, ensuring the gas reaches pipeline pressure after vaporization. For high-pressure pipelines (above 1.6 MPa), additional downstream compression is required.
