What a train contains
Strip away the gas treatment and the storage tanks and a liquefaction train is a refrigerator of extraordinary scale. Three parts matter.
The refrigerant circuits carry heat away from the gas. Refrigerant is compressed, cooled and condensed, then allowed to expand, which drops its temperature sharply. That cold refrigerant absorbs heat from the natural gas flowing past it, and the cycle repeats. Because no single refrigerant works well across a 200-degree span, plants use several stages or a blend.
The heat exchangers are where gas and refrigerant meet without mixing. Two designs dominate. Coil-wound exchangers are tall cylinders holding many kilometres of tube spiralled around a core, and they are what Air Products and Linde build. Brazed aluminium plate-fin exchangers are compact blocks of finned aluminium, used in the cascade process and in mid-scale plants, and Chart Industries is the best-known supplier.
The drivers turn the compressors. Traditionally these are large gas turbines burning some of the plant’s own gas: heavy industrial frames, or aeroderivative units such as the LM6000 and LM9000 families. Increasingly they are electric motors with variable speed drives, which decouples train size from turbine size and cuts on-site emissions when the grid is clean.
The main cycles
C3MR (propane pre-cooled mixed refrigerant). Propane chills the gas to around minus 35 degrees, then a mixed refrigerant of nitrogen, methane, ethane and propane condenses and sub-cools it in a coil-wound exchanger. This has been the workhorse of the industry for decades and remains the most common design.
AP-X. An extension of C3MR that adds a nitrogen expander loop for the final sub-cooling, which lifts a single train past 7 million tonnes a year. It is the design behind Qatar’s largest trains.
Optimized Cascade. Three cascaded circuits of pure refrigerants, propane, ethylene and methane, through brazed aluminium exchangers. Its distinguishing feature is a two-trains-in-one arrangement, where two parallel compressor sets feed one exchanger line, which keeps the plant running when one set is down.
DMR (dual mixed refrigerant). Two mixed refrigerant circuits instead of propane plus one. Mixed refrigerants can be tuned to the ambient temperature, which is why DMR turns up in very cold climates and on floating units, where a propane circuit is awkward.
Single mixed refrigerant (SMR, PRICO, IPSMR). One refrigerant loop, so fewer machines, less plot space and lower cost, at some efficiency penalty. This is the usual choice for mid-scale plants and for floating liquefaction, where deck space and simplicity dominate the economics.
Why capacity arrives in lumps
A train is an indivisible unit. You cannot build 40 per cent of one and get 40 per cent of the output. That single fact shapes the industry’s supply cycle: capacity arrives in 5 to 8 million tonne steps, several years after the decision to build, and those decisions cluster when prices are high. The result is the familiar pattern of tightness followed by a wave of simultaneous start-ups.
It also explains why our terminal pages list every train separately. A terminal described as “operating” may have two trains running, one commissioning and three more approved but not yet built. Rolling that into a single status hides exactly the information a reader needs, so we show the roll-up and the per-train detail together. Look at any large export terminal and the phase table makes the staging obvious.
The bottleneck is not the process
Process licences are not, on their own, the scarce thing. Several firms will license a cycle, and the physics is not secret. The bottleneck is further down: the manufacturing capacity for large cryogenic heat exchangers, the compressor and driver slots, and the specialised construction workforce.
Air Products has shipped more than 115 large coil-wound exchangers over the industry’s history, and Linde has delivered over a thousand coil-wound units of all sizes. These are made in a handful of factories. Similarly, the main refrigerant compressors come from Baker Hughes, Siemens Energy, MAN and Mitsubishi’s compressor arm, and the electrical equipment for an all-electric train competes with every other industrial electrification project for the same transformers and drives.
The contractors are the third constraint. Bechtel, Technip Energies, JGC, Chiyoda, KBR, Saipem and McDermott have between them built most of the world’s liquefaction capacity, and when several mega-projects reach construction at once, they compete for the same engineers, module yards and welders. Our liquefaction hub lists the firms with sources for what each supplies.
Reading a train’s status
On this site a unit’s status uses one nine-value scale, from proposed through to retired, mapped from the tracker’s own wording. Two states are worth understanding.
Pre-construction means the money is committed or permits are in hand but ground has not been broken. It is the strongest signal short of concrete that capacity is genuinely coming.
Proposed covers everything earlier, from a press release to an advanced permitting process. Historically a large share of proposed capacity is never built, so proposed tonnes and operating tonnes should never be added together. That is why the capacity cards on our terminal pages keep them in separate columns rather than showing one headline number.