A turboexpander is an axial or radial-inflow expansion turbine that expands high-pressure natural gas while simultaneously extracting mechanical shaft work. Unlike an isenthalpic Joule-Thomson (J-T) valve—which expands gas at constant enthalpy without doing external work—a turboexpander operates close to an isentropic path (constant entropy). Extracting shaft work forces the gas internal energy to plummet, producing drastic refrigeration down to -60°C to -105°C and condensing up to 90%+ of ethane (C2) and virtually 100% of propane-plus (C3+) Natural Gas Liquids (NGL).

In modern gas plants, the turboexpander and a centrifugal booster compressor are mounted on opposite ends of a single high-speed shaft inside a single casing (known as a compander). The shaft work extracted during cryogenic expansion directly drives the booster compressor without an external electric motor or gearbox, recovering 75% to 85% of the expansion power to partially recompress the residue sales gas stream.

Because turboexpander discharge temperatures plunge far below the freezing point of water (-50°C to -100°C), any residual moisture will instantly form solid gas hydrates (cage-like clathrates) or ice crystals. Upstream gas must be dehydrated using molecular sieve desiccant beds to a water dew point below -100°C (<0.1 ppmv H₂O) to prevent high-speed rotor shattering, guide vane jamming, and cryogenic heat exchanger plugging.

While modern radial-inflow turboexpander wheels feature contoured blades engineered to handle two-phase expansion, the liquid condensation fraction exiting the wheel must generally be kept below 10% to 15% by weight. Excessive liquid droplets impinging on blade surfaces at tip speeds exceeding 250–350 m/s cause catastrophic erosion pitting, rotor dynamic imbalance, and fatigue cracking.

A turboexpander achieves a temperature drop 3 to 5 times greater than an equivalent J-T valve operating across the exact same pressure ratio. For example, expanding natural gas from 70 bar to 25 bar across a J-T valve typically drops the temperature by 20°C to 28°C, whereas an efficient turboexpander drops the temperature by 65°C to 85°C due to mechanical shaft work extraction.