Transcritical

Transcritical is an adjective used to describe processes, systems, or flows that operate across the critical point of a substance—the temperature and pressure at which liquid and gaseous phases become indistinguishable. The term is used in thermodynamics, chemical engineering, and mathematics, and it contrasts with subcritical and supercritical regimes.

In thermodynamics and process engineering, transcritical operation often appears with carbon dioxide as the working fluid. CO2 has a critical point at about 31.1 degrees Celsius and 7.38 megapascals. A transcritical CO2 cycle involves compression and cooling on the high‑pressure side to conditions above the critical point, so the CO2 cannot condense into a distinct liquid phase. Heat rejection occurs via a gas cooler rather than a conventional condenser. On the low‑pressure side, the fluid expands into the subcritical region, where vaporization occurs in the evaporator. This arrangement can offer environmental and efficiency benefits in certain refrigeration and heat pump applications.

In extraction and separation technologies, transcritical operation describes processes that pass through the critical point of a solvent or mixture during phase change. Near the critical point, fluid properties change rapidly, requiring careful control of temperature and pressure to achieve the desired dissolution or separation. Transcritical fluid extraction with CO2 is a notable example, though many CO2 extractions operate in the supercritical range without a distinct liquid phase.

In mathematics, a transcritical bifurcation is a local bifurcation in which two equilibria collide and exchange stability as a parameter crosses a critical value, illustrating a different use of the term.

Etymology: trans- meaning across, combined with critical, reflecting the crossing of the critical boundary.