The flash (or partial) evaporation is one of the simplest unit operations. A liquid stream is partially vaporized in a "flash drum" at certain pressure and temperature. The result is a vapor, richer in the more volatile component than the remaining liquid.

The flash can be seen as a distillation operation with only one single-equilibrium stage. The flash operation stops when the vapor and the liquid stream reach the equilibrium compositions at flash pressure and temperature. The two streams obtained can be easily separated.

The incoming fluid is first pressurised and heated, and then fed through a throttling valve (sprayed) into the drum. Because of the large pressure drop, part of the fluid vaporizes extremely rapidly (therefore this unit operation is called "flash" evaporation). Because of the reduced pressure, the flash evaporators require less heat (and then less fuel) than normal evaporation.

The aim of a Flash evaporation is the separation of the more volatile component, which leaves (vaporizes) the initial liquid stream.