The tower is very hot at the base, and cooler rising towards the top. A furnace vaporises the crude oil. The vapour is fed into the fractionating column near the bottom. The column is divided into a series of sections. These rise through some of the petroleum vapour which has condensed.
Vapour goes through the bubble cap and bubbles through condensed petroleum.
Some of the vapour will condense. The rest will continue up the tower and also help any volatile parts of the condensate it evaporate again. Each part of the tower will contain a different mixture of hydrocarbons. These parts are called fractions. Each fraction boils within a particular temperature range.