An oil refinery may be considered as a factory that converts crude oil into a range of useable products. It is designed to produce what the market requires in the most economical and efficient manner. The first step in the manufacture of petroleum products is the separation of crude oil into the main fractions by atmospheric distillation. When crude oil is heated, the lightest and most volatile hydrocarbons boil off as vapours first and the heaviest and least volatile last. The vapours are then cooled and condensed back into liquids. This distillation process is carried out in a fractionating column. The column is divided into a series of chambers by perforated trays, which condense the vapours at each stage and allow the liquids to flow into storage tanks. Pre-heating of the crude oil is limited to 350°C to prevent the oil being thermally cracked.