After desalting, crude oil distillation takes place. The atmospheric distillation unit is the only unit in a petroleum refinery that processes the total desalted crude oil feed and therefore this unit determines the refinery capacity. The crude oil distillation are used to separate the crude oil by distillation into fractions according to boiling point so that each of the processing units following distillation will have feedstocks that meet their particular specifications. Refinery design capacity is expressed in terms of the crude oil distillation capacity of the atmospheric distillation unit, since this is the only unit that is present in all refineries and all of the crude oil has to pass through this unit. The front-end design affects how the preheated crude oil is processed before it enters the furnace of the atmospheric distillation unit. The atmospheric residue, which is the bottom product from the atmospheric distillation unit, is the feed for the vacuum distillation unit.