After desalting the crude oil is pumped through a series of heat exchangers and its temperature is raised to about 550 ° F by heat exchange with product and reflux streams. It is then further heated to about 750 ° F in a furnace and charged to the flash zone of the atmospheric fractionators. The atmospheric fractionator normally contains 30 to 50 fractionation trays. Separation of the complex mixtures of crude is relatively easy and generally 5 to 8 trays are needed for each side stream product plus the same number above and below the feed plate. Thus crude oil atmospheric fractionation tower with four liquid sidestream drawoffs will require some 30 to 42 trays.

The overhead or top distillate is the full range naphtha with a boiling point of 380 ° F. These are in gaseous form and go into the light end units.

The other sidestreams are Kerosene (380-480 ° F), Light gas oil (480-610° F), Heavy gas oil (610-690 ° F)