When processing crude oil vacuum distillation is needed in conjugation with Atmospheric distillation as well. If vacuum distillation was not used the outlet temperatures of the furnace would be so high that thermal cracking would occur. This thermal cracking would cause a loss of some of the product as well as equipment fouling. To lower the outlet temperature you have to lower the boiling point of the products. This is done be creating a vacuum. The pressure within the distillation column is lowered to about 10 mmHG. This is done by using a combination of vacuum pumps as well as an addition of steam to the furnace inlet and at the bottom of the vacuum tower. Under this low pressure the products will boil at a much lower temperature, which will prevent thermal cracking and equipment fouling. The Watson characterization factor can be use to estimate the upper temperature limit of crude oil to avoid coking in vacuum distillation. There is an empirical correlation between temperature and Watson characterization factor. From this correlation a line of where coking will occur can be drawn. But do to the variability of the composition of crude oil coking may occur below this limit. So a lower limit has to be drawn. The area between these lines is known as the decomposition zone. Where coking may occur depending on the composition of the crude oil. The temperature must be kept below the lower line to avoid coking.