Increasing the furnace outlet temperature further in the atmospheric distillation unit in order to process the heavier fractions of crude oil would result in the breaking of chemical bonds (thermal cracking) causing loss of product and equipment problems due to coking. Since boiling point decreases with a lowered pressure, transferring the heavy crudes to a vacuum distillation tower allows the refinery to separate these at lower temperatures, thereby reducing the amount of energy invested into the process. Pressures in the vacuum tower range from 25mmHg – 40mmHg, but can be lowered to 10mmHg by the addition of steam into the furnace. It is ideal for the difference in pressures at the top and bottom of the tower to be at a minimum so special packing materials are used instead of trays to improve fractionation. With all this change in pressure, it is also important to keep check on the exact temperatures being used. The Watson Characterization Factor (K_w) is used to estimate the upper temperature limit. Graphing K_w versus Temperature creates a band with an upper and lower line. It is necessary to stay below the top line, although if you need to be extremely careful (especially with paraffinic oils) it is best to stay below the bottom line as well.