The visbreaker is a mild bottoms upgrading unit. It upgrades vacuum resid by cracking it so that viscosity is reduced and the amount of high value cutter stock required to blend it into residual fuel oil is reduced. The conversion of vacuum resid into light products is very low in a visbreaker - typically less than 15% by volume. However, the cost of a visbreaker is much lower than for other, more severe bottoms upgrading units, making it an attractive investment in markets where the demand for residual fuel oil is high.