Tully Mill has six low-grade crystallisers that are divided into two parallel sets, each with three units operating in series. The paper describes modifications made prior to the 2002 crushing season, in which two crystallisers in one of the sets were modified to incorporate double helical cooling coils, variable speed drives and new control systems to replace the fixed speed, three and four shamrock type cooling coils. Comparative trials were carried out on the modified and existing sets over a 10-day period to assess the following information: • Exhaustion profiles through both refurbished and existing crystalliser sets. • Viscosities of massecuite and mother molasses through both crystalliser sets. • Heat transfer coefficients in the refurbished crystalliser units. The results and observations of the tests demonstrate the improved exhaustion and operational benefits of the new coils and drives compared with the old arrangement.