In modern installations vertical cooling crystallisers are now preferred over traditional horizontal units because of the significant benefits they offer, which include larger volumes and smaller floor space, suitability for outdoor installation, higher cooling surface to volume ratios and a better ability to handle highly viscous massecuite, amongst others. Since the first vertical cooling crystallisers were introduced, nearly 40 years ago, there has been a steady increase in their unit size from initial volumes in the 50–200 m3 range up to the present day where the most general unit size is now in the 300–400 m3 range, with even larger units becoming increasingly common. Large crystallisers present some significant design challenges and a good modern vertical cooling crystalliser design requires a robust construction of heat exchange surface, stirrer and drive units coupled with features that promote good heat transfer characteristics and uniform massecuite flow patterns. Careful attention to cooling tube and stirrer arm design and configuration are needed to achieve this, whilst the use of modern planetary gearboxes and variable frequency controlled motor drive units can provide added benefits to boost both performance and reliability. How these design features are incorporated in a modern unit is explained, focusing on cane C massecuite duty and using the Fives Cail and Fives Fletcher units as an example.
