Performance of baffled boilers with redesigned convection banks

Since the installation of many larger capacity boilers in sugar mills during the 1970s, certain designs have shown consistent and often severe erosion problems with convection bank tubes. Many mills have replaced complete convection bank tube arrays at significant expense to restore boiler availability to an acceptable leveL Convection bank tube wear is one of the major parameters which determines the upper limit for output of sugar mill boilers. An increase in output could generally be achieved if the limiting convection bank gas velocity was increased and/or the rate of tube wear decreased. Tube wear is largely responsible for the current design of new boilers, which have a relatively large furnace volume and a single gas pass baffleless convection bank. Since 1995, a computer modelling technique using the Computational Fluid Dynamics (CFD) code FURNACE has enabled the redesign of full convection banks with the aim of reducing tube wear while maintaining heat transfer levels. The performance of three boilers before and after redesign is presented including observations by sugar mill staff. The redesigns have been highly successful in reducing tube wear. Boilers are one of the largest capital cost items for a sugar mill and are often the limiting factor when considering sugar mill expansion. The convection bank design has a major effect on boiler design, capital cost and capacity upgrade capability. The implications of the new convection bank designs for building future boilers are discussed.