On the erosion of heat exchanger tube banks in fluidized-bed combustors

Abstract

Erosion (usually referred to as metal wastage) of heat exchanger tube banks in bubbling atmospheric and pressurized fluidized beds is a serious issue that may affect their reliability and economics. A simplified version of the comprehensive monolayer energy dissipation (MED) erosion model, developed by us earlier, is presented. This study illustrates the application and usefulness of this simplified MED erosion model to quickly and easily produce trends and dependencies upon independent parameters. This version can easily fit on a pocket calculator or desktop personal computer. Predicted trends of erosion rate dependency upon the particle diameter, the fluidizing velocity and bed porosity using this model are discussed. Main fluidization variables are introduced in this erosion model by interpreting the solids—fluid interphase drag as the principal means of potential erosion. This simplified MED erosion model is compared with three other erosion models: the more comprehensive MED erosion model, the Finnie impaction erosion model, and the combined ductile—brittle Neilson—Gilchrist (N-G) erosion model. The simplified MED erosion model is shown to agree reasonably well with the maximum erosion data obtained experimentally at the Grimethorpe cold model facility of the Coal Research Establishment. A two-dimensional fluidized-bed geometry embodying three immersed tubes, which represents a generic approximation of the Coal Research Establishment cold models, is used to compare erosion model predictions. Detailed hydrodynamic calculations for this bed geometry were performed using the FLUFIX computer code and were used as inputs to the four erosion models. The comprehensive MED, the Finnie and the N-G erosion models predict an order-of-magnitude agreement with each other and with limited erosion experimental data. Good agreement is obtained between the relative trends predicted by the simplified and comprehensive versions of the MED erosion model.