Drainage requirements of the crushing roll surface

The surface roughness of sugar mill rolls is recognised as being a critical factor in the feeding and juice extraction processes that occur in a milling train. During the past five years, there has been renewed interest in the application of long-life rough surfaces to minimise maintenance costs and minimise the need for roll arcing. These new surfaces usually have smaller, more numerous and more closely spaced roughness globules than the globules that are currently produced by roll arcing. The smaller size of the globules also suggests a reduction in the local cross-section available for juice drainage at the interface between the roll surface and the prepared cane or bagasse blanket. If the roll surface is not able to provide adequate drainage, then the grip, as well as other properties, will be affected to the detriment of crushing performance. This paper reports on the results of an investigation which compared the drainage properties of an arced surface to those of a smooth steel surface and a rough sandpaper surface (similar globule size and surface density to proposed new long-life surface). The analysis of the results indicates that the adoption of new long-life surfaces with small-scale roughness should not have a detrimental effect on juice drainage. Some problems with the testing methods are identified with the aim of making the test methods and geometries more rigorous/conclusive in the future. It is shown that care is required when carrying out and interpreting tests similar to historical uniaxial compression and permeability tests as the magnitude of pressure drops across drainage holes might be significant under certain conditions.