Bulk-materials handling is a process; therefore, many of the measurements taken to measure performance are not absolute values but rather individual data points in a statistical process control chart. Most of the results from the process of controlling fugitive material, like dust collection or belt cleaning, follow a classical bell curve. It follows that some of the time the results are above average and some of the time they are below average. It also follow that it is very unlikely it is possible to remove 100 percent of the dust or carryback without extraordinary cost or without unwanted consequences, but as scraping the belt so hard some of the top cover is removed.
It is best to consider the results from all of the conveyors, rather than just one, to judge the continuing performance of the plant in reducing dust and spillage.
As the control of fugitive materials is a continuing process, the more data points obtained, the more representative the results. Therefore, it is best to consider the results from all of the conveyors, rather than just one, to judge the continuing performance of the plant in reducing dust and spillage.
In a similar fashion, it is very inaccurate to refer to efficiency as a value that applies no matter what the condition of the conveyor or the properties of the bulk material. The answer is that it is all relative to the particular bulk material and equipment condition and not necessarily a function of the design of the belt cleaner. If a belt cleaner is rated as 90 percent efficient in removing a layer of carryback, is the remaining layer 100 millimeters (4 inches) thick, or is it 1 millimeter (0.04 inches) thick? Both might be 90 percent effective, but the resulting cleanup cost and operating problems are going to be dramatically different. It would be better to have a cleaner that will remove carryback down to a thickness of 0.1 millimeter (0.004 inches) rather than to the abstract 90 percent efficiency mark.
Because of the large number of variables in handling bulk materials—both in the bulk materials themselves and in the condition of the conveyors—it is physically, financially and statistically impossible to reduce fugitive emission to zero over a long period of time. In many operations, an acceptable belt-cleaning performance is one in which the fugitive material can be kept to the level that requires cleaning once a week without causing a safety problem or production loss.
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