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Systems vary from mill to mill and thus it is important to have a clear starting point and evaluate the system conditions, including pipe work, number of fittings, elevation changes and process requirements to ensure that your system operates at its best efficiency with reduced total cost of ownership.

Historically the typical filter used for this application was the Oliver-Campbell type cane mud filter, which forms the basis of our discussion. 

Conventional cane mud filters require approximately 0.7 cfm per square foot of filter area at 21” Hg at the pump, which will produce approximately 18” Hg at the filter. The filter requires approximately 10” Hg vacuum at the pick-up section, and approximately 18” Hg at the wash section. 

Typical Filter StationSeveral manufactures have introduced a rotary cane mud filter to handle a coagulated cane mud which produces a very porous cake. For these filters, 1.7 cfm per square foot of filter area at 10” Hg vacuum for the pick-up section, and 18” Hg vacuum for the wash section is recommended. 

The different operating pressures required for the pick-up and wash sections are obtained by a vacuum regulating valve installed between two filtrate receivers, one handling the wash section. A typical cane mud vacuum filter station equipped with a suitably sized liquid ring vacuum pump will ensure efficient dewatering of the filter cake and recover the much needed sucrose content. 

Rotary Vacuum Drum Filter
Rotary Vacuum Drum Filter

Additional benefits of using a filter station is that the additional sand load is handled correctly and prevented from entering the boiler house and other downstream equipment. These systems have many variables while in operation and it is recommended to always consider methods of control and automation to ensure optimal functioning of the filter station. 

Effects of incorrect liquid ring vacuum pump (LRVP) sizing are discussed below, and are only some of the many effects possible due to the complexity of the operating filter system. Capacity Undersized LRVP – insufficient capacity – typical effects noted are reduced filter efficiency, high moisture content of filter cake causing transport problems on inclined conveyor systems. 

Diagram
Schematic arrangement of a typical filter station – ‘Mud Filtration – Gre Lionnet, Sugar Milling Research Institute – 1996

Vacuum operating point too low – operating at between 10-20 kPaa – the low vacuum condition typically effects the operation of the filtrate pumps due to the reduced NPSH available at the pump suction port. This causes cavitation and reduces the operating life expected of the pump. 

Correct sizing of a suitable LRVP is essential to ensure optimal operation of the complete filter system, as well as the correct sizing of the filtrate pumps to reduce the frequency of cavitation. 

Consult KME with regards to the filtrate pump failure modes and solutions to improve the life span of these pumps.


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