Pharmaceuticals ABOVE: Ajax Equipment twin screw feeder
Both the powders require steep walled hoppers and chutes for mass flow and it was also found that both materials slip more readily against a stainless steel with a 2B Mill finish rather than a mechanical polished finish.
Plant improvements
Interpretation of the spider diagrams suggested that the existing pyramid shaped hopper under the sifter was unsuitable in terms of angles of convergence and outlet size for the material being handled and needed to be replaced with wedge shaped hopper. In general, holding equipment such as hoppers and silos require careful design with large outlets. Wedge shaped hoppers should be preferred to conical hoppers as these provide better potential for gravity flow due to single plain convergence.
Ajax replaced the sifter pyramid hopper with a wedge shaped hopper discharging into a left / right hand screw to direct material to a central outlet and maintain a 'live' condition at the bottom of the hopper, thus preventing arching. As the sifter hopper received a controlled powder flow and, under normal operation, would not be flooded with material, a single screw in conjunction with the wedge hopper was felt sufficient to ensure reliable flow. To maintain the hopper's central discharge point a right/left hand screw feeder was employed. The hopper was supplied with an open inlet and a drilled flange, for subsequent connection to the existing plant.
With respect to the pyramid hopper with a cone valve to aid discharge, the powder testing indicated that as it acted as a buffer hopper for the milled powder there was greater potential for an arch to develop. The pyramid hopper was, therefore, replaced with a wedge hopper with a working volume of 1800 litres. The hopper incorporates a left/right hand
www.solidsandbulk.co.uk March 2011 • Solids & Bulk Handling 27
twin screw feeder with a sufficiently large central outlet to ensure gravity flow. The twin screw feeder will deliver up to 700 kg/hr of product and will be inverter controlled. In addition, the powder tests showed that the milled powder had the potential for flushing if left unconfined or unsettled. Therefore special hopper and screw design features were incorporated to discourage fluidised material from flooding through the outlet. Powder testing has shown how pharmaceutical plant performance can be improved by ensuring equipment design is sympathetic to the flow properties of the process materials. Once the equipment improvements have been made, no further powder flow problems are anticipated.
For more information contact Ajax Equipment on tel: 01204 386723 or visit:
www.ajax.co.uk.
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