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Volumetric or gravimetric feeder for pasta extruders

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  • Volumetric or gravimetric feeder for pasta extruders

    Which type of feeder is better for pasta production, volumetric or gravimetric? What are the advantages and disadvantages of each?

  • #2
    In response to the question regarding volumetric versus gravimetric feeding for pasta extruders, it is important to note that in many continuous processes , such as pasta extrusion, the end process becomes a slave to the feeders, since the feeder or feeders dictate the overall rate of raw ingredients or blends to the process below. By definition, gravimetric feeders measure the flow’s weight in one fashion or another and then adjust feeder output to achieve and maintain the desired set point. Volumetric feeders do not weigh the flow; they operate by delivering a certain volume of material per unit time from which a weight-based flow rate is inferred by the process of calibration.
    Flow rate changes in a volumetric feeder are accomplished by altering screw speed. In the case of a screw feeder, three factors affect volumetric screw feeder accuracy: the consistency of delivered volume per screw revolution, the accuracy of screw speed control, and material density variability.
    Typically, volumetric feeders are open-loop devices and cannot detect or adjust to variations in the material’s density. Due to the open-loop concept, head load variations and material buildup on the feed device change the volume per-revolution relationship, throwing off calibration without any outward sign. This can result in a deviation from set point of 2-6% in the mass flow, therefore delivering inconsistent ingredient amounts to the pasta extrusion process, and then affecting the quality of the pasta produced by the extruder.
    Alternatively, gravimetric feeders automatically detect and adjust to changing densities and head load variations. In cases of screw feeding of cohesive materials, it is possible in volumetric mode to have relatively no material discharging while the screws are running, which will of course greatly affect the product quality in the extruder. In a gravimetric feeder, the control system would detect that no change in weight is occurring due to the material characteristics, and would alarm or adjust accordingly.
    Similarly, flood-through materials (such as very free flowing grains can also remain undetected since the feeder has no way of knowing the out-of-control condition. Since the feed rate in a volumetric feeder is purely a function of speed, the feeder and the process below have no way of detecting this upset condition.
    Finally, due to the fact that actual mass flow is measured with a gravimetric devices , an exact amount of ingredient delivered to the process below can be ensured and a reduction in ingredient costs versus operations with a volumetric feeder , which due to higher inaccuracies in flow may also result in excess ingredients delivered to the extrusion process below and higher overall ingredient costs. This can be especially important when feeding more expensive ingredients, such as vitamins , flavors and/or fortifiers.
    For further information on volumetric versus gravimetric feeders in food extrusion applications please see https://www.coperion.com/en/industries/food-pet-food/

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