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Thread: The Selection of Screw Feeders by Lyn Bates

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    The Selection of Screw Feeders by Lyn Bates

    The Selection of Screw Feeders
    by Lyn Bates


    Screw feeders are used for dispensing loose solids at a controlled rate from a bulk storage container, such as a silo, hopper, bin, IBC or big bag. These are normally chosen to satisfy one of two principle functions: -

    1. To discharge the product in a controlled manner, whilst generating an extraction pattern from the container outlet that generates the required flow regime in the stored material.

    2. To meter the feed at a suitable rate, uniformity and condition to meet the demands of the receiving point.

    In either case, certain basic criteria must be met.

    - Deliver the design flow rate as and when required, reliably.
    - Deliver the material in a suitable condition.
    - Deliver to the required receiving point.
    - Fit into available space.

    In Case 1, the feeder should be designed as an integral member of the container screw to optimise the geometry and performance and provide an extraction area to ensure reliable flow at an adequate feed rate and generate an extraction pattern over the feeder inlet that develops an appropriate flow regime in the container.

    In Case 2, the emphasis is on control of the feed rate, to ensure that it satisfies the accuracy and consistency required at the point of delivery.

    These differing objectives essentially separate the design process into two types of feeder. For this purpose the former is termed a ‘Screw Discharger’ and the latter a ‘Screw Feeder’. For either use the bulk container and the feeder should be designed as an integral system rather than independent items.


    ‘Screw Discharger’ design.
    This commences with the form of the storage container to be employed, the main purpose of which is usually to hold a specific volume of bulk material in temporary storage, variously described as ‘Silos’, ‘Hoppers’ or ‘Bins’. In general ‘Silos’ are externally mounted, relatively large bulk storage containers, often circular with conical outlet sections. ‘Hoppers’ or ‘bins’ are usually situated within plants and can be of rectangular or circular construction, with converging sections to the screw feeder. The storage container should be considered in two sections, that containing the main storage volume and the region approaching the feeder.

    The initial task is to establish the form of global flow regime that will be developed in the container during discharge that suits the nature of the bulk material to be handled,(1), and the size of interface necessary to ensure reliable flow.


    Other important characteristics of interest are: -

    - Whether the material is sensitive to extended time storage.
    - Does the material tend to segregate to an unacceptable degree?
    - Is the material prone to fluidise?

    Under any of these circumstances a ‘Mass Flow’ design should be undertaken for the whole unit. For materials that tend to segregate or be prone to fluidise, the feeder should also extracts as evenly as possible from an extended length of slot outlet

    - Is the material poor flowing under time compacted conditions.

    If so, but is otherwise unaffected by extended storage, an ‘Expanded Flow’ type design may be chosen that extends from the feeder to a cross section that exceeds the ‘critical rathole size’. (2). An inert product that remains free flowing under storage conditions may be stored in a ‘Funnel Flow’ type of container provided that drive power allowance is made for extracting material under a static bed.

    The second design step is to establish the minimum size of outlet required to guarantee reliable flow.

    This is dictated by the flow property of the bulk material, (3), but advantage may be taken of the facility offered by the use of a discharge screw to extract from an elongated slot and secure the many benefits of Plane Flow. These include, with features in bold that may dominate the selection criteria: -

    - Fine control of the discharge rate by a feeder
    - Discharging in a more stable condition of bulk density.
    - The potential for higher discharge rates.
    - Increased holding capacity.
    - Saving of headroom.
    - More uniform drawdown
    - Redressing fill segregation by remixing.
    - Inhibiting ‘Flushing’ and ‘Flooding’ by improved de-aeration.
    - Securing reliable flow through smaller outlets for both cohesive and lumpy products.
    - Able to deliver to two separate outlets from slot outlet, either independently or concurrently. (By proprietary design).

    To secure these full benefits, the exposed length of the screw to the stored contents must be at least three times the screw diameter and draw material down from the total outlet area. Longer outlets may be considered to enhance storage capacity. Mitigate segregation or aid de-aeration provided due design is provided for an appropriate extraction pattern. ‘Live flow’, but not necessarily uniform flow, is a mandatory condition to enable mass flow. The outlet size must be greater than the critical arching span, but excessive widths radically increase the pressures acting on the feeder, causing increased powder requirements and possible equipment wear and undue particle attrition. Flow uniformity is significant where segregation, density control or ‘flushing’ is a concern.

    Whereas screws that are uniform in pitch and diameter on a common size of centre shaft may be used as screw feeders, they are not recommended to be employed on extended lengths of bin outlets, as extraction will be limited to an initial short length of screw that fills with product that is then drawn under the remaining exposed static region of the storage units outlet section. It should be noted that feeder screws operate quite different from screw conveyors in that the axial transfer does not depend on ‘swept volume’, but is related to the flight geometry and the contact friction value of the bulk material against the surface of the screw flight and an excessive increase in pitch will result in a reduction of the screws axial transfer capacity.

    Various fabrication techniques are used to develop progressive extraction along the axis of discharge screws, such as variations in screw pitch, outside diameter, centre-shaft diameter, combinations of these and other proprietary methods. These methods enable extraction to be effective over around 8 to 10 times the diameter of the screw size selected. It is good practice for the hopper section leading to the screw to have vertical end faces, as inclined ends create pyramid type gullies that have poor flow characteristics. The efficient design of ‘Screw Dischargers’ is normally the domain of specialists as it entails measurements of the products flow related properties, experienced hopper design and a balancing of manufacturing costs against performance optimisation.


    ‘Screw Feeder’ design
    The primary function of a ‘Screw Feeder’ is to deliver a controlled rate of product at the required accuracy and reliability of dispensation and in a suitable condition.

    There may also be a need to accommodate indeterminate variations in product density and/or provide an assurance of performance. These aspects critically determine that a gravimetric system should be used. Whilst the uniformity and consistency of the rate of feed essentially depends on the design of the screw and hopper system, the verification of the feed rate requires that the discharge rate be continuously measured to ensure that it complies with the design limits. This is often confirmed by use of a Loss-in-weight feeder, where the whole dispensing unit is mounted on load cells and the change on weight during the discharge of a batch quantity is compared with the rate of change required by a chosen rate of discharge, any difference being compensated by altering the speed of screw rotation. Prior to discharge of the total batch it is necessary to top-up such a system systematically in order to provide a continuous feed. During the batch replenishment process the machine must change to a gravity mode of conveying, reverting to a gravimetric system when the input has ceased. Loss-in-weight feeders are therefore part of a system that includes a rapid batch make-up facility hence, together with the weight control system, are much more expensive than volumetric feeders.

    The need for a separate delivery facility for the batch make-up also means that gravimetric systems take up considerably more headroom than volumetric systems. Loss-in-weight feeders are normally of proprietary supply, but the make-up system generally depends on the user’s plant layout, so may be part of the feeder package or of separate supply.

    The two key features of gravimetric systems are that they can compensate for changes in the bulk density of the fed material and they can provide a record of the machines performance. The reliability of feed is determined by the inlet conditions to the feeder, especially with products that are not free flowing. A well-designed volumetric feeder has a more consistent infeed condition and can provide very uniform rates of discharge, but must be calibrated to discharge at a chosen rate and is unable to adapt to variations in bulk density or provide a record of its performance. Separate measurement of flow rate, such as impact flowmeters, can be used to provide control feedback to volumetric feeders and overcome these limitations.

    Short term discharge fluctuations may be caused by ‘avalanching’ of the bulk material emerging from the feeder due to the erratic failure of the repose surface of free flowing products at the discharge point. They may also result from cohesive materials holding together at the discharge point until the amount overhanging breaks away or by ‘pulsing’ due to the cyclic nature of the discharge screw. These effects are normally not corrected by variations of the screw speed, but can be attenuated by the design of the feeder outlet. ‘Breaker bars’ may also be fit on the screw shaft to provide a more frequent disturbance of the discharge or a proprietary construction of the screw and discharge arrangement used to spread the output more evenly.

    In summary, the effectiveness of volumetric feeders hinges on good hopper and extraction screw design. A plane flow hopper with outlet at least three times its width enjoys the flow and performance benefits previously listed, provided extraction takes place over the total outlet area and the wall inclination for Mass Flow is based on the measured wall friction value of the product. Longer outlets enable extra capacity or headroom saving to be made. Whereas standard designed feeders may be suitable for simple applications, optimum performance can only be secured by custom built equipment.



    References:

    1. See Ajax publication: – ‘Selection of Flow Regime’.

    2 To determine ‘critical rathole size’, employ Jenike method.

    3 Shear tests are needed to establish the minimum orifice size necessary to guarantee reliable flow.


    © 2016 by Lyn Bates, U.K.


    Please also see:

    Solid Sense by Lyn Bates - Contents
    https://forum.bulk-online.com/showth...e-by-lyn-bates
    Last edited by Author; 5th August 2016 at 18:51.

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