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HomeGravimetric Screw Feeders

Gravimetric feeder

Key benefits at a glance...
  • High precision, accuracies of 1% or better.
  • Robust in-built weigh system.
  • Loss in weight, gain in weight and continuous weighing.
  • Self-calibrating system.
  • Monitored process, alarms warn of up stream problems

Gravimetric feeders, often called ‘loss in weight’, feeders, have the same key parts as a volumetric feeder including the hopper, also known as the conditioning section, auger screw and agitator. If the material is very free-flowing, the metering feeder can be designed without the agitator.

Gravimetric feeding process

Gravimetric feeders’ deliver material at a weight-based rate using an advanced load cell and digital controller combination. Whether in continuous or intermitted operation this achieves a constant weight per unit of time, at high accuracies of ± 1 to 2%. Gravimetric feeders are particularly suitable for materials that fluctuate in density. As the material is released, the system weight decreases, the weigh controller constantly adjusts the speed of the feeder’s discharge rate, correcting any errors, regardless of material characteristics.

For food and pharmaceutical applications, our feeders are constructed from materials that meet all EC and FDA food contact regulations. They can be manufactured with a standard finish or crack and crevice-free with a high polish (0.2Ra) suitable for sanitary material handling.

Trantec offers an optional quick release, easy-clean, no-tool dismantling across all metering feeders, allowing total strip down in under five minutes. All parts can then be dishwasher cleaned or autoclave processed for deep cleansing. Click here for more information on our quick release feature.

 

Unsure which metering feeder fits your business needs? Click here for information on the differences between Trantec’s gravimetric and volumetric feeders.

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Gravimetric Control Schematic

Even with the most advanced gravimetric controls, the design of the feeding machine should be optimised. The gravimetric controls work best acting as a policeman, adjusting the feeder operation to correct for errors. Obviously, an error must have occurred for the controller to apply a correction. Trantec approach is to design a metering screw feeder which doesn’t have the errors in the first place. Where this can’t be achieved due to the characteristics of the material being dosed, the continuous monitoring and control provided by a gravimetric system boosts the overall performance and maintains continual accuracy.

Trantec partners with process control specialists to offer a range of gravimetric modules which can be matched to your process requirements. From cost-effective single line display units to complete recipe and factory automation systems.

Gravimetric Control in detail

Gravimetric control is aimed at applications requiring a continuous, exact, and monitored flow of product from the metering feeder. Also known as PID control (proportional integral derivative), Gravimetric controls use software algorithms to continuously sample the amount of weight lost or gained and then correct the feed rate of the Screw feeder to maintain a pre-programmed target flow.

Typically, the metering feeder will be mounted on load cells to measure the weight of the complete machine and its contents. This can be as simple as a platform scale or include a dedicated weighing support frame for larger feeders. Selection of the correct weigh system is important. The weighing capacity must be sufficient to carry the weight of the screw feeder, a full complement of bulk solid and have some spare capacity for dynamic “shock” loads or overloads. Too greater weighing capacity produces poor resolution whilst too small a capacity risks damage to the weighing transducers. Resolution is particularly important at very low feed rates. Parking a grain tanker on a weighbridge and emptying it with a thimble is an extreme analogy, how long before the weighbridge can take a meaningful reading? Gravimetric controls share the same restrictions.

During continuous operation, eventually, the metering feeder will run out of product and need refilling. During a refill cycle, gravimetric monitoring has to be suspended and the feeder goes into volumetric mode. The refilling process should be a fast as possible to minimise the period without monitored control. This is best achieved using a “surge hopper” to discharge its entire contents into the metering feeder hopper in one event. This surge hopper can then be refilled whilst the feeder is in normal “monitored” running. Where a surge hopper cannot be used, for example in limited space, a fast filling screw conveyor can be employed.

Gravimetric control feed rate graph