Gravimetric Screw Feeder
Benefits and Features
- Self-calibrating system.
- Monitored process, alarms warn of upstream problems
- High precision, accuracies of 1% or better.
- Robust in-built weigh system.
- Optional quick-release feature, for swift and easy dismantling.
- Loss in weight, gain in weight and continuous weighing.
- Available crack and crevice-free with a (0.2Ra) polish for sanitary material handling.
Gravimetric Feeding Process
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. The metering feeder can be designed without the agitator if the material is free-flowing.
Gravimetric feeders deliver material at a weight-based rate using an advanced load cell and digital controller combination. Whether in continuous or intermitted operation, loss in weight feeders 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 and the weigh controller constantly adjust the feeder’s discharge rate speed, correcting any errors, regardless of material characteristics.
Our feeders are constructed from materials that meet all EC and FDA food contact regulations for food and pharmaceutical applications. 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 the entire 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 the differences between Trantec’s gravimetric and volumetric feeders.
Gravimetric Controls In Detail
Gravimetric control is aimed at applications requiring a continuous, exact, and monitored product flow 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 screw 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 a dedicated weighing support frame for larger feeders. The selection of the correct weight system is essential. 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 a 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, the metering feeder will eventually run out of product and need refilling. During a refill cycle, gravimetric monitoring must be suspended, and the screw feeder goes into volumetric mode. The refilling process should be as 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.
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. An error must have occurred for the controller to apply a correction. Trantec’s 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 that can match your process requirements. From cost-effective single-line display units to complete recipe and factory automation systems.