Loss-in-Weight Feeder Selection: Standards, and Matching the Model to the Material

A loss-in-weight feeder is selected twice. The first time is on paper, from a rate, a bulk density and an accuracy target. The second time is when the material actually arrives and behaves nothing like the data sheet suggested. Most selection failures happen in the gap between those two moments, and they are avoidable — not by choosing a bigger machine, but by asking the right questions in the right order.

PEGE has spent around ten years building gravimetric feeders for powders that were difficult, and the procedure below is the one we apply to every enquiry. It is a standard in the sense that matters: the same six questions, in the same sequence, every time. The answer it produces is almost never a catalogue number.

There are six platforms and twenty-one models in the range, from 0.3 kg/h of micro-additive to 2,000 kg/h of granulate. The purpose of a selection procedure is to eliminate the wrong ones quickly, and then to justify the one that remains.

There is no standard feeder, only a standard procedure

Customers sometimes ask for “your standard model”. It is a reasonable question with an unhelpful answer: the standard model is the one that fits the material, and the material decides. What is genuinely standardised is the way we get there.

Every quotation is built from six inputs: rate and turndown, flow behaviour, bulk density, particle size and fragility, temperature and phase behaviour, and the plant interface. Five of them come from the material, the sixth comes from your line. None of the six can be skipped because the others happen to look favourable.

Six criteria, in the order they decide the machine

Rate and turndown. The maximum rate sets the screw diameter. The ratio between maximum and minimum sets whether one feeder can cover the range at all. A feeder asked to run at five per cent of its maximum spends most of its life at the bottom of the weighing range, where resolution is poorest. Where the turndown exceeds roughly 20:1, we would rather propose two feeders in parallel than one oversized machine pretending to be precise.

Flow behaviour. Free-flowing granulate falls out of the hopper on its own and needs nothing more than a screw. Cohesive powder bridges, floods, or coats the flights. Flakes and fragile granules break as soon as a screw compresses them. These behaviours point to different discharge principles, and no amount of control tuning converts one into another.

Bulk density, and how much it moves. Bulk density sets the volume that has to be moved per hour. Its variability sets how often the feed factor must be re-learned. A material that swings between 0.35 and 0.65 g/cm³ from one delivery to the next is a different selection problem from one that sits at 0.6 g/cm³ all year, even at the same tonnage.

Particle size and fragility. Below roughly 100 µm, powder tends to fluidise and to adhere to surfaces. Above a few millimetres, granules tend to break. Fragile material — dried granules, coated particles, some catalyst carriers — should not pass through a screw at all, which is why the vibratory tray platform exists.

Temperature and phase behaviour. Material that is hot, that solidifies at ambient temperature, or that must not exceed a temperature limit, changes the hopper, the jacketing and sometimes the feeding principle itself. A plasticiser that sets at room temperature cannot be metered through an unheated line, whatever the accuracy target says.

The plant interface. Refill method, available height, downstream connection, cleaning regime, communication protocol and hazardous-area classification decide the frame, the hopper shape and the control platform. They rarely change which principle is right, but they decide whether the installation works in practice.

From material behaviour to platform

The table below is the compressed version of the six criteria. It is a starting point rather than an answer: the final specification still depends on the numbers, and on the sample test.

Table 1   Material behaviour, and the platform it points to

The sizing numbers, and the conditions attached

We specify ±0.5% of setpoint on most platforms and ±1% on the smallest micro model, with digital weighing resolution down to 1/100,000 on the twin-screw range. Those figures assume continuous operation over at least thirty seconds, a bulk density of 0.6 g/cm³, and a feeder calibrated against the actual material.

Thirty seconds is not a formality. During refill the feeder switches to volumetric control, and for that interval the weight signal is no longer a clean measure of discharge. A well-designed refill sequence keeps the volumetric interval short relative to the gravimetric one. A poorly sized refill system makes the ratio worse and turns a good accuracy figure into an unachievable one.

Turndown deserves the same honesty. A range of 20 to 800 kg/h is 40:1, and no single screw holds ±0.5% across all of it. We state the band in which the figure applies, and we say what happens outside it.

Where the feeder ends and the line begins

Selection rarely stops at the feeder, because the feeder has to be fed. Refill hopper volume, buffer capacity and the refill device itself belong to the same calculation. A hopper that empties every ninety seconds turns a good feeder into a machine that spends half its time in volumetric mode.

Around the feeder we engineer the rest of the material path: refill hoppers and day bins sized to the refill cycle, big-bag unload stations with dust control, and pneumatic conveying in dilute or dense phase depending on the material and the distance. Where a customer wants a single point of responsibility, we deliver the dosing section as a turnkey package — from the sack tip or big-bag frame, through conveying and storage, to the metering point at the extruder throat.

That said, our centre of gravity has not moved. PEGE designs and manufactures the feeder: the weighing architecture, the screw or tray geometry, the control platform and the refill logic. Ancillary equipment is engineered around that core, not instead of it. If a project needs only a feeder, that is the project we are happiest to quote.

What we need from you

o size a feeder properly we need five things, and they are all things you already know about your own process:

The material — bulk density loose and tapped, particle size distribution, moisture content, temperature at the feed point, angle of repose if it has been measured, and whether it is fragile, adhesive, toxic or combustible.

The rate — maximum, normal and minimum throughput in kg/h, and how long the line runs between stops.

The accuracy — what the downstream process actually tolerates, expressed as a percentage of setpoint at the normal rate.

The installation — available height above the feed point, refill method, downstream connection, cleaning regime and material of construction.

The environment — hazardous-area classification, ambient temperature, and whether the line is washed down.

Send a material data sheet or a sample, together with the target rate. We will come back with the platform, the size, the accuracy that is realistic for that material, and the points where the risk sits — before you commit to a line design.