Powder feeder with loss-in-weight control: a feed screw on a weigh hopper that meters powder by weight and is refilled without stopping

01

Scale, feed screw and weigh hopper under one controller, sampling about every 50 ms

Andy Kovats of Brabender Technologie, writing in Powder & Bulk Solids, describes a loss-in-weight feeder as a scale with a dedicated controller, a feed mechanism (screw, vibrating tray or similar) with a variable-speed drive, and an integral weigh hopper; the controller measures the weight lost over time and raises or lowers the feed output to hold the set point, sampling weight about every 50 milliseconds. He cites continuous accuracies down to 0.2% of sample size at a 2-sigma confidence level, depending on the material, and lists screw conveyors, bin activators, pneumatic conveyors, slide gate valves and rotary valves as refill devices. [1]

Editorial calculation: sampling every 50 ms is 20 weight readings per second; in Kovats's example, a feeder set at 100 lb/h that lost 98 lb in an hour was 2% below set point and would raise its screw speed. [1]

Give the supplier the powder, its bulk density, flowability and any tendency to pack or flood, the feed rate range, the accuracy needed and over what sample time, how the hopper will be refilled, the headroom available, and what happens downstream if the feed rate drifts.

02

Refill: volumetric control at a held speed, and the compaction error that follows

Kovats explains that a feeder cannot control by weight while its scale is gaining weight, so modern controllers run the feed screw at a constant speed during refill, set by averaging drive tachometer readings just before refill; during that period the feeder does not respond to changes in humidity, temperature, bulk density or screw filling, so refill time should be as short as possible and repeatable, for example 15 seconds every time rather than anywhere from 5 to 45. [1] K-Tron's US patents 4,524,886 and 4,635,819 attribute the refill error to compaction: material near the bottom of the hopper compacts as material is added, raising effective density and mass flow. Their control divides the hopper's weight range into increments, stores a density-related scale factor for each increment during discharge, and during refill drives the screw at speeds set by the stored factors. [2] [3]

Editorial note: ask the supplier whether its controller holds a single averaged speed during refill or adjusts speed from stored density data, and for test data on feed rate deviation through a refill with your powder.

03

Accuracy as a percentage of sample at 2 sigma, and the 90% gravimetric-time assumption behind it

Kovats gives an example accuracy statement as repeatability in percent of sample size at a stated confidence: 0.5% at 2 sigma means that at 6,000 lb/h, 95% of consecutive one-minute samples weigh between 99.5 and 100.5 lb, assuming perfect linearity. In his example the statement holds only if the feeder is under gravimetric control for at least 90% of its cycle, where time in volumetric mode is refill time plus scale stabilisation time; high-displacement load cells can take longer to settle than low-displacement cells, and floodable powders can take longer while entrained air escapes. [1]

Kovats states that with 30 refills an hour and 5 seconds of scale stabilisation, only 7 seconds are left for each refill, and that spreading volumetric time over a larger number of short refills, such as 20 of 10 seconds rather than one of three minutes, limits its effect on the process; he asks manufacturers to quote and demonstrate scale stabilisation time. [1] Editorial calculation: 30 refills an hour is a 120-second cycle; 90% gravimetric leaves 12 seconds volumetric, and 12 − 5 = 7 seconds of refill, matching his figure. [1]

04

Send the powder, rate, accuracy and refill arrangement together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Powder: [name; bulk density; flowability; packing or flooding]
Rate: [set point range]
Accuracy: [% of sample; sample time; confidence level]
Refill: [device; capacity; frequency]
Scale: [stabilisation time; load cell type]
Site: [headroom; downstream process]

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