Gravimetric Feeding in Food Production

A food recipe spans several orders of magnitude. Flour, sugar and starch arrive by the tonne; a colour, a vitamin premix or a flavour is dosed in grams, and the small ones determine taste, appearance and label compliance.

The small ones are also the difficult powders. Flavours, colours and micronised ingredients are cohesive, they bridge in a hopper, they hang on screw flights and they change bulk density between aerated and settled by a large factor.

The quiet failure is sizing. A loss in weight feeder is accurate to a percentage of its own capability over a stated sample time, so a feeder sized generously for the largest rate in the recipe set has no useful accuracy at the smallest. The number on the datasheet is still true; it just does not describe what the plant needs.

And every recipe change means the feeder is opened. Hopper, screw, tube and outlet are all product contact parts with an allergen history, and a feeder that takes an hour to strip will not be stripped as often as the allergen matrix assumes.

Each ingredient fed by a feeder sized for its own rate range, with the accuracy stated at the rate and the sample time the process actually uses.

Refill designed so that the blind period is short, infrequent and free of mechanical shock, and so that two feeders on a shared line are never refilled at once.

A machine that can be stripped, cleaned and reassembled within the changeover window, with every product contact part named in the allergen plan.

A pet food plant adds a regulated inclusion level, so the micro feeder is not only a question of accuracy but of what has to be demonstrated.

The discharge device is matched to the powder. Twin screws are self wiping and handle cohesive flavours and colours; single screws suit free flowing sugar and salt; a vibratory tray moves flakes, granules and fragile pieces without shearing them. Getting this wrong is the single most common cause of a feeder that will not hold its rate.

Then the hopper is designed for flow rather than for volume. A steep cone, a flexible wall with external massage paddles or a vertical agitator keeps a cohesive powder moving; over agitation is its own fault, because it aerates the material and changes the feed factor continuously.

Refill is designed as a sequence. During refill the weight signal cannot be used, so the controller runs on the last known feed factor, and the longer that lasts the more the rate can drift. Fast, clean refills at a small fraction of the cycle, a refill valve that seals properly rather than trickling, and a sequence that staggers feeders on a shared conveying line are what keep the gravimetric control honest.

Finally the installation is checked for the things that corrupt a load cell: a stiff inlet sock, a cable tied across a flexible connection, vibration from a neighbouring mixer, or a pressure difference across the feeder that applies a force the controller reads as mass.

• Feeder sized for the smallest rate it has to hold, with accuracy stated at that rate and sample time.
• Discharge device matched to the powder: twin screw, single screw or vibratory tray.
• Hopper designed for flow, with agitation sufficient but not aerating.
• Refill short, clean and staggered, with a valve that seals rather than trickles.
• Flexible connections genuinely free of transmitted force, and vibration isolated.
• Every product contact part quick release and named in the allergen plan.

• Smallest inclusion rate and the tolerance on it, which sets the feeder size and often the number of feeders.
• Flow behaviour of each ingredient, measured, since the cohesive ones are usually the critical ones.
• Process sample time, because accuracy quoted over a minute means little to a process that varies over seconds.
• Allergen matrix and cleaning standard, which decide whether quick release construction is essential.
• Product fragility where granules, flakes or inclusions are fed.
• Dust explosion characteristics of the powder, including sugar and starch.