30/60 Versus 100/200 Mesh on a Volumetric Feeder: Two Very Different Settings

The same feeder, the same setpoint, two grades. With 30/60 mesh the line delivered 4.1 kg an hour against a 4.0 target. With 100/200 mesh, set identically, it delivered 6.8 kg, a 70 percent overdose that ran for two weeks before anyone noticed the gel was stiffer than usual. Nothing had drifted. The screw was simply filling differently, and the calibration curve belonged to the other product.

Mesh size changes bulk density, flowability and the fill factor of a screw, which means every grade needs its own calibration. This article gives the numbers behind that shift, the flooding behaviour of fine grades, and a 15-minute calibration that stops the drift.

Bulk Density and the Screw Fill Factor

A 30/60 mesh granule sits at 0.62 to 0.68 g/cm3 tapped and 0.58 to 0.62 loose. A 100/200 mesh powder sits at 0.50 to 0.56 tapped and 0.42 to 0.48 loose. That is a 25 percent difference in mass per unit volume before any flow behaviour is considered, and mass throughput is what the setpoint controls.

Fill factor adds to it. A coarse granule in a 40 mm screw fills 85 to 95 percent of the flight volume and stays constant with speed. A fine powder at the same screw speed fills 100 percent and can overfill in the inlet zone, so the delivered mass rises faster than the screw speed. Set the same rpm and you get 50 to 70 percent more mass with the fine grade.

Flooding With Fine Grades at 100 rpm

Above about 100 rpm a fine grade stops behaving like a metered solid and starts behaving like a liquid. The powder aerates in the hopper, the screw becomes a conveyor of aerated material, and the output depends on hopper level: full hopper gives 20 percent more than a half-full one. This is why fine-grade lines drift through a shift without anyone touching a setting.

Two fixes work. Slow the screw to 30 to 60 rpm and increase the diameter, which reduces aeration and keeps the fill factor stable. Or fit a horizontal agitator bar in the hopper that keeps the powder de-aerated, which costs about 1,200 dollars and solves the problem without changing the screw.

The 15-Minute Calibration That Stops Drift

Run the feeder at 25, 50, 75 and 100 percent of maximum speed, collecting output for 3 minutes at each point on a tared container. Weigh each and plot mass per hour against speed. A straight line through the origin with an R-squared above 0.99 means the feeder is behaving; a curve that flattens at the top means flooding, and a line that does not pass through the origin means leakage or residual material.

Do this for every grade you run, and write the slope on a label on the feeder. Two grades with different slopes should never share a setpoint, and a changeover should trigger a re-check with a 3-minute catch sample before the line goes to full rate.

When to Change Mesh Instead of the Feeder

If a fine grade is required for process reasons, fix the feeder. If the fine grade is only there because someone assumed faster absorption is better, change the grade instead. A 30/60 mesh granule reaches 80 percent of its capacity in 25 to 40 seconds in water, which is inside the residence time of almost every industrial mixer.

The exception is a process with under 10 seconds of contact, such as a spray application or an in-line static mixer. There a 100/200 mesh cut earns its handling problems. Everywhere else, the coarse grade is easier to meter and cheaper to convey.

Ask for the bulk density and the mesh distribution on the certificate, and check both against the calibration label. The classified SAP granule sizes are supplied with both figures, which makes the feeder setting a calculation rather than a guess.