Calculations

How to Calculate Extrusion, Drying, and Durability in Pet Food Manufacturing

The core math behind a kibble line, worked start to finish: specific mechanical energy, water mass balance, drying load, pellet durability index, and bag rate.

Start at the extruder, because throughput drives every downstream number. Mass throughput is simply screw output in kg/h, but the input you actually control is specific mechanical energy (SME), the drive work put into each kg of dough. SME in kWh/kg equals installed motor power times load fraction, divided by mass flow. A 90 kW drive running at 70 percent load feeding 2,000 kg/h gives (90 times 0.70) divided by 2,000, or 0.0315 kWh/kg, about 31.5 Wh/kg. Most dry dog kibble sits between 20 and 45 Wh/kg. The Extrusion Throughput calculator ties motor load, screw speed, and moisture into this single number.

SME matters because it sets expansion and cook. Below roughly 18 Wh/kg you get dense, underexpanded kibble that sinks; above 45 Wh/kg you overcook starch and burn energy. To hold SME while raising rate, you raise drive load or drop in barrel moisture. If you push flow from 2,000 to 2,600 kg/h at the same 63 kW draw, SME falls to 0.0242 kWh/kg, 24.2 Wh/kg, and you will see the kibble tighten. Read SME every batch and log it against bulk density in g/L so you can predict the product, not chase it.

Next is the water mass balance, the input the dryer depends on. Work in solids, not total mass, because solids are conserved. Kibble leaving the die at 25 percent moisture on 1,000 kg carries 750 kg dry solids and 250 kg water. Dry to 9 percent finished moisture and total mass becomes 750 divided by 0.91, or 824 kg, holding 74 kg water. Water to remove equals 250 minus 74, so 176 kg per 1,000 kg wet feed. That 17.6 percent removal is the load the dryer must clear, and it scales linearly with throughput.

Drying energy follows directly. The Kibble Drying Energy calculator multiplies water removed by an effective evaporation energy that already folds in dryer inefficiency. Pure latent heat is 2,260 kJ per kg water, but real convective belt dryers spend 4,000 to 6,000 kJ per kg once you count heated air losses and sensible heating. Take 176 kg water at 4,500 kJ/kg: that is 792,000 kJ, or 220 kWh, per 1,000 kg wet feed. On a 2,000 kg/h line that is 440 kW of thermal draw, the single largest energy line on the plant after the extruder.

Pellet durability is the strength check, reported as the Pellet Durability Index (PDI). Weigh a screened sample, tumble it in a Pfost box or hold it in an air jet for the standard time, screen out fines, and reweigh. PDI equals weight after divided by weight before, times 100. A 500 g sample that returns 480 g gives 96 percent PDI, with 20 g, or 4 percent, lost as fines. The Pellet Durability Loss calculator turns that percentage into tonnes of dust per shift. Dog kibble targets above 96 percent; below 93 percent you will see broken pieces and dusty bags.

Fines carry straight into yield, so convert PDI loss to mass. At 4 percent durability loss on 2,000 kg/h, you generate 80 kg/h of fines. Some is reworked back through the preconditioner, but rework above 6 to 8 percent of feed destabilizes the extruder and forces you to redose moisture. Track fines as a running percentage of good product and set a rework cap in kg/h, not by eye. A 2 point PDI gain from 94 to 96 percent on that line recovers 40 kg/h, roughly 0.9 tonnes across a 22 hour production day.

Finish at the bag. The Bagging Line Speed calculator converts nominal bags per minute into real output using availability. A valve bagger rated at 20 bags/min is 1,200 bags/h nominal, but at 85 percent availability you fill 1,020. At 15 kg per bag that is 15.3 tonnes/h of packed product, which must match extruder output minus fines and giveaway or you build or starve inventory between the two. If your dryer clears 1,650 kg/h finished and the bagger packs 15,300 kg/h capacity, the extruder is the constraint, so every calculation above should be tuned to feed it steadily.

One more input governs changeovers: allergen and formula sequencing. When you switch from a chicken formula to a fish or grain free line, cleanout time is a function of shared equipment surface and the validated flush mass. The Allergen Cleanout Time calculator estimates minutes from line length, number of transfer points, and whether you wet clean or push a flush batch. A dry flush of 300 to 600 kg of inert product through a 40 metre conveyor path typically clears carryover to below a 20 ppm target in 25 to 45 minutes, versus 3 to 5 hours for a full wet strip down. Sequence formulas by allergen to minimize the count of full cleans.

Published 2026-07-02.