Notes from the workshop

Three questions we are asked in almost every project, answered the way we would answer them on the plant floor: with numbers, and with the reason behind the number.

How to size a pellet mill

Almost every enquiry we receive starts with a tonnage: "we need 10 t/h". That number alone is not enough to choose a machine, because a pellet mill does not have one capacity — it has a different capacity for every recipe and every pellet diameter you put through it.

The reason is the die. Feed leaves the mill through the holes of the ring die, so the open area of the die is the real bottleneck, not the motor. A 3 mm poultry pellet passes through many small holes; a 10 mm cattle pellet passes through fewer, larger ones. Change the diameter and the same machine gives you a different tonnage.

Start from the pellet, not the motor

As a rule of thumb, a feed pellet mill needs 8 to 15 kWh per tonne of finished pellet. Soft poultry mash at 3 mm sits at the bottom of that range; a fibrous cattle ration at 6 mm, or an aqua feed pressed through a thick die, sits at the top. Multiply the tonnage you want by the kWh/t of your hardest recipe, and you have the motor you actually need — with headroom, not at 100 % load.

The mistake we see most often is a mill chosen on the easiest recipe in the book. It runs beautifully in the first week and then chokes the day the plant switches to a high-fibre formula. Size on the hardest recipe you intend to produce, not the average one.

Then check what feeds it

A pellet mill is only as good as the mash arriving at it. If the feeder and the conditioner cannot deliver a steady, evenly conditioned flow, the mill will surge: the amp meter swings, the die loads unevenly, and pellet quality moves with it. When a line underperforms, the pellet mill gets the blame roughly nine times out of ten — and it is the cause about once.

Die compression ratio, in plain terms

The compression ratio of a ring die is the length of the hole divided by its diameter. A 4 mm hole in a die with 60 mm of effective working length has a ratio of 15:1. That single number does more to determine your pellet quality — and your energy bill — than any setting on the control panel.

The longer the hole, the longer the feed is squeezed inside it, and the harder the pellet comes out. That sounds like a good thing until you look at the meter: a high ratio also means more friction, more heat, more current and a lower tonnage. Every point of pellet durability you buy with a longer hole, you pay for in kilowatt-hours.

The recipe chooses the ratio

Starch binds; fat lubricates; fibre resists. A starchy poultry mash pellets easily and needs a modest ratio. Add 4 % fat and the mash slips through the hole without forming — you need a longer one. A high-fibre cattle ration behaves the same way for the opposite reason: the fibre springs back, and the pellet falls apart unless the hole holds it under pressure for longer.

This is why a die is not a spare part you order by diameter alone. When a customer tells us the pellet is soft, our first question is never about the mill — it is what changed in the formula.

What actually kills a die

Dies rarely wear out evenly. They die from tramp iron that scores the surface, from a roller gap set too tight — which grinds the die instead of pressing feed through it — and from being left full of feed at shutdown, which sets like concrete overnight. A magnet at the intake and a disciplined shutdown routine will do more for die life than any grade of steel.

Conditioning decides pellet quality — not the mill

When a plant is unhappy with its pellets, the pellet mill is the machine that gets replaced. It is almost never the machine at fault. Pellet durability is decided before the feed reaches the die, in the conditioner, by three variables: steam quality, temperature and retention time.

What the conditioner is really doing is cooking starch. Heat and moisture make starch granules swell and burst — gelatinisation — and the freed starch is what glues the pellet together. Below roughly 75 °C very little of that happens, and no amount of pressure at the die will compensate for it.

Dry steam, and time to work

Steam has to arrive dry and at a stable pressure. Wet steam wets the mash instead of heating it: the feed goes soft, the die slips, and the pellet leaves the mill too wet to cool properly. A working separator and trap at the conditioner inlet is not an accessory — it is the difference between conditioning the feed and merely soaking it.

Retention time is the second half. A standard conditioner holds the mash for 20 to 60 seconds. A hygieniser holds it for three or four minutes at 85–95 °C, which is what kills salmonella. If you need both pathogen control and a hard pellet, you need retention — and retention is a machine, not a setting.

A number worth writing down

Every additional percentage point of moisture added as steam in the conditioner is roughly one point of moisture the cooler must take back out. Condition well and you gain durability; condition sloppily and you simply move the problem downstream, to a cooler that was never sized for it.

Bring us the recipe that is giving you trouble

Send us the formula, the pellet you need and the tonnage. Our process engineers will tell you what the line has to look like — and, when the machine is not the problem, they will tell you that too.

Talk to a process engineer