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Pellets that fall apart after the cooler are a different problem from pellets that fall apart at the die, and the difference is the point of this page. A pellet that leaves the die weak is a forming problem, while a pellet that leaves the die intact and then breaks after the cooling is a cooling, handling, or moisture-pickup problem, and the two are corrected in different places. The operator who sees the crumbled pellet in the bag and assumes the die is at fault usually adjusts the die and misses the cooling or the bagging, and the pellet keeps crumbling.

This page is about the pellet that falls apart after the cooler rather than at the die, and it covers the cooling rate, the pellet temperature at the bag, the moisture pickup, the fines, and the handling, with a diagnosis sequence and the correction order. The moisture at the die, the squeeze test, the density, the fines, and the die running-in are covered on their own pages with their own checks, and they are not repeated here.

The short answer

Pellets that fall apart after cooling come from one of four causes: the cooling is too fast, the cooling is too slow, the pellet is bagged warm and picks up the moisture in the bag, or the pellet is broken by the conveying and the handling after the cooler.

The practical rule: take a sample at the cooler outlet and a sample at the bag, compare the two on the same measure, and the sample that is worse tells you where the damage happens. A pellet that is already broken at the cooler outlet was damaged in the cooler, while a pellet that is intact at the cooler outlet and broken at the bag was damaged after the cooler, and the two are corrected in different places.

What “after cooling” tells you

The timing of the failure is the first diagnostic clue, and it is worth reading before anything is adjusted.

A pellet that is broken at the cooler outlet was damaged in the cooler, and the cause is the cooling rate, the cooling air, or the pellet that entered the cooler already weak. A pellet that is intact at the cooler outlet and broken at the bag was damaged after the cooler, and the cause is the conveying, the bagging, the moisture pickup, or the handling.

The two failures look the same in the hand and are different in the line, and the sample point is what separates them.

How to compare the two samples

The comparison is only meaningful when the two samples are measured the same way, and the measure should be chosen before the samples are taken.

Use the same measure at both points, either the fines content in the sample, the proportion of intact pellets after a light handling, or the result of the same field squeeze check, and record the measure with the sample point. A pellet that passes the cooler and fails the bag on the same measure is a handling problem, while a pellet that fails at both points on the same measure is a cooling or a forming problem.

The fines in the bag are an indicator of the handling damage rather than a cause of it, and a bag that has more fines than the cooler outlet sample has is a bag that has been handled roughly rather than a cooler that is set wrong.

The four causes and how to tell them apart

Cause	What points to it	What to check
Cooling too fast	The pellet cracks or has a hard shell and a soft core	The cooling air, the residence time, and the pellet at the cooler outlet
Cooling too slow	The pellet is soft and deforms after the cooler	The cooling air, the throughput, and the pellet temperature at the bag
Bagged warm and picking up moisture	The bag shows condensation, and the pellet softens after storage	The pellet temperature at the bag, the bag specification, and the store
Broken by conveying or handling	The pellet is intact at the cooler and broken at the bag	The conveyor, the transfer points, the drop height, and the bagging

Cooling too fast

A pellet that is cooled too fast develops a hard shell and a soft core, and it cracks as the core contracts. The cause is the cooling air that is too cold or too strong for the pellet diameter and the throughput, and the correction is to reduce the air or to increase the residence time so that the pellet is cooled gradually.

The check is on the pellet at the cooler outlet rather than on the cooler setting, because the same setting produces a different result at a different throughput, and the cooler that is correct at the design capacity may be too fast at the reduced capacity.

Cooling too slow

A pellet that is cooled too slowly stays warm and soft, and it deforms in the screen and the conveyor. The cause is the cooling air that is too low, the residence time that is too short, or the throughput that is above the cooler capacity, and the correction is to increase the air or the residence time or to reduce the throughput.

The check is on the pellet temperature at the bag, because a pellet that is still warm in the bag is a pellet that is being bagged too early. The cooler outlet reading measures the pellet at the outlet, while the bag temperature measures the pellet after the conveying and the bagging, and the bag temperature is what the customer receives.

Bagged warm and picking up moisture

A pellet that is bagged warm carries the moisture that condenses inside the bag, and the pellet that was dry at the mill is wet at the customer. The bag shows the condensation, and the pellet softens after the storage rather than at the cooler.

The check is on the pellet temperature at the bag and on the bag specification. The pellet should be close to the ambient before it is bagged, the bag should be moisture-resistant, and the store should be dry, because a warm pellet in a permeable bag in a humid store is a pellet that picks up the moisture from both the condensation and the air.

Broken by conveying and handling

A pellet that is intact at the cooler outlet and broken at the bag is a pellet that has been broken by the conveying and the handling. The cause is the free fall at the transfer points, the rough conveyor, the drop into the silo, or the drop into the bag, and the correction is in the conveying and the bagging rather than in the cooling.

The check is on the conveyor and the transfer points, and the drop height should be reduced and the cushions fitted at the points where the pellet falls. A pellet that is brittle because it is over-dried or because the die specification is wrong breaks more easily, and the pellet specification should be reviewed together with the handling.

How to diagnose a pellet that falls apart after cooling

The diagnosis moves from the bag back through the line, and the sample point is the first decision.

The first step is to take a sample at the cooler outlet and a sample at the bag, and to compare the two on the same measure. If the cooler outlet sample is already broken, the cause is in the cooler or in the pellet that entered it. If the cooler outlet sample is intact and the bag sample is broken, the cause is after the cooler.

The second step is to check the pellet temperature at the bag, and the condensation in the bag, to separate the moisture pickup from the handling.

The third step is to check the cooler air, the residence time, and the throughput, to separate the cooling too fast from the cooling too slow.

The fourth step is to check the conveyor, the transfer points, the drop height, and the bagging, to locate the handling damage.

The fifth step is to check the pellet that entered the cooler, because a pellet that is already weak at the cooler inlet is a die or a moisture problem and should be traced on the moisture and die pages rather than on this one.

What to correct and in what order

The correction follows the sample point, and it is made one variable at a time with the pellet checked after each change.

If the cooler outlet sample is broken, correct the cooling first, and adjust the air or the residence time before the throughput is changed. If the bag sample is broken and the cooler outlet sample is intact, correct the conveying and the bagging first, and reduce the drop height and fit the cushions before the cooler is touched.

If the bag shows the condensation, correct the bagging temperature, the bag specification, and the store before the cooling is changed, because the moisture pickup is a bagging and a storage problem rather than a cooling problem.

What to specify before buying a line

Before the line is specified, the cooling and the bagging should be addressed in the quotation and in the acceptance test. The quotation should state the cooling arrangement, the pellet temperature at the bag that the cooler is designed to achieve, the bag specification, and the handling at the transfer points. The acceptance test should measure the durability on a sample taken from the bag rather than only from the cooler outlet, because the pellet that reaches the customer is the pellet that has been cooled, conveyed, and bagged.

If you are producing pellets that fall apart after cooling and need to trace the cause through the cooler, the bagging, the moisture pickup, and the handling, send the samples from the cooler outlet and from the bag, the pellet temperature at the bag, the cooler setting, the bag specification, and the conveying layout, and the Shandong Changsheng Machinery team can review the line configuration and propose the correction and the acceptance test.

Bottom line

Pellets that fall apart after cooling are a different problem from pellets that fall apart at the die, and the sample point is what separates them. Take a sample at the cooler outlet and a sample at the bag, compare the two on the same measure, and if the cooler outlet sample is already broken, correct the cooling; if the cooler outlet sample is intact and the bag sample is broken, correct the conveying and the bagging. Check the pellet temperature at the bag and the condensation in the bag to separate the moisture pickup from the handling, and check the cooler air, the residence time, and the throughput to separate the cooling too fast from the cooling too slow. Correct one variable at a time, measure the durability on a sample from the bag rather than from the cooler, and state the cooling arrangement, the bagging temperature, the bag specification, and the handling in the quotation and in the acceptance test, so that the pellet that reaches the customer is the pellet that was tested.