3.6.2.5  Cooling

The pellets are additionally dried during the cooling phase. The temperature of the air flowing through the counter current cooler is increased by hot pellets. This enables air to absorb more humidity,   and  the  pellets  are  dried. This shows that high air volume is not necessary to  achieve  a  good  drying result. Large air volume means high air flow  velocity and therefore a shorter period of contact with the product. Air is heated to lesser degree and can therefore absorb less humidity,  which in turn degrades the drying result. For this reason, the air volume must be optimised relative to the required values.

Other factors

Storage

During pellet storage, temperature is crucial. Pellets must be sufficiently cooled after the pelleting  process. Pellet temperature should  be  in  a range lower  than  10°C above the ambient temperature. This will prevent condensation inside the storage bins. The storage bins should be equipped with an autonomous exhaust system to absorb major fluctuations in the climate which might affect the pellet quality.

Handling

During transportation and handling of pellets, attention must focus on minimising production of fines. As a result, agitation of the pellets must be minimised. At the same time, care must be taken to ensure movements that are easy on the pellets: low conveying velocities, low friction  and low  falling heights. The term pellet quality in this case means hardness and stability. Two basic different measurement methods exist to assess and describe the durability and stability of pellets. One is “hardness” testing which measures the force required to destroy the pellet. The other measurement method is the so-called “durability”  test. It uses test devices to investigate the actual loads acting during transportation and handling on the pellets, simulating them and measuring the weight (percentage) of fines produced or the weight of the pellets left over.