When the pellets leave the pellet mill they are very hot, soft and releasing moisture. Before the pellets can be used or stored they must first be cooled and dried. The simplest form of pellet cooling is to spread the pellets out and let them cool at room temperature.
Hot pellets should not be left to cool and dry in a heaped pile, as the temperature in the middle and at the bottom of the pile will stay hot for quite some time, preventing the pellets from properly cooling and hardening. The pellets should be spread out, as this will increase air circulation around the pellets for faster cooling. Using fans to increase air circulation will also aid cooling.
Once the pellets are cooled, they should be hard, smooth and in some cases have a surface shine. One of the best ways to describe a quality pellet is that it has similar characteristics to a crayon. If the pellets are not like this—for example, are brittle— efforts must be made to adjust the necessary parameters.
Cooling pellets is a delicate process. If the pellets are cooled too quickly, the rapid release of moisture causes stress fractures to occur. Rapid cooling also affects the lignin’s ability to properly solidify to form a hard dense pellet. Therefore rapid cooling can affect pellet quality and increase fines.
The counterflow process is the refined technology of exposing processed product to an upward moving stream of ambient airflow. As the air rises through the product, it is warmed, increasing the moisture carrying capability of the air. The product entering at the top of the cooling chamber is exposed to the warmest air available within the cooler, minimizing temperature shock. The product exiting the bottom of the unit is cooled to within 5 to10°C of the ambient temperature. The product and air have “opposite flows.” The gradual heat transfer greatly enhances product quality and reduces stressing and fines.