Each pelletizing or heat treatment line has necessarily a cooler, because it is impossible to store a warm and moist product without risks of clumping and microbial degradation.
The aim of this step is to cool and to dry pellets in a single device. In animal feed, the drying and cooling steps are combined.
These processes are carried out by creating an environment airflow within the feed mass. Drying is carried out by heat use carried by the product and cooling by adjusting the heat of the incoming air.
The cooling process is the result of coupled exchanges of heat (enthalpy) and substance (water) between the air and the product. The air is gradually warmed by heat exchange with the product as it passes through the cooler
The heat exchange coupled depends, among other things, on:
- the energy contained in the pellets at the exit of the pellet mill,
- the air velocity in the cooler,
- the production flow rate of the pelletizing line,
- the pellet size,
- the ambient air characteristics, …
In pratice, the pellets change to a temperature between 50°C and 95°C and a moisture at around 14% to 17% at the die outlet (at the cooler inlet) at a temperature of a few degrees above the ambient temperature (between 5 and 10°C above). The moisture must be lower than the prescribed value (14%) at the cooler outlet. To ensure good feed preservation, it is also necessary to appreciate the free water content and the saturation vapor pressure.
This process is done in vertical counterflow coolers or horizontal and crossflow coolers.
Air is drawn downstream from the cooler by a fan, so that it passes through the bed depth of product.
This air is charged in particles in contact with pellets or meal. Then, it must be filtered before being discharged outside the plant. The air velocity variation can be managed by a valve or a speed variation device. In order to not generate preferential flows in the bed depth of product, because of a variable thickness, a flap for layer height control is installed at the inlet of homogenizing the surface.