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How pellets are cooled?

Pellet cooling is a dynamic process that involves simultaneous heat and mass transfer between the pellets and the cooling air.


Pellet cooling occurs as a result of both evaporative cooling (the transfer of water from the pellets to the air which results in both moisture reduction and cooling in the pellets) and convective cooling (it depends on the temperature difference between the pellets and the air, the amount of pellet surface area, and the heat transfer coefficient).
The cooler performs two functions: cools and dries the pellet. Drying is required to prevent mold and to increase pellet hardness. Cooling is required to prevent mold and stabilize the residual moisture. The cooler is able to eliminate most of the heat and moisture added by the conditioning process and by the pelleting process.

Step by step this is what happens:

a) During the conditioning phase, steam is added to the meal in the conditioner. This steam increases the moisture content of the meal by 3 - 5%. In addition, during pelletization, some heat is transferred through the mechanical friction. The pellets produced are, therefore, discharged at a temperature of between 60°C for cattle and 85°C for poultry. At this point, the pellet must be cooled and dried.

b) When the pellet leaves the pellet mill, it has a relatively porous structure which allows the moisture to migrate by capillary action.

c) The cooler is designed to bring environmental air into contact with the surface of the pellets. This air, which is not 100% saturated, picks up the moisture from the surface of the pellet, thus reducing the pellet moisture content through evaporation. This evaporation process also cools the pellets.

d) The heat withdrawn from the air increases the temperature of the air itself which, in turn, increases its ability to contain water. For example, if the air entering in the cooler is 20°C with a relative humidity of 85% and this air is heated to 45°C by passing through a bed of hot pellets, its ability to contain moisture will be increased 5-fold.

e) When the moist surfaces of the pellet are inundated by the cooling air, it is dried and left in a state of imbalance. The concentration of moisture at the centre of the pellet is greater than at the surface. Because of this disequilibrium, the moisture migrates toward the surface of the pellet along with the heat. This moisture is then available and can be picked up by the cooling air.

f) This process continues until most of the moisture and heat added during the conditioning process have been removed. The moisture remaining in the pellet is usually the same as, or slightly greater than, the moisture in the initial bonding of the ingredients (that is before conditioning; this is, therefore, the intrinsic moisture). Under normal conditions, the cooling air cannot remove this binding moisture. The exception is when large volumes of extremely dry air enter the cooler as this leads to a decrease in the moisture content. Vice versa, at times the water is added to the mixture before the conditioning chamber and there is not enough heat available to withdraw the moisture remaining in the pellet even after it has gone through the cooler.

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