The Importance of Cooling Pellets in Quality Feed Processing

In order to shape or sanitize the feed, compound feed is sometimes pelleted or otherwise heat treated. The hot substance cannot be processed automatically and must be cooled first. This method is somehow taken for granted, but it is also an important part of the production of feed.

They are hot (65-85 ° C) and smooth and moist as the pellets leave the pellet mill (up to 17 percent moisture). They cannot be shipped or stored in this state without being damaged. So a cooler is mounted in close proximity to the press outlet in which the pellets can harden, evaporate moisture, and can lower the temperature to around 5 ° above ambient temperature. A side effect of cooling is the drying of the pellets by one or two percent. To ensure optimum storage efficiency, the pellets should maintain moisture content of not more than 10-12 percent.

The hot pellets have to be handled gently during transport from the pellet mill to the counterflow cooler, so they are already fragile and split quickly, which can result in a high number of fines.
Two phases take place during cooling: heat transfer and mass transfer.
The temperature variations in the contact region, the scale of the contact area, and the heat conducting character of the pellets are influenced by the heat transfer from the pellets to the air. Mass transfer means moisture is transferred from the pellets to the air in the process. The cooling effect relies on evaporating moisture.

The Time of Cooling

Cooling can be a slow process as well. If the pellets are cooled very rapidly, cracks are formed in the outer layers, which can lead to greater abrasion in turn. Cooling for too much time, the pellets become too dry, often has a negative effect on abrasion. It also has financial consequences since, more goods are consumed and less water is sold, where the cheapest ingredient in the feed is still water.


The core of the pellet is still warm and the pellets are still wet and moist because the cooling time is too short. It should be remembered that heat and moisture are released more easily by small pellets than by large pellets. Even the arrangement of pellets in the cooler optimizes the process of cooling. The airspeed does not exceed 0.5 m/s in conventional belt coolers and should remain between 0.8 and 1.5 m/s in modern counter flow coolers (Figure 1).





Heat is lost too suddenly when airspeed is too high; moisture evaporation is too low; the outer layerdries too quickly, and heat and moisture emission after cooling contributes to cracks in the surface of the pellet and hence to increased abrasion.

If air velocity is too low, heat withdrawal and elimination of moisture is too high. A schematic example of the influences on the pellet in a cooler is seen in Figure 2.

Types of Coolers

Two kinds of coolers are most widely used in practice: belt coolers and counter-flow coolers. Some other types of coolers are used on a few occasions, such as cascade coolers.

A Typical Belt Cooler:

Using the cross flow principle, a belt or horizontal cooler cools the pellets, whereby incoming air is blown in through a thin layer of product. Cooling is higher on the side of the receiving air than on the side of the exhaust. To prevent excessive cooling, the sheet of pellets should be spaced evenly and not too thickly (belt speed). A belt cooler is capable of having one or two decks. A downside of the belt cooler is that after going through a relatively shallow product bed, a large volume of air is not used for cooling and is exhausted.
 

The counter flow cooler works with an airflow that moves opposite to the product flow. In a product sheet, the product constantly travels downward while air is sucked in an upward direction through the product layer.

They share energy and also water at the moment when air and product flow travel through each other. This ensures that the coolest pellets on the bottom encounter the coolest air and air with a higher temperature is obtained by the warmer pellets on board. This removes thermal shock and cools the pellets in a soft, even, and gradual way.

A Typical Belt Cooler:

A counter flow cooler, in comparison, is more powerful than a belt cooler.

1- In a counter flow cooler, the air volume used is smaller 2- It can be as little as 60% of the air used in a horizontal belt cooler.
3-A significant transition of energy from product to air happens in a counter flow cooler, which ensures that the exhausted air has a higher temperature.