Pellet cooling system performance is affected by many variables. The first are variables found in the air being used and include:
- Air temperature
- Air relative humidity
- Airflow, the amount of air passing through the cooler
- Pellet variables include:
- Pellet temperature
- Moisture content
- Pellet size and density
- Fines present
- Retention time in the cooler
- Pellet quantity
Cooler variables include:
- Cooling bed depth
- Uniformity of bed depth (regardless of the cooler type, horizontal, vertical or counter-flow, it is very important that the bed depth is uniform where air passes through the product to keep the air flowing equally through all the product).
- System design
The cooling system must meet several requirements. It has to be designed to effectively cool a range of products by removing enough heat and moisture for further handling or storage of the pellets, but, it’s also important to avoid the over-drying of the pellets. Removal of more moisture than required involves removing the extra mass (weight) that results in losing selling weight.
The cooling system itself must be reliable and provide gentle handling of the pellets that minimizes the creation of fines in the process. In some situations, counter-flow coolers may be equipped with heat exchangers on the entering air to heat it so greater product drying may be accomplished.
| Pellets Ø 2-4 mm | 1.2 m3 /kg/h (22,0 m3 /ton/min) |
| Pellets Ø 5-7 mm | 1.4 m3 /kg/h (25,0 m3 /ton/min) |
| Pellets Ø 8-11 mm | 1.5 m3 /kg/h (27,0 m3 /ton/min) |
| Pellets Ø 12-16 mm | 1.6 m3 /kg/h (30,0 m3 /ton/min) |
If the relative humidity of the air is between 85 and 100%, the volume of air must be increased.
Air speed through the pellet = 1 m/sec for pellets Ø 3-5, 1.2 m / sec for large diameter pellets (Ø 8-16 mm)
The time required for good cooling of the pellets is called retention time and depends on the pellet dimensions:
- pellet diameter < 4 – 5 mm = 5 – 6 minutes
- pellet diameter 6 – 8 mm = 6 – 8 minutes
- pellet diameter 8 -10 mm = 7 – 8 minutes
- pellet diameter = 12 mm = 8 – 10 minutes
- pellet diameter = 20 mm = 12 minutes
- pellet diameter = 22 mm = 15 minutes
Pellets containing more than 10% liquid (in particular molasses) require a retention time which is at least 20% longer. These indicative values must nevertheless be corrected to reflect the real environmental conditions where the work is being performed. In fact, if the relative humidity of the air is between 85 and 100%, the volume of air must be increased. On the other hand, in extremely dry climates, the volume of air must be reduced.
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