The amount of air needed for cooling depends on diameter of pellet:
pellet, diameter 2 - 4 = 1.2 m3/kg h
pellet, diameter 5- 7= 1.4 m3/kg h
pellet, diameter 8 - 11 = 1.5 m3/kg h
pellet, diameter 12 - 16 = 1.6 m3/kg h
Cooling time:
Time of staying pellets in cooler depends on dimensions of pellet. For example, for a pellet of diameter about 5-7mm it is 15 min.
It is very important to remember, that speed of air flow wouldn’t be more than 1 m/s for small pellets and 1,2 m/s for big pellets.
Nowadays in fodder industry are used 3 types of coolers: horizontal, vertical and counterflow. Every type has its different advantages, but the theory is for all of them the same.
How the pellet is cooled
The cooler influences to pellets in two ways. In a moment, when pellets gets into cooler, the temperature and humidity are reduced (the degree is exactly determined). Lack of humidity and warmth affects cooler’s productivity.
Basic parameters which are valid in production process are also valid in cooling process, so if we decrease temperature by 11°C we can expect humidity reduction by 1%. Cooler is able to reduce most of warmth and moisture added during conditioning and warmth from the main motor.
Step by step – what is going on with pellets
a) steam in raw materials in conditioner in condensing and causes increasing of humidity of the raw material by 3-5%. During steam condensation a lot of warmth is produced. Then the raw material is pelleted and also during this process a lot warmth is produces.
Fresh pellet has temperature about 60-94°C. To reach finally good quality product, the pellets must be now cooled and dried.
b) When pellets leave pellets mill, it has relatively fibrous structure and this is a cause of moisture absorption. It happens due to capillary action. This is the same effect as with water absorption by paper napkin
c) The construction of the cooler lets flow the surrounding air as close as possible to pellets. This air, which is not in 100% saturated picks up the moisture from surface of pellets. The moisture is taken away by the process of evaporation, which results in cooling.
d) The warmth taken away from pellets makes air warmer. In consequence increases the air’s ability to water absorption. Moreover this warmth is needed to avoid condensation in air system, which could cause bigger amount of moisture.
For example:
If temperature of air in cooler is 70°F (20° C) by relative humidity 85%
and flowing through pellets would be heated up to 120’F (48’ C) then its ability to carry the moisture would be 5 times bigger than in initial conditions. But when the moisture in cooler was taken arises gentle balance between warmth and humidity.
e) Pellets is in unbalance state when the cooled humid surface in not entirely surrounded by air. Then the humidity level is higher inside a pellet than on the surface. It results in strange behavior of pellets which behave like wick. It is a cause of migration of steam along the pellets together with warmth. This moisture then is ready to be taken away by cooling air.
f) This process lasts until most of moisture coming from conditioning process is removed together with warmth. The humidity of pellet after cooling is usually the same (or a little higher) as humidity of mixture coming into conditioner.
This level of mixture humidity can’t be removed cooled in normal conditions. When into cooler flows big amount of very dry air then it comes to decrease of humidity or to shrinking of pellets.
Sometimes it is possible, that water is added to mixture before it gets into conditioner and we can’t reach high enough temperature to remove this moisture. It such a situation the humidity of pellets will be very high.
g) During cooling process pellets coming out from cooler will be always 5-8oC warmer than before loading into cooler. It means that if air flowing into cooler has temperature about 60’F (15’C) then discharged pallet will have temperature between 70’ F (20’C) do 75’ F (23’C).