With this our first blog article, we will refresh our knowledge about pellet mills. Pellet mills are also known as pellet presses. Let us first talk about what is a pellet.
Pellet is a product form generally in a cylindrical shape with various diameters, lengths and hardness. We produce pellets from various powder products by pushing them through dies with mechanical pressure. We also add moisture and/or heat to the product during this process.
So, which products can we pellet? We can pellet:
• Leaves, wood chips, pinecones and similar forestry products,
• Wood flour, coal flour
• Most of the grains and stalks of grains
• Shells of nuts, peanuts, almonds, and similar dried fruits
• Seeds of various fruits
• Dried animal fertilizers
• And most important for us, animal feeds like ruminant feeds, poultry feeds, fish, and shrimp feeds.
So, the answer to the question “Where do we use a pellet mill or pellet press?” is that “for pelleting above products”.
At this point, another question we need to answer is “Why do we make pellets? The answer is different for different products. But a common answer for most of the pelleted products is to make it easier to transport.
When we produce pellets from a powder or mash product, we increase the particle size and we increase the density of the product. In this sense we increase the amount of the product which we can put into any volumetric container like bags, boxes, big bags, or shipping containers. In addition to this it is also easier to fill or discharge pellet products.
As many of the pelleted products are produced because of the above reasons, we have different reasons for pelleting animal feeds. Let us remember those reasons as well:
A- We achieve starch gelatinization because of the heat coming from the steam and mechanical pressure. This means increase in the nutritional values of the feed.
B- As we change the feed form from a powder to a solid form, animal cannot choose some of the ingredients from the mixture, he eats all. Hence the programmed nutritional recipe can be applied exactly to the animal.
After replying the questions, “What do we pellet?” and “Why do we pellet?” now we can answer the question “how do we pellet?” or What is a pellet mill?” “What is a pellet press?”.
We produce pellets by using pellet mills or pellet press. First, we prepare the material for pelleting by adding water or steam and/or some liquids like molasses. We do this at our conditioner. This is for preparing the material to the stage of mechanical pressure. Then inside the pellet mill, we push the product into the holes of the die with the rollers. The friction inside the holes of pellet die gives us heat. As we already increased the temperature of the product at conditioner, with the help of this heat by mechanical pressure, the product sticks to each other and takes the shape pf the die hole. With the help of this process, our products leave our die as pellets.
After remembering which products that we pellet, why and how we pellet, let us have a detailed look into pellet mills. What are Pellet mill types?
The first classification we will see is vertical pellet mills and horizontal pellet mills. We make this classification according the alignment of the die holes with respect to ground base. Vertical pellet presses have circular and planar dies. The rollers turn on this die and hence we achieve pellets. This type of pellet mills is not very common in the world.
Most of the pellet mills in the world are horizontal pellet mills which we also produce. In this type, the die has a cylindrical form with holes on the sides. The rollers turn inside the cylinder and force the product go through the holes and go out.
Another classification is made with the transmission system between the motor and main shaft. Although this classification has its subsections, mainly we can mention two types as belt driven and gear driven pellet mills. We, as YEMTAR, produce both types. Let us have a look at specifications of belt driven pellet mills and gear driven pellet mills.
In the market, belt driven pellet mills are more common when compared to the gear driven pellet mills. The advantages of belt driven pellet mills can be listed under two topics. One: It is easier to produce belt driven pellet mills. The required technology and know-how level is less which makes it easier to produce. Two: The investment cost for belt driven pellet mills is a lot cheaper. In short, the electric motor which produces the power is connected to our main shaft via belts and pulleys.
As this is an easier and cheaper technology, most of the pellet mills in the world are belt driven pellet presses. If we look at their negative sides, then we face with the efficiency issue. The efficiency of the power transmission in belt driven pellet mills are affected from many factors.
As the belts get older, their traction ability gets lower. Hence with an old set of belts, you lost important amount of power during the transmission, because the belts start to slip.
Also, the tension of the belts is very important. The tension of the belts must be controlled and maintained continuously. Otherwise less tension causes slipping in the belts and again you lose power.
As the transmission is relied on the traction between the belts and the pulleys, it is also important to keep belts and pulleys clear from dust and oil. These must be cleaned regularly to avoid slipping belts. When we add up all these, we conclude that maintenance costs of belt driven pellet presses are very high compared to the other alternative.
When we come to the gear driven pellet mills, the power is transmitted to the main shaft is realized by a set of gears. As there is no intermediate apparatus here, the power transmission is very efficient. The production of gear driven pellet mill is a lot more difficult which causes the investment cost to go higher. But as they save energy during their operation, they pay this extra cost back very quickly.
The efficiency in power transmission is followed by easy maintenance. The level and viscosity of the oil in gear system must be controlled in the intervals suggested by oil supplier. This control can be done without any interruption in the production. Besides this, there is no maintenance need in gear driven pellet presses.
Bülent T. Helvacıkara