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What is compression ratio of pellet mill and how it works

By Jack Huang    May 08, 2014
I got lots of feedbacks from my clients who ask about what is the different of pellet die and holes for various rawmaterials, in this article i am going to give out a detailed explanation on what is compression ratio and how compression ratio influence pellet making for a pellet mill.

The state-of-the-art step-shaped die hole from GEMCO pellet mill (only for large ringdie pellet mill) has been practically proved to be the most successful and mature design for pellet production and, in particular, for wood pellet production. One factor that plays an important role affecting the pellet quality is the ”compression ratio” which is a ratio value of active length in press channel/diameter. Below is a draft drawing to give you a better understanding of how this step-shaped design works:

step shaped pellet die hole and pellet mill comparison ration diagram
step-shape die hole (watch closely)                     compresssion ratio diagram

In this photo (upper right) you could see that, the compression in a pellet die is actually L1/d1 (L1 is called the active length) at the very beginning for a new die. A higher compression ratio results in denser pellets but smaller production and higher energy consumption, while a lower compression ratio results in looser pellets but higher production and lower energy consumption. However, to make quality pellets, you may not just say which one is better as different materials require different compression ratio. What the wood pellet producers would like to see is a consistent compression ratio which leads to consistent pellet making quality.

Here is how the compression ratio works: at the first stage, d1 will gradually increase as the friction occurs when the raw material is pressed into the channel, which means the compression ratio (L1/d1) is decreasing. As soon as d1 reaches d2, the active length suddenly changes to L2. As the ratio of L1/d1 is equal to L2/d2, the compression ratio then consequently returns. In this way, the pellet quality is guaranteed and the lifespan of a die is extended.

I have to repeat it again that the compression ratio is a dynamic parameter during pellet production for a pellet mill. And for each unique material, the set compression ratio could be different. That is also the reason experience of a pellet mill manufacture matters, the longer time they devote into business, the preciser compression ratio and better total solution could be provided for customer.
Hope it helps.