ameter. As for the picture, the compression ratio would be L1/d1 (L1 is called the active length) at the very beginning for a new die.
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.
Generally, the higher the compression ratio is, the, the denser the pellets are, the lower production capability is, and more energy consumed. However, the compression ratio is a dynamic parameter for production different feed stocks. Hence pellet dies with different compression ratios should be careful selected based on the species of your materials in order to produce quality pellets.
How to check the quality of wood pellets?
1. Smell. Smell the odor of the wood pellets from a plastic bag. The high quality wood pellets should smell like the freshly cut softwood. Or you can light a long sawdust pellet at the tip. The smoke should smell of burning wood. If they smell differently, attention is needed.
2. Watch. The wood pellets made from clean conifer sawdust are light brown. Dark brown particles are a sign of bark which means increased amount of ash. Break a pellet to see the color of the pellet layers. The outer layer of the sawdust pellets are darker due to friction pressed by the pellet makers which is not a problem if the inner of the sawdust pellet are light color.
3. Test. Place a small amount of sawdust pellets into a glass of water. The wood pellets will dissolve into sawdust in a moment. If not, a binder agent might have been used or it means the feedstock may be improper. When the pellet dissolved, shake the glass and observe the bottom of the glass where the heaviest particles would be which maybe is a sign that dust from a sanding machine has been added to the pellets. It means increased amount of ash and the risk of sintering.