by Arthur vom Hofe, CPM Europe, Netherlands
ammermills are commonly used for grinding a broad range of materials used in the production of aquaculture feeds. This article is for people who always wanted to know more about a hammermill, but also especially for those who are not so much interested because they have already a hammermill which is running fineâ€Ś already for twenty years. Or also those who are going to purchase a machine and think that the purchase price is the key. The fact is that a hammermill can use >50 times its purchase price during its life time in energy, proves that energy efficiency is thus the determining factor when choosing a hammermill. Letâ€™s have a closer look to what is really happening in a hammer mill. A hammermill consists of a fast-rotating rotor with swinging hardened hammers. Product entering the grinding chamber is reduced in particles size by the impact of the rotating hammers. The particles are leaving the chamber through a screen with small holes.
Grinding chamber shape
Well understandable is that the speed differential between the product and the hammer determines the impact, which is required to reduce the particle size of the incoming product. A tear-drop shaped hammermill chamber will maintain the speed differential better than traditional circular chambers. This because the rotation of the product in the chamber which didnâ€™t escape after the first hit is effectively reduced. One of the most recent innovations is the specially designed fine grind inserts. These are abrasive resistant beater bars that follow part of the rotation of the hammers to increase grinding impact area. They are installed in the upper corners of the hammermill
grinding chamber and are replaceable. The fine grind insert system improves overall fineness of grind and efficiency. It also allows for a larger screen hole size to achieve desired product, and helps to achieve an increased screen life.
Tip speed (& relation screen hole diameter)
Depending on the application, an ideal tip speed can be selected. For more efficient fine grinding, fibrous materials at a high tip speed should be selected, while course grinding and brittle products ask for a lower tip speed. Tip speed is simply a factor of mill diameter and motor RPM; so for fine grinding the larger diameter mill is the most efficient. With a higher tip speed (larger diameter hammermill) a finer
26 | December 2018 - International Aquafeed