Grinding of raw materials for aquafeeds
By Arthur vom Hofe, CPM Europe, Netherlands
Hammermills 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 fine... 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.
Let"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 didn"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 grind can be achieved at a certain screen hole size compared to lower tip speeds (smaller diameter hammermills). It is not hard to imagine the benefits of grinding with larger diameter holes towards energy consumption and screen wear (operational costs).
Screen area (relation capacity & motor power)
The whole idea of efficient fine grinding is that the particle size reduction is done by the impact of the hammers. With a larger screen area the product escapes more effectively from the grinding chamber. Smaller sxreen surfaces keep the product longer in the grinding chamber cuasing increased energy consumption (heat) and wear. A typical design range is 120cm2/kW total screen surface or an "open hole area" of >34 cm2/kW installed motor power.
Hammer patterns (the number and distribution of the
hammers on the rotor) and positions (setting the hammer closer to or further from the screen) have a profound effect on the performance of any hammermill. Because different materials grind differently, the ideal number of hammers and clearance to the screen will need to be adjusted according to each application.
For course grinding it is most efficient to grind with a limited number of hammers, but for fine grinding aquaculture applications it is best to use an extra heavy hammer pattern to achieve the very fine finished products desired. If the rotor is equipped with a larger amount of pins (12), the total number of hammers is increased significantly. This without putting an excessive number of hammers on (four or eight) pins, which could lead to high stress and the possible failure of the rotor plates.
For optimal hammer life and most efficient operation, a hammer with a flared hard face end is preferred. Single hole hammers are generally preferred to maintain balance of the rotor and minimise the potential for catastrophic hammer failure.
It is important to realize that energy consumption of the mill increases drastically when the hammers are reaching the end of their serviceable life. The use of long-lasting tungsten carbide hard-faced hammers not only save on labor costs but also are more energy efficient.
On top of the before mentioned subjects, other fine grinding enhancers are proper screen sealing and backing screens for
longer in the grinding chamber causing increased energy consumption (heat) and wear. A typical design range is 120cm2/ kW total screen surface or an "open hole area" of >34 cm2/KW installed motor power.
protection of the thin small hole grinding screens.
Following the above guide line might help you making the right
decisions when analyzing an existing grinding system or setting up a new one.