Lesson 4 - Milling Principles
This lesson explains the objectives of milling and how mills are designed to achieve them. It details how malt is conditioned for dry milling and how we analyse grist particle size profiles. Finally, it explains how milling affects downstream brewing processes.
The Design of a Hammer Mill
If you thought the crushing and shearing of a roller mill was tough on the grain, wait until you learn about the hammer mill! In the hammer mill the grain is comminuted by multiple impacts with beaters (hammers) attached to a rotor.
The particle size profile requirement for a mash filter is simpler than that of a mash tun and lauter tun. It is also much finer. As a result, the design of a hammer mill is less complex and achieves a greater degree of comminution. Unlike roller mills, hammer mills are designed to produce a relatively uniform particle size,
Feed to the mill
Hammer mills are fed in a similar way to roller mills − from a hopper by gravity. The flow of grain coming in must be regulated below its maximum throughput, or the comminution chamber will fill with grain and the mill will jam, potentially causing damage to the mill. Some mills have a rotary valve and some use a distribution roller to meter the grain into the mill at the correct rate. The control system can change the rate of feed to maintain the same load on the motor operating the rotor. The harder the material, the more load on the motor, so the feed can be adjusted to take account of what the mill is processing.
With hammer mills, there is an increased requirement to prevent metal and other hard objects entering the mill because once in the mill they can continue to collide with the mill components until the mill stops. In the case of mills without braking systems, this can be 20-30 min. Therefore, a destoner must be installed up stream and a magnet is always fitted in the inlet. Some mills also include an area at the bottom of the screen to accommodate any large heavy objects that have entered the mill, keeping them away from the beaters (hammers).
Comminution chamber
The comminution chamber is a circular (or slightly teardrop shaped) enclosure. In most mills, the upper part is a hard solid surface and the lower part is a screen made of sheets of stainless steel with holes punched in them. The size of the holes matches the maximum particle size for the grist. Inside the chamber, a rotor with beaters attached spins at high speed. Speeds range from machine to machine, but a typical speed is 1,500 rpm.
The grain enters the comminution chamber and on entry collides with one of the beaters, which can travel at over 100 m/s (360 km/hr). This impact fractures the grain and any particles small enough can pass through the screen into the mill outlet. Larger fragments of grain are propelled by the impact into the side of the chamber where they are further fractured. They rebound off the chamber onto the beater and are smashed again. This game of malt ping pong continues until the malt particles are small enough to pass through the screen. The screen size used determines the maximum grist particle size, so selecting the correct size is essential for effective performance. Sieves used for brewing grist range from 1-4 mm. Most are in the range of 1.5 mm to 3 mm. Other factors that can be adjusted are:
• Rotor speed
• Number and length of beaters
• Beater profile
These all have a less significant effect on grist particle size, but they can be used to adjust it. A very high rotor speed risks producing a very fine grist as the force of impact of the beater on the grain causes it to disintegrate into dust.
The video below, although not about a brewing hammer mill, provides a great illustration of what happens in a hammer mill.
Please note, this is for illustrative purposes only and is not a recommendation or endorsement of the supplier’s product.
Some hammer mills have vertical rotors but most of those used in brewing today have horizontal ones.
The collisions inside the hammer mill generate heat (and a lot of noise) so air or CO2 is blown through the mill to prevent the grist heating up. This air also keeps the screen clear and maintains a negative pressure in the mill. This can be either in the same direction as the flow of material or in the opposite direction. The gas used is filtered of the copious amounts of dust it holds and this is added to the grist to prevent a significant loss of extract.
Maintenance
Like a roller mill, the hammer mill has an arduous task to perform and even if it only deals with malt, the beaters wear away. This is slowed in modern hammer mills by changing the direction of rotation each time the mill is run. The beaters in some mills can also be reversed and inverted to allow them to wear from all directions. Screens also succumb to wear and damage and need to be replaced from time to time. In some mills, this can be done while the mill is running. As screens determine the maximum particle size of the grist, it is important to inspect them frequently.