Roller mills accomplish size reduction through a combination of forces and design features. If the rolls rotate at the same speed, compression is the primary force used. If the rolls rotate at different speeds, shearing and compression are the primary forces used. If the rolls are grooved, a tearing or grinding component is introduced. There is little noise or dust pollution associated with properly designed and maintained roller mills. Their slower operating speeds do not generate heat, and there is very little moisture loss. Particles produced tend to be uniform in size; that is, very little fine material is generated. The shape of the particles tends to be irregular, more cubic or rectangular than spherical. The irregular shape of the particles means they do not pack as well. For similar-sized particles, bulk density of material ground on a roller mill will be about 5 to 15 percent less than material ground by a hammermill.
- energy efficient
- uniform particle-size distribution
- little noise and dust generation
- little or no effect on fiber
- particles tend to be irregular in shape and dimension
- may have high initial cost (depends on system design)
- when required, maintenance can be expensive
There are many manufacturers of roller mills, but they all share the following design features shown adjacent picture:
- a delivery device to supply a constant and uniform amount of the material to be ground
- a pair of rolls mounted horizontally in a rigid frame
- one roll is fixed in position and the other can be moved closer to or further from the fixed roll
- the rolls counter rotate either at the same speed or one may rotate faster; roll surface may be smooth or have various grooves or corrugations
- bar; pairs of rolls may be placed on top of one another in a frame.
To ensure optimum operation, material must be introduced between the rolls in a uniform and constant manner. The simplest feeder is a bin hopper with an agitator located inside it and a manually set discharge gate. This type of feeder is best suited for coarse processing. For grinding operations, a roll feeder is suggested. In this type of feeder, the roll is located below the bin hopper and has a manually set or automatic adjustable discharge gate. If the gate is adjusted automatically, it will be slaved to the amperage load of the main motor of the roller mill.
The rolls that make up a pair will be 9 to 12 inches (23 to 30.5 cm) in diameter, and their ratio of length to diameter can be as great as 4:1. It is very important to maintain the alignment between the roll pairs. Sizing of the material is dependent upon the gap between the rolls along their length. If this gap is not uniform, mill performance will suffer, leading to increased maintenance costs, reduced throughput, and overall increased operation costs. The gap may be adjusted manually or automatically through the use of pneumatic or hydraulic cylinders operated through a computer or programmable controller.
Each pair of rolls is counter rotating. For improved size reduction one of the rolls rotates faster. This results in a differential in speed between the roll pair. Typical differentials range from 1.2:1 to 2.0:1 (fast to slow). Typical roll speeds would be 1,300 feet per minute (~ 395 m/min) for a 9-inch (~23 cm) roll to 3,140 feet per minute (~957 m/min) for a 12-inch (~30.5 cm) roll. Usually a single motor is used to power a two high roll pair, with either belt or chain reduction supplying the differential. In a three high roll pair, the bottom pair will have a separate drive motor. In addition, the roll faces can be grooved to further take advantage of the speed differential and improve size reduction.
By placing (stacking) pairs of rolls on top of one another, two or three high, it is possible to reduce particle sizes down to 500 microns, duplicating the size-reducing capability of a hammermill for grain. For coarse reduction of grain, a roller mill may have a significant advantage (perhaps as high as 85 percent) over a hammermill in terms of throughput/kwh of energy. For cereal grains processed to typical sizes (600 to 900 microns) for the feed industry, the advantage is about 30 to 50 percent. This translates into reduced operating expense.