Particle Size Reduction For Animal Feeds: Part 5

Screens

More time, effort, and dollars are invested and lost in hammermill processing due to screen application failures than would ever be thought possible. Excessive horsepower per In2 of screen and poor screen configurations steal precious process energy and maintenance dollars. In simplest terms, the best screen for any job is the thinnest material with the most open area. Naturally, some sacrifice in efficiency must be made for the sake of endurance, yet the general rule applies.

It is easy to see how new screens improve capacity and grinding efficiency. While thicker screens will last longer, they significantly reduce the tons/hour that a mill can process. When maintenance costs are typically $0.02-$0.10/ton and electrical costs range from about $0.25/ton, to more than $1.00 per ton, saving money by not changing screens is truly false economy.

Another screen consideration is the amount of open area a particular screen offers. Factors effecting open area include hole size, stagger, angle of stagger, and hole spacing. Screens with fewer holes have less open area, are easier to produce, and generally cost less. Screens with little open area may wear a long time, but the actual grinding cost per ton is greatly exaggerated because of the increased energy cost.

Two rules of thumb apply to hammermill screens in relation to applied horsepower:
1. Never have less than 14 In2 of screen per horsepower (more is always better)
2. Never have less than 4 In2 of open area per horsepower

One very simple way of increasing hammermill capacity without affecting the finished grind or adding expense to the grinding system would be to replace the "up" wide screen with perforations that are 2/64" to 6/64" larger than the "down" side screen. This may add 10-15% to the hammermill capacity and produce no noticeable difference in the finished products. Remember that the screens must be switched when the hammermill rotation is changed to use both corners of the hammer.

Feeders

Proper feeding of a hammermill is absolutely essential if the system is to operate at maximum grinding efficiency, and with the lowest possible cost per ton. Uneven or inconsistent feeding can lead to surges in the motor load. Because the load is constantly changing, the motor cannot operate at peak efficiency and so increases the grinding costs. An additional liability that is often "hidden" is the fact that surges in the feed tend to accelerate wear on the hammers and pins by causing the hammers to "rock" on the pin.

Uneven feeding across the face of the hammermill obviously increases the wear on the working components in the areas of heaviest feeding. Because a part of the mill is being overworked, the rest of the mill is not being fully loaded and grinding efficiency is reduced. Uneven feeding also tends to cause a hammermill to go out of balance more quickly due to uneven wear. This adds to the operating cost of the mill be causing premature replacement of the wear items like hammer and pins.

Rotary Pocket Feeder - as the name indicates, rotary pocket feeders utilize a rotor mechanism much like a rotary air lock to evenly distribute the feed to the hammermill. In most cases, the rotor is segmented and the pockets are staggered to improve the distribution of the feed, and to reduce surges in the feed rate. Because the rotary pocket type feeders relies on a free flowing material to fill the pockets, they are best suited to granular materials with a density of 35#/Ft3 or more. Typical applications would be whole grains and coarsely ground meals.

Screw Feeder - screw type feeders are used when processing materials that have poor flow characteristics, or contain large bits of material that would not flow properly with a rotary pocket feeder. Screw feeders impart a surge to the feed, and so have limited applications in high capacity / high efficiency grinding situations. In the past, screw feeders were selected when rotary pocket type feeders lacked sufficient capacity to load a hammermill motor. With the advent of today’s 10" and 12" diameter rotor sections, this is no longer a problem.

Air Assist

The final application topic to be considered is the use of aspiration air to improve mill efficiency and performance. The air assist system controls the environment of the grinding chamber in the hammermill and aids in moving product from the grinding chamber through the screen perforations. A properly designed air assist allows a hammermill to grind more efficiently, producing a more uniform finished product with less heating and controls dusting around the mill. Although hammermill capacity will vary with the type of machine and operational parameters, air assisted grinding systems will out produce non-assisted systems by 15-40%.

A good rule of thumb for the amount of air required to assist product and control dusting is 1.25-1.50 CFM/In2 of screen area. Pressure drop across the mill may range from 2-5" W.C., depending on system operating conditions. In order to make an air assist system work, several items must be considered including the air flow into the mill, paths for the air and product out of the mill, separating the product from the air stream, and controlling the path of the air in the system.

Once the air is through the mill, it is necessary to allow the entrained fines to settle out before sending it along to the cyclone or filter system. To accomplish this, a plenum or settling chamber should be provided between the air/product conveyor and the pickup point. While in the past, such figures as "3-5 times the duct diameter" have been suggested, the bottom line is to reduce the velocity as much as possible to permit the fine material to settle out. If the plenum is designed so the air velocity drops below 15 times the bulk density (15 x 40 or 600 Ft/Min for most feed ingredients) the separation will usually be adequate. Larger plenums will reduce the velocity and improve the air/fines separation. For practical purposes, the plenum cannot be too large.

To make the air assist system work, it is necessary to control the path the air takes through the mill. Normally, the discharge end of the take away conveyor must include some kind of airlock to insure the air is pulled through the hammermill instead of back through the discharge system. This may be as simple as a shroud over the take away screw or as complex as a powered rotary airlock at the discharge of a drag conveyor.