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.
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 todayâ€™s 10" and
12" diameter rotor sections, this is no longer a problem.
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.