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 may 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 by 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 30#/Ft3 or more. Typical applications would be whole grains and coarsely ground
mixed rations (Figure 10).
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 may impart a surge to the feed, and so have limited applications in high capacity
/ high efficiency grinding situations. A properly designed screw feeder with multiple screws and double
flighting at the discharge end will overcome many of the negative tendencies of screw feeders.
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% (Figure 11).
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. The plenum must be designed to
reduce the velocity as much as possible to permit the fine material to settle out, and with the air pick
up point away from any swirling, turbulent fines material. If the plenum is designed so the air velocity
drops below 15 times the bulk density (15 x 35 or 525 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 air seal 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.