Basic Operational Concepts
As noted, particle size reduction in a hammermill occurs as a
result of the impact between a rapidly moving hammer and a
relatively slow moving particle. The particle breaks and is
accelerated towards the screen; depending on the particle size
and angle of approach, it either passes through the screen or rebounds from the screen into the rapidly moving hammers again. As materials move through the
grinding chamber they tend to approach hammer tip speed. Since reduction only occurs when a
significant energy is transferred from the hammer to the particle (large difference in velocities), less
grinding takes place as the particles approach full hammer tip speed (Figure 1).
While the basic operational concepts are the same
for all hammermills, the actual operating conditions
change rather dramatically depending on the
materials being processed. Grains such as corn,
wheat, and sorghum and various soft stocks like
soybean meal tend to be quite friable and easy
to grind. Fibrous, oily, or high moisture products
like animal derived proteins and wheat bran are
tough to grind and will require more energy to
reduce. To achieve the best performance, the
hammermill must be properly configured for the
specific task of fine grinding for aquaculture feeds.
The following discussion covers such factors as
tip speeds, screen hole size, hammer patterns
and position, horsepower ratios (to hammer and
screen area), and air assist systems (Figure 2).
Tip speed is simply a factor of mill diameter and
motor RPM; D x RPM = TIP SPEED. Tip speed,
in addition to screen size has a significant influence
on finished particle sizing. High tip speeds (>100
M/Min) will always grind finer than lower tip speeds.
Low tip speeds (<90 M/Min), on the other hand,
produce a coarser, more uniform granulation with
fewer fines. As a rule, smaller screen hole sizes
should be used only with higher tip speeds, and
larger screen hole sizes only with lower tip speeds
Tip Speed - Fine Grinding and Tough to Grind
Materials For fine grinding aquaculture feeds, a
high tip speed is required. Normal tip speeds for
fine grinding and fibrous materials are obtained
on 44" (1.1M) diameter hammermills operating at
1800 RPM and 22" (0.55 M) diameter hammermills
operating at 3,600 RPM or 28" hammermills
operating at 3000 RPM and 54" hammermills
operating at 1500 RPM would be about 110
M/Min Ft/Min). Recent developments in
hammermill grinding have included the use of 54"
diameter hammermills operating at 1800 RPM.
This very high tip speed (>125 M/Min) is particularly
well suited to fine grinding at high capacities and
high efficiency. Because a larger screen (hole)
sizes can be used while maintaining the fineness
of the grind, operating costs are reduced as well.
High tip speeds also help insure the hammers will
not "rock" while the machine is operating with full
motor loads, preventing excessive wear on the
hammer holes and mounting pins (Figure 4).