Hammermill air relief

Details Hammermill air relief Hammermill air relief

2001-01-05

2002-06-18

Rosenbaum, Mark (Department: 3725)

Solid material comminution or disintegration

Apparatus

Including means applying fluid to material

C241S073000, C241S074000, C241S189100, C241S194000

Reexamination Certificate

active

06405950

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved airflow hammermill assembly for grinding materials. More particularly, this invention relates to an improved airflow hammermill assembly for processing vegetable meals and cereal grains. The improved airflow hammermill assembly incorporates one or more diverging ducts communicating with the hammermill housing to increase the effective discharge area for upper portions of the collection zone and provide a more uniform negative pressure within the housing. The improved airflow hammermill assembly allows increased throughput and energy savings.
BACKGROUND OF THE INVENTION
Hammermills used for particle size reduction typically comprise a housing having a feed inlet at the top, a grinding chamber beneath the feed inlet, and a ground material outlet at the bottom of the housing beneath the grinding chamber. The grinding chamber is defined by a classification grid (usually an apertured screen) surrounding a rotor mounted on a driven shaft for rotation about an axis. When the classification grid is an apertured screen, it is commonly cylindrical (also termed “full-circle), although other configurations can be employed, including arcuate shapes, such as oval or teardrop (also termed “tear-circle”), and polygonal shapes. A number of impact members are fixedly or pivotably attached to the rotor. Typically, the impact members comprise rectangular pieces of hardened steel, commonly termed hammers, pivotably mounted to the rotor to be free-swinging when the rotor spins.
During hammermill operation, material is fed by gravity through the housing feed inlet and into the grinding chamber. Inside the grinding chamber, the rotating rotor causes the ends of the hammers to swing out and strike the material to be ground, thereby reducing particle size until the particles are fine enough to pass as finished product through the classification grid. Particles too coarse to pass through the classification grid are retained in the grinding chamber and subjected to repeated impacts until they become sufficiently reduced in size to exit.
During grinding of material in a hammermill, particle size reduction occurs as a result of the impact between a relatively rapidly moving hammer and a relatively slowly moving particle. The hammers of a hammermill typically rotate at a speed in excess of 15,000 feet per minute. By contrast, free-flowing material coming into the grinding chamber generally enters at a much lower rate of less than about 100 feet per minute. Given such a large difference in relative velocity, the initial contact of the hammer with the material produces an explosive impact, transferring sufficient energy to the material to break it into smaller particles that are then accelerated toward the classification grid. Depending on their size and angle of approach, the smaller particles either pass through the classification grid or rebound from the screen to be subjected to additional hammer impacts and further size reduction.
After the first impact, particles in the grinding chamber tend to be accelerated in the direction of hammer rotation and very quickly approach the hammer tip speed. This acceleration lessens the speed differential between the hammer and the particle, which lessens the impact force and hence lessens the size reduction that results from subsequent impacts, thereby reducing grinding efficiency.
Many devices exist for improving grinding efficiency. One approach to achieving efficient grinding is to inhibit particle acceleration. Particles are accelerated in the grinding chamber in part due to the impact itself. A common method for inhibiting particle acceleration is to redirect or interrupt the path of travel of the particle within the grinding chamber. Many hammermill designs commonly employ baffles or other deflectors within the grinding chamber for this purpose. When accelerated particles strike the baffles, they rebound or at least are momentarily halted, facilitating further high-energy impacts and effective subsequent grinding. Another common method for inhibiting particle acceleration is to employ a classification grid having a polygonal or otherwise non-circular shape. The irregular shape of a polygonal or non-circular classification grid induces flow interruptions in the same fashion as baffles, thereby increasing the particle-to-hammer speed differential in subsequent impacts. However, baffle-particle and/or classification grid-particle collisions tend to cause equipment and product heating, leading to power losses and making product more difficult to grind.
Grinding efficiency is also affected by the fanning action of the rotor on the air in the grinding chamber. One effect of rotor fanning action is that it contributes to particle acceleration; however, this problem can be addressed as described above by using baffles and/or polygonal or otherwise non-circular screens. A greater problem associated with rotor fanning action is that it produces internal recirculation of air within the hammermill, which can create a low-pressure area at the hammermill outlet. Such a low-pressure area creates a suction effect that can draw fines and other lighter ground material back into the grinding chamber, thereby reducing the capacity of the grinding chamber to receive new material. In grinding heavier materials such as corn, the ground material can have sufficient weight to discharge from the hammermill outlet without being affected significantly by a low-pressure area. But with lighter materials such as oats, or with commercial materials that are to be reduced to fine powders, a low-pressure area can have a significant effect in drawing back into the grinding chamber a substantial portion of the material that would otherwise discharge.
To prevent finished product drawback as well as other problems caused by internal air recirculation, a blower or exhaust fan is often connected with the hammermill outlet to create reduced air pressure within the unit. Such so-called negative air or negative pressure systems assist the grinding process by facilitating continuous flow of ground material out of the hammermill. However, in attempting to increase output by increasing the negative pressure on the hammermill, the increased velocity of air at the hammermill outlet tends to cause feed material to take a direct path from the hammermill inlet directly to the bottom of the grinding chamber, rather than staying in continuous suspension within the grinding chamber, and thereby decreases effective use of the upper portion of the grinding chamber and classification grid.
Under such operating conditions, ground particles tend to exit mainly from that portion of the classification grid that is positioned directly above the housing outlet, where airflow is most rapid. The effective discharge area for ground material exiting from the remaining portions of the classification grid then tends not to be the housing outlet, but rather the area determined dimensionally by the width of the classification grid multiplied by the distance between the classification grid and the housing at the narrowest gap in the vicinity of the housing outlet. Increasing the air exhaust rate worsens this phenomenon by tending to create “constrictive zones” of product buildup at the effective discharge area, where ground material passing through upper portions of the classification grid tends to become stalled and cannot freely and continuously exit the housing. Release of ground product above the constrictive zones occurs only after such product accumulates in a weight amount sufficient to overcome the nonuniform airflow conditions that create the constrictive zones. Thus a cycle of clogging and release occurs, and unwelcome power surges become a common occurrence. Throughput is greatly reduced because the relatively large fraction of the classification grid that serves upper portions of the collection zone essentially becomes nonusable.
The above and other approaches in grinding apparatus design have not been satisfactory. Consequently, further improvements in grindin

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