Removable mill throat and wear ring for pulverizer

A coal pulverizer, that supplies a coal-in-air mixture to combustion means for a power plant and comprises grinding means consisting of wheels revolving in a power driven trough from which pulverized coal is centrifugally thrust into a stream of forced air blown upward through air passages in a throat ring consisting of uniformly sized segments, is provided with a wear ring of uniformly sized segments resting flush on the throat ring segments of equal arcuate dimension. Each said wear ring has a substantially trapezoidal cross section, the base of which rests on a throat ring segment, the vertical face of which fits against the inner wall of the pulverizer and the sloping face of which rises upward from its base at an angle measured from the vertical of between ten and seventeen degrees for approximately 21/2 and 31/2 inches, so as to deflect the coal-air stream away from direct impingement on the pulverizer housing. The provision of wear-ring segments of arcuate dimension the same as that of the underlying throat ring segments serves in novel manner to permit replacement of worn throat ring segments without the necessity of removing the grinding means from the pulverizer.

This invention relates to improved coal pulverizers for coal-fired power 
plants. 
The enormous quantities of fossil fuels required by modern industrial power 
plants has placed a heavy burden on that equipment which is designed to 
pulverize solid fuel, primarily coal and to supply it, dispersed in air, 
in substantially uniform size and at a uniform rate to the furnaces of 
such power plants. Recognizably, shut-downs for repair effect a costly 
burden that must ,whenever possible, be avoided. 
Provost U.S. Pat. No. 4,874,135 teaches improvements in the mill throat 
rings in such pulverizers, which reduce air-compressor power requirements 
and "coal dribble", that is, the falling back of coal particles in the air 
channels of the throat ring. These throat rings are composed of uniformly 
sized arcuate segments, which assembled, form a ring surrounding the coal 
grinding mill of the pulverizer and provide a, plurality of air ducts 
which deliver forced air into contact with the coal particles to carry the 
same into classifier and combustion means of the power plant. Wear, or 
ledge cover, rings rest on the throat rings without blocking the outlets 
of the air channels and have been composed of segments of greater chord or 
arcuate, length than the throat ring segments so that fewer wear ring 
segments than throat ring segments have heretofore been employed in a 
single installation. This arrangement has however required the dismantling 
of the grinding wheels and truck assembly of the pulverizer to replace 
worn throat or wear rings; this dismantling at present adds to the 
operating costs as extended down-time and labor requirements. 
It is therefore a primary object of this invention to reduce the costs of 
operating such coal pulverizers by reducing the dismantling required to 
replace the throat ring and wear ring segments. 
It is another object of the invention to reduce the wear on the housing of 
the pulverizer. 
SUMMARY OF THE INVENTION 
It has been universal practice in the design and construction of coal 
pulverizers either to employ a unitary throat ring or, as in recent 
practice, to employ segments of the wear ring of arcuate lengths greater 
than the arcuate length of each segment of the throat ring. While the 
employment of segments of the throat ring rather than a unitary ring has 
greatly reduced the dismantling necessary to replace worn throat ring 
segments, the universal practice of employing wear ring segments of 
greater length than that of the throat ring segments has necessitated 
removing not only a worn throat ring segment and its associated, but 
longer, wear ring segment but also the grinding wheels and their truck 
assembly. 
By fastening the wear ring segments to throat ring segments of equal 
arcuate dimension and, incidentally, providing a threaded throat bolt hole 
in the upper face of the wear ring segments to receive a ring bolt for 
lifting the segments, the removal of a worn throat ring segment or 
segments can be readily and inexpensively accomplished. The enormous wear 
created by handling a stream of coal of, for example, from 100,000 to 
150,000 pounds per hour will eventually erode one or more segments, 
especially since coal fed to the grinding assembly is delivered in 
non-uniform amounts of non-uniform size distribution and thus effects 
non-uniform wear of the individual wear ring and throat ring segments. 
The wear ring segments, of equal arcuate dimension to that of the throat 
ring segments, are of substantially trapezoidal cross-section (the 
sections actually deviating from trapezoidal only to the degree that one 
of the approximately parallel sides actually inclines inwardly from one to 
three and one half degrees from the horizontal). The trapezoidal 
configuration as hereinafter described, acts as a buffer to redirect the 
coal air-stream from direct impingement upon the inner housing of the 
pulverizer.

DETAILED DESCRIPTION OF THE DRAWINGS 
A coal pulverizer commonly employed in the power industry to grind coal and 
classify the product so as to deliver a substantially uniform flow of coal 
in air to a power plant is shown in outline in FIG. 1. On the exterior of 
the pulverizer is the delivery end 11 of coal conveying means 11 for the 
delivery of coal to the pulverizer and the means 21 for the supply of air 
from a compressor (not shown) into a lower section of the pulverizer. On 
the exterior of the pulverizer is a "pyrites box" 14 for receiving mineral 
particles separated from the ground coal in the pulverizer. 
The lower broken away section of the pulverizer shows in cross section a 
portion of the trough-shaped grinding ring 12 which forms a revolving 
circular trough in which revolve grinding wheels 13. The grinding ring 12 
is driven by a conventional drive mechanism (not shown). Pulverized coal 
is centrifugally cast by rapid revolution of the grinding ring 12 across 
the exit ports 18 of the air channels 19 of the throat ring 20 which 
circumscribes grinding ring 12. Superposed on the throat ring 20 is a wear 
ring 24, both of which are shown in more detail in subsequent Figures of 
the drawings. Forced air is supplied through air inlet 21 to and through 
the air channels 19 at such a rate of flow as to create an air-coal flow 
of between 1500 and 3000 pounds a minute. The forced air and the design of 
the air channels 19 (as hereinafter shown in detail) impart to the coal a 
swirling motion and carries the coal upward to the classifier and feeder 
pipes (not shown) rejected large sized coal particles fall through the 
classifier cone 26 from which they are discharged in known manner to the 
coal grinding assembly. 
Referring to FIG. 2, the throat ring 20 and superposed wear ring 24 consist 
of a plurality of regularly spaced arcuate segments 27, all of uniform 
arcuate dimension so that the throat ring and wear ring segments fit 
uniformly, the wear ring on top of the throat ring segments. The wear ring 
24 covers only the outer rim of the throat ring so as to leave open exit 
ports 18. 
FIG. 3 is a vertical section taken along line I--I of FIG. 2 at the center 
of the vertical axis of the bolt hole 28, in which a bolt, not shown, 
secures each wear ring segment to each throat ring segment in uniform 
alignment. As described in the U.S. Pat. No. 4,874,135, the air channels 
19 lead to exit ports 18 across which coal is thrust and encounters a 
blast of air from the channels. The air stream is modified by arcuate 
bevel 29 and a corresponding restriction in the channel 19 so that the air 
blast encounters the coal stream at a modified angle partially concurrent 
with the coal stream; thus reducing coal dribble and the power otherwise 
required to propel the resultant coal-air mixture to the classifier 
section of the pulverizer. 
The wear ring segments 27 are designed to prevent the coal particles in the 
coal-air stream from striking directly against the walls of the pulverizer 
housing immediately above the throat and wear rings. The wear ring is of 
substantially trapezoidal cross-section, has a sloping face 48 along which 
the thrust coal in air flows. This sloping face extends upwardly and 
inwardly from the throat ring at an inclination from the vertical of from 
1/2 to 2 degrees and preferably at 1 degree. The inwardly sloping face 48 
is from about 2 to 4 inches in height and preferably about 3 inches above 
the junction of the throat ring and the wear ring. The face of the wear 
ring segment above the inwardly sloping face 48 extends upward to the 
pulverizer housing. The inner face of the wear ring 48 serves to redirect 
the coal-air stream to a vertical direction and away from direct 
impingement on the said housing. The general configuration of the inner 
face of the wear ring, which can be termed an "ablative nose", thus gives 
an upward course to the coal-air stream oblique to the inner face of the 
pulverizer housing. 
Contrary to the effect of the arcuate bevel of the throat ring described 
and claimed in Provost U.S. Pat. No. 4,874,135, which reduces the impact 
of forced air on the coal particles thrust from the grinding mechanism by 
directing the air more in the direction of the coal flow and serves to 
increase the rate of centrifugal flow of the coal stream, the ablative 
nose of the upwardly inclined face 4 of the wear ring of the present 
invention increases the impact of the coal-air stream on the wear ring and 
to a corresponding extent, decreases the impact of the stream on the inner 
face of the inner face of the pulverizer housing. The wear ring being of 
greater thickness at the major source of wear, provides an increasing life 
to the as a whole and is, moreover, susceptible to being protected by an 
abrasive resistant compound. 
FIG. 4 is a plan view of one segment of the throat ring 20 and one segment 
27 of the superposed wear ring 24 showing in particular the conformation 
of the air channels of the throat ring, whereby an upward whirling motion 
is given to the compressed air and as a result to the coal-air stream. The 
inclined or sloping face 48, which provides protection of the housing 23 
conforms at its lower edge with the outer arcuate convex edge of the 
throat ring along the line 35 as shown in the FIG. 4. The ablative nose 34 
effects novel protection of the housing 23 from the abrasive action of the 
coal-air steam by redirecting the major force of the stream. Also shown 
are the bolts 28 holding the throat ring and the wear ring segments 
together as units of equal arcuate dimension, and of a deeply threaded 
bolt 32 (preferably by inserting a helical coil in the casting of the 
segment) so as to provide for insertion of a large ring bolt for lifting 
one wear ring segment from the pulverizer.