United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-006 Feb. 1983
SERA Project Summary
Significance of Size Reduction in
Solid Waste Management:
Volume 3 - Effects of Machine
Parameters on Shredder
Performance
George M. Savage, Jonathan K. Tuck, Patricia A. Gandy, George J. Trezek, and
Ihor Melnyk
Hammermill shredders for size
reduction of refuse were examined at
three sites to determine the influence of
key machine parameters on their
performance. Internal machine
configuration and single- versus
multiple-stage size reduction were
studied. Key parameters related to
performance include the number and
volume of hammers, open volume
fraction, hammer tip speed, grate
opening, open volume, and closed
volume. The machine parameters were
related to throughput, mill holdup,
specific energy requirements, power
draw, and product size using test data
and curve-fitting analysis. Studies of
both actual and hypothetical scenarios
of single- and multiple-stage size
reduction indicated that internal
machine configuration and degree of
size reduction can significantly
affect energy requirements for refuse
size reduction.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
This study identifies the fundamental
parameters that influence refuse size
reduction and determines their effects on
the performance of refuse shredding
equipment. The work developed from
earlier efforts to define fundamental
comminution parameters and to define
energy requirements for size reduction as
a function of the size distribution of the
shredded product.
In addition to investigating the relation-
ship between energy requirement and
product size, the report examines the
influence of key machine parameters on
hammermill performance. Internal
configuration of the shredder and the
factors that determine the propriety of
single- and multiple-stage size reduction
were studied. Refuse shredders at three
sites were field tested to provide new data
and to verify earlier research. The results
of this work can be applied to the design
of energy-efficient refuse shredding
equipment and systems and to the
selection of operating parameters for
minimal energy use.
This study is one of several research
projects conducted by the Municipal
Environmental Research Laboratory
(MERL) to investigate the size reduction
of municipal solid waste (MSW). Other
reports available or in preparation are:
Significance of Size Reduction in Solid
Waste Management, Volume I, EPA-
600/2-77-131 (PB 272-096);
Significance of Size Reduction in Solid
Waste Management, Volume II, EPA-
600/2-80-115 (P6 81-107-096);
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Processing Equipment for Resource
Recovery Systems, Volume III - Field
Test Evaluation of Shredders, EPA-
600/2-80-007c (PB 81-151-557);
Determination of Explosion Venting
Requirements for Municipal Solid
Waste Shredders (to be published in
January 1983);
Engineering Design Manual for Solid
Waste Size Reduction Equipment (to be
published in February 1983).
Testing Program
Earlier research indicated that
machine parameters such as grate
spacing, number of hammers, etc., could
potentially have a significant influence on
the efficiency and performance of refuse
size reduction equipment. This study
identifies possible techniques that could
be used to optimize both single- and
multiple-stage shredding. The purpose of
the research was to verify some of the
findings presented in earlier studies and
then determine the influence of various
machine parameters on net energy
requirements and particle size. The
influence of machine parameters was
determined by field testing of shredders
located at refuse processing plants in
Appleton, Wisconsin; Odessa, Texas; and
Richmond, California.
Before the present study, published
data were not available to correlate the
performance characteristics of refuse
shredders with the internal design of the
machine. Consequently, even if the
designer knew the particle size needed
and the power required to produce it, he
lacked knowledge of the analytical
relationships that would enable him to
calculate the proper internal
configuration. The designer either
depended on previous results from
identical or similar shredders, or he
designed a machine for which operation
and performance could be varied. Thus,
an analytical method was needed to
design the internal configuration of a
shredder. The availability of such a
method would eliminate trial-and-error
solutions and would potentially
contribute to improved shredder design.
As part of the present research, data
from current and past field tests have
been combined and evaluated in terms of
characteristic machine parameters.
Consequently, throughput (mass of
material shredded), mill holdup (the
amount of material in the shredder at any
instant), power draw, specific energy (net
energy requirement per mass of
throughput), and product size were
identified as parameters that depend on
the design of the shredder cavity and the
internal hardware. Where possible, the
approach has been to identify trends and
methods for general analysis so that
performance may be predicted for other
operating conditions. The machine
parameters evaluated consist of those
typically encountered in the industry -
for example, hammer tip speed, grate
opening, and number of hammers, as
well as newly defined volumetric terms,
such as total hammer volume, closed
volume, open volume, and open volume
fraction.
One of the previous findings was that
the number of hammers appeared to
affect energy requirements for size
reduction. Specifically, the Newell*
shredder in Odessa, Texas, required less
specific energy for size reduction
compared with other horizontal hammer-
mills when the basis of comparison was
the production of an equivalent product
size. The difference in the energy
requirement noted at that time was
attributed to the relatively few hammers
used in the Newell shredder (i.e., 14
versus the 24 to 48 hammers used in the
other horizontal hammermills tested in
the earlier study).
To identify the relationship between
energy requirement and number of
hammers, the study included field
tests that varied the number of hammers
in two different shredders. The test
program established the dependence of
specific energy (kWh/Mg) on the number
of hammers through: 1) measurements of
energy requirement and product size for
different hammer complements (i.e.,
number of hammers), and 2)
measurements of mill holdup as a
function of material throughput.
Results
Total specific energy values for a matrix
of single- and mutiple-stage design
parameters were calculated using each
of the procedures described as follows.
I. Specific energy was related as a
linear function of product size.
Product size was related as a linear
function of throughput. These
linear-derived relationships were
used to describe specific energy and
particle size graphically as a
function of throughput.
'Mention of trade names of commercial products
does not constitute endorsement or recommendation
for use.
II. Specific energy was related as a
function of degree of size reduction
and characteristic product size for
the Richmond test data.
III. Specific energy was related as a
function of degree of size reduction
and grate spacing for the Richmond
test data.
IV. Specific energy was related as a
function of degree of size reduction
and grate spacing for a large array
of data collected from six
hammermills.
Procedure I data show no significant
difference in the specific energy require-
ments for single- versus multiple-stage
size reduction. But data from Procedures
II, III, and IV indicate that significant
energy savings may be possible if grate
size and mill sequence are optimized.Pro-
cedure IV is considered to be the most
accurate method for general analysis.
Analyses using this procedure indicate
that large degrees of size reduction may
require either single-stage reduction
with a 5.1 -cm grate spacing or two-stage
reduction using a flail mill and a shredder
with a 10.2-cm grate spacing for the most
energy-efficient system. Additional test
data are required to improve the accuracy
of Procedure IV.
Analyses of the Appleton West and
Odessa data indicate that no apparent
relationship exists between the number
of hammers and specific energy. But rela-
tionships were identified between the
number of hammers and characteristic
product size (screen size corresponding to
63.2 percent cumulative passing), free-
wheeling power (power necessary to
maintain constant rotation of shredder
rotor under no-load conditions), and net
power (difference between gross power
and freewheeling power).
Relationships between particle size
and number of hammers could only be
determined for the Appleton West tests.
Product particle size decreased with
increases in throughput. As the number
of hammers increased, the product size
decreased for throughputs above 20
Mg/hr.
Freewheeling power increased with
the number of hammers. For the Odessa
shredder, changing the number of
hammers from 10 to 14 increased
freewheeling power by more than 15
percent. Increasing the number of
hammers from 24 to 40 at the Appleton
West shredder produced no observable
increase in freewheeling power
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requirements. But a change from 40 to 64
hammers increased the freewheeling
power requirement by approximately 8
percent. Though additional freewheeling
power requirement will not significantly
affect the gross power requirements,
energy savings may be realized by using
the minimum number of hammers sothat
the operation and performance of the
shredder is not impaired.
The net power requirement for the
Odessa and Appleton West Mills in-
creased with throughput for all hammer
complements and decreased when more
hammers were added. An increase in the
number of hammers from 24 to 64 on the
Appleton West Mill decreased net
power by as much as 30 percent. The
corresponding increase in freewheeling
power requirement is relatively
insignificant.
Discussion
The size reduction process may be
affected by changing one or more of the
shredder parameters considered (i.e.,
hammer tip speed, grate opening,
number of hammers, total hammer
volume, closed volume, open volume, and
open volume fraction). The relationships
developed in this study may be used to
assess the potential consequences of
making these changes. If the
relationships were developed for a
particular shredder, the consequences of
varying the number of hammers, grate
opening, or open volume fraction may be
estimated for operating conditions near
those originally used in the assessment.
But the range over which the relation-
ships for a particular shredder remain
accurate must be determined through
testing.
The derived relationships and equa-
tions may be used to help optimize
shredder operations once an operating
setpoint has been determined. And
provided that a throughput or product size
constraint has been established, a group
of selected relationships may be used to
establish whether or not the machine
parameters are appropriate for obtaining
maximum performance (i.e., low energy
consumption).
Available test data were used to plot
specific energy as a function of degree of
size reductior^flHHMfipacing. This re-
lationship appw^M^Mrve value as the
basis of a method for predicting specific
energy requirements for single- and
multiple-stage size reduction.
A study of the specific energy require-
ments of various single- and multiple-
stage processes concluded that the most
energy efficient systems for size
reduction are: (a) single stage size
reduction using a 10.2-cm grate spacing,
and (b) a flail mill primary shredder
supplemented by a secondary shredder
with optimized grate spacing. The
process is for producing characteristic
product sizes ranging from 0.85 to 2.0
cm.
For shredder throughputs in excess of
20 Mg/hr of MSW, characteristic product
size increased significantly as the
number of hammers decreased on the
shredders tested. For a constant
characteristic product size, throughput
and net power requirement for size
reduction tend to increase with a
decrease in the number of hammers.
No significant changes in specific
energy requirements were observed for a
decrease in the number of hammers.
Consequently, the use of as few as 10
hammers (as opposed to as many as 64)
appears to be possible without adversely
affecting shredder operation or
performance. Decreasing the number of
hammers may have significant
maintenance and cost benefits.
The full report was submitted in
fulfillment of Contract No. 68-03-2866 by
Cal Recovery Systems, Inc., under the
sponsorship of the U.S. Environmental
Protection Agency.
George M. Savage, Jonathan K. Tuck, Patricia A. Gandy, andGeorgeJ. Trezekare
with Cal Recovery Systems, Inc., Richmond, CA 94804; and the EPA author is
Ihor Melnyk with the Municipal Environmental Research Laboratory, Cincin-
nati. OH 45268.
Carlton C. Wiles is the EPA Project Officer (see below).
The complete report, entitled "Significance of Size Reduction in Solid Waste
Management: Volume 3. Effects of Machine Parameters on Shredder Per-
formance. "(Order No. PB 83-154 344; Cost: $11.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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