S-EPA
United States
Environmental Protection
Agency
Municipal Environmental Research ,
Laboratory *«- *
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-082/083 Nov. 1982*'»T\X
Project Summary
Considerations in Selecting
Conveyors for Solid Waste
Applications
Zahid Khan, Marc L Renard, and Jay Campbell
Several types of conveyors used to
move processed fractions of munici-
pal solid waste (MSW) were evaluated
to provide reliable guidance to design
and operating engineers. Properties of
materials that affect the conveyability
of MSW and its fractions are dis-
cussed and criteria for evaluating
MSW conveyors are presented.
The report describes and analyzes
test results for three types of conve-
yors belt, vibrating pan, and apron.
Procedures are given for assessing
and operating belt conveyors with
respect to spillage rate. Experimental
results are given from a recirculating
test rig operated with six waste
fractions over a range of belt config-
urations, velocities, and flow rates.
Test results and analyses are also
presented for a vibrating pan conveyor
with six feed stocks operated over a
range of frequencies and stroke
lengths. In addition, results are evalu-
ated from experiments with a small
apron conveyor.
Screw, drag chain, and bucket con-
veyors are excluded from this study.
Pneumatic conveyors were studied as
a basis for developing a pneumatic
conveying test rig for MSW fractions.
Parameters, design, and cost of a full-
scale laboratory rig is investigated.
Material characteristics, pneumatic
system geometry, and air and solids
flows are evaluated and presented.
Cost and sizing of the pneumatic test
rig is established using the author's
evaluations and the manufacturer's
quotations. Cost comparisons for
purchasing the system for testing
materials, versus having them tested
at the manufacturer's site for a fee, are
examined.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory, Cincin-
nati. OH, to announce key findings of
the research project that is fully
documented in two separate reports
(see Project Report ordering infor-
mation at back).
Introduction
To select, design, and operate the
various types of conveyors for most bulk
solids, some knowledge is needed of
the relevant properties of the solids. But
information on the properties and
conveyability of MSW and its processed
fractions is not readily available in the
literature. This lack has hampered
design engineers and equipment ven-
dors in selecting and constructing
conveyors for MSW. Proper evaluation
and assessment procedures at existing
plants have also been compromised by
this information void. Waste-processing
facilities often experienced conveying
problems during startup and full opera-
tion. But, extensive efforts to remedy
such problems are usually based on
treatment of symptoms rather than
causes, and however ingenious they
are, they contribute little to guide the
conveyor design engineer. Problems
should be assessed and solved in the
design phase by providing a mechanism
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of tests to expand the basic knowledge
of material properties of MSW and its
processed fractions.
This program was implemented to
investigate engineering design con-
siderations in selecting conveyors for
resource recovery facilities. The first
phase of the research program included
material characterization, establish-
ment of evaluation criteria, and experi-
mental measurements and observa-
tions. The second phase consisted of
additional tests using densified, refuse-
derived fuel (d-RDF) and blends of coal
and d-RDF. Discussions were held with
representatives from several conveyor
equipment manufacturers to assess the
needs, approach, and goals of the
project. The third phase of the study
involved pneumatic conveyor systems,
which are used at most waste-proces-
sing facilities. Operation and perfor-
mance problems are more acute with
these than with mechanical conveyors.
A pneumatc conveying test rig for MSW
and its fractions was designed, con-
struction costs were estimated, and a
test plan was developed. Considera-
tions, materials, limitations, and ration-
ale for design of the test rig are included
in a separately bound portion of the
project report.
Procedures
Investigators and conveyor manu-
facturers agreed that the need for
improvement in the design and opera-
tion of conveyors for MSW fell into three
categories: (1) organized, comprehen-
sive data regarding material properties
and handling interactions; (2) known
significance of involved parameters;
and (3) acceptable criteria and tech-
niques for evaluation and field testing of
existing conveyor equipment. Controlling
spillage was of extreme importance.
Criteria Considered for Study
The key MSW properties that affect
conveyability were studied and their
importance was assessed. They are
listed as follows:
Measured properties
1. Abrasiveness*
2. Angle of external friction*
3. Angle of internal friction*
4. Angle of maximum inclination (of
a belt)
5. Angle of repose
6. Angle of slide
7. Angle of surcharge
8. Bulk density — loose
9. Bulk density — vibrated
10. Cohesiveness*
11. Elevated temperature*
12. Flowability — flow function*
13. Lumps — size — weight
14. Specific gravity*
15. Moisture content
16. Particle hardness*
17. Screen analysis and particle size
18. Presence of sized and unsized
material
Assessed properties:
1. Aeration (fluidity)
2. Plasticity or tendency to soften
3. Tendency to build up and harden
4. Corrosiveness
5. Generation of static electricity*
6. Degradability (ability to break
down in size)
7. Tendency to deteriorate in storage
(decomposition)
8. Dustiness
9. Explosiveness
10. Flammability
11. Generation of harmful dust, toxic
gas, or fumes
12. Hygroscopic nature*
13. Tendency to interlock, mat, and
agglomerate
14. Presence of oils or fats*
15. Ability to pack under pressure
16. Shape of particles
17. Stickiness (adhesion)
18. Contaminable
19. Weight (lightness and fluffiness;
ability to be windswept)
MSW fractions produced and con-
veyed in resource recovery plants
include raw and shredded wastes, light
and heavy air-classified fractions, and
ferrous metals. The fractions may vary
in properties from one plant to another
and over time at a single facility. The
most frequently found and/or most
difficult to handle fractions were chosen
for study. Test samples of processed
waste fractions were obtained from the
Baltimore County Resource Recovery
Facility in Cockeyville, Maryland. The
samples were representative of indivi-
dual processed fractions of MSW and
were reasonably consistent from test to
test.
Acceptable spillage was the main
criterion for selecting conveyors for
MSW. The most undesirable feature at
operating plants with systems to convey
MSW fractions is high spillage rate.
Here spillage is defined as MSW that
'Test methods for processed solid waste fractions
are yet to be developed.
Considered unrelated to conveyability of solid
waste
falls on the sides of conveyor belts and
drops to the floor at transfer points.
Spillage causes rotating equipment to
malfunction, and it produces odor,
sanitation, maintenance, and cleanup
problems Designating spillage as the
primary concern for conveyor design
was the consensus of manufacturers,
operators, and the authors.
Other criteria considered were power
consumption, reliability, dust emission
levels, material transfer, and through-
put. Dependence of throughput on belt
speed was studied. Power consumption
was measured at various operating
points. Malfunctions of equipment were
noted and documented. Dust levels
were recorded and evaluated. Experi-
mental observations during tests can
serve as a guide for feeding the belt with
a variety of feedstocks.
Test Rig Evaluations
Test rig evaluations were carried out
on belt and vibrating pan conveyors to
provide information on the most com-
monly used types of conveyors. Testing
of an apron conveyor was limited to
batch tests on feedstocks of d-RDF and
blends of d-RDF and coal.
A closed-loop test rig configuration
with a continuous recirculating flow
was assembled (Figure 1). The flow
speeds were fixed and a measured
quantity of test material was allowed
into the system. A constant mass flow
was obtained throughout the loop. Six
waste fractions were tested on the belt
and vibrating pan conveyors. Test
variables included inclination, velocity,
depth of burden, and mass flow rate.
A pneumatic conveying test rig for
MSW was developed (Figure 2) using
analytical techniques and evaluation
procedures acquired from studies and
field tests. Costs of test rigs were
determined based on vendor price
quotations.
Results and Discussion
Conveyor Tests
Results of most belt conveyor tests
showed that the sensitivity of the spill
rate to flow rate and/or to inclination
was much smaller in the upper ranges
of the increases in belt speeds. Higher
spill rates occurred at lower belt speeds.
When belt speed was increased at a
constant mass flow rate, spills were
reduced to a minimum. Higher mass
flow rates resulted in higher spill rates
for a given test material and belt speed.
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Return Conveyor
Chute
[
1
'/
Surge Bin/
Vari-Speed
Apron Conveyor
^»
Tesf Conveyors
(Belt, Vibrating Pan)
.__ - 1
]t
1
Vibrating
Feeder
Return Conveyor
Vari-Speed
Apron Conveyor
Variable
Incline
Figure 1. Schematic of conveyor test rig.
Top View
To Solids
Separator ,
-£\
E$~*\
Solids & J
Gas In i
I -^
fl4 1
1
^
i C-
«2 ' X'
Configuration 1
Vibrating
Feeder
/?i. . .Straight
Runs
Pressure Taps
£1.. .Elbows(90°)
Note: Elbows not
to scale
by conveyor always generated spills.
Spills are described in Table 1 by belt
section based on the type of fraction,
flow rates, and velocities.
Some types of spills encountered
were(1)fine, inert particles that became
wedged between the belt and skirting
and were blown out by material falling
on the belt, (2) crumbled fractions that
fell over the sides of the conveyor, (3)
light materials that were swept off the
belt by air currents, and (4) higher-
density fractions that bounced off the
belt.
The maximum carrying capacity of
vibrating conveyors increased with
stroke and frequency. Increasing the
amplitude from 12.7 to 22.2 mm at 540
rpm resulted in carrying capacity
increases ranging from 170% for the
heavy fraction to 262% for MSW. The
conveying speed increased with higher
frequencies at a given mass flow rate.
Similar effects occurred with changes
in stroke. When a hand-held ampmeter
was used to measure the vibrating
conveyor motor current, energy con-
sumption was not significantlydifferent
with frequency or stroke length in the
ranges covered.
Tests results on the apron conveyor
showed that carrying capacity increased
linearly with increasing conveyor speed.
Very little spillage occurred off the sides
and most of that took place at an incline
of more than 30°. These spills were
caused by materials that fell back and
off the lower end of the conveyor.
Inclines of 0° to 15° had little effect on
carrying capacity; but from 15° to 39°, a
sharp drop occurred. Only two feed-
stocks (d-RDF and a blend of d-RDF and
coal) were used for these tests.
Side View
•i-
Alternate Configuration
Vi. . . Vertical
Risers
Note: Elbows not
to scale
Vertical
Figure 2. Schematic view of pneumatic test rig (passive circuit).
The six solid waste fractions tested
showed similar spill-rate patterns, but
the rate depended on the individual
properties of each fraction. Homoge-
neous materials generally had lower
spill rates than heterogeneous ones.
Higher spill rates occurred when the
incline of the conveyor was increased
from 14° to 18°. Preferred speeds were
found for a given mass flow rate and
incline. Dense and uniform materials
were less proneto slip or roll back on the
inclined conveyor.
Belt conveyor data indicated that,
except for very low rates of throughput,
the movement of MSW and its fractions
Determining Bulk Densities of
MSW
The six materials evaluated in this
study were coded according to the
Engineering Conference of Conveyor
Equipment Manufacturer's Association
(CEMA) 1979 publication concerning
belt conveyors for bulk materials. These
published standards for determining
bulk densities of aggregates and coal
are not generally applicable to MSW,
however. Users must therefore be
aware of their limitations, which results
from the heterogeneity and variability of
the materials. Development of new,
reliable procedures for determining bulk
densities of MSW were not within the
budget of this investigation.
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Distribution of Spills
The nature and mechanism of spillage
was studied by isolating sections of the
belt conveyor to prevent carryover of
materials from one section to another.
Thus the nature and mechanisms of
spills for each belt section could be
observed. The distribution of spills in
each section may depend on the
fraction, speed, or load, or a combina-
tion; or it may have no clear trend (Table
1).
Homogeneity of MSW Versus
RDF
During the vibrating conveyor study,
an attempt was made to separate the
processed waste fractions into their
component parts. A technique was used
to split the stream of waste at the
discharge end into equaltopand bottom
layers. Particle size distribution was
performed on the two layers for three
different materials — MSW, RDF, and
the heavy fraction. In MSW and the
heavy fraction, smaller particles of
higher bulk density tended to concen-
trate in the bottom layer. In RDF,
however, the top and bottom layers
were quite similar.
Pneumatic Conveying
The materials considered for pneu-
matic conveying were shredded waste,
ferrous fraction, heavy fraction, and
RDF. Raw MSW was not included.
Studies previously reported concerning
particle size distribution of raw MSW
indicate that some components are of a
sieve size larger than 26 cm. This size
would require a duct pipe of 75-cm
diameter. The needed fan power would
place the pneumatic test rig out of the
"small" classification. The literature
reports a full-scale pneumatic conveying
system for raw waste located in Sweden
that requires five turbo-extractors o
100 kw each. But in the U.S. Resourci
Recovery Plants, RDF and shreddei
MSW are the only materials actual!'
being pneumatically conveyed. RDF hac
the first priority in both this modelm<
effort and in instructions to manufac
turers from whom price quotation;
were solicited.
Conclusions and
Recommendations
Determination of the value of infor-
mation from this work requires applica-
tion and testing of the findings on a
variety of conveyor styles, sizes, and
applications in full-scale commercial
facilities. Though projected follow-up
evaluations using the rationale and
experience from this study are not likely
to be funded, both the findings and
areas pinpointed for additional investi-
gation will benefit interested readers.
Table 1. Conveyor Spills
Spills from Conveyor Sections as Per Cent of
Product
and Flow Rate
RDF at 2.7
Mg/hr (3.0 tons/hr)
MSW at 2.7
Mg/hr (3.0 tons/hr)
Heavy fraction at
9. 1 Mg/hr
(W.O tons/hr)
Ferrous fraction at
9. 1 Mg/hr
(10.0 tons/hr)
d-RDF/coal blend
at 9. 1 Mg/hr
(W.O tons/hr)
d-RDF at 13 .6
Mg/hr (15.0 tons/hr)
Belt
Section
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
46 m/s (150 ft/min)
37.5
43.0
13.2
6.3
41.7
39.7
11.6
7.0
38.5
46.2
6.4
9.0
25.0
50.5
17.9
6.7
89.1
2.9
3.0
4.9
15.0
2.5
47.5
35.0
loiai conveyor opws
92 m/s (300 ft/min)
23.5
9.5
14.6
52.4
30.2
16.4
10.9
42.6
33.3
44.4
5.6
16.7
15.0
18.1
59.1
7.6
72.8
2.5
6.8
17.8
20.0
10.0
10.0
60.0
137 m/s (450 ft/min)
24.2
13.1
13.6
49.1
29.7
14.9
13.9
42.5
50.0
25.0
8.3
16.7
6.7
56.3
31.9
5.1
67.0
4.8
11.7
16.4
66.7
11.1
11.1
11.1
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Bulk Density Determinations
CEMA methods used to classify and
define bulk materials did not apply to
MSW and many of its fractions Users
must therefore be aware of their
limitations.
Belt Conveyors
Experimental results on belt conve-
yors indicate that (1) the sensitivity of
spill rate to flow rate generally increases
in the upper range of belt speeds,
and/or (2) the degree of incline is
smaller at higher belt speeds. The
recommendation is to operate in the
"fast and lean" range rather than under
"slow and loaded" conditions.
Measurements of discharge trajec-
tories for four fractions at fixed mass
flow and varying belt velocities corre-
spond reasonably well to the CEMA
theoretical expectations. These calcu-
lations could probably be used to predict
trajectories of MSW fractions accu-
rately. Negligible change in belt motor
current was found for a varied range of
velocities, capacities, and materials.
This lack of fluctuation in power
consumption probably indicates use of a
motor that surpasses the required
design and capability
In the vibrating conveyor experi-
ments, carrying capacity increased for
all fractions with both frequency and
stroke. The capacity curve is expected to
reach saturation level at a higher
undetermined frequency. In this study,
the operating frequency was limited to
430 to 545 c/m (cycles per minute) at a
22.2-mm stroke and 390 to 550 c/m for
a 12.7-mm stroke because of an un-
resolved imbalance in the system. No
significant difference occurred in energy
consumption with frequency or stroke
length in the above ranges. Compaction
of material as a result of pan vibration
was found in all solid waste fractions.
MSW and RDF had the highest degrees
of compaction.
Pneumatic Conveyors
Two systems for pneumatic conveying
of MSW fractions were investigated. The
first used a high-resistance, fixed
geometry system, and the other em-
ployed a simplified, variable geometry
system with restricted capabilities. In
selecting conveyors, considerations
based on user requirements and the
availability of conveyors at the manu-
facturer's sites for testing the materials
to be conveyed should enter into pur-
chasing decisions. A need for regular
tests over a period of years would justify
ownership of a complex unit. Limited
needs could be met by performing tests
at the manufacturer's sites or by
purchasing a simplified pneumatic
conveyor.
The full reports were submitted in
fulfillment of Grant No. R806709 by
National Center for Resource Recovery,
Inc., under the sponsorship of the U.S.
Environmental Protection Agency
Zahid Khan, Marc L. Renard, and Jay Campbell are with the National Center for
Resource Recovery, Inc., Washington, DC 20036.
Carlton C. Wiles is the EPA Project Officer (see below).
This Project Summary covers two reports, entitled:
"Considerations in Selecting Conveyors for Solid Waste Applications,"
(Order No. PB 83-107 482; Cost: $14.50, subject to change)
"A Pneumatic Conveying Test Rig for Municipal Solid Waste Fractions,"
(Order No. PB 83-107 474; Cost: $10.00, subject to change)
The above reports 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
Apron Conveyors
When the carrying capacity of apron
conveyors is plotted versus conveyor
speed, the maximum carrying capacity
increases linearly with increasing
velocity. The speed range evaluated is
typical of those used with apron
conveyors. The conveying surface is
rigid, and the bouncing effect seen with
the conveyor belt is absent. Very little
spillage occurred over the sides.
. S. GOVERNMENT PRINTING OFFICE: 1982/659-095/554
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United States
Environmental Protection
Aggncy
Center for Environmental Research
Information
Cincinnati OH 45268
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