Cap7_Mill and bagasse conveyors

Prévia do material em texto

175
7 MILL AND BAGASSE CONVEYORS
B. STC. MUOR
Consultant, Bosch Projects; formerly Director, Tongaat-Hulett Sugar Ltd, South Africa
7.1 MiIl intercarriers
There are tive types 01' mill intercarrier in use
loday:
Apron-type slat carrier, used where DOllllelly
chutes have not been installed;
Belt-type inlercarriers, low incline without
DOllllelly chutes;
Slat drag-type carriers, the most common for
feeding into DOllllelly chutes;
Belt intercarriers, incorporating deep pockets
when feeding DOllllelly chutes;
Fixed "Meil1ecke" chules.
mediate carrier" chain (e.g. Ewart type 5174, 90 I 01'
907 chains), 01' the type described in Section 3.5.1
and Figure 3.9. The bagasse is conveyed on the up­
per deck 01' the carrier. The slats are bolted directly
onto a web between the Y-arms (yoke) on the upper
side 01' the chain. Apron carriers are used typically
as the conveyors between closely-spaced three-roll­
er mills, where there are no enclosed feed chutes to
the mills. They are usually inclined at a small slope
(10° to 25°) to elevate the cane to the next mil!. The
conveyor slals around the headshaft are often used
effectively as a fourth 01' "feed roller" for the mil!.
Such installations are normally on relatively lighl
duties in relation to the chain strength and perform
satisfactorily. An example is shown in Figure 7.1.
Figure 7,\: Mill fccd with apron-typc intercarricr (/lI/gol 1986:78)
These carriers comprise overlapping protiled
slals mounted on two 01' lhree strands 01' short­
pitched (60 mm to 80 mm) cast Y-link type "inter-
7.1.1 Apron intcrcarricrs Speed. The speed 01' the carrier is normally set
at about 10 % to 15 % above the maximum surface
speed 01' the mill rolls 01' the following mil!. This
is to provide positive feed assistance without drop­
ping out bagasse ahead 01' the mill front rol!. The
headshaft may be driven
directly (by transmission
chain and sprockets) from
the top roll 01' the succeed­
ing mil!.
Sprockets and sup­
ports. lI' the intercarrier is
also to serve as a mill feed
roller, the slats will gen­
erally require additional
support around the head
pulley. This can be pro-
Referellces p. /88
176 7 Mill and bagasse conveyors
These carriers are now the most common type.
They comprise deep slats mOllnted either between
or olltsiele two strands of meelillm- to long-pitched
(150 to 300 mm) conveyor chain. The bagasse is
conveyeel on the lower eleck of the carrier. They are
lIsed in cases where the cane needs to be steeply el­
evated. usually at an inclination of 40° to 60°. typi­
cally as the conveyors between mills that are fed by
DO/l/lel/y chutes. Figure 5.21 in Chapter 5 illllstrates
such an application. This type of carrier can also be
used for conveying between mills filted with open
chutes. where it has advantages over the apron-type
in the steeper inclinations possible and in cleaner
operation.
Another comrnon application for drag carriers
- lIsllally horizontal - is for distribllting the feed into
rnoving bed dilTlIsers.
vided by discs with metal rims positioned between
the headshaft sprockets so as to provide support at
intervals of 300 mm to 450 mm across the width of
the carrier. The rims may be about 75 to 100 mm
wide and of a dia meter that leaves a clearance of ap­
proximately 2 mm under the slats in the new and un­
loaded condition. This is to ensure that the headshaft
sprockets continue to engage properly with the chain
under conditions of normal wear and dirt ingresso
Typically. headshaft sprockets have 600 mm to
800 mm pitch circle diameters. depending on the
mill feed configuration. Tailshaft sprockets pitch
circle diameters should be not less than 500 mm. be­
cause this will result in undue chain wear and may
open gaps between the slat overlaps.
The upper (load carrying) deck of the apron is
usually sllpported on runners lIndcrneath the chains.
Unless the carrier is long (say > 4 m shaft centers).
the lower section of the apron hangs in a free catena­
ry that provides automatic tensioning. The tailshaft
may also be adjustable for tensioning.
7.1.3 Chain-and-slat scraper inter­
carriers
Tahle 7.1: Bulk densilies in kg/Ill' of cane and bagasse on conveyors
The fllnction and characteristics of these con­
veyors is very similar to the slat-type apron carriers.
The bagasse is conveyed on a belt. either smooth top
(maximum incline 22°) or having low-height molded
or glued-on transverse ribs to assist the bagas se feed
both at pick-lIp and into the following mill (inclines
IIp to about 30°). They operate at higher speeds than
the apron-type carriers. with a commensurately low­
er levei of cane.
The Farrel belt intercarriers of this type gener­
ally incorporate an integral chain-driven overhead
feeder drllm. working in parallel with the belt over
the head plllley to assist feeding of the following
mil!. The feeder drum is sllspended on pivoting arms
so that it rides on the bagasse blan-
ket. 11 has diagonal ribs to improve
grip on the cane.
7.1.2 BeIt-type intercarriers, low in­
cline
Bulk dcnsitics (Ir canc and bagassc. A number
of sources have been referenced to establish "aver­
age" blllk elensities of knifed cane. shredded cane
anel bagasse. Data in the literatllre varies widely. This
is not surprising. since there are wide differences in
factors such as the proportion of air. cane qllality.
arnollnt of moisture/juice. degree of preparation and
presentation (e.g. free-spilleel or lightly compacted
in a chute). The data in Tablc 7.1 is therefore only
a rollgh gllideline. For more accllrate information.
measllrements of the product should be made.
A more general measure is the bulk density of
dry liber. Once it has been well shreeleled. the bulk
density of elry fiber in lIncompactcel bagasse is rea­
sonably constant. However. this fiber can absorb 5 to
6 times its own mass of water or juice. without sig­
nificant change in the bulk volume. The bagasse vol­
ume can be estimated assuming a fiber bulk density
Pb.F 01' between 55 kg fiber/m.1 (coarsely prepared
ProducI DUlyDensily rangeAverage
Knifed cane
To shredder 1Isi Illill250-480320
Shredded cane
To Isi Illilll diffuser300-550380
Bagasse
Inlercarriers. chules200-400250
Diffuser bagasse
Frolll diffuser280-380300
Final bagasse
Boiler carriers120-180140
7.1.3 Chain-and-slat scaper intercarriers 177
•.
cane) and 66 kg/mJ (well-prepared cane) (Murry
1960). With modern preparation standards, up to
70 kg/mJ may be used, particularly in bagasse that
retains some compaction from the mills. For cane
under light compression in an enclosed mill chute,
coarse and fine densities 01' 75 and 90 kg/mJ respec­
tively have been quoted, but the lower figures are
recommended for carrier calculations. The overall
bulk density Pb•F can then be estimated by adding
the mass of liquid.
Example 1
What is the bulk density 01' fine bagasse containing
51 % moislure, 3 % dissolved solids DS (Brix) and
46 % fiber?
Mass 01' dry fibcr per mJ = 70 kg (assulllcd)
Mass ofjuice (\Vater + OS) per IllJ
= 70· 54/46 = R2 kg
Total bulk density Pb,F = 70 + R2 = 152 kg/mJ.
Example 2
What is the bulk density 01' bagasse in the intercar­
rier to the final mill 01' a tandem crushing cane with
imbibition applied at 280 t water/IOO t fiber? As­
sume bagasse leaves the previous mill with 53.0 %
moisture, 7.5 % DS (Brix) and 39.5 % fiber.
Mass 01' dry fibcr pcr mJ = 60 kg (assulllcd)
Mass 01' rcsidual juice per IllJ = 60· 60.5/39.5
= 91.9kg
Mass 01' imbibition added per mJ = 60· 2ROII00
= 168 kg
Total bulk density Pb F = 60 + 91.9 + 16R
. = 319.9 kg/IllJ
Note that the density at earlier intercarriers will be
higher as less juice has been extracted.
Spccd and dimensions. For reasonable life, in­
tercarrier speeds should wherever possible be limit­
ed to < 1.0 m/s, preferably < 0.8 m/s. For good chain
life, chordal action must be limited and sprockets
should have at least 14 teeth, or 7 teeth for block
chain where there is one tooth per 2 pitches. The
overall dimensionsmust then be determined to al­
low sufficient cross-section (effective width b times
average height "B) to convey the bagasse with a
40 % free space between slats - i.e. assuming 60 %
usable volume.
The average bagasse height "B is a function 01'
the slat height ", the slat pitch p, the inclination 01'
the carrier ~ and the angle 01' repose of the bagasse
(Figure 7.2). The angle of repose a. 01' free bagas se
can vary widely (usually between 35° and 50°),
depending on texture, etc. It is easily measured by
pouring the bagasse to form a small pile.
There are two cases to consider:
a) ~ ~ a.
From the trigonometry, in the top triangle:
Side d=p' tan (~-a.)
Area AI =(d·p)/2
Area Az =P'"
Averageheight "g=(Ai-AZ)/P (7.1)
Figure 7.2: Rcprcscntation 01'bagas se conveyed by a slat
on a conveyor
hH average bagasse height;
h height 01' slats;
I' pilch 01' slals;
a angle 01' rcpose 01' bagasse;
~ angle 01' inclination 01' the carrier.
Referel/ces p. /88
178 7 Mill and bagasse conveyors
In both cases, the capacity fn (kg/s) 01' the carrier
will be:
.. a+b+p
Seml-penmeter .I' = -­
2
Area AI = [s· (s-a)· (s-b)· (.I'-p)j05,
which can be solved in terms 01' a, ~ and p.
Area A 2 = P . h
Average height hB = (AI + A2) / p
b) ~ s; a
From the trigonometry, in the top triangle:
Apex angle e = 180 - 2 a
. p'sin(a+~)Slde a = ----­
sin(l80 - 2a)
Because ~ = a, bagasse theoretically just fills to slat
height, i.e. average height hB = slat height h, the vol­
ume li in m3/s is:
.40·1000
V = 3600. 55 = 0.6 . 0.8 . 1.8· h
gasse discharge is helped by a profiled slat with the
upper section 01' the face inclined forward-of-square
to encourage the bagasse to slide off and not carry
back over the headshaft as shown in Figure 7.3 .
Examplc
What slat height h is needed to convey bagasse con­
taining 40 t fiber/h, in an intercarrier 2130 mm wide
but with most 01' the bagasse confined to the middle
1800 mm? Carrier inclination ~ = 45° and angle
01' repose 01' this bagasse a = 45°. Carrier speed
li = 0.8 m/s and assume fiber bulk density Ph F =
55 kg/m3 ..
(7.2)
p' sin(a - ~)
sin(l80 - 2a)
Side b
fil = 0.6· Pb.H • hH • b'lI (7.3) Therefore: h = 0.23 m
.....1Slats mount d
/ ~ e between ch .
/:/( .::\ ""'
Slats 01' 250 mm depth would be required. Because
the carrier inclination 01' 45° is fairly steep, forward­
inclined slats may be advised.
Were the carrier less steep, 150 mm slats would
probably suffice.
where:
0.6 "filling factor";
hB Average height 01' bagasse in m;
Pb•B bulk density 01' bagasse in kg/m3;
b effective width 01' carrier (excluding edge ef-
fects) in m;
li linear speed 01' carrier in m/s.
Drive sprocket!
head sprocket
___ Forward inclined
upper face of slat
~
Donnel/y chute
--------------- feeding into chuteBagasse
/'f
Figure 7,3: Slat profilc for SICCp inlcrcarrier
Slats. As shown in Figure
7.2, on carriers 01' low inclina­
tion, the bagas se usually will
pile to above the slat height,
but on steep carriers the ba­
gasse may not pile higher than
the slats. The slat height and
profile need to allow for this.
Actual heights will depend on
various factors (textme and
angle aI' repose, carrier incli­
nation, imbibition application,
speed, slat profile, etc.) so
should be observed for the mill
concerned.
Slats may be aI' hardwood,
steel sections, extruded alu­
minum sections or fabricated
with tines on a crossbar. On
very steep carriers (60°), ba-
7.1.3 Chain-and-slat scaper intercarriers 179
Figure 7.4: Block chain wilh attachment 510ts
l- Pitch ------!.- Pitch ~
Block linkSide bars
Top view
Chains. The chains may be 01' many types. Sug­
ar industry conveyor chains used to be predominant­
Iy of various cast malleable designs, but these have
been largely replaced by steel chains. Most suppliers
manufacture these to imperial (inch-based) dimen­
sions. The most widely used chains are 6" (150 mm)
pitch crank-sided roller chains (e.g. type 09060), 6"
(150 mm) pitch parallel-sided roller chain, 6", 8" or
12" (150, 200 or 300 mm) block-type chains (Figure
7.4) and 200 or 300 mm Y-block chains. A type not
yet widely used but strongly recommended for this
application is 150 mm cranked-link rollerless chain.
This has similar service characteristics to the roller
chains bul is cheaper because it has no (unneces­
sary) rollers and permits short length adjustments
(Figure 7.5).
An important point to note with both cranked­
linked rollerlcss and Y-block chains is the direc­
tion of traveI. To minimize wear 01' both chain and
sprocket, the driven surface on the chain should be
positioned on the sprocket periphery before engag­
ing with the driving sprocket tooth. For this, crank­
sided chain must operate wide side forward (as indi­
cated in Figure 7.5) and Y-block with the yoke (wide
section) trailing (as indicated in Figure 7.6).
Chain componcnts. Ali conveyor chains from
the main cane carrier up to the final mill operate in
wet or damp conditions and are subject to attack by
corrosive sugar juices. The load-bearing wearing
components 01' these chains (pins, bushes and pos­
sibly rollers) therefore give far superior life if made
01' corrosion-resistant stainless steel. The grade and
heat treatment 01' these components is a specialized
field 01' the chain manufacturers and varies belween
suppliers, but stainless sleel pins are typically 01'
minimum 45 Rockwell C hardness and bushes are
usually through hardened to > 35 Rockwell C hard­
ness.
Side view
Direction of travei
:.-Pitch----
Figure 7.5: Rollerless, cranked-link chain. Note direction
of traveI.Chain attachmcnts. There is a wide variety 01'
possible chain attachments. Figure 7.7 shows the
three most common types as supplied by Tsubaki
chain. Apron-type main cane carriers or intercar­
riers usually use the K-type (Figure 7.7 A), while
the most commonly used attachments for scraper­
type intercarrier and bagasse carriers are either the
A-type (side lugs with single hole - Figure 7.7 B)
for slats mounted between the chains or the C-type
or F-type (plate mounted perpendicular to the chain _____ Direction of travei
Figure 7.6:
Forged Y-block chain.
Note direction of traveI.
Refemiees p. J 88
L
180
A
(]:[] I
O O O O
(Q ~--~)
B
~ (<X=©)
7 Mill and bagasse conveyors
affect the drive. However. care is needed to ensure
that this does not leave a wedge under the chain in
the boot 01' the carrier. as this can lead to chokes
and slat breakage. Some installations use adjustable
boots to obviate this problem.
J>ower rClJlIircmcnt. The power P in kW used
by a scraper conveyor comprises the sum 01' the
powers which are needed to:
Overcome the friction 01' the empty conveyor.
with the chain-slat assembly either sliding 01'
carried on rollers (1'1);
Overcome the friction 01'bagasse on the carrier
base (to convey over the horizontal distance 01'
the conveyor) (1');
Elevate the bagasse (1'3);
Overcome the inefficiency losses in the drive
(motor. gearing. etc.).
For calculations:
where:
li1 mass t10w rate 01'product conveyed. in kg/s;
horizontal total length 01' loaded run(s) 01'
conveyor (i.e. including any loaded return
run) in m;
~ friction coefficient for cane 01' bagasse load.
This can be conservatively assumed to be 0.3
for bagasse.
where:
I/Ichain mass of chain and slats (t1ights) per m 01'con­
veyor in kg/m;
horizontal length of conveyor (tailshaft to
headshaft) in m;
~ friction coefficient for chain and slats. This
can be conservatively assumed as 0.35 for
scraping chain/slats. and 0.15 for chain run­
ning on rollers. but as rollers are often seized
01'jammed with bagasse. the 0.35 factor can
be generally used.
11 conveyor speed in m/s.
c
o (<X A ~J
Figure 7.7: Different chain attachments
A K-type attachment; B A-type attachment; C C-type 01'
F-type attachment
- Figure 7.7 C) for slats mounted below the chain.
With block-chain. slat mountings may pass through
holes in the blocks (Figure 7.4).
Great care is needed in welding slat attachments
to chain links. This cancause stress raisers and/or af­
fect heat treatment 01'the chain components. result­
ing in a weakened chain link 01' attachment failure.
11is strongly recommended that ali welded-on at­
tachments be supplied on the chain from the manu­
facturer. as the entire component can then be heat
treated to the correct procedures after welding.
Length adjllstment. The crank-sided 150 mm
chains have an advantage in that the chain length can
be adjusted in 150 mm increments. equal to 75 mm
tail-Io-head adjustment. 11 is usually preferable to
adjust by a tailshaft take-up. because this does not
I~ = g . I/Icha;n ./ . Jl . 11 li 000
P2 = .I: . li! ./ . Jl I I 000
P, = .I: . li! . li I 1000
where;
li elevation in m.
(7.4)
(7.5)
(7.6)
7.1.4 Belt-type intercarriers ISI
Ruhher helt-type intercarriers are also used.
Where only moderate inclines are needed « 20°),
With a drive efticiency of 50 %, motor power
needed is 8.5 /0.5 = 17.0 kW.
A motor of not less than 22 kW and equivalent
gearing should be installed.
P1 = 9.81 . 120· (2·7.0)·0.35·0.7/1000
4.04 kW
P2 = 9.8\ ·75·7.0·0.3/1000
1.54 kW
P1 = 9.S1 ·75·4/1000
2.94 kW
Total P1 + P2 + P, = 8.5 kW
Examplc
What power should he installed on a chain-and-slat
cane intercarrier conveying 270 t/h (= 75 kg/s) of
cane over a horil,(Jntal distance of 7 m and elevating
the cane by 4 m. The chain runs at 0.7 m/s. It re­
turns on skid rails and the chain-plus-slats assembly
weighs 120 kg/m length. Drive is hy helts and worm
reduction torque arm gearhox.
tiat surface belts can be used. However, for steep­
er inclinations (e.g. to feed into D0l111el/y chutes),
pocket belt conveyors with molded-on slats 01' pock­
ets are quite widely used. The bagasse is conveyed
on the upper deck of the conveyor.
Compared with chain-types, the advantages of
helt-type intercarriers are:
They are usually relatively low in initial cost and
in maintenance cosI.
Their power requirements are low.
They are of light weight and can be easily re­
moved as a unit for mill maintenance purposes.
Where height is limited due to the mill house
crane, a higher D0l111el/y chute is possible.
The main disadvantages of belt intercarriers are:
It is more diflicult to achieve clean operation
than with drag-type intercarriers.
They are not conducive to the efficient applica­
tion of imbibition. With tiat belts, wetted ba­
gasse would slip on the belt and with pocketed
belts imbibition surges would result in messy
spillage. For these reasons, the imbibition is of­
ten applied after the belt, e.g. into the top of the
D0l111el/y chute. With this application, distribu­
tion into the bagasse is usually poor and there
is limited time for absorption and mixing with
the residual juice. (Note that imbibition mixing
is often poor even in drag-type carriers).
The repair of damaged belts may require special­
ized resources (e.g. for splicing). For this reason.
installations should be designed for relatively
simple belt replacement. Holding spare belts for
carriers 01' differing lengths may be costly.
A design of rubber belt intercarrier supplied by Tri­
con in Louisiana has found widespread use in this
region (Figure 7.8). Belts 1.75 to 2.2 m wide at incli­
nations up 10 63° have been supplied. The cleats on
the belt are spaced between 350 and 600 mm apart,
usually 500 mm, and are 150 to 225 mm high. Drop­
ping of riddlings is prevented by the return belt run­
ning in a steel trough. The headshaft is rubber lined
with ceramic inserts to prevent slipping. The whole
unit is self-contained and can be removed as a unit
by the mill house cnme. The disadvantages are that
bagasse cannot be discharged from the mill into the
boot 01' the conveyor. and in most cases imbibition is
added at the discharge of the conveyor into the fol­
lowing mill chute. Later intercarriers have used air
helt systems. Belts usually give 30 10 40 months of
service (more than 10 years in Louisiana).
8elt-type intercarriers7.1.4
Chain spccitication. In specifying the minimum
chain rating (ultimate tensile strength) required, the
calculated operating tension must be multiplied by a
chain safety factor that depends on the chain materi­
al (malleahle or steel), speed, service hours/day. se­
verity 01' duty (steady 01' shocks) and working envi­
ronment (corrosive, dusty. abrasive). The operating
tension (in kN) can be calculated as lhe total power
P1+P2+P1 in kW divided hy the conveyor speed in
m/s (the chain tension in kg force can be calculated
as the value in kN X 1000/9.8). Manufacturers' hro­
chures provide guidance in deciding the applicahle
safety factor, which is usually between 2 and 5.
Drive cfficicncy. For drives incorporating Vee
helts 01' chains-and-sprockets together with helical
gear reducers, an overall efficiency of 85 % can be
assumed. If a worm gear reduction unit is incorpo­
rated, an overall drive efticiency 01' 50 % can be as­
sumed. For final drive sizing, it is advisable to add a
further ± 20 % to the calculated value, to allow for
starting loads and occasional chokes, etc.
-
Re{erencn {I. /88
182 7 MilJ and bagasse conveyors
Shaft mounted
drive
Adjustable take-up for belt Air inlet
tenslon and tracklng \
~
-. Bolted connections
_,7onnel/y chute
Pneumatic
bypass gate
'\
'\
'\
'\ '\
'\ '\
'\ '\
'\ '\
'\ '\
,\/
Figure 7.8: High-cleat rubber belt inten:arrier (courtesy 01' RJ.Tricon Co. LLC. Louisiana)
A fourth method of inter-mill bagasse convey­
ing is by "fixed carriers" of the Meillecke chute
type. These are used between mills that are closely
spaced. The bagasse being discharged from a mill
is kept under some compression by being con­
strained between noseplates or scraperplates on
the top and discharge rolJs. These project the ba­
gasse upwards, usually at about 30° elevation, in a
slightly diverging chute of 1.5 to 2.0 m in length.
The width of this chute at its entrance needs to be
related to the discharge work opening of the mil I,
and is usually between 5.5 and 8 times this open­
ing. From the top of this chute, the bagasse is dis-
7.1.5 Me;llecke chute conveyors charged onto a sloping (50° incline) feed pia te imo
the next mil!.
Although the Meillecke chute concept appears
attractive due to its having no moving parts, it suf­
fers from several inherent disadvantages:
The maximum possible elevation is limited and
this militates against good feeding of the suc­
ceeding milJ;
Sealing the upper noseplate against a floating
top rolJ is problematic and can give severe me­
chanical problems;
11 is not possible to inject imbibition efficiemly
imo the bagas se between mills.
For these reasons. their use is becoming less com­
mon.
----------- ~ ..•. -- -
7.2.1 Bagasse belt conveyors 183
7.2 Bagasse conveyors the bagasse may be fed onto the belt at an angle and
velocity such that it does not require to be acceler­
ated along the direction 01' belt traveI.
Surcharge angle
Figure 7.9: Belt cOllveynr cross-sectioll
~ +- - -- --t
1.2 mls
130 kg/m3
45°
30°
100%
Belt speed:
Bulk density 01' bagasse:
Trollghing angle:
Surcharge angle:
Belt till:
Power requircment. The power used by a belt
conveyor (whether for bagasse, sugar or coal in typi­
cal sugar mill applications) comprises the sum of
that which is needed:
For different belt speeds and bagasse densities, ca­
pacity will vary proportionately.
Capacity. Conveyors should always be sized for
peak loads, not average. With bagasse (or sugar or
coal) belt conveyors, the volumetric carrying capac­
ity is given by the product of cross-sectional area
times the velocity. As is seen from Figure 7.9, the
material cross-section is a function of the surcharge
angle, which is related to the angle of repose. Be­
cause of the geometry, the surcharge angle is also a
function of the inclination of the bel!. For belts in­
clined at 0° (horizontal) to 5°, the bagasse surcharge
angle is about 30°. However, this reduces to 21 ° for
belts inclined at 23°, and the capacity is therefore
significantly lower.
The usual angle oftrollghingpulleys is 20° from
horizontal, but deep-troughed arrangements (outer
idlers at 35° or 45°) are now common and well-suit­
ed to bagasse conveyance. On bagasse, deep troughs
increase the carrying capacity for a given width of
belt by 20 to 25 % for 35° idlers and by 25 to 35 %
for 45° idlers, the increase depending on the angle of
repose of the product and the inclination of the bel!.
An important benefit of deep troughs is that they
minimize the dust-generating surface.
Table 7.2 provides a guide to capacities of ba­
gasse belts of various widths. This table is compiled
based on the following assumptions:
Bagasse beIt conveyors7.2.1
The most convenient and cost-cffective means
to convey bagasse is by conventional troughed belt
conveyors. Belt speeds of IIp to 2.0 m/s can be used
satisfactorily, bllt a wider belt at 1.0 to 1.5 m/s will
give sllbstantially longer life with less dust genera­
tion.
Belt conveyors may be llsed to elevate bagasse
at inclinations 01' up to 23° from the horizontal, pro­
vided that the feed arrangements are suitable. The
feed zone of the belt may be aI a ftatter inclination or
The mechanical handling of final bagasse is
relatively simple: it is of low density, mildly cor­
rosive and llsllally rairly free-tlowing. However, it
can choke, bridge and pack tightly if not correctly
managed. 11 also prodllces dllSt, which can be both a
health hazard (causing bagassosis, an allergic reac­
tion of Illng tisslle to airborne spores from bagasse
(DalVsolI et aI. 1995)) and a seriolls tire hazard. Dust
prevention is therefore a primary consideration in
conveying bagasse.
Variolls types of conveyor may be llsed, includ­
ing pnellmatic, air belts, pipe belts and pocket belts,
bllt the greal majority of installations are conven­
lional belts on idlers or chain-and-slat drag convey­
ors.
Tahle 7.2: Capacity of horizolltal bagas se helt cOllveynrs
Belt width in mm
4505006007509001050120013501500180021002400
Capacily in tlh
12.916.725.141.361.485.4113145180264363477
Capacity ill kg/s
3.64.67.011.517.023.731.440.350.073.3100.8132.5
Referellces p. IIiIi
184 7 Mill and bagasse conveyors
where h is the elevation in m (negative if the con­
veyor lowers the product).
To e/evale the prodllct:
11' the conveyor is inclined upwards, power will
be required to lift the product. For this:
additional plough friction. Without detailed infor­
mation, an allowance 01' 0.003 . belt width (in mm)
kW per device can be allowed.
To move the empty belt;
To move the load horizontally;
To overcome any ancillaries (e.g. scrapers, trip­
pers, ploughs);
To elevate the product if the conveyor is inclined
upwards (negative power if the conveyor lowers
the product);
To overcome the inefficiency losses in the drive
(motor, gearing, etc.).
Exact power calculations are complex, but for most
sugar applications the following method for each 01'
the above components, adapted from various sourc­
es, is sufficiently accurate:
P2 = g . li! .h li 000 (7.9)
To I/love the be/t p/lIS /oad 11OriZOlltal/y:
p. = F· li (7.8)
F = g '/1' (i + 60). (111",," + li!IlI)IIOOO (7.7)
Values 01' IIlbclt in kg/m conveyor length for typical
sugar mill conveyors are:
To overeollle the illefjieieney tosses in lhe drive
(I/lotor, gearillg, ete.):
This will depend on the type 01' drive. For the
common drive type 01' induction motor, Vee belts
and gearbox, the overall drive efficiency is usually
about 85 % with helical gear reducers and 50 % with
worm-and-wheel reducers.
For final drive sizing, it is advisab1c to add a
further ± 20 % to the calculated value, to allow for
starting loads and occasional seized idlers, off-track
belt rubbing, etc.
Examplc
What power drive should be installed on a belt con­
veyor transferring bagasse to storage, the details 01'
which are:
Maximum load to be conveyed 15 kg/s; length of
conveyor 80 m, elevating bagasse by 12 m; belt
width 900 mm, speed 1.5 m/s; system includes one
tripper; drive is by motor, Vee belts and torque-arm
gearbox containing helical gears.
From above fonnulae:
To move belt + load horizontally:
F = 9.81 . 0.022 . (80 + 60) . (57 + 15/1.5)/1 000
= 2.02 kN.
Power p] = 2.02· 1.5 = 3.04 kW.
For ancillaries (tripper):
Power = 0.003· 900 = 2.70 kW.
To elevate the bagasse:
Power P2 = 9.81· 15· 12 11000 = 1.76 kW.
Total power = 3.04 + 2.70 + 1.76 = 7.5 kW.
Assuming a drive efficiency of 85 %, the installed
power requirement
= 7.5 10.85 = 8.82 kW.
Allowing 20 % for starting loads, seized idlers etc.,
a motor 01' 11.0 kW should be installed.
900 1200 1500 1800
57 90 J 28 182
600 750
32 42
Belt width in mm
11lbelt
where:
p] Power in kW;
F effective belt driving force (tension on drive
side - tension on non-drive side 01' pulley) in
kN;
li belt speed in m/s;
I-L composite friction factor 01' 0.022 for most
sugar mill belt conveyors;
conveyor length (head to tail pulley centers) in
m;
"60" equivalent length 01' fixed friction (head and
tail pulleys, take-up, loading zones, etc.) in m;
lil maximum mass flow rate 01' product to be con­
veyed in kg/s;
I/lbel• mass 01' belt conveyor's moving parts in kg per
linear m.
To overcollle allY allci/laries (e.g. serapers, trippers,
p/ollghs):
Many sugar belt conveyor applications involve
belt trippers or ploughs. In trippers, the load is usu­
ally raised and there is additional resistance from
direction-reversing pulleys; with ploughs, there is
7.2.1 Bagasse belt conveyors 185
750-900 1000-1500
3.7 4.2
1.5 1.7
Belt specifications. The standard construction
for belts used for cane, bagasse and sugar convey­
ing has a carcass of several plies 01' sljuare woven
collon duck or synthetic fabric such as rayon, nylon
or polyester. These are cemented together with a
rubber compound and covered both top and bollom
with rubber ar neoprene to seal against moisture and
resist abrasion. Some retineries specify food quality
PVC covers to prevent product contamination.
Suppliers' advice should be sought fÓr detailed
speci fications on very long or heavi Iy loaded belts,
hut for most hagasse and sugar helts (600 mm to
1500 mm wide) a four-ply construction with top
cover;:::: 5 mm and bOllom cover;:::: 3 mm thick will
surtice. Some engineers specify both covers 01' the
same thickness (5 mm) so that the bel! can be re­
versed (turned over) when worn.
General advice.
Maximum acceptable and recommended belt
speeds for bagasse are:
Widlh of heI! in mm 500-600
Maximum speed in m/s 3.0
Recommended speed in m/s 1.2
Operation at the maximum speeds will curtail
bel! life and generate dus!. Good practice would
normally be to operate at the recommended
speeds.
For accurate and reliable belt tracking, it is
essential that every idler be mounted exactly
sljuare to the conveyor center-line.
Head- and tail-pulleys should he not less than
450 mm in dia meter (preferahly 600 mm).
Head-pulleys should be lagged with a 10 to
12 mm rubber or neoprene cover for good trac­
tion.
A snub pulley should be installed immediately
behind the head (drive) pulley to increase the
angle 01' wrap and to steady the bel! from cen­
trifugal "flapping" after the head pulley (Figure
7.10).
The load side (top) troughing idlers should be
spaced :S 1.5 m apart, with closer spacing to
support the bel! in the loading zone. Return
idlers should he spaced at :S 3.001 centers.
Crowning (machining with a 5 to 10 mm larger
diameter in the center) of head- and tail-pulleys
assists tracking but stresses the bel! and should
be avoided if possible.
The tail-pulley should either be 01' open (slatted
Snub pulley
Riddlings tray
Figure 7.10: Bagasse belt scraper and riddlings tray
or spiral) construction or be titted with a scraper
to prevent build-up 01' recirculating bagasse.
For short conveyors, screw-type take-ups can be
used. For longer conveyors, a weighted gravity
take-up is recommended, in order to adjust for
temperature expansion/contraction 01' the belt,
bel! stretch and possible cutting and resplicing.
Spl iced belts last longer, create lessdust than
belts joined with clip-type fasteners and permit
the use 01' scraper-cleaners and ploughs.
Splices should extend in length by at least 1.5
and preferably 2 bel! widths.
Long, wide (heavy) conveyors and conveyors
subject to frequent stops and starts should be
provided with either an electronic soft starter or
a hydraulic clutch. This is necessary to reduce
stresses on both the drive and the bel!.
On steeply inclined conveyors, a holdback may
he needed to prevent reversing when tripped un­
der load.
Most bagasse installations warrant the use 01'
tire-retardant bel! materiais and flame-proof
motors because 01' the dusty environmen!.
Dust management. Most 01' the dust creation
occurs at transfer points - at the feed onto or dis­
charge from a bel!. Fine, dry bagasse dust can be
explosively flammable. To minimize dust:
Arrange for the feed to slide onto the belt, with
a component 01' momentum in the direction of
trave!.
Avoid high drops; if not possible. try to arrange
for sliding chutes (at 45° to 60° from vertical).
Referellee.\" {1. /88
186 7 Mill and bagasse conveyors
Dust "sprays" out from load-points with the re­
lease 01' entrained air. Enclose the head 01' the
discharging conveyor with a hood and curtain to
prevent air being entrained into the transfer zone.
Many belt cleaners - both static and "beaters"
(rotating stiff brushes or ribbed rollers that vi­
brate the belt) - are available. Ali discharge
much 01' the cleaned material immediately afler
the device, so riddlings collection trays should
be positioned for this.
A simple angle iron cleaner that packs with a
self-renewing bagasse scraper is shown in Fig­
ure 7.10. The scraper should be positioned be­
tween the head pulley and the snub pulley.
For bagasse conveyance, chain-and-slat convey­
ors are more costly initially and in maintenance than
are belt conveyors of equivalent capacity. They also
absorb more power. Their use is therefore usually
confined to applications where belts are not suitable.
Examples are:
Where elevations steeper than 23° are required.
Where multiple offtakes are required.
Where bagasse is to be transported in both direc­
tions (using both top and bottom deck 01' the slat
carrier).
For feeding bagas se into boi ler chutes.
The types 01' chain used in bagasse conveyors are
generally similar to those for mill intercarriers, with
one important exception. Whereas wear in the mill
area is primarily a juice-Iubricated corrosion-wear
process, wear in bagasse applications is mainly a dry
erosion processo As a consequence, the chain wear
components for bagasse applications should be 01'
heat-treated alloy steel, with the pins through hard­
ened and induction hardened to 2: 50 Rockwell C
and the bushes case hardened to 2: 50 Rockwell C.
These are less costly and give better service on this
duty than stainless steeI. whose pins are typically
01' 45 Rockwell C and bushes through hardened to
38--40 Rockwell C. Some suppliers case harden their
stainless bushes to 50 Rockwell C.
As with intercarriers, the recommended chain
types would usually be cranked-link rollerless chain,
block-type or Y-block chains. AS2-type side attach­
ments for the slats are the most commonly used, es­
pecially in "twin decked" arrangements.
!'o\Ver. rower calculations are as for intercarri­
ers (equations 7.4 to 7.6), with the coefficient ofslid­
ing friction for bagasse being $ 0.35. The friction
coefficient for chain running on rollers is about 0.12
but reverts towards 0.35 iI' the rollers are seized.
Feeding bagasse from conveyors into modern
boilers presents a particularly demanding chal­
lenge. For satisfactory boi ler performance, bagasse
must be fed reliably at high rates into narrow en­
closed chutes, keeping them fuI! without choking or
generating excessive dus!.
This task is usually performed either through
apertures in the floor 01' a chain-slat conveyor or us­
ing ploughs 01'1' one or both sides 01' a bell sliding in
a nat steel trough. In a review 01' this function, Moor
(2000) comments that most belt-type installations
need many adjustments before they are considered
"satisfactory" and that even then, few meet ali the
requirements. For belts in this application:
Belt must be spliced with the top cover trail­
II1g.
Slower belt speeds should be used - preferably
not more than 1.2 m/s for reasonable belt life.
Contrary to normal belts, the thinner cover
should be on topo This is to prevent the edges
from curling upwards and allowing bagas se in­
gress under the bel!.
Alternatively, a cross-stabilized weave carcass
can be used to resist the edges curling upward.
To reduce frictional drag, a "friction-backed"
belt can be used. This is a belt with no cover
on the under side. Such belts should be one ply
thicker than usual, e.g. 5-ply instead 01' 4-ply, to
maintain strength when the bottom ply wears.
Most sugar engineers agree that, fl)r boi ler feed
conveyors, good chain-slat installations are better
than good belt installations. Amongst the features
recommended for chain-slat carriers are:
Carrier speeds 01' not more than 0.8 m/s.
Conveyor sliding on the slats, not on the
chains.
The slats pitched as close together as possible
without the risk 01' bridging between slats. This
usual!y results in spacing at between 900 and
1200 mm, depending on the bagasse loading
and chain pitch.
Bagasse feeding to boilcrs7.2.3
Bagasse chain conveyors7.2.2
l
7.2.4 Bagasse sampling / References 187
Figure 7.11: Hagasse fccd gate to boilcr chutc
~
Boiler chute
Magnets7.3
Many factories have installed magnets on
the bagasse supplies to boilers in order to pratect
against tramp iron damage to boi ler feeders or mov­
ing grates. Powerful electro-magnets are generally
preferred to more limited permanent magnets. Mag­
nets cannot prevent ali damage - they do not remove
stainless steel. non-ferrous metal. timber or rocks.
Their effectiveness in removing magnetic material
is dependent on:
The power 01' the magnetic field generated;
The conljguration of the installation.
The simplest type 01' installation is for the magnet
to be suspended over a bagasse conveyor belt. For
highest separation elfectiveness, the magnet needs
to be as dose to ali parts 01' the bagasse as possible.
predicating a fast belt with thin bagasse layer. But
dwell time within the magnetic field is as important.
giving a confticting need for a slow speed. A com­
promise is therefore needed and belt speeds 01' 1.3 to
1.6 m/s can usually be used.
The overhead suspended magnets are fairly ef­
fective on well shredded cane and on bagasse. but
Masoll and Reicllllrd (1983) reported that they
did not provide satisfactory separation on coarse
(knifed) cane ahead of the shredder. Particularly on
this application. a much hetter alternative configura­
tion is to capture the tramp oU! 01' the ftow 01' bagasse
at a transfer point. This system can be very effective
on coarse or fine product, but:
It requires a magnet with a different shape 01'
magnetic field and a ledge under which the cap­
tured tramp is held until clearing;
The position and angle 01' the magnet are critical
to its performance (for details, see Masoll and
Reichard 1983).
11 is recommended that reputable magnet suppliers be
consulted for advice on any particular application.
I[ffi:--í-
\1
Feed'in-ramp
! Conveyor deck
Bagasse sampling
Direction of bagasse
conveyor ~
Side view
Plan view
7.2.4
Feeding the boi ler chutes using the arrange­
ment shown in Figure. 7.11. A feed-in ramp
upstream 01' the mouth 01' the chute allows the
bagasse to fali by approximately 150 mm before
the olTtake. The downstream cutoff is by a dou­
hle-angled plate that knifes 01'1' the bagasse and
prevents shocks on the slats. II'chute isolation is
needed. cutoff tines can be inserted through the
chute below the gate.
11 is not feasible to sample bagasse continuously
and catch samples must be taken on a routine basis.
preferahly every hour. A full width hatch sampler
(ICUMSA Method GS 5-5 [Anon.20051) should
be used when sampling fram a slat conveyor, taking
enough sample to allow the fall-Ollt 01' a complete
slat load 01' bagasse. Alternatively a full width sample
should be taken from a belt conveyor using a swing
sampler at a transfer point. sampling the full depth 01'
hagasse while in free falI. The principies involved are
much the same as those for prepared cane sampling
outlined in Section 2.3.2. Precautions to be taken
are outlined in the South African lab()ratory manual
(SASTA 2005). Good sub-sampling and prevention
01' evaporation from the sample are important.
Reference.\ 1'. /88
188
References
7 Mill and bagasse conveyors
"111
Anon. (2005): ICUMSA Mcthods Book. Vcrlag Dr. A. Bar1cns.
Bcrlin.
Daw.\'(!Il M.\v.; Sm)'/ile L.D.; Scott J.G.; Sutiler/al/d CJ. (1995):
Developments in bagasse spore detection methods. Proc. AlIst.
Soc. Sugar Canc Tcchnol. 17.279-285.
HIlIiO/ E. (1986): Handbook 01' Canc Sugar Enginccring. 3rd cd.
Elsevier, Arnsterdam.
Ma.\'(!Il v.; Reicl/(/rd S.R. (1983): An investigation into tramp iron
detection and separation. Proc. lot. Soe. Sugar Canc Tcchnol.
18.966-979.
Moor H.S/C (2000): Belt vs. chain-slat hagasse conveyors for hoi­
ler feeding. Proc. S. Afr. Sugar Technol. Ass. 74. 285-289.
Mllrr)' CR. (1960): The pressure required to feed cane mills. Par1 I
- thcorctical considerations. Inl. Sugar J. 62. 346-349.
SASTA (2005): SASTA Laboratory Manual. 4th ed. S. Mr. Sugar
Tcch. Ass. CD-ROM