Cane Preparation Equipment Cane Preparation Equipment Installed Power Specific Power Tip Speed Tip Clearance [kW/tfh] [m/s] [mm]
Leveller knives
6
50
1000
First knives
15
60
150
Second knives (heavy duty)
30
60
50
Shredder
60
100
Total
111
Southern African industry average 84
Installed Specific Power for Milling Table of required installed power for a milling tandem t andem Number of Mills
Specific Power per Mill [kW/tfh]
Four mills
22
Five mills
20
Six mills
18
Diffuser + two mills
25
Mill Capacity Calculations There are a large number of formulae for the calculation of the capacity of a milling tandem. Hugot gives the following formula:
A = 0.9 c·n·√N·(1 -0.06·n·D)·L·D2/f where
c is a factor dependent on the cane preparation equipment, c = 1.3 if the tandem is preceded by a shredder n is the mill speed in rev/min t andem N is the number of rollers in the tandem L is the length of the roll in metres D is the mean diameter of the rollers in metres f is is the fibre percent cane
The problem with M. Hugot's equation is that is a function of the square of the mill speed, which means mathematically that the mill capacity will increase with speed, reach a
maximum and then deacrease, as the speed increases. This could be interpreted in an engineering sense that the mill will have an optimum speed to operate at beyond which the capacity deacreses due to slippage of bagasse in the mill. It does not seem prudent to design the mills so that the capacity could be in the decreasing region.
Mill and Trash Plate Setting Setting a mill includes the calculation of the openings between the various mill rolls and well as the shape and position of the trashplate. The work openings are calculated first. The work openings are the gaps between the top roll and the feed roll on the one hand and the opening between the top roll and the discharge roll on the other, when the mill is in operation. The next step is to calculate the set openings, that is, what the gaps should be when the mill is empty. The positions of of the mill rolls and the trashplate are adjusted until the set openings are achieved.
Geometry of Mills
Tooth Profile
Mill Geometry Parameters Top roll mean diameter [mm]
MDT
Discharge roll mean diameter [mm]
MDD
Feed roll mean diameter [mm]
MDF
Tooth Pitch [mm]
TP
Tooth Flat [mm]
Tfl
Tooth Angle [°]
Tang
Tooth Depth [mm]
Tdepth = (TP - T fl) / (2 · tan(Tang / 2))
Roll Length [mm]
lroll
Vertical distance between top and side roll centres at rest [mm] Vrest
Mill Operating Parameters Cane throughput [ton cane/h]
tch
Fibre content of cane as a percentage
f%c
Mill lift [mm]
l
Fibre throughput[kg/min]
fibrethput = tch · 1000/60 · f%c 3
Fibre fill in the discharge opening [kg/m ]
ff D
3
Fibre fill in the feed opening [kg/m ]
ff F = ff D / millratio
Speed of top roll [rpm]
n
Ratio of feed opening to discharge opening in the working position millratio
Calculations Average peripheral velocity of top/feed roll s [mm/min]
vTF = 2 · π · n · 0.5 ·(MDT + MDF) / 2
Average peripheral velocity of top/discharge rolls [mm/min]
vTD = 2 · π · n · 0.5 ·(MDT + MDD / 2
3
Escribed volume in the discharge opening [m /min]
volEscrD = fibre thput / ff D
Discharge Work Opening [mm]
woD = vol EscrD/ (vTD · lroll)/1000 3
Escribed volume in the feed opening [m /min]
volescrF = fibrethput / ff F
Feed Work Opening [mm]
woF = volEscrF/ (vTF · lroll)/1000
Top - Feed roll Centres (Working) [mm]
TF = MD T / 2 + MD F / 2 + wo F
Top - Discharge roll Centres (Working) [mm]
TD = MD T / 2 + MD D / 2 + woD
Horizontal distance between top roll and feed roll centres [mm]
2
2
HF = √(TF - (Vrest + l) )
Horizontal distance between top roll and discharge roll centres [mm] Set feed opening (Tip to Bottom) [mm]
2
2
HD = √(TD - (Vrest + l) ) 2
2
soF = √(HF + Vrest ) - MDT / 2 + MD F / 2 2
Set discharge opening (Tip to Bottom) [mm]
2
soD = √(HD + Vrest ) - MDT / 2 + MDD / 2
Trash Plate Settings There are a number of methods of setting out a trashplate for a mill, these are discussed by Hugot, Handbook of Cane Sugar Engineering , Jenkins, Introduction to Cane Sugar Technology, 1966 and Maxwell, Modern Milling of Sugar Cane, 1932. In addition a number of papers discussing this topic have been published among them Ashe, GG, SASTA, 1963 and Van Hengel, A and Douwes Dekker, K, SASTA, 1958. Each factory that I have knowledge of has their own idiosyncratic method of laying out a trashplate, but when analysed all these methods amount to much the same thing. Hugot notes that the ideal shape of a trashplate is the logarithmic spiral, and points out that the simplest approximation to this is an arc whose center is offset from the centre point (in the working position) of the top roll along a horizontal line towards the discharge roll. The logarithmic spiral can be calculated in a spreadsheet and then set out in a cad drawing, the approximate arc can then fitted to the logarithmic spiral
Mill Pinions or Crown Wheels There are normally three mill pinions on a cane sugar mill; o ne on each of the mill rolls, namely
Top roll Feed roll Discharge roll
In mill with and underfeed roll (also known as a the fourth roll) there is often a additional pair of mill pinions. The underfeed roll is driven by a pinion mounted on the non-drive end (also called the pintle end) of the top roll. Whereas the mill crown wheels all have an equal number of teeth the gears driving the underfeed roll are o ften speed reducing. the purpose of the mill pinions is to t ransmit torque from the top roll to the other mill rolls. The feed and discharge rolls normally run at the same r otational speed as the top roll. Unlike normal gears in a normal gearbox, mill pinions need to able to accomodate a changing centre distance. This is for three reasons
The top roll floats, so the centre distance changes from second to second
The fibre content of the cane changes and to accomodate this, the mill rolls centre distance is checked and adjusted weekly. The mill rolls wear and to accomodate this wear the centre distcance will change from season to season.
The change in operating centre distance means the pinion tooth profiles need to be sufficiently flexible to accomodate these changes. Contrary to what has been written in Dr Rein's book, Cane Sugar Engineering by Bruce St C Moor it is essential that the tooth profile is involute, no other tooth profile can accomodate a changing centre distance. What is c lear though, is that a mill pinion tooth profile can not be in accordance wit h the two current AGMA tooth profile standards. The AGMA standards are based on fixed centre distances. AGMA Standard Gears and Mill Pinions Parameter
AGMA 20° and 25°
Mill Pinion
Tooth Profile
Involute
Involute
Pressure Angle
20° or 25°
16° to 20°
Addendum
1/P
1.1/P
Dedendum
1.25/P
1.5/P to 1.9/P
Fillet Radius
0.3/P
0.6/P to 0.75/P
Circular Tooth Thickness
0.5π/P
0.4π/P to 0.45π/P
While the tooth profile of a mill pinion is an involut e curve, this fact is not helpful to th pattern maker in the foundry where the pinion will be cast. The involute curve is approximated by two arcs: one from the base circle to t he pitch circle, the second arc extends from the pitch circle to the start of the tip radius of the tooth.
Mill Bearings Bearing Pressures The maximum pressure that a bearing can withstand is mainly a function of the bearing material. The bronzes that are common in sugar mills have a recommended maximum bearing pressures of up to 100 MPa for phosphor bronze and 50 MPa for tin-bronzes. Standard sugar mill practise limits t he bearing pressure to about 10 MPa.
Materials For Plain Bearings The two essential elements in a plain bearing are the bearing or bearing material itself, and the shaft or moving member. The bearing or bearing material is located in a housing or structure, and may or may not be integral with it. Separating these t wo elements is the lubricant, introduced, generally in the case of sugar mills, by external pressure feeding. The material of the shaft or journal is established from considerations of strength and rigidity, and will invariably be steel.
Because the conditions under which bearings must operate in service ma y vary over a wide range, it is necessary that bearing materials be used which have certain desirable properties. Amongst these we must include such factors as
mechanical strength; softness and low melting point; low modulus of elasticity; corrosion resistance; high thermal conductivity; and of course, economic considerations.
Since these factors cannot all be obtained to a desirable degree in a single material, it is necessary in practice to make a compromise.
The most common bearing materials consist of a. white metals, b. copperbase alloys, and c. aluminium-base alloys.
White Metal
White metals is a term used to include the tin and lead-base metals, broadly referred to as Babbitts (after Isaac Babbitt, 1839), and since such metals are highly competitive, they are recommended for most applications where the loading is not severe. Babbitt bearings are manufactured with the white metal lined onto steel, cast iron and copper base alloys. Since white metal suffers a reduction in fatigue strength with increase in temperature, and this reduction is a function of thickness, it is usual to limit the thickness to between about 0.1000.175 mm, and thicknesses of only 0.025-0.050 mm are used with copperlead over the backup material. White metal is not commonly used as a sgar mill bearing material Copper-base Alloys
Copper-base alloys including lead-bronze, gun-metal and phosphor-bronze are widely used as bearing materials. Lead-bronze is the cheapest, and is used for general service bearings. It has a low tendency to seizure, in common with the white metal bearings, and has great er fatigue strength to withstand higher temperatures. Lead bronze bushes are frequently used in the for m of single, solid units, i.e. as bushes without the supporting shell surrounding the bearing mater ial, as is required of the Babbitt or white metal bearing materials. Gun-metal provides a relatively cheap and easy to machine mater ial, having good bearing properties and capable of withstanding somewhat higher loads than the lead-bronze alloys. This alloy also has good resistance to corrosion in sea wat er. Phosphor-bronze is used for heavily loaded bearings, where high frictional stresses are likely to occur. Because of the high hardness of this mater ial, it demands the use of a harde ned steel journal.
Typical Sugar Mill Bearings Rein in Cane Sugar Engineering states that typically sugar mill bearings are tin bronzes with the following composition Cu 84% Sn 10% Pb 3% Zn 3%
Lubrication Sugar mill shafts do not turn sufficiently fast for a hydrodynamic film of lubricant to be formed between the journal and the bearing. Consequently hydrostatic lubrication is required. This is achieved by supplying lubricant to the bearing under pressure. Under these conditions, attention must be given to the adequate supply of lubricant at all t imes, and in particular to the location of oil supply holes and grooves. Bitumin based lubricants are often used in sugar mill bearings.
Bearing Loads and Sizes Specific roll loads are in the range of 2 to 3 MN per square metre of projected roll area. This together with the allowable bearing pressure mentioned above indicates t hat the total bearing area should be about 20% to 30% of the projected roll area It is usual practise to allow the top roll of a sugar mill to float in the vertical direction to:
keep a nearly constant pressure on the mat of bagasse in the mill allow some throughput variation without sacrifici ng extraction protect the mill from damage from tramp iron
Typically hydraulic rams together with a gas accumulator provide the downward force on the bearing caps to resist the upward force of the bagasse on the mill roll. The gas accumulator acts as an air spring. The hydraulic oil in the system is not compressible, but the gas in the accumulator is and it is this gas that has the give that allows the roll to float. The gas in the accumulator is precharged with a particular gas pressure. The higher the precharge pressure the softer the spring rate. A low precharge pressure will make the system very stiff and may not allow sufficient float to let tramp iron through the mill, which may cause damage. A high precharge pressure will make the system very soft and the top roll bearing may continually rise up to its maximum lift. This means the mill headstock may be subjected to very high forces, not anticipated in design. The correct precharge pressure which ensures that the top roll floats about its design position is important to ensure good extraction and to protect t he mill from damage
Sugar Mill Lubricants Castrol SMR Grades Castrol SMR lubricants are especially formulated for sugar mill roll bearings and gearboxes. They are viscous black oils fortified with load bearing additives and incorporate emulsifiers to resist the harmful effects of the inevitable contamination with sugar juices encountered in use. They also find use in other heavily loaded open gears and pinions. These grades are now lead free. SMR MEDIUM SMR HEAVY CLEAR ASMR MEDIUM* ASMR HEAVY*
Density @ 20°C
0,949
0,914
0,952
0,995
Viscosity @ 40°C (mm /s) 1205
1925
1 228
11450
Viscosity @ 100°C (mm /s) 50,5
126,0
50,5
167,0
VIE
84
160
83
74
Colour
Black
Red/Green
Black
Black
Pour Point (°C)
0
6
0
+12
Flash Point CCC (°C)
250
212
254
256
Bitumen
Yes
Nil
Yes
Yes
Compounding
Yes
Yes
Yes
Yes
Yes
Yes
Yes
2
2
EP Additives
Bagasse Calorific Value Gross calorific value, also known as the higher ca lorific value (HCV) of bagasse, is calculated from the following formula: HCV=[19 605 - 196,05(moisture % sample) - 196,05(ash % sample) - 31,14(brix % sample)]kJ.kg-1 The net calorific value, also known as the lower calorific value (LCV), assumes that the water formed by combustion and also the water of constitution of the fuel re mains in vapour form. In industrial practice it is not practicable to reduce the temperature of the combustion products below dew point to condense the moisture present and recover its latent heat, thus the latent heat of the vapour is not available for heating purposes and must be subtracted from
the HCV. By ASTM standards the HCV is calculated at atmospheric pressure and at 20°C. LCV of bagasse is calculated by the formula: LCV=[18 309 - 207,6 (moisture % sample) - 196,05 (ash % sample) - 31,14 (brix % sample)] kJ.kg-1 Do online calculations of HCV and LCV. Select the parameter to be used as the graph's xaxis by clicking the appropriate radio button
