Mill Training Course

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A TRAINING COURSE ON THE PRINCIPLES OF SUGAR MILLING 1.

INTRODUCTION The purpose of this course is to discuss the basic principles of operation of five and six roll sugar mills. Emphasis will be placed on the basic calculations that need to be carried out to establish the settings that should be used between rolls and for chutes to get the most effective performance from a milling unit. To enable calculations to be carried out it is important that we have clear definitions of the geometric and operating terms that are commonly used.

2.

DEFINITIONS OF GEOMETRIC TERMS 

MEAN DIAMETER OF ROLLER

(Refer Figure 1)

This is the diameter at the midpoint of the grooves in the roller: Mean Diameter = Outside Diameter - Depth of Groove 

WORK OPENING WO, FOR FEED, DELIVERY AND PRESSURE FEEDER (Refer Figure 2) This is the distance between the mean diameter of each pair of rollers. This includes any operations lift for mills which are hydraulically loaded Wof = feed work opening Wod = delivery work opening Wopf = pressure feeder work opening



SET OPENING So FOR FEED, DELIVERY AND PRESSURE FEEDER (Refer Figure 2) This is the distance between the tips of the teeth on each pair of rolls.

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The value can be positive or negative depending on whether the tips are open or in mesh. So = Wo - lift - half the depth of the grooving on each roll 

WORK OPENING OF TRASH PLATE Wotp This is the distance between the mean diameter of the top roll and the top of the trash plate, at the mid point. This includes any operational lift of the top roll.



MILL RATIO is the ratio of the feed work opening to the delivery work opening Wof/Wod.



VOLUMETRIC RATIO is the ratio of the work opening of the pressure feeder to the work opening of the feed roll multiplied by the ratio of the surface speed of the pressure feeder to the surface speed of the top roll. Volumetric Ratio =





Wopf x Spf Wof Sf

PRESSURE FEEDER CHUTE OPENINGS

(Refer Figure 3)

Inlet -

The distance between the top and bottom plates of the pressure chute measured at the projection point of intersection with the bottom of the grooves in the top and bottom pressure feeder rollers.

Outlet -

The distance between the top and bottom plates of the pressure feeder chute measured at the end of the chute.

CONTACT ANGLES The feed chute contact angle is the angle between the top and bottom pressure feeder rolls and exit of the feed chute.

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The pressure feeder chute inlet contact angle is the angle measured between the pressure feeder roll centres and the projected point of contact of the nose of the feeder chute with the bottom of the groove. The pressure feeder outlet contact angle (Figure 3) is the angle measured between the centre line through the top and feed rollers and the projected point of contact between the top of the trash plate and the mean diameter of the teeth on the delivery roll. The trash plate contact angle on the delivery roll is the angle measured between the centre line through the top and delivery roll and the projected point of contact between the top of the trash plate and the tip of the teeth on the delivery roll. This is illustrated in Figure 4. It is calculated using the formula: Cos Ø =

A2 + C2 - B2 2AC

A=

Dtop + Ddel + Wo d 2 2

B=

Dtop + Wo + ½ trashplate divergence 2

C=

ØDdel 2

Note: D mean is used for the calculation of A, B and C. 

ROLL SURFACE SPEED is calculated at the mean diameter of the roll. S= Where S =

Π Dm N 60

roll surface speed Dm = mean roll diameter

m/s m

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N= 3.

roll shaft speed

rev/min

MILLING PARAMETERS 

ESCRIBED VOLUME The escribed volume is the volume passing a given section of a mill in a unit of time. For the pair of rolls shown in Figure 5, the escribed volume at the nip Ve = A x L x S. A= B=

opening between rollers roll length

S =roll surface speed Ve = escribed volume

(m) (m) (m/s) (m3/s)

At a chute entry or exit the speed of the blanket that needs to be considered is the horizontal component of the roll surface speed which is the speed at which there would be no slip. For the chute configuration shown in Figure 3 the escribed volume at chute entry or exit is given by Ve = Where S =

S Cos Ø h L

speed of roll (m/s) Ø = contact angle (deg) h = height of blanket (m) L =length of roll (m) Ve = escribed volume, m3/s

At a trash plate the escribed volume is calculated using the mean setting of the plate plus half the groove depth plus lift* as the work opening, and the speed of the top roll at mean diameter. (* not applicable at NAT&L – fixed top rolls) At the entry or exit to a chute the speed of the bagasse blanket at contact is the component of the peripheral speed of the roll in the direction of the chute. This is illustrated in Figure 6.

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

COMPACTION Compaction is the basic way of measuring fibre density at any point in the milling train and is perhaps the most important parameter in reviewing how hard a mill is working. Compaction is calculated as Where

Comp = Qf/Ve Comp = compaction Qf = mass rate of fibre Ve = escribed volume

kg fibre/m3 kg/s m3/sec

Fibre rate is the basic parameter which remains constant through the milling train and is determined as -

Where

Qf =

(Qc x f)/360

Qf = Qc = f=

fibre rate kg/sec crushing rate tonne/hr per cent fibre in cane

It is important to note that compaction does not take into consideration the juice in cane or bagasse. Mill ratio and volumetric ratio have previously been defined in terms of work opening and speeds. They are readily defined in terms of compaction and this is the manner in which practising engineers usually calculate them.



Volumetric Ratio =

Compaction at feed roll Compaction at Pressure Feeder

Mill Ratio =

Compaction at delivery roll Compaction at feed roll

FILLING RATIO Filling ratio is a non-dimensional expression of fibre rating and is the ratio of compaction of fibre at a point to the no void density of fibre. This is in effect the proportion of an opening that is filled by fibre. Whilst it has been used extensively in research studies it is not commonly used in everyday

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milling studies. The no void density of fibre has been established by University studies to be 1530 kg/m3. Filling ratio is determined as Cf = Comp 1530 

PREPARATION High levels of cane preparation are a pre-requisite of good milling performance. Preparation can be measured in several ways, by bulk density, mean particle thickness, and by the percentage of cells that have been broken. Whilst all of these have been used in the past, the Pol in Open Cell (POC) measurement is now the one most used. Its measurement involves the mixing of a sub-sample with water, rotating in a drum for a fixed time, and measuring the pol of the extract. A sub-sample is also disintegrated with a fixed quantity of water and the pol of extract determined. The POC is determined by rationing the pol’s of the extracts obtained. The method is described in the BSES Methods Manual.



REABSORPTION When the no void value of bagasse exiting the delivery roll is compared to the delivery roll escribed volume it is found that the value is always greater than unity. This ratio is called the reabsorption factor. The phenomenon by which this occurs is known as reabsorption. There is debate as to the mechanism of reabsorption - forward slip of bagasse through he nip or internal shear, but the reabsorption factor can be used as a measure of milling performance. The lower the value the better the mill performance. The calculation of reabsorption factor is outlined in Appendix 1.

4.

CALCULATION OF OPERATIONAL SETTINGS FOR A MILL 

COMPACTION AT THE DELIVERY NIP Compaction at the delivery nip is the basic consideration when setting up a milling train. The selection of levels of compaction depends on several factors - the strength of mill rolls, the available torque, the degree of preparation of the cane, the variety of cane and the surface roughness of the

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roll. When extraction is plotted against compaction it can be seen that extraction initially increases with compaction but tends to flatten out at higher levels - typically 550 - 600 kg/m 3 for a No. 1 Mill and 880 to 930 kg/m3 for a final mill. The limit to achievable compaction depends on how well surface roughness of the rolls is maintained so that the increase in reabsorption does not overcome the beneficial effects of increase in compaction. For reasonable levels of preparation 85 POC the compactions in Table 1 would be considered typical at the delivery nip. Mill Number

4 Mill Tandem Delivery Roll Compaction kgf/m3

1 2 3 4

520 - 560 620 - 660 720 - 770 820 - 880 Table 1 - Range of Typical Delivery Roll Compactions

These can be modified to allow for limitations in power or roll or gearing strength. 

COMPACTION AND CONTACT ANGLES FOR THE TRASH PLATE The trash plate is a vital component of the three roll mill. Its effectiveness depends on its ability to give good drainage and deliver a good feeding blanket of material into the delivery squeeze. Most of the operational data available on trash plates is empirical and based on practical engineering experience. A high compaction allows a low contact angle to be maintained but can inhibit drainage. A high contact angle increases the requirements for a positive feeding force to the delivery nip to get the bagasse ‘up the hill’. The compactions and contact angles in Table 2 can be used as a guide for good operational performance. Trash plates are set up with divergence to assist smooth movement and gradual release of pressure across the plate. A typical divergence is 18 - 20 mm.

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Mill Number

4 Mill Tandem Trash Plate Compaction at Mid Point (kgf/m3)

Max. Contact Angle (o)

1 2 3 4

200 225 250 280

33 33 33 30

Table 2 - Typical Trash Plate Compactions and Contact Angles

There will be occasions where due to speed limitations that there may be a desire to consider increasing the recommended angles. Should this be done the need to maintain good surface roughness is essential. Trash plate contact angles above 36 degrees should be attempted only by the brave! 

COMPACTION AT THE FEED ROLL For a given delivery squeeze the top roll load in a three roll mill is governed by the mill ratio since this in turn governs the squeeze at the feed roll and the force between feed and top roll. Where strength and torque are adequate the lower the mill ratio within reasons the lower will be the bagasse moisture since this reduces the drainage duty on the delivery roll. For mills equipped with heavy duty feeders a design mill ratio of 1.6 to 1.8 would be considered normal and for light duty feeders a range of 1.9 to 2.3 would be considered normal. In practice the lowest ratio consistent with stable operation should be used.

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5.

PRESSURE FEEDERS Pressure feeders are fitted to mills to enhance both the feeding capacity and extraction performance specially at higher crushing rates. They have been classified historically into two types - heavy duty and light duty. 

HEAVY DUTY PRESSURE FEEDERS - These were developed in the early 1950's and consist of a pair of rolls of similar construction to mill rolls driven by gearing integral with the main mill gearing and attached with a robust chute to convey bagasse from the discharge of the pressure feeder nip to the feed rolls of the mill. They are designed to express significant quantities of juice at the nip.



LIGHT DUTY PRESSURE FEEDERS - Were developed in the early 1960's initially to enhance throughput of existing three rolls mills. They consisted of two hollow rolls in a relatively light frame with roller bearings and driven by simplex roller chain. As initially designed they did not express juice although most remaining units have been strengthened and do express some juice.



Both the heavy duty and light duty feeders were driven in a constant ratio to the mill. In more recent times the introduction of more appropriate drives has led to suitable independently driven pressure feeders of heavy duty construction being installed.



DESIGN FACTORS FOR HEAVY DUTY PRESSURE FEEDERS The basic design parameters as set by manufacturers of heavy duty pressure feeders are: ·

The setting of the nose scrapers should be at least 50 mm greater than the distance between the bottom of the grooving of the pressure feeder rolls. This equates to contact angles of typically 18 to 22 degrees.

·

The divergence of the pressure feeder chute should be at least 3.5 o (60 mm/m).

·

The chute contact angle on the delivery roll should not exceed 40o.

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·

The longitudinal centre line of the chute should be as symmetrical as possible with each pair of rolls.

As well as geometric considerations practical operating experience has highlighted the importance of a gradual fall off in the level of compaction between the entry and exit of the chute as a build up in compaction can lead to over pressurisation and ultimate failure of the chute. A reduction of 8 kgf/m3 across the chutes is considered desirable. The following compactions in Table 3 could be considered as a guide for a four mill tandem.

Mill Number

PF Chute Inlet Compaction kgf/m3

1 2 3 4

130 - 150 140 - 160 150 - 170 160 – 180

Figure 3 - Typical Pressure Feeder Chute Inlet Compactions for Heavy Duty Feeders

To comply with the guidelines above there will sometimes be some conflict. The consideration of divergence and contact angle should be given highest priority followed by the reduction in compaction along the chute. 

DESIGN FACTORS FOR LIGHT DUTY FEEDERS The design factors for light duty feeders whilst they follow similar principles as those for heavy duty pressure feeders are more flexible as compactions are lighter. It is possible to run light duty feeders with contact angles up to 45o. Mills filled with light duty feeders are set to operate with wider mill ratios than with heavy duty feeders as a wetter material is presented to the feed roller.

In order to get greater capacity, feeder rolls can be run faster and this can lead to reverse compactions in the chutes. Some degree of reverse compaction is tolerable provided that the compaction at chute exit is always _________________________________________________________________________

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less than at the pressure feeder nip: Reverse compactions up tp 30 kgf/m 3 can be tolerated but these can lead to chain breakages. The compactions in Table 4 can be considered as a guide for light duty feeders Mill Number

Fibre Compaction kgf/m3 Inlet

2 3 4+

110 - 130 120 - 140 130 – 150

Table 4 - Typical Light Duty Pressure Feeder Compactions

· INDEPENDENTLY DRIVEN PRESSURE FEEDERS The introduction of independent drives for heavy duty pressure feeders has been made possible by the developments of high torque hydraulic and electric drives. Whilst basic design principles remain the same the flexibility of relative speed variation between mill and pressure feeder enables a greater level of control for variations in fibre characteristics. 6.

GROOVING Good feedability at a mill requires an adequate grip on the blanket and a good coefficient of friction between the roll and bagasse. Grooving provides this as well as providing good drainage of juice when it is expressed from the blanket. The feedability provided by the grooving is enhanced by artificial roughening of the tips of the teeth by use of hard facing electrodes. Drainage is enhanced by the use of juice grooves machined into the bottom of the teeth especially in rolls that have the higher juice loadings such as pressure feeders and feed rolls. There is little absolute data on the relative performance of different grooving and most data is based on practical observations and relative performance data. Normal mill grooving has a 35o angle with a 35 mm pitch for Mills 1 and 2 and 25 mm pitch for later mills. Pressure feeder grooving is typically 40 o angle for a No 1

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Mill and 35o for later mills, with pitches to 50 mm. There is significant experimental evidence to illustrate that finer pitch gives better moistures at lower rates but pitches of less than 25 mm are not favoured due to the significant efforts that have to be put into artificial roughening. 7.

MILL FEEDING 

OPTIMUM FEED DEPTH The ability of a mill to feed is a dominant consideration in the operation of a mill. The amount of material that can be fed into a mill is the product of the escribed volume at the entry to a pair of rolls and the density of that material. The rate is calculated on the assumption of no slip between the bagasse blanket and the roll or within the bagasse blanket. Consider the geometry of the feeder entry without an underfeed roll (Figure 7), As a blanket of thickness ‘h’ approaches a pair or rolls its speed is governed by its thickness since at any point on the feed arc the approach velocity can not be greater than the resolved component of the peripheral speed in the normal direction to the line joining the roll centres. This is V Cos Ø where Ø is the initial contact angle. The volume of material per unit time per unit roll width is equal to the product V Cos Ø d and since any increase in d will produce, a decrease in Cos Ø it is apparent that a point could be reached beyond which Cos Ø may be decreasing at a faster rate than d is increasing. At this point V Cos Ø d will cease to increase and begin to decrease the value of d corresponding to this point will be an optimum. It can be shown mathematically that optimum feed depth is ½ (Wo + D) where Wo is the work opening. There is no point in attempting to run with a feed depth greater than this optimum.



FEED FIBRE COMPACTIONS AT THE FEEDER MOUTH OR FEED CHUTE EXIT The level of compaction at the exit of a feed chute has a dominant influence on the potential performance of a milling unit. For vertical or non vertical chutes the compaction that can be achieved depends largely on gravity and thus depends on the height of the chute. As chute heights increase the

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magnitude of these frictional forces increase such that from a practical point of view there is little benefit in chute heights above three metres. Other factors that influence compaction are the level of preparation and the position of the mill in the train. Figure 8 shows a general relationship that exists between chute height, compaction and position of the mill in the train. It must be emphasised that the relationship illustrated is a generic one and can vary significantly from factory to factory and from season to season. By plotting a mill's calculated compaction however, against this template it is possible to monitor closely performance and help identify problems. Figure 9 shows the relationship between feed chute setting and crushing rate for a No. 1 Mill at two levels of feed chute compaction and clearly illustrates the importance of obtaining a good compaction. It is important to note that in order to control the mill the operating setting for the chute must be less than the optimum setting. Figure 9 was developed for a 2140 mm long No 1 Mill operating at 230 mm/sec with a fibre per cent cane of 15 per cent. Optimum chute setting was 624 mm 8.

UNDERFEED ROLL CONFIGURATIONS AND FEED CHUTES In the evolution of milling to achieve higher capacities an 'underfeed' roller was added to the three roll mill to assist in turning the blanket to achieve a deeper feed to the mill. Most pressure fed mills are now fitted with an underfeed roll to enhance feeding capacity still further. Most feed chutes are near vertical - typically 85o and the underfeed roll provides a 'live' surface to turn the blanket to be symmetrical to the pressure feeder nip., Because of the larger centre distance of the pressure feeder roll and underfeed roll compared to the top and bottom pressure feeder rolls this configuration provides an enhanced optimum feed depth so that the material fed to the pressure feeder can be increased without an increase in roll speed. Whereas with small underfeed rolls it was common to operate at a higher peripheral speed than the top pressure feeder roll, it has been established best performance is achieved with underfeed rolls of approximately the same diameter as the top pressure feeders roll.

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A typical relationship between the optimum setting of the underfeed roll as a function of pressure feeder roll setting is shown in Figure 10. The relationship between the compaction at the entry to the pressure feeder to the compaction in the feed hopper is called the "effectiveness". A detailed discussion of the estimation of effectiveness is beyond the scope of this course. Effectiveness can vary from 1.0 where no underfeed roll is fitted to 1.7 where optimum settings are used on underfeed rolls of equal diameter to the top pressure feeder roll. 9.

CALCULATION OF COMPACTIONS THROUGH THE MILLING TRAIN When analysing the performance of a milling train or determining the settings for a new mill or new operating conditions it is necessary to calculate the basic parameters for mill performance such as compaction, contact angles and chute divergences. Determining the settings the first time involves an interactive approach to get the best results, whereas checking the settings of an existing mill can be done directly. Because the calculations used in both cases are the same we will use as an example here, the calculations involved in checking the settings of a No. 1 Mill. EXAMPLE DESIGN CONDITIONS - NO 1 MILL Given the mill data in Table 5 calculate the compactions and contact angles throughout the mill. The exercise will be restricted to the first mill only. However, for completeness, data is supplied for the four mills in a train.

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INPUT DATA CRUSHING RATE (T/H) FIBRE % CANE MILL SPEED (RPM) RATIO PF: TOP RATIO UNF: PF OD. PFA. (MM) OD. PFB (MM) OD. TOP (MM) OD. FEED (MM) OD. DEL (MM) OD. UNF (MM) MILL ROLL LENGTH (MM) DEEP GROOVE PFA (MM) DEEP GROOVE PFB (MM) DEEP GROOVE TOP (MM) DEEP GROOVE FEED (MM) DEEP GROOVE DEL (MM) DEEP GROOVE UNF (MM) TRASH PLATE DIVERGENT (MM) TOP ROLL LIFT (MM) SETTINGS (MM) DEL SETTING FEED SETTING PF SEEDING PF CHUTE INLET SETTING PF CHUTE OUTLET SETTING TRASH PLT SET TRASH PLT TOE SET TRASH PLT HEEL SET UNF SETTING @FEED CHUTE AT BASE RATIOS MILL RATIO VOLUMETRIC RATIO PLT WO/FEED WO COMPACTIONS (KG/M 3) DEL FEED PF PF CHUTE IN PF CHUTE OUT TRASH PLT COMP PF CHUTE CHANGE ANGLES

#1 320.00 12.9 2.7 1.1 1.13 1067.00 1067.00 1039.00 1043.00 1050.00 1000.00 2140.00 35.00 35.00 35.00 35.00 35.00 35.00 20.00 0.00

#2 320.00 12.90 2.52 1.10 1.13 1067.00 1067.00 1043.00 1048.00 1056.00 1000.00 2140.00 35.00 35.00 35.00 35.00 35.00 35.00 20.00 0.00

#3 320.00 12.9 2.41 1.10 1.13 1067.00 1067.00 1044.00 1053.00 1051.00 1000.00 2140.00 35.00 35.00 35.00 35.00 35.00 35.00 20.00 0.00

#4 320.00 12.9 2.30 1.10 1.13 1067.00 1067.00 1049.00 1052.00 1042.00 1000.00 2140.00 35.00 35.00 35.00 35.00 35.00 35.00 20.00 0.00

37 81 142 267 395 157 147 167 488 761

29 67 127 250 371 143 133 153 464 749

23 55 113 240 351 131 121 141 439 736

17 47 107 242 348 123 113 133 429 731

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PF CHUTE INLET PF CHUTE OUTLET TRASH PLT

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10.

CALCULATION OF MILLING PARAMETERS 

FIBRE RATE The determination of fibre rate is basic to all milling parameters as it is constant through the milling train. Fibre rate =



320 x 12.9 360 = 11.47 kg/sec

CALCULATION OF DELIVERY COMPACTION -

Mean Diameter Top Roll

= =

1039 - 35 1004 mm

-

Mean Diameter Delivery Roll

= =

1050 - 35 1015 mm

-

Mean Diameter Feed Roll

= =

1043 - 35 1008 mm

-

Top Roll Speed S

= =

3.14 x 1.004 x 2.7 60 .142 m/sec

-

Delivery Work Opening Wod

= =

37 + 35 72 mm

-

Roll Length L

=

2140 mm

-

Escribed Volume

= = =

S L Wod .124 x 2.14 x .072 .02188 m3/sec

-

Compaction

=

11.47 .02188 524 kg/m3

=

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

CALCULATION OF FEED COMPACTION -

Feed Work Opening

Wof

= =

81 + 35 116 mm

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

-

Escribed Volume

= = =

-

Compaction

=

-

Mean Diameter Top PF roll

= =

-

Mean Diameter Bottom PF Roll = 1067 - 35 = 1032 mm

-

PF Roll Speed Spf

11.47 .0352 = 325 kg/m3 CALCULATION OF PRESSURE FEEDER COMPACTION

-

PF Work Opening Wopf

-

Escribed Volume

-

Compaction

= =

=

1067 - 35 1032 mm

=

Π Dm n 3.14 x 1.032 x 1.1 x 2.7 60 .160 m/sec

= =

142 + 35 177 mm

S L Wopf = .160 x 2.14 x .177 = .0606 m3/sec = =



S. L. Wof .142 x 2.14 x .116 .0352 m3/sec

11.47 .06 06 189 kg/m3

CALCULATION OF MILL AND VOLUMETRIC RATIOS -

Mill Ratio

= = =

Delivery Compaction Feed Compaction 524 325 1.61

-

Volumetric Ratio

= = =

Feed Compaction PF Compaction 325 189 1.72



CALCULATION OF TRASH PLATE COMPACTION -

Trash Plate setting

=

-

Trash Plate Work Opening

=

157 + 35/2 = 174 mm

-

Escribed Volume

=

S L Wotp = 1.42 x2.14 x .174 = .0529 m3/sec

-

Compaction

= =



157 mm

11.47 .0529 217 kg/m3

CALCULATION OF TRASH PLATE CONTACT ANGLE (Refer to Figure 4) A

= = =

B

C

= =

½ (Mean Dia Top Roll) + Work opening of trash plate + half divergence 502 + 174 + 10 = 686 mm

= = =

½ Mean Diameter of Delivery Roll ½ x 1015 507 mm

Cos Ø = = = 

½ (Mean Dia Top Roll + Mean Dia Delivery Roll) + Delivery Work Opening ½ (1004 + 1015) + 72 1081

A2 + C2 - B2 2AC 10812 + 5072 - 6862 2 X 1081 X 507 .871

Ø = 29.4O CALCULATION OF PRESSURE FEEDER COMPACTION AND CONTACT ANGLE ·

Contact Angle Diameter of pressure feeder rolls at root of tooth PF Chute Inlet setting hi Distance between roots of PF teeth

CHUTE

= = =

1067 - 2(35) 997 mm 267 mm

= =

Wo + depth of tooth 177 + 35

INLET

= Cos Ø

·

Ø Escribed Volume at Inlet Peripheral Speed at root

=

D + W - hi D = 997 + 212 - 267 997 = .945 = 19.1o = = =

Escribed Volume Compaction

= = = = =



212 mm

Π Drn 3.14 x .977 x 1.1 x 2.7 60 .152 m/sec Sr Cos Ø h L .152 x .945 x .267 x 2.14 .0821 m3/sec 11.47 .0821 139 kg/m3

CALCULATION OF PRESSURE FEEDER CHUTE OUTLET COMPACTIONS AND CONTACT ANGLE ·

Contact Angle Mean Diameter of top roll Chute Outlet setting Cos Ø

= = =

1004 mm 395 mm Dm+ Wof- ho ` Dm

= = =

Ø



Peripheral Speed at mean dia =

Π Dm n = 3.14 x 1.004 x 2.7 60 = .142 m/sec

-

Escribed Volume

= = =

Sm Cos Ø ho L .142 x .722 x .395 x 2.14 .0867 m3/sec

-

Compaction

= = =

11.47 .0867 132 kg/m3

CALCULATION OF COMPACTION DROP ALONG PF CHUTE Compaction Drop

=

Inlet = =





1004 + 116 - 395 1004 .72 43o

Compaction Compaction 139 - 132 7 kg/m3

-

Outlet

CALCULATION OF PF CHUTE DIVERGENCE Divergence

= = = = =

Height out - Height In 395 - 267 128 mm Assume length of PF Chute 1.5 m Divergence 128__ 2 x 1500 = .043 = 2.5o CALCULATION OF REQUIRED COMPACTION AT ENTRANCE TO THE PRESSURE FEEDER ROLLS -

Optimum feed depth

= = =

½ (Wo + D) ½ (177 + 1032) 605 mm

Contact Angle Cos Ø

=

½ (Wo + D) D 605 1032 .586 540

= Ø

= =

Escribed Volume

= = =

Compaction Required

=

S x Cos Ø x ho L .160 x .586 x .605 x 2.14 .121 m3/sec

11.47 .121 = 94.8 kg/m3 To determine the compaction required at the base of the feed chute it will be assumed that the feeding arrangement has an effectiveness of 1.65. Required Compaction at base of feed chute

= =

11.

94.8 1.7 56 kg/m3

ROLL LOADS AND TORQUES The principal factor in setting mills is the level of compaction that is required at the delivery squeeze to achieve a given performance for mill. Generally the compaction is determined by strength considerations of the top roll load and the power of the prime mover although factors such as maintenance of surface roughness in some cases limit the maximum loads used. The relationship between roll load compaction and torque was extensively studied using the experimental mill at the University of Queensland in the early 1960's. This work established the basic relationship R = where

Pr L D (Cf - 0.1) R is Roll Load (N) Pr is the proportionality constant (Pa) L is the roll length (m) D is the roll diameter (m) Cf is the filling ratio at the nip of the two rolls

In practice values of Pr are derived from measurements on operating mills. The University experimental work also established the relationship between torque, milling geometry and filling ratio. This relationship is G = Pr (Wo/D) 0.5 Cf 0.21 R where G is the roll torque (Nm) Pn is the roll torque factor (m) Relationships were also developed for pressure feeder torque although these are considered to be less accurate.

Operating power and load curves can be developed for a specific size of mill, however it must be considered that the characteristics of the cane and preparation also affect the absolute values predicted. The form of the relationship between compaction, milling torque and power for a 2.14 m mill with a heavy duty pressure feeder is shown in Figure 11. 12.

FACTORS AFFECTING MILLING PERFORMANCE Whilst the level of compaction and speed of operation are dominant factors in the performance of an individual mill, factors such the level of cane preparation, level of maceration and number of mills in the train play an important part in the overall performance of the milling train. The effect of preparation level on the performance of a No. 1 is typically that for every unit of increase of POC the No. 1 mill extraction increases by approximately 0.5 units at a nominal preparation level in the range of 83 to 88 POC. The overall extraction of a 4 mill train would be expected to increase by approximately 0.05 unit per unit increase in P.O.C. An increase in the level of maceration applied will have a beneficial effect on the level of extraction achieved. An increase in maceration per cent fibre from 200 to 300 would be expected to increase overall pol extraction by approximately 0.5 to 0.6 units. The law of diminishing returns applies with an increase from 450 to 550 maceration per cent fibre giving only a 0.2 unit increase in extraction.

13.

The number of mills in a milling train also significantly influences the level of extraction. Obviously the level of extraction achieved by adding an extra unit depends on the strength of the individual milling unit. In one series of trials carried out bypassing existing intermediate mills in a six mill train the decrease from 6 mills to 5 mills dropped the extraction by 0.5 units and the reduction from 5 to 4 units dropped the extraction a further 0.75 units. MILLING TRAIN CONTROL There are two main objectives for the control of a mill in a milling train.

-

To control the loads on the mills to ensure a high crushing load is maintained, which should deliver good extraction.

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To control the crushing rate through the mill.

For a conventional five or six roll mill with a single drive load can be controlled by the mill roll hydraulics, but there are a range of control systems that have been developed to control the load in a more controlled way. These rely on a sensor to detect the load and a controller to act to limit the load. The range of sensors that have been developed since the mid 1960's includes the measurement of pressure feeder torque, turbine chest pressure or pressure feeder chute deflection. Use of chest pressure is satisfactory for a No. 1 mill which runs at constant speed but has restricted application on intermediate mills which are independently speed controlled and suffer chest pressure disturbances due to change of speed. The 'standard' form of control for six roll mills that developed during the 1970's and early 1980's was, for rate control: ·

A chute height monitor consisting of eight probes in the side of the chute which could be arranged in a way that they could sense the height of bagasse in the chute and depending on the change give a control signal, to a device to maintain the set points. In the case of a No.1 Mill this was invariably the feeder carrier which would change speed or for an intermediate mill the signal could cause the drive (invariably a turbine) to change speed. For torque control the sensor (being a torque or chest pressure sensors) moved a flap in the feed chute controlled by a hydraulic ram which in times of high torque would restrict the depth of feed to the mill. It is important to note that the set up of this flap is important as the maximum opening must be less than optimum feed depth (Refer Figure 9). In more recent times with the introduction of more cost effective drives especially with new installations the use of a variable speed drive on the pressure feeder has more readily been adopted as a control device. Independent drives on all rolls have been gradually introduced on new installations since the mid 1980’s. Early installations had the independently driven pressure feeder speed used to directly control the mill torque and a flap to control the pressure feeder torque. This system suffered from poor response to rate changes. Modifications to this system included changing the torque control to drive the ratio of pressure feeder speed to mill speed.

Where all rollers are independently driven the control of pressure feeder torque overload is effected by increasing feeder speed. Whilst some control of chute height is lost the mill torque is maintained and pressure feeder torque is controlled. An advantage of these drives is that their speed control is infinite and the mill can hold its torque at very low speed. A second advantage of these drives is that a wide range of crushing rates can be achieved without a change of settings. The author is grateful to Harvey Flanders who has offered advice on the operation strategy at NAT & L where the pressure feeder speed is ratio controlled to the speed of the mill with torque control for the mill drives being able to be biased up and down to maintain a preset maximum torque. If the mill drive on any roll reaches the preset maximum torque then the pressure feeder drive will shut off via a controllable time delay and this is repeated till the mill torque stays below the preset limit on the pressure feeder rolls stop. The control of six roll mills with independent drives is being studied on many fronts at the present time and advantage will be taken during the course to have open discussion on this topic.