Molino

Proceedings of The South African Sugar Technologists' Association-April 1963

101

CALCULATION OF MILL SETTINGS
By G. G. ASHE
The following is a simplified method of getting
approximate mill settings, using first principles. For
the purpose of these calculations we assume that cane
consists of three ingredients, i.e. fibre, sucrose and
water. Also that the S.G. of fibre, sucrose andwater
is 1.35, 1.55 and I respectively. It is necessary to have
these values so that volumes in cubic feet can be
calculated. The figures used in the following calculations are those for the 84" x 39" mill at the Umfolozi
Co-operative Sugar Planters' factory at Riverview.
The following figures are the expected averages for
the season:
13 %Sucrose (S.G.=1.55)
13 %Fibre (S.G.=1.3)
74 %Water(S.G. = 1 )

So far we have worked on one ton of cane and as
mill roll speeds are usually given in feet/minute we
must now establish a crushing rate in tons per minute.
Crushing Rate= 170 tons cane per hour
=170 tons cane per minute
60
='2.83 tons cane per minute. ..

We now have to calculate the peripheral speed of
the mill. In this example we have taken the average
outside diameter ofthe three rolls, whereas the mean
diameter of the top roll only is sometimes used.
The following figures and calculation are based on
our No.6 (last) mill.
'

The cubic footage for I ton (2,000 lbs.) of cane can
be calculated as follows:
Sucrose=2,000 lbs. x .13= 2601bs.
Fibre =2,000 lbs. x .13= 2601bs.
Water =2,000 lbs. x .74=1,480 lbs.

Top
39"

Feed

Discharge

AverageDiameter

39"

39"

39"

Circumference e-Dia. x 3 .14=39 x 3 .14= 10.02 ft.

12
Average revolutions per minute at which the mill runs
=3.6 r.p.m
(5)

2,000Ibs.

Peripheral speed = Circumference x r.p.m.

Sucrose
The weight of sucrose is 260 lbs. and the specific
gravity is 1.55. Therefore, in order to get the volume,
we divide the weight by S.G. x 62.5, thus:
Vol. of Sucrose/tonCane =

260
1.55x62.5
=2.69cu.ft.

(I)

Fibre
The weight of fibre is 260 lbs. and the specific gravity
is 1.35. Therefore, in order to get the volume, we
divide the weight by S.G. x 62.5, thus:
Vol. of Fibre/ton Cane

260
1.35x62.5
=3.07 cu. ft.

(2)

Water

"

(4)

The weight of water is 1,480 Ibs. and the S.G. of
water is I. Therefore, in order to get the volume, wedivide the weight by the S.G. x 62.5, thus:
Vol. of Water/ton Cane

1,480
lx62.5
23 . 7 cu. ft. . . .... (3)

We can summarise as follows:
Sucrose =2,000 x .13= 260Ibs.= 2.69 cu. ft.
Fibre =2,000 x .13= 260Ibs.= 3.07 cu. ft.
Water =2,000 x .74=1,480 Ibs.=23. 7 cu. ft.
2,000 Ibs.;=29.46 cu. ft.

=1O.02x3.6
=37 feet per minute. . . . . . . . ..

(6)

Width of Roller = 7 feetEscribed Area = Peripheral speed x Width of
Roller
=37x7
=259 sq. ft.
(7)
Escribed area can be described as the area that the
roller will cover if rolled on the floor for 3.6 turns.
From the graph shown in figure A it will be seen that
we expect an extraction of 96.5 % and a moisture of
49 % from No. 6 mill. In order to achieve this it
means that the bagasse passing out of this mill mustcontain only 3.5 % of the original weight sucrose in
the cane, all of the fibre and only have a moisture
content of 49 %. Therefore the volume of these three
ingredients must be the volume passing through the
discharge opening.
This volume is calculated as follows:

Sucrose
From (I) we see that the total sucrose weighed
260 lbs. and had a volume 2.69 cu. ft.
. '.3.5 %of this sucrose willweight 260 x .035 lbs.
=9.1 lbs.. .. .. ... (8)
The volume occupied by this sucrose will be
2.69x .035 cu. ft.
= .094 cu. ft.

102

Proceedings of The South African Sugar Technologists' Association-Apri/1963

Fibre

Feed Opening

The bagasse contains all the original fibre. Therefore
from (2) we get weight of fibre which is 260 lbs. and
has a volume of 3 .07 cu. ft. . . . . . . . ....