Way to improve yarn realisation and control wastes?
In the above case, if the mill produces combed
yarn with a noil extraction of 18%, then the expected yarn realisation will be
70.8%. In both the cases, it is assumed that the mill reuses the usable wastes
in the same mixing.
Process waste
Blow room
The amount of waste
extracted in blow room is mostly determined by the trash level in cotton. In
modern blow room lines greater importance is attached to the opening of cotton
than cleaning. Hence, cleaning efficiency of about 60% in cottons with high
trash content and 50% in cottons with low trash level can be considered to be quite
satisfactory in these lines. For good cleaning efficiency, the waste extracted
in blow room should be about the same as the trash in mixing. If, however, the
cleaning efficiency achieved is less than 50 - 60%, then the total waste
extracted should also be low. It should be ensured that the overall lint in
waste is no more than 40% in cottons with high amount of trash and 50% for
cottons with low level of trash. The expected lint loss can be estimated using
the following formula:
Wb = (t - t L) 100 .................. (1) (100 - L) (t - t L)
L = 100 1 - .................. (2)
Where
t = trash in mixing (%)
t L = trash in lap (%)
Wb = waste extracted in blow room (%)
L = % lint in waste
Illustrative examples
1. Cleaning efficiency of blow room: 40%
Trash in mixing:3.5%
Trash in lap:1.5%
Waste extracted:3.2%
Calculate the lint loss in waste.
Refer equation (2),
(t - t L)
Lint loss (L)= 100 1 - Wb (3.5 -1.5) = 100 1 - 3.2 = 38%
2) Trash in mixing: 5%
Trash in lap:2%
Cleaning efficiency :50%
Expected lint loss:40%
Estimate the amount of waste to be extracted in blow room
Refer equation (1),
(5 - 2) x 100 Wb = (100 - 40) = 5%
Presently many mills are equipped with Automatic
Waste Evacuation System (AWES) for blowroom, cards and combers, which removes
wastes from these machines either continuously or intermittently. This system
not only reduces the man power required to collect and transport wastes but
also reduces the incidence of fly and fluff generation in these departments and
improves yarn quality, particularly short thick faults.
In mills not equipped with filters in blow room a
proper estimate of gutter waste should be made, since gutter cleaning is not
done that regularly at the end of every month. A proper estimate of gutter
waste could be made based on the quantum of waste collected and number of days
the blow room has worked.
Cards
The waste extracted in cards is usually in the
range of 4% to 7% depending upon the type of card and counts. Between same type
of cards and mixing, the waste % should not vary more than 0.5% from the
average. The card waste is also governed by the cleaning efficiency achieved at
blow room. Thus while assessing the waste, combined waste extracted at blow
room and cards should be taken into account. The combined cleaning efficiency
generally varies from 85 - 92% with SHP cards, 88 - 95% with HP cards and 90 -
98% with VHP/SuHP cards. To illustrate, for 4% trash in cotton and 0.12% trash
in sliver (SuHP card), the combined cleaning efficiency is 97%.
Combers
Generally, all cottons respond well to combing
for noil extraction up to 16%. For levels beyond 16%, the law of diminishing
returns operate and the improvement in yarn quality is not commensurate with
the additional cost of production. Higher levels of waste should be extracted
only in such cottons where combing performance is satisfactory or where the end
use requires yarns of very high quality. Under good working, for every 1%
increase in comber waste, yarn lea strength will increase by 1% and unevenness
is expected to improve by 0.15 U%. The variation in noil % between combers must
be maintained within 0.5% and between heads it must be within 1.5%.
Yarn waste
The yarn waste in a spinning mill should not
normally exceed 0.1% with conventional cone winding. In the case of automatic
cone winding, the yarn waste generally varies from 05% in winders fitted with
round magazine feed to 0.8% in winders with auto bobbin feed system. However,
if the yarn under goes additional processes such as reeling, doubler winding
and TFO twisting/ring twisting, the waste would be somewhat higher. A high
incidence of yarn waste, apart from leading to a loss of Rs.6 to Rs.15 per
spindle per year for every 0.1% waste (savings increase with decrease in
count), is an indication of poor machinery condition and maintenance, and
inappropriate work practices of operatives. A number of factors such as
vibrating spindles, spindle out of center, soft cops, oil stain on yarn,
improperly built cop bottom, yarn left over in cops during winding and
operatives using excess length while piecing, leads to high yarn waste. For
further information on yarn waste control reference may be made to SITRA
publication “Measures to Control Hard Waste in Spinning” Vol. 41, No.11 March
1996.
In the production economics of a spinning mill,
yarn realisation plays a significant role. To illustrate, in the prevailing
cotton cost and yarn selling price, even 1% improvement in yarn realisation
would lead to a saving of Rs 20 lakh per year for a 30,000 spindle mill
manufacturing 40s count says D Shanmuganandam
Two decades ago SITRA published a focus on “Yarn
Realisation and Process Waste Control”. During this intervening
period, remarkable changes have taken place in
the industry with regard to technology of machinery, yarn quality and workers’
efficiency. Keeping the above in mind, the formulae for estimating yarn
realisation and norms for different categories of wastes have been updated and
presented in this focus.
This focus also deals with control of yarn
realisation and wastes. Besides controlling process wastes such as blowroom and
card droppings, flat strips, comber noil, sweep waste and yarn waste,
equal emphasis should also
be laid on the control of reusable wastes (soft waste) such as blow room lap
bits, sliver bits, roving ends and pneumafil and bonda waste. This is because
apart from loss in production, reprocessing of soft wastes involves extra
handling and deteriorates yarn quality.
Yarn
realisation
The establishment of norms
for wastes is a prerequisite for a successful waste control in a mill. The yarn
realisation (YR) is largely governed by the level of trash in cotton and type
of machinery. The expected yarn realisation can be obtained using the following
formulae:
A. For mills reusing the entire usable wastes in the same mixing
YR = 97.5 - t - Wk - Wh ... for carded counts
= (100 - t - Wk) (1 - Wc/100) - Wh - 2.5 ... for
combed counts
B. For mills not reusing the usable wastes in the same mixing
YR = 97.5 - t - Wk - Wh - Wu ... for carded
counts
= (100 - t - Wk) (1 - Wc/100) - Wh - Wu - 2.5 ...
for combed counts
Where
t=trash in mixing (%) Wk=card waste (%) Wc=comber
waste (%) Wh=yarn waste (%) Wu=usable waste (%)
For example, for t = 3%, Wk = 7% and Wh = 0.5%,
the expected yarn real
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