How to improve yarn realisation and control wastes?

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 realisation is 87%.

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.

Sweep waste

The sweep waste in all departments of a spinning mill together should be within 1%. A high sweep waste arises invariably due to operatives throwing away the wastes like bonda waste, lap bits, sliver bits, roving ends etc. on the floor and generation of fly and fluff. The fly frame and ring frame tenters should be provided with hip bags and it should be ensured that the bonda waste and roving ends are deposited in the bags during piecing. Good waste, if any, should be picked before sweeping instead of sorting out the waste later. A high price fetched for sweep waste would give an indication of the presence of good fibers in the waste.

Invisible loss

Invisible loss in a spinning mill occurs due to a number of factors such as short fibers (fluff) escaping from the departments, improper accounting of wastes produced, weighment errors in cotton purchased and wastes sold, excess give away of yarn and inaccuracies in the estimates of stock held in process. Since it would be difficult to accurately assess the process stock, it is suggested that the invisible loss be assessed only once in 4 months for control purposes. This will help in minimizing the variation in invisible loss due to errors in process stock estimate. From the data compiled every month, a cumulative average could also be taken for control purposes. However, not much importance should be given for estimates made from data less than 4 months. To maintain the invisible loss within 0.5%, the Mill should also ensure that moisture content in the finished goods is at par with the level prevailed in cotton at the time of purchase.

Usable waste

By exercising good control over

• end breaks in various machines
• material handling and storage and
• work practices of operatives a mill could maintain the usable waste below 5%.

Conclusion

In many mills, there is good scope for improving yarn realisation and reducing wastes (as revealed by inter-mill studies as well as consultancy studies by SITRA). The following 4 steps would be helpful to improve yarn realisation.

Step 1:Calculate actual yarn realisation and different categories of wastes

Step 2: Using the formulae given in this focus estimate the expected yarn realisation for the existing working conditions.

Step 3:Compare the actual yarn realisation with the expected value and actual wastes with norms.

Step 4 : Analyse the causes for deviation and initiate corrective action. Create awareness among workers and technical staff about the importance of waste control. Good supervision and proper maintenance of machinery would help reduce the waste.

(The author is with The South India Textile Research Association, Coimbatore)