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  • 2.1.7-COTTON VALUE ADDITION-DRYING

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  • Drying

    Chapter 2 - Cotton value addition - Impact of varieties and production practices 

     
     
    The moisture content of seed cotton is very important in the ginning process. Seed cotton with too high a moisture content will not clean or gin properly and will not easily separate into single locks but will form wads that may choke and damage gin machinery or entirely stop the ginning process. Seed cotton with too much moisture will also form tight twists known as ‘fish hooks’ that remain in the ginned lint and degrade appearance. Excess moisture is removed by 38 Chapter 2 – Cotton value addition Figure 2.6: Representative cross-sections of typical types of gin machinery arrayed in a sequence used for spindle-picked cotton exposing the cotton to heated, dry air. Drying systems can seriously overdry cotton and must be used properly to avoid reducing cotton quality. Drying at low temperatures is much less harmful than drying at high temperatures.

    Cotton with too low a moisture content may stick to metal surfaces as a result of static electricity generated on the fibres, and cause machinery to choke and stop. Fibre damage is especially likely at moisture contents below 5%. Dry cotton requires more force and power to compress than does moist cotton. When pressing and baling such low-moisture cotton, it is often difficult to achieve the desired bale weight and density without adding moisture.

    Dryers should be adjusted to supply the gin stand with lint having a moisture content of 6%–7% to preserve fibre quality. Cotton at this moisture level is more able to withstand the stresses of ginning without breaking. However, cotton at 5% moisture content will result in better cleaning and a smoother appearance, which is erroneously preferred by many classing and marketing systems. Gin cleaners remove more trash at moisture levels below 6%–7% but not without more fibre damage. Fibre moisture higher than 7% preserves fibre length but results in ginning problems and poor cleaning.

    Fibre length preservation can best be attained with fibre moisture of 6.5%–8%, but both cleaning efficiency and ginning rate are reduced at higher moistures. As a compromise, moisture contents of 6%–7% are feasible. Ginning below 5% moisture can cause serious damage to the fibres, while ginning above 8% may produce rougher lint, decreased gin capacity, and less effective cleaning. The effects of ginning cotton below 5% moisture are decreased yarn strength and yarn appearance, and increased short fibres in the card sliver.