Steaming in textile processing — a manufacturer's view

  • View
    214

  • Download
    2

Embed Size (px)

Transcript

  • PAPERS

    Steaming in textile processing - a manufacturers view Claus Tischbein

    Babcock Textilmaschinen GmbH, Postfach 3148, 0-21 05 Seeuetal3, West Germany

    INTRODUCTION Steaming is one of the most important processes in textile finishing, as well as being widely used in preparation, dyeing and printing processes. In view of the fact that steamers and reaction accumulators play a key part in many textile processes, there is a requirement not only for efficient, practical machines, but also for appropriate auxiliary devices such as steam coolers, air detectors, steam volume control devices and others to guarantee reliable and economical production. Even the best steamer cannot produce high-grade fabric if, for example, the steam used is superheated. Frequently only a few degrees of superheating, which may easily result in the expansion of originally saturated steam from high pressure to normal pressure, are enough to mar results. Many plants also employ turbine-tapped steam, which has to be super- heated for technical reasons.

    Superheated steam can cause severe defects in the fabric. In the alkaline scouring stage, for instance, even slight superheating of the steam can cause migration of the caustic soda in the fabric in the reaction accumulator, leading to permanent marks and uneven dye uptake in dyeing.

    Superheating of the steam can, however, also result in sizes, lint, fibres, etc. dying onto the fabric guide rollers. This may be highly detrimental to smooth, crease-free fabric running. Such encrustations on the fabric guide rollers may also occasionally be transported back to the fabric and thus cause marks that can be hard to remove. Many processes require the complete absence of air in the steaming or reaction chamber; such processes include dyeing or printing with vat dyes sensitive to oxygen or discharge processes in printing, and also scouring with strongly alkaline liquors, where there is a risk of oxycellu- lose formation. It is very costly to ensure air exclusion by means of an excessive throughput of steam, especially since a high steam throughput still does not absolutely guarantee air exclusion. Very much more effective is the continuous testing for the presence of air with the aid of a special device.

    Assuring product quality through a controlled supply of steam optimally adapted to varying operating conditions should be a matter of course for all cost-conscious textile processors today. Steam volume control devices, which have also already found wide acceptance, not only guaran- tee product quality but also save substantial costs by limiting steam throughput to the minimum. The aim must be to supply the minimum volume of steam necessary for the process in a uniformly optimum condition. It is always

    possible to increase the minimum steam volume by a certain percentage for the sake of greater processing reliability.

    U-BOX REACTION ACCUMULATORS IN TEXTILE PREPARATION The U-box reaction accumulator was first developed more than 20 years ago. Continuous practice-oriented further development has led to widespread acceptance, so that approx. 90 U-box accumulators have now been supplied to customers all over the world. The machine (Figure 1) is reliable in operation and produces consistent results. The U-box is used for dwelling times of 10 to 40 min, i.e. in the medium dwelling-time range.

    \

    I

    & Figure 1 - Babcock Rapid Relax U-Box reaction accumulator for medium-duration treatments on fabric in tensionless crimps

    Treatments carried out on the U-box are desizing, scouring and peroxide bleaching. It may also be used for bleaching with sodium hypochlorite, but here special precautions are called for, such as changing the gas

    JSDC Volume 105 December 1989 431

  • content several times per hour and carefully rinsing all interior metal parts after the operation to prevent heavy corrosion occurring. The fabric is able to shrink and develop its volume due to the treatment in tensionless crimps on a driven roller passage. By extending the dwelling time to more than 5 min, even in scouring processes, excellent absorbency of the fabric can be achieved.

    In addition, a true white, as well as a print- and dye- white, is achievable by the optimum use of bleaching chemicals. Crease marks, a much-feared defect, occur mainly in the scouring stage. They are, however, restricted to twisted-yarn fabrics, very dense goods and fabrics with weights exceeding 300 g/m2 (cotton) or 200 g/m2 (poly- ester/cotton). The different capacities of the U-box, ad- justable according to requirements, mean that it has many varied uses. Fabric entry is in the floor (steam dome principle). This results in automatic exclusion of air without the need for a large surplus volume of steam, and abrasion- free steam-tight fabric entry.

    At the exit the fabric is fed directly into the washing compartment through a steam-tight connection duct with- out cooling. This prevents coagulation of the dissolved escort substances in the fabric. If the fabric is kept hot these substances can be washed-off much more easily.

    The U-box operates with saturated steam under atmos- pheric pressure fed into the rear of the U-box. The steam fills the accumulator from above, thereby forcing the air out at the fabric entry position. The necessary slight surplus volume of steam (steam barrier) leaves the U-box through a slot at the fabric entry position. This steam, mixed with the air carried along by the fabric, is exhausted into the open air via a chimney without a ventilator. Sometimes the waste steam has to be extracted with the aid of a small amount of pressurised steam blown into the chimney.

    The impregnated fabric enters the U-box through the low-lying fabric entry and is fed to a traction unit followed by a studded roller and a chute, driven by a separately governed d.c. motor, which allows crimp formation (and thus the thickness of the fabric stack) to be controlled. Particularly narrow fabric stacks can be prevented from tipping sideways by an additional supporting device, ad- justable from the exterior. The stacks are carefully trans- ported by driven rollers to the fabric take-off. During transport the stack is turned to ensure smooth take-off by a tension bar and fabric guiding rods. A long free-standing section centres the fabric and expands it by special fabric guiders adjustable from the outside.

    A second traction unit is installed in the steam-tight transfer shaft to the washing machine. The housing consists of several elements made of stainless steel (1.4571 AISI 316). U-boxes are available for fabric widths of 1.2 to 3.2 m and fabric capacities of 400, 600, 1000 k d m .

    ROLLER STEAMERS IN FABRIC PREPARATION Normally used in pad-steam processes, the roller steamers (Figure 2) can also be used in preparation treatments, especially for processing crease-prone fabrics. It is essen- tial, above all in the scouring stage, for such fabrics to run smoothly and without creases. The risk of crease marking is particularly high here due to the high caustic soda

    concentration and extreme swelling of the cotton fibres. Economical dwelling times in the roller steamer are be- tween 1 and 5 min. Under these conditions desizing, scouring and bleaching processes can be run, the latter with the use of higher peroxide concentrations.

    Figure 2 - Babcock roller steamer for short-duration treat- ments in contactguiding

    The steam dome principle is implemented in roller steamers too, thus guaranteeing the exclusion of air. This is vital for the alkaline scouring process, since the presence of air may here lead to fibre damage due to oxycellulose formation. The roller steamer is connected to the washing unit following it in the same way as described for the U- box.

    The roller steamer operates with saturated steam under atmospheric pressure, fed in at the rear. On starting up the air is forced out at the low-lying fabric entry slit. Here a slight surplus of steam keeps out any air that may be carried in by the running fabric. All upper rollers are driven by the elastic endless belts that equalise variations in tension in the fabric, in combination with the pendulum rollers at every 25-30 m.

    The housing of a roller steamer may be composed of up to five sections, joined together by steam-tight flanges; the material is high-quality stainless steel (1.4571 AISI 316). The fabric can be observed during operation through large windows positioned at all important points. Large doors are provided for cleaning and servicing. Fabric capacity is up to 150 m for a singleJoop threading-up arrangement and up to 250 m for a double-loop threading-up arrange- ment..

    The Babcock roller/roller-bed steamer is a multi-pur- pose reaction accumulator for continuous preparation treatments (Figure 3). This has a roller passage for contact guiding and a roller bed underneath, on which fabrics that are not crease-prone can be deposited free of tension. It is used for desizing, scouring and bleaching. The dwell times resulting from fabric capacity and production speed are in the range 1-3 min in the roller passage, i.e. in the short- duration range, and 5-20 min in the roller bed, i.e. in the medium-duration range. This steamer is suitable for proc- essing all dimensionally stable fabrics in cotton, polyester/ cotton, viscose and polyester/viscose .

    Figure 3 - Babcock roller/roller-bed steamer for preparation treatments, installed at floor level

    432 JSDC Volume 105 December 1989

  • Normally both passages are employed, the use of the roller bed depending on the tendency of the fabric to retain crease marks. An important advantage is the fact that the fabric content in the roller bed can be varied as required, since the ratio of the speed of the roller-bed rollers to the production spee