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.
& 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
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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
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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 speed is infinitely variable. This allows exact adjustment to specific processing formulations.
Crease-prone fabrics are run through the roller passage only. This applies above all to the scouring stage because of the increased tendency of the fabric to retain crease marks when treated with high concentrations of caustic soda. A special version of the Babcock roller/roller-bed steamer is available for continuous ranges used to run preparation or pad-steam processes. On the exit side an integrated wash unit acts simultaneously as water lock and washing zone (Figure 4). A heated steamer ceiling stops condensation forming on the ceiling, which could lead to drip marks on the fabric. The roller/roller-bed steamer can be installed at floor level. Fabric entry is then at the front and fabric exit at the rear of the steamer. This arrangement is possible only for working widths up to a maximum of 2.2 m, due to the short distance from the steamer ceiling to the fabric guiders at the exit.
Figure 4 - Babcock roller/roller-b~d steamer with heated steamer roof and integrated washing compartment for prepa- ration and padsteam dyeing
In another version the steamer is raised to save floor space. Fabric entry and exit are here again separated. In this arrangement the preceding and following machines in the range can each be used separately for other opera- tions. The elevated arrangement is possible for every working width. The fabric entry lock implements the same design principle as shown in Figure 3. The exit is in the form of a steam-tight duct with integrated traction unit at the rear of the steamer and runs from the steamer down to the following washing compartment installed at floor level (Figure 5).
In another raised version, available for every width, fabric entry and exit are both located at the front of the steamer. As a result the downstream wash units can be positioned underneath the steamer. The communication duct with integrated traction units runs here from the floor of the steamer (at the front) down to the washing compart- ment installed at floor level. The fabric entry lock is located inside the communication duct, close to the floor. The washing compartment acts as a water lock for the fabric exit.
As with all Babcock steamers, the roller/roller-bed steamer is directly supplied with saturated steam for rapid heating of the steamer and short down-times. The steam
Figure 5 - Babcock roller/roller-bed steamer for preparation treatments, elevated/fabric entry and exit separated
dome principle guarantees air exclusion and reduced steam consumption. The roller passage for contact-guid- ing is available for single-loop or double-loop threading. The resulting fabric capacities, dwell times and fabric speeds open up interesting possibilities for the processing of crease-prone fabrics in short-duration treatments.
The large roller diameter (160 mm) in the roller passage for single-loop threading-up, or in the outer row of rollers in the roller passage fdr double-loop threading-up, plus the short distance between the axes of these rollers, are designed to ensure reliable fabric guiding.
The Babcock Relax drive for the upper rollers of the roller passage for both roller arrangements automatically ensures uniform fabric tension in all speed ranges, without cumulative build-up of tension. It also compensates for alterations in fabric length occurring in the roller passage. Every section has its own Relax drive with a pneumatically loaded pendulum compensator for its speed control. The rollers of the roller bed have a non-slip drive. Their circumferential speed is automatically adjusted to the given production speed. The steamer has a stainless steel hous- ing (4571 AISI 316), which may be insulated to save energy and give better operating conditions. Working widthsavailablearefrom 1.2 to3.2minstepsof 200 mm.
Fabric capacities in the roller passage are 30 m per section with a single-loop threading-up arrangement and 50 m per section with double-loop threading-up. Reduced capacity in the roller passage is possible. The fabric capacity of the roller bed, at a fabric weight of 200 g/m2, is up to 300 m per section. Fabric content and dwelling time are selectable over a wide range by changing the speed of the roller-bed rollers.
STEAMERS FOR FABRIC DYEING The only steamer used in dyeing is the pad-steam dye steamer (Figure 6) , which operates under saturated steam conditions at atmospheric pressure. It also implements the steam dome principle and thus remains automatically free of air. The excess steam needed to maintain a clean atmosphere inside the steamer is extracted via an ante- chamber and extraction pipes.
The steam-heated double ceiling reliably prevents any droplet formation on the ceiling of the steamer chamber. Any condensate running off the walls or liquid dripping from the fabric is discharged through an air-tight opening in the steamer floor into the drain. At the exit point the fabric passes through a water lock. The water flowing
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Figure 6 - Babcock pads team dye steamer
through this reduces fabric temperature to the level re- quired by the process and prevents an excessive build-up of concentration of dye in the bath. A Relaxdrive here also drives the upper rollers, thus eliminating problems arising from slackening or tension build-up in the fabric.
The fabric content in single-loop threading is 30, 45, 60,90 and 120 m and 50,75,100 and 150 m in double- loop.
The reaction chamber of the pad-roll dyeing range can also be called a steamer. The fabric passes through a padder, and is impregnated with dye, squeezed, heated up with direct steam and wound up in the reaction chamber. Since the reaction time can be extended indefinitely, this technique is of special interest for processes using dyes requiring fixation times in excess of 2 min.
Temperatures between 70 and 100C can be set in the chamber. Either a steam/air mixture at a certain tempera- ture below 100C or pure steam is fed into the chamber. The set temperature can be maintained during filling of the chamber and during the dwelling time of 1 to 5 h by control devices in the chamber itself. To preclude drip marks, the entry lip and the roof of the chamber are steam-heated.
Fabric entry is in the floor of the chamber, thus also permitting the use of processes that require air exclusion. The details of the steamer are shown in Figure 7.
Figure 7 - Babcock pad-roller dyeing range
LOOP STEAMERS IN DYE FIXATION The loop steamer (Figure 8) offers a variety of possibilities in dye fixation after printing. Dye fixation take place in the treatment medium best suited to fibre type and dye class, i.e. in either: - Saturated steam (100C) - Superheated steam in the low-temperature range
- The high-temperature range (1 15-185C and higher) - Hot air (150-185C and higher).
Figure 8 - Babcock btm 5621 loop steamer
The steamer is equipped for the treatment of woven or knitted fabrics and non-wovens made from all common types of fibres in all weightsusual for clothing. The steamer housing consists of a special steamer steel, the surface of which becomes covered criith a dense, strongly bonded oxide coating. As additional protection all inner walls are coated with a special grease. Both these measures are designed to protect the steamer against corrosive attack, e.g. stress corrosion.
Insulation of walls and roof reduces heat losses to a minimum. An aluminium skin on the outside protects the insulating material. The roof of the steamer can be heated to prevent formation of condensation, while installation of the steamer clear of the floor eliminates the need for a foundation under the complete machine.
The fabric transport system is one of the most crucial components for loop steamers. In the Babcock steamer the fabric enters the steam chamber from below through a lock in the floor. A transporting roller lifts the fabric only half-way up the steam chamber. This means that the tension in the fabric resulting from its own weight is halved, so that fabric tension is very low from the beginning. The fabric is taken up with minimum slip because of the slightly roughened surface of the transporting roller. If necessary, a steam-blowing pipe above the roller supports fabric transport with gentle pressure against the fabric. Endless belts hold the fabric away from the transport roller, so that even very light fabrics never become wound round it. The fabric falls into a loop between the transporting roller and a stainless steel carrying rod, transported by a round steel chain, as a result of its own weight. As the rod is lifted towards the steamer ceiling the loop grows to its full length in a flowing movement without any force being applied to the printed face of the fabric. The printed surface is not touched as the fabric runs through the steamer and there is no fluttering of the loops, which might lead to such contact.
As soon as the rod reaches its highest position with the full loop, it is deposited onto slide bars located on both sides of the steamer, in cams attached to parallel-running roller chains. Each rod is held separately by the cam attachments and transported to the end of the steamer in the appropriate dwelling time. During transport the rods are turned at short intervals, so that the area of fabric in contact with the rod frequently changes, so that the risk of marking is minimised. At the end of the steamer two coupled pulling rollers draw the fabric out of the loop. The speed of the pulling rollers and of the transport rollers of the plaiter outside the steamer are governed by a control- ling bar provided with a proximity switch that scans the
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position of the loops when they arrive at the delivery end of the steamer. As in all Babcock steamers, the loop steamer implements the steam dome principle.
Fabric entry and fabric exit locks are both located in the floor of the steamer. Thus the steam consumption of the steamer is limited to: - The amount needed to heat up the fabric - The amount needed to make up unavoidable heat losses - The amount needed to replace the treatment medium.
Continuous replacement of the treatment medium keeps it free of reaction products. To save energy, the exchange is kept to the lowest level permitted by the requirements of the process. The used steam is extracted through both locks by means of a fan and blowing through a pipe to the open air. Heat recovery from the used medium is possible.
The steam dome principle automatically keeps the steamer free of air so that the user does not need to consume extra steam to maintain this condition. The reliable exclusion of air is highly important in the fixation of dyes sensitive to oxygen and in discharge processes.
The internal heating system, with steam- or oil-heated radiators and automatic temperature control, ensures that temperatures are precisely maintained and that consump- tion of steam and heating energy is economical, and permits use of the particularly low-cost hot-air processes. Two-speed fans circulate the treatment medium and pro- vide for uniform temperature and moisture over the complete fabric width, ensuring ideal processing condi- tions. In addition to the saturated steam fed into the steamer in the low-temperature range, condensate spray can be injected under temperature-controlled conditions into the circulating medium.
Working widths between 1.2 and 3.6 m are possible and the fabric capacity is 70 m per intermediate section and web.
AUXILIARY DEVICES FOR STEAMERS
Steam conditioning The effectiveness of every process in which steam is used as a treatment medium depends largely on achieving and maintaining the supply of steam in a constant state. Pressure control devices or reducing valves should be used in every operating system. Wet steam should be passed through a steam dryer. Cooling almost to the saturated state is necessary in many cases if the steam is super- heated.
A simple steam-cooling device has been developed by Babcock. This permits the elimination of excess heat resulting from the expansion of saturated steam (with at most 10 bar) and for cooling superheated steam with less excess pressure (3 to 4 bar) (Figure 9). In the upper part of the device, which must be installed vertically, exterior water cooling causes condensation in the interior of the steam pipe. The condensate runs downwards onto an orifice plate on top of the mixing pipe. The orifice plate is dimensioned in such a way that the steam flows through it at the speed of sound (450 m/s). This speed is achieved if the pressure before the orifice plate is at least double the pressure behind it. The high speed of the steam atomises the condensate on the edge of the orifice plate into extremely fine droplets, and transports it into the mixing
pipe. The cross-section of the pipe increases very rapidly behind the orifice plate, leading to intensive turbulence and mixing of condensate and hot steam. The steam cooler and steamer are directly connected so that the pressure in the mixing pipe is approximately equal to the operating pressure in the steamer. The intensive mingling means that the condensate evaporates quickly, in this way cooling the steam. Non-evaporated condensate is removed via a steam trap or separated inside the steamer.
4- Uncooled steam
J, &-+= Cooling water i nf eed I 1 -cooling zone
I l l discharge
Figure 9 - Babcock steam cooling device
The device has a maximum steam throughput of 500 k g h , but parallel operation of two or three steam coolers is possible. The device has proved successful in many roller steamers, bleaching accumulators and similar ranges. For greater amounts of steam a steam saturator can be provided. Here the superheated steam is dispersed as finely as possible and directed through a layer of conden- sate approx. 0.5 m thick stored in a vertical cylinder of 1.2 m diameter. Here the degree of superheating is partially reduced. Above the condensate layer the steam meets atomised condensate sprayed in by a multiple spray nozzle at 10 bars excess pressure. The steam saturator operates at near atmospheric pressure and its maximum capacity is 1400 kg/h.
Contol of steam supply Operating conditions can vary widely in textile processing. Differences in fabric weight, width and moisture content and varying fabric speeds call for a different amount of steam supplied to the steamer in every case, unless the maximum requirement is used throughout. This maximum demand would have to be determined by calculation or
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experiment and the equipment adjusted accordingly; the method would not be economical because in many cases far too much steam would be supplied and then exhausted wastefully through the chimney. Babcock has developed a special probe to ensure that any steam reaction chamber kept full of steam, or else ensure the throughput of a certain amount of steam independent of its processing state and (within certain limits) of supply pressure (Figure 10).
c I1 I
-1 I I
._ % 1 Motorized valve
Bypass + valve -Temperature sensor
Steam outlet r 3-
-Controlled air infeed
Figure 10 - Babcock steam supply control device
The steam supply control requires a tap just above the lowest point at which steam should always be present in a chamber. When there is sufficient steam, a small amount flows from a pipe of 1.25 cm (1/2 in) diameter into an external measuring device. This steam is mixed with the amount of air necessary to produce a temperature of 55 to 70C. The use of a steam/air mixture is the distinguish- ing feature of this control system, since at this temperature the variation in heat transfer coefficient on varying the steam content is very slight. This makes it possible to meas- ure the temperature of the steam/air mixture with a sensitive probe and to keep it constant, via control of the steam supply to the steam chamber.
This control system can quite easily equalise variations in steam requirement resulting from heating of the equip- ment, fast or slow running of light or heavy fabrics, variations of supply pressure, etc. Where a higher through- put of steam through a range is required, the ideal temperature is set to a higher value, e.g. 75"C, on the
control instrument. Steam supply has proved successful in very many continuous bleaching and pad-steam dyeing ranges. Apart from the important aspect of ensuring a constant steam supply to the equipment, this control system also means that substantial amounts of steam are saved.
Air detection devices In the fixation of dyes sensitive to oxygen, e.g. vat or sulphur dyes, in discharge printing or in the treatment of cotton or viscose or blends of these with synthetic fibres with strongly alkaline solutions, the presence of even very slight amounts of air in the steam may lead to faulty dyeings or to fibre damage because of oxycellulose formation. In other finishing processes that are not sensitive to oxygen, the presence of air may still be undesirable because at a lower partial pressure of steam the temperature falls below saturation temperature at ambient pressure. Since all steamers or reaction accumulators contain air when op- eration begins, it is important to know when, after steam in-feed has begun, the steamer is ready to start production.
Starting production too soon can easily lead to a defective product, while too late a start, i.e. waiting too long for the steamer to become free of air, is costly because output is lost. On the basis of knowledge gained through years of experience with a very simple air-detection device, Babcock has now developed a new instrument - Air-Tec - operating according to the same proven principles.
Air- Tee Air-Tec consists of a housing, on top of which an unbreak- able measuring cylinder with adjustable level-mark and float switch is mounted. The housing contains four sole- noid valves and the electrical control system (Figure 11). On the front are a selector switch and indicator lamps. The device is connected to a supply of cold fresh water. A tube leads from the selected measuring point in the steamer and is joined to the device with an air-tight connection. A discharge tube leads out of the instrument.
The operational cycle begins by filling the measuring cylinder with cold water, then the valves in the measuring and in the discharge tubes are opened. The water running out through the discharge tube draws in medium from the measuring point, which should, if possible, be located at the lowest point in the steamer; since air is heavier than steam, any air present in the steamer would collect here. If there is no air in the steam, the level of the water in the measuring cylinder does not fall.
The more air there is in the steam, the lower the level in the measuring cylinder drops during the measuring time of 60 s. Since the instrument indicates even slight amounts of air very clearly, a certain drop in the water level can be tolerated in most cases. The level-mark and the switching point of the float should be adjusted on an empirical basis. If the air content in the steamer is higher than permissible, an audible or visible signal is given when the water level falls below the set point.
The Air-Tec device automatically continues to repeat the complete cycle roughly every 3 min. In this way air entering the steamer at any time during operation is automatically indicated. The appropriate moment to begin production can also be determined accurately.
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Y Figure 11 -Babcock automatic Air-Tec device for determina- tion of air content in steamers
Steam-Trac When fabric is transferred from a transport system with contact-guiding (roller passage) to, for example, transport on a roller bed, there should be a drop of a certain height for the fabric down from the last fabric guiding roller or reel, especially at higher fabric speeds. If this drop is not high enough, as for instance in the roller/roller-bed reac- tion accumulator from a speed of 100 m/min, the fabric is no longer deposited evenly in crimps, and there is a risk that the slightly sticky fabric may become wound round the last roller.
The Steam-Trac device (Figure 12) was developed to solve this problem; it is a variety of steam jet pump for use with fabrics. In an injector-like duct steam jets blowing
Figure 12 - Babcock Steam-Trac device for fabric transport
obliquely downwards from pipes on both sides of the fabric pull the fabric down. The steam pressure needed is approx. 0.3-0.6 bar. In this way the fabric is pulled away from the last fabric guide roller at speeds of up to 180 m/ min and deposited in even crimps on the roller bed. An accompanying swivel roller with an apron between the guide roller and shaft also allows contact-guiding only of the fabric, without transfer. The apron permits the steam flow from the Steam-Trac device to be suitably discharged, thus ensuring that the fabric is correctly deposited.
CONCLUSIONS The design, engineering, manufacture and operation of steamers and reaction accumulators for a wide range of applications in textile finishing require comprehensive technical/physical knowledge, far-reaching experience and meticulous care. Even minor engineering faults, care- lessness or insufficient expertise in operation can result in poor-quality or faulty products, leading to losses which, in the case of large outputs, may be very substantial.
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