Drying in textile industry

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1. OBJECTIVE


Drying is necessary to eliminate or reduce the water content of the fibres, yarns and fabrics following wet processes. Drying, in particular by water evaporation, is a high-energy-consuming step (although overall consumption may be reduced if re-use/recycling options are adopted) (BAT for the Textiles Industry, July 2003).


2. FIELD OF APPLICATION


Drying can be applied to the following textile materials (BAT for the Textiles Industry, July 2003):

  • loose fibre
  • hanks
  • yarn packages
  • fabric


3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT


  • Classification of drying techniques: (BAT for the Textile Industry, July 2003)

Drying techniques may be classified as mechanical or thermal. Mechanical processes are used in general to remove the water which is mechanically bound to the fibre. This is aimed at improving the efficiency of the following step. Thermal processes consist in heating the water and converting it into steam. Heat can be transferred by means of:

  • convection
  • infrared radiation
  • direct contact
  • radio-frequency

In general, drying is never carried out in a single machine; normally drying involves at least two different techniques.


  • Loose fibre drying: (BAT for the Textile Industry, July 2003)

The water content of the fibre is initially reduced by either centrifugal extraction or by mangling before evaporative drying.

  • Centrifugal extraction:
Textile centrifugal extractors (hydroextractors) are essentially a more robust version of the familiar domestic spin dryer, and normally operate on a batch principal, although machines capable of continuous operation may be used in very large installations.
When using conventional batch hydroextractors, fibre is unloaded from the dyeing machine into specially designed fabric bags which allow direct crane loading of the centrifuge. An extraction cycle of 3-5 minutes reduces residual moisture content to approximately 1,0 l/kg dry fibre (in the case of wool).
  • Mangling:
Pneumatically loaded mangles may be used to reduce the water content of dyed loose fibre. Such equipment is often associated with a fibre opening hopper which is designed to break up the dyepack and present the fibre to a continuous dryer as an even mat. Mangling is invariably less efficient than centrifugal extraction.
  • Evaporative drying:
All hot air evaporation dryers are of essentially similar design consisting of a number of chambers through which hot air is fan circulated. Consecutive chambers operate at different temperatures, fibre passing from the hottest into progressively cooler chambers. Fibre may be transported on a brattice or conveyer belt or may be carried through the machine on the surface of a series of “suction drums”. High efficiency dryers with perforated steel converter belt have been developed which even out the air pressure drop across the fibre matt. This design results in more even drying and lower thermal requirements.
While the majority of dryers are steam heated, a number of manufacturers supply radio frequency dryers. Fibre is conveyed on a perforated polypropylene belt through the radio frequency field and air flow is fan assisted. With these machines the fibre is not subjected to such high temperatures and the moisture content of the dried material can be controlled within fine limits.
Radio frequency dryers are reported to be significantly more energy efficient than steam heated chamber dryers. However, the higher efficiency is not always gained if a more global analysis is made, comparing the primary energy needed for production of electric power with methane gas consumed for thermal energy production. Radio frequency dryers are mainly used where the cost of electricity is low.


  • Hanks drying: (BAT for the Textile Industry, July 2003)
  • Centrifugal extraction:
Drained hanks from the dyeing machine can contain (in the case of wool) up to 0,75 kg water per kg of dry fibre (or higher depending on the hydrophilicity of the fibre). Moisture content is normally reduced by centrifugal extraction prior to evaporation drying using equipment identical to that described for loose fibre, above. Yarn is normally unloaded from the dyeing machine into fabric bags held in round carts to facilitate direct crane loading of the centrifuge. Hydroextraction reduces the moisture content approximately 0,4 litres/kg dry weight.
  • Evaporative drying:
Evaporative dryers consist of a number of heated chambers with fan assisted air circulation, through which the hanks pass suspended on hangers or poles or supported on a conveyer.
The hank sizes employed in carpet yarn processing require a slow passage through the dryer to ensure an even final moisture content, and a residence time of up to 4 hours is not uncommon. Air temperature is maintained below 120°C to prevent yellowing (wool yellows above the boiling temperature).
All designs are capable of continuous operation. Thermal input is normally provided by a steam heated exchanger and many designs incorporate air-to-air heat exchangers on the dryer exhaust to recover heat.
Less commonly, hanks may be dried by employing a dehumidifying chamber. Moisture is recovered by condensation, using conventional dehumidification equipment. In comparison to evaporative dryers, yarn residence time tends to be longer, but energy consumption is lower.


  • Yarn packages drying: (BAT for the Textile Industry, July 2003)

The moisture of dyed packages is initially reduced by centrifugal extraction. Specially designed centrifuges, compatible with the design of the dyeing vessel and yarn carriers are employed.

Traditionally packages were oven dried, very long residence times being required to ensure adequate drying of the yarn on the inside of the package. Two methods are currently used, rapid (forced) air drying and radio frequency drying, the latter sometimes being cobined with initial vacuum extraction. Forced air dryers generally operate by circulating hot air from the inside of vacuum extraction. Forced air dryers generally operate by circulating hot air from the inside of the package to the outside at a temperature of 100°C. followed by conditioning, in which remaining residual moisture is redistributed in a stream of air passing from the outside to the inside of the package. Radio frequency dryers operate on the conveyer principle and are perhaps more flexible than the types mentioned above. Lower temperatures can be used and energy efficiency is said to be high (comments made for evaporative drying of loose fibre apply in this case too).


  • Fabric drying: (BAT for the Textile Industry, July 2003)

The drying process for fabric usually involves two steps: the first one is aimed at removing water which is mechanically bound to fibres, while the second one is necessary to dry completely the fabric.

  • Hydro-extraction by squeezing:
The fabric is squeezed by means of a padding machine through two or three rollers covered with rubber. This process cannot be applied to delicate fabric.
  • Hydro-extraction by suction:
The fabric is transported flat over a “suction drum” which is linked to a pump. The external air is sucked through the fabric and thereby removes the excess water. The resulting residual humidity is still about 90%.
  • Centrifugal hydro-extractor:
The design of this machine is similar to the one described earlier for loose fibre and yarn hydro-extraction. With heavy fabric, an horizontal axis machine may be used.
This is the most efficient method for mechanical water removal, but it cannot be applied to delicate fabrics prone to form permanent creases.
  • Stenter:
This machine is used for full drying of the fabric. The fabric is conveyed through the machine in open width. A hot current of air is blown across the fabric thereby producing evaporation of the water.

The fabric is sustained and moved by two parallel endless chains. The fabric is hooked undulating and not taut to allow its shrinking during drying.

Most common stenter designs are horizontal and multi-layer, but many new designs exist. In the horizontal stenter machine, the fabric enters wet from one side and exits dried from the other. In the multi-layer type it enters and exists from the same side. While in the first one the fabric moves horizontally without direction changes, in the second it is derivated many timers, which makes this equipment unsuitable for delicate fabrics. On the other hand horizontal stenter frames occupy more space and are less efficient (in terms of energy consumption).
  • Hot-flue dryer:
This machine is composed of a large metallic box in which many rolls derivate the fabric (in full width) so that it runs a long distance (about 250 m) inside the machine. The internal air is heated by means of heat exchangers and ventilated.
  • Contact dryer (heated cylinder):
In this type of machinery the fabric is dried by direct contact with a hot surface. The fabric is longitudinally stretched on the surface of a set of metallic cylinders. The cylinders are heated internally by means of steam or direct flame.
  • Conveyor fabric dryer
The fabric transported within two blankets through a set of drying modules. Inside each module the fabric is dried by means of a hot air flow.
This equipment is normally used for combined finishing operations on knitted and woven fabrics when, along with drying, a shrinking effect is also required in order to give the fabric a soft hand and good dimensional stability.


  • Air dryer-use in fabric processing: (BAT for the Textile Industry, July 2003)

This machine can be used for washing, softening and drying operations on woven and knitted fabrics in rope form. During the drying phase the fabric in rope form is re-circulated in the machine by means of a highly turbulent air flow. Water is thus partly mechanically extracted and partly evaporated. Thanks to the particular design of this machine it is possible to carry out in the same machine wet treatments such as washing. In this case the bottom of the machine is filled up with water and the required chemicals and the fabric is continuously soaked and squeezed. The capacity of this machine is determined by the number of channels (from 2 to 4).


  • Best practice: Hot air drying – stenters:

(http://www.e4s.org.uk/textilesonline/content/6library/report5/15_hot_air_drying_stenters.htm)

Stenters have an important role to play in a dyeing and finishing works. As well as drying, heat setting and curing fabric they also has an effect on the finished length, width and properties of the fabric. Fabric can be processed at speeds from 10 - 100 metres/ minute and at temperatures up to and in excess of 200°C. Sophisticated feed and transport mechanisms mean that the fabric is presented to the oven in a way to ensure that the finished product meets customer requirements. Stenters can be heated in a variety of ways. The most common means of heating nowadays is by direct gas firing, with the burnt gas fumes being fed into the stenter oven. A few units are indirect gas fired but their efficiencies are poor when compared to direct fired systems. Gas fired stenters are highly controllable over a wide range of process temperatures. Thermal oil heating is another method. But this requires a small thermal oil boiler (usually gas fired) and all its associated distribution pipework. Less efficient than direct gas firing with higher capital and running costs. Again can be used over a wide range of process temperatures. Oil itself can be used as a means of heating stenters. Because of the problems with incomplete combustion this can only be done indirectly via a heat exchanger. This, as with indirect gas firing is relatively inefficient. Very few stenters nowadays use this mode of heating. Finally there are a number of steam heated stenters. But because of temperature limitations (usually a maximum of up to 160°C) they can only be used for drying and not for heat setting or thermofixation. The air is heated, forced against the fabric and then recirculated. A fraction of this air is exhausted and made up with fresh air. To offer better control stenters are split up into a number of compartments, usually between 2 and 8 three metre sections each fitted with a temperature probe, burner/heat exchanger, fans, exhaust and damper. For a typical hot air drying job on a stenter the energy breakdown would include the following components:


Drying textile.jpg


The energy breakdown with hot air drying processes is dominated by both evaporation and air heating. It is therefore imperitive to reduce the moisture content on the fabric and to reduce the exhaust airflow. A lot of stenters are still poorly controlled in that they rely on manual adjustment of exhausts and on some, estimation of fabric dryness.

The main opportunities for energy saving on this type of machine can therefore be classified as follows:

a) Use less energy intensive methods first:
As with the contact drying it is important to use less energy intensive methods first such as the mangle, centrifuge, suction slot, air knife or drying cylinders. Even though drying cylinders are about five times more energy intensive than a suction slot, they are still about 1½ to 2 times less energy intensive than a stenter. Drying the fabric down to about 25-30% regain before passing it through the stenter still makes it possible to adjust the fabric width to the customers requirements.
Other techniques used to reduce drying costs include infra-red and radio frequency drying. Gas fired infra-red has been used for the pre-drying of textiles prior to stentering. This can have the effect of increasing drying speeds by upto 50%, thereby relieving production bottlenecks which tend to be around stenters. Typically you could expect the infra-red drying energy requirement to decrease by as much as 50 - 70% when compared to conventional stenter drying.
If an efficient means of pulling the fabric out to width could be devised for a short hot zone length then infra-red could be used to do all the drying. Radio frequency drying is used extensively for the drying and dye fixation of loose stock, packages, tops and hanks of wool and sewing cotton. The energy requirement for radio frequency drying when compared to conventional drying in a steam heated dryer can be as much as 70%. It is however, limited to loose stock and packages and cannot be modified, as yet, to accommodate knitted or woven fabric since the traditional stenter transport mechanism, pins and clips would interfere with the RF drying field causing discharge.
b) Do not overdry:
As with the contact drying of textiles it is important not to overdry. More so on stenters since it is a more energy intensive drying technique. There are automatic infra-red, radio active (* source) or conductivity based systems which can be linked to the stenter speed control to achieve as close as possible the fabric regain.
c) Turn off exhausts during idling:
Commission dyers and finishers tend to operate with relatively small batch sizes, and so in some extreme cases the operatives may be required to change over to different fabric qualities every hour. It is common practice to leave the exhausts on during these changeovers, which may take 10 - 15 minutes or more. With the large air heating requirement it is important to isolate the exhausts, or at least partially close them down, wherever possible during periods of idling.
d) Dry at higher temperatures:
If the fabric allows then drying at a higher temperature means that radiation and convection losses become relatively smaller compared to evaporation energy.
e) Shut and seal side panels:
On older machines the side panels may be damaged thereby upsetting the delicate air balance within the machine. All faulty panels should be repaired or replaced to provide an effective seal around the oven. f) Improving insulation is usually not practicable. Although on some older machines it may be cost effective to insulate the roof panels.
f) Insulation
g) Optimise exhaust humidity:
When drying there is an optimum exhaust rate which should be adhered to. Since a significant number of stenters still rely on manual control of exhausts, which basically means 'fully open all the time', the potential for energy saving is considerable. :Manual control of exhausts is generally very difficult since the expected airflow patterns and the ones found in practice vary considerably. Hence the tendency to leave them fully open.
Optimisation of exhausts can be achieved by controlling the exhaust humidity to between 0.1 and 0.15 kg water/ kg dry air. This is called the Wadsworth criterion. It is not unusual to come across stenters where the exhaust humidity is 0.05 kg water/ kg dry air. Which means a considerable waste of energy. Instruments are available which automatically control the dampers to maintain exhaust humidity within this specified range thereby cutting air losses without significantly affecting fabric throughput. These vary from wet/dry bulb temperature systems to fluidic oscillators measuring the variation in sound through a special filter head.
Where drying of solvent based work is required then the high air losses may not be avoidable for safety reasons. Although many solvent based systems have now been replaced by aqueous systems because of the Environmental Protection Act.
h) Heat recovery:
Exhaust heat recovery can be achieved using air to air systems such as the plate heat exchanger, glass tube heat exchanger or heat wheel. Efficiencies are generally about 50-60%, but there can be problems with air bypass, fouling and corrosion.
If other measures are applied first, such as fabric moisture control and exhaust humidity control, then there is usually no or little economic case for such systems.
Air to water systems such as a spray recuperator avoids fouling and cleans the exhaust, but there may be problems with corrosion. There is also the need for secondary water/water heat exchange and of course the problem of coinciding utilisations.
Where stenters are exhausting prohibitive amounts of volatile organics or formaldehyde, then a form of scrubber, electrostatic precipitator or even an incinerator may be required to comply with the statutory limits set under the EPA process guidance notes. In these cases it makes sense to incorporate heat recovery so that at least the installation costs can be recovered.
i) Direct gas firing:
Compared to other stenter heating systems direct gas firing is both clean and cheap. When it was first introduced there were fears that oxides of nitrogen, formed to some extent by exposure of air to combustion chamber temperatures, would either cause fabric yellowing or partial bleaching of dyes. This has since been shown to be unjustified.
Unlike steam and thermal oil systems there are no distribution losses to worry about. Heating up times are shorter and thermal capacities less, all leading to lower idling losses.


4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS


a) Changes in the process
  • “How to dry textile without over-drying”, Kaisa Bengtsson, Kathrine Segel, Henrietta Havsteen-Mikkelsen (pdf-file)
Click here.
  • “Energy saving in textile processing“, Nandish Mehta (Technical Director)
Harish Enterprise Pvt. Ltd. (pdf – file)
Click here.


b) Changes in the energy distribution system
No information is available.


c) Changes in the heat supply system
No information is available.


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