Textile products are subjected to various finishing processes during the production process and the moisture taken into their body during these processes is removed by drying. In general, drying of textile products is in the form of pre-drying with mechanical methods and then the main drying by heat transfer without losing the hygroscopic moisture required on the product. The most widely used drying machines in the textile industry in terms of drying the product and not damaging its structure; conveyor dryers, high frequency dryers and stenters. Stenters are tensioned drying machines and are used in drying, condensation and heat-fixing processes, and with the desired width and length adjustment, non-shrinkage can be achieved in the fabric. Conveyor belt dryers, on the other hand, are tensionless drying machines and the desired width and length adjustment cannot be given to the fabric, but they are preferred for drying fabrics with sensitive surfaces. High frequency dryers are also preferred for drying bobbin, hank, open fiber, yarn and ready-made clothing products (socks, etc.) and provide homogeneous low temperature drying in a short time.
Stenters (Tensioned Drying Machines) Stenters are drying machines in which the fabrics are attached transversely from the edges by needles or pallets in the machine, the movement of the fabric is ensured by a pair of walking chains and hot air is sent to the fabric in the meantime.
Although the initial investment and operating costs of the stenters are high, they are the most preferred drying machines because the dimensional form of textile products can be controlled and they can be used in drying, condensation and heat-fixing processes. In the stenters, the desired width and length adjustment can be given to the fabric, the wrinkles in the fabric can be removed and the fabric can be passed without touching anything except the holders on the fabric edge.
It is a drying machine that works on the basis of ramous convection drying.
In this drying method;
The textile product only comes into contact with the heated air or drying gas, there is no contact with the drying surface. Heated air is generally used in drying with this method. The textile product is in contact with this heated air. During contact, heat transfer from the gas (heated air) to the textile product and water vapor transfer from the textile product to the gas takes place. Since the product temperature is lower, a heat transfer occurs in direct proportion from the air to the product.
With the effect of heat passing from the air to the product, the water on the product evaporates. Water vapor passes into the air.
- For heat transfer, the gas must be hotter.
- There is a certain amount of water that the air can contain as steam without condensation or becoming mist. This is about temperature. At low temperature this amount is very small.
Convection drying is the most used drying method in textiles today. Most of the machines used today work on this principle.
The main reasons for this;
- 1-Low risk of contamination of the fabric
- 2-Easy to keep tension under control
- 3-It is suitable for all types of fabrics
Since all of the water in the textile product is not on the surface and is attached to the fabric by weak physical forces, heat and mass transfer during drying do not occur at the same speed and rate during the process. While it is possible to easily remove the water on the surface, it is very difficult to easily remove the water in the fibers.
Since there is sufficient water on the surface of the yarns at the beginning of drying, it does not exceed the cooling limit temperature. This temperature does not exceed 50-80°C depending on the amount of water in the fabric. The transfer of water vapor from the surface of the fabric to the drying gas obeys the laws of mass transfer.
In this step of drying, the drying rate depends on the operating conditions of the dryer.
- Temperature of drying gas
- drying speed,
- Drying water vapor pressure
It is bound and does not change with the properties of the product to be dried. Drying (water removal) is fastest at this step, as only an air boundary layer at the surface has to be crossed for heat and mass transfer. For a cotton fabric, this quick-drying fabric 35-40% humidity continues until the rest.
As the drying progresses, the water in the wide and capillary pipes in the product is absorbed and comes to the surface and evaporates on the surface. As the resistance in the capillary pipes increases over time, it becomes difficult to transport water to the surface and evaporation begins to take place in the product, and water vapor is carried to the surface by diffusion. In this case, as the distance required for the water to evaporate and pass into the hot air increases, drying slows down. In this step;
- no need heat
- For mass transfer
The hot air must pass both the air boundary layer and the dried part of the product, that is, the distance between the surface and the point where evaporation occurs in the product. Therefore, the distance to be covered for heat conduction and diffusion increases as the point where evaporation takes place moves into the product. In the mass and heat transfer equations, the mass and heat transfer coefficients are replaced by the heat permeability and vapor permeability coefficients.
In this step, the drying rate is low as the required path for heat and mass transfer becomes longer and harder. As well as the working conditions of the dryer, the structure of the product is also effective on the drying speed.
After capillary water is removed, swelling water and hygroscopic moisture are removed. In this step, there is a diffusion of the water in the fiber first to the fiber surface and then to the product surface, and therefore this is the slowest and most difficult step of drying. In addition, since the amount of water in the fabric is very low, the fabric temperature approaches the temperature of the drying air. This last portion of water remaining in the fibers, due to both the heating of the fabric and the slow removal of water, (of hygroscopic moisture) Removal, that is, over-drying, not only negatively affects the quality, but also significantly increases the drying cost. Stenter machine, also known as truss dryerIt is the most important and most used drying machine among the machines that work according to both convection drying and other drying methods in textile finishing enterprises. This machine is not only used for drying, but also for drying and condensation after any finishing process or for thermofixing of fabrics..
Like many textile finishing machines, stenters do not have a standard type and size. Each finisher has to determine the most suitable machine for his own business and production conditions.
The following main factors must be taken into account when choosing a stenter.
- Production quantity (number of cabins)
- Maximum and minimum fabric widths to work with
- Production quantity (number of cabins)
- Maximum and minimum fabric widths to work with
In convection dryers, the transmission of the hot drying gas to the fabric can be done in different ways. in ho-flue Drying is provided by passing hot air parallel to the textile product, and passing through the fabric in the dryers of the suction drum. In the stenter, hot drying gas (air) is sprayed vertically onto the fabric surface through holes or slits called nozzles.
The ability to adjust the width and length of the stentered fabric, even though there is chemical substance on the fabric during the finishing processes, the fabric is carried to the dryer outlet by holding it from both sides without touching anything, providing an effective drying, positive effect on the fabric handle and shrinkage values with the effect of the hot air hitting the fabric during drying. It has many advantages such as Having an indispensable place in the finishing of woven fabrics for a long time, the stenter has become a very important machine for both knitted and woven fabrics, with the tendency to work from tube form to open width in knitted fabrics. In stenters, the transport of the fabric in the machine is done with the help of endless chains consisting of large links on both sides of the stenter. There are pins and/or latches on the links of the chain. The fabric moves with the chain by attaching to these pins from both sides or being held by the pegs. It is possible to increase the width of the fabric by increasing the gap between the chains in the section from the entrance of the machine to the first drying chamber, as well as to ensure that the fabric can be pulled freely transversely by decreasing this gap gradually. It is also possible to adjust the length of the fabric in stenters with needle carriers. If the fabric is conveyed to the machine at a higher speed than the chain movement speed (advance), it is attached to the chains in a large amount and shortens in length by pulling in the warp direction while passing through the dryer. Therefore, the products sewn from this fabric do not shrink much when washed while being used. Stenters are the main thermal energy consumers in textile finishing businesses. Ramöz is the most important and most used drying machine among the machines working according to both convection drying and other drying methods in textile finishing enterprises. As in all convection dryers, if the hot air entering the stenter comes into contact with the textile product and transfers some heat energy to the product and takes some water vapor from the product, if it is thrown out, it takes much less water vapor than it can (can be transported without the risk of condensation), that is, from itself. The hot air, which has not been sufficiently utilized, is constantly sent to the sky with the heat energy it contains. To prevent this situation, some of the air passing through the dryer is mixed with fresh air and reused after being heated. How much of the air passing through the dryer is thrown out and how much is reused can be adjusted by opening and closing the flaps at the exhaust air outlets. Heating of the waste air used in drying constitutes a significant part of the energy consumed for the operation of drying machines. For this reason, the hot air/steam ratio is a helpful factor in determining the economic efficiency of drying processes and should be taken into account as energy costs are increasing day by day. The amount of water evaporated per unit time, fabric weight. The moisture of the fabric at the inlet and at the end of drying depends on the fabric width and passage speed, so a constant flue flow cannot be mentioned.