After finishing, the moisture content of the non-woven fabric exceeds the permissible range of quality requirements. In addition to affecting the quality, shelf life, measurement accuracy and packaging quality of the product, the excess moisture will also cause the product to breed bacteria and become moldy. Therefore, drying is an important link in the production process of non-woven fabric finishing. Its function is to remove excess water in the product and ensure product quality. For some special products, the drying process is still a process to activate the function of the treatment agent. In the production of non-woven fabrics, heating drying is mostly used, and there are four commonly used heating drying methods as follows.
1. Far infrared drying
The far-infrared dryer is a drying equipment designed using the principle of infrared radiation. The infrared generating devices of the far-infrared dryer include infrared lamps, infrared electric heating tubes, electric heating plates, far infrared electric heating tubes, electric heating plates, etc. In order to improve the efficiency of infrared drying, the dryer is often equipped with a forced convection ventilation system to take away the hot air attached to the surface, reduce the humidity on the surface of the product, and accelerate the evaporation of water. Since far-infrared drying does not require an intermediate medium, the energy penetrates into the interior of the product to directly heat the product, so its efficiency is higher than that of hot air drying.
The infrared dryer has the advantages of compact structure, easy installation, high output energy, high drying efficiency and good sanitary conditions. However, since the surface temperature of the heating element is far beyond the melting point of the non-woven fabric, if the product is in a static state or directly in contact with the heating device, it is easy to melt the product or cause a fire accident. Therefore, corresponding security measures should be set in the equipment.
2. Hot air drying
Hot air drying is a process in which the water in the product is heated, evaporated and taken away by the air flow by using the energy of the hot air flow. The products dried by hot air are limited by the temperature of the air flow of the hot air. Usually, the method of increasing the contact time between the hot air and the product is used to improve the heat energy utilization rate and drying efficiency of the hot air. Therefore, the product needs to be wound in a reciprocating and circuitous form in the drying equipment, and the size of the drying box is large.
Hot air drying mainly relies on the convection heat exchange of hot air. Although the drying efficiency is not high, the drying process is mild, the temperature is uniform, and the dried product still maintains a good hand. Therefore, in the post-finishing and drying process of non-woven fabrics, hot air drying is also a commonly used process.
3. Cylinder drying
Drying cylinder drying is to make the non-woven fabric with high moisture content directly close to the surface of the high-temperature drying cylinder, and heat the non-woven fabric through heat conduction, so that the moisture in it is vaporized and evaporated to achieve the purpose of drying. Drum drying is a drying process widely used in the paper industry and a drying method used in nonwoven finishing production lines.
In the drying cylinder dryer, the cylinder is made of red copper with good thermal conductivity. When the finishing agent is corrosive or the product has high hygienic requirements, it needs to be made of stainless steel. The dryer is generally equipped with multiple drying cylinders, and the drying cylinders can be arranged horizontally or vertically. The non-woven fabric and the drying cylinder are often wound in a route with a large wrap angle (such as Ω type) and a large contact area, so that the front and back sides of the product can be dried.
The drying efficiency of the drying cylinder dryer is related to the temperature and ambient humidity of the drying cylinder surface. The drying cylinder can be heated by steam or heat transfer oil. The higher the temperature, the higher the drying efficiency, but the higher the energy consumption of processed products. During the drying process of the product, there will be a large temperature difference between the temperature of the side that is in direct contact with the surface of the drying cylinder and the other surface. Yellow, affecting the quality of the product.
In order to meet the requirements of high-speed operation of the production line, the drying cylinder dryer is equipped with a large number of drying cylinders, which occupies more site and space on site, and its drying efficiency is not as good as that of the hot air penetration dryer. Therefore, in spunbond products, It is less used in the drying process of spunbond meltblown composite products.
4. Hot air through drying
Hot air penetration drying uses the principle of air convection to allow hot air to directly penetrate the non-woven fabric, and at the same time transfer heat to the non-woven fabric to evaporate the moisture and take away the moisture, realize the process of heat and mass transfer, and make the product fast dry.
The core component of the hot air penetration dryer is the drying drum, and the surface of the drum is a porous structure with high air permeability. The negative pressure draws the hot air outside the drum through the non-woven fabric into the drum, the hot air heats the cloth, the moisture in the cloth evaporates and is then taken away by the air flow in the drum, and the product is dried. A small part of the hot and humid airflow extracted from the drum is discharged into the atmosphere through the dehumidification valve, and most of it enters the main circulation pipeline, and then enters the air heater again to heat up and dry the product continuously. The air heater can use steam, gas or heat transfer oil as an energy source, adjust the temperature of the hot air by changing the flow rate of the steam, gas or heat transfer oil, and control the drying effect.
Hot air penetration dryer not only has the advantages of accurate temperature control, uniform temperature distribution, drying process, energy saving, etc., but also the dried product has good hand feeling, controllable sanitary conditions and small additional tension. The hot air through dryer occupies a small area and can not only be used for off-line finishing of products, but also can be configured as the main process equipment for on-line drying in the production line.