Basic principles of drying
In the whole process of fabric drying, the following two conditions must be met to meet the drying process.
(1) The heat necessary to evaporate the moisture in the fabric.
(2) The partial pressure gradient of water vapor necessary to take away evaporated water.
When the above two conditions are coordinated, the ideal drying effect can be achieved.
Figure 5-1 is the relationship curve between temperature, moisture, drying speed and drying time of the fabric during the drying process. It can be seen from the figure that when the fabric enters the dryer, its temperature is T1 and its moisture content is d1. After a period of t1 drying, the temperature of the fabric rises to T2 and the moisture content drops to d2.
During this period of time t1, most of the heat energy provided to the fabric by the dryer is used to heat the fabric, and very little heat energy is consumed for drying. The moisture in the fabric is d2 ≈d1. Therefore, the time interval t1 is called the increasing speed drying zone. When the fabric temperature reaches T 2, drying officially begins. During the period of t 2, almost all the heat given to the fabric is used to evaporate the moisture in the fabric, causing it to drop from d2 to d3 sub>, and the temperature of the fabric T3 ≈T2, the time interval t2 is called the constant speed drying zone. Most of the fabric drying process is completed in this area. As the drying process continues, when the moisture in the fabric reaches d 3, it approaches its critical value, the drying speed decreases, the fabric temperature continues to rise to T4, and the fabric moisture content reaches its critical value. When the critical value d4 is reached, the drying process is stopped. The time interval t3 is called the slow-speed drying zone. Therefore, the drying speed curve has trapezoidal characteristics.
The temperature rise of fabrics during the drying process is achieved by relying on three basic forms of heat exchange: heat conduction, heat convection and heat radiation.
When a high-temperature object and a low-temperature object are in direct contact, heat is transferred from the high-temperature object to the low-temperature object, or in the same object, part of which has a higher temperature and another part has a lower temperature, and the heat is transferred from the high-temperature part to the low-temperature part. The process is called conductive heat transfer or thermal conduction. The drum dryer uses the heated metal surface to contact the fabric surface to transfer heat to the fabric, vaporize the moisture in the fabric, and dry it.
When a part of the fluid is heated, it flows due to changes in density, causing the transfer of heat, which is called convective heat transfer or thermal convection. The flow caused by the different densities of various parts of the fluid itself is called “natural convection”. If the flow of fluid is caused by a fan or the like, it is called “forced convection”. Hot air dryers, baking machines and heat setting machines use heated air to blow onto the surface of the fabric to transfer heat. The moisture vaporized from the fabric is still taken away by the hot air.
The process of transferring heat through space in the form of electromagnetic waves is called radiative heat transfer or thermal radiation. In dryers, radiation heat transfer such as infrared or far-infrared is generally used.
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