In most parts of China, post-harvest water is generally higher than safe water (14%), which is not conducive to storage and storage. The use of natural precipitation is not only time-consuming, laborious, and takes up space, but it is also difficult to achieve the desired effect. Therefore, many food banks use artificial precipitation.
According to the method of grain drying, the dryer can be divided into: a convection heat dryer, a conduction drying dryer, a radiation dryer and a high frequency electric field dryer. Among them, the conductive drying dryer was widely adopted by grain enterprises in the 1980s and early 1990s. The dryer is also called steam boiler, storage granary, preheating tower, drying tower, cooling tower and various conveying equipment. The advantage is that the precipitation is large, and the water can be reduced by 10%-15% at a time, the production volume is large, and the degree of automation is high. The disadvantages are high cost, high fuel consumption, and inconvenient maintenance.
The working principle of the dryer is:
In this drying method, the steam is not directly in contact with the grain, but only through the metal pipe, the grain flows outside the pipe to absorb heat, and the temperature of the grain rises after being heated, so that the internal moisture is transferred, and the surface vaporizes and vaporizes the moisture and is carried away by the drying medium. , thereby reducing the moisture of the grain.
According to its working principle, the cost reduction method is as follows:
1. Improve the drying medium. When the grain is dry, the substance that can be mixed with water vapor and carried away is called a drying medium. According to the definition of the drying medium, if the drying medium can heat the food in the drying process to a large extent, and the continuously vaporized water vapor is taken away, the food is dried. This depends on the package capacity of the drying medium for water vapor, ie the humidity of the air. The humidity of the air at different temperatures is different. At a certain temperature, the amount of water vapor contained in the humid air has a certain limit. When the amount of water vapor contained in the atmosphere and the amount of water vapor in the air condense into a dynamic balance, the water is contained. The amount of steam reaches a large limit, and the air at this time is called saturated air, and its humidity is called saturated humidity. The same kind of air has different saturated humidity values at different humidity levels, such as a saturated humidity value of 17.2 g/m3 at 20 °C and 37.3 g/m3 at 34 °C. It can be seen that the larger the saturated humidity value, the stronger the drying ability.
In the traditional steam dryer, the flue gas is put into the steam after passing through the preheating tower, which not only pollutes the environment, but also wastes heat energy. In order to improve the utilization of heat energy, flue gas can be directly used as a drying medium. The specific modification method is to directly introduce the flue gas into the heating tower from the cigarette box after preheating. In this way, the temperature of the drying medium can be effectively increased without increasing the heat source. It should be noted that the temperature of the furnace gas should be paid attention to during production. If the temperature is too high, the quality of the grain will be affected. Under normal circumstances, the temperature of the drying medium should be controlled between 80 °C and 130 °C.
2. Using drying - ventilation drying method. In conventional drying methods, the grain is usually dried to a moisture content of 14% to 15% and then stored for cooling. The drying-ventilation drying method firstly heats the food at a high temperature. The water content is reduced to 18%, and then the moisture is fed into the silo for 5-7 hours, so that the temperature and moisture between the inside and the surface of the grain tend to balance, and then the air is slowly ventilated, and the air volume is 25m3 - 50 m3 / h, ventilation time is about 8 hours, so that the temperature of the grain is the same as the atmospheric temperature, the higher the temperature of the grain usually entering the warehouse, the more water is removed (about 4%).
The advantages of this method are:
(1) increase the amount of drying. Reduce water consumption by 18% instead of 14%;
(2) Reduce energy consumption. Because the grain has a low precipitation rate in the equipment; in the process of slowing down the Soviet Union, when the slow ventilation is carried out, the heat accumulated by the grain itself is used to further reduce the water.
(3) Guarantee food quality. Because the direct cooling of the grain after baking often causes the grain to burst, and this method can reduce the bursting rate and the breaking rate;
(4) the drying cost can be reduced by about 13%.
