The spray dryer uses hot air to achieve the drying of the misty product. The hot air for drying enters the drying tower from the tower, and the exhaust gas after the completion of the drying operation is evacuated by the induced draft fan. During the operation of the spray dryer, the control of the temperature (hot air temperature) and the outlet temperature (exhaust air temperature) when the hot air enters the tower is very important. It determines the quality of the product and the dry bulk density. control.
In the spray dryer operation, the exhaust air temperature is related to the moisture content of the product and should be determined according to the moisture allowed by the product. The higher the hot air temperature, the higher the thermal efficiency and the better the economy. However, too high a hot air temperature will cause the product quality to deteriorate, so the hot air temperature must be appropriately increased under the premise of ensuring product quality.
The hot air temperature of the spray dryer will have a significant effect on the bulk density of the product. High-temperature hot air tends to produce low bulk density due to the rapid drying of hot air, which hardens the surface of the particles and swells the residual moisture, thereby promoting the formation of balloon-like hollow particles. If high bulk (low specific volume) or solid particles are required, high temperature hot air should not be used in contact with newly formed droplets. In the drying of chemicals, the hot air temperature is as high as 538 ° C, and the product particles are large. It can be found that the residual steam has sufficient pressure to blow the outer surface into a void. In food spray drying, the hot air temperature generally does not exceed 177 ° C, so the initial diameter of the hollow particles formed is not as large as the chemical.
The dry bulk density of the spray dryer is also called the bulk density, which plays a decisive role in the drying effect of the product. Therefore, determining the dry bulk density is an important task. Generally, when designing a spray dryer, the dry bulk density should be set so that the product can achieve a better drying effect. The dry bulk density of the spray dryer is not fixed at random, and various factors should be considered when determining, as follows:
(1) Equipment type
If a parallel flow type spray dryer is used, a product with a higher bulk density can be obtained, so it is necessary to select according to actual needs.
(2) The influence of additives
If you want to get a low bulk product, such as instant coffee, instant tea, some paint powder, etc., you can add nitrogen or carbon dioxide to the dry material. However, if an inorganic salt such as sodium chloride is added, the bulk density will be increased, so it should be added cautiously, and it cannot be mistaken because it may be counterproductive.
(3) Feed concentration
The feed concentration is proportional to the bulk density. When the feed concentration is small, the bulk density is small, and the productivity is also reduced.
(4) Hot air and exhaust air temperature
If the temperature difference between the hot air and the exhaust air temperature is large, the looseness of the product is lower; when the temperature difference is small, the bulk density is high. However, the temperature difference should not be too small, because the temperature difference is small, the amount of air required is large, so that the required power and heat energy are increased, which adversely affects, so it cannot be too small.