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Design Of Air Dryer
Mar 28, 2018

The basic principle of design is that the residence time of material in the dryer must be equal to or greater than the required drying time. The design calculation mainly adopts four equations: material balance, heat balance, speed relation and balance relation. In the design of a dryer, the determination of the operating conditions of a dryer usually needs to be measured by experiment or in accordance with the general principle of general selection.

Selection of 1. drying medium

The selection of dry medium depends on the process of drying and the heat source that can be used. The basic heat source has saturated water vapor, liquid or gaseous fuel and electrical energy. Air, inert gas, flue gas and superheated steam can be used in a convective drying medium.

When the drying temperature is not too high, and the presence of oxygen does not affect the performance of the dried material, hot air can be used as a drying medium. The inert gas should be used as a drying medium for some easily oxidizing materials or vaporizing the explosive gas from the material. The flue gas is suitable for high temperature drying, but it requires that the dried material is not afraid of pollution, and does not affect the gases such as SO2 and CO2 in the flue gas. Because of the high temperature of the flue gas, the drying process can be strengthened and the drying time can be shortened. In addition, the economy and source of the medium should also be considered.

2. choice of flow mode

In the current operation, the flow direction of material movement direction and the media on the contrary, drying impetus the whole drying process is uniform, it is suitable for: in the water content of the material is high, with fast drying is not allowed; in the late stage of drying, high temperature resistant material requirements; dry product moisture content is very low.

In the wrong flow operation, the direction of motion between the drying medium and the material is perpendicular to each other. The location of each of the materials on the medium with high temperature and low humidity of the contact, so drying force is relatively large, and the higher gas velocity, so the drying speed is high, suitable for both in water containing high or low, can be fast drying and high temperature materials, but due to resistance; the dryer or structural requirements is not suitable for cocurrent or countercurrent operation.

3. the temperature of the drying medium into the dryer

Improving the drying process in order to strengthen and improve economic efficiency, drying medium inlet temperature should be maintained at the highest temperature within the allowable range of materials, but also should consider the change of physical and chemical materials to avoid discoloration decomposition, etc.. For the same material, the inlet temperature of the permissible medium varies with the type of dryer. For example, in the dry box, because the material is static, therefore should choose medium inlet at a lower temperature; in the rotating cylinder, boiling, airflow dryer, the turning materials continuously, resulting in higher drying temperature, more uniform, speed and short time, so the medium inlet temperature can be higher.

4. relative humidity and temperature of drying medium leaving the dryer

Relative humidity of drying medium which was increased 2 to leave the dryer, reduce air consumption and heat, can reduce operating costs; but with 2 increase is medium water vapor partial pressure increased, the average force of drying process decreased, in order to maintain the same drying capacity, need to increase the size of the dryer that is, to increase the cost of investment. So, the most suitable Phi 2 value should be decided by economic balance calculation.

For the same material, if the type of desiccator selected is different, the suitable 2 values are also different. For example, for a pneumatic dryer, because the residence time of material in the device is very short, it requires greater driving force to improve the drying rate. Therefore, generally, the partial pressure of water vapor in the gas leaving the dryer is lower than that of 50%~80% on the surface of the outlet material.  For some desiccator, it is required to guarantee a certain air speed. Therefore, considering the relationship between gas volume and phi 2, that is, in order to meet the requirements of larger gas velocity, more air can be used to reduce the value of 2.

The temperature of drying medium leaving the dryer T2 and phi 2 should be considered at the same time. If the T2 is reduced and the phi 2 is higher, the wet air may precipitate water droplets in the equipment and pipeline behind the dryer, thus destroying the normal operation of the dry. The air dryer generally requires that T2 is more than 10~30 C at the outlet temperature of the material, or the T2 is higher than the adiabatic saturation temperature of the inlet gas at 20~50 C.

5. the temperature of the material leaving the dryer

Improving the material outlet temperature is 2 and is related to many factors, but mainly depends on the material moisture content Xc and the mass transfer coefficient of the second drying stage. The lower the Xc value, the lower the material outlet temperature, theta 2, and the higher the mass transfer coefficient, the lower the theta 2.

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