The size of the tube dryer is measured by the heat transfer area. According to the material balance, the heat consumption Q of the drying process (ie the heat consumed by the dryer per unit of time) can be determined. And Q = KA △ t, so the size of the dryer heat transfer area depends on the total heat transfer coefficient K and temperature difference △ t. The temperature difference Δt can be adjusted by adjusting the inlet temperature of the steam or hot carrier fluid, and the total heat transfer coefficient K is mainly determined by the following factors:
1 Material characteristics
2 Mixing level of material in the dryer (effective contact rate)
3 heat exchange efficiency
1 Particle heat transfer model From the working principle, the calculation of the heat transfer coefficient of the tube dryer can be attributed to the heat conduction of the tube bundle on the agitated particle bed. The main method for calculating the heat transfer coefficient of the agitated bed is to use Schlunder's "particle heat transfer model".
The theory holds that the heat transfer between the moving heating surface and the bed of particles to be dried is mainly controlled by three mechanisms:
1 Heat transfer between heated walls and particles;
2 Heat transfer in packed bed;
3 Thermal convection in the matrix due to particle motion.
The movement of the particles in the dryer material bed is very complex and has not yet been fully understood. In the case of simplified complete mixing, there is no temperature distribution in the bed, and the effect of heat resistance caused by particle convection on the heat transfer coefficient can be neglected. The main factor affecting the thermal resistance is whether the internal bed particles of the dryer are completely mixed.
2 The calculation of the actual heat transfer coefficient The complexity of the particle heat transfer model is segmented. It is necessary to calculate the heat transfer coefficient of the whole dryer in different regions according to the drying characteristics curve of different materials, and then heat according to each segment. The surface ratio gives the total heat transfer coefficient.
According to the above calculation method, the actual test was performed on the 180, 210, 350, and 500 m2 tube dryers. According to the test report, the contact rate between the material and the heating tube bundles was 20%, and the overall heat transfer coefficient of the 210 m2 dryer was:
h= 101. 3 × 20 % = 20. 3W/ (m2·K)
1 %。 Compared with the measured value of the heat transfer coefficient of 19. 1W / (m2 · K) compared to the theoretical calculation of the heat transfer coefficient value calculated according to the measured value is greater than 6.0%. This is due to the fact that the material is incompletely mixed under actual conditions and the actual heat transfer coefficient value is slightly smaller than the theoretical calculation.
It can be seen that the key to improving the heat transfer coefficient is 1 to increase the contact rate between the material and the tube bundle (ie, the particle coverage factor fR), 2 to increase the void gas thermal conductivity, 3 to reduce the particle size, and the first two points can be achieved by completely mixing. Status gets.
3 heating medium
At present, the most widely used heating medium is saturated steam. Tube dryers commonly use 0.3-0.6 MPa saturated water vapor, and the temperature is controlled in the 120-150°C range. The production environment of high pressure steam should be used after decompression measures. When the pressure exceeds 1.0MPa, the structure and production safety of the dryer should be taken into account. Generally, it should not be adopted directly.
After passing through the saturated steam in the dryer, the condensate must be discharged in a timely manner after condensation, otherwise the drying effect and the utilization of steam will be affected.
Saturated water vapor mixed with air can also affect the drying effect and reduce the actual temperature of the heating medium. Generally mixed with 10% -30% proportion of air, the drying temperature will be reduced by 3% -10%. Therefore, when designing a drying system, it is necessary to consider the timely discharge of condensate and the sealing effect so as not to mix in air.
1 Material characteristics
2 Mixing level of material in the dryer (effective contact rate)
3 heat exchange efficiency
1 Particle heat transfer model From the working principle, the calculation of the heat transfer coefficient of the tube dryer can be attributed to the heat conduction of the tube bundle on the agitated particle bed. The main method for calculating the heat transfer coefficient of the agitated bed is to use Schlunder's "particle heat transfer model".
The theory holds that the heat transfer between the moving heating surface and the bed of particles to be dried is mainly controlled by three mechanisms:
1 Heat transfer between heated walls and particles;
2 Heat transfer in packed bed;
3 Thermal convection in the matrix due to particle motion.
The movement of the particles in the dryer material bed is very complex and has not yet been fully understood. In the case of simplified complete mixing, there is no temperature distribution in the bed, and the effect of heat resistance caused by particle convection on the heat transfer coefficient can be neglected. The main factor affecting the thermal resistance is whether the internal bed particles of the dryer are completely mixed.
2 The calculation of the actual heat transfer coefficient The complexity of the particle heat transfer model is segmented. It is necessary to calculate the heat transfer coefficient of the whole dryer in different regions according to the drying characteristics curve of different materials, and then heat according to each segment. The surface ratio gives the total heat transfer coefficient.
According to the above calculation method, the actual test was performed on the 180, 210, 350, and 500 m2 tube dryers. According to the test report, the contact rate between the material and the heating tube bundles was 20%, and the overall heat transfer coefficient of the 210 m2 dryer was:
h= 101. 3 × 20 % = 20. 3W/ (m2·K)
1 %。 Compared with the measured value of the heat transfer coefficient of 19. 1W / (m2 · K) compared to the theoretical calculation of the heat transfer coefficient value calculated according to the measured value is greater than 6.0%. This is due to the fact that the material is incompletely mixed under actual conditions and the actual heat transfer coefficient value is slightly smaller than the theoretical calculation.
It can be seen that the key to improving the heat transfer coefficient is 1 to increase the contact rate between the material and the tube bundle (ie, the particle coverage factor fR), 2 to increase the void gas thermal conductivity, 3 to reduce the particle size, and the first two points can be achieved by completely mixing. Status gets.
3 heating medium
At present, the most widely used heating medium is saturated steam. Tube dryers commonly use 0.3-0.6 MPa saturated water vapor, and the temperature is controlled in the 120-150°C range. The production environment of high pressure steam should be used after decompression measures. When the pressure exceeds 1.0MPa, the structure and production safety of the dryer should be taken into account. Generally, it should not be adopted directly.
After passing through the saturated steam in the dryer, the condensate must be discharged in a timely manner after condensation, otherwise the drying effect and the utilization of steam will be affected.
Saturated water vapor mixed with air can also affect the drying effect and reduce the actual temperature of the heating medium. Generally mixed with 10% -30% proportion of air, the drying temperature will be reduced by 3% -10%. Therefore, when designing a drying system, it is necessary to consider the timely discharge of condensate and the sealing effect so as not to mix in air.