The reason why this technology is called indirect heating drum drying technology is that the wet sludge and heating medium do not contact directly in the dryer, but exchange heat through the metal wall. Like the tubular heat exchanger, the mass and heat transfer relationship between the material and the heating medium is as follows.
1. Material balance
Because of indirect heating, there is no mass transfer between sludge and heating medium, so the flow rate and moisture content of heating medium have not changed. On the material side, part of the water in the material evaporates into water vapor.
Schematic diagram of drying calculation 2. Energy balance
The heat transfer of indirect heating drum dryer is similar to the heat transfer of shell and tube heat exchanger. The heat transfer formula of drum dryer can be derived from the heat transfer calculation formula of shell and tube heat exchanger.
The heat exchange between the material and the heating medium is in the form of countercurrent, and the relevant parameters are shown in the figure.
Heat transfer diagram of dryer (1) Heat balance equation
The heat balance equation reflects the relationship between the heat absorbed by the heated medium in the equipment and the heat released by the heating medium. The heat loss coefficient of the equipment is set as 1. If the heat loss is not considered in the calculation, the heat absorbed by the heated medium is equal to the heat released by the heating medium.
(2) Heat transfer coefficient
The heat transfer mode of indirect heating drum drying technology is similar to that of shell and tube heat exchanger.
3. Drive power
The transfer power of drum drying is mainly determined by the resistance of the scraper acting on the cylinder, the resistance of the air inlet sealing device and the bearing resistance. Generally, the viscous resistance of materials in the film forming process can be ignored. For double drum or multi drum dryers, the power is transmitted by the driving drum through the relative meshing gear, and only one transmission device is set. The driving power can be calculated according to the following conditions, and the motor power can be determined on this basis.
4. Other calculations
(1) Film thickness
The film thickness of wet materials in the actual production of drum drying can be calculated from the material balance.
(2) Effective drying area
According to the rotating direction of the drum, after the drum leaves the material, the drying of the drum begins. At this time, the material starts to spread film on the drum until the position of the scraper point. The drum rotates at θ angle. The area of the drum within the θ angle is the preferred drying area A of the drum. Within the remaining (360 ° - θ) immersion range, the drum is heated by the heating medium, Therefore, the materials in this section belong to the preheating section.
(3) Average drying rate
For ribbon materials or wet fabrics with a drying rate of 6~15kg/(m2 · h) and for liquid materials, empirical formulas are used.
