Overview of Vacuum Disc Drying Machine Equipment
Disc dryer, also known as multi-layer disc dryer, is a type of multi-layer disc, rotary rake stirring, vertical continuous drying device, belonging to the conductive drying type contact dryer. This equipment has been continuously improved and developed based on fixed bed conduction dryers and rake type stirring dryers. The drying process is to introduce a heat carrier into a fixed multi-layer hollow disk, indirectly heat the wet material in contact with the metal disk surface through conduction, and under the mechanical stirring effect similar to a plow shaped rake blade, continuously roll and move the moisture inside the material to the boiling point of the operating state for evaporation and gasification. The steam leaves the equipment through the moisture outlet, thus obtaining qualified dried products at the bottom.
Disc type continuous dryers are mainly used in the pharmaceutical and food industries with sealed and vacuum types. Vacuum type dryers can be used for heat sensitive materials, and sealed type dryers can be used for other pharmaceutical or solvent recovery materials. They are also suitable for drying paste like materials. In addition, compared with other vacuum drying equipment, the vacuum disc dryer reduces the production time required by 80% compared to the box type vacuum drying oven under the condition of equal equipment volume, and increases the production capacity by 6-10 times compared to the double cone vacuum dryer.
Vacuum disc dryer can be applied to drying operations in industries such as pharmaceuticals, chemicals, pesticides, food, and feed. In recent years, Changzhou Baide Drying Engineering Co., Ltd. has borrowed from the German Krauss Maffei company's TT/TK (atmospheric pressure type), GTT (closed type) VTI、 The PLG series disc continuous dryer, developed based on the (vacuum type) series disc dryer, fully understands the principle of disc continuous dryer, especially the structural details, and maximizes its effectiveness. With the continuous recognition of its advantages, multiple sets of vacuum disc dryers have been successfully applied to the drying of powder and granular materials in industries such as pharmaceuticals and food.

Structure and working mechanism of vacuum disc dryer
The vacuum disc dryer mainly consists of a shell and frame, a hollow heating plate, a main shaft and mixing arm with rake blades, upper and lower bearings, couplings, variable speed drive devices, feeders, heat carrier inlet and outlet pipes and their control instruments, inspection doors, and discharge devices. Vacuum operation takes into account the force situation of the equipment, and the host is usually cylindrical. Internally equipped with a frame to securely place multiple horizontal annular hollow heating plates. The distance between the upper and lower plates is fixed, and the hollow part of the hollow heating plate can be filled with heat carriers such as steam, hot water, or hot oil. A partition or support rod is added in the middle to increase rigidity and strength. There are inlet and outlet pipes on each heating plate, which can be assembled in series, parallel or series parallel. The temperature of each heating plate can be controlled separately and the temperature distribution inside the equipment can be adjusted. If required by the process, a coolant can be introduced into the bottom heating plate to reduce the product temperature, recover heat, and avoid the phenomenon of material temperature tending to the wall temperature of the heating plate and overheating and deterioration in the later stage of the fixed bed.
The heated moisture inside the chamber overflows from the material and is discharged through the moisture outlet located at the top of the chamber, while the moisture evaporated by the vacuum type disc continuous dryer is extracted by the vacuum extraction system through the exhaust port at the top of the chamber.
Each heating plate is equipped with 2-4 mixing arms, which are equipped with several plow shaped rake blades. The mixing arms are placed horizontally, fixed in a staggered manner on the central spindle, and driven by an external variable speed drive device to rotate at a speed of 1-8r/min. The form and quantity of the rake blades depend on the requirements of the working conditions and the residence time. Their structure should be able to move elastically, so that their bottom edges float along with the plate surface.

The function of the plow shaped rake blades in the vacuum disc dryer:
(1) Continuously moving the material on each layer of the heating plate, making the material move along the Archimedean spiral trajectory and evenly spread on the surface of the plate, thereby increasing the length of contact heating and achieving the purpose of continuous conveying and drying.
(2) Continuously stirring and stirring the materials to effectively mix the materials within the same batch layer, avoiding uneven material temperature and preventing boiling between the materials and the heating plate. Avoiding material adhesion to the surface reduces thermal resistance and improves heat transfer coefficient and efficiency.
(3) Continuously flipping and stirring the material is beneficial for improving the mass transfer rate.

Characteristics of Vacuum Disc Drying Machine Equipment
(1) During the vacuum drying process, the pressure inside the chamber is always lower than atmospheric pressure, resulting in fewer gas molecules, lower density, and lower oxygen content. Therefore, it can dry easily oxidized drugs and reduce the chance of material contamination;
(2) Due to the fact that the temperature of water during vaporization is directly proportional to the steam pressure, the moisture in the material can vaporize at low temperatures during vacuum drying, which can achieve low-temperature drying and is particularly suitable for the production of thermosensitive materials;
(3) Vacuum drying can eliminate the surface hardening phenomenon that is easily caused by atmospheric hot air drying. This is because the pressure difference between the inner and surface of the vacuum dried material is large, and under the pressure gradient, moisture quickly moves to the surface, without surface hardening;
(4) During vacuum drying, the temperature gradient between the inside and outside of the material is small, and the reverse osmosis effect allows the solvent to move and collect independently, effectively overcoming the solvent separation phenomenon caused by hot air drying.

Improvement of structural details of vacuum disc dryer
The drying effect of vacuum disc dryer is related to temperature control and vacuum control factors, and the factors affecting the drying speed include:
(1) The condition of the dried material, including its moisture content, density, viscosity, and other properties. Generally speaking, if the material particles are fine and uniform, stacked loosely, and thin in thickness, the internal moisture is prone to diffusion. If measures such as increasing the initial temperature of the material, vacuum filtration pretreatment, and reducing the moisture content of the material are taken, the vacuum drying speed can be improved;
(2) The higher the vacuum degree, the more favorable it is for water to vaporize at lower temperatures, but excessive vacuum is not conducive to heat conduction and can affect the heating effect on the material. To improve the drying speed of materials, the setting of vacuum degree should be comprehensively considered based on the characteristics of the materials.
(3) Scraper device with rake blades
The scraper device with rake blades is composed of a rake arm, plate ears, and rake blades. The plate ears have installation holes and are fitted onto the rake arm. The rake blades and plate ears are fixed together, and the rake arm is installed vertically with the rotating shaft. Under the driving of the rotating shaft, the rake blades move in a circular motion on the heating disk surface. Due to the angle (installation angle) between the rake blade and the rake arm, the material in front of the rake blade moves radially along the spiral line while flipping. There are various forms of scraper devices with rake blades, which can be determined according to the properties of the material, but it is better to use rake blade surfaces as curved or spiral surfaces. The characteristics of curved rake blades are: they can improve the stirring function of the rake blades, ensure sufficient mixing of materials, increase drying rate, and have a simple structure and easy processing, suitable for materials with low internal friction; The characteristics of helical rake blades: materials move in a spiral trajectory on the rake blades, suitable for materials with high internal friction. The installation angles of these two forms of structures can be adjusted.
(4) Structure of heating disc
Due to the introduction of heating medium into the heating disc, it provides a heat source for the dried material. Although there are various forms of heating discs, such as support plates, baffle plates, and stamping plates, stamping heating discs are preferred. The upper plate is welded to the lower plate that is formed by stamping and has a regular distribution of protrusions. The protrusions not only increase the stiffness and strength of the upper and lower plates, but also increase the movement of the heating medium, thereby improving thermal efficiency.
(5) Feeding device
The feeding device is generally a spiral feeding device, consisting of a silo, a spiral feeder and its driving mechanism, a feeding chamber, a gate valve, etc. The rotational speed of the spiral feeder is an important parameter that restricts the amount of feed. If the rotational speed is too fast, it will cause the material layer to become too thick, and the rake blades will not be able to effectively flip and stir, which is not conducive to drying; If the speed is too slow, it will cause discontinuity in material distribution, which in turn affects the continuity of production. The optimal setting is that the speed of the spiral feeder can be adjusted, usually between 5 and 10 r/min, depending on the characteristics of different feed materials.

Basic technical parameters of vacuum disc dryer
(1) Heating disc design pressure: generally 0.4MPa, up to 1.6MPa^ High operating pressure: generally ≤ 0.4MPa, up to 1.6MPa; Heating medium: steam, hot water, thermal oil. When the temperature of the heating disc is ≤ 100 ℃, heat it with hot water; Heating with saturated steam or superheated steam of ≤ 0.4MPa at 100-150 ℃; Heat with thermal oil at 150-320 ℃ Heating methods such as electricity and molten salt can be used at 320 ℃.
(2) Main spindle speed of material conveying system: 1-10r/min (electromagnetic or variable frequency stepless speed regulation); Rake arm: There are 2-8 rake arms fixed on the main shaft on each heating disc layer; Rake blade: connected to the rake arm, can float up and down with the disc surface to maintain contact, and has various types.
(3) Vacuum type shell: cylindrical, designed for an external pressure of 0.1MPa, with the main inlet and outlet pipelines for the heating medium inside the shell.