A vacuum rake dryer is a type of conductive heat transfer dryer. The material does not directly contact the heating medium and is suitable for drying small quantities of high-temperature-sensitive and easily oxidized mud-like or paste-like materials with a moisture content of 15%–90%. The blades of the horizontal rake agitator inside the dryer are made of cast iron or steel and mounted on a square shaft, with half of the blades facing left and the other half facing right. The shaft rotates at 7–8 r/min, driven by a motor with a gearbox. An automatic steering device changes the direction of the agitator's rotation every 5–8 minutes.
The vacuum rake dryer mainly consists of a shell, a rotating shaft, and rake teeth. Unlike paddle dryers, the rotating shaft and rake teeth of a vacuum rake dryer are not used as heating surfaces; they only serve to agitate the material and renew the surface. The rake dryer operates under vacuum. First, wet material is added to the vacuum rake dryer, and a heating medium (usually steam or hot water, but heat transfer oil can also be used) is circulated through the jacket. Then, the vacuum pump is started, and once the specified vacuum level is reached, the stirring device is activated. The forward and reverse rotation of the rake teeth continuously pushes the material towards the center and both ends during the drying process. Simultaneously, four stainless steel rods (seamless steel pipes) can be placed between the rake teeth according to user requirements. These rods move up and down continuously during shaft rotation, vibrating the material adhering to the dryer wall and breaking up any clumps. These measures ensure timely renewal of the heat transfer surface, thereby accelerating the rate of heat and mass transfer. When the material reaches the specified moisture content, heating is stopped, the vacuum system is shut off, and the dried material is removed, completing one cycle. This type of dryer is suitable for drying slurry, paste, granular, and fibrous materials, especially heat-sensitive materials and drying operations requiring the recovery of organic vapors. Vacuum rake dryers have two basic rake tooth configurations: left-handed and right-handed. Both configurations have irregular and paddle-type rake teeth. During installation, adjacent rake teeth are positioned 90 degrees apart. Irregularly shaped rake teeth are installed at both ends of the shaft, while paddle-type rake teeth are installed at the rest. When the shaft rotates, the material moves to both sides and then towards the center under the action of the stirring shaft, ensuring that the material remains in a uniformly stirred state throughout the drying process.
The commonly used drying media for vacuum rake dryers are steam, heat transfer oil, or hot water at 0.1–0.3 MPa. The vacuum degree inside the dryer is 50–90 kPa, the material filling rate is 30%–80%, and the thermal efficiency is 70%–80%. The shaft speed is infinitely variable from 6–30 rpm.