Working Principle of Low-Speed Agitating Dryer

Working Principle of Low-Speed Agitating Dryer，低速搅拌干燥机，结晶干燥机

# Working Principle of Low-Speed Agitating Dryer The Low-Speed Agitating Dryer is an integrated all-in-one device that combines PET pre-crystallization and deep dehumidifying drying, specially developed for recycled PET flakes, mixed PET materials and semi-dull PET chips. Its whole working cycle can be divided into four sequential stages: feeding, pre-crystallization, continuous low-dew-point drying and uniform discharging, with low-speed agitation running through the entire process to solve caking, bridging and uneven drying problems. ## 1. Feeding & Uniform Material Distribution Stage Wet PET flakes or pellets are conveyed into the insulated drying tank via a vacuum loader with cyclone dust removal. Fine dust attached to material surfaces is separated in advance to avoid scratching finished products. The PLC system monitors the real-time material level inside the tank, and automatically stops feeding once the filling volume reaches the optimal range of 60%–75% of tank capacity. The cone bottom equipped with scraping paddles prevents raw materials from accumulating in dead corners. ## 2. Low-Speed Agitation Assisted Pre-Crystallization Stage A high-torque variable frequency reducer drives composite ribbon-rake paddles to rotate at a low speed of 5–20 RPM. The low-shear stirring design gently turns materials without producing excessive fine powder caused by violent friction. Circulating hot air heats the tank to a constant temperature of 140–150℃. Under continuous slow flipping, the amorphous molecular chains on the outer layer of PET pellets rearrange regularly to form a dense microcrystalline shell, raising the softening point of PET effectively. This crystallization process lasts 30–60 minutes, controlling the crystallinity between 40% and 48%. The hard crystalline surface prevents the PET from softening, melting and sticking to the tank wall in the subsequent high-temperature drying stage. ## 3. Closed-Loop Low-Dew-Point Deep Drying Stage After pre-crystallization, the system adjusts the hot air temperature to 160–175℃ and supplies closed-loop dehumidified air with dew point ≤ -50℃. Dry hot air penetrates the moving material layer vertically and horizontally. Unlike static drying hoppers where internal pellets cannot contact hot air evenly, constant low-speed agitation ensures every flake and pellet is fully exposed to dry airflow in all directions. Moisture locked inside PET particles is heated and evaporated, then carried away by circulating hot air. The damp air passes through a molecular sieve dehumidifier for moisture removal and waste heat recovery, and clean dry hot air is sent back into the tank for cyclic utilization. The total residence time of materials in this stage lasts 120–240 minutes, steadily reducing the moisture content of PET to below 50ppm. Sufficient deep drying avoids PET hydrolysis during extrusion, which would lead to intrinsic viscosity drop, bubbles, silver streaks and brittle finished products. ## 4. Stable Continuous Discharging Stage After completing the whole crystallization and drying process, dry crystalline PET materials are pushed to the discharge outlet by rotating bottom paddles. The discharging speed matches the feeding speed to realize uninterrupted continuous production. Temperature, dew point and stirring torque data are uploaded to the control panel in real time; once abnormal conditions such as over-temperature, material shortage or blockage occur, the system will trigger automatic alarms and interlock protection to safeguard stable operation. ## Core Functional Logic of Low-Speed Agitation Low-speed stirring is the core design that distinguishes this machine from separate pre-crystallizers and static drying hoppers. It achieves three key effects simultaneously: 1. Eliminate material bridging and blockage, especially for irregular thin rPET flakes that are prone to stacking; 2. Achieve all-round heat exchange between materials and hot air to realize uniform drying without local damp zones; 3. Maintain low shear force to reduce material abrasion and powder loss, lowering raw material waste for mass production lines.