What is the Working Principle of a Grain Dryer?

　　The core working principle of grain dryers is "hot air convection drying" — using heated clean air as the drying medium, forced ventilation enables full contact between hot air and wet grain. By leveraging the moisture gradient and temperature gradient between air and grain, the vaporization, diffusion, and removal of grain moisture are achieved, ultimately reducing the grain moisture content to a safe storage level (typically 12%-15%, varying slightly by grain type).

　　The entire process follows a dual physical mechanism of "heat transfer + mass transfer" and can be decomposed into 4 core stages. Based on the common logic of equipment structures (e.g., tower-type, box-type, drum-type), a detailed analysis is provided below:

　　I. Core Principle Framework: Synergy of Heat Transfer and Mass Transfer

　　The essence of drying lies in the simultaneous occurrence of "heat transfer" and "mass transfer," both of which are indispensable:

　　Heat Transfer Process: Hot air (high temperature, low humidity) transfers heat to wet grain (low temperature, high humidity), causing moisture inside the grain to vaporize (change from liquid to gas) after absorbing heat.

　　Mass Transfer Process: Vaporized water vapor diffuses from the interior of the grain to the surface under the action of a "concentration difference," then is carried away by flowing hot air and discharged outside the equipment.

　　Cycle Logic: Hot air is continuously supplied (supplementing heat and removing moisture), and grain is continuously in contact with hot air (or in relative motion) until the grain moisture and hot air humidity reach "dynamic equilibrium" (at this point, the grain moisture is the safe moisture content).

　　Key Premise: Hot air must maintain the characteristics of "high temperature and low humidity" — the higher the temperature, the more water vapor the air can hold (stronger moisture absorption capacity); the lower the humidity, the greater the driving force for moisture diffusion (higher drying efficiency). Therefore, dryers must simultaneously control "hot air temperature" and "ventilation speed" to avoid grain damage from high temperatures or uneven drying due to insufficient ventilation.

　　II. 4 Core Working Stages (Taking Mainstream Tower-Type Dryers as Examples; Universal Logic Applies to All Models)

　　Tower-type dryers fully reflect the entire drying process due to their complete structure (including lifting, drying, tempering, cooling, and grain discharge systems). Other models (box-type, drum-type) only differ in "grain movement mode" or "stage integration," with consistent core stages:

　　1. Pretreatment and Distribution Stage: Uniform Feeding and Impurity Removal

　　Working Logic: Wet grain is transported to the top of the equipment via an elevator, and impurities such as straw, soil, and stones are removed through a "cleaning device" (screens, dust collectors) to avoid duct blockage or equipment wear.

　　Key Function: A "distributor" (rotary or louver-type) evenly distributes the grain in the drying bin, ensuring that subsequent hot air can penetrate the entire grain layer (avoiding local "dead zones" that cause uneven drying).

　　2. Drying Stage: Hot Air Penetration and Moisture Vaporization (Core Stage)

　　Working Logic:

　　The heat source system (biomass furnace, gas furnace, electric heater, etc.) heats cold air to the target temperature (40-120℃, adjusted according to grain type), and the hot air is evenly sent to the bottom of the drying bin through "air supply ducts."

　　Under the forced action of a fan, hot air penetrates the evenly distributed grain layer upward, making full contact with wet grain: hot air releases heat (temperature decreases), and grain absorbs heat (moisture vaporizes), forming a hot air change from "high temperature and low humidity" to "low temperature and high humidity."

　　Moist hot air is discharged from the air outlet at the top of the bin (some models recover waste heat to reduce energy consumption).

　　Key Designs:

　　Grain layer thickness control (usually 30-80cm): Excessively thick layers increase hot air penetration resistance, leading to incomplete local drying; excessively thin layers cause hot air waste and higher energy consumption.

　　Hot air temperature control: Heat-sensitive grains such as rice and seeds require low temperatures (40-60℃) to avoid increased cracking rate or decreased germination rate; temperature-resistant grains such as corn can use high temperatures (60-80℃) to improve drying efficiency.

　　3. Tempering Stage: Internal Moisture Diffusion and Crack Prevention (Key to Quality Assurance)

　　Working Logic: After the drying stage, the surface moisture of the grain has vaporized rapidly, but the internal moisture is still high (forming an "outer dry, inner wet" state). When the grain enters the "tempering stage," hot air heating stops, and only slight ventilation is maintained to allow slow diffusion of internal grain moisture to the surface.

　　Key Function: Prevents grain particle cracking (e.g., rice cracking, corn breakage) caused by "excessive internal and external moisture difference," and prepares for subsequent secondary drying (if moisture reduction is high) or cooling.

　　Supplementary Note: Small-batch dryers (e.g., small box-type) may not have an independent tempering stage, achieving tempering effects through "intermittent ventilation"; large tower-type dryers usually have 1-2 tempering stages to adapt to high moisture reduction requirements (e.g., wet corn from 25% to 13%).

　　4. Cooling Stage: Temperature Reduction and Moisture Regain Prevention

　　Working Logic: The grain temperature after tempering is relatively high (usually 40-60℃). Direct storage will cause condensation and moisture regain in the bin (hot grain contacts cold air, and water vapor in the air condenses on the grain surface).

　　Key Design: Normal-temperature air is introduced into the cooling stage for forced convection cooling, reducing the grain temperature to within ±5℃ of the ambient temperature; the cooled moist hot air is directly discharged to avoid mixing with hot air in the drying stage (which affects drying efficiency).

　　III. Working Principle Differences Between Different Models (Consistent Core Stages, Only "Grain Movement Mode" Varies)

　　Core Commonality: Regardless of the model, all follow the core logic of "uniform distribution → hot air heat transfer → moisture vaporization → tempering diffusion → cooling and moisture regain prevention." The only difference lies in the design of "how to achieve fuller contact between grain and hot air."

　　IV. Key Control Parameters: Affecting Drying Effect and Grain Quality

　　To achieve "efficient drying + low damage," dryers need to precisely adjust the following parameters through the control system, echoing the core logic of the working principle:

　　Hot Air Temperature: Determines heat transfer speed; set according to grain type (e.g., rice ≤60℃, corn ≤80℃).

　　Air Volume: Determines mass transfer speed; larger air volume accelerates moisture discharge but increases energy consumption (balance between efficiency and cost is required).

　　Grain Residence Time: Adjusted by grain discharge speed (tower-type) or batch duration (box-type) to ensure grain completes the entire "drying - tempering - cooling" process in the equipment.

　　Final Moisture Control: Real-time monitoring of grain moisture via moisture sensors; automatic shutdown or grain discharge when safe moisture is reached (precision requirement: ±0.5%).