Abstract: This article systematically explains the working principles of belt dryers, key technologies for uniform hot-air distribution, structural design optimization methods, and energy-efficiency improvement measures. Through in-depth analysis of drying kinetics principles and airflow organization optimization techniques, it provides comprehensive technical guidance for enterprises to achieve efficient, energy-saving, and uniform drying production.
Chapter 1: Working Principle of Belt Dryers
1.1 Overview of Basic Working Principles
A belt dryer is a continuous convective drying equipment that achieves convective heat and mass transfer through direct contact between hot air (or other hot medium) and wet materials on a conveyor belt. Its core working principle is based on the following basic physical processes:
1. Hot air and material contact: Hot air passes through the material layer on the belt at a certain speed, transferring heat to the wet material through convection while carrying away evaporated moisture.
2. Moisture evaporation and diffusion: Moisture inside the material diffuses to the surface under heat and evaporates into water vapor on the material surface, which is carried away by flowing hot air.
3. Segmented drying control: According to the drying characteristics of the material, the dryer is divided into a preheating section, a constant-rate drying section, a falling-rate drying section, and a cooling section, each with different temperature and airspeed parameters.
4. Continuous conveying and processing: Materials are continuously conveyed on the belt, passing through each drying section in sequence, achieving continuous drying from feeding to discharging.
1.2 Fundamentals of Drying Kinetics
The belt drying process follows classical drying kinetics principles, mainly including the following three stages:
| Drying Stage | Moisture Characteristics | Mass Transfer Driving Force | Controlling Factors | Recommended Temperature Setting |
| Preheating Stage | Material temperature rises, surface moisture begins to evaporate | Temperature gradient | External heat transfer conditions | 50-70°C (avoid surface crust formation) |
| Constant-rate Drying Stage | Surface moisture saturation, evaporation rate constant | Surface vapor pressure difference | External mass transfer conditions | 70-90°C (maximize evaporation rate) |
| Falling-rate Drying Stage | Internal moisture diffusion control | Internal moisture gradient | Material internal diffusion | 60-80°C (prevent overheating damage) |
Key drying parameter relationship: Drying rate formula: m = h·A·(T_air – T_material) / λ + k·A·(P_surface – P_air)
Where: m – drying rate (kg/s), h – convective heat transfer coefficient (W/m²·K), A – contact area (m²), T – temperature (K), λ – latent heat of vaporization (J/kg), k – mass transfer coefficient (m/s), P – water vapor partial pressure (Pa)
Chapter 2: Key Technologies for Hot Air Uniform Distribution
Importance of Hot Air Uniformity
Uniform distribution of hot air inside the drying chamber is a key factor affecting drying efficiency, product quality, and energy consumption. Studies show that reducing the non-uniformity of hot air speed from 20% to 10% can decrease drying energy consumption by 15-25% and improve product moisture uniformity by over 30%.
2.1 Basic Principles of Airflow Organization
The airflow organization design of belt dryers is based on fluid dynamics principles, aiming to achieve uniform distribution of wind speed, temperature, and humidity throughout the entire drying area:
Pressure balance principle: By reasonably designing changes in duct cross-sectional area, maintain basically the same static pressure at each air supply outlet, ensuring uniform air volume at each outlet.
Airflow mixing principle: Install a mixing section before airflow enters the drying chamber to fully blend airflows of different temperatures and speeds, eliminating local nonuniformity.
Guide vane design: Reasonably arrange guide vanes to direct airflow in predetermined directions, avoiding eddies and dead zones, improving airflow uniformity.
Flow equalization devices: Use perforated plates, grids, or honeycomb flow equalizers to disperse large airflow into multiple small streams, improving distribution uniformity.
2.2 Hot Air Distribution System Design
System composition: Hot air generator → main duct → distribution box → branch ducts → flow equalization device → drying chamber → return duct
Design points: The main duct is tapered, the branch ducts are evenly spaced, and the flow equalization devices are selected based on airflow characteristics.
2.3 Key Design Parameters and Optimization
| Design Parameter | Recommended Range | Impact on Uniformity | Optimization Measures |
| Supply outlet velocity | 2.0-4.0 m/s | Too high causes jet flow, too low causes uneven distribution | Control by adjusting outlet area |
| Wind speed non-uniformity | < 15% | Directly affects drying uniformity | Optimize duct structure, add flow equalizers |
| Duct width-to-height ratio | 2:1 – 4:1 | Affects airflow stability | Determine based on drying chamber width |
Chapter 3: Structural Design and Optimization
3.1 Drying Chamber Structural Design
Cross-section shape optimization: Adopt a tapered-expanded cross-section design to ensure uniform velocity distribution before airflow passes through the material layer, avoiding low-speed corners.
Layered drying design: Multi-layer belt design with material flipping between layers, improving drying uniformity and making it especially suitable for difficult-to-dry materials.
Temperature zone control: Divide the drying chamber into multiple independent temperature zones along the length, each independently temperature-controlled to accommodate different drying-stage requirements.
3.2 Duct System Design Points
Main duct design: Adopt a tapered design to ensure uniform pressure loss along the path, and to control the static pressure difference between outlets within 10%.
Branch duct arrangement: Branch ducts evenly spaced, cross-sectional area precisely calculated based on required air volume. Smaller cross-section near fan, appropriately larger at far end to compensate for pressure loss.
Flow equalizer selection: Select appropriate flow equalizers according to airflow characteristics: perforated plates suitable for low-speed airflow (<3m/s), grids suitable for medium-speed airflow (3-6m/s).
3.3 Belt and Airflow Coordination Design
| Belt Parameter | Impact on Airflow | Optimization Suggestions | Suitable Materials |
| Belt mesh count | Higher mesh count increases airflow resistance but improves distribution uniformity | Select based on material particle size, use larger mesh count while ensuring no material leakage | Small particle materials |
| Belt open area ratio | Open area ratio affects airflow permeability and distribution uniformity | Open area ratio 30-50%, evenly distributed holes | General |
| Number of belt layers | Multi-layer belts can improve airflow penetration | For thick material layers, use multi-layer belts with airflow channels between layers | Thick layer materials |
Chapter 4: Hot Air Distribution Optimization Technology
Expected Optimization Effects
Through systematic optimization, hot air distribution non-uniformity can be reduced from traditional 25-40% to 8-15%, drying efficiency improved by 20-35%, unit product energy consumption reduced by 15-25%, and product moisture non-uniformity improved from ±3% to within ±1%.
4.1 CFD Simulation and Optimization
Computational Fluid Dynamics (CFD) simulation has become a core technical means for hot air distribution optimization:
Flow field simulation analysis: Establish a 3D model of the drying chamber; simulate airflow velocity, pressure, and temperature distributions; and identify low-speed zones, eddy zones, and other non-uniform areas.
Structural optimization design: Based on simulation results, optimize duct shape, guide vane position angles, flow equalizer parameters, etc., to achieve uniform airflow distribution.
Solution verification and comparison: Compare effects of multiple optimization solutions, select design solution with optimal technical-economic performance.
4.2 Guiding and Flow Equalization Technologies
Curved guide vane technology: Install curved guide vanes at airflow turning points to reduce airflow separation and eddy formation, resulting in pressure loss reductions of 40-60% compared to right-angle turns.
Variable-aperture perforated plate technology: Perforated plates use a variable-aperture design with small holes in the center and larger holes at the edges to compensate for the natural tendency toward higher center airflow velocity, improving overall uniformity.
Honeycomb flow equalizer: Use a honeycomb structure to divide large airflow into numerous parallel small streams, effectively eliminating large-scale eddies, and improving uniformity by over 50%.
4.3 Intelligent Control and Adjustment
| Control Technology | Implementation Method | Optimization Effect | Application Scenario |
| Zoned wind speed control | Independent fan or damper control for each drying section | Adapt to different drying stage requirements, uniformity improved by 30% | Multi-stage drying processes |
| Adaptive air volume adjustment | Adjust air volume based on material moisture feedback | Energy saving 15-20%, prevent over-drying | Moisture-sensitive materials |
| Airflow direction switching | Periodically switch between upward and downward air supply directions | Improve drying uniformity between upper and lower material layers | Thick layer drying |
Chapter 5: Energy Efficiency Improvement and Best Practices
5.1 Key Energy Efficiency Improvement Technologies
Heat recovery technology: Exhaust heat recovery efficiency can reach 40-60%, using heat pipes, plate heat exchangers, etc., to recover exhaust waste heat and preheat incoming air.
Variable-frequency speed control technology: Fans and belt-drive motors use variable-frequency control, automatically adjusting speed to match load, saving 20-40% energy.
High-quality insulation technology: Use high-performance insulation materials (ceramic fiber, nano-aerogel, etc.) to reduce heat loss by 50-70%.
Solar-assisted heating: Integrate solar air collectors to provide 20-40% of the thermal energy demand during sufficient sunlight.
5.2 Best Practice Cases and Effects
| Optimization Project | Pre-implementation Status | Optimization Measures | Post-implementation Effect | Investment Payback Period |
| Duct system renovation | Wind speed non-uniformity 32%, temperature difference ±12°C | CFD optimized design, added guide vanes and perforated plates | Non-uniformity reduced to 11%, temperature difference ±4°C | 8-12 months |
| Intelligent control system | Fixed air volume operation, high energy consumption | Installed variable frequency drives and intelligent control system | Energy consumption reduced by 28%, product uniformity improved | 10-15 months |
| Heat recovery system | Exhaust temperature 120°C, thermal energy wasted | Installed heat pipe heat exchanger to recover waste heat | Heat recovery rate 52%, fuel saving 24% | 14-18 months |
Economic Benefit Analysis (Taking annual 3000-ton nut drying line as an example)
Investment cost: The total investment for hot air distribution optimization renovation is about ¥450,000 (including CFD analysis, structural renovation, and control system upgrades).
Annual operating benefits: Energy saving ¥280,000 + production capacity increase benefit ¥320,000 + quality improvement benefit ¥150,000 = ¥750,000/year.
Investment payback period: About 7.2 months, with excellent economic feasibility.
Implementation Suggestions and Precautions
1. System diagnosis first: Before renovation, comprehensive airflow distribution testing and problem diagnosis must be conducted to avoid blind renovation.
2. Stepwise implementation strategy: Prioritize projects with low investment and quick results (such as guide vane optimization), then implement major investment projects.
3. Professional team support: Complex renovation projects should hire experienced professional teams to ensure technical solutions are scientifically feasible.
4. Operation training: After renovation, systematic training must be provided to operators to ensure optimization effects are sustained.
5. Continuous monitoring and improvement: Establish long-term monitoring mechanism, regularly evaluate equipment performance, continuously improve and optimize.
Summary
The key to efficient operation of belt dryers lies in uniform distribution of hot air within the drying unit. By deeply understanding drying kinetics principles, optimizing airflow organization design, applying modern CFD simulation technology and intelligent control systems, hot air distribution uniformity can be significantly improved, thereby enhancing drying efficiency, reducing energy consumption, and improving product quality.
Key Success Factors Summary
Scientific design is the foundation: Reasonable duct design based on fluid dynamics principles, guide device arrangement, and flow equalizer selection.
Precise control is the guarantee: Zoned temperature and wind speed control, adaptive adjustment to ensure optimal parameters in each drying section.
Technological innovation is key: Application of new technologies such as CFD simulation optimization, intelligent control systems, and efficient heat recovery.
Continuous optimization is the driving force: Establish monitoring and evaluation mechanisms, continuously identify and improve problems, achieving continuous performance improvement.
