The airflow pre-distributor is a key device installed at the end of the air supply system and above the material layer in a belt dryer. Its core function is to rectify, equalize pressure, and direct airflow from the main duct (or recirculation duct), which may have uneven speed and direction, before it reaches the material surface, ensuring it passes through the material layer as uniformly, stably, and vertically as possible.
Simple analogy: Just like a shower head, which disperses and distributes a strong stream of water from the pipe into numerous fine, uniform water columns, providing a comfortable and even shower. The airflow pre-distributor is the “shower head” for drying hot air.
Why is an Airflow Pre-Distributor Needed?
Without a pre-distributor, hot air would directly rush from the duct outlet (usually a slot or large opening) towards the material at high speed and in a turbulent state, leading to:
- Uneven drying: High wind speed and fast drying directly opposite the air outlet, low wind speed and slow drying at the edges or away from the outlet. This results in inconsistent material moisture, with some parts over-dried and others under-dried.
- Local blowing away: High-speed airflow may displace light, thin, or small materials, leading to product loss and contamination.
- Increased energy consumption: To compensate for uneven drying, it may be necessary to extend the drying time or increase the temperature, which can increase energy consumption.
- Uneven air distribution: In multi-layer belt dryers, the “wind grabbing” phenomenon on the upper layer is severe, while the lower layer lacks sufficient airflow.
The goal of the pre-distributor is to address all the problems above.
Main Types and Structures
Airflow pre-distributors come in various designs, with the following being common:
Perforated Plate / Multi-hole Plate Type
Structure: A metal plate with uniformly distributed small holes.
Principle: When airflow passes through the small holes, resistance is generated, creating a pressure difference across the plate. This pressure difference helps equalize the static pressure in front of the plate, making the wind speed flowing out of each small hole essentially consistent.
Characteristics: Simplest structure, acceptable flow equalization effect. However, resistance is relatively large, and holes are easily clogged by dust or materials, requiring regular cleaning.
Guide Vane + Flow Equalizing Plate Combination Type
Structure: This is the most classic and efficient design.
- Guide vane (or diffusion plate): Typically, a set of inclined blades that change the direction of the main airflow from the duct from horizontal to vertical downward and provide initial diffusion.
- Flow equalizing plate (grid or honeycomb plate): Installed below the guide vane, composed of many vertical, dense, parallel thin plates or honeycomb-shaped channels.
Principle: The guide vane changes the airflow direction, reducing dynamic pressure loss. The key to the flow equalizing plate lies in its aspect ratio (channel length / hydraulic diameter). A sufficiently long channel can “cut” large eddies in the airflow into numerous small, parallel streams, greatly suppressing transverse flow (turbulence), forcing the airflow to flow vertically downward, achieving excellent flow equalization.
Characteristics: Best flow equalization effect, highest airflow verticality, standard configuration for high-performance dryers. However, the structure is relatively complex, and the cost is slightly higher.
Nozzle / Air Duct Type
Structure: Connect a series of downward small air ducts or nozzles to the main duct to directly guide airflow to specific locations.
Principle: Distribute air volume by precisely designing the layout, diameter, and length of the nozzles.
Characteristics: High distribution accuracy, suitable for materials requiring special airflow distribution (such as strong wind at specific positions). However, the design is complex, flexibility is poor, and it is prone to clogging.
Variable Cross-section Duct + Plenum Chamber Type
Structure: Design the duct before entering the drying chamber as a gradually expanding cone (plenum chamber), sharply increasing its cross-section, suddenly reducing wind speed, converting dynamic pressure into static pressure, forming a relatively stable high-pressure zone inside the plenum chamber.
Principle: Utilize the “plenum chamber” effect to make the static pressure at various points inside the chamber basically equal, then discharge air through openings or flow equalizing plates at the bottom of the chamber.
Characteristics: Commonly used in the main entrance duct of the drying chamber as primary pre-distribution. Needs to be used in conjunction with flow equalizing plates inside the chamber.
Comparison of Different Types of Pre-Distributors
| Type | Advantages | Disadvantages | Applications |
| Perforated Plate | Simple, low cost | Prone to clogging, moderate effect | General drying, non-dusty materials |
| Guide Vane + Equalizing Plate | Excellent uniformity, high verticality | Complex, higher cost | High-performance drying, sensitive materials |
| Nozzle / Air Duct | Precise distribution | High design difficulty, easy to clog | Special distribution requirements |
| Plenum Chamber | Good static pressure uniformity | Large space required | Primary distribution, large dryers |
Design Points and Considerations
Wind Speed and Resistance
The pre-distributor creates airflow resistance. The total system air pressure needs to be calculated during design to ensure the fan capacity is sufficient.
Uniformity Indicators
The key to measuring pre-distributor performance is the velocity non-uniformity coefficient. Excellent designs can control the non-uniformity coefficient within ±15%.
Adjustability
Advanced pre-distributors are designed with adjustable mechanisms to accommodate different material characteristics and drying-stage requirements.
Easy Cleanability
When processing materials containing sugar, starch, or fibers, the structure must be easy to disassemble and clean.
Wind speed design reference: The inlet wind speed is usually set at 2-5 m/s, and the wind speed after passing through the material layer is generally 0.5-1.5 m/s.
Material selection: Should be selected based on drying temperature (ambient to over 200°C) and material corrosiveness, such as stainless steel, aluminum alloy, or galvanized carbon steel.
Summary
The airflow pre-distributor is one of the “soul” components of a belt dryer. Although it does not directly heat or carry materials, it determines the efficiency and uniformity of energy transfer (hot air).
- For users: When purchasing a belt dryer, be sure to pay attention to its airflow distribution system, as this is key to distinguishing between equipment grade and performance.
- For operators: Keeping the pre-distributor clean and unobstructed is fundamental to ensuring long-term, stable dryer operation and product quality.
- For designers: Optimizing the pre-distributor is one of the most effective ways to improve the overall machine’s energy efficiency and save operating costs.
A well-designed airflow pre-distributor is the fundamental guarantee for achieving energy-saving, efficient, and high-quality drying.
