Mechanical Classification, Mechanical Principles, and Mechanical Structure of Air-Flow Dryers

Airflow Dryers—Mechanical Classification

Vertical Tube Airflow Dryer

Wet material is fed into a vertical tube via a feeder. Air is drawn into a finned heater by a blower, heated to a certain temperature, and then blown into the vertical tube. The velocity of the air inside the tube depends on the size and density of the wet particles and is generally greater than the settling velocity of the particles (approximately 10–20 meters per second). The dried particles are carried out by the strong airflow and sent to two parallel cyclone separators for separation, then discharged via a screw conveyor, while the exhaust gas is vented through a bag filter. Due to the short residence time, a two-stage or multi-stage series process is often required for certain products.

Cyclone Airflow Dryer

A type of cyclone airflow dryer. A hot airflow carrying the material particles to be dried enters the cyclone dryer tangentially, generating rotational motion along the hot walls, which keeps the material particles in a state of suspended rotation during drying. A steam jacket may be installed on the dryer walls as needed. This significantly intensifies the drying process. Furthermore, the impact of the particles against the vessel walls causes some fragmentation, increasing the contact area between the gas and solid phases and further enhancing the drying process. It is particularly suitable for heat-sensitive, free-flowing granular materials that are highly hydrophobic and resistant to fragmentation. However, it is not suitable for materials with high moisture content, high viscosity, low melting points, a tendency to sublimate or explode, or a tendency to generate static electricity.

Pulsed Airflow Dryer

The pulsed airflow dryer is a type of airflow dryer. During the drying operation, the tube diameter alternately narrows and widens, causing the airflow and particles to flow at different speeds. This results in a high relative velocity between the airflow and the particles, as well as a large heat transfer area, thereby enhancing the rates of heat and mass transfer. Additionally, the airflow velocity decreases significantly in the widened section of the tube, which correspondingly increases the drying time.

Air-Flow Dryer—Mechanical Principles

Wet material enters the dryer via a conveyor simultaneously with heated ambient air, where the two mix thoroughly. Due to the large heat and mass transfer area, evaporation and drying are achieved in a very short time.

The dried product is discharged from the cyclone separator, while a small portion of fine dust is recovered and reused via a cyclone dust collector or baghouse filter. The Q-type airflow dryer operates under negative pressure, with the material not passing through a fan; the QG-type airflow dryer operates under positive pressure, with the material passing through a fan that performs a crushing action; the FG-type airflow dryer is a tail gas recirculation type; and the JG-type airflow dryer is an enhanced airflow dryer that integrates flash drying and airflow drying.

Fans with a dispersing function are particularly suitable for airflow drying of heat-sensitive materials. The high-speed, rapidly rotating fan impeller can break up wet or even agglomerated material until it is fully dispersed; during this process, the material is simultaneously stirred and mixed, after which it flows in parallel with the hot air stream. This type of equipment is suitable for drying filter cakes and other materials with surface moisture, where the moisture content is ≤40%. If the processing volume is large or the finished product must be dried to below 15%, two-stage airflow drying may be employed. When the material’s moisture content exceeds 40%

but is ≤60%, feeding becomes difficult; a mixer should be used to reduce the feed moisture content by blending in dry material. In this case, the total output of the drying equipment will decrease significantly, which is not economically viable. Therefore, users should first use mechanical methods (centrifugal dewatering or filter pressing) to reduce the feed moisture content as much as possible to ensure the smooth operation of the drying process.

Air-Flow Dryer—Mechanical Structure

Direct Feeding

This is currently the most widely used method, suitable for situations where the wet material has good dispersibility and only surface moisture needs to be removed. Examples include the drying of synthetic resins, certain pharmaceuticals, organic chemical products, coal, starch, and flour. If the wet material has a high moisture content and tends to clump during feeding, a portion of the dried product can be used as recirculated material and mixed with the wet material in a mixing feeder to facilitate the drying process.

With a Disperser

In airflow drying systems equipped with a disperser, a cage-type disperser is installed below the drying tube to break up the material. It is suitable for lumpy materials with low moisture content and good flowability, such as filter cakes from centrifuges and filters, as well as phosphogypsum, calcium carbonate, sodium fluorosilicate, clay, coffee grounds, sludge residue, and corn residue.

Equipped with a Grinding Unit

An airflow drying system with a pulverizer features an impact hammer mill installed below the drying tube to pulverize wet material, reduce particle size, increase surface area, and enhance drying efficiency. Consequently, a significant amount of moisture is evaporated during the pulverization process; under normal conditions, this can account for 80% of the total moisture vaporized. This allows for the use of higher inlet air humidity, thereby achieving greater production capacity and higher heat transfer efficiency.