Rotary dryer

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A rotary dryer functions as an effective industrial dryer, essential for minimizing moisture content in various materials by exposing them to a heated gas. The design typically comprises a rotating cylinder known as a "drum" or "shell," coupled with a drive mechanism and a supportive frame, usually formed from concrete or steel. The cylinder is slightly inclined, causing the discharge end to be lower than the feed end, allowing gravity to aid in the movement of material through the dryer. As the drum rotates, material to be dried is lifted by a series of internal fins or flights. Upon reaching a certain height, the material cascades back down, interacting with the hot gas stream as it descends.

Applications

Rotary Dryer units are highly versatile, finding primary application within the mineral sector for drying various materials including sand, stone, soil, and ore. They are similarly utilized in the food industry to dry granular items such as grains, cereals, pulses, and coffee beans.

Design

Various rotary dryer designs are tailored for specific applications, where factors such as gas flow, heat source, and drum design influence the dryer’s efficiency and compatibility with different materials.

Gas Flow

The hot gas stream can travel towards the discharge end from the feed end (termed co-current flow) or in the opposite direction from the discharge end to the feed end (referred to as counter-current flow). The flow direction, combined with the drum’s slope, significantly impacts the material transit speed through the dryer.

Heat Source

Gas streams in rotary dryers are primarily heated via burners utilizing gas, coal, or oil. When combustion gases mix with air to create the gas stream, the dryer is classified as "directly heated." Conversely, in cases where the gas stream consists of preheated air or other inert gases without burner gases entering the dryer, it is referred to as "indirectly-heated." Indirectly heated options are particularly advantageous when preventing product contamination is critical. Some designs also employ a combination of both heating methods to optimize efficiency.

Drum Design

Rotary dryers can feature a single shell or multiple concentric shells, where configurations with more than three shells are seldom necessary. Multi-drums enhance space efficiency by delivering the same throughput while being more compact. These multi-drum systems are often heated directly by oil or gas burners, and incorporating a combustion chamber at the feed end guarantees optimal fuel usage and consistent drying air temperatures.

Combined Processes

Certain rotary dryer designs allow for the simultaneous execution of additional processes such as cooling, cleaning, shredding, and separating alongside drying.

The rotary dryer offers exceptional capacity, low fuel consumption, and economical drying costs. This high-temperature operational ability enables quick drying of materials. With scalable designs that can accommodate slight production increases without necessitating equipment replacement, it offers considerable efficiency.

Encouraging smooth operation, the dryer adopts a spherical tractor structure, which effectively minimizes wear and energy usage. The specially designed retaining wheel structure significantly lessens horizontal thrust due to the equipment's inclination. This results in robust overload resistance and dependable performance of the cylinder.

With a thermal efficiency of 80% to 90%, the rotary dryer utilizes heat transfer effectively, maintaining a consistent heat transfer direction throughout the process. Consequently, the majority of the heat provided is dedicated to vaporizing moisture from the material.

The drying mechanism maintains a high evaporation rate, generally between 30 and 70 kg/(m.h), thanks to the substantial temperature gradient enabling effective heat and mass transfer from within the material film on the drum wall.

Due to controllable heating methods, the internal temperature and heat transfer rate remain stable, ensuring consistent product quality during the drying process. However, one drawback includes higher surface humidity, which may adversely affect the flavor or color of sensitive products due to overheating. Additionally, while vacuum dryers are an option, they can prove cost-prohibitive and are reserved for materials sensitive to heat.