Direct dryers are used more frequently than their indirect counterparts, because of the efficiency they offer. And while direct rotary dryers are most often the selected drying method, indirect rotary dryers are a valuable alternative for specific processing requirements.

OPERATION

While both types of dryers are comprised of a rotating drum in which the processing occurs, these two types of dryers utilize very different methods to process the material.

Direct dryers rely on direct contact between the material and drying air to efficiently dry materials. Efficiency is further increased by the addition of lifting flights affixed to the internal walls of the dryer. Conversely, indirect dryers do not utilize direct contact with the drying air to reduce the material’s moisture content. In fact, this is the precise advantage of an indirect rotary dryer; indirect drying allows you to tightly control the processing environment—an ideal characteristic when working with materials that can be combustible in certain settings.

Indirect dryers rely on the heat transferred through the drum’s shell to dry the material via conduction and radiation. Subsequently, indirect dryers would not benefit from the addition of lifting flights, and instead utilize tumbling flights, which aid in material rotation and help to ensure process consistency.

Indirect rotary dryers also require an internal flow, referred to as sweep air. Sweep air carries the evaporated moisture, along with dust particles from inside the dryer, to the exhaust system at the discharge breeching. This process ensures that the material is being dried to the required moisture percentage.

Furthermore, because the furnace exhaust is kept separate from the drying air, indirect dryers have the added benefit of requiring less exit gas treatment. This exit gas can be used to pre-heat the combustion air and increase burner efficiency.

MATERIAL OF CONSTRUCTION

Unlike their direct counterparts, which are typically constructed of carbon steel, indirect rotary dryers are constructed using a high-temperature alloy, in order to maintain the integrity of the drum, despite the constant exposure to high temperatures that the shell must endure.

INDUSTRIAL USE/ APPLICATIONS

Direct rotary dryers account for most of the bulk solids drying applications,and are used throughout a multitude of industries, including agriculture, mining, specialty chemicals, and more. They offer heavy-duty processing and consistent reliability.

Indirect rotary dryers are better suited for niche applications, and are ideal for processing dusty fine materials, or materials that cannot be exposed to drying air during processing. This is commonly seen with highly combustible materials, organics such as grains and pulps, and fine materials such as pigments.