Humans from time immemorial have been involved in drying using natural means — the sun and the wind. As time passed, we adopted the ovens for better drying. All these methods are used on a small scale level. Large scale drying used by industrialists involves a rotary dryer. The rotary dryer is versatile making it suitable for application over a wide range of materials from chemicals to food items.
The main function of a rotary dryer is the removal/reduction of moisture from the material fed into the dryer. It works using heated gas. Some materials do not get damaged if they come in contact with the heated gas, so these materials adopt the direct heated rotary dryer. While some other materials do get damaged when exposed to the heated gas, in this instance such materials are heated indirectly using an indirect heated rotary dryer.
The type of rotary dryer to use depends on the material to be dried, including the intended end product.
This brings us to the two main types of rotary dryers: the direct heated and the indirect heated rotary dryer.
Rotary dryers are simple machines, and easy to operate. They are all shaped cylindrically and are usually four times as tall as they’re wide. The reason for this is to give the gas more room for interaction with the material inside the dryer. Lining its inside are metallic fins used to propel and lift the material around inside the dryer. Quantity fed into the dryer is usually 10-15% of its full capacity. The cylindrical rotary dryer is supported by four rollers.
Both types of dryer carry out essentially the same functions; however key differences exist between them.
Direct heated rotary dryer works by releasing hot gas into the cylinder after feeding the material into the cylinder. The hot gas passes through the material, drying it in the process before exiting in the opposite direction from where it was released.
The airflow that passes through the dryer, in the direct heated rotary dryer, is what dries the material within the dryer. This airflow is of two types: concurrent and countercurrent.
Concurrent airflow is also known as parallel airflow because the materials and air move in the same direction. The gas is usually of very high temperature and is used for materials with high moisture content. Concurrent airflow moves at high speed, making it suitable for fast drying of very wet materials. However, the materials that concurrent airflow can be applied to must be materials that do not change states under high heat, nor get damaged when exposed to high heat.
Concurrent airflow rotary dryer is used for materials such as chemicals, sludge, fertilisers, food items and mostly ores.
Countercurrent airflow, on the other hand, is air flowing in the opposite direction to where the material is exiting from the cylinder. This method is used for very rapid drying of the materials as the airflow temperature is usually at the highest. However, the materials that countercurrent airflow can be applied to must be those that require to end up in a different state resulting from the heat applied.
Countercurrent airflow rotary dryer is used for materials such as clay, plaster, and sodium.
Indirectly heated rotary dryers are used for materials that could easily get damaged if brought into direct contact with hot gas or oxidation. Radiation and heat conduction is used to transfer the heat from the body of the dryer cylinder to the material inside it. As a result, indirectly heated rotary dryer does not employ any airflow that needs to pass through the dryer. The cylinder itself is heated through a furnace built around the cylinder.
Indirectly heated rotary dryer is the correct choice if you need to control the amount of gas exhaust, or are dealing with a wet, finely powdered material that could generate a large amount of dust when dried, or when you need to recover material susceptible to sudden change.
Indirectly heated rotary dryer can also be used for materials that countercurrent airflow rotary dryer is used for, including silicone, ammonium, and sugar.
The main limitation of the indirectly heated rotary dryer is that if the material to be dried sticks to the cylinder walls, it becomes difficult to dry such material properly.
The majority of rotary dryer manufacturer offer precise engineering or customising services to their clients. This is not surprising as the type of material, its density, and size of the material to be dried vary greatly. While most general design elements are adhered to, certain specifications are incorporated to better meet the needs of the client and achieve excellent results.
Ravi Kumar is a mechanical engineer with over a decade of work experience in the manufacturing industry. He is a key member of the product architecture team at Chanderpur Group. He is always keen to study and try out new technologies.