Principle

A Rotary Tube Bundle Dryer (RTBD) is a type of indirect heat transfer dryer used extensively in the chemical, food, fertilizer, and mineral industries for drying free-flowing and granular materials. It operates on the principle of conduction, where heat is transferred indirectly from steam (or another heating medium) flowing through a bundle of rotating tubes to the wet material surrounding them. The moisture from the product evaporates without the material coming into direct contact with the heating medium, making it ideal for heat-sensitive, dusty, or contaminable products.

Principle of Operation

The principle of a rotary tube bundle dryer is based on indirect heat transfer by conduction and mild convection.

• The dryer consists of a rotating shell containing a large number of steam-heated tubes (the “tube bundle”).
• The wet feed material is introduced into the rotating shell and continuously tumbled and mixed by the rotating motion of the drum.
• Steam flows through the tubes, transferring heat through the tube walls into the surrounding wet material.
• As heat is conducted from the tube surfaces to the product, moisture evaporates and the vapour is carried away by a small stream of low-temperature exhaust air or inert gas.
• The dried product is discharged continuously from the other end of the drum.

Since there is no direct contact between product and heating medium, the process is clean, controlled, and suitable for heat-sensitive or oxidizable materials.

Construction

A rotary tube bundle dryer consists of the following major components:

1. Shell (Outer Drum):

The main body of the dryer is a hollow, cylindrical shell that rotates slowly around its axis. The shell is slightly inclined (1°–5°) to allow the material to move gradually from the feed end to the discharge end by gravity. It is fabricated from carbon steel or stainless steel (SS 304 / SS 316), depending on the product characteristics. The shell is usually steam-jacketed externally to prevent heat loss and maintain consistent temperature.

The length-to-diameter ratio typically ranges from 4:1 to 8:1, and the drum rotates at a speed of 2–10 rpm, driven by a motor and gear system.

2. Tube Bundle Assembly:

The heart of the dryer is the tube bundle, which consists of hundreds of small-diameter tubes (typically 25–75 mm OD) fixed at both ends to tube plates and mounted inside the rotating shell. Steam (or another heating medium) flows through these tubes.

• The tubes are arranged parallel to the drum axis, covering the entire interior of the shell.
• The number and layout of tubes depend on the dryer capacity and heat transfer requirement.
• The rotation of the tube bundle ensures continuous agitation and uniform mixing of the material, exposing fresh surfaces for heat transfer.

3. Steam Inlet and Condensate Outlet System:

Steam is supplied through a rotary joint to one end of the tube bundle. The steam flows through the tubes, condenses on the inner walls, and releases latent heat to the material outside. The condensate formed is removed through a condensate collection header and discharged via the same rotary joint or a separate outlet.

A steam trap is installed to ensure continuous condensate removal and maintain maximum heat transfer efficiency.

4. Feed and Discharge Arrangements:

• The wet feed is introduced into the higher end of the drum through a feed chute or screw feeder that ensures uniform loading.
• As the drum rotates and the tube bundle agitates the material, the product gradually moves toward the discharge end, where the dried material exits through a discharge chute or screw conveyor.
• The residence time of the material inside the drum can be adjusted by controlling the drum speed and feed rate.

5. Air / Vapour Exhaust System:

Although the dryer works on indirect heating, a small amount of air or inert gas is introduced to carry away evaporated moisture vapours. The moist air exits through an exhaust duct connected to a cyclone separator or bag filter to recover fine particles before venting. In some cases, vapour condensers are used when solvent recovery is required.

6. Drive Mechanism:

The drum is mounted on support rollers or trunnions and rotated by an electric motor with a gearbox via a chain, belt, or girth gear drive. The drive system is designed to handle the combined load of the drum and the tube bundle while maintaining smooth rotation.

7. Insulation and Enclosure:

The entire outer shell is insulated with glass wool or mineral wool and covered with a protective steel sheet to minimize heat loss and ensure safe operation.

8. Instrumentation and Control System:

The RTBD is equipped with pressure gauges, thermocouples, condensate flow meters, and temperature controllers to regulate the steam pressure, drum speed, and feed rate.
Modern systems are automated using PLC–HMI panels for precision control, data logging, and safety interlocks.