A drum dryer is a rotating cylinder that uses steam or hot air to heat feedstock and reduce the moisture content during a manufacturing process. The configuration may include one or more drums. A drum dryer may use atmospheric pressure or function under a vacuum. Drum dryers have many applications in the food industry because they are very efficient for drying pastes or gelatinous materials.
In the simplest configuration, a single drum dryer is supported on a frame so it can rotate on its axis. Steam is forced through the inside. A thin layer of feedstock is then applied to the outside of the drum. As the temperature rises, the moisture in the feedstock reaches boiling point, begins to evaporate, and leaves the feedstock. The dried layer of feedstock is removed from the drum with a scraper.
Depending upon the products it will be processing, a drum dryer is typically made from cast iron or stainless steel. The length of time the feedstock remains on the drum depends on the original moisture content and the desired ending moisture content. The range is usually between a few seconds and half a minute. Feedstock may be applied to the drum dryer by rollers, dipping the lower edge of the drum into a tray of feedstock, or spraying it on to the drum surface.
A double drum dryer consists of two drums mounted side by side and rotating toward each other. The layer of feedstock is controlled by the distance between the two drums. Feedstock is applied to the two drums from above. This configuration can increase production capacity.
If the feedstock is subject to damage from excess heat, a vacuum drum dryer may be used. The entire drum is encased in a vacuum chamber where air can be evacuated; this reduces the boiling point, and the drying can be accomplished at a lower temperature. Vacuum systems are more expensive to operate and are more commonly used for higher value products such as pharmaceuticals.
Another type of rotary drum dryer is a double or triple pass system. In this configuration, the drums are nested one inside the other. The feedstock is introduced to the innermost hottest drum and is moved along with channeling flights mounted to the drum interior and with hot air. After the feedstock loses a certain amount of moisture, the air flow carries it to the next drum, which is cooler, where it continues to lose moisture. If there is a third drum, it is even cooler with slower air flow, thus continuing the drying process while protecting the feedstock from heat damage.