Drum dryers use steam or hot air to reduce moisture content in raw materials during manufacturing. They have many food industry applications and can be made of cast iron or stainless steel. Different configurations include single, twin, vacuum, and double/triple pass systems.
A drum dryer is a rotating cylinder that uses steam or hot air to heat raw material and reduce the moisture content during a manufacturing process. The configuration can include one or more drums. A tumble dryer can use atmospheric pressure or operate under vacuum. Drum dryers have many applications in the food industry because they are very efficient for drying paste or gelatinous materials.
In the simplest configuration, a single tumble dryer is supported on a frame so it can rotate on its axis. The steam is forced through the interior. A thin layer of raw material is then applied to the outside of the drum. As the temperature increases, the moisture in the raw material reaches boiling point, begins to evaporate and leaves the raw material. The dried layer of raw material is removed from the drum with a scraper.
Depending on the products that will be processed, a drum dryer is usually made of cast iron or stainless steel. The length of time the raw material remains on the drum depends on the original moisture content and the desired final moisture content. The interval is usually between a few seconds and half a minute. The raw material can be applied to the drum dryer by rollers, by dipping the bottom edge of the drum into a tray of raw material or by spraying it onto the drum surface.
A twin drum dryer consists of two drums mounted next to each other and rotating towards each other. The layer of raw material is controlled by the distance between the two drums. The raw material is applied to the two drums from above. This configuration can increase production capacity.
If the raw material is prone to excessive heat damage, a vacuum drum dryer can be used. The entire drum is enclosed in a vacuum chamber where the air can be evacuated; this reduces the boiling point and drying can be done 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 inside each other. The raw material is introduced into the innermost hotter drum and is moved along the duct flights mounted inside the drum and with hot air. After the raw material has lost a certain amount of moisture, the airflow carries it to the next, cooler drum, where it continues to lose moisture. If there is a third drum, it is even cooler with slower airflow, thus continuing the drying process while protecting the raw material from heat damage.
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