Do you know all seven types of indirect batch dryers? Knowing, understanding, and comparing the various types of dryers available will help you make informed choices when it comes time to select the right equipment for your process. This article details some of the types and uses of the different indirect batch dryers on the market today. We’ll also touch on the unique design elements that differentiate each model from the others.
First, it’s important to define the two main classifications for drying equipment – indirect drying versus direct drying. Direct dryers utilize heated air or inert gas that is continuously applied directly to the product. Indirect dryers, on the other hand, are typically run as a batch operation (though they can be continuous) and involve hot fluid circulating through a jacket surrounding a sealed vessel that contains the product.
The indirect drying method is usually preferred for small or medium batches whose products risk contamination or degradation by exposure to the atmosphere or feature toxic and/or hazardous chemicals that should not be released into the environment. Do any of your processes include pharmaceuticals, photochemicals, agricultural chemicals or explosives? If you answered “yes” to this question or if you work with another specialty chemical that has sensitive characteristics, then one of the following indirect dryers could be suited for your application!
1. Conical (Double Cone) Tumble Dryer
Maybe the simplest and probably the most cost-effective dryer is the conical tumble dryer. It gets its name from its basic shape (though several variations are available). The dryer is mounted on a horizontal axis, enabling a full 360 degree rotation during operation to provide good mixing and regular contact of the product with the heated wall. The machine is moved to a vertical position after drying is achieved to discharge solids from a nozzle at the bottom of the cone. Materials of construction include glass-lined steel, which provide excellent corrosion resistance.
2. Paddle Dryer
Operating in the horizontal plane, the paddle dryer features a cylindrical vessel equipped with a multi-paddled agitator supported at both ends. In most designs the heating fluid that circulates through the vessel wall also flows through the agitator. The movement of the agitator in combination with the heating element makes this machine ideally suited for efficient mixing and heat transfer. Furthermore, the agitator’s support structure allows for the handling of high-shear or lumpy solids. Product discharge is achieved through an outlet on the bottom of the cylindrical shell, but due to the shape total discharge is not possible.
3. Disc Dryer
Known to be extremely versatile, the disc dryer can be used in batch or continuous mode and can process a variety of solids including wet cake or slurry. The equipment consists of a vessel shell that encompasses a vertically or horizontally-mounted rotor. A series of discs are set at an angle and attached to the tube-shaped rotor. Heating fluid flows through not only the vessel wall but the rotor and discs as well, creating a high heat transfer area (additional heat via hot-gas blow through is also capable in direct conduction mode). The complexity of this design, however, hinders its cleanability so it is not typically used for high-value solids.
There's a lot more ground to cover with Batch Dryers and drying processes. Be sure to check out Part 2 of this post where we'll discuss the four remaining types of indirect dryers (can you name them?...see the bottom of this post for the answer!). To find out even more, read our free White Paper, “Batch Drying: The Indirect Solution to Sensitive Drying Problems” where you can learn about the guidelines on specifying an indirect dryer and how to optimize drying performance through peripheral equipment.