Concluding my series on general drying systems, in this column I will continue my discussion of rotary dryers. In my last column, I left off discussing direct rotary cascade dryers, so I'll pick it up there.
As I explained last month, direct rotary cascade dryers have internal lifters or flights to elevate the feed and drop it in a curtain from the top to the bottom, cascading along the length of the dryer. The carrier stream (hot gas) may be co- or countercurrent with the primary flow being through the "bed" or curtain, and, in this instance, multiple curtains in the longitudinal direction. As you can imagine, the formation of each curtain is intermittent. Therefore the design should allow for successive curtains to be formed in advance, promoting a continuous exposure of the feed to the carrier. Secondary crossflow occurs on the surface of the bed material on the bottom of the drum.
Some rotary cascade dryers are double- and triple-pass units where each drum is nested inside the previous drum. They have similar lifting arrangements, but they offer the added benefits of increased residence times for the same physical floor area and conductive heat transfer between the nested drum surfaces and product. This technology, however, can only be used for specific products and applications - dependent on product characteristics.
Rotary Louver Dryers. This type of rotary dryer has the feed materials supported and moving over a set of louvers mounted to an external rotating drum. The hot gas is introduced into a tapered bustle below the louver ring. The air passes through the louvers and the product (through the bed) before being exhausted from the dryer in a co-current or countercurrent flow. The drum rotation causes the material to roll and mix, providing intimate contact with the drying gas. There is a certain amount of fluidization that occurs in a rotary louver dryer, leading to this technology being thought of as a combined fluid-bed rotary dryer. The technology provides a very gentle method of handling the material and is especially well suited for fragile and crystalline materials.
In my next column, I will discuss rotary tube furnace dryers as well as the process flexibility and limitations of rotary dryers in general.