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.
|
|
Multiple rows of tubes are heated
internally by steam. Material cascades through the tube bundle as the
drum rotates, allowing efficient conductive heat transfer to ensue.
Photo courtesy of Drytech Engineering |
|
Rotary Steam Tube Dryers. Rotary steam tube dryers operate in a
similar fashion to conventional rotary cascade dryers with the
exception that the heat transfer is indirect (principally conductive)
with the material cascading through a rotating nest of tubes that are
internally heated by steam or other thermal transfer fluid. The
lifters, if used, are on the peripheral circumference of the drum.
Otherwise, the tubes actually act as the lifters or conveying medium
that elevate the feed to the top of the drum and release it to contact
and tumble through the tube bundle directly. Many have spirals
installed to assist in moving the materials forward. Only evolved
vapors are exhausted from the drum, requiring a lower volume of air for
the process.
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.
