Rotary Dryers, Part 1 By Darren A. Traub January 1, 2002

Spiral flights quickly move the material out of the feed section. Lifting flights elevate the material to produce a curtain. The drum is supported by a riding ring. Photo courtesy of Drytech Engineering

Concluding my series on general drying systems, in this column I will cover rotary dryers. Rotary dryers potentially represent the oldest continuous and undoubtedly the most common high volume dryer used in industry, and it has evolved more adaptations of the technology than any other dryer classification. Rotary dryer technology includes direct rotary cascade dryers, indirect (steam) tube rotary dryers, multipass rotary dryers, rotary tube furnace dryers, and rotary louver dryers. Drum dryers are sometimes referred to as "rotary" drum dryers and paddle dryers are sometimes referred to as "rotary" paddle dryers, but the technology behind these dryers is distinctly different and will not be included in this family.

In simple terms, a rotary dryer introduces wet feed into one end of a tube and a hot gas into the same or opposite end. The tube rotates and the hot gases and feed are intimately mixed while being transported down the tube, producing a dry product and a wet exhaust. Let's slow down -- it's not that easy!

As I have mentioned many times in my columns, the presentation of the feed to the carrier in the most intimate manner possible is a fundamental art aspect of designing drying systems. The better this is achieved, the more efficient and effective the operation. It is arguably more difficult to achieve this with rotary cascade dryers than most any other drying technology.

Because the heat transfer and presentation aspects of the different variations of these dryers are not the same for each configuration, these will be discussed individually. All rotary dryers have the feed materials passing through a rotating cylinder termed a drum. The drum is mounted to large steel rings, termed riding rings, or tires that are supported on fixed trunnion roller assemblies. The rotation is achieved by either a direct drive or chain drive, which require a girth gear or sprocket gear, respectively, on the drum. The drum expands at operating temperature, so it is important that only one side, usually the feed end, be constrained with thrust rollers in the longitudinal direction.

The drum normally is inclined down from feed to discharge at an angle of 1 to 4 degrees. The initial section of the drum has helical screw or spiral flights to rapidly move the material out of the feed section. Material moves from one end of the dryer to the other by the motion of the material falling "forward" or rolling "downhill" due to the angle of inclination of the drum as well as other dynamics associated with the angle of inclination and the rotation of the drum. Frequently, there also is a discharge spiral section to prevent blocking of the dryer discharge. Rotary dryers can process extremely high volumes of product: The drums can have diameters ranging from less than half a yard for laboratory units to in excess of 13' (4 m) for large-scale applications.

Direct Rotary Cascade Dryers. The most common type of rotary dryer, 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 -- hence the name rotary cascade dryers. These flights need to be carefully designed to prevent cross-sectional asymmetry of the curtain. The flights are arranged in repeating patterns, and the dryer should have several rows of distinctively designed flights that are indexed and offset to form numerous simultaneous curtains along the drum length.

Rotary Dryers, Part 2

This co-current rotary cascade dryer is chain driven and uses a fabric reverse-pulsing dust collector for emission control. The dryer processes clay-type minerals that are used for the paper industry. Photo courtesy of Drytech Engineering

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.

Rotary Dryers, Part 3

This conventional rotary cascade dryer is used to dry Anthracite (for a reactor) and has an integral cooler built in to the end of the drum. The unit is co-current and uses a cyclone followed by a wet scrubber for dust collection.

Concluding my series on general drying systems, in this column I will continue my discussion of rotary dryers.

Rotary Tube Furnace Dryers. An indirect dryer that allows a high degree of temperature control, a rotary tube furnace (RTF) dryer consists of a muffle furnace with a steel drum passing through it. Tumbling or rolling flights rather than the lifting flights such as those in the cascade rotary dryer are fitted to the inside of the drum. In operation, the particles are exposed to the drum surface, which is heated from the outside by a suitable heat source such as gas burners or electric elements. The internal flights tumble and mix the product, constantly exposing new surfaces to the heated drum surface. In addition there is a high degree of conduction between the product particles to enhance operation efficiency. This same principle is the basis for indirect calciners and reactors, which will be a topic for a future column.

In all rotary dryers the speed of advancement of the material -- and hence its retention time in the dryer -- is determined by the rotational speed of the drum as well as its angle of inclination. By varying these parameters, the residence time can be controlled accurately. The amount of material in the drum at any one time -- the drum fill -- is relatively low as a percentage of the cross-sectional area or total volume of the drum. They are typically of the order of 8 to 15 percent of the total volume of the drum.

Rotary dryers are continuous processing machines that can effectively process feeds that are classified as powders, granules, nonfriable agglomerates and large solid particles. Some -- the direct cascade and tube furnace, for example -- are able to cope with wide variations in the feed such as particle size and moisture. Depending on the configuration of the particular dryer, the feed and carrier inlets, discharges (or both) need to be well sealed to prevent the introduction of cold air into the system or the expulsion of hot, dust-laden air to the atmosphere. They operate at varying feed rates from several pounds to hundreds of tons per hour.

Process Flexibility

Some of these dryer designs increase productivity because they are able to integrate specific process requirements. For example, integral cooling is easily accommodated in all of the types described except steam tube and multipass cascade units. For high temperature or corrosive applications, internal refractory linings can protect the fabric of direct-cascade units. Hammers, chains and knockers will relieve the buildup of sticky products to the flights and internals of cascade, multipass and tube furnace dryers.

Heat sources include steam, electricity, coal, liquid fuels and gas. In the United States, however, most large-scale rotary cascade and louver dryers use gas due to the relatively high cost of electricity and the costs associated with treatment of the products of combustion for coal and heavy fuel oil burners to meet EPA requirements. Indirect rotary tube furnaces use gas frequently and use electricity more often than conventional cascade type dryers. Obviously, steam tube dryers use steam but may also use other thermal oils.

As discussed above, rotary dryers have a dynamic airflow that can principally be either co-current or countercurrent. In either instance, an induced-draft exhaust fan normally achieves extraction of the carrier. For conventional direct rotary dryers such as the cascade or louver, dust collection systems are essential. The technology for these systems includes primary and secondary collection devices such as cyclones, bag houses, scrubbers and/or electrostatic precipitators.

Control of these systems is either by PLC or solid-state controllers. The system traditionally modulates the inlet temperature (energy) based on the discharge temperature. Controls for the modulation of feed also can be incorporated into the design architecture.

Limitations

Rotary dryers do have some significant limitations, especially if the technology is misapplied. Wet and sticky products cause clogging of the inlet and transfer section of the dryer drum. Flights often are clogged, reducing their carrying capacity and the volume of the curtains. Chains and knockers can mitigate this somewhat, but industry spends far too much valuable processing time digging out "blocks" of built-up material.

Like the hammers and chains, processing large particles causes noise, and the impact of the particles from the fall may cause size reduction. Cascade dryers have difficulty providing accurate temperature control, particularly if there is a variation in the feed characteristics. This results in variations in the dried product characteristics and, most commonly, the final moisture. Direct cascade dryers are simple machines with aggressive material handling. This can result in significant wear and high maintenance costs. In addition, a large rotating mass such as a drum has numerous high maintenance aspects and components. Well designed, engineered and maintained, these are controlled. This,

however, is more the exception than the rule, and quick-fix patches do nothing to improve the reputation of these systems.

Because the dryer drum is large and rotating, insulation is frequently omitted for cost or other reasons. Although insulation is only really effective in the first 25 to 30 percent of the length due to the rapid decrease of temperature, not having insulation compromises the system's thermal efficiency, with high losses associated with the drum shell. Large volume machines require large real estate.

There is no other technology that can offer the same complete benefits to a high volume process with wide variations in the feed. Rotary cascade dryers are the "donkeys" of the drying industry: They are relatively low cost machines that are effective when applied correctly for the specific operation.

The rotary dryer family offers a comprehensive choice in processing. Each has a specific niche and when properly applied offers significant overall advantages for the application.