When using a rotary drum, there can be a number of challenges that many processes face depending on the material. Three of the most common are: buildup, corrosion, and abrasion. Keeping these challenges under control will help to ensure that a rotary drum, whether a rotary dryer, cooler, kiln, or granulator, will last to its fullest potential. The first challenge we will discuss is buildup in a rotary drum.
Controlling Buildup in a Rotary Drum
Buildup is caused by material sticking to itself, or to the interior of the drum, and is almost always the result of a sticky material. Dealing with buildup in a rotary drum is a common factor faced in many industries, from fertilizer to mineral processing. Depending on the material, and the process, there are several ways to control buildup.
One option for controlling buildup in a rotary drum is to use a liner on the interior of the drum. There are many types of liners, from rubber liners, to stainless steel liners, all the way to ceramic tiles, the choice of which is dependent upon the material and the purpose of the liner.
Adding a liner is mainly used as an option in the case of granulators, coolers, and agglomerators, when heat is not involved in the process. Heat does not work well with most types of liners. Even a stainless steel liner can run into troubles when in a heated operation, because the heat causes liner and base materials to expand at different rates. A stainless steel liner inside a carbon steel shell could have a potential for cracking. When heat is involved, such as in the case of a rotary dryer or rotary kiln, external means can be taken to help prevent buildup, as opposed to using any sort of liner.
Another option for controlling buildup in a rotary drum is to “knock” off buildup through external means. The use of external means is typically employed in the case of rotary dryers, because, liners can pose problems in heat applications.
There are three basic knocker designs,: the ball and tube knocker, hammers, and pneumatic hammers.
Ball and tube knockers consist of tubes attached to the drum that hold heavy balls (think shot put)that drop on the drum by gravity as the drum rotates. A wear plate protects the shell, while still allowing for enough force to knock material off of the interior of the drum.
Hammer knockers are a similar set-up, using gravity as the delivery mechanism. A hammer is set up on a pivot, so as the rotary drum turns, gravity causes the hammer to fall, knocking the drum, and causing buildup to fall off. A wear band around the drum protects the shell, while still allowing for enough force to knock material off of the interior of the drum.
Pneumatic hammers – . A “hammer,” set up on a hydraulic system, is mounted to the ceiling or support structure above the rotary drum. This hammer can be programmed to knock the drum at set time intervals. A wear band around the drum protects the shell, while still allowing for enough force to knock material off of the interior of the drum.
In the fertilizer industry for rotary granulators, the liner of choice is often a flexible rubber liner. Flexible rubber liners work by having strips of rubber fixed to the interior, running parallel to the drum. As the drum rotates, gravity causes the rubber to flex down, sloughing off any material that was caked on. Historically, the fertilizer industry typically used scrapers on the interior of the drum to control buildup. However, the potential for the drum to “jump,” or jar, due to the scraper coming in contact with a large, heavy, built-up mass of material.. The move from scrapers to flexible liners was two-fold for fertilizer manufacturers, because not only did these liners help control buildup, but they also offered some corrosion protection.
Although there are many ways to keep buildup under control, the best way to control buildup is to prevent it from the start. There are a couple of ways to approach this.
One way to prevent buildup is to adjust the feedstock through what is called back mixing. Back mixing takes place when dry material is added to the wet, raw material in order to make the material less sticky. However, this is not always an option, as some materials remain sticky for a wide range of moisture levels. In cases such as this, a heated screw could be a more efficient choice over a rotary dryer.
Another way to prevent buildup is to design a less aggressive internal flight or lifter. Because buildup tends to occur in sharp corners, a less aggressive flight can lessen the propensity for buildup to occur. In this situation, some efficiency is lost, but the decline in buildup can be a compensating factor. Additionally, adding a bald section in the beginning of the drum allows the feedstock to dry a little before it hits the flights, also lessening the chances for buildup to occur.
While buildup is a common challenge faced by those utilizing a rotary drum in their process, there are many methods to combat buildup, or prevent it from occurring. Next we will look at corrosion and abrasion problems in rotary drums.
FEECO has been an industry leader in custom-engineering rotary dryers, coolers, kilns, and agglomerators, for over 60 years. Our equipment is robust, and built for longevity. With process design and optimization services, our equipment and systems are built to suit material needs, so you get the best process for your material. Contact us today to learn more about our rotary equipment capabilities.
View Part 2: Overcoming Challenges with Rotary Drums Part 2
Check back soon for the second half of this two-part series on the challenges faced in the use of a rotary drum.