Rotary drum dryers are the workhorse of the drying industry because of their capacity, efficiency and ability to adapt to new purposes. This article will discuss what a rotary drum dryer is, what applications a rotary drum dryer is best suited for, and common concerns and solutions regarding rotary drum dryers.
Rotary drum dryers can be chosen for projects as large as evaporating half a million tons of water to as small as evaporating several tons of water. They come in all sizes to accommodate many products, quantities and moisture-content ranges. Although they can be larger, 18-foot-diameter drums seem to be the largest practical size for manufacturing, transport and installation purposes. Depending on length, an 18-foot drum could evaporate 120,000 pounds per hour of water. If, for example, the product to be dried starts at 50 percent moisture-content wet basis (mcwb), this drum could handle 120 tons per hour or more. At 8,400 operating hours per year, you’re looking at a million tons of product infeed going through every year, or about half a million tons per year of final product.
An 18-foot dryer drum is not the only option. Rotary dryer drums as small as 4-foot diameter can evaporate anywhere from 200 to 500 pounds per hour. Rotary drums much smaller than this have efficiency losses, so an alternate drying system may be more beneficial. Using the same example as above, with a product at 50 percent mcwb, a small rotary drum dryer can take care of 400 to 1,000 pounds per hour of product, or 1,680 to 4,200 ton/year infeed, and approximately half that of product out. Obviously, these numbers depend on a variety of variables — not the least of which is the moisture content of the infeed and the desired moisture content of the final product.
Rotary drum dryers can be designed to withstand extremely high temperatures and corrosive materials. A basic carbon steel drum is designed to withstand inlet temperatures from 1000 to 1100°F (537 to 593°C). Of course, rotary drum dryers can be designed to handle different inlet temperatures. Stainless steel can be used in place of carbon steel if the product is known to be corrosive. Other materials can be substituted as well depending on the nature of the product to be dried.
Rotary drum flighting is the primary material-handling mechanism in a drum. Flighting can be designed to accommodate different flow characteristics of the materials to be dried. Some materials clump and potentially form balls that need to be broken up while others flow like water and need to be slowed down. Other pieces might be long and stringing and easily can get trapped. Different flighting designs can solve these and other problems.
With so many specialized dryers available today, it seems that every product has a different dryer designed specifically for it. What happens if you quit making a particular product that had a specialized dryer? Can the dryer be modified to dry something else? If it is a rotary drum dryer, the answer is “yes.” Another wonderful feature of a rotary drum dryer is that the flighting system can be changed to repurpose a used or existing dryer. A flighting designer can look at the flow characteristics of the material in question and determine what kind of flights are necessary, thereby converting the dryer to suit a new purpose at a smaller cost than buying a new dryer.
|Figure 1. Products that have nonuniform particle sizes such wood are good candidates for a rotary drum dryer. However, the flighting system would be different for the sample on the left than for the sample on the right.|
What Products Do Best in a Rotary Drum Dryer?
Rotary drum dryers can dry many different types of products. Not only are they great for uniform solid particles, they are also a viable solution for nonuniform solid particles. The number one factor in deciding on a dryer is whether the material handling of the dryer matches the needs of the product.
Products that have nonuniform particle sizes such wood are perfect candidates for a rotary drum dryer (figure 1). However, the flighting system would be different for the sample on the left than for the sample on the right. It is important to know the characteristics of the infeed product to choose the best flighting design.
A rotary drum dryer’s greatest asset is its ability to accommodate a product that has multiple size particles and moisture contents by utilizing the right flighting package. As shown in figure 1, the wood is in a variety of shapes and sizes. The larger, heavy particles will be carried less distance by the drying gases than the smaller, lighter particles. This means that the smaller particles can exit the dryer before they become overdry and the larger particles can remain in the drum until they have dried a sufficient amount. The flights can be designed to keep the larger pieces in the drum longer and allow the smaller pieces to flow more quickly. The right flighting package can help achieve uniform product quality and also can lead to reduced emissions.
Why Doesn’t Everyone Use a Rotary Drum Dryer?
Unfortunately, rotary drum dryers seem to have fallen into a niche market. Dryer manufacturers and purchasers have been shying away from rotary dryers because of perceived reliability, quality and versatility issues.
One of the perceived issues is lack of reliability. Drum shell, headplate or tire flexing can lead to premature failure. Rotary drum dryers get a bad rap due to some drums not being designed and built correctly or drums being repurposed without upgrading them for the new product or application. Without proper internal and external support, the drum has a tendency to flex while rotating — in some instances, causing shaking and vibrating. Some drums have been measured to continually be 0.25" wider than tall, flexing every time they rotate. Imagine if you will, a water balloon rolling down the sidewalk. When steel flexes to that degree, eight times per minute (based on 4-rpm), it does not take long before it breaks. However, given the right design, construction materials and fabrication techniques, a well-built rotary drum dryer should last more than 20 years without any major structural issues such as cracks in the shell or headplates.
Lack of versatility is another perceived issue associated with rotary drum dryers. The importance and flexibility of different flighting packages is often misunderstood in the industry. Many rotary drum dryers available today only have an option for one type of flighting package. To get the most out of a rotary drum, find a manufacturer that offers flighting packages for your applications and products.
Another challenge facing rotary drum dryers is a reputation of poor product quality and excessive emissions. This can be overcome in a few ways, two of which are discussed here. The first way, already discussed in this article, is by utilizing the right flighting package for your product.
The next method is through the use of exhaust gas recycle (EGR) — also known as flue gas recycle (FGR) — where the product can be protected from hot, dry, drying gases by recycling water vapor back through the system and increasing the wet-bulb temperature. The higher wet-bulb temperature allows the particle internals to heat at the same rate as the externals, thereby allowing an even evaporation rate. If the wet-bulb temperature is low because the system does not have enough moisture, the outside of the particle can flash dry and act as an insulator to the inside, not allowing the moisture to migrate to the surface. The particle can reach the end of the drum with an overdried or charred outside and wet inside. (Think of it like putting a burger on the grill when the coals are too hot.) Overdrying or charring the outside gives off volatile organic compounds (VOCs), which will cause a haze and bad smell outside the plant if a back-end cleanup system has not been utilized.
Not overdrying the outside decreases the chance of creating VOCs, which results in lower emissions. Another benefit of EGR and the right flighting package for your product is the potential to lower the inlet temperature. High inlet temperatures cause most of the overdrying, so lowering the inlet temperature will also reduce the potential for VOC emissions.
Rotary drum dryers are the workhorses of the drying industry because of their longevity, capacity and ability to be adapted to new purposes. Do not overlook a rotary drum dryer when trying to decide how to improve or expand your drying equipment. As you can see, you can overcome any perceived problems and achieve effective drying.
|Troubleshooting a Rotary Drum Dryer|
A drying system can have many different types of problems that require routine maintenance, repairs or replacement parts. Below are a few common problems and solutions.
Vibration. Many rotary drying systems have problems with vibration. The vibrations come from the rolling surfaces between the tires on the drum and the trunnion wheels and the drive system and chain. Most people notice these vibrations first by feeling them in the floor while they walk around the drum or in supporting equipment when they touch it. The vibrations will shorten the life of some system components. The most common problem is shortening the lifespan of the trunnion wheel bearings. Vibration also can do damage to the trunnion bases, causing them to come loose, and to the drive assembly such as the gearbox, teeth and chain. The best way to prevent vibration from the rolling surfaces is to align the trunnion wheels and have the tires and trunnion wheels resurfaced every two years. If the vibration is coming from the drive assembly, it is likely time to replace the teeth, sprocket or chain.
Cracks in the Head Plates, Tires or Shell. Many rotary drying systems have problems with cracking head plates, tires and shells. This is likely caused by flexing. Flexing is when the drum changes shape while it rotates. Think of it like a rolling water balloon, which will always be wider than it is tall. Drums that flex will eventually work harden and become brittle and break. If not corrected, the effects dramatically decrease the drum?s useful life.
There are several ways to stop the flexing and fix the damage. The exact solution needed can be different for each system. If flexing is allowed to continue, it will cause more damage to the drum, which will increase the maintenance cost and the likelihood that the drum will need to be replaced. As soon as you notice problems with the head plate, shell or tire, you need to address this issue.
Trunnion Wheel Failures. A trunnion wheel failure is most commonly the failure of a trunnion wheel bearing. It can also be caused by the axle shaft failing or simply by the trunnion wheel wearing out over time. Did you know that according to manufacturers? specifications, the typical trunnion wheel bearing should last more than 20 years? Are yours lasting that long? If not, it is important to find out why.
|Trunnion bearing failure most often is caused by three different issues:|
Are your trunnion wheel shafts breaking? This is often caused by thrust in the system being applied to the trunnion wheels incorrectly, causing the shaft to fail. To correct this problem, the nature of the misapplied thrust should be investigated to determine how to correct the problem.