As is true with most mechanical equipment, not all dryers and kilns are created equal. Perhaps even more dramatic is how the design of flighting systems — the fins or baffles installed inside dryer drums to direct airflow and product within the dryer and optimize drying — have changed over time. As flighting designs have evolved, many of the units have become outdated compared to the new technology.
For the purpose of this article, the term “flighting” will include any material flow management components located inside the processing unit. Given that, here are the 10 tips for finding a flighting system to increase productivity and reliability while decreasing the plant maintenance cost.
1. Optimize Heat Transfer
Heat transfer is key. The ideal flighting system must provide efficient heat transfer from the source in order to heat and dry material and allow the removal of water vapor and products of combustion. In addition, an optimal flighting system will protect the primary shell, the thickness of the shell, the carrier rolls (tires), support rollers (trunnions), drive systems and other components. Drying efficiency, productivity and long-term performance can be increased (or sacrificed) based upon design decisions about the flighting system. The type of flights, the pattern of flights, the material of construction and other specifics associated with process demands impact overall heat transfer.
2. Recognize When to Upgrade
Typically, older units require new, improved flights to accommodate current operating conditions. From a general industry observation of both kilns and dryers, more than 70 percent of them are being operated above original design capacity. Given that, the original equipment flighting cannot keep up with the increased demands for operating the dryer or kiln. Cost savings could be realized by improving the flighting pattern and improving the design of the entire system. These improvements will, in turn, impact combustion, heat transfer and wear.
3. Evaluate Current Flighting Setup
A thorough flighting evaluation is important. Establishing baseline performance and efficiency for the flighting will include items such as:
• Measuring the CO and O2 levels at multiple stages of operation.
• Measuring the temperature of the shell at multiple points on the unit.
• Measuring multiple temperatures on the exhaust to see if there is a significant range of temperatures from one side of the exhausting duct to the other side of the duct, and other points in between.
By establishing this baseline, it will help determine whether and how much the operation has changed over a certain period of time. If there are significant increases in temperatures or decreases in combustion efficiency levels, these often can be attributable to the flighting system.
4. Select Flighting Materials of Construction Carefully
How abrasive is the material being processed? Another major item to consider regarding the flighting system is the type of material being processed: its abrasiveness, the rate of airflow passing through the unit and the wear resistance of the design.
When extremely abrasive materials are processed, the flighting systems selected must offer abrasion resistance. This is critical in order to maintain the proper processing rate of flow as well as efficiency.
If the flighting system will be subjected to chemical or acid attack, it may call for specialty alloy steels. Depending upon the level of abrasiveness in the material being processed, specially engineered flights or extra reinforcement of the existing flights should be considered.
5. Factor In the Size of the Material Being Processed
The next major consideration in flighting is the material gradation. It is important to know if the gradation of materials being processed will vary greatly. For example, processing a 0.25" or less material size would require a different flighting pattern, depth of bed, material of construction and veiling patterns than processing a 0.75" or less sized material.
If a dryer or kiln unit is going to process multiple sizes and varying materials, custom flighting systems may be needed where zoned flighting handles one size material in one zone and alternates with another size in another zone.
6. Consider the Fuel Type
One recent development in flighting systems is the emergence of overlapping internal combustion zones. If the dryer or oven is direct fired and the flame actually occurs within the envelope of the shell, then many steps must be evaluated to properly ensure adequate volume for complete combustion without impinging the flame. The rates of flow — in pounds or tons per hour being processed — and air system are important but so is the selection of fuel type. Burning a heavy oil or waste oil will require a significantly different pattern of combustion flighting than if fuels such as natural gas or propane are used.
7. Optimize Multi-Function Units
A number of rotary dryer/kiln/calciner units actually perform multiple functions within the shell. A unit may perform drying in one section, introduce other materials in the next section, effect chemical or thermal changes in another, and mix or add a liquid in the final stage. In these multi-purpose units, the flights must be selected carefully to perform the different functions. For example, some of them may actually slow the progress of moving material. Others may have reversing flights to perform a mixing or churning action as opposed to a progressive movement or veiling action.
8. Learn about Zone Flighting
In zone flighting, many different types, styles and lengths of flights create a curtain of material and promote heat transfer most efficiently. Many older drying units had few flights — only six or eight — and often long flights — 10 to 12’ long — around the circumference. This is extremely inefficient when it comes to fuel usage and energy costs.
In modern dryers, ovens and kilns, with the increasing desire for efficiency, the same circumference may have 18 or 25 flights that measure only 3 to 5’ long. The flights are arranged in staggered patterns to promote a much denser veil of the material. The flights are constructed in many shapes and sizes to provide initial moisture removal or maximum moisture removal through both conductive and convective heat.
9. Check the Seal System
The seal on an oven, dryer or kiln helps reduce bleed-in air, provides more consistent combustion, helps reduce the cost of heating and may help cut production losses due to bleed-in (fugitive) air loss. To be effective, the flighting for both the intake and discharge ends must be arranged with the seal system in mind. Doing so helps ensure that the material to be processed can be carried into or out of the drying unit quickly and minimize air losses.
Many designs use a spiral or intake unitized flight arrangement that quickly sweeps in material to avoid leakage and interference with the air seals. Sticky materials that tend to agglomerate can be separated by a swinging or pivoting flight that moves or rotates with the rotation unit. Chains are another alternative for releasing and freeing the product that also pick up heat from the hot gases and conductively transfer it to the material.
On discharge flighting arrangements, certain styles call for side discharges or even a high lift discharge. Having the flighting arranged to consistently provide the dried material without getting it re-entrained in the airstream is another important design consideration. Remember that most seal systems are designed to seal air only and not product. The flighting system must keep the product away from the seal in order for the process to be efficient.
10. Minimize Material Waste
There is an optimal speed in velocity — or feet per minute — for the air going through the dryer and through the flights. If the speed is too high, the material becomes entrained in the airstream and is carried over to the primary dust collector or to the secondary air pollution control equipment. This extra material actually is carried out of the dryer unit and, depending on the process, is wasted or circulated back into the system. Designing the right flighting system can help maximize the process and reduce material waste.
Now that you are aware of flighting considerations, you can be better prepared to talk to the experts: a qualified company with experience working on ovens, dryers or kilns used to dry or process your type of material. These suppliers can incorporate the modern technology and custom features needed to increase the efficiency of the flighting system and thereby your overall drying process.
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