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THE TECHNICAL CORNER

10 Tips for Your Flighting System
Dryer flights for asphalt plants

As is true with most mechanical equipment, not all dryers/drums are created equal. Perhaps even more dramatic is that flighting systems are so varied and many of the units are outdated from their original design compared to the new technology currently available. For the general purpose of this article, the term "flighting" will also cover "material flow management component located inside the processing unit."

Here are the Top 10 Tips for finding improved flighting systems that will increase productivity, reliability and decrease the plant maintenance cost:

  1. 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 removal of water vapor. In addition, it will protect the primary shell, the thickness of the shell, the tires (carrier rolls), trunnions (support rollers), drive systems and more. There can be significant increases in efficiency, productivity and long term performance based upon the selection of the type of flights, the pattern of flights, the material selected and other specifics associated with process demands.

  2. OLDER UNIT, NEW & IMPROVED FLIGHTS. From a general industry observation of dryers/drums, over 70% of them are being operated above original design capacity. With this being the case, the original equipment flighting cannot keep up with the increased demands for operating the dryers/drums and significant cost savings could be realized by improving the flighting pattern and improving the design of the entire system which, in turn, will affect combustion, heat transfer, wear and more. Flighting efficiency is the best return for your investment.

  3. A THOROUGH FLIGHTING EVALUATION IS IMPORTANT. Establishing baseline performance and efficiency for your flighting will include items such as:
    • Measuring the temperature of the shell at multiple points on the unit.
    • Measuring multiple temperatures on the exhaust gas housing to see if there is any significant range of temperatures from one side of the exhausting duct vs. the other side of the duct and other points in between.
    By establishing this baseline, it will help determine if and how much the operation has changed over a certain period of time. If there are significant increases in temperature or combustion inefficiency, this can often be attributable to the flighting system.

  4. HOW ABRASIVE IS YOUR MATERIAL? Another major item to consider regarding your flighting system is the type of material being processed: its abrasiveness, the rate of airflow passing through the unit and the wear resistant design. When extremely abrasive materials are being processed, flighting systems must be selected to take into consideration their abrasiveness resistance. This is critical in order to maintain the proper processing rate of flow as well as efficiency. Depending on the level of abrasiveness in your material, you may even consider specially engineered flights, or extra reinforcement of the existing flights.

  5. SIZE OF MATERIAL MATTERS. The next major consideration in flighting is the material gradation. You must ask if the gradation of materials being processed will vary greatly. For example, processing a 1/4" minus size material would require a different flighting pattern, different depth of bed, material and veiling patterns as opposed to processing a 3/4" minus size material or variations within that range. If a dryer/drum is going to process multiple sizes and varying materials, custom flighting systems may be needed where zoned flightings handle one size material in one zone and alternate with another size in another zone.

  6. CONSIDER YOUR FUEL TYPE. One of the most outstanding recent developments in flighting systems is the emergence of overlapping internal combustion zones. If the unit is what is considered "direct-fired" and the flame actually occurs within the envelope of the shell, then many steps have to be evaluated to properly have enough volume for complete combustion to be generated without impingement on the flame. Burning a heavy oil or waste oil will require a significantly different pattern of combustion flighting than if natural gas or propane may be used.

  7. WHAT ABOUT THOSE MULTI-FUNCTION UNITS? A number of rotary dryer/drum units actually perform multiple functions within the embodiment of the shell. Your unit may perform drying in one section, introducing recycle materials i.e. – RAP and RAS in the next section, and adding a liquid in a final stage. In these multi-purpose units, the flights must be carefully selected to perform the different functions. As an example, some of them may actually slow the progress of moving material while some 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, there are many different types, styles and lengths of flights that create a curtain of material and promote heat transfer most efficiently. Many older drying units had very few flights and often very long. It was not unusual to see only six or eight 10- or 12-foot long flights around the circumference of the unit, which is extremely inefficient when it comes to fuel usage and energy costs. Today, with the increasing desire for efficiency, the same circumference may have 18 or 25 flights that measure only 3 to 5 feet long and in staggered patterns in order to promote the much more dense veil of the material. These flights may appear in all different shapes and sizes to do initial moisture removal or dramatic moisture removal through both conductive heat and convective heat.

  9. HAVE YOU CHECKED YOUR SEAL SYSTEM? The seals on your unit help reduce bleed in air, provide more consistent combustion and reduces the cost of heating and loss in production by this bleed in (fugitive) air loss. But in the same manner, the flighting for both the intake and discharge ends needs to be arranged with the seal system in mind, in order to get the material into the drying unit or discharged. Many designs use a spiral or intake unitized flight arrangement that quickly sweeps material in 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 with revolution of the unit. Chains are another alternative to release and free material build up. On discharge flighting arrangements, certain styles call for side discharges or even a high lift discharge. Having the flighting arranged to consistently provide this dried material without getting it reintraned in the air stream is another important design consideration. REMEMBER that most seal systems are designed to seal air only, and not product. Your 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 velocity, represented by feet per minute for the air going through the dryer and through the flights. If the speed is too high, the material becomes entraned in the air stream and is carried over to the primary dust collector and/or to the secondary air pollution control equipment. This extra material is actually being carried out of the dryer unit and, in some processes, wasted or circulated back into the system. Designing the right flighting system can help maximize the process and reduce the waste of material.
BONUS TIP: Stay abreast to the latest changes in Technology. The hotmix asphalt industry has changed as dramatically as most any industrial drying and processing industry. These rotary dryers/drums now must super heat aggregates for conductivity heating RAP and RAS. These same units have the severe duty of multiple stops/starts on any given day. As the funding for federal and state highway spending fluctuates so does the demand for a given hotmix plant and the rate of individual dryer/drum requirements.

In the end, nothing beats speaking to the experts! Talk to a qualified company with experience working on units that are drying and processing your type of material about the most recent developments in technology and what custom features you may need to increase the efficiency of your flighting system.
Contact us today!