Belt Sludge Dryer vs. Thin Film Sludge Dryers
LCI Corporation
Provider of Thermal Separation Systems and Powder Processing Equipment
Belt Dryers and Thin Film Dryers have successfully dried sewage sludge for many decades. However, these dryers operate on a completely different evaporation principle with a significantly different mode of operation. Some of these differences include:
Principle of Operation
Belt Dryers use a large volume of heated air (known as direct drying) streamed through a plastic or stainless-steel belt which conveys the sludge through the dryer. The principle is like a convection or pizza oven. The dryers heat the air via heat exchangers using hot water, steam, thermal oil, or direct heated air from a furnace.
Thin Film Dryers use a heated surface to transfer heat to the sludge (known as indirect drying) and evaporate the water using very little airflow. The principle is like boiling water in a pot or cooking in a frying pan. The Thin Film Dryer uses thermal oil, steam or hot water.
Complexity in Design
Overall, the Belt Dryer is a more complex piece of equipment compared to a Thin Film Dryer. A Belt Dryer can require over 20 drive systems to operate depending on the manufacturer. In contrast,?the Thin Film Dryer only requires 4 drive systems?as shown below:
Maintenance
As the Belt Dryer has more moving parts, it also requires more maintenance. Wear Parts and Maintenance Items include:
Some of these systems can be costly and difficult to maintain over the course of the Belt Dryer’s operational life. Many times, a factory technician is required to complete the service. These items require maintenance every 1 to 5 years of operation.
The?Thin Film Dryer’s only wear part is the low cost, simple to replace blades on the rotor. These can be easily replaced by plant maintenance staff. Depending on the application, service could be required every 1-2 years.
Size
As Belt Dryers require a large airflow to dry the sludge and operate at lower drying temperatures, they are larger than Thin Film Dryers. Belt Dryer size will vary by manufacturer, but they can be greater than 15ft wide, 20ft tall and 100ft long. Thus, they require 3-4 stories and are difficult to fit in existing buildings. On the other hand, Thin Film Dryers are?less than 12ft wide, 12ft tall and 70ft long. This means they can fit in a 1-2 story building and more?easily fit into existing spaces, saving on construction costs.
The Thin Film Dryer does require the auger to be pulled for maintenance which requires a maintenance area equal to the machine’s length for service. However, this area can be outdoors, limiting the size of the dryer inside the building. Belt Dryers will vary in the maintenance space required depending on manufacturer.
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End Product
The product which comes out of the Belt Dryer and Thin Film Dryer vary significantly as shown below:
A big advantage of the?Thin Film Dryer is its ability to partially dry sludge between 30-70% and create a homogenous dryness. This can be useful in applications like landfilling, composting, and thermal destruction where 90%DS material is not required. Most Belt Dryers, due to the long retention time, cannot dry below 70%DS. Also, with the Thin Film Dryer, as the sludge is spread in a thin film (8-10mm) and agitated, the?entire layer is evenly dried and mixed. In contrast, a Belt Dryer places a large sludge layer on the belt (11 inches or 200mm) and creates no agitation. So, when the sludge is partially dried, the outside layer will be significantly drier than the inside layer.
The other main difference is the size and shape of the end product. The Belt Dryer has a lower density, larger “Cheeto like” shape which breaks apart more readily. Its material, in basic form, is difficult to spread in an agricultural fertilizer but could be better for soil blending. The Thin Film Dryer material is a smaller, spherical shape with a higher density. This makes the material, like commercial fertilizers, allowing it to be spread with typical agricultural spreaders.
A required item of the Thin Film Dryer when drying to 90% solids, that is not required of the Belt Dryer, is a cooling conveyor. This is to cool the hotter denser product of the Thin Film Dryer.
Safety
Both the Belt Dryer and the Thin Film Dryer are safe to operate, but they rely on different methods for safety. For a fire to occur, you must have oxygen, a fuel source (i.e. sludge) and an ignition source. For combustion, you need a confined space and dispersed material (i.e. a dust cloud). The Belt Dryer relies on low temperature operation and assumes this will keep the sludge from igniting, eliminating the ignition source. The Thin Film Dryer relies on an inert atmosphere in which the oxygen level is below a level for ignition, taking oxygen out of the equation.
Both dryers also limit the fuel (i.e. dust) in their processes. A Belt Dryer does not agitate the material as it only lays on the belts, causing very little dust to be produced. With a Thin Film Dryer, the counter flow between the vapor and the product pulls the dust back to the feed end of the dryer where the conditions are moist, precipitating much of the dust back into the process.
Safety is a concern for all drying systems and subsystems associated with drying and must be thoroughly designed and engineered. Completing a Dust Hazzard Analysis with an industry professional is recommended for the entire drying system.
Conclusion
The Belt Dryer and Thin Film Dryers have been used for many years to dry sewage sludge. However, they are significantly different in design and operation which allows each design to have its advantages. It's important to understand their advantages so that you know which machine will best fit your drying application. The Belt Dryer excels at using low temperature waste heat sources, having lower frequency maintenance and a smaller maintenance area. However, the Thin Film Dryer has the advantages of lower energy consumption, reuse of recovered heat, simpler and more cost-effective maintenance, has a smaller vapor flow, has the ability to accept difficult sludges, is partially dry, and can fit in a smaller operation space.
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Chip Pless, LCI Corporation
Sales Manager, Sludge Dryers
Gesch?ftsführer bei Buss-SMS-Canzler GmbH
1 年I only can confirm Chip's conclusions. Please thonk about that and conclude yourself.