Energy-efficient drying of bricks with the Paraflow dryer from KELLER HCW
[November 2010]
Since the beginning of the nineties, KELLER HCW has been successfully using Ecorapid dryers. The name is a composition of Economic and Rapid, thus meaning economic rapid drying. In times of exploding energy costs and increased quality requirements combined with the pursuit of higher cost-effectiveness and environment-protecting processes, KELLER HCW looked for potential for improvement for all dryer types and created a new type of dryer – the Paraflow dryer. The name is derived from the words parallel and flow. It refers to the parallel air flow from recirculated air and the longitudinal air flow within the brick setting.
For example, this dryer type was installed in the brick factory Heluz in the Czech town of Hevlin in 2009. The output of the new factory for common bricks lies with up to 1000 tons per day of fired goods with an apparent density of the bricks of 0.7 kg/dm³. The increased use of porosity agents allows the production of bricks with an apparent density of up to 0.6 kg/dm³.
How is the Paraflow dryer designed?
In general, the Paraflow dryer consists of following components (pictures 1 and 2):
1. Foundation with rails and "sand seal".
2. Exterior walls with one or several layers, in the wet area with additional insulation on the inside.
3. Concrete ceiling with insulation and floating floor with the necessary openings required for process engineering purposes.
4. Intermediate ceiling, made of metal, non-metal, concrete or synthetic material with a plant-specific number of suction and injections slots.
5. Separation walls for the recirculation systems, made of metal, non-metal, synthetic material, concrete or masonry.
6. Recirculation fans, made of metal or non-metal, or with special anti-corrosion coating, if required.
7. Air supply systems, consisting of hot air or secondary air pipes.
8. Cars: Kiln cars may be used when the direct setting method is applied, if not, dryer cars with insulated car deck and sand seal are used.
9. Pallets: Large pallets or stacking pallets are primarily used.
The highly intensive mixture of longitudinal and recirculating air flows in the setting, together with the knowledge how to dimension the necessary free air flow sections result in a perfect air flow pattern with a very high quality concerning the air distribution across the setting height and width. On the one hand, this optimised air flow behaviour leads to a homogeneous drying process and, on the other hand, it provides a high degree of saturation of the air with an ideal energetic use. The necessary hot and/or secondary air is fed into the process through the suction side of the recirculation fans and is thus added in the best possible way. The quantity of the added air is regulated by humidity-controlled or temperature-controlled motor valves installed in the pipe.
A CFD (Computational Fluid Dynamics) flow survey of the Paraflow dryer was made at KELLER HCW in the context of a diploma thesis. For this purpose, a segment of the dryer was rebuilt in a representative test stand and air flow measurements were made by detecting pressure and speed. The next step was to simulate a numeric model adapted to the experimental model using the software tool FLUENT. This was the basis for the complete simulation of the air flow diagram for the drying process (picture 4). Unlike the quantity of recirculated air, the longitudinal air flow is different in each dryer section, and this also leads to different static pressure losses in each one of the dryer sections. These variable pressure losses were simulated in the CFD survey, where different examples were calculated: The simulation started with a simple recirculation air flow, the percentage of the longitudinal air flow was then gradually increased in small steps and separately evaluated. The aim was to use the simulated pressure losses as a forecast of the static pressure losses over the total dryer length for the future drying operation.
After completion of the diploma thesis and after successful commissioning of the Heluz plant, the simulated results were compared with the real results from the working dryer. For this purpose, a comprehensive series of measurements was initiated at the Heluz plant. A comparison of the results showed a high degree of conformity, which was more than originally expected and which is a proof for KELLER HCW to continue carrying out CFD simulations in order to develop more potential to make the energetic systems even more efficient in the future.
Which are the characteristics of the Paraflow dryer?
The Paraflow dryer:
• is suitable for a large variety of products, such as common bricks, facing bricks, pavers, split tiles and klinker strips
• is suitable for drying sensitive products
• is suitable for variable drying times (product-dependent 4 to 32 hours)
• is suitable for high dryer outputs (100 to 1600 tons of fired material per day)
• is suitable for setting widths between 5 and 10 metres
• is suitable for high operating temperatures
• has a low specific electric energy consumption (8.5 kWh/t fired)
• has a low thermal energy consumption (820 kcal/kg H2O)
• has a maintenance-friendly structure (no moving parts in the dryer)
• has a very homogeneous flow distribution in the setting and simultaneously uses the longitudinal air flow in the best possible way
• offers a very good accessibility to the setting room (sample taking)
• achieves a very high product quality through a homogeneous drying process
• does not need a separate transfer car technology for the dryer cars
• does not need sophisticated downstream and upstream machinery
The KELLER HCW Paraflow dryer does not only fulfil the requirements for energy-efficient brick drying and the request for environment-friendly drying processes but is an economic dryer with many structural advantages that meet the high quality standards of our customers.
The maintenance-friendly conception is as important as the easy access to the setting room during the drying process to be able to make a visual check of the drying progress.
The CFD simulation we made is not only restricted to an individual recirculation circuit but detects the air flow across the total length of the dryer, thus obtaining excellent results in view of the air flow conditions and the pressure losses in the real dryer. Such knowledge is a decisive benefit for the calculation of the dimensions of the process-engineering equipment. The results of the simulation could be confirmed by extensive measurements in the real dryer, established confidence in the reliability of the simulation programmes and convinced us also to detect future air flow processes first with the CFD method. KELLER HCW will continue to use this facility to create further potential for improvements for the benefit of their customers.
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