What Are Front Rake Bars?
In the waste-water treatment sector, there are specific machines designed to filter out unwanted waste and debris from water. These screening systems can be found for instance in rivers, sewage channels, dams, and other water streams. The cleaning elements of the screening systems are metal rake bars, placed in the front of the machine. As influent water passes through, the screen catches all large objects and items such as sticks, cans, rags, plastic bags, etc. which can then be easily discharged.
Rake bars are used to manufacture coarse screening systems for both municipal as well as industrial wastewater. The result is a continuous flow of filtered water.
The Development of the Rake Bar Design
Historically, the sections designed for rake bar screens used to be hot rolled steel profiles with a rectangular shape. These rectangular profiles or flat bars (original design) were and are far from perfect: they experience high friction levels against streaming water and they are known to jam more easily than hydrodynamic rake bars; not only with hard debris but also in case of muddy flows. In an attempt to improve the speed at which water flows through the rakes and the efficiency in which it gets filtered, engineers and technicians started analysing the impact of the bar’s shape on the hydrodynamic throughput capacity. Following numerous studies, they came up with a trapezoid shape for rake profiles (1st evolution). This new profile geometry proved to have a considerable reduction in head loss as compared to flat bars, which meant a much better performing wastewater screening machine. The trapezoidal profiles have proofed to be a huge step up from the use of flat bars, but are best employed in systems that are placed in wastewater settings, where there are no small debris, such as stones and gravel, because these tend to get stuck in the screen due to the trapezoidal shape of the rake bars, which then could cause jams.
In an effort to avoid downtimes of the screening system and high maintenance costs, the research for a universal front rake bar that would not jam under any circumstance was initiated. This brought about the development of a third design of rake bars: the fish belly shape (2nd evolution). Research studies show a reduction in friction up to 30% and the profile effectively prevents solid and liquid jamming, which makes the fish belly (also called teardrop shaped) profile the most hydrodynamic screen bar, which ensures a higher quality front rake system as well as an effective way to avoid downtime and maintenance costs.
The trapezoidal as well as the fish belly shaped profile have given Montanstahl a big push in becoming market leader in the production and distribution of front rake profiles. Montanstahl leadership in the wastewater treatment sector can be attributed to one key factor: thanks to our precision hot rolling mill we are able to produce the streamlined profiles with proper rounded head, which is a distinctive feature in comparison to other suppliers. It means that the head of the profile, which is the first contact point of the screen with the water and therefore the part that determines the hydro-dynamicity of the system as whole, has a smooth surface, no rolling burr and guarantees a maximized water flow.
Montanstahl Wastewater Hydrodynamic Profiles Program
Given the growing demand of filtering rakes for the waste water industry, over the years we have developed a wide range of highly hydrodynamic profiles for our customers that meet new market demands and specifications.
Our range of front rake profiles have reached a staggering number of approximately 30 different hydrodynamic sections and Montanstahl is regarded as the “One stop solution provider” for this product category. The program includes trap bars (or trapezoid bars) as well as high performance tear shaped sections.
The rake sections are available in carbon steel, such as S235J2 and S355J2, as well as in stainless steel, like grades 304 or 316.
The adequate material selection depends on circumstances of the rake bar’s final application. On one hand, carbon steel is generally less expensive as compared to stainless steel grades. On the other hand, the advantage of selecting stainless steel over carbon steel is lower maintenance costs of the rake-screening machine in the future. This is because carbon steel needs to be galvanized to be protected from corrosion while stainless steel has naturally high corrosion resistance and is therefore a more durable material over time.
Montanstahl Steel Rake Bars: Distinctive Features and Advantages
Our hydrodynamic profiles are characterized by high geometrical precision and good surface finish (low roughness).
For hot rolled sections, the standard norm requires a surface roughness equal to Ra 7.5µm. Our hot rolled fish belly profiles and trap bars have
surface roughness of approximately Ra 4µm.
We manufacture small to medium size sections by means of hot rolling and larger profiles (>100 mm) by means of hot extrusion, which are both cost-efficient production technologies.
As previously mentioned, thanks to the expertise gained over many years we are able to manufacture the profiles with a very accurate and smooth “head”, free from rolling burrs, which results in an excellent hydrodynamic throughput.
Furthermore, we can ensure extremely tight tolerances if needed, which of course also helps customers during the production process and ensures a front rake screen with high precision on the gaps. Here are the specific figures of maximum tolerance levels of our tear shaped product range:
- Thickness tolerance = ± 2 mm
- Width tolerance = ± 5 mm
Following is the technical drawing of one of our best sellers, namely the Tear Shaped Profile 60x8x5mm:
The hydrodynamic sections can be supplied in custom lengths from 3 to 8 meters to meet customer’s requirements. This greatly reduces scrap, simplifies the initial assembly as well as further replacement works of the rake screening machine.
Available on Stock
We supply rake bars across the world and make it a priority to keep delivery times as short as possible. Therefore, several profiles (such as the bestseller illustrated above) are available from stock in stainless steel grades 304 and 316 as well as in carbon steel grade S235JR. If desired by the customer, we also arrange JIT agreements that ensure the availability of specific custom sections on stock throughout the year.
For new custom profiles, generally we are able to deliver the finished sections within 4-6 weeks from the order date (depending on the availability of the raw material).