Choosing the right bucket elevator can improve productivity, reduce maintenance, and contribute to profitability. But what criteria should you use to specify the best bucket elevator for your application?
By Claus Weyhofen
Bucket elevators are essential for the vertical transport of a wide range of bulk materials, from powders like raw meal and cement to coarse materials like clinker. Using a belt or chain mechanism, these high capacity, heavy duty machines rely on a combination of centrifugal and gravimetric forces to eject the bulk material out of each bucket when it reaches the desired height.
Belt and chain bucket elevators use different transmission mechanisms. Each has its own advantages and disadvantages.
A belt bucket elevator uses an endless belt with a pulley, from which the buckets are suspended. The belt comprises a series of steel cords, which take the load, encased in rubber plates to protect the steel from environmental impacts. Centre distances of up to 250 m are possible, with conveying capacities in excess of 3000 m3/h to a height of 200 m. This technology typically exhibits low wear, and higher conveying capacities, making it a cost-effective choice in terms of both CAPEX and OPEX. Belt bucket elevators are also relatively smaller than a chain belt equivalent, which saves on steelwork and so reduces environmental impacts.
A chain bucket elevator uses an endless chain with a non-toothed sprocket and the buckets are attached by means of vibration-absorbing angular bucket holders to the chain. Centre distances of 70 m or more are achievable, with conveying capacities of up to 1900 m3/h to heights of around 70 m. Chain bucket elevators really come to the fore in transporting hot, abrasive, or granular bulk material, such as limestone, slag, and refuse derived fuel (RDF). However, chain bucket elevators are generally more expensive to purchase and to maintain and have a shorter lifespan under the same strain when compared to belt bucket elevators.
Ultimately, the selection of belt or chain bucket elevator comes down to three simple criteria: temperature, grain size, and material density.
Temperature is the single most important factor in choosing between a belt or a chain bucket elevator. Belt elevators can operate happily for more than 10 years in lower temperature environments (<130°C). They can even cope with temporary exposure to higher temperature peaks. However, rubber deteriorates when subjected to high temperatures over long periods. If it gets too hot, the rubber on a belt bucket elevator will become hard, shiny and form cracks in its surface, an effect known as elephant skin. Ultimately, it will flake and crack, exposing the inner steel cords to moisture that leads to corrosion.
Chain bucket elevators, by contrast, will operate effectively without deterioration in 250°C heat and can withstand peaks of up to 350°C. So, for extreme temperatures – when the bulk materials reach temperatures above 130°C – you should always opt for a chain bucket elevator.
If conditions mean you opt for a belt bucket elevator, grain size is the second consideration. This factor will determine whether you should opt for a standard or heavy-duty model. Typically, grains of up to 25 mm can be handled with a standard belt bucket elevator. Grain sizes between 80 mm-120 mm require a heavy-duty version.
One further operating parameter to consider in belt bucket elevator specification is the bulk density. For applications of less than 2,3 t/m3, standard belt elevators are perfectly adequate. Applications up to 1,2 t/m³ density are likely to require a high capacity belt bucket elevator.
By applying these three simple criteria, cement manufacturers can determine whether a belt or a chain bucket elevator is the best solution for their application, and then further refine the type of belt bucket elevator they need. Working with their preferred supplier, they can then further refine parameters such as the bucket width and depth, corrosion resistance and any necessary explosion proofing to ensure that their chosen technology will deliver optimum return on investment.
