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As plant capacities in the polymer/downstream petrochemical industry have increased over recent decades, the volume flow of the polypropylene/polyethylene (PP/PE) gas phase reaction process has similarly increased. The head requirements, however, have remained the same. On the one hand, the recycle compressor used in this process is traditionally a radial centrifugal compressor. On the other, handling the ever-increasing volume flow is a challenge for the recycle compressor, and especially for the impeller and diffusor design.
One solution is to increase the impeller’s diameter until the flow coefficient is again within the flow coefficient range of the impeller. This is achieved by either reducing the rotational speed of the impeller via the application of a gearbox or by using an impeller with a lower-head coefficient to get a bigger impeller diameter. The downside of a bigger impeller is a bigger casing and increased investment costs.
A more economical solution is to develop the impeller in a way that ensures it is suitable for a higher flow at the same impeller outlet diameter. The efficiency level has to be maintained and operational costs should not increase. Generally, a high-flow and low-head impeller suggests the use of an axial shape. Nevertheless, experience with radial stages shows that an intermediate approach makes sense. Moreover, a less radial shape also results in lower head and allows higher flows. To cope with the increasing plants sizes and subsequent increasing flows, therefore, a further development of the half-axial impeller needs to be taken.
The latest developments mainly focus on increasing flow capacity and keeping the same efficiencies. The result is an optimized flow path with an even more axial shape. The new impeller design, with a more axial shape, requires a complete review of the flow path of the impeller and reshape of the impeller blade. To cope with the new conditions, the flow path and blade shapes are designed with the help of state-of-the-art CFD tools.
Due to economic pressures, increased plant capacities are expected to further increase the volume flow requirements of many processes in the petrochemical world. Applications with traditionally centrifugal compressors will need to adapt compressor capacities in an economical way. The specific development of the radial impellers to be able to handle higher volume flows helps to successfully master the challenge.
The new more axial-shape impellers provide an optimal solution to the volume-flow challenge. And the general principle of extending centrifugal radial impellers for higher volume flows can be applied to similar applications with high flow and low head.
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