Usually a sophisticated simulation should be applied in a trial-and-error procedure to find the outer rim of any impeller for even a simple radius trimming. Use caution The Affinity Laws apply satisfactorily for pumps and fans (incompressible flow) but do not accurately describe the performance of turbocompressors. In other words, a combined speed adjustment and impeller trimming could be employed to achieve head reduction while the flow is kept constant.
For a constant flow, speed adjustment might be required in addition to impeller trimming. The flow is not a constant after trimming and all these parameters are related. As a rough indication, the formula “head” proportional to “(radius) 2” can be used. In this case, the prediction of the trimmed radius is crucial to a compressor impeller. The goal is often to adjust the head and sometimes to reduce the head.Ī simple trimming option is the trimming of the impeller radius to adjust the head.
#Point by point method centrifugal impeller design driver
Impeller trimming is often used to change the performance characteristics (head-versus-flow) and is sometimes employed to limit driver loading. Turbulences could increase at the vane tips as the impeller is trimmed because the shroud-to-casing clearance increases. Trimming can affect the clearances between the impeller and casing, which could increase internal flow recirculation, cause head loss, lower overall efficiency, etc. Trimming should be limited to certain levels because excessive trimming can result in operational and reliability problems such as a mismatched impeller and casing. Trimming often involves machining the different parts and sections of an impeller to modify some dimensions or angles. For such a compressor casing, speed adjustment might not solve the operational problem and impeller trimming is necessary. However, special circumstances such as compressors with side-streams and others where accurate head generation of each set of impellers may be present. In a compressor casing or a compressor train where all impellers are operated with the same speed, too often some impellers generate more head than is required, while others produce head that is less than what is required. In many variable speed compressors, the trimming of the impeller should also be studied and employed. Impeller trimming is usually the best or only option. For a fixed speed centrifugal compressor, limited options are available to deal with such an issue. Centrifugal compressor impellers can sometimes be oversized. Sometimes the head is more than required, but often it is less than what it should be. For 3D impellers – particularly 3D semiopen impellers or to a lesser extent for 3D closed-type designs – the impeller might be trimmed axially so the exit blade height is reduced, or along the entire meridional length of the blades so that the passage area is reduced.īecause of conservative engineering practices, inaccurate impeller designs or other errors in the engineering or fabrication of a compressor, the impeller flow or the impeller-generated head might be different than what they need to be. This method can be used for nearly all types of compressor impellers. This article offers practical guidelines for the impeller trimming of centrifugal compressors.įor a given design, the impeller may be trimmed radially so the exit radius is reduced. Few works are published on impeller trimming topics. Impeller trimming can be an effective method of modifying the performance of a centrifugal compressor without significantly compromising the efficiency. Each impeller type requires a specific set of rules for trimming. Impellers used in centrifugal compressors are two-dimensional (2D) closed-type impellers, three-dimensional (3D) closed-type impellers and 3D semiopen impellers. Using properly designed impeller trimming to avoid weeks or months of delay could offer significant advantages. Delays in compressor delivery and plant startup are extremely costly. This trimming process can offer considerable commercial benefits since the impeller or compressor redesign and remanufacturing are expensive. A possible solution is slightly trimming one or some impellers to achieve reasonable compressor performance and avoid impeller redesign and refabrication. Sometimes in a centrifugal compressor performance test at the shop or site, performance does not meet expectations. Trimming of impellers can be an important way to change the characteristics of a centrifugal compressor.