Range Versus Efficiency - Striking The Proper Balance

The paper addresses the balance between peak attainable efficiency and overall operating range that must be addressed when specifying, designing and/or selecting centrifugal compressors. The relative roles of the various compressor components; i.e., impellers, diffusers, guide vanes, and return channels; in achieving the proper balance are discussed. Finally, the importance of proper component and stage aerodynamic matching is emphasized. INTRODUCTION Two of the most important considerations in centrifugal compressor performance are efficiency and overall flow or operating range. A compressor’s efficiency has a direct impact on the power requirement for the process because higher efficiency yields lower power consumption or allows more product to be made for a given amount of energy input. The overall flow range limits the compressor’s ability to operate at other than the design condition; i.e. off-design conditions. Ideally, compression equipment would provide both high peak efficiency and wide overall operating range. Unfortunately, efficiency and flow range are quite often mutually opposing forces in the real world. The very features that contribute to high peak efficiency (i.e., vaned diffusers) can and do cause a reduction in overall operating range. Likewise, the design approaches used to obtain wide operating flow range typically do not provide the maximum achievable peak efficiency levels. As a result, the designer must determine the proper balance between overall flow range and peak efficiency when developing new stages and/or specifying components for a compressor application. This paper is discusses the factors that must be considering in striking this balance in industrial centrifugal compressors. The paper briefly describes the parameters commonly used to assess flow range and efficiency of centrifugal turbomachinery. This paper touches on the impact of range and efficiency on machine cost. That is, it might be possible to maximize both range and efficiency but only via non-standard components that add to the complexity and, therefore, the cost of the equipment. However, in some applications, the additional range provided might justify the added expense. For example, if by providing additional range, a bundle change-out can be avoided, the additional upfront cost of the compressor might be offset by the reduction in long-term costs that would result from the bundle changes, production interruptions, and other related expenses. The paper also provides insight into the design choices made by manufacturers of industrial turbo-compressors. The cost to build the compressor and/or other manufacturing concerns/limitations strongly influence the design choices made by OEMs and said choices can have significant impact on equipment performance. The impact on the design philosophy for impellers, diffusers, guide vanes and other components is discussed. For example, it is common knowledge that channel vaned diffuser (i.e., wedge diffusers) provide high peak efficiencies but at the expense of overall flow range. However, it is less commonly known that good flow range is possible with a wedge diffuser if the upstream impeller is designed to promote such. Also, in the past 20 years, various styles of