Empirical Design Considerationsfor Industrial Centrifugal Compressors

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This paper provides some empirical information for designing industrial centrifugal compressors with a focus on the impeller. A ported shroud compressor basic design guideline is also discussed for improving the compressor range.

Transcript of Empirical Design Considerationsfor Industrial Centrifugal Compressors

  • Hindawi Publishing CorporationInternational Journal of Rotating MachineryVolume 2012, Article ID 184061, 15 pagesdoi:10.1155/2012/184061

    Research Article

    Empirical Design Considerations forIndustrial Centrifugal Compressors

    Cheng Xu and Ryoichi S. Amano

    Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53212, USA

    Correspondence should be addressed to Ryoichi S. Amano, amano@uwm.edu

    Received 22 February 2012; Revised 29 April 2012; Accepted 30 April 2012

    Academic Editor: Ashwani K. Gupta

    Copyright 2012 C. Xu and R. S. Amano. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

    Computational Fluid Dynamics (CFD) has been extensively used in centrifugal compressor design. CFD provides further optimi-sation opportunities for the compressor design rather than designing the centrifugal compressor. The experience-based design pro-cess still plays an important role for new compressor developments. The wide variety of design subjects represents a very complexdesign world for centrifugal compressor designers. Therefore, some basic information for centrifugal design is still very important.The impeller is the key part of the centrifugal stage. Designing a highly eciency impeller with a wide operation range can ensureoverall stage design success. This paper provides some empirical information for designing industrial centrifugal compressors witha focus on the impeller. A ported shroud compressor basic design guideline is also discussed for improving the compressor range.

    1. Introduction

    New compressor designs always must meet the customersneeds with the shortest time to market, low cost, andimproved performance. To push the design to state of the artaerodynamic performance, the structure design also needsto meet a suitable performance life of the compressors.Mechanical integrity is one of the important parts of the cen-trifugal compressor design. Mechanical constraints are usu-ally negative factors for aerodynamic design, for example,mechanical constraints require thick blade for reliability buthurt impeller eciency. The purposes of the mechanicalanalyses are to provide all compressor components withina reasonable time duration to sustain the aerodynamic andcentrifugal force, and eigen frequencies do not match criticalexcitation frequencies [1]. The safety factors of the mechan-ical design had been reduced dramatically compared withold fashioned design. Due to the nature of the Finite Ele-ment Analysis (FEA) tools and material property improve-ments, the safety factor of a modern industrial compressordesign normally is set to 7 to 12%. The mechanical require-ments need structure designers to have better practice toallow more freedom to aerodynamic designers and to keepall the components at the lowest weight and the lowest cost.

    Design of a long lifetime single component of compres-sors is not a goal for designers. Emphasis on improvingeciency has been a primary issue, but this also is not asimportant as in the past. The development cost and develop-ment time is also a key factor that needs to be considered fora modern compressor design. Industrial compressor designexpects a state-of-the-art performance compressor withoutmaking a second build for less cost and short developmenttime. For achieving this goal, compressor design engineersneed to have multidiscipline knowledge of centrifugal com-pressor design. Detailed design considerations can reduce thetime to perform the advance design studies and laboratoryinvestigations. The wide variety of design subjects representsa very complex design world for compressor designers. Onepurpose of this paper is to provide information in an aero-dynamic point of view to understand the overall designbefore starting the detailed design process of a centrifugalcompressor. The paper also summarizes important aspects ofthe centrifugal compressor design for industrial compressordesigners and scientists.

    The compressor market and business model has changedin the last few decades. Industrial compressor design nowrequires designing for success in the marketplace, not justfor scientific experiments. In the past, compressor designers

  • 2 International Journal of Rotating Machinery

    developed a new compressor in the development group andpassed the design to manufacturing. The manufacturinggroup would evaluate how to make it at the lowest cost, andsome designs were rejected because they could not meet themarket requirements. The new development model requiresthe compressor designer to design for market, manufactur-ing, and end users. New business concepts have been pro-posed [2, 3] in which the design also considers an integratedsystem of manufacturers and end users. The new compressordevelopments become a complex system task. Minimizingmanufacturing cost of the compressor design is not enough.The compressor design must consider all aspects of themanufacturing and end users. If surplus is defined as thetotal profit of manufacturing, end users, and aftermarket,the compressors new development will focus on the designfor maximum surplus. Therefore, in the compressor designstage, many choices of design options need to be consideredbefore the final design, and discussions must consider thesurplus value. It is essential that design engineers begin toperform a compressor design with full understanding of allaspects of the design considerations [48].

    To reduce manufacturing cost, many high volume com-pressor manufacturers, for example turbocharger, often usethe flow cut for dierent applications. The flow cut uses thesame defined blade geometry for multiple flows and a similaror lower pressure ratio. This is dierent from scaling, in thatthe impeller blade and any diuser vane geometry maintainthe same definition. A brief introduction of the flow cut isalso discussed in this paper.

    With the development of computational science andcomputer hardware, design engineers rely on quality modelsto establish the physical relationships among diverse ther-modynamic, geometric, and fluid dynamic parameters thatgovern turbomachinery performance [926]. Although CFDhas helped to design many successful industrial compressorsand has become an important tool in industrial compressordesign, multidomain optimisation is still very time consum-ing. Most CFD optimisations still focus on the component[22]. When the compressor inlet flow is reduced, the com-pressor experiences an unsteady flow phenomena surge androtating stall. These instabilities can cause noise nuisance andcritical operating conditions with strong dynamical loadingon the blades. Therefore, they cannot be tolerated duringcompressor operation. With the reduction of the compressorinlet flow, a rotating stall occurs in the impeller, or diuseror scroll. If the compressor inlet flow continues to reduce,the rotating stall eventually will drive the compressor intoa surge [2732]. A rotating stall is an unsteady and three-dimensional flow phenomenon. CFD simulation is still a bigchallenge. The flow range of a centrifugal compressor canbe extended by allowing gas to bleed from a ring of holesor a circular groove port around the compressor casing ata point slightly downstream of the compressor inlet. Thistype of the compressor called ported shroud compressor.Ported shroud forces a simulation of impeller stall to occurcontinuously, allowing some air to escape at port locationinhibits the onset of surge and widens the operating range.The flow inside port is unsteady and complicated when com-pressor stalls. CFD guides ported shroud design is still very

    time consuming and less reliable. Some design practicesfor a ported shroud impeller casing [33] are discussed forimproving the compressor operational range.

    2. Industrial Centrifugal Compressor

    Centrifugal compressors are widely used in automotive,marine turbocharging, oil and gas, aerospace, and distribu-ted power applications because of their compact design andhigh stage pressure ratio.With dierent types of applications,the structural characteristics of the compressors have twobasic types, that is, horizontal split and vertical split, asshown in Figure 1. Horizontal split type compressors areapplied for low-to-medium pressure service, as shown in Fig-ure 1(a). This type of casing is split along the rotor shaft andbolted at the split line. The bearing and seal sections alloweasy disassembly and assembly via the inspection cover, with-out having to remove the upper casing. A vertical split com-pressor is easy to access the gears, bearings, seals, and berepaired on site. However, due to the large crossing area in thesplitting surface, it is dicult to prevent gas and lubricant oilleakage. A vertical split compressor is applied for medium-and high-pressure service, as shown in Figure 1(b). This typeof compressor consists of an inner casing and an outercasing. The inner casing forms a single unit with the head,bearing, and seal and is fixed to the outer casing by shearing.The nozzle can be attached to the top, bottom, or side inaccordance with client specifications. Both bearings and sealscan be inspected without removing the inner casing. How-ever, the manufacturing cost and installation cost may behigher than for a vertical split compressor. It is also not easyto access the gear, bearings, and seal. For combining theadvantages of both split types, a hybrid split has becomepopular. For some applications, the gearbox can be a verticalsplit, and the compressor stage can use a horizontal split, asshown in Figure 1(c).

    3. Impeller Design Methodologies

    The impeller is a key component to influence overall perfor-mance of a centrifugal compressor [5]. The eciency of cen-trifugal compressors has increas