Technology & Engineering
Isentropic Efficiency of Compressor
The isentropic efficiency of a compressor is a measure of how effectively the compressor is able to compress a gas with minimal energy loss. It is expressed as the ratio of the actual enthalpy increase to the ideal enthalpy increase in an isentropic process. A higher isentropic efficiency indicates that the compressor is operating more effectively.
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eBook - PDFRadial Flow Turbocompressors
Design, Analysis, and Applications
- Michael Casey, Chris Robinson(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
4 Efficiency Definitions for Compressors 4.1 Overview 4.1.1 Introduction Two examples should suffice to whet the reader’s appetite for the importance of clarity in efficiency definition in radial compressors. First, the commonly used isentropic effi- ciency – which is sometimes called adiabatic efficiency – compares the actual work transfer to that which would take place in an ideal isentropic adiabatic process with no losses and no heat transfer. Unfortunately, the isentropic efficiency does not actually represent the real quality of a machine at all well. For example, consider a two-stage turbocharger with a pressure ratio of 2 in both stages. If each stage achieves the same isentropic efficiency of 80%, then on combining them to a two-stage compressor with a pressure ratio of 4, the isentropic efficiency is then lower than that of the individual stages (78.1%). How strange and disorientating. The so-called polytropic efficiency overcomes this problem and, in this case, if both stages have 80% polytropic efficiency, the two-stage compressor would have the same polytropic efficiency as its individual stages. Second, a radial compressor impeller may have, at the same time, a total–total polytropic impeller efficiency of over 90%, a static–static isentropic efficiency of well below 60% and an impeller wheel efficiency of 30%. You can guess which definition a sales engineer would prefer to use to sell his product. It turns out that one can misuse efficiencies just as one can misuse statistics. In this chapter, the background to the systematic definition of the isentropic, polytropic and isothermal efficiencies, with and without kinetic energy, is considered. This chapter concerns only the different definitions of the efficiencies, and Chapter 10 gives more detail on the source of losses and expected efficiency levels.
eBook - ePub- A. Barber(Author)
- 1997(Publication Date)
- Elsevier Science(Publisher)
T is the temperature of the air at the point of use, which can be any value between ambient and compressor delivery. If air can be used at high temperature, there is more potential power available but, as with conventional systems, the temperature normally falls to the ambient either in the receiver or in its passage along the distribution mains.The definition of potential efficiency follows from potential power: Isentropic power has been chosen as the denominator, but any calculation of power can be used – isothermal if the compressor works on that cycle or polytropic if the index of compression is known. Now since isentropic power is given by the potential efficiency becomes:[3]T is normally equal to T2 , but the expression indicates that if the air can be used at a higher temperature than ambient, the efficiency is improved. This is rarely likely to be possible in practice, but if circumstances are such that it can be used in that way, there are advantages to be gained. The second conclusion to be drawn from this expression is that the efficiency is a function of pressure ratio.A plot of the potential efficiency is given in Figure 1 .FIGURE 1 – Variation of compressor efficiencies with pressure ratio for two level mode.On the same figure is a plot of the overall efficiency measured from experiments performed at FPC on a modified vane compressor. The overall efficiency, taking into account mechanical losses in the compressor drive, is rather less than the potential efficiency.When comparing theoretical efficiency with the overall efficiency, it appears that the mechanical efficiency alone is of the order of 67%. On the face of it this seems rather lower than one would expect, but the values are obtained at an early stage in the development of the concept, and it is likely that they would improve with experience.
eBook - PDFAnalysis of Engineering Cycles
Thermodynamics and Fluid Mechanics Series
- R. W. Haywood, W. A. Woods(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
3.6. Imperfections in the actual plant—the effect of irreversibilities All irreversible processes result in lost opportunities for producing work, so these will cause the performance of an actual plant to fall below that of the ideal. Hence, for the best possible performance, Simple Closed-Circuit Gas-Turbine Plant 37 FIG. 3.4. Enthalpy-entropy diagram for irreversible cycle, taking account of inefficiency of turbine and compressor. For the purpose of illustrating the effects of irreversibilities on the plant performance, only inefficiencies in the turbine and compressor will be considered. This is a matter of convenience only, and it must not be taken to imply that frictional effects in the heat exchangers and ducting are, in practice, unimportant; such is far from being the case. The cycle is now as shown in Fig. 3.4, the turbine work output being less and the compressor work input being greater than under frictional pressure drops in the heat exchangers and ducting must be minimised to an extent which is economically profitable. Larger ducting will give smaller gas velocities and hence smaller parasitic pressure drops, but it will also cost more and will be an embarrassment if too large. Frictional effects in the turbine and compressor must also be minimised; this means that the isentropic efficiencies of both must be as high as possible. 38 Simple Power and Refrigerating Plants ideal conditions, when both processes were isentropic; they are still assumed to be adiabatic, but in both the entropy now increases. Thus, whereas the turbine isentropic efficiency η τ is defined as the ratio of the actual to the isentropic enthalpy drop, the compressor isentropic efficiency r/ c is defined as the ratio of the isentropic to the actual enthalpy rise. Thus η τ = -j—r-and r c = A3-A4 h 2 —hi ' For given values of T± = T a and T 3 = T b , it is a simple matter to calculate T v and T A , for various values of q p when η τ and r c are specified.
eBook - PDF- Pavel Tsvetkov(Author)
- 2018(Publication Date)
- IntechOpen(Publisher)
Section 2 Energy Conversion Chapter 4 Exergy Flows Inside Expansion and Compression Devices Operating below and across Ambient Temperature Mikhail Sorin and Mohammed Khennich Additional information is available at the end of the chapter http://dx.doi.org/10.5772/intechopen.74041 Abstract The various definitions of the coefficient of exergy efficiency (CEE), which have been pro-posed in the past for the thermodynamic evaluation of compression and expansion devices, operating below and across ambient temperature as well as under vacuum conditions, are examined. The shortcomings of those coefficients are illustrated. An expression for the CEE based on the concept of transiting exergy is presented. This concept permits the quantitative and non-ambiguous definition of two thermodynamic metrics: exergy produced and exergy consumed. The development of these CEEs in the cases of an expansion valve, a cryo-expander, a vortex tube, an adiabatic compressor and a monophasic ejector operating below or across ambient temperature is presented. Computation methods for the transiting exergy are outlined. The analysis based on the above metrics, combined with the traditional analy-sis of exergy losses, allows pinpointing the most important factors affecting the thermody-namic performance of sub-ambient compression and expansion. Keywords: exergy efficiency, expansion, compression, sub-ambient, across ambient 1. Introduction Cooling is part of twenty-first century life. Air conditioning, food conservation, industries such as steel, chemicals, and plastics depend on cooling. By mid-century people will use more energy for cooling than heating [1]. Almost all cold is produced by vapor-compression refrig-eration and requires large amounts of electricity for its production. And since electricity is still overwhelmingly produced by burning fossil fuels, the rise in cold production will inevitably © 2018 The Author(s).
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