The CRC Handbook of Mechanical Engineering. Chapter 2. Engineering Thermodynamics (776125), страница 19
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It is an external combustion engine.© 1999 by CRC Press LLC2-86Section 2sFIGURE 2.19 (A) Carnot, (B) Ericsson, and (C) Stirling cycles.© 1999 by CRC Press LLCEngineering Thermodynamics2-872.7 Guidelines for Improving Thermodynamic EffectivenessThermal design frequently aims at the most effective system from the cost viewpoint.
Still, in the costoptimization process, particularly of complex energy systems, it is often expedient to begin by identifyinga design that is nearly optimal thermodynamically; such a design can then be used as a point of departurefor cost optimization. Presented in this section are guidelines for improving the use of fuels (naturalgas, oil, and coal) by reducing sources of thermodynamic inefficiency in thermal systems. Furtherdiscussion is provided by Bejan et al. (1996).To improve thermodynamic effectiveness it is necessary to deal directly with inefficiencies related toexergy destruction and exergy loss. The primary contributors to exergy destruction are chemical reaction,heat transfer, mixing, and friction, including unrestrained expansions of gases and liquids.
To deal withthem effectively, the principal sources of inefficiency not only should be understood qualitatively, butalso determined quantitatively, at least approximately. Design changes to improve effectiveness must bedone judiciously, however, for the cost associated with different sources of inefficiency can be different.For example, the unit cost of the electrical or mechanical power required to provide for the exergydestroyed owing to a pressure drop is generally higher than the unit cost of the fuel required for theexergy destruction caused by combustion or heat transfer.Since chemical reaction is a significant source of thermodynamic inefficiency, it is generally goodpractice to minimize the use of combustion.
In many applications the use of combustion equipment suchas boilers is unavoidable, however. In these cases a significant reduction in the combustion irreversibilityby conventional means simply cannot be expected, for the major part of the exergy destruction introducedby combustion is an inevitable consequence of incorporating such equipment.
Still, the exergy destructionin practical combustion systems can be reduced by minimizing the use of excess air and by preheatingthe reactants. In most cases only a small part of the exergy destruction in a combustion chamber can beavoided by these means. Consequently, after considering such options for reducing the exergy destructionrelated to combustion, efforts to improve thermodynamic performance should focus on components ofthe overall system that are more amenable to betterment by cost-effective conventional measures. Inother words, some exergy destructions and energy losses can be avoided, others cannot. Efforts shouldbe centered on those that can be avoided.Nonidealities associated with heat transfer also typically contribute heavily to inefficiency.
Accordingly, unnecessary or cost-ineffective heat transfer must be avoided. Additional guidelines follow:• The higher the temperature T at which a heat transfer occurs in cases where T > T0, where T0denotes the temperature of the environment (Section 2.5), the more valuable the heat transfer and,consequently, the greater the need to avoid heat transfer to the ambient, to cooling water, or to arefrigerated stream.
Heat transfer across T0 should be avoided.• The lower the temperature T at which a heat transfer occurs in cases where T < T0, the morevaluable the heat transfer and, consequently, the greater the need to avoid direct heat transfer withthe ambient or a heated stream.• Since exergy destruction associated with heat transfer between streams varies inversely with thetemperature level, the lower the temperature level, the greater the need to minimize the streamto-stream temperature difference.• Avoid the use of intermediate heat transfer fluids when exchanging energy by heat transfer betweentwo streamsAlthough irreversibilities related to friction, unrestrained expansion, and mixing are often secondaryin importance to those of combustion and heat transfer, they should not be overlooked, and the followingguidelines apply:• Relatively more attention should be paid to the design of the lower temperature stages of turbinesand compressors (the last stages of turbines and the first stages of compressors) than to theremaining stages of these devices.© 1999 by CRC Press LLC2-88Section 2• For turbines, compressors, and motors, consider the most thermodynamically efficient options.• Minimize the use of throttling; check whether power recovery expanders are a cost-effectivealternative for pressure reduction.• Avoid processes using excessively large thermodynamic driving forces (differences in temperature,pressure, and chemical composition).
In particular, minimize the mixing of streams differingsignificantly in temperature, pressure, or chemical composition.• The greater the mass rate of flow, the greater the need to use the exergy of the stream effectively.• The lower the temperature level, the greater the need to minimize friction.Flowsheeting or process simulation software can assist efforts aimed at improving thermodynamiceffectiveness by allowing engineers to readily model the behavior of an overall system, or systemcomponents, under specified conditions and do the required thermal analysis, sizing, costing, andoptimization. Many of the more widely used flowsheeting programs: ASPEN PLUS, PROCESS, andCHEMCAD are of the sequential-modular type. SPEEDUP is a popular program of the equation-solvertype.
Since process simulation is a rapidly evolving field, vendors should be contacted for up-to-dateinformation concerning the features of flowsheeting software, including optimization capabilities (ifany). As background for further investigation of suitable software, see Biegler (1989) for a survey ofthe capabilities of 15 software products.ReferencesAhrendts, J. 1980. Reference states. Energy Int. J.
5: 667–677.ASHRAE Handbook 1993 Fundamentals. 1993. American Society of Heating, Refrigerating, and AirConditioning Engineers, Atlanta.ASME Steam Tables, 6th ed. 1993. ASME Press, Fairfield, NJ.Bejan, A., Tsatsaronis, G., and Moran, M. 1996. Thermal Design and Optimization, John Wiley & Sons,New York.Biegler, L.T. 1989. Chemical process simulation. Chem. Eng. Progr. October: 50–61.Bird, R.B., Stewart, W.E., and Lightfoot, E.N.
1960. Transport Phenomena. John Wiley & Sons, NewYork.Bolz, R.E. and Tuve, G.L. (eds.). 1973. Handbook of Tables for Applied Engineering Science, 2nd ed.CRC Press, Boca Raton, FL.Bornakke, C. and Sonntag, R.E. 1996. Tables of Thermodynamic and Transport Properties. John Wiley& Sons, New York.Cooper, H.W. and Goldfrank, J.C. 1967. B-W-R constants and new correlations. Hydrocarbon Processing. 46(12): 141–146.Gray, D.E.
(ed.). 1972. American Institute of Physics Handbook. McGraw-Hill, New York.Haar, L. Gallagher, J.S., and Kell, G.S. 1984. NBS/NRC Steam Tables. Hemisphere, New York.Handbook of Chemistry and Physics, annual editions. CRC Press, Boca Raton, FL.JANAF Thermochemical Tables, 3rd ed. 1986. American Chemical Society and the American Instituteof Physics for the National Bureau of Standards.Jones, J.B. and Dugan, R.E. 1996. Engineering Thermodynamics. Prentice-Hall, Englewood Cliffs, NJ.Keenan, J.H., Keyes, F.G., Hill, P.G., and Moore, J.G.
1969 and 1978. Steam Tables. John Wiley &Sons, New York (1969, English Units; 1978, SI Units).Keenan, J.H., Chao, J., and Kaye, J. 1980 and 1983. Gas Tables — International Version, 2nd ed. JohnWiley & Sons, New York (1980, English Units; 1983, SI Units).Knacke, O., Kubaschewski, O., and Hesselmann, K. 1991. Thermochemical Properties of InorganicSubstances, 2nd ed.
Springer-Verlag, Berlin.Kotas, T.J. 1995. The Exergy Method of Thermal Plant Analysis, Krieger, Melbourne, FL.Lee, B.I. and Kessler, M.G. 1975. A generalized thermodynamic correlation based on three-parametercorresponding states. AIChE J. 21: 510–527.© 1999 by CRC Press LLCEngineering Thermodynamics2-89Liley, P.E. 1987.
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