Principles of naval engineering - Historic Naval Ships Association

Chapter 8-INTRODUCTION TO THERMODYNAMICSstatement indicates that water will not freezewhen heat is applied. Note that the Clausiusstatement includes and goes somewhat beyondthe common observation that heat flows onlyfrom a hotter to a colder substance.The statement that no process is possiblewhere the sole result is the removal of heatfrom a single reservoir and the performanceof an equivalent amount of work is known as theKelvin-Planck statement of the second law.Among other things, this statement says thatwe cannot expect the heat of friction to reverseitself and perform mechanical work. Morebroadly, this statement indicates a certain onesidednessthat is inherent in thermodynamicprocesses. Energy in the form of work can beconverted entirely to energy in the form of heat;but energy in the form of heat can never beentirely converted to energy in the form ofwork.A very important inference to be drawnfrom the second law is that no engine, actualor ideal, can convert all the heat supplied to itinto work, since some heat must always be rejectedto a receiver which is at a lower temperaturethan the source. In other words, therecan be no heat flow without a temperature differenceand there can be no conversion to workwithout a flow of heat. A further inference fromthis inference is sometimes given as a statementof the second law: No thermodynamic cycle canhave a thermal efficiency of 100 percent.We must say, then, that the first law ofthermodynamics deals with the conservation ofenergy and with the mutual convertibility ofheat and work, while the second law limits thedirection of thermodynamic processes and theextent of heat-to-work energy conversions.THE CONCEPT OF ENTROPYThe concept of reversibility and the secondlaw of thermodynamics are closely related tothe concept of entropy . In fact, the second lawmay be stated as: No process can occur in whichthe total entropy of an isolated system decreases;the total entropy of an isolated systemcan theoretically remain constant in some reversible(ideal) processes, but in all irreversible(real) processes the total entropy of an isolatedsystem must increase.From other statements of the second law, weknow that the transformation of heat to work isalways dependent upon a flow of heat from a hightemperature region to a low temperature region.The concept of the unavailability of a certainportion of the energy supplied as heat to anythermodynamic system is clearly implied in thesecond law, since it is apparent that some heatmust always be rejected to a receiver which isat a lower temperature than the source, if thereis to be any conversion of heat to work. Theheat which must be so rejected is thereforeunavailable for conversion into mechanical work.Entropy is an index of the unavailability ofenergy. Since heat can never be completely convertedinto work, we may think of entropy as ameasure or an indication of how much heatmust be rejected to a low temperature receiverif we are to utilize the rest of the heat for theproduction of useful work. We may also think ofentropy as an index or measure of the reversibilityof a process. All real processes areirreversible to some degree, and all real processesinvolve a "growth" or increase of entropy.Irreversibility and entropy are closelyrelated; any process in which entropy has increasedisan irreversible process.The entropy of an isolated system is at itsmaximum value when the system is in a stateof equilibrium. The concept of an absoluteminimum— that is, an absolute zero—value ofentropy is sometimes referred to as the thirdlaw of thermodynamics (or Nernst's law). Thisprinciple states that the absolute zero of entropywould occur at the absolute zero of temperaturefor any pure material in the crystallinestate. By extension, therefore, it should bepossible to assign absolute values to the entropyof pure materials, if such absolute values wereneeded. For most purposes, however, we areinterested in knowing the values of the changesin entropy rather than the absolute values ofentropy. Hence an arbitrary zero point forentropy has been established at 32° F.Entropy changes depend upon the amount ofheat transferred to or from the working fluid,upon the absolute temperature of the heat source,and upon the absolute temperature of the heatreceiver. Although actual entropy calculationsare complex beyond the scope of this text, oneequation is given here to indicate the units inwhich entropy is measured and to give therelationship between entropy and heat and temperature.Note that this equation applies only toa reversible isothermal process in which Tj =T2.S S Q^2 ~ ''l = T181