Discussion on ‘selecting a working fluid for a Rankine-cycle engine’

Abstract

The authors state the usual assumption that 'the theoretical maximum effectiveness of transforming heat into mechanical work is the Carnot efficiency, namely: r/c c = 1 TCOr~D/TEv, where TEV and TCOND are the maximum and minimum absolute temperatures of the cycle, corresponding to the saturation temperatures, respectively, of the evaporator and condenser'. While this is true of the particular circumstances of this project, where the source of heat is condensation of steam at constant pressure, it is not generally true of either primary or waste-heat sources. These are commonly either a stream of hot gas or of hot liquid, both of which in yielding up their heat may vary in temperature. Hence the ideal cycle to utilise such variable-temperature heat is not a Carnot cycle, nor any other ideal cycle such as the Stirling cycle, which embodies isothermal heat reception. It is no accident that the majority of practical heat engines use largely or entirely non-isothermal heat reception processes--all gas turbines and internal-combustion engines and even steam turbines are supplied by 'boilers' in which the bulk of the heat is supplied over a range of temperature--feed heating, superheating and reheating. Nevertheless, the belief survives--and is perpetuated in all text books of thermodynamics-that the Carnot-cycle efficiency is the ultimate ideal for which all heat engines should strive. As this writer has pointed out,1 Carnot himself clearly laid down as the basis for his cycle both an infinite heat source and an infinite heat sink. Under such circumstances